Contact Potential Difference under Illumination CPD dark Contact Potential Difference in Complete Darkness

Keywords:

During this thesis, model donor-acceptor (DA) systems for organic photovoltaics have been studied by non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). To enhance the understanding of the optoelectronic processes on the nanoscale, DA systems with better defined structural and electronic properties than the one of most bulk heterojunction blends (BHJ), have been studied.

With DA phase-separations of below 10nm in organic photovoltaic systems, the highest possible resolution has to be achieved by KPFM to investigate optoelectronic processes. It has been shown that nc-AFM/KPFM measurements in the regime of short range (SR) forces can increase imaging resolution. In preparation of such investigations, the influence of the interaction regime on the topographic measurement via KPFM has been studied for a self-assembly of P3DDT on HOPG. It is demonstrated that imaging in the SR-regime not only increases the lateral resolution, but also assures correct topographic height values.

In a next step, DA blends of FG1:[70]PCMB have been studied by KPFM. For these BHJs, the structure and the scale of the DA phase-separation can be tuned via the liquid crystal behavior of the donor FG1. The in dark potential contrasts are consistent with surface and bulk morphology. The relationship between the surface photovoltage (SPV) and the tip-sample interaction regime has been analyzed. Increased SPV resolutions and image contrasts are obtained in the SR-regime. Furthermore, due to an optical field exaltation caused by the AFM tip, an amplified SPV signal is observed at small tip-sample distances.

Finally, a new generation of DA dyads with donor and acceptor moieties has been studied. Its self-assembly on HOPG has been determined via a comparative study by scanning tunneling microscopy and nc-AFM plus molecular mechanics and dynamics simulations. By KPFM the charge carrier generation and collection has been analyzed down to the level of a single molecular layer. A clear relationship between the dyads' molecular assembly and their photovoltaic properties can be established. ix

Resumé

Pour cette thèse, des systèmes donneur-accepteur (DA) modèles pour le photovoltaïque organique ont été étudiés par microscopie à force atomique en mode non contact (nc-AFM) et microscopie à sonde de Kelvin (KPFM). Ces systèmes DA présentent une structure et des propriétés électroniques mieux contrôlées que dans la plupart des hétérojonctions DA en volume.

Afin, d'étudier les propriétés optoélectroniques d'architectures DA présentant une séparation de phase à l'échelle de la dizaine de nanomètres, il est indispensable d'optimiser la résolution des modes nc-AFM/KPFM. Dans ce travail, l'influence du régime d'interaction pointe-surface sur les mesures a été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes.

Ensuite, un système DA à base de FG1:[70]PCBM a été étudié. Pour ce mélange DA, la nanostructure et l'échelle de la séparation de phase peuvent être ajustées grâce aux propriétés cristal liquide du composé donneur FG1. Les potentiels mesurés dans le noir sont consistants avec la morphologie attendue en surface et en volume. La relation entre le photo-potentiel de surface (SPV) et le régime d'interaction pointe-surface a pu être précisément analysée. Une meilleure résolution et un meilleur contraste des images SPV sont obtenus dans le régime à courte portée. De plus, à cause d'une exaltation du champ optique causée par la pointe, un signal SPV amplifié est mesuré pour des petites distances pointe-surface.

Enfin, une nouvelle génération de diades comprenant des groupements donneur et accepteur a été étudiée. La nature de l'auto-assemblage sur HOPG a été établie sur la base d'études comparatives de microscopie à effet tunnel et de nc-AFM, avec le support de simulations en mécanique et dynamique moléculaire. Les mesures de photo-potentiel de surface ont ensuite permis de démontrer qu'il était possible d'étudier les mécanismes de photo-génération des porteurs jusqu'à l'échelle de la mono-couche moléculaire.

One of the biggest challenges of the 21st century consists in finding a solution to the world's increasing energy demand and in limiting the human impact on the earth's climate. From 1993 to 2011 the world population increased by 27% and currently over 7 billion humans are living on our planet [1]. The energy demand of this growing population is to a great part answered by the consumption of the earth's limited fossil fuel resources. In 2013, for example Germany's primary energy consumption of about 14 ¨10 18 Joule was only satisfied by about 12% via sustainable and renewable energy sources like bio mass, hydro power and solar energy (Figure 1 Joule with different contributions. The renewable energies satisfy only 11.8% of the total primary energy consumption [2].

This extreme consumption of fossil fuels is not only problematic because of the limited resources, but also because of the emission of the greenhouse gas CO 2 upon the conversion into energy. In recent years a broad consensus developed among scientists that the rise in the global temperature is human-made and caused by these carbon dioxide emissions [3]. The thereby induced climate change will have severe effects on our planet, like the rise of the sea level, desertification, drink water shortages and even possible changes of the ocean currents. To counteract these developments, the use of renewable energies represents the only reasonable solution.

Among the different renewable energy sources, the sun is the most abundant one. If one would install photovoltaic (PV) panels of 10% efficiency in the Sahara (local solar irradiation 285W {m 2 [4]), the annual world energy need of about 550 ¨10 18 Joule [2] could be produced on roughly 7% of this desert's size (Sahara surface " 9.4 ¨10 6 km 2 ). But even in regions of weaker solar irradiation photovoltaic plants have a huge potential and can contribute significantly to a country's electrical power supply. In Germany in 2013 the share of photovoltaics in the renewables-based electricity supply was of 19.7% and hence higher than the one of hydro power with 13.9% [5].

Most of today's installed solar panels are single junction cells based on crystalline silicon. However there is a multitude of other photovoltaic technologies and systems with continuously increasing efficiencies (Figure 1.2). For monocrystalline silicon PV cells with efficiencies of up to 25% already half of the theoretically achievable performance was achieved [6,7]. Thin layer solar cells have been developed for a reduced material cost and show with efficiencies of up to 20% for CIGS and CdTe competitive performances (Figure 1.2). The highest efficiency so far with values over 40% has been achieved by multi-junction solar cells based for example on a InGaP/GaAs/InGaAs structure [7]. The focus of this thesis is directed towards another type of photovoltaic system based on organic π-conjugated materials (see Chapter 3). These organic (semi-)conductors create a promising complementary branch in view of silicon technologies. Organic photovoltaic (OPV) devices are mostly based on a combination of polymers (polythiophenes, polytriphenylamines,...) plus fullerene derivates or small π-conjugated molecules [8]. Recently, You et al. achieved a new efficiency record of 10.2% with a polymer tandem solar cell. Besides such relatively high performances, OPV cells also offer numerous advantages compared to inorganic solar cells. In contrast to the latter they can be produced without any energy intensive manufacturing process and consequently they possess a short energy payback time. A simple and cheap fabrication via roll-to-roll printing or other low-temperature deposition techniques can be applied [8]. Furthermore, these OPV cells are of light weight and can potentially be constructed on flexible substrates thereby offering possibilities for new applications like the integration in mobile devices. Finally OPV devices are characterized by a good low-light performance due to the relatively high absorbances of organic semiconductors which makes them also very interesting for use in diffuse light and indoor conditions. [9]. transparent OPV glass facades for buildings to smaller flexible organic solar cells for the integration in bags or mobile devices.

Although first OPV devices are on the market, a comprehensive and clear understanding of all parameters necessary for a full quantitative description of the physical processes within an organic solar cell is still missing [10]. In this endeavor, model organic donor-acceptor (DA) systems have been studied during this thesis. These, in comparison to commonly used bulk heterojunction blends, better controlled DA systems allow the engineering of electronic and structural properties and facilitate thereby the investigation of the optoelectronic processes on the nanoscale. Among the different characterization techniques for organic photovoltaic systems, scanning probe microscopy techniques play an important role. Since the invention of the scanning tunneling microscope (STM) by Binnig, Rohrer, Gerber and Weibel [11] and the invention of the atomic force microscope (AFM) by Binnig, Quate and Gerber [12] these surface characterization instruments have had a huge impact in material science. However, to further enhance the understanding of charge carrier generation and collection in the active photovoltaic layer it is essential to probe not only the morphological nano-structure but also the local electronic properties. For this purpose, Kelvin Probe Force Microscopy (KPFM) [13] proved to be a very powerful tool. It enables the simultaneous investigation of topography and surface potential with an exceptional level of resolution [14] and has therefore been mainly used for this thesis.

| Outline of the Thesis

In the following a short outline of this thesis will be presented:

Chapter 2 introduces the employed scanning probe microscopy techniques. In particular the different tip-sample interaction forces will be discussed together with the contact potential difference in the long-range and short-range regime. An overview of the used ultra high vacuum AFM/STM system will be given.

In Chapter 3 the basics of organic photovoltaics will be introduced including all elements needed for the discussion of the experimental results. After the presentation of π-conjugated molecules and polymers, the metal-organic and organic-organic interface will be addressed being of huge importance for organic electronic devices including OPV. Finally the optoelectronic processes in donor-acceptor systems for organic solar cells will be discussed in detail.

Chapter 4 will present the laser illumination setup which has been employed in combination with the scanning probe microscopy setup. The correct measurement of surface photovoltages via Kelvin probe force microscopy will be discussed. Furthermore the context of the following KPFM investigations of organic photovoltaic systems under illumination will be given on the basis of important results taken from literature.

With DA phase-separations on the nanoscale in organic photovoltaic systems, the highest possible resolution has to be achieved by KPFM to investigate optoelectronic processes. It has been shown that nc-AFM/KPFM measurements in the regime of short range (SR) forces can increase imaging resolution [15]. In preparation of such investigations, the influence of the interaction regime on the topographic measurement via KPFM will be studied in Chapter 5 for a self-assembly of the π-conjugated polymer P3DDT on highly oriented pyrolitic graphite (HOPG).

In Chapter 6, DA blends of FG1: [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCMB will be studied by KPFM. For these "model" bulk heterojunction blends, the structure and the scale of the DA phase-separation can be tuned via the liquid crystal behavior of the donor FG1. The potential contrast in dark and under illumination will be discussed with regard to the surface and bulk morphology. In particular also the relationship between the surface photovoltage and the tip-sample interaction regime will be analyzed.

Finally, a new generation of DA dyad with donor and acceptor moieties will be discussed in Chapter 7. Its self-assembly on HOPG will be investigated via a comparative study by scanning tunneling microscopy and nc-AFM plus molecular mechanics and dynamics simulations. By KPFM the charge carrier generation and collection will be analyzed down to the level of a single molecular layer in order to establish a clear relationship between the dyads' molecular assembly and their photovoltaic properties.

The conclusion in Chapter 8 will summarize the thesis, discuss current projects and describe future perspectives. Techniques Contents Les techniques expérimentales utilisées dans ce travail sont présentées dans ce chapitre, à savoir la microscopie à effet tunnel (STM), la microscopie à force atomique en mode non contact (nc-AFM) et la microscopie à sonde de Kelvin (KPFM). En introduction, nous rappelons brièvement le contexte historique du développement des techniques de champ proche et donnons quelques illustrations de résultats obtenus par des groupes internationaux au meilleur niveau de l'état de l'art. Les principes de base des microscopies STM et AFM sont ensuite introduits, les modes dynamiques (AM-AFM et FM-AFM) étant plus spécifiquement détaillés dans le second cas. Les différentes forces d'interaction pointe-surface (forces électrostatiques, de van der Waals et chimiques) sont décrites, ainsi que les principes de fonctionnement de la microscopie à sonde de Kelvin. On rappelle notamment les principales différences entre les modes AM-KPFM et FM-KPFM, et la notion de contact de potentiel local (LCPD) est introduite. Enfin, nous donnons une présentation synthétique du système expérimental utilisé pour réaliser les études présentées dans ce manuscrit.

| Context

In March 1981 Binnig, Rohrer, Gerber and Weibel opened the door to the nanoworld with the invention of the Scanning Tunneling Microscope (STM) [1] and the field of Scanning Probe Microscopy (SPM) was born. Five years later Binnig and Rohrer were awarded the Nobel Price in physics for their discovery. The invention of the STM represents a major breakthrough in surface-and nanoscience because it enabled for the first time the investigation of matter on an atomic scale. Individual surface atoms could be made visible on flat surfaces in real space. Due to this capability the STM helped to solve one of the most fascinating and controversially discussed problems of that time, the determination of the surface reconstruction of Si(111) 7x7. Binning and Rohrer were able to image this reconstruction by STM in 1983 [2] (see Figure 2.1 (a)) and shortly after the dimer-adatom-stacking fault (DAS) model [3] could be developed for Si(111) 7x7 [4]. Nowadays, STM is employed in a huge variety of research domains. As one example for an important finding acquired by this technique the quantum corral in Figure 2.1 (b) is shown [5,6,7]. In 1990, Eigler et al. discovered that the STM can not only be used for imaging but also for manipulations on the atomic scale [8]. For the quantum corral, Crommie et al. arranged a circle of iron atoms on a copper(111) surface. The confined 2D electron gas within the iron circle exhibits a quantum mechanical interference pattern that was finally imaged by STM.

A disadvantage of STM is however the need for conductive samples because this technique is based on a tunneling current between the STM tip and the investigated surface. Already early STM experiments revealed though that in close approach significant forces can act between tip and sample additionally to the tunneling current [4]. This finding motivated Binnig, Quate and Gerber in 1985 to use these interaction forces for imaging and to invent the Atomic Force Microscope (AFM) [12]. With this new instrument the imaging on insulators became possible. In AFM a flexible cantilever with tip is scanned across the sample surface. The cantilever probes the tip-sample interaction forces and thereby traces the sample's topographic contours. Binnig and Quate already anticipated that atomic resolution can be achieved via AFM since its invention. However it took several years until the first atomic 111) surface with 7x7 reconstruction [9]. (b) Chemical identification of individual surface atoms by AFM via chemical force measurements [10]. (c) Chemical structure of the molecule pentacene (upper image) revealed by nc-AFM with CO-functionalized AFM tip (qPlus tuning fork sensor, T " 5K) [11].

resolution was obtained on inert surfaces like KBr(001) [13] or calcite(1014) [14]. The development of the dynamic AFM by Martin et al. [15] and Albrecht et al. [16] represented another major step forward for the AFM. For the dynamic AFM, the cantilever is used as mechanically oscillating resonator and the tip-sample interaction forces can induce a shift in its resonance frequency proportional to the force gradient. This new mode finally allowed F. Giessibl in 1995 to achieve true atomic resolution also on a reactive surface like the well-known Si(111) 7x7 (see Figure 2.2 (a)) [17].

Another important finding obtained via AFM that proofs the large possibilities of this scanning probe microscopy technique is shown in Figure 2.2 (b). Sugimoto et al. demonstrated the possibility to determine the chemical identity of individual surface atoms by spectroscopic force measurements above every single atom [10]. In the year 2000, another milestone for the AFM was set by F. Giessibl due to the introduction of the tuning fork AFM [18,19]. This AFM is based on a self-sensing quartz crystal resonator (similar to the one used in watches) which is with typical force constants of about 1800N {m very stiff and allows oscillation amplitudes of below 0.1nm. Due to these very low amplitudes, the tuning fork sensor is very sensitive to short-range forces, which exhibit decay lengths in the same range, and very high resolutions can be obtained. With these tuning fork sensors and at the very stable conditions of ultra high vacuum (UHV) and temperatures of 5K, Gross et al. were able to image the chemical structure of a molecule for the very first time [11] (see Figure 2.2 (c)).

Besides such impressive topographic investigations on the nanoscale, the atomic force microscope has still more to offer. Gross et al. showed that via tuning fork AFM it is possible to measure even the charge state of an adatom on a surface and to probe single charges (see More generally, in order to image potential distributions on a surface, the Kelvin Probe Force Microscopy (KPFM) has been introduced by Nonnenmacher et al. in 1991 [22]. This AFM variant enables work function measurements down to the nanometer scale based on the detection of electrostatic forces. These forces arise from work function differences between tip and sample and are compensated by applying a bias which equals the so-called contact potential difference (CPD) [23]. In UHV conditions, KPFM is mostly combined with a dynamic mode non-contact AFM in order to simultaneously investigate the topography and the CPD. When performing high resolution KPFM imaging in the regime of short-range forces, the interpretation of the CPD is more sophisticated and it is referred to the local CPD (LCPD) [24]. A recent breakthrough in KPFM imaging has been achieved by Mohn et al. who were able to image the charge distribution within a single molecule [21]. This result, obtained for the molecule naphthalocyanine, is shown in Figure 2. 3 (c,d,e) together with the frequency shift image and the DFT calculation of the electrostatic field.

Having described the historical background and some major experimental findings obtained by scanning probe microscopy, the working principle of these microscopy techniques will be discussed in the following sections.

| Scanning Tunneling Microscopy

STM was the first SPM technique which emerged over thirty years ago and which is still intensively used in research groups all around the world. During this thesis it was mainly used as complementary technique for the AFM and KPFM investigations and it will therefore only be briefly introduced.

For STM, a sharp tip is scanned via a piezoelectric tube scanner (xyz scanner) over the sample surface with sub-atomic precision [4] (see Figure 2.4 (a)). Prior to the experiment, the tip, which is usually a W, PtIr or Ir wire, has been sharpened via chemical etching or mechanical grinding. A so-called tunneling bias V tunneling is applied between tip and sample. For the measurements performed during this thesis, the sample was grounded and V tunneling was applied to the tip. The STM is based on the principle of electron tunneling between two conductive electrodes in close approach (typically several Ångström). From the point of view of classical physics no current flow should be possible, even when applying a bias of several Volts between both of them. However, since the 1920s, the phenomena of quantum tunneling is known [26] and electron tunneling between two metal electrodes separated by a thin potential barrier has first been experimentally confirmed by Giaever et al. in 1960 [27]. The energetic situation of such an electron tunneling process between tip and sample across the vacuum is illustrated in Figure 2.4 (b) with the work functions Φ and Fermi level E F of tip and sample and the applied tunneling bias V tunneling . Typically obtained tunneling currents I tunneling between STM tip and sample lie in the range of pA to nA.

In order to understand the tip-sample distance z dependence of I tunneling , a quantum mechanical model calculation can be performed. The time dependent Schrödinger equation can be solved for the one-dimensional tunneling process depicted in Figure 2.4 (c) with a rectangular potential barrier of diameter a and height V 0 . The wavefunction solution after the potential barrier corresponds to Ψpzq " A exppikzq with the wavenumber k " a 2m e E{� 2 [28]. In these equations, m e corresponds to the electron mass and E ă V 0 to the electron energy. The transmission coefficient T for tunneling through a relatively high potential barrier can be estimated by T " 16EpV 0 ´Eq{V 2 0 ¨expp´2Kaq with K " a 2mpV 0 ´Eq{� 2 [28]. This transmission probability, which decreases exponentially with z, also induces an exponential dependence of the tunneling current: I tunneling pzq " I 0 expp´2Kzq.

(2.1)

For an exact quantitative description of the electron tunneling between tip and sample, however, a three-dimensional description of this quantum mechanical problem is needed [29]. This calculation confirms the exponential z-dependence and also includes the influence of the local density of states, because electron tunneling is only possible from occupied states of the one electrode into unoccupied states of the other.

Due to this very strong, exponential distance dependence of I tunneling , most of the tunneling current passes through the front atom of the tip apex. This current is used as feedback signal for the topographic STM imaging. In general, two different imaging modes are possible: constant current mode and constant height mode [4]. In the first mode, I tunneling is kept constant and the tip follows the topographic contours of the sample (ñ zpx, y, I tunneling " constq). In the second mode, the height z of the tip is kept constant and the current is recorded (ñ I tunneling px, y, z " constq). In the constant height mode the scan speed is not limited by the STM feedback loop, however, this mode is only possible for atomically flat surfaces to avoid tip crashes. Finally it remains important to say, that the STM image contrast does not only depend on the samples topography, but also on the local density of states which is probed via the electron tunneling [25]. Consequently special attention has to be paid when interpreting the acquired STM images. Figure 2.4 (d,e) shows two STM images on the reference surfaces HOPG and Si(111) 7x7 that have been measured during this thesis for instrument calibration.

| Atomic Force Microscopy

Contrary to STM, the AFM does not rely on a tunneling current but on the interaction forces between tip and sample. Consequently, the AFM can also be used to investigate non-conductive samples. The flexible AFM cantilever with tip is scanned across the sample surface and thereby traces the sample's topography. In most cases silicon cantilever are used with their specific spring constant k, resonance frequency f 0 and quality factor Q (see below). To precisely measure the cantilever bending during the measurement, the beam deflection system has been introduced by Meyer et al. in 1988 [30]. Nowadays it is the most common detection system and relies on the reflection of a laser beam by the cantilever with following beam detection via a four quadrant photo-sensitive diode (PSD) (see also Figure 2.11 (c)). In the case of the self-sensing tuning-fork sensors such an additional detection system is not needed.

For AFM imaging, different modes have been established over the past decades. They will be discussed in the following section with a particular emphasis on the frequency-modulation (FM) AFM mode that has been employed during this thesis.

| AFM Operation Modes

Basically there are two modes for the AFM operation, the static mode (contact mode) or the dynamic mode. In the contact mode, the topography is directly measured via the static cantilever deflection, which is proportional to the tip-sample interaction force [31]. However, due to the normally large contact area, true atomic resolution can hardly be achieved and the sample surface can potentially be modified by the tip. Due to these limitations, dynamic mode AFM has been developed. In the latter mode a mechanical actuator excites the cantilever at its resonance frequency f 0 and stable imaging conditions without a (pronounced) tip-sample contact can be obtained. The two existing dynamic modes, amplitude modulation (AM) and frequency modulation (FM) mode, are illustrated in Frequency modulation mode, which uses frequency changes as feedback signal (illustration according to [24]).

In the AM-AFM mode (also called tapping mode), the cantilever oscillates under constant excitation and at a fixed frequency, which corresponds to its resonance frequency f 0 [31]. Upon variations in the tip-sample distance, and thus variations in the tip-sample interaction, the cantilever's oscillation amplitude A changes. The amplitude change is measured and used as feedback signal in order to keep the tip-sample distance at the demanded setpoint. Thereby the cantilever follows the sample's topographic contours and the surface is imaged. An analytical description based on the harmonic oscillator model can demonstrate that the amplitude change is directly related to the tip-sample interaction forces. Thus, in AM-AFM, the force F int between tip and sample is directly measured [4,32].

In the FM-AFM mode (non-contact mode), the cantilever amplitude A is maintained at a fixed setpoint and the oscillation frequency is used as feedback signal (Figure 2.5 (b)). Upon changes in the tip-sample distance, and thus changes in the tip-sample interaction, the cantilever's resonance frequency is shifted. In FM-AFM mode, the feedback regulation tries to keep the oscillating cantilever at a fixed frequency shift Δf " pf ´f0 q, i. e. a fixed deviation from its initial resonance frequency f 0 (far away from the surface) is maintained [33]. Therefore the cantilever's z-position above the sample is adapted to keep the oscillation at a constant frequency of f " pf 0 `Δf q and the surface topography can be imaged. As it will be shown below, the frequency changes directly depend on the force gradient between tip and sample (in the limit of small A and for a cantilever restoring force large compared to F int ). Thus, in FM-AFM, the force gradient BF int {Bz is detected rather than the force F int itself, which induces a increased spatial resolution compared to the AM-AFM mode [32,33].

A very important characteristic quantity in dynamic mode AFM is the quality factor Q of the vibrating cantilever. It strongly influences the imaging resolution and characterizes the bandwidth or the FWHM (full width half maxima) of the cantilever resonance relative to f 0 :

Q " f 0 F W HM . (2.2)
A high Q factor indicates a low rate of energy loss due to dissipation relative to the stored energy in the cantilever oscillation [32]. In air, cantilever have typical Q values of several 100. However in UHV, due to the absence of the surrounding viscous air, Q can exhibit values in the order of 10 4 . The expression for the minimum detectable frequency shift δpΔf q min by FM-AFM

δpΔf q min " d f 0 k B T B 4 π k Q xz 2 osc y (2.3)
directly visualizes the impact of the Q factor on the measurement resolution [16,32]. Here, k B T denotes the thermal energy, B the measurement bandwidth and xz 2 osc y the mean-square amplitude of the driven cantilever oscillation. Obviously the measurement sensitivity increases with Q which makes the AFM operation in UHV particularly advantageous. However, not both dynamic modes are equally suited for the use in UHV and high Q factors. The time scale of an amplitude change in AM-mode τ AM « 2Q{f 0 exhibits a linear Q dependence. In contrast, the time scale of a frequency change in FM-mode τ F M « 1{f 0 is independent of Q. Consequently, measurements via AM-AFM in UHV are extremely slow. FM-AFM allows higher scan rates and is the mode of choice in UHV conditions.

Having employed the FM-mode for all AFM experiments during this thesis, this mode will be discussed more in detail. Figure 2.6 illustrates the functional principle and the feedback system of a FM-AFM setup. The deflection signal coming from the PSD is preamplified and passed through a bandpass filter. On the one hand the automatic gain control measures the amplitude A, compares it to the amplitude setpoint A 0 and adapts the excitation amplitude A X of the cantilever actuator accordingly in order to assure an oscillation with the demanded amplitude setpoint. The energy lost via dissipation effects (damping) is thereby restored. On the other hand, a phase-locked loop (PLL), measures the frequency shift Δf coming from the bandpass filter and adapts the cantilever z-position via the z feedback circuit to always maintain the demanded frequency shift setpoint Δf 0 . Furthermore A cantilever, which is mechanically excited at its resonance frequency, is scanned via the piezo tube scanner over the sample surface. The tip sample interaction forces can induce a frequency shift Δf of the cantilever oscillation. This shift Δf represents the imaging signal and can for example be detected by a beam deflection system with photo sensitive diode (PSD). The black boxes correspond to the different components of the FM-AFM feedback system. Red boxes stand for amplitude, frequency shift and phase setpoints and the blue box contains all obtained physical observables (schema according to L. Nony [34]). it assures that the cantilever is always oscillating at its resonance frequency with the help of a phase shifter. Therefore the latter takes the incoming detected oscillation signal and shifts it by the phase setpoint ϕ " ´π{2 before feeding it back to the cantilever actuator. Together, automatic gain control and the PLL plus phase shifter are generating the driving signal for the cantilever oscillation with A " A 0 and ϕ " ´π{2. The topographic signal is obtained from the z feedback circuit and generates a topographic map of constant Δf .

From a theoretical point of view, far away from the sample surface and thus without tip-sample interaction forces F int , the cantilever corresponds to a forced harmonic oscillator with damping. It can be described by the following equation of motion [32,34]:

: zptq `2 π f 0 Q 9 zptq `p2 π f 0 q 2 zptq " p2 π f 0 q 2 A X cosp2 π f tq. (2.4)
After the acceleration : zptq, the next term from the left describes the influence of the intrinsic dissipation of the system, followed by the restoring term being linear in z for a harmonic oscillator. On the right side of the equation the excitation term can be found with the driving frequency f and the excitation amplitude A X . f 0 " 1{p2πq a k{m ˚denotes the resonance frequency of the harmonic oscillator without external excitation and m ˚corresponds to the cantilever's effective mass. With the harmonic solution zptq " Acosp2 π f t `ϕq of this differential equation one obtains the resonance curves Apf q and ϕpf q of the driven cantilever:

Apf q " A 0 b Q 2 p1 ´pf {f 0 q 2 q 2 `pf {f 0 q 2 ϕpf q " arctan ˆf f 0 Q pf 2 ´f 2 0 q ˙. (2.5)
The last equation confirms that a phase-shift of ϕ " ´π{2 has to be applied between cantilever oscillation and excitation in order to keep a resonant cantilever oscillation (f Ñ f 0 ).

If the cantilever is now brought in close approach to the sample surface, tip-sample interaction forces F int occur and an additional term has to be added to the system's motion equation [32,34]:

: zptq `2 π f 0 Q 9 zptq `p2 π f 0 q 2 zptq " p2 π f 0 q 2 A X cosp2 π f tq `p2 π f 0 q 2 F int pzptqq k . (2.6)
Because of the different tip-sample interaction forces (see Section 2.3.2 below), which are not linearly dependent on the tip-sample separation z, this last term adds a nonlinear z-dependent component to the differential equation. Consequently this equation can not be analytically solved without any approximations and the amplitude and the phase will have an additional z-dependence (ñ Apf, zq, ϕpf, zq) [34]. The whole system is completely described by the five variables A X , A, ϕ, f and z, with the latter describing the mean tip-sample distance.

In FM-AFM, two of these parameters are fixed to values of the free oscillator (A " A 0 , ϕ " ´π{2) and the frequency shift Δf " pf ´f0 q becomes the most important measurement parameter. For small amplitudes and not too strong force gradients BF int {Bz, the interaction forces can be linearized and one can directly determine F int from Δf via [16,35]

Δf f 0 " ´1 2 k BF int Bz . (2.7)
This equations applies for example well for the stiff tuning fork sensors with amplitudes A 0 of 20 to 100pm. However for normal cantilever-based AFM, oscillation amplitudes lie normally well above 1nm. Consequently the tip probes a significant part of the nonlinear force-field and BF int {Bz is not constant over one cantilever oscillation cycle (see also Figure 2.7 below). For this case F. Giessibl introduced an approach via canonical perturbation theory in the limit of a large maximum restoring force in comparison to the maximum interaction force (kA " F int,max ). This approach results in first order to the following expression [35,36]:

Δf f 0 kA " ż 2π 0 dφ 2π F int pz `A cosφq cosφ. (2.8)
In this equation, the integral with integration parameter φ represents the integration over one oscillation cycle. All quantities on the left side can be experimentally obtained and then be compared to the integrated force [36]. In order to obtain an inverse version F int pΔf q of the above mentioned equation, approximative inversion procedures have been developed, for example by Sader and Jarvis [37,38].

| Tip Sample Interaction Forces

In the following section the different force contributions to the total force between tip and sample F int pzq will be discussed. F int pzq with its typical tip-sample distance z dependence is depicted in The tip probes with its amplitude A cantilever a certain part of the force curve around the setpoint. Repulsive and attractive forces can appear between the probe and the sample. The long-range electrostatic (LRE) and the longrange (LR) van der Waals (vdW) forces dominate the force curve at increased tip-sample distances, the different short range (SR) forces dominate at small z. Depending on the chosen setpoint, measurements can be performed in the contact and the non-contact regime.

In ultra high vacuum, the total interaction force consists in general of long-range (LR) electrostatic forces F el , LR van der Waals forces F vdW and a sum of different short-range (SR) forces F SR , like chemical forces (including Pauli-repulsion) and SR electrostatic forces [36]:

F int " F el `FvdW `FSR .
(2.9)

Other forces like magnetic forces or capillary forces, when measuring in liquid environment, will not be considered. For tip-sample distances over typically 1nm, the attractive LR-forces dominate F int pzq, whereas in close approach the different attractive and repulsive SR-forces with decay lengths in the order of several Ångström dominate the tip-sample interaction force. In contact-mode AFM, imaging is performed at very small tip-sample distances and repulsive forces (F int ą 0). For non-contact AFM, however, the regime of attractive forces (F int ă 0) at larger z is chosen [31]. As already mentioned, due to the cantilever amplitudes A of normally more than 1nm in dynamic mode AFM (to avoid a jump-to-contact [32]), the cantilever tip probes a whole section of the nonlinear F int pzq curve during one oscillation cycle instead of measuring one single point of it [33]. Consequently, for measurement setpoints with small enough z, the tip passes through both, LR and SR interaction regimes, and averages the different force contributions. In order to mainly probe the SR-forces, which are due to their short decay lengths responsible for high resolutions [4], very small amplitudes would have to be used like in the case of tuning fork sensors (A " 20 to 100pm). Furthermore, due to the occurrence of attractive and repulsive force contributions, F int pzq exhibits a non-monotonic distance dependence (BF int pzq{Bz ă 0 for small z and BF int pzq{Bz ą 0 for larger z) much in contrast to STM imaging and the tunneling current [31]. Consequently, stable z feedback conditions can only be achieved for one branch of the F int pzq curve at a time (normally the attractive one is used).

Assuming the additivity of the different forces F int " F el `FvdW `FSR , the total frequency shift Δf (see Equation 2.8) can be determined in first order as the sum of the Δf of all contributions (Δf " Δf el `Δf vdW `Δf SR ) [36]. Analytical model for each force contribution will be given in the following.

Electrostatic Forces

LR electrostatic forces occur in case of a potential difference between a conductive AFM tip and the sample. In order to analytically model this situation, the system out of tip and sample is generally described as a capacitor with z-dependent capacitance Cpzq [4]. The electric energy of a capacitance corresponds to W el " 1{2 C V 2 ef f . In the case of a capacitance connected to a generator, i. e. the electrodes lie at fixed potential values, the electrostatic force is the positive gradient of W el [34]:

F el pzq " BW el Bz " 1 2 BCpzq Bz V 2 ef f " 1 2 BCpzq Bz pV DC ´CP Dq 2 .
(2.10)

In this equation, V ef f denotes the effective tip-sample potential difference. The latter is the difference of the bias V DC applied between both electrodes and the contact potential difference CP D, which is the potential difference created upon contact of tip and sample due to the Fermi level alignment. In this thesis, the CPD will be defined as CP D " pΦ tip ´Φsample q{|e| with the work functions of tip Φ tip and sample Φ sample . This is in contrast to the CPD convention found in many publications where the CPD is the difference of Φ sample minus Φ tip [24,34]. Nevertheless, this differing definition will be used because it corresponds to an "electrostatically friendly" definition and facilitates the interpretation of the KPFM results. With V DC being applied to the tip, as in the case of the employed setup, a positive charging of the sample surface corresponds to a positive CPD change (for details see Sections 2.4 and 4.2).

The gradient BCpzq{Bz is tip dependent but always negative because the capacitance decreases with increasing electrode distance. Consequently F el is negative and the capacitive electrostatic force always an attractive contribution to the total tip-sample force. F el can be easily compensated by applying a static bias that equals the contact potential difference (see Equation 2.10).

In a simple model, the tip apex can be represented as a sphere with radius R being located at a distance z above an infinitely large sample plane [4]. In the limit of close approach (z ! R) this results for the interacting electrostatic force to [39,40]

F el pzq " ´π � 0 ˆR2 zpz ´Rq ˙pV DC ´CP Dq 2 « ´π � 0 R z pV DC ´CP Dq 2 9p1{zq (2.11)
with the vacuum permittivity � 0 . For typical values (R " 50nm, z " 0.5nm, V ef f " 1V ) the electrostatic force lies in the range of F el " ´1nN . With the simplified sphere-plane tip-sample model and Equation 2.8, Guggisberg et al. derived an expression for the electrostatic frequency shift contribution [36]:

Δf el f 0 kA " ´π � 0 R ? 2 z A pV DC ´CP Dq 2 . 9p1{ ? zq (2.12)
As the force, Δf el is quadratically dependent on the effective potential difference V ef f " pV DC ´CP Dq and decays furthermore with 1{ ? z.

