Heejae Lee
email: heejae.lee@polytechnique.edu

Jean-Charles Vanel

Abderrahim Yassar

Gael Zucchi

Chloe Dindault

Minjin Kim

Dr Warda Hadouchi

Dr Anna Shirinskaya

Dr Mark Chaigneau

Dr Jongwoo Jin

Dr Taewoo Jeon

Dr Chang-Seok Lee

Dr Chang-Hyun Kim

Dr Heechul Woo

Dr Sungyup Jung

Seonyong Park

Sanghuk Yoo

Junha Park

Hyeonseok Sim

Gookbin Cho

Analysis of Current-Voltage Hysteresis and Ageing Characteristics for CH 3 NH 3 PbI 3-x Cl x Based Perovskite Thin Film Solar Cells

The three years have already passed, and I want to prepare another new start. It was a doctoral course that started with curiosity and worry,

The synthesis of the halide perovskite materials is optimized in a first step by controlling the deposition conditions such as annealing temperature (80°C) and spinning rate (6000 rpm) in the one step-spin-casted process. CH 3 NH 3 PbI 3-x Cl x based perovskite solar cells are then fabricated in the inverted planar structure and characterized optically and electrically in a second step.

Direct experimental evidence of the motion of the halide ions under an applied voltage has been observed using glow discharge optical emission spectroscopy (GDOES). Ionic diffusion length of 140 nm and ratio of mobile iodide ions of 65 % have been deduced. It is shown that the current-voltage hysteresis in the dark is strongly affected by the halide migration which causes a substantial screening of the applied electric field. Thus we have found a shift of voltage at zero current (< 0.25 V) and a leakage current (< 0.1 mA/cm 2 ) in the dark versus measurement condition. Through the current-voltage curves as a function of temperature we have identified the freezing temperature of the mobile iodides at 260K. Using the Nernst-Einstein equation we have deduced a value of 0.253 eV for the activation energy of the mobile ions.

Finally, the ageing process of the solar cell has been investigated with optical and electrical measurements. We deduced that the ageing process appear at first at the perovskite grain surface and boundaries. The electrical characteristics are degraded through a deterioration of the silver top-electrode due to the diffusion of iodides toward the silver as shown by GDOES analysis.

Abstract (in French)

Les perovskites organiques-inorganiques en halogénures de plomb sont des matériaux très prometteurs pour la prochaine génération de cellules solaires avec des avantages intrinsèques tels que leur faible coût de fabrication (grande disponibilité des matériaux de base et leur mise en oeuvre à basse température) et leur bon rendement de conversion photovoltaïque. Cependant, les cellules solaires pérovskites sont encore instables et montrent des effets d'hystérésis courant-tension délétères. Dans cette thèse, des résultats de l'analyse physique de couches minces de pérovskite à base de CH 3 NH 3 PbI 3-x Cl x et de cellules solaires ont été présentés. Les caractéristiques de transport électrique et les processus de vieillissement ont été étudiés avec différentes approches.

Dans une première étape, la synthèse du matériau pérovskite a été optimisée en contrôlant les conditions de dépôt des films en une seule étape telles que la vitesse de rotation (6000 rpm) de la tournette et la température de recuit des films (80 °C). Dans un second temps, des cellules solaires perovskites à base de CH 3 NH 3 PbI 3-x Cl x ont été fabriquées en utilisant la structure planaire inversée et caractérisées optiquement et électriquement.

Grace à l'utilisation de la spectroscopie optique à décharge luminescente (GDOES), un déplacement des ions halogénures a été observé expérimentalement et de façon directe sous l'application d'une tension électrique. Une longueur de diffusion ionique de 140 nm et un rapport de 65% d'ions mobiles ont été déduits. Il est montré que l'hystérésis courant-tension dans l'obscurité est fortement affectée par la migration des ions halogénures provoquant un écrantage substantiel du champ électrique appliqué. Nous avons donc trouvé sous obscurité un décalage de la tension à courant nul jusque 0,25 V et un courant de fuite jusque 0,1 mA / cm 2 en fonction des conditions de mesure. Grâce aux courbes courant-tension en fonction de la température, nous avons déterminé la température de transition de la conductivité ions/électrons à 260K et analysé les résultats expérimentaux en utilisant l'équation de Nernst-Einstein donnant une énergie d'activation de 0.253 eV pour les ions mobiles.

Enfin, le processus de vieillissement de la cellule solaire a été étudié avec des mesures optiques et électriques. Nous avons déduit que le processus de vieillissement apparaît d'abord à la surface des cristaux de pérovskite ainsi qu'aux joints de grains. Les mesures GDOES nous indiquent que les caractéristiques électriques des cellules pérovskites sont perdues par une corrosion progressive de l'électrode supérieure en argent causée par la diffusion des ions iodures.

Chapter 1. Introduction

Solar cells operation

Photovoltaics

The worldwide consumption of energy has increased every year by several percentage over the last thirty years 1 . Due to an overall growth of world population and a further development in area like Asia, Africa and Latin-America, it is believed that the growth will persist or even accelerate over the coming decades 2 . Most of this energy is nowadays supplied by fossil on the one hand and by nuclear energy on the other hand. However, these resources are limited and their use has a serious environmental impact, which extends probably over several future generations 3 . This situation poses an enormous challenge already for the present generation to start up a transition in energy consumption and production. More efficient usage of produced energy could possibly lead to a decreased consumption whereas new technologies could steer this transition toward a more sustainable energy production.