Besides the LR electrostatic forces, also a SR electrostatic interaction is possible, for example in the case of ionic crystals [13]. It has been shown that this bias dependent SR electrostatic forces decay exponentially [START_REF] Nony | On the relevance of the atomicscale contact potential difference by amplitude-modulation and frequency-modulation Kelvin probe force microscopy[END_REF][START_REF] Bocquet | Polarization effects in Non-Contact Atomic Force Microscopy : a key to model the tip-sample interaction above charged adatoms[END_REF]. This SR electrostatic force contribution is responsible for high (atomic) CPD image resolutions potentially obtainable by KPFM.

Van der Waals Forces

The van der Waals forces (vdW) are another contribution to the total tip-sample interaction force F int . They are caused by fluctuations of the local dipole moment of an atom or molecule and the interaction of these dipoles with surrounding induced or permanent dipoles [4,25]. There are three contributions to the vdW interaction. First, there is the interaction energy of two dipoles that are free to turn with dipole moments µ 1 and µ 2 and at temperature T , the Keesom energy E Keesom , and second there is the interaction energy of an induced dipole with a permanent dipole, the Debye energy E Debye [START_REF] Jerome | Modelisations et Experimentations en Microscopie a Force Atomique Dynamique en Ultra Vide[END_REF]:

E Keesom " ´µ2 1 µ 2 2 3 p4 π � 0 q 2 k B T z 6 E Debye " ´µ2 α p4 π � 0 q 2 z 6 .
(2.13)

The respective dipoles are located at a mutual distance z and α denotes the molecular polarizability. Contrary to the Keesom energy, E Debye , which relies on the existence of permanent dipoles, does not vanish at high temperatures. Besides the two contributions, E Keesom and E Debye , that originate from classical model, there is also the London interaction energy E London [START_REF] London | The general theory of molecular forces[END_REF]. It is an induced dipole -induced dipole interaction that relies on electron cloud fluctuations due to the electron's quantum-mechanical zero point (T " 0K) energy and the exchange of virtual photons. The London dispersion energy between two molecules with ionization energies hν 1 and hν 2 and polarizabilities α 1 and α 2 is [43]

E London " ´3 2 h ν 1 ν 2 pν 1 `ν2 q α 1 α 2 p4 π � 0 q 2 z 6 .
(2.14)

Among the different contributions to the vdW energy, it is typically the biggest one. When neglecting retarding effects (Casimir forces) [START_REF] Casimir | The influence of retardation on the London-van der Waals forces[END_REF], which only appear for tip-sample distances large compared to the wavelength of the virtual photons (z ą 100nm), the total vdW energy results in

E vdW " ´pC Keesom `CDebye `CLondon q z 6 " ´CvdW z 6 (2.15)
with a common 1{z 6 dependence of all vdW contributions. The different C denote positive constants. According to Equation 2.15 the vdW force between two atoms/molecules, being the negative gradient of E vdW , is proportional to 1{z 7 and always attractive.

In order to determine the vdW interaction for macroscopic objects like tip and sample, the so-called Hamaker approach [START_REF] Hamaker | The London -van der Waals attraction between spherical particles[END_REF] can be made under the assumption that the forces are additive. Therefore it is integrated over the volumes of the macroscopic objects. With the already used sphere-plane geometry, one obtains for the vdW tip-sample interaction energy in first order

E vdW pzq " ´H R 6pz ´Rq . (2.16)
R denotes again the tip radius and H corresponds to the material specific Hamaker constant. With F vdW being the gradient of the interaction energy and for tip-sample distances z ! R one finally obtains

F vdW pzq " ´BE vdW pzq Bz " ´H R 6pz ´Rq 2 « ´H R 6 z 2 . 9p1{z 2 q (2.17)
F vdW decays quadratically with z and can be quite large. For typical values (R " 50nm, z " 0.5nm, H " 1eV ), the van der Waals force lies in the range of F vdW " ´10nN . Consequently, it can potentially strongly influence the topographic measurements performed by nc-AFM as discussed in Chapter 5.

Similar to the case of F el , one can determine the frequency shift caused by the vdW interaction Δf vdW according to Equation 2.8 which decays with the tip-sample distance with 1{z 3{2 [36]:

Δf vdW f 0 kA " ´H R 12 ? 2 A z3{2 . 9p1{z 3{2 q.
(2.18)

Chemical Forces

Finally, the short-range chemical forces are still of great importance for AFM measurements in the vacuum. They occur at small tip-sample distances where the electron wavefunctions of tip and sample overlap and they can be attractive or repulsive [4]. On the one hand, the Pauli exclusion principle creates a strong repulsive force at very close approach when occupied electron orbitals overlap. The quantum mechanical Pauli principle excludes that two identical fermions occupy the same quantum state. On the other hand, an attractive force can occur between two atoms that are approached and that are capable to develop a mutual chemical bond. Typical decay lengths κ of these forces lie in the order of 1 to 2 Ångström and for that reason they are mainly responsible for high (atomic) resolution by AFM in the SR-regime [4].

Typically these SR-forces are described via empirical interaction-energy model. One common model is the Morse potential [36,[START_REF] Israelachvili | Intermolecular and Surface Forces[END_REF]

E M orse " ´Ebond " 2 exp ˆ´pz ´z0 q κ ˙´exp ˆ´2 pz ´z0 q κ ˙ (2.19)
which describes the potential energy of two atoms in close approach. E bond is the bonding energy, κ the decay length and z 0 the equilibrium distance of both atoms. The first term is an attractive term responsible for the chemical bond formation, the second a repulsive term which is induced by the Pauli principle. A second common model is the Lennard-Jones potential [START_REF] Israelachvili | Intermolecular and Surface Forces[END_REF] E LJ " ´Ebond

" 2 ´z0 z ¯6 ´´z 0 z ¯12  (2.20)
which also consists out of a first attractive term followed by a repulsive term. The attractive term is more long-ranging and can be attributed to the vdW interaction. The repulsive SR-term is again caused by the Pauli exclusion principle.

Both potential model, lack however a description of the anisotropy of chemical bonds. According to the Morse potential, one obtains for the chemical SR-force F SR the following expression:

F SR " ´2 E bond κ " exp ˆ´pz ´z0 q κ ˙´exp ˆ´2 pz ´z0 q κ ˙ . (2.21)
The SR-force decays exponentially with the tip-sample distance. In the case of nc-AFM imaging, one mostly chooses a setpoint in the attractive SR-regime and avoids the regime of Pauli repulsion that could damage the cantilever tip. Therefore, as an approximation, the repulsive term in F SR can be neglected and one obtains for the contribution Δf SR [36]:

Δf SR f 0 kA " ´?2 E B ? π A κ exp ˆ´pz ´z0 q κ ˙. (2.22)

| Kelvin Probe Force Microscopy

In 1898, Lord Kelvin realized that if two metals are brought in electric contact a current will flow until equilibrium (with aligned Fermi level) is attained and a contact potential difference is created between both of them [START_REF] Kelvin | V. Contact electricity of metals[END_REF]. Thereby the work function of a material can be determined relative to a reference material with known work function. This Kelvin method was refined in 1932 by W. A. Zisman by using an oscillating Kelvin probe as reference electrode at a certain distance above the sample and by performing measurements in vacuum [START_REF] Zisman | A new method of measuring contact potential differences in metals[END_REF]. Due to the vibrating Kelvin probe, an ac voltage which is directly related to the CPD, could be measured between both electrodes. In 1991, Nonnenmacher et al. introduced the KPFM, a variant of the nc-AFM, where the vibrating AFM cantilever is used as reference electrode [22], enabling like this work function measurements with highest spatial resolution. The work function Φ of a material is defined as the minimal amount of energy required to remove an electron from its electronic ground state. For a metal, the work function corresponds to Φ metal " pE vac ´EF q.

Instead of measuring a displacement current of both electrodes in contact, as in the case of Lord Kelvin's original method, KPFM detects the electrostatic forces between tip and sample. In order to distinguish F el from the other force contributions, a bias modulation V AC is applied additionally to the static compensation bias V DC between both electrodes (V bias " V DC `VAC sinpω K tq) [24]. This bias modulation induces an oscillating electrostatic force. According to Equation 2.10 the electrostatic force results in

F el " 1 2 BC Bz pV bias ´CP Dq 2 " 1 2 BC Bz rpV DC ´CP Dq `VAC sinpω K tqs 2 . (2.23)
The resulting oscillating force has three spectral components [24]:

F el, DC " 1 2 BC Bz " pV DC ´CP Dq 2 `1 2 V 2 AC  (2.24) F el, ω K " BC Bz pV DC ´CP Dq V AC sinpω K tq (2.25) F el, 2ω K " ´BC Bz V 2 AC 4 cosp2 ω K tq. (2.26)
F el, DC is a static component that adds an offset to the total tip-sample interaction force F int . F el, 2ω K is the second harmonic component, which can be used for capacitance spectroscopy at 2ω K because V AC and ω K are known [START_REF] Sadewasser | Kelvin Probe Force Microscopy: Measuring and Compensating Electrostatic Forces[END_REF]. Finally, by adapting the compensation bias V DC (in our case applied to the tip) until the detected electrostatic force component F el, ω K is nullified (V DC " CP D), the contact potential difference CP D " pΦ tip ´Φsample q{|e| (2.27) can be determined. When using an AFM cantilever with known work function Φ tip , the local sample work function Φ sample can be measured. As mention above, an electrostatically friendly CPD definition is used here, that deviates from the one found in other publications.

Figure 2.8 illustrates the measurement of the CPD via KPFM for the case Φ tip ą Φ sample . In the absence of an electrical contact between tip and sample (a), vacuum level E vac alignment is obtained between both electrodes. When put into electrical contact (b), it becomes favorable for the electrodes to align their Fermi level E F . Therefore a current flow i will take place until this equilibrium condition is attained. The thereby induced charging of tip and sample creates electrostatic tip-sample interaction forces F el and offsets the electrode's E vac by the contact potential difference CPD. Via the compensation bias V DC , the Kelvin feedback loop tries to compensate the detected electrostatic forces (c). The compensation of F el, ω K and the realignment of E vac is achieved according to Equation 2.25 for V DC " CP D and thus the CPD and the work function difference are obtained. Furthermore, Figure 2.8 (b) illustrates again the simplicity of the chosen "electrostatically friendly" CPD definition. An additional positive charging of the sample offsets the E vac of tip and sample even more and creates an increase of the measured CPD.

V DC = CPD V DC = CPD (a) (b) (c) E F E F E F E F E F E F
The KPFM can detect the electrostatic forces F el, ω K by two different modes, an amplitude modulation (AM) mode or a frequency modulation (FM) mode [24].

AM-KPFM

In AM-KPFM, a V AC modulation frequency f K " ω K {2π equal to the second harmonic resonance frequency f 2nd of the AFM cantilever is chosen [START_REF] Sadewasser | Kelvin Probe Force Microscopy: Measuring and Compensating Electrostatic Forces[END_REF]. Consequently, the V AC -induced electrostatic forces F el cause a resonant oscillation at f 2nd « 6.3 ¨f0 additionally to the one at the first resonance frequency f 0 which is used for the topographic AFM regulation [34]. The second cantilever resonance is broader and has a reduced quality factor in comparison to the first resonance, but with Q 2nd in the order of 10.000 in UHV it is still relatively high [START_REF] Sadewasser | Kelvin Probe Force Microscopy: Measuring and Compensating Electrostatic Forces[END_REF]. By detecting the amplitude of the second resonance, F el,ω K is directly obtained. Via V DC the electrostatic forces F el,ω K and hence the amplitude of the second cantilever resonance is suppressed and the CPD can be determined.

FM-KPFM

For FM-KPFM, an arbitrary but much lower V AC modulation frequency f K is chosen, which lies typically in the range of a few 100Hz to several kHz. V AC induces a modulated electrostatic force F el (see Equation 2.25 and 2.26) that shifts the cantilever resonance and thereby creates a modulation of the frequency shift Δf at the frequency f K . This modulated electrostatic component of the total frequency shift is measured and nullified by adapting V DC " CP D and thus the sought contact potential difference is obtained. The modulation frequency f K in FM-KPFM has to be chosen high enough to avoid crosstalk with the topographic feedback channel but at the same time low enough to be in the bandwidth of the FM demodulator [23].

There have been controversial discussions on which KPFM mode allows for better CPD imaging results [START_REF] Sadewasser | Kelvin Probe Force Microscopy: Measuring and Compensating Electrostatic Forces[END_REF][START_REF] Sadeghi | Multiscale approach for simulations of Kelvin probe force microscopy with atomic resolution[END_REF][START_REF] Zerweck | Accuracy and resolution limits of Kelvin probe force microscopy[END_REF][START_REF] Nony | On the relevance of the atomicscale contact potential difference by amplitude-modulation and frequency-modulation Kelvin probe force microscopy[END_REF]. It is important to mention the following points when comparing AM-and FM-mode:

• A priori, the FM-mode should achieve a higher spatial resolution compared to the AM-mode because it detects the force gradient BF el {Bz and not the force F el itself. Detecting the force gradient corresponds to a short-range detection via the tip apex, whereas detecting the force itself represents a long-range detection via the whole tip (including tip trunk) thereby inducing averaging effects and a reduced spatial resolution [START_REF] Zerweck | Accuracy and resolution limits of Kelvin probe force microscopy[END_REF][START_REF] Sommerhalter | Kelvin probe force microscopy in ultra high vacuum using amplitude modulation detection of the electrostatic forces[END_REF]. However, very high (atomic) CPD resolutions have been demonstrated by both techniques [23,[START_REF] Enevoldsen | Atomic Scale Kelvin Probe Force Microscopy Studies of the Surface Potential Variations on the TiO2(110) Surface[END_REF]. Apparently, due to the SR-electrostatic forces which possess very short decay length and which are responsible for the atomic CPD resolution, FM-and AM-mode have in practice similar spatial resolutions [START_REF] Nony | On the relevance of the atomicscale contact potential difference by amplitude-modulation and frequency-modulation Kelvin probe force microscopy[END_REF].

• Due to the measurement of the CPD via the second cantilever resonance, AM-KPFM achieves a better signal-to-noise ratio for the CPD detection. Consequently, the energy/work function resolution might be better in AM-mode than in FM-mode and typical resolution limits down to 10meV can be obtained [START_REF] Nony | On the relevance of the atomicscale contact potential difference by amplitude-modulation and frequency-modulation Kelvin probe force microscopy[END_REF][START_REF] Sommerhalter | Kelvin probe force microscopy in ultra high vacuum using amplitude modulation detection of the electrostatic forces[END_REF][START_REF] Glatzel | Amplitude or frequency modulationdetection in Kelvin probe force microscopy[END_REF].

• Due to the resonant amplification of the oscillation amplitude at f K " f 2nd in AM-mode, even small V AC " 100mV are sufficient for a precise CPD detection [START_REF] Sommerhalter | High-sensitivity quantitative Kelvin probe microscopy by noncontact ultra-highvacuum atomic force microscopy[END_REF]. The FM-KPFM mode demands higher bias modulations in the order of V AC " 1000mV . The use of such a high modulation bias can potentially induce artifacts, as for example in semiconductors where a voltage induced band bending can be observed [START_REF] Leng | Atomic ordering of GaInP studied by Kelvin probe force microscopy[END_REF].

• In the case of a beam deflection system with a low limited bandwidth (typically 500kHz), only soft low frequency cantilever can be used in AM-KPFM due to f 2nd « 6.3 ¨f0 [23]. Such a limitation does not exist for the FM-mode where f K lies at maximum just several kHz above the first cantilever resonance f 0 . However, recently, more and more KPFM setups are upgraded to achieve a larger bandwidth and to be able to use stiff high frequency cantilever also in AM-mode. Our setup exhibits a maximum bandwidth of 3M Hz.

For this thesis, most of the KPFM investigations have been performed in the FM-mode. This originates from the fact that for most of the measurements stiff high frequency cantilever have been employed. The stiff cantilever routinely used in our laboratory exhibit according to several test performed in our group, but also in the group of C. Loppacher and L. Nony (UMR7334, IM2NP, CNRS, University Aix-Marseille III, France) surprisingly a very weak second resonance with a poor quality factor. Therefore, these cantilever were preferably used in the FM-KPFM mode.

In Figure 2.9 the AM-and FM-KPFM including feedback circuits is schematically illustrated. The black circuit is identical to the one of the normal nc-AFM introduced above (Figure 2.6). The pink circuit is the regulation system needed additionally for KPFM measurements. Deviations in the circuit for AM-and FM-mode are indicated. The V AC coming from the lockin amplifier is applied to the conductive tip, whereas the sample is grounded. In FM-KPFM mode the Δf signal is split after the Δf demodulator and guided towards the Kelvin regulations lockin amplifier. The lockin amplifier extracts the signal at f k and feeds it into the KPFM controller. The latter nullifies the lockin output signal by adapting V DC , which is applied additionally to V AC to the tip. In AM-KPFM mode, a V AC with f K " f 2nd is applied to the tip in order to excite the cantilever at its second resonance by the modulated electrostatic force. The bandpass filter sorts out this high frequency component which lies well above the first resonance used for the topographic feedback. The signal is fed into the lockin amplifier and the KPFM controller adapts V DC to compensate the modulated electrostatic forces F el, ω K and thus suppresses the second cantilever resonance. Thereby the CPD is obtained.

Local Contact Potential Difference

In atomically resolved KPFM measurements, the CPD is referred to as Local Contact Potential Difference (LCPD) and depends on a bias-dependent SR electrostatic interaction [24]. The origin of such a SR-force is highly dependent on the investigated system and is mainly responsible for atomic CPD contrasts. For example in ionic solids, the SR-force can be attributed to the ionic bonding force, whereas in the case of semiconductors the SR-force corresponds to a covalent bonding force [24].

LCPD contrast in KPFM imaging are still an intensively discussed subject. Different groups developed model to describe the measured LCPD contrast quantitatively [23,[START_REF] Nony | On the relevance of the atomicscale contact potential difference by amplitude-modulation and frequency-modulation Kelvin probe force microscopy[END_REF][START_REF] Sadeghi | Multiscale approach for simulations of Kelvin probe force microscopy with atomic resolution[END_REF]. A description of these model is beyond the scope of this thesis. However, one has to keep in mind that in order to achieve the highest CPD resolutions by KPFM one has to leave the conventional LR interaction regime in favor of the SR regime [START_REF] Spadafora | Local contact potential difference of molecular self-assemblies investigated by Kelvin probe force microscopy[END_REF]. In the latter additional forces occur, which may induce additional effects and make the interpretation of the LCPD less straightforward. The photovoltaic model systems investigated during this thesis represent very interesting samples for an interpretation of the origin of the LCPD contrast. They enable a selective charging of specific nanoscale surface areas under illumination.

To conclude this introduction of the basic principles of KPFM it remains to say that there is an important alternative CPD acquisition method without the need for any ac-modulation bias. As presented above, the electrostatic force F el and also the frequency shift Δf el exhibit a quadratic dependence on the effective tip sample bias V ef f " pV DC ´CP Dq (Equation 2.12). Consequently, when performing Δf pV DC q spectroscopy, the CPD can be directly determined because it lies at the maximum of the parabola. This method is very slow, because after the topographic measurement a Δf pV DC q spectrum has to be taken above every pixel of the image. For stability reasons such CPD maps are nearly exclusively taken at very low temperatures (typically T " 5K) and can take several hours. However, they offer a more straightforward interpretation compared to CPD measurements with V AC and exhibit very high measurement sensitivities. The CPD map of naphthalocyanine shown in Figure 2.3 has for example been obtained this way with a tuning fork sensor at T " 5K [21].

| Summary of the Employed SPM Setup

In the last section a short overview of the used measurement setup will be given. It is a commercially available Omicron VT SPM for AFM and STM investigations under UHV conditions (p " 10 ´11 to 10 ´10 mbar) . The whole setup with corresponding control computers is shown in Figure 2.10. The UHV chambers are divided into a preparation chamber and an analysis chamber. The former is equipped with a heating stage including optical pyrometer for temperature control, an argon ion sputtering unit, a thermic STM tip treatment stage and a Knudsen molecular evaporator. The analysis chamber possesses besides the variable temperature (VT) SPM unit also a low energy electron diffraction (LEED) system and a photoelectron spectroscopy system (UPS and XPS) with corresponding UV-and x-ray sources and a hemispherical electron energy analyzer. For the control of the SPM setup an Omicron Matrix control unit is used (right side of Figure 2.10). The Kelvin lockin amplifier is an essential part of the Kelvin regulation system for the compensation of the electrostatic tip-sample Figure 2.11 illustrates in detail the used Omicron SPM unit. The SPM head is protected against vibrations via an eddy current vibration isolation system. The SPM head comprises the AFM's piezo tube scanner with tip mount located underneath the sample plate holder. The xyz-scanner has a scan range of 10µm ˆ10µm ˆ1.5µm. In Figure 2.11 (c) a schema of the integrated beam deflection AFM system is depicted. The laser beam from the external light source is guided via an optical fibre into the vacuum chamber and pointed via optical mirrors through the piezo tube scanner on the cantilever back. From there it is reflected and guided by the two mirrors to the photo-sensitive diode, a four quadrant photo-detector. In Figure 2. 11 (d-f ) two different sample plates and a chip carrier with mounted cantilever are shown. With the variable temperature sample holder, the investigated sample can be heated or cooled covering like this a temperature range from 25K to 1500K.

Figure 2.12 (a) shows the backside of the preparation chamber with the load lock for the sample introduction and the argon sputtering unit. In order to introduce a sample into the UHV chamber, the load lock is pumped with a rotary roughing pump and a turbomolecular pump down to pressures of 10 ´8mbar. Subsequently the sample is transferred into the preparation and the analysis chamber which are permanently pumped via ion pumps. In order to achieve a vacuum of pressures down to 10 ´11 mbar the ion pumps are supported by titanium sublimation pumps. The ion source ISE 5 (Figure 2.12 (b)) is also connected to the preparation chamber. It can be used for the argon sputtering of tip or sample. In the case of silicon cantilever, the argon sputtering removes the native oxide and possible contaminants. Another possibility for the treatment of AFM cantilever consists in a thermal annealing in vacuum on the heating stage. STM tips can also be treated via argon sputtering or alternatively by the home-made thermal tip treatment stage. In the latter, a current of several ampere passes through the STM tip and heats it up in order to remove the native oxide and contaminants. Ce chapitre a pour objectif d'introduire le domaine de l'optoélectronique organique, en présentant certaines notions essentielles relatives aux propriétés des molécules et polymères π-conjugués, à la physique des interfaces organique-métal et organique-organique, et aux cellules photovoltaïques organiques. Après un bref rappel sur les niveaux électroniques dans les systèmes moléculaires π-conjugués, nous soulignons certains points spécifiques des quasi-particules impliquées dans les processus de photo-génération et de transport des porteurs de charges. La physique des interfaces métal-organique et organique-organique est introduite du point de vue des mécanismes de transfert de charge et des processus d'alignement des niveaux d'énergie, et les modèles de transfert de charge entier (ICT) et de densité d'état d'interface induite (IDIS) sont introduits. Les mécanismes de photo-génération des porteurs aux interfaces donneur-accepteur sont ensuite présentés, ainsi que les principes de base de fonctionnement des cellules solaires organiques.

| Context

Nowadays, organic electronic devices are on the rise and have already entered our day-to-day life for example in form of displays, light emitting diodes and first organic solar cells. The way for this impressive break through, however, has already been paved in the 1970s by Hideki Shirakawa, Alan G. MacDiarmid and Alan J. Heeger, who discovered the conductivity of the polymer polyacetylene upon doping [1,2]. In the year 2000 they received the Nobel Price in Chemistry for the "discovery and development of conductive polymers". These polymers owe their conductivity to the delocalized electrons in the π-conjugated system and to the electric transport via charge hopping between localized states (see below). In their pristine undoped state the conjugated polymers are semiconductors (SC) or even insulators with large band gaps and low electrical conductivity. Typical representatives are polythiophenes and polyacetylenes with pristine conductivities of about 10 ´8S{cm. Upon chemical doping, for example with oxygen, the conductivity increases significantly and can even reach values of up to 10 5 S{cm as reported for polyacetylene [3]. (Copper, for comparison, has a conductivity of 6 ¨10 9 S{cm). Furthermore, these organic SC can be "electrostatically" doped like in the case of organic field effect transistors or polymer light emitting diodes. In this case charges are accumulated for the electric transport within the organic film at the organic SC/dielectric interface by applying an electric potential to the substrate (gate).

These conjugated polymers are also of great importance in the field of organic photovoltaics (OPV). In 1986 Tang et al. introduced the first organic heterojunction solar cell [4]. Tang developed a bilayer device based on copper phthalocyanine and a perylene tetracarboxylic derivative and achieved a power conversion efficiency of about 1%. Todays organic solar cells can be mainly divided in two groups, the one based on a combination of conjugated polymers and fullerenes and the one based on small molecules. Apart from this, (organic) dye-sensitized solar cells [5] have been developed, that are based on a different functional principle and will not be addressed in this thesis. In general, organic solar cells are prepared by solution processing. In the case of small molecule devices, however, preparation can also be done via vapor deposition, which is not possible for polymers. Furthermore, in comparison to small molecules, polymers have no fixed weight, but rather a weight distribution because of the varying number of monomers that they comprise. This results in a more defined chemical and even electronic structure of small molecules compared to polymers [6]. Moreover, the solar cell fabrication can be performed by two different nano fabrication approaches, "top-down" and "bottom-up" [7]. For top-down one uses conventional micro fabrication methods such as photolithography in order to shape the desired structure from a larger block of matter. The bottom-up approach relies on the chemical properties of molecules to achieve the aimed structure/morphology. Molecular self-assembly, for example via π-π interactions, is the key concept of supramolecular chemistry [8] and will also be largely used in this thesis to obtain organized photovoltaic systems in a bottom-up approach. Making use of such nano fabrication techniques and upon optimization of the used organic SC, You et al. recently achieved a new efficiency record of 10.2% for a polymer:fullerene tandem solar cell [9]. This demonstrates the potential of the latest OPV technology which becomes a more and more interesting alternative to the established silicon technologies.

In order to further improve the efficiency of OPV devices it is essential to have a detailed understanding of organic semiconductors and of the optoelectronic processes on the nanoscale. Therefore, some basic principles of organic SC will be reviewed in the following. Furthermore, different model for the energy level alignment at interfaces with organic semiconductors will be discussed being of great importance for OPV devices. Finally the latest understanding of the mechanisms of photo-induced charge carrier generation and charge collection will be presented together with the different OPV device architectures and characteristics.

| Conjugated Molecules and Polymers

In order to obtain electric conductivity in a material, delocalized electrons have to be present which can receive energy from an applied electric field. In inorganic crystalline semiconductors a strong delocalization is observed due to the formation of energy bands. Organic semiconductors, like conjugated molecules and polymers, can exhibit a much weaker but still significant electron delocalization due to π-conjugated systems.

A π-conjugated system relies on the creation of π-bonds between single atoms upon molecule formation like in the case of ethene C 2 H 4 in Figure 3.1. Atoms can form molecules via covalent bonds by sharing their valence electrons (outer shell electrons) with neighboring atoms in the and bonding (π) molecular orbitals. The double bond between the two C-atoms consists out of one σ-bond and one π-bond. The phase of the p orbitals is indicated by black and grey color. (d) Energy level diagram of the carbon atoms' 2p z orbitals before bonding and after formation of the common π-bond. The electrons are located in the energetically lower π state. Illustrations according to [11].

molecule to obtain the energetically very stable noble gas configuration (octet rule). In order to obtain this most stable conformation and to minimize the electron pair repulsion for adjacent orbitals, new orbitals can be formed upon covalent bonding. This so called hybridization is induced by the overlap of electron wave functions and creates in the case of ethene three new hybridized 2sp 2 carbon orbitals out of the former ground state orbitals 2s, 2p x and 2p y (see Figure 3.1 (a)). For this sp 2 hybridized carbon an electron from the 2s orbital was lifted into the p z orbital. This energy cost is instantly compensated by the following creation of covalent bonds and the achievement of the nobel gas configuration. The covalent bond out of sp 2 orbitals is called a σ bond the one out of the p z orbitals is called a π bond. Because of the double bond (σ plus π bond) between ethene's carbon atoms and the orientation of the atomic orbitals a planar molecule is obtained with the p z orbitals perpendicular to the molecular plane (Figure 3.1 (b,c)). From a quantum mechanical point of view, covalent bonds are created by a linear combination of the single-atom orbitals described by the LCAO method [10]. This leads to a splitting of the energy eigenvalues. Depending on the phase of the overlapping wave functions, bonding orbitals with lower energy and anti-bonding orbitals with higher energy are obtained. In the case of ethene these are for example the π and π ˚molecular orbitals created out of the p z orbitals of the individual carbon atoms. The π bond orbital is lower in energy and is thus filled with the two electrons (Figure 3.

(d)).

A molecule with alternating double bonds and single bonds is said to have a conjugated system. In case of hydrocarbon molecules this means that every C atom forms three sp 2 orbitals and one perpendicular p orbital (like in the case of ethene). The overlap between the p orbitals creates an extended π-conjugated system with delocalized electrons. Aromatic compounds are known to have a very extended π-conjugated system with a strong electron delocalization which enables electron transport. The most simple aromatic representative is benzene (C 6 H 6 ) for which the π-conjugated system is built out of six p orbitals (Figure 3.2 (a)). There are two possible resonant (mesomeric) realizations of benzene's Lewis structure with different double bond positioning. However, the real chemical situation lies in between with all six carbon atoms contributing to the same extend to the delocalized π system instead of having three localized double bonds. The wave function overlap between the six atomic p orbitals results in The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are indicated. The higher the wave function's phase overlap, the higher is the energy of the π orbitals. Illustrations according to [11].

six molecular π orbitals of distinct energy because of the different possible phase combinations (see Figure 3.2 (b)). Depending on the degree of the wave functions' phase overlap the molecular orbitals are higher or lower in energy. The three bonding (π) orbitals are filled with the six electrons, whereas the three anti-bonding (π ˚) orbitals are unoccupied. The energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) depicted in Figure 3.2 (b) is with about 6eV relatively large and the molecule will absorbs in the UV regime at 250nm wavelength [12,11]. When increasing the size of the π-conjugated system, for example by adding additional aromatic rings, a further splitting of the energy eigenvalues will occur and the HOMO-LUMO gap will be decreased. For instance pentacene with five aromatic rings exhibits a lower HOMO-LUMO gap of just 2.2eV and absorbs already in the visible spectra at 560nm [12,11]. This behavior can be understood analogous to the particle in a box model in quantum mechanics, where the Schrödinger equation yields a reduced distance in the energy eigenvalues for a wider potential well [13].

In general these π-conjugated organic molecules and polymers can be seen as organic SC with an energy band gap. Upon chemical "doping" like in the case of the conducting polymer polyacetylene or upon electrical or optical excitation charge transport gets possible. However, when forming an organic solid out of π-conjugated molecules or polymers there are some clear differences to crystalline inorganic SC. In the case of organic solids, the organic compounds nearly exclusively interact by the relatively weak and attractive van der Waals interaction (see Chapter 2.3.2) which holds the organic "soft matter" together. Other stronger intermolecular bonds are normally absent due to the fully occupied molecular bonding orbitals. As a consequence, the wave function overlap between electronic states of adjacent molecules is normally weak. This induces electronic bands with narrow intermolecular bandwidth (below 0.1eV ) or even a complete localization of electronic states on the individual molecules/polymers [14]. Inorganic SC with their crystalline structure and the covalent bonding between neighboring atoms have a large bandwidth and can absorb, like for example silicon (E gap pSiq " 1.1eV ), the whole visible spectrum up to 1000nm [15]. Furthermore, electric transport in organic semiconductors is strongly influenced by polarization and relaxation of the organic matrix. As explained above, isolated organic molecules and polymers possess in their ground state a closed electronic shell with completely filled bonding orbitals. Thus in order to have free electrons or holes for charge transport we have to withdraw or to add an electron. In a first approximation, the holes would be transported in the HOMO and the electrons in the LUMO. However withdrawing or adding charges to the molecule, and thus charging it, shifts the molecule's charge transport level. The excess charge induces polarization effects and is electrostatically screened due to the rearrangement of the neighboring molecules' charges. Additionally, nuclear relaxations of the charged molecule and its surrounding organic matrix (changes in their geometric structure) further modify the charge transport level. In total this results in a reduced HOMO-LUMO gap (or more precisely transport gap) of an excess charge in the organic solid in comparison to the one of the single isolated molecule or polymer. Based on these findings, one can define a negative (positive) polaron which is the electron (hole) with its corresponding polarization [14]. It is the relevant charge transport species in organic materials and generally transported via charge hopping between localized states [16]. The negative and positive polaron level lie in the former HOMO-LUMO gap of the single molecule/polymer and determine the energy transport level for excess charges in organic semiconductors [16].

Another quasi-particle besides the polaron which is of great importance for the description of organic photovoltaic systems is the exciton. It is a localized electronic excitation of a molecule and is created upon photon absorption [16]. An exciton can also be seen as a Coulomb-bound electron-hole pair with the exciton binding energy BE X being the difference between the exciton energy and the energy of a completely separated electron-hole pair. In organic semiconductors, due to the low dielectric constants (� ORG " 3 to 6), the electron-hole pairs are relatively strongly bound with BE X of 0.1 to 1eV [17]. The binding energies of these so-called Frenkel excitons are about one order of magnitude larger than the available thermal energy at room temperature (k B T " 25meV ) [15]. Inorganic semiconductors have relatively high dielectric constants (� IN ORG " 13 to 16) which results in a BE X in the order of k B T [18]. Thus, in the case of inorganic SC, free electron and holes can directly be generated upon appropriate optical excitation. Excitons can be transferred to neighboring molecules and thereby provide energy transport by migration [11]. Typical exciton diffusion lengths for organic semiconductors lie in the order of 10nm [15].

It remains important to say, that for organic SC the optical and the transport gap are not the same, contrary to inorganic SC [19,20]. The optical band gap is given by the absorption and represents the optical excitation of a neutral molecule/polymer. The transport band gap with the corresponding polaron level, however, refers to a potential diagram for electrons in a single-particle picture [11]. This results in a general problem to visualize electronic excitations (excitons) and charge transport (via polarons) in the same energy level representation. Having this in mind, we will nevertheless need such representations in this manuscript to illustrate the optoelectronic mechanisms in OPV devices (see Figure 3.6 (a) below).

| Metal-Organic and Organic-Organic Interfaces

Having reviewed some principle characteristics of charge transport in organic SC, the next section will focus on the energy level alignment at metal/organic and organic/organic interfaces in equilibrium without illumination. The exact interface band structure is of the utmost importance for organic photovoltaic devices. The organic/organic donor-acceptor (DA) interface enables the dissociation of the photo-generated excitons and the interface of the organic photovoltaic layer with the metal electrodes influences strongly the charge collection.

It is an experimental evidence that for metal-organic but also for organic-organic interfaces the vacuum level E vac alignment, as proposed by the "Schottky-Mott" model, is not always achieved due to the creation of interface dipoles [14,17,21]. These interface dipoles can result from a charge transfer at the interface, from induced (polarization) and permanent molecular dipoles/multipoles or even due to charge redistributions caused by a wave function overlap at the interface (pillow effect) [16,17,22,23]. The resulting total interface dipole offsets E vac at the heterojunction and shifts thereby the electronic energy level of one material (HOMO/LUMO, polaron level) relatively to the other. Consequently, when choosing materials for an organic electronic device, one must not only look on the energy level of the isolated compounds which would neglect the influence of interface dipoles on the energy level alignment [23].

Two established model, that have been developed for the organic/metal interface, will be presented in the following, the Integer Charge Transfer model (ICT) and the Induced Density of Interfacial States model (IDIS). They apply respectively in the case of physisorption and weak chemisorption of the organic SC and will cover most of the systems discussed in this thesis. Both model have also been applied to the organic/organic interface as discussed below (Section 3.3.3).

| Integer Charge Transfer Model

The ICT model [14,24,25] describes interfaces that are characterized by a negligible mixing (hybridization) of the molecular orbitals with the substrate wave functions. This is for example the case of solution processed polymers/molecules, organic deposits on passivated or weakly interacting substrates (oxidized metals, HOPG,...) or organic/organic interfaces. However, even without a significant wave function overlap, charge transfer can occur between the organic SC and the metal substrate via tunneling. In this case an integer amount of charge (one or several electrons) will tunnel from or into well-defined charge states on the polymer or molecule [14]. These well-defined intrinsic charge states correspond to the already defined negative and positive polaron states which include electronic and geometric relaxations. In the ICT model they are named positive and negative charge transfer states, E ICT `and E ICT ´, and are the relevant energies to determine the energy level alignment. Figure 3.3 illustrates the three regimes for the energy level alignment at the metal/organic interface according to the ICT model. The relative positioning of the charge transfer states in the organic SC and the metal Fermi level E F determines if an integer charge transfer occurs or not. For a Fermi level lying in between the negative and positive charge transfer states (E ICT ´ą E F ą E ICT `) no charge transfer occurs and the vacuum level E vac align (Figure 3.3 (a)). On the one hand, the metal electrons cannot tunnel into the energetically higher and empty E ICT ´states. On the other hand, electrons in the E ICT `level cannot be transferred into the already occupied band states below E F in the metal. In the second regime, E ICT `lies energetically above E F and electrons can move from the positive charge transfer state to free states in the metal (Figure 3.3 (b)). This charge transfer lowers the energy level on the organic SC side and occurs until E ICT `and E F are aligned (Fermi level pinning). An interface dipole is created which points away from the surface and offsets the vacuum level at the interface by Δ " pE ICT `´E F q. In the third regime,

E F ą E ICT ´,
electrons can tunnel from the filled metal states into the empty E ICT ´states (Figure 3.3 (c)). This charge transfer rises the molecular energy level and results in a Fermi level pinning of the E ICT ´level. The interface dipole Δ " pE F ´EICT´q points this time towards the substrate.

In the two Fermi level pinning regimes the substrate's initial work function Φ SU B is effectively modified by the organic layer. The resulting new work function Φ ORG{SU B corresponds to Φ SU B minus (plus) the dipole barrier Δ in case of E ICT `(E ICT ) pinning. In the E vac alignment regime

Φ ORG{SU B equals Φ SU B . A representation of Φ ORG{SU B over Φ SU B is depicted in Figure 3.3 (d) which
shows the in literature commonly named "mark of Zorro" behavior [14]. Interestingly, in the E F pinning regimes, the size of the dipole barrier Δ compensates the change in the substrates work function to keep the resulting Φ ORG{SU B pinned to the intrinsic E ICT `{´o f the organic SC. Such a condition is of particular interest when choosing the best anode/cathode material for an OPV device because a selective E F pinning to the donor/acceptor level in the active layer assures ohmic contacts and a best possible charge collection at the electrode interface [14] (see Section 3.4 below).

E vac LUMO HOMO E vac E F Φ SUB E ICT- E ICT+ (a) contact E ICT-> E F > E ICT+ : metal organic metal organic (c) charge transfer e - contact E F > E ICT-: ∆ + _ metal organic E vac LUMO HOMO E vac E F Φ SUB E ICT- E ICT+ (b) charge transfer e - E vac LUMO HOMO E vac E F Φ SUB E ICT- E ICT+ contact E ICT+ > E F : ∆ interface dipole + _ metal organic metal organic E vac LUMO HOMO E vac E F Φ SUB E ICT- E ICT+ E vac LUMO HOMO E vac E F Φ SUB E ICT- E ICT+ metal organic E vac E F Φ SUB LUMO HOMO E vac E ICT- E ICT+ (d) ∆ = E ICT+ -E F interface dipole ∆ = E F -E ICT- substrate workfunction Φ SUB organic/substrate workfunction Φ ORG/SUB E ICT+ E ICT- flatband E vac -alignment E F -pinning E F -pinning Φ ORG/SUB = Φ SUB (slope = 1)

| Induced Density of Interfacial States Model

The IDIS model [26,27] applies, in contrast to the ICT model, in the case of a somewhat stronger metal-molecular interaction that leads to a significant orbital overlap and to orbital mixing (hybridization) [22]. Such a relatively strong electronic coupling between organic SC and metal substrates is for example obtained when evaporating organic compounds on clean nonreactive substrates [14]. It leads to a broadening of the delta-like distribution of states of molecular energy level into lorentzian peaks and creates a non-negligible DOS in the HOMO/LUMO band gap [28] (Figure 3.4 (a)). If this broadened DOS is filled with the number of electrons of the neutral isolated molecule/polymer the energetically highest filled state can be defined as the charge neutrality level (CN L). The CN L lies in the former energy gap and its relative position to the metal Fermi level defines the direction of the partial charge transfer. Here, partial charges can be transferred, in contrast to the ICT model's integer charge transfer, due to the mixing of the metal and organic SC orbitals which induces the "sharing" of electrons [22]. For a CN L level above the metal E F as depicted in Figure 3.4 (a,b), charge transfer (CT) occurs from the organic SC towards the substrate. This CT lowers the molecular energy level, creates an interface dipole and reduces the energetically distance between CN L1 and E F to

HOMO LUMO energy E density of states DOS

CNL Φ SUB S(Φ SUB -CNL) Φ ORG/SUB E vac LUMO HOMO E vac E F ∆ + _ metal ∆ = (1-S)(Φ SUB -CNL) Φ SUB CNL organic Φ ORG/SUB S(Φ SUB -CNL) (b) (a)
pΦ ORG{SU B ´CN Lq " SpΦ SU B ´CN Lq. (3.1)
The difference between the metal work function and the CN L is screened by the polarization of both materials with the screening power (or interface slope parameter) S " dE F {dΦ SU B (S P r0, 1s) [26]. It depends on the density of induced interface states at the Fermi level DpE F q:

S " dE F dΦ SU B " 1 1 `4πe 2 DpE F qδ{A (3.2)
with the metal-molecule distance δ and the area associated with one organic molecule A [27]. The dipole barrier at the interface can be determined by Δ " p1 ´SqpΦ SU B ´CN Lq.

(3.3)

For S " 1, vacuum level alignment is obtained and for S " 0 the Fermi level is pinned to the organic SC CNL, which occurs for very large DpE F q. Similar to the polaron level used in the ICT model, the CNL is an intrinsic property of the organic material and independent of the used metal substrate [28].

| Energy Level Alignment at the Organic-Organic Interface

The two model for the energy level alignment at the metal/organic interface are well established and have achieved in many cases a good agreement with experimental results. Consequently it has been tried to employ both also for the organic/organic interface in a first approximation.

The ICT model should apply well for organic/organic interfaces, because organic materials generally physisorb on top of each other without very strong intermolecular interactions (mainly vdW forces) and without a significant mixing of their molecular orbitals. As already mentioned, the ICT model's charge transfer level are an intrinsic material property. Their relative positioning at the interface should determine the energy level alignment and the appearance of eventual interface dipoles [14]. Figure 3.5 (a) illustrates the situation at an organic/organic interface according to the ICT model where the E ICT `of the organic SC on the left is already pinned to the E F of the metal substrate. In this case due to the prior CT to the metal substrate, states are available in the E ICT `level and a further integer CT can occur via tunneling across the organic/organic interface from right to left. The E ICT `of both organic SC align and an interface dipole is created. It has been shown that the alignment at the metal/organic interface, as well as the deposition sequence (SC 1 /SC 2 or SC 2 /SC 1 ) influences the energy level alignment at the organic/organic interface [14].

LUMO 1 HOMO 1 CNL 1 LUMO 2 HOMO 2 CNL 2 S oo (CNL 1 -CNL 2 ) initial ∆ = (1-S oo )(CNL 1 -CNL 2 ) initial ∆ E ICT- E ICT+ partial charge transfer q<0 E vac + _ E vac HOMO 1 HOMO 2 LUMO 1 LUMO 2 ∆ E vac E vac E ICT- E ICT+ + _ integer charge transfer e - org. SC 1 org. SC 2 (a) (b) org. SC 1 org. SC 2 metal E vac E F Figure 3.5: (a)
In the ICT-model at the organic/organic interface the polaron transport level E ICT tend to align via an integer charge transfer and an interface dipole can be created. This alignment process is however also influenced by the work function of the underlying substrate. (b) Energy alignment at the organic/organic interface according to the IDIS model. The charge neutrality level of the two components approach upon contact due to partial charge transfer and an interface dipole with Δ " p1 ´Soo qpCN L 1 ´CN L 2 q initial is created with the screening parameter S oo .

Also the IDIS model has been employed to describe the organic/organic interface [17,28,29]. Figure 3.5 (b) illustrates the energy level alignment between two organic SC according to the IDIS model. Every organic SC has its own intrinsic CNL which plays the role of an electronegativity. The lower the CNL lies in the gap of the organic SC, the stronger is its attraction for electronic charges [29]. Upon contact a partial charge transfer can take place that approaches both CNL to

pCN L 1 ´CN L 2 q f inal " S oo pCN L 1 ´CN L 2 q initial (3.4)
with the screening parameter S oo for the organic/organic interface [28]. If one assumes that both organic SC equally screen the potential offset at the interface, S oo can be calculated by

S oo " 1{2 ˆ1 � 1 `1 � 2 ˙(3.5)
with the dielectric constants of both materials [28]. The created interface dipole exhibits a dipole barrier of

Δ " p1 ´Soo qpCN L 1 ´CN L 2 q initial . (3.6)
Due to the lower dielectric constants � 1 and � 2 of organic materials in contrast to inorganic ones, generally higher screening parameters S oo are obtained which induce lower vacuum level shifts. This IDIS model for the organic/organic interface has been quite successful, however it is based on an induced density of interfacial states originating from orbital hybridization due to strong interactions. For a pure organic SC interface, such strong interactions and orbital mixing are not expected though, because of the closed electronic shells and the only weak intermolecular interactions via vdW forces creating a generally small electronic bandwidth [16,22]. Therefore, for the analysis of the experimental results presented in this manuscript, the ICT model will be mostly employed.

So far, both presented model describe the energy level alignment and the creation of interface dipoles by a charge transfer across the heterojunction. They ignore the exact contact geometry and the molecular arrangement at the interface [23]. However, as already mentioned, interface dipoles can also originate from arranged induced and permanent dipoles (multipoles) at the interface without any charge transfer [16,22,23]. This contribution is more complicated to assess but can nevertheless play a significant role for the energy level alignment. Stijn Verlaak et al. demonstrated by numerical calculations that such contributions can have a great importance for example in the case of a pentacene/C 60 bilayer [16]. At the organic/organic interface, the microelectrostatic environment is changed compared to the bulk material and exhibits an abrupt discontinuity due to changing dielectric constants (� 1 Ø � 2 ) and due to arranged molecular dipoles and quadrupoles. For example pentacene as planar π-conjugated molecule exhibits a significant quadrupole moment in contrast to the completely absent quadrupole moment of the fullerene C 60 on the other side of the interface. Similar findings have been discussed for the P3HT:PCBM [30] and TTF:TCNQ [22] interface. In general, in order to exactly determine the contribution of permanent and induced molecular dipoles and quadrupoles to the interfacial energy level alignment, extended numerical calculations would have to be performed for each investigated system. For the following discussions in this manuscript it will therefore be referred to the established, yet more simplistic, ICT or IDIS model to describe the energy level alignment. One has however to bear in mind the possible additional contributions to the total interface dipole neglected by these model.

| Charge Carrier Generation in Organic Photovoltaic Systems

After the presentation of the basic characteristics of organic semiconductors, the introduction of the quasi-particles polaron and exciton and the discussion of the energy level alignment at interfaces, the following section will address the principle mechanisms within an OPV heterojunction. To date, there is still no clear understanding of all parameters necessary for a full quantitative description of an OPV cell [17]. Especially revealing the mechanism for an Bulk heterojunction with crystalline interpenetrating DA network with high interfacial area and small DA phase-separation. (e) Ideal situation of an organized heterojunction with a DA phase-separation in the order of the exciton diffusion length L D and an active layer thickness in the order of the inverse absorption coefficient 1{a. Illustrations according to [11,17].

S 0 S 1 S n hν CT n CT 0 k CT hν k D CS (b) - + - + BE x Coulomb potential k XD - -BE CTS + - k T k PA k CC k CT donor acceptor k D ∆LUMO ≈ (a) k PA - + - + k XD + - + k CT anode recombination - + k D - donor acceptor k T k CC cathode (c) cathode cathode anode anode (d) (e)
efficient electron-hole pair dissociation represents the main challenge [31]. The latest state of understanding will be presented in the following.

In with the exciton's ground state S 0 being placed at the position of the donor's lower transport level. The exciton binding energy BE X is the difference between the exciton energy and the energy of the completely separated electron-hole pair. In comparison to inorganic SC, the BE X is with values of 0.1 to 1.0eV [17] relatively high due to the lower dielectric constants � of organic compounds and lies well above the available thermal energy k B T . Thus, the photo-generated excitons are tightly bound and not immediately dissociated into free charges as in the case of inorganic SC. In order to overcome the mutual Coulomb attraction, the electron-hole pairs depend in organic SC on the favorable electronic landscape of a donor-acceptor interface [30].

Via diffusion the excitons reach the donor-acceptor interface with a rate k XD . In organic SC, the excitons exhibit a typical diffusion length L D of about 10nm [15]. Consequently, the donor-acceptor phase-separation in an OPV cell should not be larger than L D so that the excitons can reach the DA interface before a possible recombination.

At the interface the exciton's electron is transferred to the acceptor (in the case of an exciton generation in the donor) at a rate k CT and experiments have shown that this process is very efficient and fast [32,33,34].

This charge transfer at the interface creates a so-called charge transfer state (CTS) of still coulomb-bound electron and hole across the interface. In literature different names can be found for such a CTS, like CT complex, polaron pair, CT exciton, exciplexes or geminate pairs [20]. In Figure 3.6 (a) the mutual attractive coulomb potential of the CTS's electron and hole is indicated (violet dashed curve). The CTS has a binding energy BE CT S which is slightly lower than the one of the exciton, but it is still one order of magnitude larger than the available thermal energy k B T [17]. The probability to overcome the coulomb attraction of several 100meV by pure thermal activation goes to zero according to a simple estimation by a Boltzmann law (probability 9 expp´BE CT S {k B T )) [20,35]. To dissociate the CTS into free charges with rate k D the energetic gain for the dissociation has to be larger than BE CT S . Interestingly the simple energy offset between the donor and acceptor transport level is not enough to explain the CTS dissociation because even for smallest energy offsets the mutual coulomb trap can be overcome [35]. This separation into free charges at the DA interface is seen as the essential process in organic solar cells. It is not yet completely understood and controversially discussed in literature [36]. There are several approaches that probably in combination can help to explain the reduced CTS binding energy and the non-negligible CTS dissociation rate k D :

• For the creation of the CTS, the exciton's electron tumbles down in the energy transport level of the acceptor what results in a certain amount of excess energy. This excess energy induces so called hot charge transfer states CT n , that are higher energetic states with lower binding energy and higher delocalization (see Figure 3.6 (b)) [16,30]. These non-thermalized states could significantly account for a more facile CTS dissociation. However, Bakulin et al. showed that the higher delocalization and the increased average electron-hole distance of the hot CTS plays probably are more significant role than the excess energy itself to overcome the mutual Coulomb attraction [35].

• Furthermore entropic gain could favor the CTS separation [36]. The equilibrium condition in a DA system is not just determined by the electron energy landscape but more exactly by the free energy landscape. Entropy favors the situation of an increased configuration space with the charges far away from the DA interface and thereby it could effectively lower BE CT S .

• Moreover, the microelectrostatic environment at the DA interface can play a significant role. The local electric fields caused by charge transfer and polarization can favor the CTS dissociation [16,30]. The polarization stabilizes the separated charges much more in the bulk than at the interface. Furthermore the interaction with permanent interface dipoles and quadrupoles can help to separate the bound electron-hole pair at the interface. For instance a pentacene/C 60 interface possesses a quadrupole field discontinuity that assists the CTS separation (quadrupole moment of pentacene vs. absent quadrupole moment for C 60 ) [16]. In this context, Arkhipov et al. developed the so-called "dark dipole model" that explains how an array of discrete organized interface dipoles can enhance the CTS dissociation via the dipole field [37,31].

• An applied external field, but also the built-in field, can assist the CTS dissociation [15].

• Finally the CTS dissociation depends on the extend of crystallinity [17]. Local crystallinity in the donor and/or the acceptor phase provides a delocalization contribution to the CTS wave function, which effectively reduces BE CT S and also increases the charge carrier mobility [20].

All these above mentioned effects are assumed to lower the CTS's binding energy and to assist the CTS dissociation. The available thermal energy k B T serves thereby as activation energy.

After a successful CTS dissociation, the free positive and negative polarons are transported to their respective electrodes with a rate k T (Figure 3.6 (a,b)). In organic semiconducting materials this happens via charge hopping between localized states. The recombination of electron and holes represents the only possible loss mechanism.

When the charges arrive at their respective electrodes they can there finally be collected with a rate k CC . Figure 3.6 (c,d,e) illustrates the different organic solar cell architectures. The planar heterojunction is the most simple configuration. However, as we have seen before, the exciton diffusion length L D in organic semiconductors lies typically at around 10nm whereas typical absorption length are at about 100nm. Therefore, in planar heterojunctions, a significant part of the photo-generated excitons will recombine before they reach the DA interface [15]. A milestone for organic photovoltaic devices was consequently the development of the bulk heterojunction (BHJ) in the early 1990 (see Figure 3.6 (d)). These DA blends exhibit in the best case a DA phase-separation in the order of the exciton diffusion length and possess a crystalline interpenetrated DA network for an efficient charge transport and collection at the electrodes [15]. However, the exact DA morphology in a BHJ blend, which has been solution processed and deposited for example via spin coating, remains random. Consequently the charge collection in this random DA structures is in general more difficult than for bilayer devices and a higher recombination is obtained for too fine phase-separations [15,36]. Furthermore, in order to obtain an efficient BHJ cell, a favorable concentration gradient of donor and acceptor has to be achieved within the DA blend. Upon illumination the donor has to transfer the photo-generated holes to the anode and the acceptor the electrons to the cathode. Consequently for a good charge collection it is important to have next to the anode (cathode) a higher concentration of the donor (acceptor) which then decreases towards the opposite electrode [38]. All in all a good morphology and phase-separation control is essential. They can be influenced via the right choice of solvents, thermal annealing or the already mentioned bottom-up approach via molecular self-assembly [36]. The latter includes for example the use of block copolymers, DA dyads and double cable motifs and will also be used within this thesis (see Chapter 6 and 7). The ideal DA structure one would like to achieve is the organized heterojunction which is depicted in Figure 3.6 (e). It corresponds to a high aspect ratio nanostructure where donor and acceptor are well connected to their respective electrodes. The phase-separation lies in the range of the exciton diffusion length L D and the length of the crystalline donor and acceptor channels, which favor the charge transport and collection, is in the order of the inverse absorption coefficient 1{a in order to absorb most of the light in the active layer.

| Characteristics of Organic Photovoltaic Devices

Finally it will be shortly discussed how the macroscopic characteristics of an OPV cell relate to the described optoelectronic processes on the nanoscale. A typical current density j versus voltage V curve of an inorganic solar cell in dark and under illumination is depicted in Figure 3.7 [18].

Inorganic solar cells as well as organic ones show the rectifying behavior of a diode. Under illumination, one can define the current density under short circuit condition j SC and the voltage under open circuit condition V OC . In short circuit, as well as in open circuit, no power can be extracted from the solar cell because the power density P out is the product of bias and current and one of both quantities is equal to zero in both conditions. P out also corresponds to the rectangular area defined by j and V within the current density voltage curve (see Figure 3.7).

It reaches its maximum at the maximum power point (MPP) where the comprehended area is the biggest. For this case one can define the fill factor F F , the ratio of the area at the MPP over the area defined by j SC and V OC :

F F " V M P P j M P P V SC j SC . (3.7)
The photovoltaic power conversion efficiency η is the ratio of the maximum output power density at the MPP over the incident light power P in in [W {m 2 ] and can also be expressed via the fill factor:

η " P out,M P P P in " V M P P j M P P P in " V OC j SC F F P in . (3.8)
For an ideal inorganic solar cell (pn-junction) the described current-voltage behavior under illumination is described by the Shockley equation [15,18]

j " j S ˆexp ˆe V k B T ˙´1 ˙´j P H (3.9)
and reproduces the observed curve shape in Figure 3.7. j S corresponds to the saturation current density in reverse operation (V ă 0) without illumination, e to the elementary charge and j P H represents the photocurrent induced by the photo-generated charges. In short circuit condition (V " 0) the current density j SC equals j P H , whereas in the open circuit case (j " 0), one can estimate the open circuit voltage to [11,18]:

V OC « k B T e ln ˆjPH j S ˙jS " j S0 e p´E gap k B T q " E gap e ´kB T e ln ˆjS0 j P H ˙. (3.10)
The case of a relatively strong illumination with j P H " j S has been assumed. As result a logarithmic dependence of V OC from j P H and thus the light intensity is obtained. Furthermore, with the temperature-activated j S , which follows a Boltzmann law including the energy gap E gap of the inorganic SC, the linear dependence of V OC from E gap and T becomes evident.

Organic semiconductor solar cells are often described in a similar manner, however there are several distinctive differences. The two most determinant physical steps within an operating OPV cell is the CTS dissociation at the DA interface and the following charge transport to the respective electrodes [15]. As discussed before the separation of the coulomb-bound CTS is much increased with an applied electric field, much in contrast to inorganic SC where the light absorption directly induces free charge carriers (BE X " k B T ). Furthermore, also the charge transport in organic semiconductors via hopping depends strongly on the applied electric field. Altogether, this leads to a strong electric field dependence of the photocurrent j P H pV q and the simple Shockley equation can not be applied any more [15]. Additionally, due to the thermally activated hopping transport, the charge transport mobility in organic SC and hence j P H increases with T. This is in contrast to the case of inorganic SC where a higher temperature increases scattering and reduces the charge carrier mobility [18]. Different model have been developed for an analytical description of the charge hopping transport (Bässler Gaussian Disorder Model [15]) and the field and temperature dependence of j ph . Their discussion is out of the scope of this thesis and for more details it is referred to literature [15,20,36]. Apart from the photocurrent j P H pV, T q, also the open circuit voltage V OC strongly depends on the internal optoelectronic processes. There is experimental evidence that V OC increases linearly with the effective DA bandgap E DA gap " pLU M O acceptor ´HOM O donor q [20], equivalent to the case of the inorganic solar cell where V OC 9 E gap (see Equation 3.10 above). In general, the magnitude of V OC is strongly influenced by the generation and recombination of photo-induced charge carriers. In open circuit situation the generation rate equals the recombination rate because of the absence of any current flow [20]. Several analytical model have been developed for V OC that describe the linear dependence with E DA gap and include generation and recombination rates [15]. According to the latest understanding, the maximum achievable V OC is determined by the energy of the charge transfer state E CT S via V OC,max " E CT S {e (see Figure 3. 6 (a,b)) [20,39]. However, besides the energy landscape at the DA interface, also the energy level alignment at the electrode interfaces can influence the externally measured V OC . To avoid any further reduction of V OC , ohmic contacts have to be achieved with, in the best case, E F -pinning to the respective transport level of the organic SC (see Section 3.3) [20].

| Conclusion

In order to understand the properties of organic electronic devices and to enhance their efficiencies model have been developed to describe the fundamental processes on the nanoscale. The energy level alignment at interfaces between two organic SC and at the interface of an organic SC with a metal electrode plays a crucial role. The ICT model and the IDIS model describe such interfaces quite well and allow the prediction of an eventual charge transfer across the interface and the creation of interface dipoles. Especially the ICT model, which applies in the case of a weak interaction and a negligible hybridization of the different compounds at the interface, will be widely used during this thesis to interpret the obtained CPD image contrasts. However one has always to bear in mind the limitations of such model which are solely based on a charge transfer across the interface and do not account for the microelectrostatic environment at the interface caused by induced and permanent dipoles.

In organic SC, charge transport is happening via charge hopping between localized states. Adding or withdrawing charges to a molecule changes however their geometric and electronic conformation and the additional positive or negative charges are screened by the surrounding molecules. Consequently the polaron, which corresponds to a charge with its surrounding polarization, has been defined as the charge transport species in organic SC. An optical electronic excitation on the other hand is described via the exciton, a Coulomb-bound electron-hole pair. Contrary to inorganic SC, where free charges are directly generated upon illumination, an optical excitation in organic SC results in these tightly bound electron-hole pairs with binding energies of several 100meV because of the lower dielectric constants of organic SC. For the exciton dissociation, the favorable electronic landscape of a DA interface is needed. Upon the exciton's arrival at the DA interface, a charge transfer state is created. The dissociation of this CTS represents, together with the subsequent charge transport to the electrodes, the determinant process in an organic photovoltaic cell. Different origins for an efficient CTS dissociation have been proposed, but the exact mechanism is still subject to ongoing discussions. Furthermore, due to the limited diffusion length of excitons of typically 10nm in organic SC, the scale of the DA phase-separation is of great importance. Nowadays, the most common organic solar cell architecture is the "bulk heterojunction" which offers in the best case a DA phase-separation in the order of the exciton diffusion length. However, efforts are made to develop better defined "organized heterojunctions" which improve additionally the transport of the photo-generated charges to the respective electrodes. An organized heterojunction solar cell can for example be achieved via molecular self-assembly, an approach which is also followed in this thesis.

Two very characteristic parameters of an organic solar cell are the open circuit voltage V OC and the short circuit current density j SC . The V OC , on the one hand, is strongly dependent on the effective DA energy gap E DA gap and its maximum value is defined by the energy of the charge transfer state. Furthermore, the V OC is in good approximation proportional to the logarithm of the illumination intensity. j SC , on the other hand, depends via a Shockley-like law on the device voltage V and the photocurrent j P H . The j P H of OPV cells depends on V and the temperature T . This is because j P H relies on the thermally activated charge hopping transport and on the CTS dissociation at the DA interface, which are both field-and thus voltage-dependent. With the different developed model for the nanoscale processes of charge carrier generation and transport, it is in a simplistic manner possible to understand the macroscopic characteristics of OPV devices. However, to date, a full quantitative description of an organic photovoltaic cell based on these model is still out of reach. Ce chapitre est spécifiquement consacré à la discussion de la mise en oeuvre des techniques de microscopie à sonde de Kelvin (KPFM) pour les études de potentiel de surface sous illumination. Les spécificités du dispositif expérimental utilisé pour ce travail ainsi que les protocoles de mesure seront discutés. La convention de signe adoptée dans ce manuscrit pour présenter les données de potentiel de contact sera détaillée, ainsi que la méthodologie de mesure mise au point pour s'assurer de l'absence d'artefacts dus au cantilever lors des mesures de photo-potentiel de surface. Enfin, des mesures sur des systèmes servant de référence aux travaux ultérieurs seront présentées, ainsi que des résultats issus de la littérature. Ces derniers seront utilisés comme motivations à la source des recherches menées dans le cadre de cette thèse.

| Context

In Chapter 3 organic semiconductors have been introduced and the basic principles of organic photovoltaics have been discussed. For the investigation of the electronic and optoelectronic processes on the nanoscale in such materials and devices, Kelvin probe force microscopy (Chapter 2.4) has been chosen for this thesis. In the following chapter some experimental and technical aspects of KPFM investigations on illuminated samples will be discussed from a general point of view, but also specifically for the setup employed during this thesis. First of all, the used CPD sign convention will be clarified being the basis for all further discussions. Then the used illumination setup will be introduced and possible procedures discussed that prevent KPFM artifacts under illumination related to the cantilever itself. Finally, KPFM reference measurements on OPV systems will be presented together with relevant results from literature that illustrate the context of the research conducted during this thesis.

| CPD Sign Convention

In recent years, Kelvin Probe Force Microscopy became more and more popular with research groups all around the world. However, in many publications, depending on the commercial setup and the specific configuration employed by each group, the definition of the sign of the contact potential difference is not always clearly defined. The compensation bias V DC of the Kelvin regulation can be applied to either the tip or the sample and CPD could be defined as potential difference between sample and tip or between tip and sample. A clear and first of all consistent definition of the CPD is of great importance in order to correctly interpret the experimental observings, like the orientation of interface dipoles and the positive or negative charging of donor and acceptor domains upon illumination.

As explained in Chapter 2.4 the following definition of the contact potential difference will be used within this thesis: CP D " V DC " pΦ tip ´Φsample q{|e| " E vac, tip ´Evac, sample (4.1) with the compensation bias V DC physically applied to the KPFM tip (see also Figure 2.8). This is in contrast to the CPD convention found in many publications where the CPD is the difference of Φ sample minus Φ tip [1,2]. Nevertheless, this differing definition will be used because it corresponds to an "electrostatic friendly" definition of the CPD. A positive charging of the sample surface lowers E vac, sample relative to E vac, tip and is detected as a shift towards more positive CPD values.

To verify the sign of the software's CPD output, several tests have been performed. First of all an additional potential to the sample via an external and independent battery pack has been applied. KPFM imaging was performed on the HOPG sample and every 20 image lines the additionally applied bias was increased by 500mV moving the sample potential towards more positive values (Figure 4.1). The CPD image cross-section in Furthermore, the CPD sign will be discussed in the case of interface dipoles of well-known molecular assemblies on HOPG. , which is typically used as a electron-donor polymer in BHJ blends with PCBM, and its self-assembly on HOPG can be found in Chapter 5. The monolayer domains of self-assembled P3DDT appear in the CPD image as bright regions and exhibit thus higher CPD values than the surrounding HOPG substrate. This corresponds to a charge transfer from the "donor" polymer to the graphite substrate, a positive charging of the polymer domains and finally to the creation of an interface dipole pointing away from the sample surface. In order to verify this experimental finding with the theoretical predictions, the energy level arrangement at the metal-organic interface has been determined according to the ICT-model [3] presented in Chapter 3.3.1 (see Figure 4.2). P3DDT is very similar to P3HT and their main-difference is just the different length of the alkyl-chains attached to the thiophene polymer backbone. Consequently they should have very similar molecular energy level and we can approximate P3DDT's E ICT `level by the one of P3HT of around ´4.0eV , which is available in literature [4,5,6]. The Fermi level E F of HOPG is located at about ´4.7eV [7,8,9,10] and lies below the polymer's E ICT `. Due to this relative positioning, charge transfer occurs from P3DDT towards HOPG upon contact until the energy level are aligned and an interface dipole is created pointing away from the sample surface. This energy level alignement creates also an offset between HOPG's and P3DDT's vacuum level E V AC , the dipole barrier Δ, which is finally measured as the CPD difference between HOPG and P3DDT. In total, the experimental results and the theoretical predictions are in full agreement concerning the direction of the charge transfer and confirm the used CPD sign convention. However, in this case, the ICT-model is apparently not able to predict the exact value of the CPD difference between polymer and substrate. The alignement of P3DDT's E ICT `" ´4eV and of HOPG's Fermi level E F " ´4.7eV should result in a CPD change of about 0.7V . This is however, with measured CPD differences of below 100mV , experimentally not observed. This deviation may have its origin in the limited capability of the ICT-model to exactly reproduce the energy level alignment at the interface. As discussed in Chapter 3.3, the ICT-and also the IDIS-model do not include the microelectrostatic environment at the interface. Arranged permanent and induced dipoles may affect the energy level alignment even without a charge transfer across the interface. Furthermore, the experimental energy level values for E ICT `are mostly determined by photoelectron spectroscopy of complete monolayers or even multilayers of the organic material. Using these values in the case of a sub-monolayer may also introduce deviations of the expected CPD difference between polymer and HOPG.

Another example for a metal-organic interface, is the molecular self-assembly of C 60 on HOPG presented in Figure 4.3. The fullerene C 60 can be used as electron-acceptor in OPV blends in combination with other donor molecules or polymers. The KPFM results on HOPG reveal the lower CPD values obtained for the C 60 domains in comparison to the surrounding graphite. For this system, apparently an electron charge transfer must have happened from the HOPG towards the fullerene layer. This corresponds to a negative charging of the molecular assembly and the creation of an interface dipole pointing towards the sample surface. With the illustration in Figure 4.3 the energy level alignment at the HOPG/C 60 interface will be discussed. On the far right side the energy level for C 60 are depicted before contact with typical values taken from literature [3,11]. The E ICT ´of C 60 lies with ´4.5eV above HOPG's Fermi level and according to the ICT model a flatband situation with aligned vacuum level E vac would be expected. Different experimental techniques like photoelectron spectroscopy, scanning tunneling spectroscopy and Kelvin probe proof nevertheless clearly a charge transfer and an interface dipole oriented towards the surface for HOPG and nobel metals with similar work functions [12,13,14,15,16,17]. It was demonstrated that screening effects at the organic/metal interface narrow C 60 's (HOMO-LUMO) gap and lower the LUMO position [13,18,19,15]. Furthermore, studies revealed that hybridization can occur between C 60 's LUMO and the metal substrate's energy bands. This causes a broadening of the LUMO states and also effectively lowers the E ICT ´position. In total, upon contact with the HOPG substrate, C 60 's E ICT ´is lowered to such extend that charge transfer becomes possible from HOPG towards the C 60 domains (see Figure 4.3 (c)). An interface dipole pointing towards the substrate is obtained and the energy level alignment at the organic-metal interface confirms once again the experimental KPFM results and the consistency of the chosen CPD sign convention. Due to the relatively strong interaction of C 60 with the underlying substrate, hybridization of the metal bands with C 60 's LUMO occurs and effectively approaches LUMO and HOMO level. Furthermore also screening effects upon contact with the metal substrate lower the LUMO level. The subsequent charge transfer from the substrate to the fullerene creates an interface dipole whose orientation is in agreement with the experimental results (p2000nmq 2 , Δf " ´7Hz, A cantilever " 10nm, f cantilever " 251kHz, FM-KPFM, Nanoworld SSS-NCH cantilever).