Sustainable development meets the needs of today without jeopardizing the future. In this respect, renewable energy sources fit in very well. Besides their environmental friendliness, they offer several advantages 4 . Diversification of energy supplies can lead to more economical and political stability. Moreover, countries and regions can become more independent by supplying their own energy from renewable instead of having to import fuels or electricity from large production plants. Finally, a transition from traditional fuel based energy production to renewable energy resources can even lead to substantial increase of employment.

Several renewable energy resources are under development or even already introduced on the market. Still, they make up only a limited part of the total energy production 5 . Among these renewable energy resources, the direct and indirect use of solar energy is believed to have much larger possible application than used nowadays. The total amount of solar irradiation per year on the earth's surface equals 10000 times the world's yearly energy need. This solar energy can on the one hand be applied passively as lighting resource and space heating in buildings. Besides this, active applications concern the heating of water or heat fluids through concentrator systems for domestic use or even in industrial processes. The Where is the angle from the vertical (solar zenith angle). When the sun is directly overhead, the air mass is 1. By passing through the earth's atmosphere, light scattering and absorption occur such that the spectrum and the intensity at the earth's surface have been altered. Combining these effects with the complication that besides a component of radiation directly from the sun also a significant part is scattered as indirect or diffuse light, other AMstandards are developed. A commonly used reference radiation distribution is the global spectrum. This global spectrum combines a direct AM1.5 part with a (= 100 mW/cm 2 ). It accords rather well to circumstances for Western European countries on a clear sunny day in summer. common to list the short-circuit current density (mA/ ) rather than the short-circuit current. Second condition is the number of photons. J SC from a solar cell is directly dependent on the light intensity. So the spectrum of the incident light is also important. Third condition is the optical properties with absorption and reflection of the solar cell. It can be controlled directly by the thickness of active layer. And last condition is the collection probability of the solar cell. It depends chiefly on the surface passivation and the minority carrier lifetime in the base.

When comparing solar cells of the same material type, the most critical material parameter is the diffusion length and surface passivation. In a cell with perfectly passivated surface and uniform generation, the equation for the short-circuit current can be approximated as:

= ( + ) (1.5)
When G is the generation rate, and L n and L p and the electron and hole diffusion lengths respectively. Although this equation makes several assumptions which are not true for the conditions encountered in most solar cells, the above equation nevertheless indicates that the short-circuit current depends strongly on the generation rate and the diffusion lengths.

Open-circuit voltage (V OC )

The open-circuit voltage, V OC , is the maximum voltage available from a solar cell, and The above equation shows that V OC depends on the saturation current of the solar cell and the light-generated current. While J SC typically has a small variation, the key effect is the saturation current, since this may vary by orders of magnitude. The saturation current, I 0 depends on recombination in the solar cell. Open-circuit voltage is then a measure of the amount of recombination in the device. The V OC can also be determined from the carrier concentration 11 :

this
= ( ∆ )∆ (1.7)
Where kT/q is the thermal voltage, N A is the doping concentration, n is the excess carrier concentration and n i is the intrinsic carrier concentration. The determination of V OC from the carrier concentration is also termed implied V OC .

Fill factor & Power conversion efficiency (PCE)

The J SC and the V OC are the maximum current and voltage respectively from a solar cell.

However, at both of these operating points, the power from the solar cell is zero. The fill factor (FF) is a parameter which, when considering the J-V curve in conjunction with V OC and J SC , determines the maximum power from a solar cell. The FF is defined as the ratio of the maximum power from the solar cell to the product of V OC and J SC . Graphically, the FF is a measure of the squareness of the J-V curve of the solar cell, so is the area of the largest rectangle inscribed in the J-V curve in Figure 1.3b.

% = × ×100 = × × ×100 (1.8)
The PCE is the most commonly used parameter to compare the performance of one solar cell to another. Efficiency is defined as the ratio of electrical power output from the solar cell to input incoming optical power from the sun. In addition to reflecting the performance of the solar cell itself, the efficiency depends on the spectrum and intensity of the incident sunlight and the temperature of the solar cell. Therefore, conditions under which efficiency is measured must be carefully controlled in order to compare the performance of one device to another. The efficiency of a solar cell is determined as the fraction of incident power which is converted to electricity and is defined as following equation.

PCE

= -exp -1 -(1.10)

In ideal case, R S become zero and R SH become infinity. To understand the effect of resistances on solar cell properties, Figure 1.4 shows the equivalent circuit and example of J-V characteristics with poor R S and R SH . 12

Introduction of Perovskite Solar Cells

While the organic-inorganic halides have been of interest since the early twentieth century 13 , the first report of perovskite-structured hybrid halide compounds was published by Weber in 1978 14,15 . He reported both CH 3 NH 3 PbX 3 (X = Cl, Br, I) and the CH 3 NH 3 SnBr 1-x I x alloy. In the subsequent decades, these materials were studied in the context of their unusual chemistry and physics [16][17][18] with the first solar cell appearing in 2009 19 .

The notable achievements in the photovoltaic applications of hybrid perovskites have been the subject of many reviews. The basics of the perovskite crystal structure are introduced and the unique dynamic behaviors of the hybrid organic-inorganic materials are presented in this chapter, which underpins their performance in photovoltaic devices that will be discussed in the later chapters. Walsh et al. is the leading group that verified the molecular motion and dynamic crystal structure of hybrid halide perovskite [20][21][22][23][24][25][26][27][28][START_REF] Leguy | The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells[END_REF] .