The observed charge transfer and the resulting interface dipole can also be understood via the IDIS-model presented in Chapter 3.3.3. Due to the interaction with the underlying metal substrate, a certain density of states will be induced for the C 60 fullerenes within the gap between HOMO and LUMO [20]. This states will be filled-up until the C 60 charge neutrality level aligns with HOPG's Fermi level [3]. Consequently C 60 's molecular energy level are shifted and an interface dipole is created.

| Laser Setup and Illumination Geometries

In the following the employed laser setup and the sample illumination geometries will be presented. During the first one and a half years of this thesis a simple laser diode of 532nm wavelength and 5mW output power was used. In order to perform more complex KPFM measurements under illumination, a new multiple wavelength laser setup was installed. The "LightHUB" from Omicron Laser is connected to the VT-AFM via an optical glass fibre (see 515nm and 685nm wavelength. Their optical output can be regulated from the sub-mW regime to about 30mW measured at the exit of the optical fibre. All experiments have been performed under continuous illumination. However, experiments under pulsed illumination, which are also possible with the presented laser setup, are intended in the near future.

For the illumination of the samples in the UHV AFM setup two different geometries have been used: the illumination in "frontside" and in "backside" configuration. In general measurements on two different types of substrates have been performed. Non-transparent HOPG, has been investigated in frontside illumination and the transparent ITO on glass substrates in backside illumination geometry. The sample holder shown in (a) is a standard Omicron VT-AFM sample holder and the organic photovoltaic sample (here on ITO) has been fixed to the sample plate via a tungsten clamp. The red organic deposit has been removed on the left side of the sample in order to contact the ITO electrode via the tungsten clamp to the ground of the UHV system. For the frontside illumination, the laser beam is directed onto the sample and the AFM cantilever is approached to the surface for the measurement within the ellipsoidal laser spot (see Figure 4.5 (b,c)). For this illumination geometry, it is not possible to avoid a partial irradiation of the cantilever by the laser beam. However due to the small beam diameter only a small fraction of the cantilever chip next to the prong is concerned.

In Figure 4.6 the backside illumination configuration for transparent samples is presented. For the indirect backside illumination, the sample is resting on a small MACOR ceramic plate and is held via a tungsten sheet on top that is screwed into the Omicron sample plate underneath. A mirror that is glued directly on top of the sample plate reflects the incoming laser beam towards the sample's ITO/glass electrode above. The laser beam traverses the transparent electrode, which has been functionalized by the PEDOT:PSS space layer and the organic photovoltaic layer. This way the active OPV layer is irradiated from behind meanwhile AFM imaging is possible from underneath on the illuminated zone (see Figure 4.6 (b,c)). Due to this illumination geometry much less light is directly shining onto the AFM cantilever and possible shadowing effects caused by the cantilever prong and tip can be completely excluded.

| KPFM on Illuminated Samples

The combination of Kelvin Probe Force Microscopy and laser illumination offers interesting possibilities for the investigation of the sample's optoelectronic properties. In order to assure reliable experimental results acquired by the complex measurement setup out of AFM stage, cantilever, sample and laser beam, reference measurements have to be performed.

| Avoiding Cantilever Artifacts upon Illumination

It is important to precisely know the influence of the AFM cantilever upon illumination in order to avoid possible measurement artifacts. First of all the illumination of the cantilever can create thermic heat within the AFM probe. This slightly shifts its mechanical properties and consequently changes the cantilever's resonance frequency. This "thermic detuning" is observed directly after the engaging of the illumination source and takes place until the cantilever has found its new thermic equilibrium under constant irradiation. Without adequate measures this resonance frequency detuning changes the imaging setpoint and like this hinders a reliable comparison of the KPFM images in dark and under illumination. For example if the CPD is dependent on Δf and the tip-sample distance, a change of the Δf setpoint will directly introduce a CPD shift without any photovoltaic influence. Furthermore, in the worst case, the detuning can even lead to a tip crash into the sample surface. Finally, in AM-KPFM mode, a shift in the resonance frequency will also introduce a (even larger) shift of the next higher harmonic of the resonance, which is used for the CPD determination. Because the Kelvin regulation can not follow the second resonance dynamically, artifacts can be introduced or simply good CPD measurement conditions are lost. In Figure 4.7 (a) the detuning upon "frontside" illumination is shown for a PtIr-coated cantilever (PPP-EFM) and a non-coated silicon cantilever (SSS-NCH) for two different illumination intensities of 1mW and 10mW . In all cases the sudden resonance frequency shift upon illumination and its stabilization after about 20min is observed. For higher illumination intensities a higher thermic detuning can be asserted. Moreover, the PtIr-cantilever with the reflecting metallic coating seems to be less influenced by this effect because it reflects most of the incident light. In total, to avoid artifacts due to thermic detuning in frontside illumination, but also in backside illumination configuration, a waiting period of up to 30min followed the engaging of the laser illumination until the cantilever was correctly stabilized. Then the new resonance frequency was determined and the AFM system recalibrated before starting the KPFM imaging.

As already mentioned, in the backside illumination geometry, much less laser light irradiates the AFM cantilever. This reduces the thermic detuning significantly by nearly one order of magnitude as it can be seen in Figure 4.7 (b). Thus, for transparent substrates, it is highly advantageous to image in backside illumination geometry.

Another possible artifact for KPFM imaging under illumination concerns a possible photovoltage of the cantilever itself. Most commercial AFM cantilevers are made out of the semiconductor silicon which is in most cases highly doped in order to assure a good conductivity and metal-like behavior. Some cantilever, like the PPP-EFM, are silicon cantilever which possess an addi- tional metal coating. In the case that the AFM probe is sensible to the laser illumination and would shift its work function Φ tip under continuous illumination, the measured CPD values by KPFM would be directly affected (CP D " pΦ tip ´Φsample q{|e|). A global CPD-shift would occur independent of the investigated sample. In order to exclude such an effect, reference measurements have been performed before and after the actual experiment with the same AFM probe. One possible reference surface that shows no photo-response itself is bare HOPG. In Figure 4.8 such a reference measurement is presented with the topographic KPFM image and the corresponding CPD images in dark and under illumination on the same zone. Obviously there are no contrast changes in the CPD images and also the CPD image histograms exhibits no significant CPD change upon illumination. The ΔCP D of below 4mV lies in the uncertainty of our CPD measurement. Thus, for this cantilever photovoltage artifacts can be excluded. For each investigation an adequate reference sample can be chosen. In the case of photovoltaic systems on ITO/glass, the reference sample of choice could be the bare ITO substrate.

| Surface Photovoltage Measurements on Organic Photovoltaic Systems

Before the investigation of more complex photovoltaic systems it is reasonable to make a proof of principle with a well known system that has already been characterized by other means. As such system a complete organic photovoltaic polymer solar cell with PBTFB: [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM (ratio 1:2) as active layer has been investigated [21]. The OPV blend has been deposited onto a PEDOT:PSS/ITO/glass electrode (anode) and was covered by a topelectrode of TiOx/Al. The solar cell was mounted in backside illumination configuration and the CPD change of the topelectrode (cathode) has been measured via KPFM (see This CPD change is also referred to as the (surface) photovoltage and corresponds to the difference of the CPD values under illumination and in dark:

P V " pCP D light ´CP D dark q. (4.2)
Upon illumination (515nm) the photo-generated excitons dissociate at the DA-interface and the acceptor [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM charges negatively and transfers electrons to the topelectrode (cathode).

The donor polymer PBTFB takes the holes, charges positively and is connected to the ITO bottom electrode. The whole cell is in open circuit condition and the ITO anode is connected to the UHV setup's ground. Thus, upon continuous illumination the topelectrode should charge negatively relative to the ground of the system until attaining equilibrium (drift current = diffusion current). This is confirmed by the experiment in Figure 4.9 (c) where the CPD change of the topelectrode is plotted over the illumination intensity. The measurement has been performed for silicon and Pt-coated cantilever and in FM-and AM-KPFM mode. In all cases, a negative CPD shift has been observed confirming the expected negative charging of the topelectrode. This furthermore proofs again the consistency of our CPD sign definition discussed in Section 4.2. The investigated "freshly" fabricated solar cell achieves already the highest CPD shift at 1mW illumination intensity being a sign for a still defect free and non-oxidized OPV blend with low recombination rates. Moreover, in principle, the measured CPD shift corresponds to the open circuit voltage V OC measured between anode and cathode. However, in order to learn more about the optoelectronic processes on the nanoscale, one has to investigate OPV samples without topelectrode and to measure directly on top of the active layer. In open circuit condition, the photo-induced charge carriers fill the donor and acceptor layer respectively and create due to this charge separation an electric field. The charging continues until the drift current caused by the electric field cancels out the diffusion current of charge carriers diffusing away from the DA interface and equilibrium is attained. The negative charging of the acceptor layer, shifts its molecular energy level (including E vac ) upwards and a ΔCP D is measured. This ΔCP D does not correspond directly to V OC as in the case of the complete solar cell in Figure 4.9 with anode and cathode. For a complete OPV cell, the energy level alignment at the organic-metal interface may modify the externally measured open circuit voltage. In case of non-ohmic contacts between the electrodes and the organic compounds, the maximum achievable open circuit voltage V OC,M AX is reduced to V OC , the difference of the electrodes' Fermi level [22] (see The lower half of Figure 4.10 illustrates the more complex situation of a BHJ blend without topelectrode. The molecular energy level of the inter-penetrating DA network are presented in a simplified way in the band structure diagram as energy level going from anode to cathode. In the ideal case, most of the donor is located next to the anode with a decreasing concentration towards the upper surface. The opposite is true for the acceptor component. Consequently, upon illumination the acceptor (donor) charges negatively (positively) and an increasing electron concentration is obtained from the anode to the top surface. A global negative CPD shift should be detected by KPFM at the upper blend surface. However, a BHJ blend represents a relatively random DA mixture with important local structural variations that strongly influences the CPD values acquired locally by KPFM. Additionally to the complications of energy level alignment and interface dipoles, averaging effects can potentially occur when measuring on top of a donor acceptor blend with nano-phase separation (see which represents an equipotential plane for the potential measurement via KPFM [23].

A comprehensive treatment of KPFM CPD imaging on a BHJ blend with respect to the blend's morphology will be presented in Chapter 6 using the example of self-assembled DA system. In the following, three examples for KPFM investigations of organic photovoltaic blends will be shortly discussed. These results taken from literature illustrate well different findings being important for all further discussions and serve as motivation for the experiments performed during this thesis.

In These are buried PCBM cluster which are covered by a "skin-layer" out of the donor polymer MDMO-PPV as it is demonstrated by the SEM cross-section image in Figure 4.11 (d). PCBM cluster, which are not covered by such a skin-layer exhibit lower work function values (red ellipse). This demonstrates the influence of the exact sample morphology on the measured CPD image contrasts. Upon illumination the photo-induced excitons dissociate at the DA interface and a work function change is observed for the skin-layer covered PCBM clusters. These results nicely showed that it is possible to image the photovoltaic charge carrier generation and to understand the optoelectronic response of the OPV blend via KPFM.

Another instructive study on the same DA system MDMO-PPV:PCBM (mass ratio 1 : 4) was performed by Maturová et al. [23]. The corresponding KPFM results of this BHJ blend are presented in Figure 4.12 for two different transparent (bottom) electrodes. In one case the ITO electrode was functionalized by a space-layer of PEDOT:PSS and in the other case by a space-layer of ZnO, before depositing the active photovoltaic layer.

First of all a clear global CPD shift is obtained for the BHJ blends on both substrates upon illumination. When comparing the CPD images "in dark" before and after the illumination interval, one recognizes that the CPD shifts are not completely reversible. The authors assign this effect to charge trapping in the organic compounds, a finding that has also been observed by other groups on similar systems like for example Chiesa et al. on F8BT/PFB blends [25]. Another important result concerns the importance of the energy level alignment at the electrode interface for the observed CPD contrasts in dark and also for the CPD shifts upon illumination. They claim that both organic components are in direct contact with the electrode's space layer underneath and that band bending [26] or interface dipoles at this interface create the "in dark" contrast. In case of the PEDOT:PSS space layer, no energetic barrier exists between the donor polymer's HOMO level and PEDOT:PSS's Fermi level. Positive charge transfer from the electrode towards the donor polymer (or electron transfer in the opposite direction) occurs and band bending is observed at this interface. However, PCBM's energy level are positioned in such way that E vac alignment occurs and no charge transfer takes place (see Figure 4.12 (g)). In total, an "in dark" contrast with different CPD values for MDMO:PPV and PCBM is observed. For the ZnO space layer on the contrary a hole barrier exists at the interface with MDMO:PPV and the positive charge transfer towards the bottom electrode is significantly reduced. This explains the much reduced "in dark" CPD image contrast between donor and acceptor for the ZnO functionalized electrode. Furthermore, the energy level alignment also explains the observed CPD shifts upon illumination. In case of the PEDOT:PSS space layer, photo-generated holes can easily be transferred to the bottom electrode (anode), however, the electrons remain in the acceptor and charge the whole blend negatively. A negative CPD shift is observed. In case of the ZnO space layer, the hole barrier with MDMO:PPV hinders the positive charge transfer to the bottom electrode and the photo-generated holes remain in the blend. However, the corresponding electrodes can be transferred from PCBM's LUMO towards the ZnO-functionalized bottom electrode. A positive charging of the BHJ blend and a global positive CPD shift is observed. All in all, the results of Maturová et al. demonstrate the importance of the energy level alignment with the electrodes for the observed CPD contrast in dark and under illumination.

Finally KPFM results on P3HT:PCBM will be presented that have been performed in our group in the context of a preceding thesis [27]. The investigated DA blend showed a phase-separation on the 10nm-scale, ideal for an efficient dissociation of the photo-generated The surface photovoltage image on a smaller area in Figure 4.13 (c) proofs the photo-induced charge carrier generation within the DA blend and the opposite charging of the donor P3HT and the acceptor PCBM. These results demonstrate that a PV image resolution on the nano-scale can be achieved. However, it is questionable if the identification of acceptor and donor domains only based on the varying damping signal is reliable enough. Furthermore, in order to have a clear damping/dissipation signal one has to image in the regime of SR-forces. It is still not clear, though, how the chosen interaction regime influences the CPD and PV values obtained by KPFM. Finally, the measured average PV values are, with values of some tenths of mV , pretty low compared to open circuit voltage obtainable for OPV devices and a quantitative interpretation seems difficult. To enhance the understanding in this field it seems therefore helpful to study model DA systems with better defined structural and electronic properties than the one found in ordinary BHJ blends.

| Conclusion

In this chapter technical and experimental aspects of KPFM investigations under illumination have been presented. Besides the correct definition of the CPD sign, it has been illustrated how cantilever artifacts can be avoided upon laser irradiation. The laser setup with the two different employed illumination geometries has been introduced and the KPFM measurement situation on organic photovoltaic systems clarified. With selected KPFM results on different BHJ blends taken from literature, different important findings have been illustrated. However, still a lot of open questions remain. Can we clearly identify nanoscale donor acceptor domains in order to understand the measured contact potential differences on this scale and to relate them to the optoelectronic processes? Is it possible to increase the resolution of the CPD and PV images by KPFM by measuring in the SR interaction regime without creating artifacts? How influences the chosen interaction regime the investigation of the photo-generated charge carriers in general? In order to study these questions in detail it should be of great advantage to investigate model DA systems with better defined structural and electronic properties. Two different model systems of this kind have been characterized by KPFM during this thesis. One is a BHJ blend of FG1: [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCMB for which it is possible to tune the DA phase-separation via the liquid crystal behavior of the donor FG1 (see Chapter 6). The other is a new generation of DA dyads with donor and acceptor moieties which self-assembles in form of bicontinuous DA-segregated stacks (see Chapter 7). En AFM non contact, il est possible d'imager la surface dans différentes conditions d'interaction pointe-surface. La réalisation de mesures de potentiel de surface en très haute résolution nécessite a priori de travailler dans le régime d'interaction à courte portée avec des distances pointe-surface minimales. Avant de réaliser des mesures de photo-potentiel de surface, il est indispensable d'étudier comment le choix du régime d'interaction pointe-surface (i.e. à longue ou à courte portée) peut modifier l'acquisition des données topographiques et de potentiel de surface. Ce chapitre est ainsi dédié à l'étude d'un système modèle par nc-AFM/KPFM sous la forme de films sub-monocouches de P3DDT auto-assemblé sur substrat HOPG. Sur ces échantillons, nous montrerons que la mesure de la hauteur d'empilement des domaines de P3DDT sur le substrat est fortement influencée par le choix du régime d'interaction. Bien que la compensation des forces électrostatiques à longue portée soit assurée par la régulation KPFM, nous montrerons que la variation des forces de van der Waals entre les domaines du polymère semi-conducteur et les zones nues du substrat induit une sous-estimation de la mesure de hauteur dans le régime à longue portée. Un modèle sera proposé pour rendre compte des résultats expérimentaux de spectroscopie en fonction de la distance pointe surface. Cette étude permettra de conclure que des mesures topographiques plus précises peuvent être réalisées par nc-AFM/KPFM dans le régime d'interaction à courte portée.

| Context

As discussed in Chapter 2.3.2 there are several forces acting between AFM tip and sample that can be classified in short-range (SR) and long-range (LR) interaction-forces. Depending on the chosen frequency shift setpoint Δf 0 it is therefore possible to image the sample surface in different interaction regimes. For an increased lateral resolution it might be advantageous to investigate the sample in the regime of SR-forces (SR-regime). This is true for the topographic resolution, but also for the resolution in the contact potential difference (CPD) image in the case of Kelvin probe force microscopy (see LCPD, Chapter 2.4). This increased resolution in the SR-regime can be extremely helpful for example in the case of organic photovoltaic devices with their donor-acceptor phase-separation on the sub-10nm scale (Chapter 3). However, in order to obtain reliable results by AFM and KPFM, it is essential to know the influence of the chosen interaction regime on the performed measurements.

In the following chapter, as a starting point, the influence of the interaction regime on the topographic measurement will be discussed. Exact topographic measurements by nc-AFM are of the utmost importance not only in the rising field of organic electronics [1, 2, 3] and when characterizing the structure of functional molecular assemblies on surfaces. For a proper understanding of the molecular arrangement, correct topographic height measurements have to be performed and a comprehensive knowledge of the interacting tip-surface forces is required.

In this context, Sadewasser et al. [4,5] performed height measurements of monolayers of self-assembled C 60 on HOPG (see Figure 5.1). On such heterogeneous samples, each material possesses its proper characteristic contact potential difference (CPD) with the AFM-tip (Figure

z [nm]

5.1(b))

. It was demonstrated that the step-height between domains of different CPD is strongly affected by the varying long-range (LR) electrostatic interaction between AFM-tip and sample. Depending on the applied tip-sample compensation bias, different step-height values were obtained for C 60 on HOPG (Figure 5.1(c)). In contrast, C 60 -C 60 and HOPG-HOPG steps with no CPD change and a constant electrostatic tip-surface interaction are not affected. Thus, in order to assure reliable topographic height values, it is essential to compensate the electrostatic forces by KPFM via the appropriate compensation bias (V DC " CP D) at all times. This general fact has been confirmed for different multi-component samples [4,6]. However, besides the electrostatic interaction also other forces act between tip and sample, like LR van der Waals (vdW) forces and short range (SR) forces [7,8,9,10]. Therefore, for many heterogeneous systems a simple Kelvin compensation of the electrostatic interaction may not be enough to assure correct height values. In the following, the effect of the chosen interaction regime and of the uncompensated LR-forces on the measured step-heights will be investigated using the example of an auto-organized semi-conducting polymer on graphite (HOPG). Thereby, we want to support the determination of more reliable topographic height values by KPFM.

| Experimental Details

All experiments have been performed with the VT-AFM/STM setup presented in Chapter 2.5 in UHV (p " 10 ´11 mbar) and at room temperature (T " 300K). The HOPG substrates have been freshly cleaved before the drop-cast of the polymer poly(3-dodecylthiophene) (8µg{ml dissolved in chloroform CHCl 3 ) onto the surface [11]. Silicon super sharp cantilevers (Nanoworld, used in FM-KPFM mode) have been argon-sputtered in vacuum in order to remove the oxide layer and possible contaminants for the KPFM measurements. Tungsten STM tips have been chemically etched and thermally treated in vacuum to remove the oxide layer.

| Self-Assembled Polymer Submonolayer as Model System

For the presented work samples with a self-assembled submonolayer of poly(3-dodecylthiophene) (P3DDT) on HOPG have been chosen [11] (chemical structure P3DDT in The estimated size of one monomer is indicated (compare [14]). (c) Self-assembly of P3DDT on highly oriented pyrolytic graphite (HOPG). The 2D polycrystalline structure exhibits three-fold oriented mono-domains connected by regular, 120 ˝or 60 ˝chain folds. The auto-organization is induced by the epitaxy of the lateral alkyl chains on HOPG, which promotes the formation of single monolayers.

The conjugated polymer P3DDT finds its application for example in photovoltaic BHJ blends and organic field effect transistors and has been used and characterized in our laboratory SPrAM for several years. In the context of a preceding PhD thesis (Lorette Scifo) a comprehensive STM study of this polyalkylthiophene was performed on HOPG. Its 2D polycrystalline structure exhibits three-fold oriented mono-domains connected by regular, 120 ˝or 60 ˝chain folds. This auto-organization is induced by the epitaxy of the lateral alkyl chains on HOPG, which promotes the formation of single monolayers [11,12,13] (see Figure 5.2 (c)).

The height of the polymer on HOPG is well-defined by the height of π-stacking [15,16] of the π-conjugated polymer with the π-orbitals of the underlaying graphite. Therefore this system represents an ideal model-system for topographic height measurements. In preparation of the nc-AFM investigations, STM measurements have been performed to determine a polymer-height value for comparison with the AFM results. However, one has to bear in mind the physical differences between STM and AFM measurements when comparing the obtained height values. STM is sensible to the sample's density of states (DOS), AFM is sensible to (the gradient of) the various forces acting between tip and sample (see Chapter 2). Prior to the STM measurements, Scanning Tunneling Spectroscopy (STS) was performed on HOPG and on the polymer domains in order determine a tunneling voltage V tunneling at which HOPG and P3DDT exhibit a similar density of states [13]. By using this tunneling voltage (V tunneling applied to the tip) for the STM imaging, possible height artifact related to a dissimilar DOS can be excluded. For example, imaging at a V tunneling for which P3DDT possesses a much lower DOS than HOPG would cause an underestimation of the polymer height on HOPG.

The bright lines in the STM image in

STM -Topography

| Influence of the Interaction Regime on Height Measurements

In The CPD image is nearly identical for both interaction regimes besides a very minor shift in the average CPD which may be caused by the apparition of SR-electrostatic forces [20]. In contrast, the topographic image (Figure 5.4 (a)) shows in the regime of damping contrast not only a better resolution [20] but also a significant difference in the measured height for the polymer layer on HOPG. According to the histograms in both regimes (Figure 5.4 (d) and 5.4 (e)) a height value of p1.9 ˘0.1q Å is obtained in the LR-regime and in the SR-regime a value of p4.7 ˘0.4q Å. Compared to typical values for π-conjugated systems on HOPG [21,22,16], the height value in the LR-regime is much too low and even falls below the layer-to-layer distance in bulk HOPG [23,24]. Also the STM results indicate much larger polymer heights (bearing in mind the physical differences in STM and AFM). On the contrary, the measured step-height in the SR-regime proves to be more realistic. It agrees well with the 4.4 Å obtained by a preceding Qplus-study on the same system [25] and is, in comparison to the value obtained in the LR-regime, also much more consistent with the obtained STM results. The falsification of the measured polymer height in the LR-regime occurs despite a proper Kelvin regulation which is supposed to compensate electrostatic forces while scanning the surface. For a perfect Kelvin regulation any error in the height measurement due to an electrostatic influence could be excluded [4]. Zerweck et al. [26] showed how accurate a real CPD regulation can be when scanning across an edge of a KCl island on Au( 111) and what finite distance is needed for the Kelvin regulation to follow the sudden CPD change at the edge. Therefore a very precise Kelvin regulation has to be assured, especially with regard to the quite small average size of the P3DDT domains of about 50nm. In our case the polymer domains are found to be very well resolved in the CPD images. Figure 5.5 shows CPD and topographic profiles across a P3DDT domain. The Kelvin loop follows very rapidly any change in the CPD and the CPD rises at the P3DDT edge within about 10nm (which corresponds approximately to the measured topographic width of the edge). Thus, the quality of the regulation is even better than in the former reported publication [26]. This is true for the LR-and the SR-part of the CPD images considering the equivalent quality and contrast of the CPD images in both regimes. Furthermore it has even been shown that the height difference effect between SR-and LR-regime does not depend on the amplitude of the bias modulation A bias,modul of the Kelvin regulation (see Figure 5.6). This is why it is believed that the electrostatic components are properly compensated during the presented measurements and do not falsify the measured height of the polymer layer. In conclusion, other long-range forces apart from the compensated LR electrostatic interaction that are dissimilar for P3DDT and HOPG should cause the observed step height effect.

In a next step, Δf pzq-spectroscopy has been performed to study the nature of the LR-forces and to confirm their influence on the measured step-heights (see Figure 5.7). As before, the electrostatic forces have been compensated during the performed tip-retraction spectra. The spectra show a clear difference in the LR-regime with a less pronounced slope for P3DDT than for HOPG. The uncompensated LR-interaction of the AFM-tip with the polymer domains seems to be inferior to the one with the bare substrate. The insert in Based on these spectra, it is illustrated in magnitude of Δf pzq in the LR-regime for P3DDT than for HOPG, the AFM tip has to approach the polymer layer more than the bare graphite in order to maintain the same frequency shift set-point Δf 0 . Consequently, the measured experimental height h exp lies below the real value h real (see Figure 5.8 (b)). However, for a Δf 0 in the SR-regime, the LR-interaction is weak compared to the one of the SR-forces. Moreover, slight differences in the SR-interactions above HOPG and the polymer do not cause a large height-effect due to the short decay lengths.

Altogether, an accurate height measurement gets possible.

To confirm the developed model, height measurements on bare HOPG were performed (Figure 5.9). Compared to the heterogeneous sample P3DDT on HOPG, no step-height difference is observed for the bare HOPG sample. The measured HOPG double-step exhibits the same height in the LR-and SR-regime within the accuracy of the measurement. Due to the identical tip-sample interaction on both sides of the step-edge, the LR van der Waals forces do not falsify the topographic measurement and the correct height value is obtained independent of the interaction regime. Furthermore, based on the spectroscopic results above (Figure 5.7), one can also exclude that the observed difference in the height measurement in the LR-and SR-regime is caused by any CPD averaging effect. A such averaging effect could potentially originate from the rather small P3DDT domain size in comparison to the dimension of the tip apex and would induce artifacts. However, in the spectroscopic results and the topographic imaging data for a stepwise increase of Δf 0 (Figure 5.10), a sudden change of the measured step-height was observed when entering the SR-regime. The onset of SR-forces is sharp and occurs within a very small change of the tip-sample distance. A distance change that does not affect the overall geometric arrangement of tip versus P3DDT islands on HOPG to such extend that the observed height difference in the LR-and SR-regime could be explained by any averaging effect or imperfect CPD compensation.

| Role of vdW-Interaction in Height Measurement Effect

So far, the experimental results have been discussed in detail and a model has been proposed illustrating how dissimilar LR-interaction forces can affect the measured height of the polymer domains on HOPG. In case of the presented model system P3DDT on HOPG and in UHV conditions only two principal LR-interactions occur, LR-electrostatic forces and LR-vdW forces. It has been shown that the electrostatic forces are properly compensated and that averaging effects can be excluded. Thus, LR-vdW forces are suggested to cause the observed effect on the height measurement by nc-AFM.

In the following two key aspects of this study will be discussed which underline the assumption that the differing LR-interaction observed for bare HOPG and P3DDT on HOPG is mainly caused by LR-vdW-forces. The presented discussions are based on the work performed in collaboration with Franck Bocquet, Laurent Nony and Christian Loppacher from the IM2NP UMR7334 CNRS and the University Aix-Marseille III (see supplementary information of [27]).

First of all, it will be estimated if one polymer-monolayer can influence the LR-vdW part to the extent observed in the experiments. Therefore the ratio R " Fepzq Ftotpzq of the vdW force which is caused by an effective layer of thickness e, named F e pzq, divided by the total vdW force F tot pzq between the tip and the whole sample (effective layer plus substrate of infinite height) will be calculated. The proposed tip sample model is depicted in Figure 5.11. The cone-shaped tip apex of height b is connected to a cylindric tip trunk of infinite length and z corresponds to the working distance between the tip and the effective layer. infinitely long tip trunk. The tip radius is described by rpηq. The effective layer is the surface layer with thickness e for which the contribution to the total force between tip and sample will be determined. It can be for instance the P3DDT polymer layer. The whole sample is a semi-infinite plane and the substrate is of infinite thickness.

In a simple model to describe the vdW-interaction (Chapter 2.3.2), the force is proportional to the Hamaker constant H and the different force contributions are additive. The total vdW force between tip and sample is the sum of the vdW force of the effective layer plus the contribution of the infinitely high substrate (F tot pzq " F e pzq `F8 pz `eq). With this additivity the ratio R gets:

R " 1 ´F8 pz `eq F tot pzq .

(5.1)

When integrating the van der Waals interaction between points in tip and sample over the infinitely large and thick substrate and the tip, one obtains for the total van der Waals force at working distance z:

F tot pzq " π H 4 ż tip rpηq 2 pz `ηq 4 dη. (5.2)
With the used tip model (r " a ¨η, for r P r0 ď η ď bs and r " a ¨b, for η ą b) and the ratio x " e z between effective thickness and working distance, the force ratio R finally results in:

R " 1 ´z pb `zq 2 pe `zqpe `b `zq 2 " 1 ´1 p1 `xqp1 `x 1`b{z q 2 . (5.3)

When R is averaged for typical b{z values in the range of r4, 40s (for example b " 40nm and 1nm ď z ď 10nm), one can determine that roughly 80% of the vdW-interaction between tip and surface originates from a surface layer with a thickness in the order of " 3 times the working distance z. For the used experimental parameters, a polymer-layer of 4 Å thickness would result in a modification of the measured frequency shift Δf of up to 0.2Hz due to vdW interaction. This is in good agreement with the spectroscopic results presented in Figure 5.7, where the Δf pzq difference between P3DDT and HOPG is of similar magnitude.

The second aspect under discussion concerns the shape of the experimentally obtained Δf pzq-curves and hence also the vdW-interaction for different tip-sample distances. Figure 5.12 displays a logarithmic plot of the experimental Δf pzq-curves. The z-scale has been adjusted in such way that the short-range regime has its onset at 6 Å. In the LR-interaction regime, for tip-sample distances z ă 3nm, the observed slope in the logarithmic plot is ´0.5 whereas for the largest distances it is around ´2.5. A qualitative agreement is observed with the experimental curves concerning the magnitude of the observed difference between P3DDT and HOPG curves, as well as concerning the varying distance dependencies for z ! A cantilever , z " A cantilever and z " A cantilever . The overall shape of the curves can only be matched with the experimental results when using more complex tip geometries such as the ones of the used high aspect ratio "Super-Sharp Silicon" tips. The experimental Δf pzq data show in contrast to the calculated pure vdW curves also the onset of SR-forces at 0.6nm.

ln(z) [ln(nm)]

At first glance, this is in contradiction to the distance dependence of a force following a pure power law. In the latter case a constant (linear) slope should be observed in the logarithmic plot when using a conical tip with large radius at small tip-sample distances (see Figure 3 in [8] by Guggisberg et al.). There are two explanations for the shape observed in our Δf pzq-curves. The first one concerns the tip geometry, the second the averaging of the interaction due to the large oscillation amplitudes [28].

A Δf pzq which follows a pure power law would only be measured for a tip with homogeneous geometry. All tip-geometries with different geometry for apex and trunk, like the ones described by Guggisberg et al. [8], do not fulfill this criteria. For example a pure spherical tip with radius R would follow a force law according to z ´2 when z ! R, and a z ´4 dependence for z " R as in the latter case, the interaction can be described by a point above a semi-infinite plane [7]. Though, for small distances, the vdW interaction is determined by the tip apex -sample interaction, and for large distances by the tip trunk -sample interaction. Note that in the experiments by Guggisberg et al. the z ! R condition is satisfied over the complete data range and a constant slope is observed although the tip geometry is conical with spherical end.

The second cause for the experimentally observed Δf pzq curve shape is related to the use of larger oscillation amplitudes. Giessibl et al. already explained clearly how the latter influence the measured Δf pzq-curves via the averaging of the tip-sample interaction [28,29] . For a power law of the force which strictly follows a z ´n distance dependence, the measured Δf pzq would show a z ´pn`1q dependence for z " A cantilever , whereas for z ! A cantilever it would follow a z ´pn´0.5q dependence.