Perovskite

Alkali-metal lead and tin halides had been already synthesized in 1893 [START_REF] Wells | Uber die Caesium-und Kalium-Bleihalogenide[END_REF] , yet the first crystallographic studies that determined that cesium lead halides had a perovskite structure with the chemical formula CsPbX 3 (X = Cl, Br or I) were only carried out in 1957 by the Danish scientist Christian Møller [START_REF] Møller | Crystal structure and photoconductivity of caesium plumbohalides[END_REF] . He also observed that these coloured materials were photoconductive, thus suggesting that they behave as semiconductors. In 1978, Dieter Weber replaced cesium with methylammonium (MA) cations (CH 3 NH 3 + ) to generate the first threedimensional organic-inorganic hybrid perovskites [START_REF] Weber | CH 3 NH 3 PbI 3 , a Pb(ǁ)-system with Cubic Perovskite Structure[END_REF][START_REF] Weber | CH 3 NH 3 PbI 3 , a Pb(ǁ)-system with Cubic Perovskite Structure[END_REF] . The general crystal structure of these materials is shown in Fig. 1.5.

the undistorted facets of the cuboctahedral cavity. Correspondingly, molecules belonging to different planes are anti-aligned with a head-tail motif. Such an anti-ferroelectric alignment is expected from consideration of the molecular dipole-dipole interaction 21 . In the lowtemperature orthorhombic phase, the CH 3 NH 3 + sub-lattice is fully ordered (a low entropy state). The ordering may be sensitive to the material preparation and/or cooling rate into this phase, i.e. the degree of quasi-thermal equilibrium. It is possible that different ordering might be frozen into the low-temperature phase by mechanical strain or electric fields.

At 165 K, MAPI goes through a first-order phase transition from the orthorhombic to the tetragonal space group, which continuously undergoes a second-order phase transition to the cubic phase by ca. 327 K 16,[START_REF] Weller | Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352[END_REF] . As with the orthorhombic phase, this can be considered a 2× 2×2 expansion of the cubic perovskite unit cell. The molecular cations are no longer in a fixed position as in the orthorhombic phase. CH 3 NH 3 + is disordered between two non-equivalent positions in each cage. [START_REF] Wasylishen | Cation rotation in methylammonium lead halides[END_REF] With increasing temperature the tetragonal lattice parameters become more isotropic. The molecular disorder also increases to the point where a transition to a cubic phase occurs around 327 K. The transition can be seen clearly from changes in the heat capacity, 17 as well as in temperature-dependent neutron diffraction [START_REF] Weller | Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352[END_REF] . Indeed, for the bromide and chloride analogues of MAPI, pair-distribution function analysis of X-ray scattering data indicates a local structure with significant distortion of the lead halide framework at room temperature.

Hysteresis characteristics and device stability 1.2.3.1. Hysteresis

The rapid rise in performance of the cells has been accomplished essentially by minor modifications in the device structure, morphology of the films and fabrication methods, etc.

However, there are several fundamental issues despite the excellent device efficiencies. Many observations indicate that the perovskite films suffer from un-stabilized optoelectronic performance. They are hysteresis in current and voltage curves, wide distribution in performance, performance durability, difficulties in reproducing the results, etc. These issues require deeper scientific understanding and demand serious attention. Among all the above problems, hysteresis has been apparently considered as a major. It has been widely observed that perovskite solar cells show substantial discrepancy between the current density-voltage (J-V) curves measured on forward scan (from short circuit to open circuit) and backward scan (from open circuit to short circuit).

J-V hysteresis can be found in dye-sensitized solar cells (DSSCs), organic thin film solar cells (OSCs), and Si solar cells, when the voltage scan is too fast [START_REF] Naoki | Methods of measuring energy conversion efficiency in dye-sensitized solar cells[END_REF] . This hysteresis is explained by the effect of capacitive charge, including space charges and trapped charges.

When the scanning speed is faster than the release rate of traps, or faster than the space charge relaxation time, hysteresis is seen. In organic-inorganic perovskite solar cells, hysteresis behavior is much slower but more complex and anomalous. This anomalous property of the perovskite solar cells creates confusion about the actual cell performance [START_REF] Editorial | Solar cell woes[END_REF][START_REF] Editorial | Bringing solar cell efficiencies into the light[END_REF][START_REF] Editorial | Perovskite fever[END_REF] .

Hysteretic J-V curves imply that there is a clear difference in transient carrier collection at a given voltage during the forward and backward scan. It is generally known that backward scan measures higher current than the forward scan, independent of scan sequence. This confirms that carrier collection is always more efficient during the backward scan. In general, carrier collection (or current) in the device depends on carrier generation, separation, and its transport at recombination in the bulk of the layers and across different interfaces in the device. As carrier generation and separation are considered fast processes and depend only on illumination (not on the voltage scan) any difference in initial collection must be influenced by transport and/or transfer at the interfaces.

Parameters affecting hysteresis

As carrier collection depends on the conductivity of perovskite and other layered materials and the connectivity at their interfaces, hysteresis is observed to be affected by many factors that can slightly change the characteristics of the layers in perovskite device. Therefore, the diversity in device structure and the fabrication methods, and even changes in measurement conditions result in wide variation in the trends of hysteresis. As a result, the problem of hysteresis becomes too complex to be understood completely.

Device structure and process parameters

Perovskite devices of different architectures using the same perovskite but different electron and hole collecting layers show different magnitudes of hysteresis [START_REF] Kim | Control of I-V Hysteresis in CH3NH3PbI3 Perovskite Solar Cell[END_REF] . For instance, the standard planar heterojunction architecture; FTO / TiO 2 compact layer / perovskite / spiro-OMeTAD / Ag, FTO / PCBM / perovskite / spiro-OMeTAD / Ag and FTO / TiO 2 -PCBM / perovskite / spiro-OMeTAD / Ag (PCBM is [6, 6]-phenyl-C61-butyric acid methyl

In addition, temperature and light intensity can also alter the J-V hysteresis significantly.

For a planar perovskite solar cell (FTO/TiO2 compact layer/perovskite/spiro-OMeTAD/Au), as reported by Ono et al. [START_REF] Ono | Temperature-dependent hysteresis effects in perovskite-based solar cells[END_REF] , J-V hysteresis is large at low (250 K) and room temperatures (300 K) whereas the behavior becomes feeble at higher temperature (360 K). Grätzel et al. [START_REF] Meloni | Ionic polarization-induced current-voltage hysteresis in CH3NH3PbX3 perovskite solar cells[END_REF] also witnessed hysteresis of iodide based perovskite cells increasing with decrease in temperature. The other interesting fact that was noticeable here is that the reverse scan shows minor dependence on the temperature while the forward J-V curve is strongly affected by change in temperature. Even, in case of hysteresis-free perovskite cells of inverted architecture, remarkably large hysteresis comes up when the device is measured at low temperature [START_REF] Bryant | Observable hysteresis at low temperature in "Hysteresis Free" organicinorganic lead halide perovskite solar cells[END_REF] .

Like in other solar cells, photocurrent of perovskite cells increases linearly with light intensity. With increase in photocurrent, the gap between the forward and backward J-V curves increases proportionately. For a planar perovskite cell (FTO/TiO2 compact layer/CH 3 NH 3 PbI 3-x Cl x /spiro-OMeTAD/Au), as observed in our lab, the difference between the forward and backward performance increases with light intensity. However, when normalized with the photocurrent, the J-V hysteresis looks almost unchanged. For a TiO2 mesoscopic device (FTO/TiO2 dense layer/TiO2 mesoporous layer/ CH 3 NH 3 PbI 3-x Cl x /spiro-OMeTAD/Au), as reported by Grätzel et al., the shape and magnitude of J-V hysteresis remain independent of light intensity. Such independence of hysteresis on light intensity rejects direct involvement of the photo-generated carriers in hysteresis (Fig. 1.14).

Piezoresponse force microscopy (PFM) of CH 3 NH 3 PbI 3 at different poling (prebiasing in dark) conditions shows ferroelectric domains in CH 3 NH 3 PbI 3 [START_REF] Yasemin Kutes | Direct observation of ferroelectric domains in solution-processed CH3NH3PbI3 perovskite thin films[END_REF][START_REF] Kim | Ferroelectric polarization in CH3NH3PbI3 perovskite[END_REF][START_REF] Chen | Interface band structure engineering by ferroelectric polarization in perovskite solar cells[END_REF] . Such poling of the perovskite films alters the cell performance and J-V hysteresis dramatically, which is attributed to polarization of the ferroelectric domains that leads to modification of band structure at the interfaces [START_REF] Chen | Interface band structure engineering by ferroelectric polarization in perovskite solar cells[END_REF] .

It While some results evidence ferroelectric property of perovskite and support its strong effect on hysteresis some other reports contradict the presumption of hysteresis being caused by ferroelectric polarization [START_REF] Snaith | Anomalous hysteresis in perovskite solar cells[END_REF] . Fan et al. [START_REF] Fan | Ferroelectricity of CH3NH3PbI3 perovskite[END_REF] reported that perovskites are not ferroelectric at room temperature although their theoretical calculations predicted the material to be mild ferroelectric. Therefore, exhibition of ferroelectric behavior by the perovskite compound at the operating conditions of the device still remains a debate. Furthermore, the effect in the actual device consisting of thin layers can be different from the ferroelectric property of bulk perovskite. The explanation of transient effect caused by ferroelectric polarization must be consistent when all the different interfaces are included. Therefore, it is reasonable that the interface properties play equally important role in causing hysteresis.

Unbalanced carrier extraction

Although polarization by pre-biasing perovskite was found to enhance hysteresis, Jena et al. [START_REF] Jena | The interface between FTO and the TiO2 compact layer can be one of the origins to hysteresis in planar heterojunction perovskite solar cells[END_REF] discovered that hysteresis is also exhibited by a cell made of non-ferroelectric PbI2. (Fig. This result indicated that ferroelectric property couldn't be the only reason for hysteresis.

1.16)

Examination of J-V characteristics of different interfaces in simplified structures like FTO/TiO2 compact layer (CL)/Spiro-OMeTAD/Au and FTO/TiO2 CL disclosed that the interface between FTO and TiO2 can be one of the contributors to hysteresis. Besides, it strongly suggested that the interface between perovskite and TiO2 could be a major player for hysteretic J-V curves.

In devices made of perovskite, modification of the interface between perovskite and electron collecting layer apparently affects hysteresis. For instance, modification of TiO2 compact layer (CL) with C60 83 reduces hysteresis. Incorporation of Zr [START_REF] Nagaoka | Zr incorporation into TiO2 electrodes reduces hysteresis and improves performance in hybrid perovskite solar cells while increasing carrier lifetimes[END_REF] and Au nanoparticles [START_REF] Yuan | Hot-electron injection in a sandwiched TiOx-Au-TiOx structure for highperformance planar perovskite solar cells[END_REF] in the TiO2 compact layer also reduces hysteresis. In addition, no/negligible hysteresis observed in the perovskite-based cells using organic electron collecting layers [START_REF] Im | 18.1 % hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells[END_REF][START_REF] Tao | 17.6 % steady state efficiency in low temperature processed planar perovskite solar cells[END_REF] instead of TiO2 compact layer sandwiched between FTO and perovskite also supports the fact that interface properties can be crucial for hysteresis Carrier extraction depends strongly on physical and electrical contact at the interfaces.