A complete fit of the experimental data with a realistic tip model is out of the scope of this thesis. Even more as for the acquired experimental data it would be very difficult since the z " A cantilever regime is not achieved. Nevertheless, the simple tip geometry depicted in Figure 5.11 will be used in the following to compare the experimental data with the performed calculations. The calculated Δf pzq curves based on the vdW-interaction between such a tip and the bare HOPG surface (blue line with squares) as well as the HOPG surface covered by a 4 Å thick polymer-layer (red line with triangles) is displayed in Figure 5.12 in comparison with the experimental data. For the calculation, a ratio of the Hamaker constants of H P 3DDT {H HOP G " 0.65 was assumed. Furthermore, a pre-factor which includes the Hamaker constant H HOP G as well as the opening angle of the conical tip apex was adjusted in order to align the calculated curves reasonably with the experimental data (resulting conical tip apex b " 45nm). Taking into account the particular tip-sample geometry used in the experiments, the overall shape of the observed LR-interaction can be fairly reproduced. The calculations confirm that the integral shape of the Δf pzq-curves measuring pure vdW interaction can show different distance dependencies for z ! A cantilever , z " A cantilever and z " A cantilever . Furthermore, in combination with the fact that about 80% of the vdW-interaction originate from a surface layer of " 3 times the working distance z, these simple calculations demonstrate that a single polymer-monolayer can significantly change the tip-sample interaction in the long-range regime.

The fact that both, the shape of the experimental Δf pzq-curves as well as the variations between the clean HOPG substrate and the one covered by a polymer-layer are in good agreement with the calculations, underlines the conclusion that the observed effect in the topographic height measurement is caused by the LR-vdW interaction.

| Conclusion

In this chapter height measurements via KPFM have been presented for a submonolayer of P3DDT on HOPG. This study demonstrated what impact dissimilar LR-interactions can have on the measured step-height of polymer self-assemblies on graphite. A very significant difference for the topographic height values was found depending on the interaction regime. As a consequence a simple compensation of electrostatic forces like in KPFM is essential [4], however, no guaranty for a correct height measurement. The presented results and calculations suggest, that the observed height differences are due to the differing vdW-interaction above the polymer layer. In order to obtain reliable topographic height values it can be advantageous to measure in the SR-regime where the fast-decaying SR-forces dominate the LR-contribution. Consequently, with respect to the following nanoscale investigations of organic photovoltaic DA systems, it should be advantageous to image in the SR-regime in order to obtain a better resolved and more accurate topography.

To further investigate and confirm this influence of LR-vdW forces, measurements of molecular assemblies with very different Hamaker constants and thus different vdW tip-sample interaction could be performed. For example, the pure carbon material C 60 might show a fairly different height effect when changing the interaction regime than polyalkylthiophenes with long alkylchains and a completely different molecular composition. Furthermore, the critical influence of different tip geometries on the observed Δf pzq-curves could be studied by using different types of cantilevers. Le premier système modèle utilisé pour les études de photo-potentiel de surface consiste en des films d'hétérojonctions en volume donneur-accepteur basés sur un dendrimère cristal liquide (FG1) et sur le [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM. Nous montrerons comment la morphologie des hétérojonctions FG1: [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM peut être modifiée par recuit in situ sous ultra-vide grâce aux propriétés cristal liquide du matériau donneur d'électrons (FG1). Des échantillons modèles sous la forme d'agrégats de [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM uniformément recouverts par une matrice de fils moléculaires auto-assemblés de FG1 sont ainsi obtenus. L'analyse de contrastes du potentiel de contact (CPD) «dans le noir» sera menée dans le cadre des modèles de transfert de charge entier aux interfaces donneuraccepteur. L'influence du régime d'interaction pointe-surface (i.e. à longue ou à courte portée) sur les contrastes topographiques, de dissipation, de potentiel de contact et de photo-potentiel de surface sera ensuite étudiée au travers d'une série d'images réalisées à amplitude de vibration constante et décalage en fréquence variable. Nous montrerons que la résolution latérale de l'ensemble des signaux est accrue dans le régime à courte portée, et que les mesures de photopotentiel de surface peuvent être a priori réalisées à des distances pointe-surface minimales sans générer d'artefacts. De plus, un effet remarquable d'augmentation du photo-potentiel moyen dans le régime à courte portée sera mis en évidence et interprété en termes d'exaltation de l'intensité du champ optique par la sonde de champ proche.

| Context

As illustrated in Chapter 4.4, it is advantageous to study model organic DA systems in order to enhance the understanding of the processes of photo-induced charge carrier generation and collection on the nanoscale. Compared to conventional bulk heterojunctions, like the well-known P3HT:PCBM blend [1], these model system possess better defined electronic and structural properties. Therefore, in the following, a first donor-acceptor model system based on a fluorene-core based dendrimer (FG1) and [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM will be presented [2].

After a complete morphological study, this DA system will be examined with regard to the photovoltaic charge carrier generation upon illumination. It will be aimed to relate the obtained photovoltage contrast to the blend's morphology and to find the best measurement conditions for an optoelectronic analysis. In the previous Chapters 2.4 and 5 it has been clarified that KPFM investigations can be performed in different interaction regimes and that it can be advantageous for topographic and CPD imaging to measure in the regime of short-range forces. Thus, in the following, the influence of the chosen tip-surface interaction regime on the detection and the imaging of photo-induced charge carriers by KPFM will be of particular interest.

| Experimental Details

Substrates of Indium-Tin-Oxide (ITO) have been functionalized with PEDOT:PSS via spincoating. In a second step, equally by spin-coating, the active photovoltaic layer out of FG1 and [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM (ratio 1 : 2.5) was deposited on top. The obtained samples were immediately introduced in the UHV setup. All experiments have been performed with the VT-AFM/STM setup presented in Chapter 2.5 in UHV (p " 10 ´11 mbar) and at room temperature (T " 300K). Sample annealings have been performed at pressures of 10 ´10 mbar and under temperature control with an optical pyrometer. The annealing is expected to remove remaining solvent of the solution processed samples. The AFM cantilever (Nanoworld SSS-NCH) have been Argon sputtered in vacuum to remove the oxide layer and possible contaminants. KPFM measurements have been performed in FM-mode with the compensation voltage V DC applied to the cantilever (V DC " CP D " pΦ tip ´Φsample q{|e|, see Chapter 2.4). The dual laser source with 515nm and 685nm wavelength and variable light intensities (presented in Chapter 4.3) has been used for the KPFM measurements in backside illumination geometry. To verify the absence of a photo-voltage generated by the cantilever, reference measurements have been performed with the same cantilever on samples of pure [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM and pure FG1 on PEDOT:PSS/ITO.

| Tailored Donor-Acceptor Phase-Separation

The presented donor-acceptor system is a bulk heterojunction blend with FG1 as donor molecule and [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM as acceptor (see due to the liquid crystalline properties of FG1. The liquid crystal phase is an intermediate state of matter in between liquids and crystals that can appear for molecules with anisotropic shape, fairly rigid backbones and strong dipoles (and/or easily polarizable groups) [3]. In the liquid crystalline state the molecules have the tendency to auto-organize and to point along a common axis due to the dipolar interaction. The fluorene-core of FG1 in the middle of the molecule exhibits a strong electric dipole caused by the high electron negativity of oxygen (dipole indicated in Figure 6.1 (a)). It has already been shown for FG1 on HOPG that this dipole significantly influences and promotes the molecular self-assembly for this molecule [4]. In the BHJ blend with [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM, it should therefore be possible to influence directly the structural molecular organization via well-defined sample annealings that enable FG1's liquid crystalline ordering.

In Figure 6.2 topographic and CPD-images of the FG1: [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM blend are shown before and after annealing steps at 120 ˝and 150 ˝Celsius for 15min. The influence of the annealing is clearly visible and a higher structural order can be seen in the topographic, but also in the CPD images. Thus, due to the liquid crystalline ordering and the molecular self-assembly, it is not only possible to change the blends morphology, but also the donor-acceptor phase-separation which is essential for the photo-induced charge carrier generation and collection (see Chapter 3). Because of its molecular properties, FG1 tends to assemble in lamellar structures as it has been confirmed by preceding studies [2,4]. Therefore, the "zig-zag" pattern of adjacent lamella visible in Figure 6.2 (e) (and also in the topographic and damping images below) indicates that these surface areas are nearly exclusively covered by FG1 and contain only a small amount of PCBM. Because of the FG1:PCBM ratio of 1 : 2.5 for the photovoltaic blend, it is suggested that most of the fullerene-derivates have to be situated deeper in the active layer with a concentration increase towards the ITO/PEDOT:PSS electrode. This observing has also been found for other BHJ systems and is caused by the different surface energies of FG1 and PCBM. Depending on the exact blend preparation, for example the well-known P3HT:PCBM blend can show similar concentration gradients [5,6].

Nevertheless, the large-scale KPFM images in Figure 6.3 (a,b) show circular domains of higher CPD that can be identified as PCBM-rich areas, thus regions where PCBM starts to 53.93 nm 0.00 nm (a) This corresponds to a donor-acceptor phase-separation on the µm-scale (p5000nmq 2 , Δf " ´10Hz, A cantilever " 9nm, f cantilever " 275kHz). Closer topographic (c) and CPD (d) images reveal buried PCBM cluster (small bright spots in CPD) which are intercalated between the FG1 lamellae. This corresponds to a donor-acceptor phase-separation on the nm-scale (p2000nmq 2 , Δf " ´10Hz, A cantilever " 9nm, f cantilever " 275kHz). (e) Illustration of the different possible donor-acceptor phase-separations on the µm-scale (large-scale), the nm-scale and both at the same time (multi-scale).

aggregate and crystallize. The aggregation of PCBM upon annealing has been stated before and stems from thermally induced diffusion [7,8]. When having a closer look on the surroundings of one of the PCBM-rich areas (Figure 6. 3 (c,d)), one notices in the CPD image the appearance of bright little spots in between the FG1 lamellae. These spots with diameters in the range of 10nm exhibit the same CPD as the large-scale PCBM-rich aggregates. They correspond to PCBM domains that are intercalated in between the FG1 lamellar structure. Most of these small PCBM crystallization areas have to be buried within the self-assembled FG1 matrix because the small PCBM clusters are only visible in the CPD and not in the topographic images (and neither in the damping images as shown hereafter). Furthermore, even the larger PCBM aggregates seem to be covered, at least partially, with lamellar bundles of FG1 giving rise to fibrillar structures on top (Figure 6.3 (a,b)). In total, the PCBM-FG1 blend exhibits a donor-acceptor phase-separation on the µm and the nm-scale at the same time. This corresponds to the multi-scale phase-separation illustrated in Figure 6.3 (e) [9].

To understand the CPD contrasts in dark it is essential to look at the energy level alignment at the metal-organic and organic-organic interface (see Chapter 3.3). In Figure 6.4 (a) the HOMO and LUMO molecular electronic level, which have been estimated by cyclic voltammetry, are given for FG1 and PCBM [2]. Furthermore, according to the Integer Charge Transfer (ICT) model [10], their positive and negative integer charge transfer states E ICT `and E ICT ´are indicated at their approximative positions. Finally, also the Fermi level E F of the PEDOT:PSS/ITO electrode is given [6]. Before the contact of all components, their vacuum level E vac are aligned and, based on the experimentally observed potential contrast between FG1 and [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM (vide infra), the E ICT `level of FG1 has to lie energetically above the E ICT ´of PCBM (Figure 6.4 (a)). Because of this relative positioning, electron charge transfer (CT) occurs from FG1 towards PCBM upon contact until both level are aligned (Figure 6.4 (b)). This CT creates an interface dipole and lowers FG1's vacuum level by the dipole barrier energy Δ. In contrast, at the interface between PCBM and PEDOT:PSS, a flatband situation with aligned E vac is achieved. This is due to the relative positioning of PCBM's charge transfer level and PEDOT:PSS's E F . On the one hand, no charges can be transferred from PCBM's E ICT `to the PEDOT:PSS electrode because all electron states below E F are already filled. On the other hand, the electrons in the PEDOT:PSS electrode do not have enough energy to be transferred into PCBM's empty E ICT ´states. All in all, the energy level alignment at the FG1/[70]PCBM/PEDOT:PSS interface is very similar to the one of P3HT/PCBM/PEDOT:PSS [6]. However, in order to obtain an efficient OPV cell, normally the inverse sequence of donor and acceptor layer is aspired. The donor FG1 should be located next to the PEDOT:PSS/ITO anode for an efficient collection of the photo-generated holes.

In Figure 6.4 (c) the total morphological situation within the FG1: [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM blend is illustrated. Like mentioned above, most of the PCBM has to be located next to the PEDOT:PSS/ITO electrode and the PCBM concentration decreases towards the upper surface. That way, the formation of a virtually completely closed PCBM layer is expected next to the bottom electrode like in the case of a bilayer photovoltaic cell. The PCBM aggregates in the surface-near region are buried within the matrix of auto-organized FG1 and are covered by a thin layer of self-assembled FG1. The appearance of such a "skin-layer" covering the blend's PCBM cluster has also been found for other DA blends like MDMO-PPV:PCBM [11] (see Chapter 4.4). In the following, the interface dipoles and the CPD contrasts of the surface-near buried PCBM cluster ("surface interface dipoles") and of the complete PCBM bottom layer with the FG1-rich layer on top ("bulk interface dipole") will be discussed in detail. the acceptor. Consequently the PCBM aggregates charge negatively and the surrounding FG1 positively. First of all, it is underlined that for a perfectly spherical PCBM cluster the interface dipoles, pointing in all directions, would cancel each other out (see Figure 6.5 (a)). For this situation, electrostatic modeling performed with COMSOL1 in collaboration with Thierry Melin (IEMN, CNRS, France) has shown that the buried PCBM aggregates will not be detected via Another anisotropy resulting in a net electrostatic surface dipole may be caused by the exact molecular assembly and the microelectrostatic environment at the interface. Here a situation is illustrated where CT is only possible from the FG1 molecules towards the PCBM cluster if these are aligned parallel to the FG1/PCBM interface. The partial CT across only specific sections of the PCBM sphere creates a net surface dipole and the PCBM cluster can be detected.

Surface Interface Dipoles

KPFM. Nevertheless, the experimental CPD images proof that these buried acceptor cluster possess net electrostatic dipoles pointing away from the sample surface. Consequently, the PCBM aggregates must show a certain electrostatic anisotropy. In a first scenario, illustrated in Figure 6.5 (b), one could imagine an anisotropic, non-spherical shape of the buried PCBM volume. For example in the case of an ellipsoid oriented with its long axis perpendicular to the surface, the surface-near DA interface dipole is much closer to the tip than the one on the ellipsoid's opposite side. Considering the relatively fast decay of the electrostatic dipole-field, a net dipole moment will be measured by the KPFM tip. A second possible explanation for the detection of the buried PCBM cluster lies in an uneven charge transfer across different sections of the spherical PCBM interface (Figure 6.5 (c)). As explained in Chapter 3 and as observed by numerical calculations [12,13,14], the exact molecular alignment at the DA interface strongly influences the charge transfer across the interface and the thereby induced interface dipoles. Verlaak et al. demonstrated for a system of C 60 /pentacene that the energy level alignment and the created interface dipole depends on the molecular assembly and the orientation of the molecular dipoles at the interface [13]. In Figure 6.5 (c) an exemplary situation is depicted where charge transfer is only possible from FG1 towards the PCBM cluster if the FG1 molecules are aligned parallel to the FG1/PCBM interface. The partial CT across only specific sections of the PCBM sphere creates a net interface dipole and the PCBM cluster can be detected. Altogether, one could imagine also a combination of both effects illustrated in Figure 6.5 (b) and (c). With a net electrostatic dipole pointing away from the sample surface, KPFM detects the buried PCBM-rich areas as regions of higher CPD. Further electrostatic modeling is in progress to address this issue more quantitatively.

Bulk Interface Dipole

Apart from the surface-near region, charge transfer also takes place in the bulk material at the interface of the bottom PCBM layer and the FG1-rich layer on top (see Figure 6.4 (e)). Again electrons are transferred from FG1 towards PCBM and thereby a bulk interface dipole is created.

The average dipole moment is pointing upwards, away from the anode. This bulk interface dipole increases the measured global CPD of the FG1:PCBM blend in comparison to the one of the bare PEDOT:PSS electrode to CP D F G1:P CBM " ´700mV . To confirm the developed model for the bulk interface dipole, the global mean CPD has also been determined for pure [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM on PEDOT:PSS and for pure FG1 on PEDOT:PSS (see measurements in Figure 6.9 in the next section). A comparison of the energy level alignments according to the ICT model for the different interfaces is shown in Figure 6.6. In the case of a pure [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM functionalized electrode (a), E vac alignement is obtained between PCBM and PEDOT:PSS and no bulk interface dipole is created. The obtained experimental value CP D P CBM " ´1100mV should correspond to the CPD of the bare PEDOT:PSS electrode and lies below CP D F G1:P CBM . Only samples of pure FG1 on PEDOT:PSS will show even higher global CPD values due to the alignement of FG1's E ICT `level with the electrode's Fermi level E F which creates a large dipole barrier Δ (Figure 6.6 (b,c)). An experimental average CPD value of CP D F G1 " ´500mV has been measured. Apparently the experimental CPD values for the different functionalized electrodes reflect well the direction and the order of magnitude of the interface dipole predicted by the ICT model. Consequently, also the developed model for the FG1:PCBM blend's bulk interface dipole is confirmed. All in all, based on the FG1:PCBM blend morphology and with the ICT model for the energy level alignement at interfaces, a consistent model could be developed for the interpretation of the measured "in dark" CPD images. This model explains the blend's global CPD dark value as well as the nanoscale image contrasts.

In order to further prove the intercalation of PCBM within the lamellar FG1 structure, comparative KPFM investigations have been performed for pure FG1 and for the FG1:PCBM blend, both on PEDOT:PSS/ITO. In Figure 6.7 topographic, damping and CPD images of the same scale are presented for the two samples. In both cases lamellar structures are visible. The , Δf " ´20Hz, A cantilever " 9nm, f cantilever " 275kHz). Topographic (d) and damping (e) images of FG1:PCBM show also a lamellar assembly, however with an increased periodicity of around 10nm due to intercalated PCBM. The CPD image (f ) shows a relatively strong contrast for FG1:PCBM with thin stripes and bright spots corresponding to buried PCBM-rich cluster (p300nmq 2 , Δf " ´24Hz, A cantilever " 9nm, f cantilever " 275kHz).

measured periodicity for pure FG1 is of 4.5nm and corresponds very well to the value found for monolayers of pure FG1 on HOPG [4]. In contrast, the FG1:PCBM blend shows an increased average periodicity of about 10nm. This increased lamellar width has to be related to the added PCBM. On the one hand PCBM might be directly intercalated between the self-assembled lamellar FG1 stacks thereby increasing the assembly's periodicity. On the other hand, the integration of PCBM within the FG1 matrix may change the microelectrostatic environment and thereby induce another type of molecular auto-organization of FG1 with increased lamellar width. Furthermore, there is also a significant difference in the CPD images (Figure 6.7 (c,f )). In the case of pure FG1 a nearly homogeneous CPD is obtained besides the deviation of the mean CPD at the grain boundaries between different FG1 crystallites. For the BHJ blend, however, a stronger and very heterogeneous contrast is observed. Spots of bright CPD correspond again to buried PCBM crystallization areas, a darker CPD to FG1-rich regions. Additionally bright and dark stripes appear with a periodicity similar to the one of the observed lamellae. On the one hand, these stripe-like CPD contrasts may originate simply from the periodic intercalation of PCBM between the FG1 lamella. However, on the other hand, it is also known that FG1 can exhibit different types of lamellar assemblies which can give rise to such a CPD contrast depending on the orientation of their molecular dipoles [4]. In Figure 6.8 modeling results for FG1 in vacuum are presented. As already seen for the samples of pure FG1 (Figure 6.7 (a-c)), FG1 tends to create lamellar stacks. For energetic reasons, the molecules in one single FG1 stack will align their molecular dipoles in a parallel manner (Figure 6.8 (b)). For assemblies of neighboring FG1 lamella, different conformations with parallel or alternating dipole orientation are possible (Figure 6.8 (c)). In general, such different assemblies may also give rise to the observed stripe-like nanoscale CPD contrasts. However, one has to bear in mind that the exact structure of the FG1:PCBM blend differs a lot from the one of the lamellar FG1 stacks in vacuum presented in Figure 6.8.

| KPFM of Illuminated FG1:PCBM Blend

By means of the already presented laser setup, KPFM measurements under illumination have been performed. First of all, reference samples of pure FG1 and pure [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM on PEDOT:PSS/ITO have been investigated. The pure components of the BHJ are not expected to exhibit any CPD-shift or photovoltage (PV) upon illumination, due to the absence of any DA-heterojunction for the exciton dissociation. This is confirmed by the experimental results presented in Figure 6.9.

53.58 nm 0.00 nm (a) Figure 6.9: KPFM investigation of reference samples in dark and under illumination (1mW , 515nm). Topographic and CPD image in dark of pure FG1 (a,b) and pure PCBM (d,e) (p2000nmq 2 , Δf F G1 " ´8Hz, Δf P CBM " ´6Hz, A cantilever " 9nm, f cantilever " 275kHz). The CPD image histograms in dark and under illumination show no significant CPD shift for pure FG1 (c) and pure PCBM (f ) on PEDOT:PSS/ITO.

Both CPD image histograms, for pure FG1 and pure PCBM demonstrate that no significant CPD-shift is obtained between the situation in dark and under illumination (1mW , 515nm). This also excludes any photovoltage generated by the silicon cantilever itself which would cause a global CPD-shift.

In Figure 6.10 KPFM investigations under illumination (515nm, 1mW ) of the FG1:PCBM bulk heterojunction are shown. KPFM imaging was performed on a flat FG1-rich sample area with buried (intercalated) PCBM. Besides the topographic results, the CPD images in dark and under illumination are presented (Figure 6.10 (b,c)). First of all, a global positive CPD-shift of more than 100mV is obtained for the whole sample. This shift is completely reversible when turning the illumination off, as it can be seen in the histogram in Figure 6.10 (e). Apparently no charge trapping [15] occurs for this organic semiconductor blend. Apart from the global positive charging, also the CPD contrast changes upon illumination. Some areas charge more positively than others which is clearly visible in the PV image in Figure 6.10 (d). As already mentioned in Chapter 4.4, the photovoltage image is the subtraction of the CPD images under illumination and in darkness (P V " CP D light ´CP D dark ).

In organic photovoltaic blends, the photo-generated excitons depend on the favorable electronic landscape at the DA interface to overcome the mutual Coulomb attraction (see Chapter 3.4). The generated negatively charged electrons are transferred to the acceptor ([70]PCBM) and the positively charged holes remain in the donor (FG1). Thereby a charge transfer state (CTS) is created of still Coulomb bound electron and hole across the DA interface. This CTS will subsequently be dissociated and free charge carriers (polarons) are created. In Figure 6.4 (a,b), the energy level alignment without illumination has been derived for the PCBM/FG1 interface based on the ICT model. In agreement with the experimental results an electron transfer from FG1 towards PCBM and the creation of an interface dipole was predicted. Intuitively, the induced interface dipole would counteract a further negative charge transfer towards the acceptor under illumination. This aspect, as well as the exciton and CTS dissociation at the DA interface in general, is still subject to ongoing controversial discussions. It is arguable if the ICT-model, which has been developed for the determination of the energy level alignment at interfaces in equilibrium and hence without optical excitations, is also applicable for illuminated interfaces. As already mentioned in Chapter 3, for organic semiconductors, there is a general problem to represent optical excitation (excitons) and charge transport (via polarons) in the same energy level representation. This is because, contrary to inorganic SC, the optical gap and the transport gap are not the same. Adding or withdrawing charges for charge transport modifies the molecular energy level, whereas an exciton is the optical excitation of a neutral molecule or polymer. Consequently, one would have to develop a modified ICT-model for the non-equilibrium situation to describe the modified energy level alignment under illumination. In order to precisely understand the energy level alignment under illumination, advanced numerical modeling would be necessary that also includes the exact microelectrostatic environment at the DA interface [14,13,12]. Concerning the "in dark" interface dipole, Arkhipov et al. demonstrated with their so-called "dark dipole model" that interface dipoles nevertheless can enhance the overall dissociation probability of electron-hole pairs at the DA interface [16,17]. Once a CTS is created, the dipole field at the interface reduces the CTS binding energy and facilitates the CTS dissociation. All in all, the dissociation of the photo-generated electron-hole pairs is possible and the acceptor [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM domains will charge negatively whereas the donor FG1 domains will charge positively.

With most of the PCBM located near the bottom electrode and with the FG1-rich top-layer next to the top-surface, a global positive CPD shift is obtained by KPFM. This is even the case if no negative charges have been transferred to the bottom electrode because of the creation of a strong oriented dipole created with the photo-generated charges similar to the "in dark" bulk interface dipole illustrated in Figure 6.4 (e). Additionally to this global CPD shift, the PV image in Figure 6.10 (d) indicates regions that shift less positive than others. These dark areas in the PV image appear mostly as bright areas in the CPD images. This is even more clearly visible in Figure 6.11 and the high resolution KPFM image Figure 6.12 discussed below. Brighter spots in the CPD images, however, correspond to buried PCBM-rich aggregates. Thus, these surface-near cluster of the acceptor PCBM must have taken the photo-generated electrons, charged negatively, and like this counteract locally the sample's global positive CPD shift. After illumination, all present photo-generated charges will recombine in the dark DA blend and the mean CPD level, but also the CPD image contrast, will return to their initial state (see Figure 6.10 (e)). The FG1:PCBM blend constitutes hence a completely reversible DA systems which is ideal for investigations of the photo-induced charge carrier generation and collection. The fact that different CPD shifts are obtained for the red and the green wavelength is simply related to the different absorbances for the two wavelengths in this organic semiconductor blend [2]. The saturation of the open circuit voltage V OC with increasing intensity is related to the equally rising electron-hole recombination rate. As deduced from Equation 3.10 in Chapter 3.4.1, theory predicts a logarithmic increase of V OC with the illumination intensity [18]. This is in good agreement with the experimental findings (Figure 6.10 (f )).

In order to undoubtedly confirm the negative charging of the surface-near buried PCBM aggregates, experiments on a region with a large PCBM crystallization area have been performed (Figure 6.11 (a)). A zoom (topography, CPD dark , PV) on the corner of the large buried PCBM cluster is shown in Figure 6. 11 (b-d). In the topographic image the lamellar structure is visible for the whole zoom area, indicating thereby that the PCBM is in fact buried underneath. Three regions of different PCBM concentration can be identified in the CPD image: a FG1-rich area ①, an area with the buried PCBM-rich cluster underneath ③ and a region with an intermediate PCBM concentration ②. When comparing the CPD dark and PV images, the regions of higher PCBM concentration appear again as dark regions in the PV image. Upon illumination, the PCBM aggregate takes the negative charges and confines them spatially within the volume of this isolated acceptor cluster. In contrast, the corresponding photo-generated holes disperse over the whole surrounding FG1 network. In total, KPFM probes the negatively charged acceptor-rich spot through the thin positively charged FG1 cover layer and detects a local reduction of the Δf " ´22Hz, A cantilever " 9nm, f cantilever " 275kHz). Topographic (b), CPD dark (c) and PV (d) images of buried PCBM-rich area and surroundings. The images have been taken on the zoom area indicated in (a). A FG1-rich area ①, an area with the buried PCBM-rich cluster underneath ③ and a region with an intermediate PCBM concentration ② are marked. The PV image demonstrates that PCBM (acceptor) rich areas take the electrons and counteract the global positive charging of the sample under illumination (1mW , 515nm). They charge less positive as it can also be seen in the CPD image histograms (e) (p400nm ˆ350nmq, Δf " ´20Hz, A cantilever " 9nm, f cantilever " 275kHz).

global positive CPD shift. This different photovoltage response for the different sample regions ① to ③ becomes in particular visible in the CPD image histograms for the sample in dark and under illumination in Figure 6.11 (e). The FG1-rich ares ① and ② exhibit a positive CPD shift of more than 100mV whereas the area with the buried PCBM cluster ③ shows a much inferior surface photovoltage and seems to nearly compensate the global positive CPD shift.

| High Resolution Photovoltage Imaging

For different reasons, the FG1: [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM blend is a perfect donor-acceptor model system for the investigation of the photo-induced charge carrier generation and collection. First of all, as we have seen above, the observed photovoltaic effect in this organic semiconductor blend is completely reversible and charge trapping does not perturb the investigation of the optoelectronic properties. Furthermore, the FG1-rich sample regions shown in Figure 6.10 (a) are, with a surface roughness of RM S " 0.6nm, topographically extremely flat. Thus, eventual cross-talks [19] between topography and CPD channel should not play any significant role and the observed CPD contrasts must have a physical origin and be related to the DA blend. Finally, due to the virtually complete coverage of the surface with the lamellar FG1-rich assembly, a sample surface with a very homogeneous chemical composition is obtained. Consequently also the tip-sample interactions, like vdW-or SR-forces, are laterally more homogeneous compared to a blend where both, donor and acceptor domains, are exposed at the sample surface. As a result, the variations of the electrostatic contribution, which is the one responsible for the detection of photo-generated charges, have a higher impact on the total tip-sample interaction force. In order to study the CPD contrasts and the surface photovoltages more in detail, high resolution KPFM images have been acquired on the blend's very flat FG1-rich areas with longer acquisition times and at higher Δf setpoints (see Figure 6.12). Topographic and damping images show the lamellar structure of the self-assembled FG1-rich sample areas with great detail. topography representation illustrating that the photovoltage is not directly correlated to the topography (no cross-talk artifacts) and that the PCBM has to be buried near the surface within the lamellar FG1 matrix (p400nmq 2 , Δf " ´24Hz, A cantilever " 9nm, f cantilever " 275kHz).

The CPD images in dark and under illumination show a clear contrast change upon illumination and even very fine structures are resolved (Figure 6.12 (d,e)). As mentioned, the CPD contrasts are independent of topography and damping at the surface and have to be related to the blend's DA interface structure and the photo-physical properties. The PV image, obtained via subtraction of the two CPD images, indicates clearly which sample areas charge more than others. For the comparison of the PV and CPD dark images in Figure 6.12 (c,d), buried PCBM-rich cluster have been indicated by red contours and FG1 areas with very low PCBM concentration by blue ones. Buried and intercalated PCBM cluster appear as bright spots in the CPD image and as darks spots in the PV image because theses areas shift less positively. For relatively pure FG1 areas the inverse effect is observed. Consequently, these experimental results are in full consistency with the presented predictions for this DA blend. In a first step, based on the determination of interface dipoles with the ICT-model, FG1-and PCBM-rich areas could be identified in the CPD dark images. In a second step, the photo-generated charge carriers have been detected within these donor and acceptor domains that charge in opposite directions under illumination. Figure 6.12 (f ) presents a 3D topographic representation with superimposed PV color code. Once again, the independence of topography and PV channels becomes clear for this flat DA model system. The dark patches have to be related to PCBM-rich acceptor cluster buried below the lamellar structures of self-assembled FG1.

The smallest features in the CPD and PV images of Figure 6.12, like the stripe-like structures in the images' center bottom part, are just below 10nm and testify to the high resolution of this photo-physical KPFM investigation. In order to verify, that even these smallest features in the potential images have a physical origin, a cross-correlation analysis has been performed [20] (see Figure 6.13). Cross-correlation image with high aspect ratio maximum next to the coordinate system's origin (a). An average absolute drift value of below 1nm was obtained which lies fairly below the size of the dark stripe features in the PV image (a) (p400nmq 2 , Δf " ´24Hz, A cantilever " 9nm, f cantilever " 275kHz).

Damping -Cross-Correlation

In the high resolution photovoltage image in Figure 6.13 (a) some of the very thin stripe like features have been indicated by blue arrow. As mentioned before, the PV image is obtained via subtraction of the CPD images in dark and under illumination. In principal this subtraction could create PV image artifacts if both potential images are not exactly superimposable for example due to drift or piezo creep. For the cross-correlation analysis, the two corresponding high-contrast damping images in dark and upon illumination have been used (Figure 6.13 (b,c)). The calculated cross-correlation image is presented in Figure 6.13 (d). The higher the aspect ratio of the maximum at the origin, the better the congruence of the KPFM images. Furthermore, the offset of the maximum from the origin gives an average drift vector. This absolute drift value lies for the presented data set below 1nm. Together with the chosen image pixel density, this determines the possible highest resolution of the PV images. Here, every pixel has a size of 1.3nm ˆ1.3nm. After image subtraction the uncertainty should hence be about twice this size and is apparently more important than the one caused by drift. All in all, the resolution limit of this high resolution PV image should be at around 3nm. Consequently, the indicated stripe like features should have a physical origin. On the one hand they could be related to intercalated PCBM in between the self-assembled FG1. On the other hand, one could also imagine that the stripes are caused by different periodic molecular arrangements of FG1. If the photo-generated charges are not distributed or evacuated in the same way for the different molecular stacks, a photovoltage contrast could be observed.

| Photovoltage Measurements at Different Interaction Regimes

Finally the initial question of how the measurement in different interaction regimes influences the photovoltage investigations will be addressed. In order to determine this influence, KPFM measurements have been performed on the exact same sample area for increasing frequency shift setpoints Δf but constant vibration amplitude. For each setpoint an image in dark and under illumination (515nm, 1mW ) has been acquired to determine the corresponding PV image. Figure 6.14 shows topography, damping, CPD dark and PV image for three representative frequency shift setpoints in the LR-regime (Δf " ´4Hz), at the onset of the SR-regime (Δf " ´14Hz) and finally in the SR-regime (Δf " ´24Hz). Δf " ´4Hz in the LR-regime, at Δf " ´14Hz (e,f,g,h) and at Δf " ´24Hz in the SR-regime (i,j,k,l) (p800nmq 2 , A cantilever " 9nm, f cantilever " 275kHz). For increasing Δf -setpoints a clearly enhanced resolution can be observed for all KPFM channels including the photovoltage image. The damping contrast at higher Δf indicates the appearance of additional (short-range) forces.

The damping images clearly state the occurrence of additional interaction forces when going towards more negative frequency shifts Δf . Far from the surface in the LR-regime no damping contrast is observed, at the onset of SR-forces the first contrast patches can be seen, and in the SR-regime at ´24Hz a clear and sharp damping contrast related to the SR-interactions can be stated. Furthermore, for all channels, a significant increase in resolution and contrast is detected at stronger Δf . In Chapter 5 it has already been shown that besides a more precise height measurement, also higher topographic resolutions are obtained in the SR-regime [21]. In order to quantify more precisely this enhanced image quality for the CPD and PV images, image profiles have been taken at exact the same position but for different frequency shift setpoints. In These images have been taken as zoom from Figure 6.14 (c,k) together with the third image at Δf " ´14Hz and their profiles are depicted together in Figure 6.15 (c). For the profile analysis a slight profile smoothing has been applied to remove noise and to facilitate the analysis. To obtain a measure for the image resolution and contrast, the slope ratios a{b of the peaks along each profile are determined and averaged (Figure 6.15 (c)). Taking the CPD image in the SR-regime at Δf " ´24Hz as a reference (fl 100%), at a Δf setpoint of ´14Hz only about 80% and at Δf " ´4Hz only about 40% of the reference resolution and contrast are achieved. These calculations are based on one single profile sample, however, they reflect well the apparent image quality at the different setpoints (see Figure 6.14 (c,g,k) and 6.15 (a,b)). Similar to the CPD images, one can also assess by this method the image resolution and contrast for the different photovoltage images. At a Δf setpoint of ´14Hz only about 70% and at Δf " ´4Hz only about 60% of the image resolution and contrast in the SR-regime (Δf " ´24Hz) are obtained for the PV image set presented in Figure 6. 14 (d,h,l). For the resolution analysis of CPD dark and PV, the very thin stripe like features that are only visible at Δf " ´24Hz have been left out and are not reflected in this quantitative values. It is important to note, that the general (rough) features in the CPD and PV images stay the same at the different setpoints. These potential contrast have therefore to be of physical origin and are just better resolved when going towards more negative Δf setpoints.

The increased potential image resolution and contrast in the SR-regime can be understood by two arguments. On the one hand, the additional SR-forces at small tip-sample distances and strongly negative Δf setpoints can increase imaging resolution. As discussed in Chapter 2.3.2 different system-dependent SR-forces can occur between tip and sample. The presence of these additional forces is nicely visible in Figure 6.14 due to the sudden onset of a sharp damping contrast and the strong increase in topographic resolution. Also the potential image resolution is affected by short range forces. At these small tip-sample distances, also short range electrostatic forces [22,23] may contribute to the total SR-interaction. These forces are mainly responsible for atomic CPD contrasts, achieved for example on Si(111) 7x7 [24], and can increase the CPD and PV imaging resolution significantly. In this case, however, the concept of a local CPD (LCPD) [25,4] has to be introduced (see also Chapter 2.4). On the other hand, the increased potential image resolution can be caused by an decreased "averaging effect" in the SR-regime at small tip-sample distances (see illustration in Figure 6.16). In the LR-regime at weakly negative Δf , the cantilever tip is relatively far from the surface. Thus, due to the long-rang electrostatic forces, adjacent charges on the surface are detected together by the KPFM tip. This averaging effect decreases the CPD image contrast and resolution. On the contrary, for strongly negative Δf , the tip is in close approach to the sample surface and sees the adjacent charges separately. A CPD image with good contrast and high lateral resolution is obtained (Figure 6.16 (b)). In total, it seems to be advantageous to perform CPD and PV investigations in the SR-regime at strong frequency shifts due to the increased resolution and contrast. However, it is important to know if the Δf setpoint also affects the mean values of CPD and photovoltage. In Figure 6.17 (a,b) CPD image histograms in dark and under illumination (515nm, 1mW ) are plotted for different frequency shift setpoints. Apparently the mean CPD under illumination increases with more negative Δf setpoints until it finally saturates in the SR-regime. In contrast to CPD light , the mean CPD in dark shows no significant Δf dependency and remains in good approximation constant (Figure 6.17 (b)). Just a larger FWHM of the CPD dark histogram peaks is observed in the SR-regime due to the already discussed increased image contrast. With P V " pCP D light ´CP D dark q, the photovoltage rises consequently from P V " `90mV at Δf " ´4Hz in the LR-regime to P V " `177mV at Δf " ´24Hz in the SR-regime.

The Δf -dependence of CPD dark , CPD light and the PV is illustrated in the upper part of until its saturation in the SR-regime has to be directly related to the Δf -dependence of CPD light .

Based on these findings, it is suggested that the increase of CPD light and of PV with more negative Δf is related to an optical field enhancement effect in the nanogap between SPM tip and sample [26,27,28,29]. This effect is well known and comes to use for example in Surface Enhanced Raman Scattering (SERS) [30,31,32] and Scanning Near-field Optical Microscopy (SNOM) [33,34,35]. The amplification of the incident electromagnetic field due to the presence of a metallic nanotip arises from two different origins. The first one, called the lightning-rod effect, is of pure geometric origin and is based on the enhancement of the electric field near metal surfaces with preferably small curvature radius (antenna effect) [28,36]. The second origin stems from the resonant excitation of local surface plasmons in the tip (and the sample) [28,37]. The incident electromagnetic field induces coherent oscillations of the free electrons along the direction of the electric field [27]. The confinement of the electromagnetic waves at the SPM tip-apex and the local field exaltation make the probe a highly localized and powerful near-field light source [38,39]. To quantify this field exaltation effect, one can define the Field Enhancement Factor FEF [28] F EF "

E loc E 0 (6.1)
with the locally enhanced electric field at the tip apex E loc and the external electric field E 0 of the electromagnetic wave. It has been shown that silicon AFM tips, similar to the one used in this thesis, can be used as optical near-field sensor [28,40]. However, semiconductor tips (even if strongly doped) show a lower electromagnetic field exaltation effect than metallic ones [39]. One of the most advantageous materials for the local laser field enhancement would be silver due to its strong surface plasmon resonances [38,28]. Additionally, a strong electromagnetic near-field coupling can occur between tip and sample, which further increases the electromagnetic field exaltation [27,29]. This coupling effect is again biggest for metallic substrates and less pronounced for dielectric samples. All in all, the field exaltation depends strongly on the material and the geometry of tip and sample, the optical wavelength of the incident light and last but not least on the tip-sample distance z [26,27,37,38]. In order to observe such a field exaltation with a significant surface plasmon resonance and tip-sample coupling, one has to measure at small z. The FEF decreases strongly with increasing z and typically at tip-sample distances of several tens of nm no field exaltation is observed any more [26]. Figure 6.18 (b) shows a typical tip-sample z dependence of the field enhancement factor and illustrates the tip-sample geometry under frontside illumination (backside illumination analog [39]). In Figure 6.18 (c) modeling results for the field exaltation of a gold tip above a flat gold substrate are shown for different z (images adapted from Roth et al. [26]). The incident light (λ " 633nm) arrives with an angle of 30 ˝relative to the sample-plane normal. Far away from the surface (z Ñ 8) only a weak field enhancement directly next to the apex is observed. At smaller tip-sample distances the FEF is strongly increased due the plasmon resonance and the tip-sample coupling and reaches values of up to F EF " 100 at z " 8nm.

In the case of the strongly doped "metallic" silicon tips used in this thesis a much less pronounced, but still significant, field exaltation effect is expected compared to the case of the gold tip presented in Figure 6.18 (c) [28]. This field enhancement of the incident laser radiation can create a locally higher effective illumination intensity that increases with decreasing z. As demonstrated by the results in Figure 6.10 (f ), CPD light and consequently the photovoltage increases for higher light intensities. This explains the increase of CPD light and PV with decreasing z and Δf . The saturation in the SR-regime can have two possible explanations. On the one hand, it is due to the logarithmic dependence of CPD light with the illumination intensity Figure 6.10 (f ). At high enough light intensities even a stronger field exaltation at smaller z will not increase CPD light further. On the other hand, the PV-saturation in the SR-regime may arise from the typical shape of the Δf pzq-curve depicted in Figure 6.18 (lower part). In the LR-regime, at tip-sample distances z above 1nm, a small variation in the frequency shift setpoint Δf will induce a strong change in z due to the weak slope of the Δf pzq-curve. In contrast, in the SR-regime, due to the very small decay lengths of SR-forces of several Ångström, even a strong additional decrease of Δf will not significantly reduce the tip-sample distance z (see also Chapter 5). Consequently, in the case of a FEF which decays more slowly than Δf pzq, the SR-regime is limited to a thin z-interval of the F EF pzq-curve. A decrease of the Δf -setpoint in the SR-regime will not affect much FEF and saturation occurs (SR-and LR-regime are indicated in Figure 6.18 (b)). As a result, CPD light (Δf ) and PV(Δf ) saturate in the SR-regime.

All in all, due to the field exaltation effect, higher average surface photovoltages are measured by KPFM on organic photovoltaic blends in the SR-regime. This facilitates the detection of the photopotentials via KPFM for a given illumination intensity. However, at the same time, a quantitative analysis of the measured photovoltage is significantly complicated because of its strong dependence on the frequency shift setpoint. For a more quantitative analysis, advanced numerical simulations would be needed that also include the z-dependent field exaltation effect caused by the SPM tip. Furthermore, due to the predicted strong dependence of the optical field enhancement on the geometric arrangement of tip, sample and laser beam and on the shape and the material of the tip, it would be interesting to experimentally study their influence on the measured photovoltages.

| Conclusion

In this chapter, a donor-acceptor model system based on FG1 and [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM has been presented. The FG1:PCBM blend offers the possibility to influence the DA phase separation via sample annealings due to the liquid crystalline properties of FG1. Based on the ICT model for the energy level alignement at the organic-organic interfaces, the "in dark" CPD contrasts can be understood. Due to the perfectly reversible photovoltaic effect, the molecular self-assembly and the high flatness of the sample surface, this DA blend is a perfect model system for the investigation of the photovoltaic charge carrier generation and collection. Upon illumination a significant global positive CPD shift and a CPD image contrast change is observed. CPD and photovoltage images indicate a positive charging of the FG1-rich regions and a negative charging of the PCBM cluster which are buried within the FG1 structure close to the top surface. These experimental findings are in full consistency with the developed model for the DA blend's morphological structure. The highest resolved PV images show a very small resolution limit down to about 3nm for the optoelectronic investigations by KPFM at room temperature.

Furthermore PV investigations have been performed at different frequency shift setpoints in order to determine the influence of the interaction regime. At more negative Δf an increased lateral resolution and contrast is obtained for the CPD and PV images (but also for topography and damping). This finding has been attributed to a reduced averaging effect in the SR-regime and to the occurrence of additional SR electrostatic forces. Furthermore, the results prove a raise of the measured photovoltage with increasingly negative Δf until its saturation in the SR-regime. It is suggested that a field exaltation effect caused by the SPM tip creates a locally enhanced optical field, which increases with decreasing tip-sample distance z (or more negative Δf ). This higher effective optical field increases CPD light and thereby the measured photovoltage.

In total, photovoltage imaging via KPFM in the SR-regime offers an increased resolution and contrast as well as an amplified PV signal due to the optical field exaltation induced by the SPM tip. However, for a quantitative analysis of the measured photovoltages advanced numerical modeling would be necessary which includes the Δf -dependent optical field exaltation.

CPD contrast observed by KPFM could be related to the blend's morphology with the help of the ICT model for the energy level alignment at interfaces. Furthermore, highly resolved photovoltage images could be acquired that demonstrate the opposite charging of PCBM and FG1 under illumination.

Another path to tailor the morphology and the electronic properties within the active photovoltaic layer is to apply a so called "building block approach". In this case molecules with covalently coupled electron donor and acceptor units are used in a bottom-up approach to create an organized DA-heterojunction via molecular self-assembly [1]. These DA model systems intrinsically possess well-defined structural and electronic properties and found great interest in recent years [2,3]. In the case of DA dyads, Bu et al. demonstrated the structural organization in form of DA lamellae by transmission electron microscopy [4]. Furthermore, similar assemblies of DA molecules have also been characterized by STM [5,6] and AFM [7,8]. However, to further enhance the understanding of charge carrier generation and collection in the active photovoltaic layer, it is essential to probe not only the morphological nano-structure but also the local electronic properties. Consequently, Kelvin Probe Force Microscopy will again be employed for the following optoelectronic investigations like in the foregoing chapters. As mentioned in Chapter 4.4, several studies demonstrated that it is possible to investigate the photovoltaic charge carrier generation for BHJ blends by KPFM [9,10,11,12]. Also the results on FG1:PCBM, proof this finding in the case of a well-controlled liquid crystal BHJ blend. However, to date, no optoelectronic KPFM investigations of the better controlled DA molecule assemblies can be found in literature. At the time of writing, an impressive result was obtained by Schuler et al. on single DA molecules via tuning fork sensor at low temperatures (T " 5K) [13]. They performed LCPD mapping of DA dyads and triads with high intramolecular resolution. However, the experiments have been conducted without illumination and no optoelectronic characterization has been done. The imaging of the photo-potentials of DA molecules under illumination has been the goal of this thesis and the results will be presented in the following.

In the present study, a new generation of DA dyad based on a perylene-diimide electron accepting unit (PDI) and a fluorene-terthiophene electron donor unit is investigated (Figure 7.1 (a), synthesis in supporting information of [Fuchs et al., submitted to Adv. Mat., 2014]). Following the concept of Li et al. for a guided self-assembly via sidechain incompatibility [14], hydrophilic di-ethylene glycol (DEG) sidechains have been attached to the PDI units and hydrophobic alkyl-chains to the opposite side of the DA dyad. The hydrophilic-hydrophobic interaction between the sidechains can lead to a bicontinuous DA segregated assembly, which is also the favorable situation for a photo-induced charge carrier generation (Figure 7.1 (b)). At the same time, an alternating DA stack, which could be caused via charge transfer interaction between acceptor and donor moieties [15], can be avoided (Figure 7.1 (c)).

In a first step the molecular self-assembly on highly oriented pyrolytic graphite (HOPG) has been identified by STM and nc-AFM and compared to molecular mechanics (MM) and molecular dynamics (MD) simulations. Subsequently, the self-assembled monolayers were investigated in UHV by KPFM upon illumination at different wavelengths. Thereby the photo-induced charge carrier generation and collection at the electrode interface were analyzed with regard to the DA dyad's molecular assembly. To the best of our knowledge, this represents the first KPFM study on DA dyads probing optoelectronic properties and morphology of this new class of materials down to the level of one molecular monolayer.

| Experimental Details

The DA dyads dissolved in toluene have been deposited via solution drop cast on freshly cleaved HOPG. A subsequent solvent vapor annealing has been applied for two hours before introduction in UHV [16]. At pressures of 10 ´10 mbar, the samples have been annealed for 1hour at 180 ˝C to remove remaining solvent and to promote the molecular self-assembly.

All experiments have been performed with the VT-AFM/STM setup presented in Chapter 2.5 in UHV (p " 10 ´11 mbar) and at room temperature (T " 300K). The gold STM tips have been chemically etched. The AFM cantilever (Nanoworld SSS-NCH, Nanosensors Si-PPP-NCL) have been Argon sputtered in vacuum to remove the oxide layer and possible contaminants. KPFM measurements were performed in frequency modulation (FM) mode with the compensation voltage V DC applied to the cantilever (V DC " CP D " pΦ tip ´Φsample q{|e|, see Chapter 2.4). Complementary measurements in ambient conditions have been performed with a Bruker Icon AFM in Peak Force Mode [17].

The dual laser setup presented in Chapter 4.3 with 515nm and 685nm wavelength and a laser with 532nm and 5mW have been used for the KPFM measurements under illumination. All measurements with laser irradiation have been performed with SSS-NCH cantilever. To verify the absence of a photovoltage generated by the cantilever, reference measurements on bare HOPG have been performed with the same cantilever (see Additional STM investigations reveal the self-assembly of the DA dyads in form of lamellae with a width of p5.6 ˘0.5qnm (Figure 7.3 (a)). In the case of nc-AFM imaging, the resolution of the lamellae is hindered by the moving sidechains which perturb STM imaging less due to their low density of states. Complementary measurements by Peak Force AFM [17] at higher interaction forces also confirmed the lamellar assembly (Appendix 9.1 Figure 9.1). Because of the lamella layer's height of about 2.7nm, the molecules cannot be in a face-on conformation but have to adopt an edge-on orientation on the surface [18]. Molecular modeling (see Section 7.4 below) evidences that these edge-on lamellae correspond to the intended bicontinuous DA stacks with a periodicity equal to the one measured by STM. In Figure 7.3 (b,c) the measured periodicity is discussed in detail. As already mentioned, STM is sensitive to the density of states of the measured sample surface (Chapter 2.2). Consequently the π-conjugated backbone of the DA molecule with acceptor and donor moieties appears higher in the STM image than the DEG and alkyl sidechains which possess a lower electronic density of states. Due to the sidechains' incompatibility, the lamellar DA stacks should be oriented in such way that alkyl chains of adjacent stacks are in contact as well as their DEG sidechains. The lattice constant of such an assembly (DAADDAAD...) corresponds to the length of the DAAD repeating unit. Depending on the nature of the DEG-DEG and alkyl-alkyl interdigitation between neighboring lamellae, the distance between topographic maxima D max may be equal to the distance between topographic minima D min , or may vary (see This finding is of great interest for the fabrication of potential future devices based on these molecules. An organization of the bicontinuous DA stacks in form of 3D crystals brings us closer to the ideal case of an ordered heterojunction for organic photovoltaics (see Chapter 3.4).

In the case of the 3D crystals as well as for the lamellar monolayer depicted in (c) STM image of face-on buffer layer on HOPG with higher deposition concentration (C DA " 1{3mg{ml). In comparison to the results presented in (a, b) no amorphous regions of unorganized dyads are found due to a higher surface density that forces the molecules to organize perfectly into various face-on domains covering the whole substrate region (p150nmq 2 , I tunnel " 3pA, V tunneling " `1.9V ).

DA dyads, which are lying flat on the HOPG substrate [19]. This buffer layer decouples all further DA layers from the influence of the already weakly interacting HOPG [20]. Hence, they can self-assemble into lamellar DA stacks without being perturbed by the interaction with the underlying substrate.

To investigate in detail the self-assembly of the buffer layer alone, samples with a lower surface coverage were prepared (C DA " 0.1mg{ml). Self-assembled face-on domains were observed by both STM and nc-AFM (Figure 7.5 (a,b)). These organized domains with stripe-like structure are surrounded by areas of unorganized dyad molecules or even holes in the monolayer that expose the bare substrate (HOPG). The "amorphous" regions of unorganized DA molecules only appear at surface sections with a locally lower molecular density. In contrast, samples with higher surface coverage, show a closed and completely organized face-on layer (see Figure 7.5 (c)). In this case, the lower average intermolecular distance induces a more pronounced interaction with neighboring molecules that forces the DA dyads into perfect organization.

Via STM and AFM resolutions up to the submolecular level were obtained within the face-on domains (Figures 7. 6 (a,b)). In the STM image, the unit cell with two DA face-on dyads is indicated. As shown by the numerical simulations below (Section 7.4), the orientation of neighboring molecules is alternated and the bright lines in the STM images correspond to aligned PDI and terthiophene units (red and blue arrow in

| Molecular Mechanics and Molecular Dynamics Simulation

For a deeper understanding of the DA dyad self-assembly within the stacks and the face-on layer, MM and MD simulations with the Gromacs package [21] and the OPLS force field [22] have been performed (for details see Appendix 9.2). When performing a dynamic on a dimer of DA dyads, the formation of π ´π interactions between their PDI units is observed defining that way an intermolecular distance of 0.45nm [23]. Furthermore, due to the sidechains' hydrophobic-hydrophilic incompatibility, the molecules are oriented along the same direction. The modeling of several DA dyads in vacuum resulted in the lamellar DA stack in These values are in fair agreement with the experimentally determined parameters bearing in mind that the modeling was performed for a single isolated stack. Due to the bulky hindrance caused by the alkyl-chains, the organization on the donor-side is less perfect than on the acceptor-side. Based on these results, the experimentally found lamellae can finally be identified as the aspired bicontinuous DA-segregated stacks. In Figure 7.7 (c) the modeling of the face-on dyads on graphite is presented. The flat lying PDI units are π-stacked on the topmost HOPG layer, whereas the fluorene units are turned sidewards. Consequently, the dihedral angle between donor and acceptor moieties within one dyad is of about 90 ˝leading to electronically fully decoupled dyad moieties. The orientation of neighboring molecules is alternated, implying that for the face-on layer the interaction with the substrate overcomes the sidechain incompatibility. Furthermore, in contrast to the hydrophobic alkyl-chains, the hydrophilic DEG-chains are preferentially pointing away from the hydrophobic graphite surface. The characteristic parameters of the face-on assembly's unit cell (blue quadrangle Figure 7.7 (c), a " p6.1 ˘0.2qnm, b " p2.0 ˘0.1qnm and θ " p67 ˘4q ˝) are in very good agreement with the experimental results (see also Appendix 9.1 Figure 9.2).

| Optoelectronic KPFM Study of the DA-Assembly

After a complete morphological characterization it will now be focused on the optoelectronic properties of the self-assembled dyads probed by KPFM. Different CPD level are obtained for the bare HOPG, the amorphous regions and the organized face-on domains. Due to the lack of strong internal molecular dipole-moments, the modification of the substrate's CPD in the areas of molecular coverage has to originate from an interface dipole with the underlying HOPG [24]. DA dyad covered areas exhibit a positive deviation from the HOPG CPD level, hence negative charges have been transferred from the dyads towards the graphite substrate (i.e. interface dipole pointing away from the surface).

In Figure 7.9 (a-c) the topographic image of a lamella monolayer is presented together with the corresponding CPD images in dark and under green laser illumination (515nm, 10mW ). It can be clearly seen that the lamellar DA stack domains shift upon illumination towards more positive CPD values, corresponding to a net positive charging of the molecular layer (Kelvin compensation bias V DC applied to the tip). This has also been checked for samples with lower surface coverage and smaller lamella domains as shown in Figure 7.9 (d-f ). In contrast, HOPG areas that are just covered by face-on dyads, i.e. holes in the lamella monolayer, do not exhibit any CPD change (Figure 7.9 (b,c) and (e,f )). This result has also been confirmed on a separate sample with a pure face-on coverage (see Figure 7.10). The fact that the lamella domains' CPD changes upon illumination meanwhile other regions on the same sample remain at the same potential excludes a general CPD shift generated by a cantilever photovoltage (see Chapter 4.4). Actually, for all KPFM experiments under illumination, reference measurements on bare HOPG have been performed additionally in order to exclude any influence of the cantilever on the measured CPD values (Figure 4.8 in Chapter 4).

The lamellae's photo-response has been analyzed for different illumination intensities (Figure 7.11 (a)). The CPD image histograms have been taken on the exact same zone shown in Figure 7.9 (a). With increasing intensities the DA stacks charge more positively in contrast to the face-on regions that remain at the same CPD level. Additionally, the potential changes upon illumination have also been tested at 685nm (Figure 7.11 (b)). Bare HOPG does not show any significant response and the lamellae only charge positively for the green wavelength at 515nm. This is in full agreement with the DA dyads' absorption spectrum determined by UV-vis spectroscopy (Figure 7.11 (c)). The latter shows that the 515nm wavelength is well absorbed in the PDI acceptor, whereas the red wavelength at 685nm lies beyond any absorption regime of the DA molecule. The wavelength dependent CPD response represents another evidence for a photo-induced effect related to the molecular DA stacks.

Having shown that a photovoltage is created by the self-assembled dyad stacks upon appropriate illumination, the physical origin of this CPD-change is discussed in the following. First of all, the photo-induced exciton creation alone would not change the overall potential due to charge neutrality, even after dissociation into an intramolecular charge transfer state and intermolecular charge delocalization [25]. In order to observe a surface-potential change for the molecular monolayer a charge imbalance has to occur via unequal transfer rates for holes and electrons towards the substrate. For the face-on layer it is suggested that the molecule-substrate interaction modifies the dyad's electronic level at the HOPG interface and accounts for the absence of any detectable photovoltage. As shown above in Figure 7.8, the face-on dyads perform an electron charge transfer towards the substrate and an interface dipole is created with the underlying HOPG [24,26]. This process is illustrated by the energy level diagram of the organic-metal interface between face-on dyads and HOPG presented in Figure 7.12 (a,b). The Fermi level of HOPG, which is located at about ´4.7eV [27,28,29,30], and the molecular electronic level of donor and acceptor moieties are indicated. The donor's HOMO level and the acceptor's LUMO level have been estimated by Cyclic Voltammetry (CV) with the molecules in solution (see Appendix 9.1 Figure 9.3). The other two HOMO and LUMO level are not accessible by CV and have been very roughly estimated by the electronic level found in literature for PDI and a similar donor unit [31]. Furthermore, the charge transfer level of acceptor and donor E A{D,ICT `and E A{D,ICT ´according to the Integer Charge Transfer (ICT) model are indicated [32] (see Chapter 3.3.1). Because of the negative charge transfer (CT) towards the substrate, E D,ICT `initially has to lie above HOPG's Fermi level. The CT energetically lowers the donor's electronic level (including the vacuum level E vac ) by Δ, until E D,ICT `is aligned with E F (Figure 7.12 (b)). At the same time, the face-on dyad's acceptor level are untouched and in E vac alignment with the underlying graphite because of the complete electronic decoupling of donor and acceptor units. This decoupling only occurs for the face-on conformation due to the dihedral angle of about 90 ˝between PDI and donor units (see modeling results, Figure 7.7 (c)). The relative shift of the electronic energy level of acceptor and donor, approaches E D,ICT `and E A,ICT `and reduces thereby the effective DA energy gap E DA gap (see Chapter 3). Consequently, the available energy for the dissociation of the photo-generated excitons and charge transfer states is lowered. As discussed in Chapter 3, exciton and charge transfer state binding energies in organic semiconductors are much higher than in inorganic ones, mainly due to lower dielectric constants that increase the Coulomb interaction between electron and hole [33,34]. In literature, exciton binding energies ranging from 300meV to 700meV can be found, depending on the chosen π-conjugated organic material [35,36,37]. Thus, based on the estimated energy level of the studied system and the relatively high binding energies, it is suggested that the decrease of pE D,ICT `´E A,ICT `q significantly hinders the dissociation of the photo-generated electron-hole pairs. Without the possibility of dissociation, the excitons and charge transfer states will recombine and no CPD shift can be measured for the face-on assembly (Figure 7.12 (b)). However, it has to be noticed that the developed model, even if it is in full agreement with the experimental results, is based on a simple estimation of the energy level alignment at the dyad-HOPG interface. The rough estimation of the molecular energy level via cyclic voltammetry as well as the limitations of the ICT model discussed in Chapter 3.3.1 hinder a complete quantitative description of the exact energy level alignment at the interface.

In the case of the lamellar DA stacks, on the contrary, the net positive charging indicates an efficient charge generation and separation and a subsequent electron transfer to the graphite electrode. In contrast to the face-on layer, the molecular energy level and thus the photovoltaic properties of the lamella domains are preserved due to the decoupling from the substrate via the face-on buffer layer. The lamellae's donor and acceptor energy level are in E vac alignment with the HOPG substrate. Moreover the face-on assembly acts as a hole blocking layer [38]. The hole barrier between the E D,ICT `level of face-on dyad and lamella, equals the shift of the vacuum level Δ and hinders the transfer of holes from the lamellae towards the substrate (blue dashed arrow in Figure 7.12(c)). In contrast, the relative positioning of the E A,ICT ´level facilitates the extraction of the photo-generated electrons (red arrow). Furthermore, the bicontinuous stacks represent transporting channels especially for the electrons which are delocalized within the more perfectly stacked PDI units. As a consequence, under continuous illumination, an accumulation of positive charge carriers in the donor moieties is detected by KPFM as the lamella domains' global positive CPD shift (Figure 7.12(d)).

| Conclusion

In this chapter a model system has been presented based on DA dyads, which include donor and acceptor moieties within the same molecule. With the hydrophilic and the hydrophobic sidechains grafted to the molecule, a guided molecular self-assembly in form of bicontinuous DA stacks has been aspired. Via STM and AFM a precise characterization of the molecular auto-organization on HOPG has been achieved, which confirmed the creation of lamellar DA stacks. Interestingly, these DA lamella are not directly organized on top of the graphite substrate, but rather resting on a buffer layer of face-on lying molecules. Via both SPM techniques a submolecular resolution has been obtained within this very first monolayer of face-on dyads. Molecular mechanics and molecular dynamics simulations have confirmed the experimentally found self-assembly and have attested that the DA lamella are indeed corresponding to bicontinuous DA segregated stacks. With KPFM, the creation of an interface dipole between the face-on layer and the HOPG has been revealed. Upon illumination, the lamellar DA stack domains show a significant and wavelength dependent positive charging due to the photo-induced charge carrier generation.

In contrast, the face-on layer does not show any photovoltaic effect. Based on the KPFM results under illumination and using the ICT formalism, a comprehensive model has been developed explaining the DA system's optoelectronic behavior and relating it directly to the molecular self-assembly. Due to the underlying buffer layer, the lamellar DA stack are decoupled from the substrate's influence and keep their photovoltaic properties. Because of the interface dipole created with the face-on dyads donor moiety, its internal energy level are shifted, inhibiting a photovoltaic effect of the buffer layer itself. Furthermore the shifted internal energy level make the face-on layer a hole-blocking layer for the overlying DA stacks. It blocks the hole transfer towards the sample thereby inducing the DA lamella's positive CPD shift upon illumination. To the best of our knowledge, this represents the first KPFM study demonstrating a photovoltaic effect of a single self-assembled monolayer. It paves the way for local investigations of the optoelectronic properties of all kinds of molecular DA systems and shows how photovoltaic properties can depend on the molecular self-assembly.

In the following a concluding overview of the experimental results obtained during this thesis will be given. Furthermore current research projects and a short outlook will be presented.

The global idea of this thesis was to study organic photovoltaic systems via KPFM and to investigate the photo-induced charge carrier generation in these systems under illumination. As discussed in Chapter 3, model have been developed to describe the principle mechanisms of charge carrier generation and collection in organic DA systems. However, especially the dissociation of charge transfer states at the DA interface is still not completely understood and a complete quantitative description of an organic photovoltaic system remains a challenge. Today, the highest organic solar cell efficiencies of over 10% are achieved with donor acceptor bulk heterojunction (BHJ) blends [1]. These DA blends owe their relatively high performances among other things to their high DA interfacial area and a DA phase-separation in the order of the exciton diffusion length. However, common BHJ are due to their random morphology and structure far from ideal to investigate the optoelectronic properties on the nanoscale. Consequently, this thesis was concentrated on the investigation of better structurally and electronically defined "model" donor acceptor systems. The investigated photovoltaic systems organize via molecular self-assembly into well-defined DA structures that facilitate the interpretation of the acquired potential images in dark and under illumination. With the obtained surface potential image, one can map the photo-induced charge carriers and gain an insight into the optoelectronic processes on the nanoscale.

As detailed in Chapter 4, different technical upgrades and an extensive series of reference measurements have been performed in preparation of the KPFM investigations under illumination. A new multiple wavelength laser system has been installed and a backside illumination system has been developed for the use with transparent substrates. In order to ensure an artifact free measurement under illumination, the influence of the laser irradiation on the cantilever has been studied for the different illumination geometries. Furthermore reference measurements on HOPG and on a complete and tested organic solar cell have been performed. These experiments allowed for the development of a detailed measurement procedure and furthermore confirmed the CPD polarity for a correct interpretation of the KPFM images.

There is evidence in literature that imaging in the regime of short-range forces can enhance the lateral resolution. This is true for the topographic channel, but also for CPD imaging via KPFM. Compared to the regime of long-range forces, the SR interaction regime is somehow a less conventional imaging regime because different system-dependent SR-forces occur additionally. They can dominate the tip-sample interaction and make an interpretation of the nc-AFM and KPFM results less straightforward. However, with a DA phase-separation in OPV systems on a nm-scale, imaging in the SR-regime is a necessity in order to achieve the highest possible resolutions. Consequently, one has to make sure that the imaging in the SR-regime does not introduce potential artifacts in both, topography and CPD images. Chapter 5 discussed the influence of the interaction regime on the topographic measurement. It extended the work of Sadewasser et al. [2], who demonstrated that the compensation of electrostatic forces via a Kelvin compensation is essential to obtain reliable topographic height values. The model system for topographic height measurements used in this thesis consisted out of an self-assembled polymer which is π-stacked on the HOPG substrate. It has been shown that a strong height difference for the polymer domains is obtained between the LR-and SR-regime. Because of the proper compensation of the electrostatic forces and with support of theoretical sample-calculations, this height measurement effect has been attributed to the influence of LR van der Waals forces. In the LR-regime these forces dominate the tip-sample interaction and much too low height values have been obtained, whereas in the SR-regime the vdW forces are dominated by SR-forces and a precise topographic height measurement gets possible. This study demonstrated in the case of an organic submonolayer on HOPG, that an artifact-free and even more exact topographic imaging can be achieved in the SR-regime.

Having clarified the influence of the interaction regime on the topographic images, the investigation of more sophisticated photovoltaic systems could be undertaken with the aim to correctly image photo-induced charge carriers. Therefore, a model DA system based on self-assembled FG1 and [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCBM has been studied in Chapter 6. This BHJ blend offers the possibility to influence the DA phase-separation via sample annealings due to the liquid crystalline properties of FG1. On the basis of the ICT model, the explanation of the nanoscale "in dark" CPD contrasts obtained via KPFM has been possible. The PCBM crystallization areas, which are intercalated and buried within the lamellar FG1 matrix have been clearly identified. Under illumination a perfectly reversible photovoltaic effect has been observed with a significant global CPD shift and with local CPD contrast changes that have been attributed to the opposite charging of acceptor and donor domains. It has been shown that these experimental findings are in full consistency with the developed model for the DA blend's morphological structure. Furthermore, a very high photovoltage image resolution down to about 3nm has been achieved for these optoelectronic investigations at room temperature. In order to understand more in detail the impact of the interaction regime on photovoltage imaging, KPFM measurements at different frequency shift setpoints have been performed. For more negative Δf an increase in lateral resolution is obtained for the photovoltage images, but also for the topography, damping and CPD channels. This finding has been attributed to a reduced averaging effect in the SR-regime and possibly to the occurrence of the additional SR electrostatic forces. Additionally, a raise of the measured photovoltage with decreasing frequency shift Δf has been obtained. This observing has been related to the exaltation of the local optical field due to the presence of the AFM tip. An effect which increases, in agreement with theory, with reduced tip-sample distance. In total, this study demonstrates that photovoltage measurements by KPFM in the SR-regime can be performed without artifacts and that they even offer higher resolutions and higher contrasts than the one in the LR-regime.