Therefore, conductivity of the layers and their morphology and the interface connectivity play a role in causing hysteresis. Gaps at the interfaces can act as capacitors due to carrier accumulation and thereby alter carrier extraction significantly [START_REF] Cojocaru | Origin of the hysteresis in I-V curves for planar structure perovskite solar cells rationalized with a surface boundary-induced capacitance model[END_REF] . Although ferroelectrics can be involved in the above-mentioned interfacial phenomena, it is not solely responsible for hysteresis. The carrier dynamics at the interface, which might be influenced either by ferroelectric polarization or ion migration, holds responsible for causing hysteresis.

Ion migration

In halide perovskites, smaller organic cations can diffuse faster than larger organic cations and the halide anions are considered to have significantly high mobility compared to the heavy metal cation (Pb2+). The calculated activation energy for migration of halide vacancies (iodide vacancy) is significantly lower than the organic cation vacancies [START_REF] Haruyama | First-principles study of ion diffusion in perovskite solar cell sensitizers[END_REF][START_REF] Eames | Ionic transport in hybrid lead iodide perovskite solar cells[END_REF] .

Methyammmonium cation (MA+) and iodide anion (I-) are freely diffusible in MAPbI 3 at room temperature. Even, MA+I-is thermally unstable and it can be evaporated from the crystal structure at elevated temperature [START_REF] Alberti | Similar structural dynamics for the degradation of CH3NH3PbI3 in air and in vacuum[END_REF] . These ions in halide perovskite never undergo reaction with photo-generated carriers and contribute to self-organization electrostatically and geometrically in crystal structure formation. However, migration of the ions in perovskite is considered to cause carrier localization that gives rise to J-V hysteresis. More importantly, the process of ion motion can affect stability of perovskite devices through continuous change in chemical and physical properties of the perovskite under photovoltaic operation.

Such ionic migration under applied voltage to the device has been discussed in relation to generation of hysteresis. Snaith et al., based on photocurrent behavior of perovskite cell influenced by pre-biasing, proposed that under forward bias, MAPbI 3 becomes polarized due to accumulation of positive and negative space charges near electron and hole collector interfaces [START_REF] Zhang | Charge selective contacts, mobile ions and anomalous hysteresis in organic-inorganic perovskite solar cells[END_REF] . This charge accumulation is assumed to cause n-type and p-type doping at the interfaces (formation of p-i-n structure), which temporarily enhances photocurrent generation.

Accumulation of migrated ions at the interfaces can change the polarity [START_REF] Zhao | Anomalously large interface charge in polarity-switchable photovoltaic devices: an indication of mobile ions in organic-inorganic halide perovskites[END_REF] (Fig. 1.17) and the dynamic change in accumulation of these charges with the scanning voltage (changing internal field) is assumed to generate hysteresis in photocurrent of the perovskite devices [START_REF] Zhang | Charge selective contacts, mobile ions and anomalous hysteresis in organic-inorganic perovskite solar cells[END_REF][START_REF] Zhao | Anomalously large interface charge in polarity-switchable photovoltaic devices: an indication of mobile ions in organic-inorganic halide perovskites[END_REF] .

Figure 1. 17. Device and mechanism of switchable polarity in perovskite photovoltaic devices [START_REF] Zhao | Anomalously large interface charge in polarity-switchable photovoltaic devices: an indication of mobile ions in organic-inorganic halide perovskites[END_REF] Li et al. [START_REF] Li | Iodine migration and its effect on hysteresis in perovskite solar cells[END_REF]

Trap states

Among all possible causes of hysteresis, trap states are also considered to be major. Even though the recent reports strongly support the hypothesis of ion migration being responsible for hysteresis, most of the proposed models cannot rule out involvement of trap states. While some results partially agree with the assumption that trap sates must be causing hysteresis some other results certainly indicate their participation in the process. Trap states on surface and grain boundaries of perovskite have been proposed to be the origin of hysteresis [START_REF] Shao | Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells[END_REF] .

Fullerene deposition on perovskite is believed to passivate these trap states and thereby eliminate the notorious hysteresis in photocurrent (Fig. 1.18). While ion migration is being talked much in relation to J-V hysteresis in the present time, lack of more direct evidences and comprehensive models makes it hard to conclude that ion migration is the sole cause of hysteresis. In fact, according to a model proposed by Snaith et al., only ion migration cannot explain hysteresis but ion migration and interfacial trap states together explains hysteresis [START_REF] Van Reenen | Modeling anomalous hysteresis in perovskite solar cells[END_REF] . The reduction of density of either the mobile ions or the trap states can reduce hysteresis. However, there are few facts that conflict with the theory of trap states being responsible for hysteresis. One of them is the observed low trap density in perovskite [START_REF] Shi | Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals[END_REF][START_REF] Oga | Improved understanding of the electronic and energetic landscapes of perovskite solar cells: high local charge carrier mobility, reduced recombination, and extremely shallow traps[END_REF] . Relatively small trap density in perovskite is expected to result in slight/no hysteresis if trap states are the only players in hysteresis. In addition, the effective change in trap density with illumination intensity must be reflected as altered hysteresis in the perovskite cells. As trap states are better filled under light of higher intensity, hysteresis is expected to be less in the cases of higher intense illumination. On the contrary, it has been observed that hysteresis increases with light intensity, in proportion to the photocurrent.

Therefore, the presumption that trapping-detrapping of existing trap sates in perovskite causes hysteresis is not completely convincing. Further investigation is needed to find more direct and distinct evidences for active participation of trap states in creating hysteresis. Also, mechanism of generation of the trap states and their density in perovskite need to be examined further to support the models based on trap states.