Finally, another photovoltaic model system has been investigated in Chapter 7 based on DA dyads, which include donor and acceptor moieties within the same molecule. With the hydrophilic and the hydrophobic sidechains grafted to the molecule, a guided molecular self-assembly in form of bicontinuous DA stacks has been aspired. Via STM and AFM a precise characterization of the molecular auto-organization on HOPG has been performed which confirmed the creation of lamellar DA stacks. Interestingly, these DA lamella are not directly organized on top of the graphite substrate, but rather resting on a buffer layer of face-on lying molecules. Via both SPM techniques a submolecular resolution has been obtained within this very first monolayer of face-on dyads. Molecular mechanics and molecular dynamics simulations have confirmed the experimentally found self-assembly and have attested that the DA lamella are indeed corresponding to bicontinuous DA segregated stacks. Via KPFM, the creation of an interface dipole between the face-on layer and the HOPG has been revealed. Upon illumination, the lamellar DA stack domains show a significant and wavelength dependent positive charging due to the photo-induced charge carrier generation. In contrast, the face-on layer does not show any photovoltaic effect. Based on the KPFM results under illumination and using the ICT formalism, a comprehensive model has been developed explaining the DA system's optoelectronic behavior and relating it directly to the molecular self-assembly. Due to the underlying buffer layer, the lamellar DA stack are decoupled from the substrate's influence and keep their photovoltaic properties. Because of the interface dipole created with the face-on dyads donor moiety, the donor's internal energy level are shifted, inhibiting a photovoltaic effect of the buffer layer itself. Furthermore the shifted internal energy level make the face-on layer a hole-blocking layer for the overlying DA stacks. It blocks the hole transfer towards the sample thereby inducing the DA lamella's positive CPD shift upon illumination. To the best of our knowledge, this represents the first KPFM study demonstrating a photovoltaic effect of a single self-assembled monolayer. It paves the way for local investigations of the optoelectronic properties of all kinds of molecular DA systems and shows how photovoltaic properties can depend on the molecular self-assembly. So far, all presented studies have shown the photovoltaic charge carrier generation for domains of several DA dyads or for whole PCBM cluster embedded in a donor polymer matrix with typical cluster sizes of 10nm. Even if very good CPD and photovoltage resolutions have been obtained, an investigation of the charge carrier generation within one single DA stack or even for one single molecule was still out of reach. A lot of efforts are done on an international level to reveal a photovoltaic effect on a molecular level and this represents also one part of our current research interests.

In collaboration with the group of Stéphane Méry and Nicolas Leclerc from the University of Strasbourg (UMR7504 and UMR7515 CNRS) a KPFM investigation of longer DA dyads and triads has been started. The group in Strasbourg has an important chemical background in the field of organic photovoltaics and recently published several DA dyads and triads synthesized in their laboratory including transmission electron microscopy results [3]. In very first nc-AFM results of the DA dyad A-δ `-d 3 [4] on HOPG obtained in our lab and the dyad's molecular structure are shown. Contrary to the DA dyads investigated in Chapter 7, submolecular resolution has been obtained via nc-AFM within the self-assembled lamellar DA stacks. Due to the longer DA molecule of 15.1nm length, it has been possible to reveal the (L)CPD contrasts within one molecular stack (results not shown here). In a next step a comprehensive investigation by KPFM under illumination will be performed.

Another current project pointing in a similar direction represents the investigation of inorganic DA nanorods. In The different CPD of core and shell can already be seen in the CPD image (c). Currently, KPFM investigations of these single isolated nanorods under illumination are performed. At sufficient high optical excitation energies, an exciton will be created in the CdS shell that subsequently transfers its hole to the CdSe core [6]. Thereby a charge transfer state is created within this single nanorod heterostructure. This charge transfer state might be made visible via KPFM due to the positive charging of the CdSe core and the negative charging of the CdS shell upon illumination. While writing this thesis, an impressive result has been published by Schuler et al. at IBM Zürich [7]. At very low temperatures (T " 5K) and with qPlus tuning fork sensors [8], they have been able to reveal the internal molecular structure and the LCPD contrast of single isolated DA dyads and triads (see Figure 8.3).

In a next ambitious step one would like to image single DA molecules under illumination via KPFM with similar resolutions. As mentioned in Chapter 3, the creation and dissociation of the charge transfer state represents probably the most determinant step of the photovoltaic charge carrier generation and still has to be better understood. Consequently, the measurement of a modified molecular LCPD under illumination, which clearly states the creation and dissociation of a CTS within a single molecule, would be an important breakthrough. In order to detect the photo-generated charges via KPFM, one relies on the dissociation of the CTS state (if created in the first place) and the subsequent opposite charging of the molecule's donor and acceptor units. However, besides the technical challenge, it is an open question if the CTS dissociation in a single molecule will take place. As discussed in Chapter 3, the possible delocalization of the charge transfer state's electron and hole on their respective sides of the DA interface can significantly contribute to the CTS dissociation. Consequently, also the investigation of more complex molecular DA assemblies, as the DA dyads in Chapter 7 or potentially the one presented in Figure 8.1, are also of great interest. In this case, the formation of bicontinuous donor-acceptor stacks and the intermolecular delocalization of charge carriers in the stack's donor and acceptor channel may enhance the CTS dissociation and the detection of the separated electron-hole pairs via KPFM. and a stack height of 1.2nm on the acceptor side and of 2.5nm on the donor side was obtained. were added and the dimension of the considered box was reduced to achieve a surface density close enough to the experimental value. The obtained face-on assembly is displayed in Figure 9.10 (top-view) and Figure 9.11 (side view). The perylene-diimide moiety is adsorbed flat on the graphite surface (π-stacking), while the fluorene moiety is turned on the side. The direction of DA molecules in one row is alternating. The lattice parameters are calculated along a trajectory of 10ns for the 16 molecules (1000 snapshots) and converged to a " p6.1 ˘0.2qnm, b " p2.0 ˘0.1qnm and θ " p67 ˘4q ˝. They agree well with the experimental scanning probe microscopy data.

Figure 1 . 1 :

 11 Figure 1.1: Primary energy consumption of Germany in 2013 of 14 ¨10 18 Joule with different contributions. The renewable energies satisfy only 11.8% of the total primary energy consumption [2].

Figure 1 . 2 :

 12 Figure 1.2: Research Solar Cell Efficiency Records confirmed and published by the National Renewable Energy Laboratory (NREL).

Figure 1 . 3

 13 shows several examples for organic photovoltaic applications. They range from semi-

Figure 1 . 3 :

 13 Figure 1.3: (a) Flexible organic photovoltaic cell from Heliatek ® (Dresden, Germany). (b) The semi-transparent OPV can be integrated in glass facades of buildings. Here, the dye solar cell facade of the SwissTech Convention Center (EPFL, Switzerland) fabricated by Solaronix ® is shown. (c) Mobile multifunctional solar panels for indoor and outdoor use, for example for charging mobile devices (image taken from www.phys.org). (d) Solar-powered bus stop in San Francisco with OPV solar module on top from Konarka Power Plastic ®. (e) Bag with integrated OPV panel for the charging of mobile devices by Neuber ®.

Figure 2 . 1 :

 21 Figure 2.1: (a) First STM image of the silicon(111) 7x7 reconstruction obtained by Binnig and Rohrer in 1983 [2]. (b) Quantum corral with circular arranged iron atoms on a copper(111) surface. The iron atoms have been positioned by STM. The STM image of the quantum corral reveals a quantum mechanical interference pattern of the confined 2D electron gas [5, 6, 7].

Figure 2 . 2 :

 22 Figure 2.2: (a) FM-AFM image of the silicon(111) surface with 7x7 reconstruction [9]. (b) Chemical identification of individual surface atoms by AFM via chemical force measurements [10]. (c) Chemical structure of the molecule pentacene (upper image) revealed by nc-AFM with CO-functionalized AFM tip (qPlus tuning fork sensor, T " 5K) [11].

  Figure 2.3 (a,b)).

Figure 2 . 3 :

 23 Figure 2.3: (a) Investigation of the charge state of gold adatoms on a NaCl layer via qPlus tuning fork AFM. (b) The different electrostatic interaction of the AFM tip with the charged and uncharged adatom is measured and reveals the charge state with single-electron charge sensitivity [20]. Imaging the charge distribution of a molecule via Kelvin Probe Force Microscopy: (c) Constant height frequency shift image of a single naphthalocyanine molecule. (d) Kelvin Probe Force Microscopy map of the local contact potential difference with submolecular resolution acquired with a CO-functionalized tip. (e) DFT-calculated z component of the electrostatic field above a free naphthalocyanine molecule for comparison with (d) [21].

Figure 2 . 4 :

 24 Figure 2.4: (a) Schematic illustration of a scanning tunneling microscope with piezo tube scanner and tunneling bias V tunneling applied between tip and sample. (b) Energy diagram of the tip-sample system with vacuum potential barrier in between. The Fermi energies E F , the vacuum level E vac and the work functions Φ of tip and sample are indicated. The energy difference between the two Fermi level e V tunneling is defined by the applied tunneling bias and enables the tunneling of electrons across the potential barrier. (c) Real part of the wave function solution of the Schrödinger equation for a particle traveling from left to right and crossing a rectangular potential barrier of height V 0 and thickness a. (Illustrations (a,b,c) according to [25]) (d) Constant current 3D STM image of HOPG (p2nmq 2 ,V tunneling " 0.4V , I tunneling " 110pA, V tunneling applied to the tip). (e) Constant current STM image of Si(111) 7x7 reconstruction (p48nmq 2 ,V tunneling " ´1.0V , I tunneling " 250pA).

Figure 2 Figure 2 . 5 :

 225 Figure 2.5: Illustration of the dynamic AFM operation modes. (a) Amplitude modulation mode, which uses the amplitude as feedback signal to maintain a constant tip-sample distance. (b)Frequency modulation mode, which uses frequency changes as feedback signal (illustration according to[24]).

Figure 2 . 6 :

 26 Figure 2.6: Functional principle of a frequency modulation AFM.A cantilever, which is mechanically excited at its resonance frequency, is scanned via the piezo tube scanner over the sample surface. The tip sample interaction forces can induce a frequency shift Δf of the cantilever oscillation. This shift Δf represents the imaging signal and can for example be detected by a beam deflection system with photo sensitive diode (PSD). The black boxes correspond to the different components of the FM-AFM feedback system. Red boxes stand for amplitude, frequency shift and phase setpoints and the blue box contains all obtained physical observables (schema according to L. Nony[34]).

Figure 2 . 7 .Figure 2 . 7 :

 2727 Figure2.7: Total tip-sample interaction force F int over tip-sample distance z. The tip probes with its amplitude A cantilever a certain part of the force curve around the setpoint. Repulsive and attractive forces can appear between the probe and the sample. The long-range electrostatic (LRE) and the longrange (LR) van der Waals (vdW) forces dominate the force curve at increased tip-sample distances, the different short range (SR) forces dominate at small z. Depending on the chosen setpoint, measurements can be performed in the contact and the non-contact regime.

Figure 2 . 8 :

 28 Figure 2.8: Functional principle of a Kelvin compensation by KPFM. (a) Tip and sample with their corresponding work functions Φ tip and Φ sample and Fermi level E F before contact at a tip-sample distance z. (b) Upon electronic contact a current i flows until charge equilibrium is obtained and the E F of tip and sample are aligned. The charging of both electrodes creates capacitive electrostatic forces F el between tip and sample and offsets the vacuum level of both materials with a contact potential difference CP D " pΦ tip ´Φsample q. (c) The Kelvin feedback loop compensates the electrostatic forces between tip and sample by applying a compensation bias V DC to the tip that equals the CPD and realigns the vacuum level E vac .

Figure 2 . 9 :

 29 Figure 2.9: Illustration of the Kelvin Probe Force Microscope and its feedback circuit. The black circuit is identical to the one of the simple nc-AFM depicted in Figure 2.6. For the KPFM, the cantilever has been electrically contacted and an additional feedback loop (pink circuit) assures the compensation of the CPD between tip and sample via a compensation bias V DC . The two possible KPFM modes, amplitude modulation (AM) KPFM and frequency modulation (FM) KPFM, with their corresponding circuits are indicated.

Figure 2 . 10 :

 210 Figure 2.10: Employed Omicron UHV setup with a variable temperature (VT) SPM unit, a low energy electron diffraction (LEED) unit, a photoelectron spectroscopy system and different surface and tip preparation units. On the right, the SPM control system is shown with the Omicron Matrix control unit, the SPM remote box and the used lockin amplifier for the Kelvin regulation.

  forces and the determination of the CPD. The sample laser illumination setup which is connected to the UHV SPM setup is introduced in detail in Chapter 4.3.

Figure 2 . 11 :

 211 Figure 2.11: Beam deflection VT-AFM system from Oxford Instruments Omicron Nanotechnology. (a) VT-AFM/STM unit with eddy current system and SPM head. (b) SPM head with sample plate holder and piezo tube scanner including tip mount. (c) Omicron beam deflection system with laser source, optical components and photo sensitive diode (PSD). (d) Standard stainless steel sample plate. (e) Variable temperature SPM sample plate. (f ) AFM cantilever mounted on chip carrier. Images (a) and (c-f) taken from www.omicron.de.

Figure 2 .

 2 Figure 2.12: (a) VT-AFM preparation chamber with load lock and ion source for argon sputtering. (b) ISE 5 ion source used for the argon sputtering of tip and sample (image taken from www.omicron.de).

Figure 3 . 1 :

 31 Figure 3.1: (a) Electron occupation of ground state and sp 2 -hybridized orbitals of a carbon atom. (b) Lewis structure of ethene C 2 H 4 . The planar molecule exhibits a double bond between its carbon atoms and an angle of 120 ˝between the hydrogen atoms. (c) Illustration of ethene's anti-bonding (π ˚)

Figure 3 . 2 :

 32 Figure 3.2: (a) Molecular structure of Benzene. The Kekulé structure (middle) consists out of two possible mesomerisms (structural representations). The six C atoms form a common π-conjugated system with delocalized π electrons. (b) Molecular energy level created by the different overlap of the six p z orbitals.The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are indicated. The higher the wave function's phase overlap, the higher is the energy of the π orbitals. Illustrations according to[11].

Figure 3 . 3 :

 33 Figure 3.3: Illustration of the energy level alignment at the metal-organic interface according to the ICT-model. (a) For E ICT ´ą E F ą E ICT `no charge transfer occurs upon physisorption of the organic SC on the substrate. Their vacuum level E vac align. (b) For E ICT `ą E F , integer charge transfer occurs from the org. SC to the metal, the molecular energy level are shifted downwards until E ICT `and E F align and an interface dipole with Δ" pE ICT `´E F q is created. (c) For E F ą E ICT ´,charge transfer occurs from the metal towards the organic SC and pins the E ICT ´level to E F . The interface dipole with Δ " pE F ´EICT ´q point this time in the opposite direction towards the substrate. (d) Work function of the functionalized substrate Φ ORG{SU B over work function of the bare substrate Φ SU B . A "mark of Zorro" behavior is observed with E vac alignment in the middle and the E F pinning regimes at the extremities of the curve. Illustration according to[14].

Figure 3 . 4 :

 34 Figure 3.4: Illustration of the induced density of interfacial states model at a metal/organic interface. (a) Upon contact with the metal substrate, the discrete molecular energy level of the organic deposit (blue bars) mix with the metallic energy bands and create a Lorentzian-broadened density of states (black curve) with a non-negligible DOS in the former energy gap between HOMO and LUMO. (b) Depending on the relative positioning of the charge neutrality level (CNL) and Φ SU B , a partial charge transfer occurs into or from the induced density of states until pΦ ORG{SU B ´CN Lq " SpΦ SU B ´CN Lq with the screening parameter S. Depending on the direction of the CT, the molecular energy level are up-or down-shifted and an interface dipole with Δ " p1 ´SqpΦ SU B ´CN Lq is created.

Figure 3 . 6 :

 36 Figure 3.6: (a) Band structure illustration of the photo-induced charge carrier generation at the DA interface with indicated rate constants k , binding energies of exciton BE X and charge transfer state BE CT S and the mutual Coulomb potential of the electron and hole pair (violet) that has to be overcome to dissociate the charge transfer state (CTS). (b) Excited state diagram of the singlet exciton's ground state S 0 and excited states S n that can convert with the rate k CT into a charge transfer state (states CT 0 to CT n ) until a possible dissociation into charge separated polaron states (CS) with rate k D . The non-thermalized, "hot" charge transfer states CT n are suspected to mainly contribute to the CTS dissociation due to their excess energy and higher average electron-hole separation. Red arrow correspond to optical excitation, violet ones to possible recombination paths and green arrow indicate relaxation paths within each manifold of states. Triplet excitons are not shown. (c) Planar (bilayer) DA heterojunction with indicated charge generation and collection steps introduced in subfigure (a). (d)Bulk heterojunction with crystalline interpenetrating DA network with high interfacial area and small DA phase-separation. (e) Ideal situation of an organized heterojunction with a DA phase-separation in the order of the exciton diffusion length L D and an active layer thickness in the order of the inverse absorption coefficient 1{a. Illustrations according to[11, 17].

Figure 3 .

 3 6 (a,b,c) the essential steps of photo-induced charge carrier generation and collection are illustrated. Upon photon absorption, excitons are created, for example in the donor, with a rate k PA . As already mentioned before, it is problematic to indicate the optical molecular excitation (excited states diagram in Figure 3.6 (b)) in the energy level diagram with transport level (a) because they are on separate scales. A representation has been chosen

Figure 3 . 7 :

 37 Figure 3.7: Current density over voltage curve of a solar cell in dark (blue) and under illumination (red). A solar cell shows the rectifying behavior of a diode. The electrical output power density P out is the biggest at the maximum power point MPP. The fill factor F F is the ratio out of the comprehended rectangular area at the MPP over the area defined by the short circuit current density j SC and the open circuit voltage V OC .

Contents

4. 1

 1 Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.2 CPD Sign Convention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.3 Laser Setup and Illumination Geometries . . . . . . . . . . . . . . . . . . . . . . 55 4.4 KPFM on Illuminated Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.4.1 Avoiding Cantilever Artifacts upon Illumination . . . . . . . . . . . . . . . . . . . 57 4.4.2 Surface Photovoltage Measurements on Organic Photovoltaic Systems . . . . . . . 59 4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Figure 4 . 1 :

 41 Figure 4.1: Application of an external bias to a HOPG sample by means of a battery in order to determine the correct CPD sign of the KPFM setup. (a) Topographic KPFM image of HOPG. (b) CPD image corresponding to (a). Every 20 image lines the sample potential was increased by 500mV . (c) Image cross-section along blue line in (b) that shows the stepwise potential increase. For the used KPFM setup a positive charging of the sample surface corresponds to a CPD increase (p2000nmq 2 , Δf " ´10Hz, A cantilever " 24nm, f cantilever " 81kHz, AM-KPFM, Nanosensors PPP-EFM cantilever).

  Figure 4.1 (c) shows this stepwise CPD increase when moving the sample towards more positive potentials. Hence, a measured positive CPD shift corresponds to a potential increase or a positive charging of the sample surface.

  Figure 4.2 (a) and (b) shows the topography and the CPD image of the π-conjugated polymer poly(3-dodecylthiophene) (P3DDT) on HOPG obtained by KPFM. More details concerning P3DDT

Figure 4 . 2 :

 42 Figure 4.2: Energy level alignment and interface dipole at the P3DDT/HOPG interface. (a) Topographic image of auto-organized P3DDT domains on HOPG. (b) Corresponding CPD image with P3DDT domains at more positive CPD values than the surrounding substrate indicating an interface dipole pointing away from the sample surface. (c) Energy level of the HOPG/P3DDT interface according to the ICT-model. Due to the relative positioning of P3DDT's E ICT `andHOPG's E F level, charge transfer occurs from the polymer to the graphite substrate. An interface dipole is created whose orientation is in agreement with the experimental results (p570nmq 2 , Δf " ´16Hz, A cantilever " 7nm, f cantilever " 286kHz, FM-KPFM, Nanoworld SSS-NCH cantilever).

Figure 4 . 3 :

 43 Figure 4.3: Energy level alignment and interface dipole at the C60/HOPG interface. (a) Topographic image of auto-organized C 60 domains on HOPG. (b) Corresponding CPD image with C 60 domains at more negative CPD values than the surrounding substrate indicating an interface dipole pointing towards the sample surface. (c) Energy level of the C 60 /HOPG interface according to the ICT-model.Due to the relatively strong interaction of C 60 with the underlying substrate, hybridization of the metal bands with C 60 's LUMO occurs and effectively approaches LUMO and HOMO level. Furthermore also screening effects upon contact with the metal substrate lower the LUMO level. The subsequent charge transfer from the substrate to the fullerene creates an interface dipole whose orientation is in agreement with the experimental results (p2000nmq 2 , Δf " ´7Hz, A cantilever " 10nm, f cantilever " 251kHz, FM-KPFM, Nanoworld SSS-NCH cantilever).

Figure 4 .

 4 4 (a)). The glass fibre guides the laser beam towards one of the UHV setup's optical windows and illuminates like this the sample on the AFM sample stage. Different laser units of different wavelength can be integrated in the LightHUB and the laser beams are guided by the beam combiner unit to the common optical output of the illumination source (see Figure 4.4 (b,c)). For the presented experiments the LightHUB was equipped with two laser units of

Figure 4 . 4 :

 44 Figure 4.4: (a) Used Omicron VT-AFM with Omicron Laser LightHUB with 515nm and 685nm wavelength. The laser beam is guided with a glass fiber to one of the UHV system windows for sample illumination. (b) LightHUB: multiple wavelengths laser source from Omicron Laser. (c) Opened LightHUB with integrated laser modules of different wavelength and beam combiner unit.

Figure 4 . 5

 45 illustrates the case of frontside illumination.

Figure 4 . 5 :

 45 Figure 4.5: (a) Sample plate with functionalized ITO sample for frontside illumination. (b) Illustration of frontside illumination geometry with laser beam. (c) Photography of frontside illuminated organic photovoltaic sample and cantilever with indicated laser spot.

Figure 4 . 6 :

 46 Figure 4.6: (a) Backside illumination sample holder for photovoltaic samples on transparent substrates with mirror for indirect illumination. (b) Backside illumination geometry with laser beam, typically composed OPV sample and AFM cantilever. (c) Photography of backside illuminated OPV sample and cantilever with indicated laser spot.

  Figure 4.7: (a) Thermic detuning of the cantilever resonance frequency upon illumination (515nm) for a silicon super sharp cantilever (Nanoworld SSS-NCH) and for a PtIr-coated cantilever (Nanosensors PPP-EFM) at two different illumination intensities (1mW and 10mW ). The reflecting, metal coated PtIr-cantilever detunes less. After about 20min stabilization is obtained. (b) Comparison of thermic detuning for PtIr-cantilever in frontside and backside illumination configuration at 1mW and 10mW .In backside illumination geometry the thermic detuning is reduced by nearly one order of magnitude.

Figure 4 . 8 :

 48 Figure 4.8: KPFM reference experiment on bare HOPG in dark and under green laser illumination (532nm, 5mW ). Topographic (a) and CPD images in dark (b) and under illumination (c). The CPD histograms shown in (d) demonstrate the absence of significant CPD changes upon illumination (ΔCPD below 4mV ) for the cantilever and the reference sample HOPG. (p1200nmq 2 , Δf " ´14Hz, A cantilever " 10nm, f cantilever " 291kHz, FM-KPFM, Nanoworld SSS-NCH cantilever).

Figure 4 .

 4 9 (a,b)).

Figure 4 . 9 :

 49 Figure 4.9: SPV measurements on topelectrode of whole OPV cell by KPFM. (a) OPV Cell with two quadratic evaporated Al topelectrodes on backside illumination holder. (b) Illustration of SPV measurements on photovoltaic cell with topelectrode. Upon illumination (515nm) the cathode (top) is charging negatively. This negative charging is measured by KPFM as a negative CPD shift (c). The same ΔCPD is observed for different cantilever and in FM-and AM-KPFM mode. For the investigated "fresh" OPV cell the maximum SPV response is already obtained at illumination intensities of 1mW .

Figure 4 .

 4 10 (a,b) illustrates this in the more simple case of a DA bi-layer in open circuit condition under illumination. The bottom ITO electrode is connected to the ground and upon illumination the acceptor layer on top charges negatively. The band structure diagram explains the optoelectronic processes happening at the DA interface.

Figure 4 .

 4 10 (b)). As discussed in Chapter 3, V OC,M AX corresponds only in first approximation to the effective donor acceptor bandgap E DA gap " LU M O A ´HOM O D . More precisely it is defined by the energy of the charge transfer state at the DA interface (V OC,M AX " E CT S {e). For the sake of simplicity the E ICT `{´l evel according to the ICT-model have been left out in Figure 4.10 (b,d).

Figure 4 .Figure 4 .

 44 Figure 4.10: (a) Organic photovoltaic bi-layer device without topelectrode under illumination investigated by KPFM. (b) Band structure of bi-layer device in (a) under illumination in open circuit condition. Acceptor (donor) layer charges negatively (positively) due to the photovoltaic charge carrier generation until diffusion and drift currents cancel each other out. The negative charging of the acceptor layer is measured by KPFM as ΔCP D. Depending on the energy level alignment of electrodes and organic compounds the measured V OC can lie below the maximum possible V OC,M AX . For a less complex illustration the E ICT `{´l evel have been left out. (c) BHJ blend device without topelectrode under illumination investigated by KPFM. Because of the donor-acceptor phase-separation on the nanoscale, averaging effects can occur when the tip probes acceptor and donor domains together. (d) Band structure of BHJ blend device under illumination in open circuit conditions.

Figure 4 .

 4 11 KPFM results on a MDMO-PPV:PCBM blend (mass ratio 1:4) performed by Hoppe et al. are presented[24]. Next to the topographic image, the work function images in dark and under illumination (442nm) are shown. The vast majority of the topographic elevations appears as bright spots in the work function image (example marked by blue ellipse).

Figure 4 . 11 :

 411 Figure 4.11: SPV measurements by KPFM of toluene-cast MDMO-PPV:PCBM blend with mass ratio 1:4. Topographic (a) and work function images in dark (b) and under illumination at 442nm (c). Most of the topographic hills show high work functions and corresponds to buried PCBM cluster (blue ellipse) that are covered by a thin "skin-layer" of MDMO-PPV as can be seen in the SEM cross-section image (d). PCBM cluster without skin-layer exhibit lower work function values (red ellipse). Upon illumination the photo-induced excitons dissociate at the DA interface and a work function change is observed for the skin-layer covered PCBM clusters. Images adapted from Hoppe et al. [24].

Figure 4 . 12 :

 412 Figure 4.12: Comparative SPV investigation of MDMO-PPV:PCBM blend (weight ratio 1 : 4) on two different space layers on the ITO substrate: PEDOT:PSS and ZnO. CPD images in dark (a), under illumination (b) and in dark afterwards (c) of OPV blend on PEDOT:PSS. A clear CPD shift is obtained upon illumination, however, the photovoltaic charging is not completely reversible due to charge trapping (compare (a) and (c)). CPD images in dark (d), under illumination (e) and in dark afterwards (f ) for the same OPV blend on ZnO. Again, a CPD shift is observed upon illumination, however in the opposite direction. Due to the different energy level alignment at the blend/electrode interface (g) different "in dark" contrasts and photovoltages are obtained for the OPV blend on PEDOT:PSS and ZnO. Images adapted from Maturová et al. [23].

Figure 4 .

 4 Figure 4.13: (a) Topographic image of P3HT:PCBM bulk heterojunction blend with donor acceptor phase-separation on the 10nm scale. (b) 3D Topography with superimposed damping color code. The dissipation signal can help to identify P3HT and PCBM domains. Areas of higher tip-sample dissipation can be tendentially identified as the mechanically "softer" P3HT polymer domains. (c) 3D Topography (zoom) with superimposed SPV signal (SP V " CP D light ´CP D dark ). The P3HT domains receive the holes and charge positively upon illumination, whereas the PCBM domains charge negatively and lie at negative SPV values. Images adapted from Spadafora et al. [27].

Figure 4 .

 4 13 (a,b) shows the topographic image of the BHJ blend together with a 3D topographic representation with superimposed damping signal. The authors claim to be able to identify P3HT and PCBM domains by their different tip-surface dissipation. Bright areas in the damping image are assigned to P3HT and darker areas to the mechanically stiffer PCBM.

Contents 5 . 1

 51 Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 5.3 Self-Assembled Polymer Submonolayer as Model System . . . . . . . . . . . . 73 5.4 Influence of the Interaction Regime on Height Measurements . . . . . . . . . 75 5.5 Role of vdW-Interaction in Height Measurement Effect . . . . . . . . . . . . . 80 5.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Figure 5 . 1 :

 51 Figure 5.1: Importance of the Kelvin compensation of electrostatic forces for a correct heightmeasurement by nc-AFM. (a) Topographic KPFM image of C 60 on HOPG with Kelvin compensation of electrostatic forces and height profile (profile along blue line in image). (b) CPD image corresponding to KPFM image in (a) with CPD profile along blue line. (c) Step-height results for different tip-sample compensation voltages for C 60 -C 60 , C 60 -HOPG and HOPG-HOPG steps. Steps between materials with different CPD are influenced by the applied compensation voltage. The correct step height C 60 -HOPG is only obtained when the electrostatic forces are correctly minimized and the tip-sample compensation voltage corresponds to the CPD at all times like in the case of an active Kelvin compensation (Results adapted from Sadewasser et. al. [4]).

Figure 5 .Figure 5 . 2 :

 552 Figure 5.2: (a) Chemical structure of one monomer of poly(3-dodecylthiophene) (P3DDT). The alkyl-chains are connected laterally to the thiophene polymer backbone. (a, b)The estimated size of one monomer is indicated (compare[14]). (c) Self-assembly of P3DDT on highly oriented pyrolytic graphite (HOPG). The 2D polycrystalline structure exhibits three-fold oriented mono-domains connected by regular, 120 ˝or 60 ˝chain folds. The auto-organization is induced by the epitaxy of the lateral alkyl chains on HOPG, which promotes the formation of single monolayers.

Figure 5 . 3

 53 are the π-conjugated backbones of the polymer-chains (height z high ) with a high density of states. The alkyl-chains that are connected to the polymer backbone are situated in between (height z low ). Both, histogram and image profiles (Figure 5.3 (b) and 5.3 (c)) show a mean stacking height for the thiophene cycles on HOPG of z high ´zHOPG " p3.8 ˘0.3q Å.

Figure 5 . 3 :

 53 Figure 5.3: (a) STM image of P3DDT on HOPG (80nm 2 , low current STM at V tunneling " 1.9V , I tunneling " 1pA). (b) Height histogram of area inside blue rectangle (a) with peaks for alkyl chains (z low ) and π-conjugated cycles (z high ) of the polymer. (c) Profile along blue line in (a). P3DDT exhibits a chain-to-chain distance of about 2.5nm and a stacking height of 3.8 Å.

Figure 5 .

 5 4 (a-c) KPFM results are shown for the same system. The lower part of the KPFM image has been recorded in the SR-regime where SR-forces are dominant, the upper part at less negative Δf 0 in the LR-regime. The damping image (Figure5.4 (b)) shows only a contrast in the SR-regime (damping values calculated in W att[17]). This abrupt change in energy dissipation indicates that additional forces occur when entering the SR-regime. The CPD image (Figure5.4 (c)) shows a CPD-difference of about 30mV between P3DDT domains and bare HOPG which is related to the interface dipole created by P3DDT with the underlying HOPG substrate (see Figure4.2 in Chapter 4.2). This dipole offsets P3DDT's and HOPG's vacuum level and causes thereby the measured CPD difference[18, 19].

Figure 5 . 4 :

 54 Figure 5.4: (a) Topographic (b) damping and (c) CPD image of P3DDT on HOPG. Damping legend in (b) valid for lower half of image (SR-regime), damping in upper half (LR-regime) at about 26.3f W (FM-KPFM, 320nm 2 , f cantilever " 286kHz, A cantilever " 10nm, A bias,modul " 1.0V ). Lower half of image recorded at Δf 0 " ´20Hz with damping contrast (SR-regime), upper half at Δf 0 " ´15Hz without damping contrast (LR-regime). Height histograms of P3DDT on HOPG in the LR-regime (d) and SR-regime (e) show very different values.

Figure 5 . 5 :

 55 Figure 5.5: (a) Topographic and (b) CPD image of P3DDT on HOPG (FM-KPFM with silicon super sharp cantilever, f cantilever " 286kHz, A cantilever " 9nm, Δf 0 " ´24Hz, A bias,modul " 1.0V ). (c) CPD and topographic profile at position marked with blue line in (a) and (b). The well regulated Kelvin loop allows the CPD to follow very rapidly the CPD change when scanning across the P3DDT domain. For both, topographic and CPD channel, the step edge is about 10nm wide.

Figure 5 . 6 :

 56 Figure 5.6: (a) Topographic image of P3DDT on HOPG with corresponding P3DDT height histograms (based on respective blue rectangles in topographic image) in the long-range (b) and the short-range (c) regime (FM-KPFM, 1400nm 2 , f cantilever " 280kHz, A cantilever " 6nm). KPFM-image recorded at low modulation amplitude of Kelvin compensation A bias,modul " 0.2V . Lower half of image recorded at Δf 0 " ´7Hz (SR-regime), upper half at Δf 0 " ´5Hz (LR-regime). The test with lower Kelvin modulation amplitude confirms the results obtained for a modulation amplitude of A bias,modul " 1.0V .

Figure 5 . 7

 57 shows the simultaneously recorded damping curves. The onset of dissipation in the damping curves coincides with the abrupt decrease in the Δf pzq-curves and marks the onset of SR-forces. tip retraction distance z [nm] -regime frequency shift ∆f [Hz]

Figure 5 . 7 :

 57 Figure 5.7: Δf pzq-spectra and damping(z)-spectra (insert) for HOPG and P3DDT (f cantilever " 289kHz, A cantilever " 11nm, A bias,modul " 0.2V ). The onset of damping occurs when entering the SR-regime (sudden strong decrease in Δf pzq). Due to different LR-forces, Δf pzq decreases more slowly for P3DDT than for HOPG in the LR-regime.

Figure 5 . 8 Figure 5 . 8 :

 5858 Figure 5.8: Illustration: (a) the differing uncompensated LR-interaction above P3DDT and HOPG is reflected in the different shape of the Δf pzq curves. (b)For the same Δf 0 , the tip has to approach the P3DDT layer more than the bare HOPG and the experimental polymer height h exp falls below the real value h real . In the SR-regime the height values are more precise because the SR-forces dominate the LR-interaction. Moreover slight differences in the SR-interactions do not cause a large height-effect due to the short decay lengths.

Figure 5 . 9 :

 59 Figure 5.9: (a) Topographic (b) damping and (c) CPD image of bare HOPG. Damping legend in (b) valid for upper half of image (SR-regime), damping in lower half (LR-regime) at about 3.2f W (FM-KPFM, 300nm ˆ100nm, f cantilever " 286kHz, A cantilever " 13nm, A bias,modul " 1.0V ). Upper half of image recorded at Δf 0 " ´10Hz with damping contrast (SR-regime), lower half at Δf 0 " ´5Hz without damping contrast (LR-regime). Height histograms of HOPG double-step (in the middle of KPFM image) show with good precision the same step-height values in the LR-regime (d) and the SR-regime (e). For homogeneous samples the Δf pzq-dependence is a priori constant for the whole sample and a precise height determination gets possible regardless of the chosen Δf 0 set-point.

  shift ∆f [Hz]

Figure 5 .