Stability

In order to check the performance stability, Jena et al. measured photocurrent density of the cells (not encapsulated) operated across the load (600 Ω) with cyclic on and off of light (1 sun). [START_REF] Jena | Steady state performance, photoinduced performance degradation and their relation to transient hysteresis in perovskite solar cells[END_REF] The time for each on and off cycle was fixed to 3 min and all the devices were measured for 10 cycles, enduring almost 1 h for the complete measurement. As can be seen in Fig. 1.20, hysteresis increased and performance stability decreased with thickness of the perovskite film in the planar perovskite solar cells, indicating that hysteresis may be directly related to such performance instability. *PEDOT = PEDOT :PSS

Motivation

First motivation of the thesis is the lack of theoretical understanding on J-V hysteresis for organic-inorganic hybrid perovskite films and devices. In spite of impressive progress in terms of device performance, current knowledge on many fundamental phenomena is still incomplete. In a historical point of view, this problem might be due to the urgent need to prove that perovskite device can compete with existing technologies in the photovoltaic field.

Tremendous research efforts have thus mainly been put into the experimental works, which could finally meet such requirements. However, many theoretical questions remain unsolved.

To answer these questions, various analysis technologies are used in the thesis. In conclusion, direct experimental evidences of special characteristics for organic-inorganic hybrid perovskite films are verified. These scientific evidences can provide the ability to interpret the experimental results and even predict them based on proper physical insights. Such process can form a positive feedback for experimental design and fabrication.

Second motivation is the understanding the ageing mechanisms for overcoming the poor stability issue in the organic-inorganic hybrid perovskite solar cells. The organic devices, such as display (OLED; organic light emitting diode) and photovoltaics (OPV; organic photovoltaics) have been studied since 1990s. The ageing characteristic is the incorrigible issue for the organic materials and devices. For solving the stability issue, most academic groups still focus on the encapsulation technologies or verifying the ageing mechanisms. I already studied the innovative encapsulation technology for OPV device. [START_REF] Lee | Solution processed encapsulation for organic photovoltaic[END_REF] However, it is not the fundamental solution for solving stability issue. We need more radical understanding in this issue. In the thesis, diverse electrical and optical measurements are used for understanding the ageing mechanisms. This study makes a step forward in the quest of elucidating ageing phenomena usually observed in perovskite-based solar cells.

Thesis overview

This Thesis is titled 'Electrical transport characteristics and ageing characteristics of Perovskite thin film'. As the title implies, our foremost aims have been on development of fabrication techniques for perovskite solar cells and investigation of perovskite film characteristics for photovoltaic devices using various measurement systems. Here, a brief description of each chapter is given for overview.

Chapter 1 Introduction provides short summaries on the most basic physical backgrounds for understanding perovskite thin film and perovskite solar cells devices. The origin of the semiconducting properties in organic-inorganic hybrid materials is briefly described. The parameters affecting hysteresis, and mechanism of origin to hysteresis are described in this chapter.

We explain in Chapter 2 (Experimental Methods) the main techniques or methods that have been applied to verify electrical and optical characteristics of perovskite thin film and device. Seven main methods used for analyzing thin film are UV-Vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), ellipsometry, transmission line measurement (TLM), steady state photo-carrier grating (SSPG) and atomic force microscopy (AFM). The two main methods used for analyzing solar cells device are current densityvoltage (J-V) and glow discharge optical emission spectroscopy (GD-OES). As will be seen in the result chapters, all these methods have been complementarily used. An introduction and practical knowledge for each method is given here to understand the results obtained by using each method.

In Chapter 3 (Development of Cells Performance by Studying Film Characteristics),

we will discuss the development of cells performance with various thin film analyzing techniques. The analysis techniques have been studied for analyzing two different categories of characteristics of the perovskite thin film; electrical and optical characteristics. For verifying electrical characteristics, contact resistance and conductivity are measured by TLM system and diffusion length is estimated by SSPG measurement. With the optical characteristics, we can check the quality of crystallinity (by using XRD and SEM) and energy bandgap (by UV-Vis Spectroscopy). Furthermore, all thin film measurement systems are used for understanding the ageing characteristics of the perovskite thin films. After one and half year efforts, the PCE of perovskite solar cells has risen from about 6 % to 12.7 % with great reproducibility.

In Chapter 4 (Ionic migration in CH 3 NH 3 PbI 3-x Cl x based perovskite solar cells), direct experimental evidences of ionic migrations are verified by using GD-OES systems.

The applied voltage (from -2.5V to +2.5V) induced the perovskite thin film to generate the ionic migrations. Lead (Pb), Nitrogen (N), Iodine (I), and Chlorine (Cl) are the main atoms for studying ionic transport. We confirm that I ions in the perovskite thin film consist of mobile and fixed ions. In addition, the average length of ionic migration is estimated by the GD-OES system.

In Chapter 5 (Hysteresis characteristics in CH 3 NH 3 PbI 3-x Cl x based perovskite solar cells), the J-V hysteresis is studied not only in light but also in dark conditions for understanding the original material characteristics without considering photo-generated (excess) carriers. The initial voltage, the scan rate and the temperature are the parameters to control the hysteresis. In conclusion, we confirm the J-V hysteresis even with inverted planar structure in dark condition. In addition, we found the ionic freezing temperature by studying J-V analysis at low temperature.

Chapter 6 (Ageing Characteristics in CH 3 NH 3 PbI 3-x Cl x based Perovskite solar cells)

shows that the results of device performance are presented with the film characteristics for understanding their operation mechanism. The J-V curve and GD-OES system are used for studying device performance. TLM, XRD, and UV-Vis spectroscopy are used for studying the film characteristics. Finally, not the variation at the bulk but the significant alternations at the interface between perovskite and PCBM is proposed.

In Chapter 7 (Conclusion and Outlook), major results that are found and analyzed in the thesis are summarized with concluding remarks. Limitation encountered during the research and related suggestions for the future work will be also specified.

Chapter 2. Experimental methods

Introduction

In this chapter, descriptions of fabrication details and analysis methods will be presented.

The chapter will be separated in three parts; device fabrication, film characterization, and device characterization. In the first part, device fabrication details will be explained, from substrate/solution preparations to all layer deposition processes using different techniques.

This part will also include a description of the different materials used in this study.

In the second part, the characterization methods of perovskite film will be explained in two types, optical and electrical characteristics. The working principles of used equipment for characterizations are described in this part.

Finally, the electrical characteristic measurements of perovskite solar cells and device analysis method will be discussed in the last part.

Organic-inorganic hybrid perovskite solar cells device fabrication

Substrate and solution preparation

Substrate preparation is important to maintain reproducibility of the device performance.

According to following preparation process, indium tin oxide (ITO) substrate is prepared.

(Figure 2.1.(a)) The ITO coated glass substrates were purchased from Xinyan Technologies (inf. 20ohm/sq). The wet etching process is used to pattern the purchased ITO glass. The substrate was masked with 3M scotch tape and the ITO, uncovered with mask area was etched with acid solution of hydrochloric acid (HCl) and zinc powder (Zn). 1 The tape mask was removed after etching, and substrates were cleaned in sequential ultrasonic baths of detergent (Liqui-Nox Phosphate-Free Liquid Detergent, Alconox, Inc.) 1% diluted in deionized water, pure deionized water and 2-propanol (IPA) (30 min each). Nitrogen gas (N 2 )

was used to dry the substrates after each bath. majority of the ink is flung off the side. Airflow then dries the majority of the solvent leaving a plasticized film before the film fully dries to leave the useful molecules on the substrate.

The rotation of the substrate at high speed means that the centripetal force combined with the surface tension of the solution pulls the liquid coating into an even covering. 5 During this time the solvent then evaporates to leave the desired material on the substrate in an even covering.

In this study, the spin coating technique was used to deposit PEDOT:PSS (HTL) and PCBM (ETL) in air or N 2 condition.

Dynamic dispense (Spin casting)

In a dynamic dispense, the substrate is first started spinning and allowed to reach the desired spin speed before the solution is dispensed into the center of the substrate. The centripetal force then rapidly pulls the solution from the middle of the substrate across the entire area before it dries.

In general, a dynamic dispense is preferred as it is a more controlled process that gives better substrate to substrate variation. 6 This is because the solvent has less time to evaporate before the start of spinning and the ramp speed and dispense time is less critical (so long as the substrate has been allowed time to reach the desired rpm). A dynamic dispense also used less ink in general although this does depend upon the wetting properties of the surface.

The disadvantage of a dynamic dispense is that it becomes increasingly difficult to get complete substrate coverage when using either low spin speeds below 1000 rpm or very viscous solutions. 7 This is because there is insufficient centrifugal force to pull the liquid across the surface, and the lower rotation speed also means that there is increased chance that the ink will be dispensed before the substrate has completed a full rotation. As such, it is generally recommended using a static dispense at 500 rpm or below with either technique a possibility in a region between 500-1000 rpm. For the majority of spin coating above 1000 rpm, it is normally used a dynamic dispense as standard unless there are any special circumstances or difficulties.

For more controlled process, the perovskite film was deposited by dynamic dispense process instead of spin coating process in this study.

intensity beams by a half-mirrored device. One beam passes through a sample containing a target film onto the transparent substrate. The other beam, the reference, passes through an identical transparent substrate that used at sample. The intensities of these light beams are then measured by electronic detectors and compared. The intensity of the reference beam, which should have suffered little or no light absorption, is defined as I 0 . The UV region scanned is normally from 200 nm to 400 nm, and the visible portion is from 400 to 800 nm.

UV-Vis spectroscopy is based on the principle of electronic transition in atoms or molecules upon absorbing suitable energy from an incident light that allows electron to excite from a lower energy state to higher excited energy state. While interaction with infrared light causes molecules to undergo vibrational transitions, the shorter wavelength with higher energy radiations in the UV and visible range of the electromagnetic spectrum causes many atoms/molecules to undergo electronic transitions.

If the sample compound does not absorb light of a given wavelength, I=I 0 . However, if the sample compound absorbs light then I is less than I 0 , and this difference may be plotted on a where h is the Planck constant, is the wave frequency and c light speed in vacuum.

Experimentally, the optical band gap E opt of the thin film is estimated by linear extrapolation from the absorption feature edge to A =0 and subsequent conversion of the wavelength (nm) into energy value versus vacuum (eV). In conclusion, the E opt can be determined by absorbance spectra. 11 In this study, UV-Vis spectroscopy was used for calculating the E opt value and studying ageing characteristics of the perovskite thin film.

Scanning electron microscopy (SEM)

Scanning electron microscopes (SEM) use a beam of highly energetic electrons to examine objects on a very fine scale. In a SEM, when an electron beam strikes a sample, a large number of signals are generated. This examination can yield the information such as topography (the surface features of an object), composition (the elements and compounds that the object is composed of and the relative amounts of them) and crystallographic information (how the atoms are arranged in the object). 13 The combination of high magnification, large depth of focus, good resolution, and the ease of observation makes the SEM one of the most widely used equipment. Figure 2.7 shows the schematic illustration of SEM measurement system.