 5 Figure 5.10: (a) Topographic (b) damping and (c) CPD image of P3DDT on HOPG in line mode for different frequency shifts. Stepwise increase of the frequency shift Δf 0 from ´22Hz to ´10Hz. (d) P3DDT polymer domain height on HOPG plotted over the frequency shift. Each height value was obtained by an histogram of the respective topographic image stripe in (a).A steplike increase of the polymer height is obtained when entering the SR-regime at around Δf 0 " 16Hz with a sharp onset of the SR-forces as also confirmed by Δf pzq spectroscopy (FM-KPFM, 272nm ˆ140nm, f cantilever " 286kHz, A cantilever " 10nm, A bias,modul " 1.0V ).

Figure 5 . 11 :

 511 Figure 5.11: Proposed tip sample model: the tip consists of a conical tip apex with height b and an infinitely long tip trunk. The tip radius is described by rpηq. The effective layer is the surface layer with thickness e for which the contribution to the total force between tip and sample will be determined. It can be for instance the P3DDT polymer layer. The whole sample is a semi-infinite plane and the substrate is of infinite thickness.

  onset SRregime ∆f(z) HOPG experimental ∆f(z) P3DDT experimental ∆f(z) HOPG calculated ∆f(z) P3DDT calculated ln(∆f(z)) [ln(Hz)]

Figure 5 . 12 :

 512 Figure 5.12: ln ´ln plot of the experimental Δf pzq data in Figure 5.7 and calculated pure-vdW Δf pzq curves for the tip model in Figure 5.11 with a conical tip apex of height b " 45nm (Hamaker constant ratio of H P 3DDT {H HOP G " 0.65 for calculation).A qualitative agreement is observed with the experimental curves concerning the magnitude of the observed difference between P3DDT and HOPG curves, as well as concerning the varying distance dependencies for z ! A cantilever , z " A cantilever and z " A cantilever . The overall shape of the curves can only be matched with the experimental results when using more complex tip geometries such as the ones of the used high aspect ratio "Super-Sharp Silicon" tips. The experimental Δf pzq data show in contrast to the calculated pure vdW curves also the onset of SR-forces at 0.6nm.

Figure 6 .Figure 6 . 1 :

 661 Figure 6.1: Chemical structure of the donor molecule FG1 (a) and the acceptor molecule [70]PCBM (b) of the investigated bulk heterojunction blend. FG1's molecular dipole due to the fluorenone unit with the strongly electronegative oxygen is indicated by a red arrow.

Figure 6 . 2 :

 62 Figure 6.2: Morphologic study of FG1:[70]PCBM blend before and after different annealing steps. Topography (a) and CPD (b) image before annealing, topography (c) and CPD (d) image after annealing at 120 ˝C for 15min, topography (e) and CPD (f ) image after final annealing step at 150 ˝C for 15min. Due to the liquid crystal properties of FG1 the annealing has great influence on the sample's morphology and nano phase-separation. (p2000nmq 2 , Δf " ´10Hz, A cantilever " 9nm, f cantilever " 275kHz).

Figure 6 . 3 :

 63 Figure 6.3: Multi-scale phase-separation of FG1:PCBM blend after annealing at 150 ˝for 15min. Large scale topographic (a) and CPD (b) images show PCBM-rich areas where PCBM starts to crystallize (visible as bright spots in CPD image) surrounded by areas dominated by self-assembled FG1.This corresponds to a donor-acceptor phase-separation on the µm-scale (p5000nmq 2 , Δf " ´10Hz, A cantilever " 9nm, f cantilever " 275kHz). Closer topographic (c) and CPD (d) images reveal buried PCBM cluster (small bright spots in CPD) which are intercalated between the FG1 lamellae. This corresponds to a donor-acceptor phase-separation on the nm-scale (p2000nmq 2 , Δf " ´10Hz, A cantilever " 9nm, f cantilever " 275kHz). (e) Illustration of the different possible donor-acceptor phase-separations on the µm-scale (large-scale), the nm-scale and both at the same time (multi-scale).

Figure 6 .

 6 Figure 6.4 (d) illustrates the situation of the surface-near PCBM cluster, which are buried within the FG1 matrix. The ICT model predicts an electron transfer from the donor towards

Figure 6 . 4 :

 64 Figure 6.4: (a) Molecular electronic level of FG1 and PCBM (HOMO and LUMO level plus charge transfer states E ICT `and E ICT ´) and Fermi level E F of PEDOT:PSS/ITO. Situation before contact and charge transfer (E vac level aligned). (b) Molecular electronic level upon contact. PCBM's E ICT ánd FG1's E ICT `level are aligned and an interface dipole with dipole barrier Δ is created. (c) Suggested morphologic structure of the FG1:PCBM blend. The concentration of PCBM is highest next to PEDOT:PSS and decreases towards the upper surface. Therefore, and due to the high PCBM:FG1 ratio of 2.5 : 1, PCBM is suspected to create a complete layer on top of the bottom electrode. (d) PCBM-cluster near the surface are covered by at least one FG1-layer. Due to the interface dipole they appear as regions of higher CPD. (e) With the virtually closed PCBM layer just above the bottom electrode, the bulk interface dipole at the FG1-PCBM interface creates also a global positive CPD shift relative to the CPD of the bare electrode.

Figure 6 . 5 :

 65 Figure 6.5: On the detection of buried PCBM cluster: (a) For a perfectly spherical PCBM domain with symmetrical charge distribution the interface dipoles (indicated by black arrow) will cancel each other out and no CPD contrast is obtained for the buried PCBM cluster. (b) For an anisotropic shape of the PCBM domain, here an ellipsoid, the tip will measure a net surface dipole because the ellipsoid's upper dipole is much closer to the tip and the electrostatic dipole-field decreases relatively fast. (c)Another anisotropy resulting in a net electrostatic surface dipole may be caused by the exact molecular assembly and the microelectrostatic environment at the interface. Here a situation is illustrated where CT is only possible from the FG1 molecules towards the PCBM cluster if these are aligned parallel to the FG1/PCBM interface. The partial CT across only specific sections of the PCBM sphere creates a net surface dipole and the PCBM cluster can be detected.

Figure 6 . 6 :

 66 Figure 6.6: Energy level alignment according to the ICT model for (a) pure [70]PCBM on PEDOT:PSS, (b) pure FG1 on PEDOT:PSS and (c) a bilayer FG1/[70]PCBM on PEDOT:PSS.For the case of pure PCBM, E vac alignment is predicted, in the other cases the creation of an interface dipole. In the case of pure FG1, Δ is more pronounced than for the bilayer FG1/PCBM. The experimental values CP D P CBM " ´1100mV , CP D F G1 " ´500mV and CP D F G1:P CBM " ´700mV reflect well the direction and the order of magnitude of the interface dipole.

Figure 6 . 7 :

 67 Figure 6.7: Comparison between pure FG1 and the FG1:PCBM blend, both on PEDOT:PSS/ITO. Topographic (a) and damping (b) image for pure FG1 show lamellar self-assembly of the FG1 molecules with a periodicity of 4.5nm. The CPD image (c) is relatively homogeneous (p300nmq2 , Δf " ´20Hz, A cantilever " 9nm, f cantilever " 275kHz). Topographic (d) and damping (e) images of FG1:PCBM show also a lamellar assembly, however with an increased periodicity of around 10nm due to intercalated PCBM. The CPD image (f ) shows a relatively strong contrast for FG1:PCBM with thin stripes and bright spots corresponding to buried PCBM-rich cluster (p300nmq 2 , Δf " ´24Hz, A cantilever " 9nm, f cantilever " 275kHz).

Figure 6 . 8 :

 68 Figure 6.8: Molecular mechanics modeling of lamellar FG1 assemblies in vacuum without PCBM. (a) Single FG1 molecule with indicated length scales and molecular dipole moment. (b) Single FG1 molecular stack (top-view). (c) Multiple neighboring lamellar FG1 stacks. Two different possible conformations arise from the modeling results. One with parallel oriented dipoles of neighboring stacks and one with alternating dipoles of neighboring lamella. The stack periodicity is different in the two cases. Modeling performed by Mathieu Linares (Linköping University, IFM, Sweden) [4].

Figure 6 . 10 :

 610 Figure 6.10: Topographic (a) and CPD images in dark (b) and under illumination (c) (1mW , 515nm) of FG1:PCBM blend. Upon illumination a global positive CPD-shift and a contrast change in the CPD images is observed. The photovoltage image (d) is the subtraction of the CPD images under illumination and in dark (P V " CP D light ´CP D dark ) and shows a clear contrast with areas that charge less positively (PCBM-rich) and more positively (FG1-rich) (p1000nmq 2 , Δf " ´10Hz, A cantilever " 9nm, f cantilever " 275kHz). The positive CPD shift is completely reversible (e) and returns to its initial value in dark after the illumination interval. (f ) The CPD-shift is dependent on the wavelength and saturates with increasing illumination intensity.

Figure 6 .

 6 Figure 6.11: (a) Overview CPD image with FG1-rich area and buried PCBM-rich cluster (p1000nmq 2 , Δf " ´22Hz, A cantilever " 9nm, f cantilever " 275kHz). Topographic (b), CPD dark (c) and PV (d) images of buried PCBM-rich area and surroundings. The images have been taken on the zoom area indicated in (a). A FG1-rich area ①, an area with the buried PCBM-rich cluster underneath ③ and a region with an intermediate PCBM concentration ② are marked. The PV image demonstrates that PCBM (acceptor) rich areas take the electrons and counteract the global positive charging of the sample under illumination (1mW , 515nm). They charge less positive as it can also be seen in the CPD image histograms (e) (p400nm ˆ350nmq, Δf " ´20Hz, A cantilever " 9nm, f cantilever " 275kHz).

Figure 6 . 12 :

 612 Figure 6.12: High resolution KPFM investigation of flat FG1:PCBM blend area. Topographic (a), damping (b), photovoltage (c) and CPD images in dark (d) and under illumination (e) (1mW , 515nm). Besides the global positive charging of the sample under illumination of 177mV , a clear contrast change is observed in the CPD images. Buried PCBM cluster (bright spots and stripes in CPD images (d, e), examples marked with red contours) appear mainly as dark areas in the photovoltage image (c) and are hence less positively charged (PCBM accepts the photo-generated electrons). Examples for FG1-areas with very low PCBM concentration are indicated by blue contours. (f ) Photovoltage on 3Dtopography representation illustrating that the photovoltage is not directly correlated to the topography (no cross-talk artifacts) and that the PCBM has to be buried near the surface within the lamellar FG1 matrix (p400nmq 2 , Δf " ´24Hz, A cantilever " 9nm, f cantilever " 275kHz).

Figure 6 . 13 :

 613 Figure 6.13: Cross-correlation analysis to verify the physical origin of sub ´10nm features in the photovoltage image (a) like the thin dark stripes indicated by blue arrow. The cross-correlation was performed between the high-contrast damping images in dark (b) and under illumination (1mW , 515nm) (c).Cross-correlation image with high aspect ratio maximum next to the coordinate system's origin (a). An average absolute drift value of below 1nm was obtained which lies fairly below the size of the dark stripe features in the PV image (a) (p400nmq 2 , Δf " ´24Hz, A cantilever " 9nm, f cantilever " 275kHz).

Figure 6 . 14 :

 614 Figure 6.14: KPFM investigation of FG1:PCBM blend in dark and under illumination (1mW , 515nm) at different Δf -setpoints. Topographic, damping, CPD dark and photovoltage image (a,b,c,d) atΔf " ´4Hz in the LR-regime, at Δf " ´14Hz (e,f,g,h) and at Δf " ´24Hz in the SR-regime (i,j,k,l) (p800nmq 2 , A cantilever " 9nm, f cantilever " 275kHz). For increasing Δf -setpoints a clearly enhanced resolution can be observed for all KPFM channels including the photovoltage image. The damping contrast at higher Δf indicates the appearance of additional (short-range) forces.

Figure 6 .Figure 6 . 15 :

 6615 Figure 6.15: Quantitative analysis of the CPD image resolution at different setpoints via profiles. (a) p400nmq 2 zoom into the CPD dark image shown in Figure 6.14 (c) at a setpoint of Δf " ´4Hz. (b) Zoom on the same area into the CPD dark image shown in Figure 6.14 (k) at a setpoint of Δf " ´24Hz. (c) Profiles on the same position (marked by blue and red line in (a,b)) for different setpoints (Δf " ´4Hz, ´14Hz, ´24Hz). A slight smoothing has been applied to the profiles to remove noise. As indicator for the CPD image resolution and contrast, the slope ratios a{b of the peaks along each profile are calculated and averaged. At a frequency shift setpoint of Δf " ´4Hz, the image resolution and contrast is only about 40% of the one in the SR-regime at Δf " ´24Hz.

Figure 6 . 16 :

 616 Figure 6.16: Illustration how a decreased tip-surface distance z can enhance the lateral resolution in CPD and photovoltage images. In the long-range regime at low Δf -setpoints and large z (a) an averaging effect occurs because the tip measures the positive and negative charge together. This results in an decreased CPD image contrast and lateral resolution. In the short-range regime and at high Δf -setpoints (b), however, the tip is more closely to the surface and images the two charges separately resulting in an increased image contrast and a superior resolution.

Figure 6 .Figure 6 .

 66 Figure 6.17: (a) CPD image histograms in dark (Δf " ´4Hz) and under illumination (1mW , 515nm) at different Δf -setpoints (p800nmq 2 , A cantilever " 9nm, f cantilever " 275kHz). For more negative frequency shifts the absolute value of the photovoltage increases until its saturation in the SR-regime. In contrast the CPD histograms in dark (b) are practically independent of the Δf -setpoint. Only the FWHM of the histogram peaks increases slightly for more negative Δf setpoints due to the enhanced image contrast in the SR-regime.

Figure 6 .

 6 photovoltage PV(∆f)

Figure 7 . 1 :

 71 Figure 7.1: (a) Illustration and chemical structure of the studied donor-acceptor dyad. (b) Schematic representation of donor-acceptor dyad stack with hydrophobic sidechains on the one side and hydrophilic sidechains on the other side. Due to the hydrophobic-hydrophilic sidechain incompatibility, the DA dyads are assembled in the form of bicontinuous donor-acceptor stacks, which corresponds to the favorable situation for a photovoltaic charge carrier generation and charge transport. (c) Stack of DA dyads without incompatible sidechains. Due to the charge transfer interaction between donor and acceptor part an alternating DA stack is created.

Figure 4 . 8 in

 48 Chapter 4.4).

7. 3 |

 3 DA-Dyad Self-Assembly on HOPGTo investigate the morphology of the DA dyad deposit on HOPG, STM and AFM measurements have been performed. The nc-AFM image in Figure7.2 shows large scale domains of different size (DA solution concentration C DA " 1mg{ml). All domains exhibit the same height of p2.7 ˘0.1qnm on HOPG and indicate thereby already the molecular auto-organization.

Figure 7 . 2 :

 72 Figure 7.2: (a) nc-AFM topographic image of DA dyads on HOPG assembling in organized domains (p3000nmq 2 , Δf " ´18Hz, A cantilever " 10nm, f cantilever " 286kHz, drop cast with C DA " 1mg{ml). (b) Height histogram of molecular domains on HOPG based on AFM image of area marked with blue square in (a). The dyad domains exhibit a height of 2.7nm.

  Figure 7.3 (b,c)). For both cases, however, D min stays the same. The inter-lamellae distance D min has been determined via STM profiles and Fast Fourier Transformation (FFT) to p5.6 ˘0.5qnm. However, with the obtained image resolution,

Figure 7 . 3 :

 73 Figure 7.3: (a) STM image of sample in Figure 7.2 (a) revealing the DA stack domains' lamellar structure (p600nmq 2 , I tunnel " 3pA, V tunneling " `1.9V ). (b,c) Model for the z topographic profiles in constant current STM imaging of the edge-on lamellae. Due to a reduced density of electronic states, z minima occur over the areas covered either by alkyl or diethylene glycol (DEG) groups. The distance between topographic minima D min is always equal to the half of the lattice periodicity. Depending on the nature of the DEG-DEG and alkyl-alkyl interdigitation between adjacent lamellae, the distance between topographic maxima D max can be equal to D min (a), or may vary as shown in (b).

Figure 7 . 4 :

 74 Figure 7.4: (a) STM image of DA multilayer crystallites. Every layer is a perfect assembly of adjacent lamellar DA stacks (p400nmq 2 , I tunnel " 1pA, V tunneling " `1.9V , C DA " 6mg{ml). (b) Face-on buffer layer with thereon organized edge-on lamellae (p200nmq 2 , I tunnel " 5pA, V tunneling " `1.9V ).

Figure 7

 7 .3 (a), the DA stacks are however not arranged directly on top of the underlying graphite. As shown in Figure 7.4 (b), they are rather located on a so-called buffer layer of face-on organized2

Figure 7 . 5 :

 75 Figure 7.5: Face-on buffer layer of sample with lower surface coverage (C DA " 0.1mg{ml) (a) STM image of face-on organized domains surrounded by unorganized dyads and regions of bare HOPG (p230nmq 2 , I tunnel " 3pA, V tunneling " `1.9V ). (b) nc-AFM image of sample in (a) with face-on domains of different orientation (p300nmq 2 , Δf " ´12Hz, A cantilever " 15nm, f cantilever " 163kHz).(c) STM image of face-on buffer layer on HOPG with higher deposition concentration (C DA " 1{3mg{ml). In comparison to the results presented in (a, b) no amorphous regions of unorganized dyads are found due to a higher surface density that forces the molecules to organize perfectly into various face-on domains covering the whole substrate region (p150nmq 2 , I tunnel " 3pA, V tunneling " `1.9V ).

Figure 7 . 6 :

 76 Figure 7.6: Face-on assembled DA dyads on HOPG (C DA " 0.1mg{ml) (a) STM image with submolecular resolution within face-on domains. The unit cell with parameters a, b, θ and two DA dyads is shown. The red arrow indicates aligned PDI units, the blue arrow the aligned terthiophenes (p16nmq 2 , I tunnel " 5pA, V tunneling " `1.7V ). (b) High resolution nc-AFM image of the face-on assembly (p50nmq 2 , Δf " ´17Hz, A cantilever " 15nm, f cantilever " 163kHz).

Figure 7 Figure 7 . 7 :

 777 Figure 7.7: MM and MD simulation of dyad assembly. (a) Bicontinuous lamellar DA stack with π-stacked PDI-units and height h lamella (b) Illustration of several neighboring lamellae with width d lamella . (c) Face-on assembly with unit cell (blue) and characteristic parameters. PDI-units π-stacked on underlying HOPG, fluorene-units turned on the side. The orientation of neighboring dyads is alternated.

Figure 7 .

 7 8 (a) and (b) show the topography and the CPD image of the face-on buffer layer in the case of the sample with low molecular surface coverage (C DA " 0.1mg{ml).

Figure 7 . 8 :

 78 Figure 7.8: Topographic (a) and CPD (b) image of face-on domains, amorphous regions and bare substrate (hole in the monolayer) with their different characteristic CPD level (Δf " ´25Hz, A cantilever " 10nm, f cantilever " 280kHz).

Figure 7 . 9 :

 79 Figure 7.9: (a) Topographic KPFM image of lamellar DA monolayer with several holes down to the face-on covered HOPG. Corresponding CPD images in dark (b) and under laser (515nm, 10mW ) illumination (c). Upon green laser illumination the lamella domains shift towards more positive CPD values, meanwhile the CPD of the holes in the monolayer remains constant (p2000nmq 2 , Δf " ´6Hz, A cantilever " 9nm, f cantilever " 275kHz). (d) Topographic image showing mono-and double-layer lamella DA domains. The CPD images in dark (e) and under illumination at 532nm and 5mW (f ) show again a clear CPD-shift of the lamella domains towards more positive values corresponding to a positive charging of the domains. The CPD of the surrounding face-on dyad covered HOPG does not change. This also excludes measurement artifacts like a global CPD-shift induced by a cantilever photovoltage. (p1200nmq 2 , Δf " ´7Hz, A cantilever " 10nm, f cantilever " 291kHz).

Figure 7 . 10 :

 710 Figure 7.10: KPFM results on face-on buffer layer. (a) Topographic image showing organized face-on domain, amorphous regions and a hole in this first monolayer exposing the bare HOPG. The CPD images in dark (b) and under illumination at 532nm and 5mW (c) show no significant CPD difference for the bare graphite, the amorphous regions and the organized face-on domain. This is even more clearly visible in the CPD histograms shown in (d). (p350nmq 2 , Δf " ´10Hz, A cantilever " 10nm, f cantilever " 291kHz).

Figure 7 .

 7 Figure 7.11: (a) Histograms of CPD images on area depicted in 7.9 (a) for increasing laser intensity (515nm). The lamellae's positive CPD shift increases with higher optical intensities. At the same time the CPD of the face-on dyad covered HOPG is virtually unchanged. (b) CPD change over illumination power (515nm and 685nm) for DA lamella domains and bare HOPG. The only significant CPD shift occurs for the lamellar DA stacks at 515nm. (c) UV-vis spectroscopy of DA dyad and its acceptor and donor building blocks. For the spectroscopy the molecules have been dissolved in CHCl 3 (10 ´5M). The absorption of the DA dyad is a superposition of the absorption of its building blocks. For green laser light at 515nm and 532nm the photons are absorbed in the PDI acceptor unit. The red wavelength at 685nm is not absorbed, being beyond the absorption peaks of the compounds.

Figure 7 .

 7 Figure 7.12: (a,b) Proposed model for the energy level alignment at the interface of face-on dyads and HOPG. HOMO and LUMO level have been estimated by cyclic voltammetry or taken from literature. Donor and acceptor charge transfer states E D{A,ICT `and E D{A,ICT ´are indicated. For the face-on dyads, acceptor and donor moieties are electronically decoupled. Due to electron charge transfer towards the substrate (a) an interface dipole is created and the donor energy level are lowered by Δ relative to the acceptor ones (b) to align E F and E D,ICT `. This lowering reduces the effective DA gap E DA gap and thereby strongly decreases the probability of a charge transfer state dissociation. (c) Schematic energy diagram for the lamellae, the buffer layer and the substrate. Due to the buffer layer, the edge-on stacks are electronically decoupled from the substrate. Furthermore, the buffer layer also acts as hole blocking layer. The photo-generated holes in the lamellae's donor segments cannot be transferred to the HOPG, because of the hole barrier (offset) between face-on dyads' and lamellae's E D,ICT `level (blue dashed arrow). In contrast the positioning of the E A,ICT ´level promotes an efficient electron transfer towards the graphite (red arrow). (d) Illustration of the substrate, the buffer layer and the lamellae. Under illumination the lamellae's donor moieties are charging positive and a positive CPD is detected by KPFM. The interface dipole between face-on dyads and HOPG is indicated (violet arrow).

Figure 8 Figure 8 . 1 :

 881 Figure 8.1: Lamellar self-assembly of A-δ `-d 3 DA dyad on HOPG revealed by nc-AFM. (a) Monolayer domains of auto-organized edge-on DA molecules (p1500nmq 2 , Δf " ´20Hz). (b) The resolution of the DA lamella within the edge-on domains is achieved by nc-AFM (p268nmq 2 , Δf " ´35Hz). (c)High resolution nc-AFM image of molecular stacks with submolecular resolution. The DA stacking is apparently not perfect creating gaps within the lamellar assembly (p142nmq 2 , Δf " ´39Hz) (A cantilever " 7nm, f cantilever " 300kHz for (a-c)).

Figure 8 .

 8 2 a single CdSe@CdS DA nanorod imaged via KPFM is shown. The nanorods of about 10nm ˆ80nm size possess a CdSe core and a CdS shell and have been synthesized by Peter Reiss et al. in our group (PhD thesis E. Rossitto[5]).

Figure 8 . 2 :

 82 Figure 8.2: Topographic (a), damping (b) and CPD image (c) acquired by KPFM of a CdSe@CdS DA nanorod on HOPG. The nanorod of about 10nm ˆ80nm size has a CdSe core and a CdS shell. The different work function of the CdSe core can also be seen in the CPD image (p60nmq 2 , Δf " ´30Hz, A cantilever " 10nm, f cantilever " 309kHz).

Figure 8 . 3 :

 83 Figure 8.3: Recent work of Schuler et al. demonstrating the resolution of the molecular structure of the triad TTF-PYZ 2 by (constant height) nc-AFM and the molecule's corresponding LCPD contrast acquired by KPFM [7]. The measurements have been performed at low temperatures of 5K and with a CO-terminated tuning fork sensor tip. The LCPD contrast is in agreement with the donoror acceptor-character of the molecular subunits. The next important challenge would be a KPFM investigation of such molecules under illumination with similar submolecular LCPD resolutions.

Figure 9 . 7 :

 97 Figure 9.7: DA stack before dynamic in the NVT ensemble. Typical distances are indicated.

Figure 9 . 8 :

 98 Figure 9.8: Side-view of DA stack after dynamic in the NVT ensemble for 50ns.

Figure 9 . 10 :

 910 Figure 9.10: Top-view of face-on DA assembly on graphite. PDI-units are π-stacked with π-orbitals of topmost graphite layer. Fluorene-units are turned on the side. The direction of DA molecules in one row is alternating.

Figure 9 . 11 :

 911 Figure 9.11: Side-view of face-on DA assembly on graphite. PDI-units lay perfectly flat on top of the substrate. Alkyl-chains are preferentially lying on top of the graphite, the hydrophilic DEG chains are pointing away from the surface.

  .1).

		Braunkohle brown coal	primary energy consumption
		Steinkohle stone coal	11,6%	Germany 2013 (14 005 Petajoule)
		12,7%	nuclear
		1.625	Kernenergie energy
		1.779		7,6%
			1.058	Andere 0,7% others	Biokraftstoffe 1,2% biofuels	0,7% Abfälle+Deponiegas waste
			100	renewable		128	Solarthermie 0,2% solar thermal
	Erdgas 22,5% gas natural	3.152	1.654	Erneuerbare 11,8% energies	1.015	76 27 102 1 100 24	Fotovoltaik 0,7% photovoltaics Geothermie 0,007% geo thermal
						180	Wärmepumpe heat pumps
							0,2%
				Biomassefest/ gasförmig biomass 7,2%	Windkraft 1,3% wind	Wasserkraft 0,5% hydro power
		4.637
		Mineralöl
		33,1%
		mineral oil

  2.1 Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 2.2 Scanning Tunneling Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3 Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3.1 AFM Operation Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.2 Tip Sample Interaction Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.4 Kelvin Probe Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2.5 Summary of the Employed SPM Setup . . . . . . . . . . . . . . . . . . . . . . . 23 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

  Contents 3.1 Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.2 Conjugated Molecules and Polymers . . . . . . . . . . . . . . . . . . . . . . . . . 32 3.3 Metal-Organic and Organic-Organic Interfaces . . . . . . . . . . . . . . . . . . . 35 3.3.1 Integer Charge Transfer Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 3.3.2 Induced Density of Interfacial States Model . . . . . . . . . . . . . . . . . . . . . . 37 3.3.3 Energy Level Alignment at the Organic-Organic Interface . . . . . . . . . . . . . . 39 3.4 Charge Carrier Generation in Organic Photovoltaic Systems . . . . . . . . . . 40 3.4.1 Characteristics of Organic Photovoltaic Devices . . . . . . . . . . . . . . . . . . . . 43 3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

	6 | Detection of Photo-Induced Charge Carriers in Organic DA Blends by
	KPFM

Contents 6.1 Context . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 6.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 6.3 Tailored Donor-Acceptor Phase-Separation . . . . . . . . . . . . . . . . . . . . . 90 6.4 KPFM of Illuminated FG1:PCBM Blend . . . . . . . . . . . . . . . . . . . . . . 99 6.4.1 High Resolution Photovoltage Imaging . . . . . . . . . . . . . . . . . . . . . . . . . 102 6.4.2 Photovoltage Measurements at Different Interaction Regimes . . . . . . . . . . . . 106 6.5

  2

CN L is here defined as a positive quantity as well as the work function Φ.

COMSOL Multiphysics Software: www.comsol.com

All the Molecular Mechanics (MM) and Molecular Dynamics (MD) simulations were performed with the Gromacs package [1] and the OPLS force field [2] by Mathieu Linares from the IFM at Linköping University in Sweden. For MD simulations in the NVT (canonical) ensemble the temperature was maintained by the Nosé-Hoover thermostat [3]. Periodic boundary conditions were used to form an infinite stack to study the face-on monolayer formation on graphite. For the latter system, four layers of graphite were used and maintained frozen during the MD simulation.

| Dimer

To study the formation of stacks of DA molecules, we started by performing dynamics of the dimer in vacuum. At the beginning of the simulation the distance between the two DA molecules is about 5nm. After 1ns, the two molecules come close to each other. After roughly 2.5ns, and due to the hydrophilic-hydrophobic interactions, the perylene-diimide moieties of the molecules are well stacked (Figure 9.4). For the fluorene part of the molecule, a less organized stack is observed, due to bulky hindrance caused by the alkyl-chains connected to the donor moiety. In order to reduce this hindrance and consequently obtain more organized stack, it seems possible to alternate the orientation of the fluorene moiety. In the side view of the dimer (Figure 9.5), the molecule in the back is represented in orange.

| Donor Acceptor Stack Formation

By optimizing simultaneously the molecules and the size of the box, we found an organized stack with an intermolecular distances of 4.5 Å (Figure 9.6).

Typical distance are presented in Figure 9.7 below. The width of the stack is about 5.39nm and the height varies from 2.04nm on the DEG side to 3.37nm on the fluorene-terthiophene side. Experimentally the width is about p5.6 ˘0.5qnm and the height of p2.7 ˘0.1qnm. However, the calculated stack presents no defect and has been calculated in vacuum. To obtain a more realistic stack, we performed a dynamic in the NVT ensemble for 50ns. The results can be seen in Figure 9.8 (side-view stack) and Figure 9.9 (top-view stack). A stack diameter of 5.0nm

| Face-on Monolayer on Graphite Surface

The study of the self-assembly of the DA dyads on graphite (HOPG) represents a real challenge because of the size and complexity of such a system. To determine the face-on monolayer organization, we performed a trial and error approach starting by putting two molecules on a large graphite surface to obtain a seed for the self-assembly. Step by step more molecules

Acknowledgement

My time as PhD student at the LEMOH in Grenoble has been an enrichment in both, professional and personal respect. On the one hand I had the opportunity to learn new techniques and to deepen my scientific knowledge. On the other hand I got to know great colleagues and friends and could enjoy the common time in the lab to the fullest. Therefore I would like to thank all members of the laboratory for their support and fine collegiality throughout the past three years. I has been a pleasure to work with you all!

In particular I would like to thank: Benjamin Grévin, my PhD advisor. It was very enriching to work with you over the past three years. I could always be sure of your support and your scientific enthusiasm motivated me to the best. Your research experience and guidance helped me to clearly shape and focus my ideas and I will keep in mind a lot of nice memories of the common time... thank you for everything! Clemens Barth, Sébastien Gauthier, Thilo Glatzel, Pierre Mallet and Guillaume Schull, that they kindly accepted to participate in my PhD thesis committee and reviewed my PhD manuscript.

Renaud Demadrille for all the discussions, the support and many marvelous samples from him and his chemistry crew.

All the PhD students, Postdocs, research trainees and permanents that I had the pleasure to work and live with. Without you my time in Grenoble would not have been the same! Antoine Chassin de Kergommeaux, Aurélie Lefrancois, Axel Maurice, Cécile Philippot, Chiara Ottone, Clément Thomassé, Damien Joly, David Aradilla, David Djurado, Dimitry Aldakov, Dorian Gaboriau, Frédéric Chandezon, Fleur Thissandier, Florent Caffy, Gijo Raj, Jean-Pierre Travers, Jérôme Faure-Vincent, Jinhyung Park, Kars de Vet, Loïc Laplatine, Louis Vaure, Luca Assumma, Lucia Hartmann, Lucia Mattera, Maria Méndez Màlaga, Maxime Godfroy, Mohammad Jouni, Olga Burchak, Onintza Ros Bengoetxea, Pascale Chenevier, Patrice Rannou, Peter Reiss, Rania Makrygenni, Tim Senden, Yann Kervella and Zaireen Yahya.

Céline Deleval and Charles Picot, for their assistance with all different kinds of organization questions.

Christian Lombard, Mahjoub Fakir and Philippe Montmayeul for their help with a multitude of technical tasks.

Lukasz Borowik, Sylvain Pouch, Denis Mariolle and Nicolas Chevalier for the collaboration on KPFM investigations of organic photovoltaics samples and the common measurement days in their lab.

Abstract

During this thesis, model donor-acceptor (DA) systems for organic photovoltaics have been studied by non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). To enhance the understanding of the optoelectronic processes on the nanoscale, DA systems with better defined structural and electronic properties than the one of most bulk heterojunction blends (BHJ), have been studied.

With DA phase-separations of below 10nm in organic photovoltaic systems, the highest possible resolution has to be achieved by KPFM to investigate optoelectronic processes. It has been shown that nc-AFM/KPFM measurements in the regime of short range (SR) forces can increase imaging resolution. In preparation of such investigations, the influence of the interaction regime on the topographic measurement via KPFM has been studied for a self-assembly of P3DDT on HOPG. It is demonstrated that imaging in the SR-regime not only increases the lateral resolution, but also assures correct topographic height values.

In a next step, DA blends of FG1: [START_REF] Afin | été étudiée dans le cas d'auto-assemblages de P3DDT sur substrat HOPG. Nous avons ainsi démontré que l'imagerie dans le régime d'interaction à courte portée améliore non seulement la résolution latérale, mais permet également de réaliser des mesures de hauteur plus réalistes[END_REF]PCMB have been studied by KPFM. For these BHJs, the structure and the scale of the DA phase-separation can be tuned via the liquid crystal behavior of the donor FG1. The in dark potential contrasts are consistent with surface and bulk morphology. The relationship between the surface photovoltage (SPV) and the tip-sample interaction regime has been analyzed. Increased SPV resolutions and image contrasts are obtained in the SR-regime. Furthermore, due to an optical field exaltation caused by the AFM tip, an amplified SPV signal is observed at small tip-sample distances.

Finally, a new generation of DA dyads with donor and acceptor moieties has been studied. Its self-assembly on HOPG has been determined via a comparative study by scanning tunneling microscopy and nc-AFM plus molecular mechanics and dynamics simulations. By KPFM the charge carrier generation and collection has been analyzed down to the level of a single molecular layer. A clear relationship between the dyads' molecular assembly and their photovoltaic properties can be established.