Secondary electrons (SE), corresponding to the most intense emission due to electronic impact, are produced when an incident electron excites an electron in the sample and loses some of its energy in the process. 14 The excited electron moves towards the surface of the sample and, if it still has sufficient energy, it excites the surface and is called a secondary electron (non-conductive material can be coated with a conductive material to increase the number of the secondary electrons that will be emitted with energies less than 50 eV).

Alternatively, when the electron beam strikes the sample, some of the electrons are scattered (deflected from their original path) by atoms in the specimen in an elastic fashion (no loss of energy). These essentially elastically scattered primary electrons (high-energy electrons) that rebound from the sample surface are called backscattered electrons (BSE).

The mean free path length of secondary electrons in many materials is round 10 Ǻ. Thus, although electrons are generated throughout the region excited by the incident beam, only those electrons that originate less than 10 Ǻ deep in the sample escape to be detected as secondary. This volume of production is very small compared with those associated BSE and X-rays. Therefore, the resolution using SE is better than either of these and is effectively the same as the electron beam size. The shallow depth of production of detected secondary electrons makes them ideal for examining topography. The secondary electron yield depends on many factors, and is generally higher for high atomic number targets, and at higher angles of incidence. 15 BSE can be used to generate an image in the microscope that shows the different elements present in a sample. 16 In our study, we used ellipsometry analysis for investigating the crystallinity and ageing characteristics of the perovskite thin film.

Electrical characterization

Transmission line measurement (TLM)

Transmission line measurement (TLM) is a technique used in semiconductor physics and engineering to determine the contact resistance between a metal and a semiconductor. 20 The technique involves making a series of metal-semiconductor contacts separated by various distances. (Figure 2. AFM operation is usually described three modes, according to the nature of the tip motion : contact mode, also called static mode (as opposed to the other two modes, which are called dynamic modes) ; tapping mode, also called intermittent contact, AC mode, or vibrating mode, or, after the detection mechanism, amplitude modulation AFM ; non-contact mode, or, again after the detection mechanism, frequency modulation AFM.

In this study, AFM measurement was used for identifying the surface roughness and the ageing mechanisms of the perovskite thin film.

Perovskite solar cells device characterization

Current density-voltage (J-V) characterization

The J-V characteristics of perovskite solar cells were measured in N 2 glove box using a source meter (Keithley 2635) in the dark and under illumination conditions. An AM 1.5 solar simulator SC575PV with 100mW/cm 2 was used as the light source. Key parameters of perovskite solar cells could be extracted by using the methods mentioned in chapter 1.1.2.

V OC and J SC were directly obtained from X and Y intercept of J-V characteristics, respectively.

The FF was calculated as follows :

= × × (2.

3)

Where, V max and J max refer to the values when the photovoltaic device generates maximum power. Finally, PCE could be written as :

= × = × × (2.

4)

Furthermore, based on the equivalent circuit diagram model, series resistance and shunt resistance could be calculated from J-V characteristic. In high forward voltage (V OC region)

In 1968, Werner Grimm introduced a glow discharge tube as a light source for spectroscopic analyses investigating the chemical composition of metallic materials.24,25

The so-called Grimm discharge tube is characterized by a special arrangement of the electrodes: The two electrodes of the DC current source are set up of a cylindrical hollow anode and the sample as cathode which is sealing the anode tightly. (Figure 2.12) Since then, this technique and its applications have been continuously refined, an important development relevant for our work has been the introduction of pulsed RF sources. With pulsed RF, not only non-conductive specimen and layers can be measured but also fragile and heat sensitive materials. [26] In 1968, Werner Grimm introduced a glow discharge tube as a light source for spectroscopic analyses investigating the chemical composition of metallic materials. 24,25 The so-called Grimm discharge tube is characterized by a spec. Today, GDOES is one of the most precise methods of elemental analysis and layer thickness measurement.

The Glow Discharge Source is generally filled with argon gas under low pressure (0.5 -10 hPa). As shown in figure 2.12 (which describes the simple historical configuration with dc source), a high direct voltage (DC) is applied between the anode and the sample (cathode).

Due to the DC voltage, electrons are released from the sample surface and accelerated towards the anode gaining kinetic energy. By inelastic collisions the electrons transfer their kinetic energy to argon atoms, which causes them to dissociate into argon cations and further electrons. This avalanche effect triggers an increase in the charge carrier density making the insulating argon gas conductive. The resulting mixture of neutral argon atoms and free charge carriers (argon cations and electrons) is called plasma. The argon cations are accelerated towards the sample surface because there is a high negative potential. Striking the sample surface the argon cations knock out some sample atoms. This process is referred to as sputtering. The sample surface is ablated in a plane-parallel manner.

The knocked out sample atoms diffuse into the plasma where they collide with highenergy electrons. During these collisions, energy is transferred to the sample atoms promoting them to excited energy states. Returning to the ground state, the atoms emit light with a characteristic wavelength spectrum.

Passing through the entrance slit, the emitted light reaches a concave grating where it is dispersed into its spectral components. These components are registered by the detection system. The intensity of the lines is proportional to the concentration of the corresponding element in the plasma.

In this study, GD-OES analysis was used for investigating the direct experimental evidences of ionic migration in perovskite thin film, which is one of the key factors of current voltage hysteresis and poor stability.

evaporated PSCs have been studied since March 2016. The PCE has increased from 0% to 11.1% in 6 months with bad reproducibility. The devices show different performances even though the perovskite thin films are deposited at the same time. Finally, the 1-step spincoating process was chosen in this thesis for the investigation of the perovskite solar cells.

Introduction

Inorganic-organic hybrid perovskite solar cells have attracted great attention due to its solution-processing and high performance. The hybrid perovskite solar cell was initially discovered in a liquid dye sensitized solar cells (DSSCs). 1 Miyasaka and coworkers were the first to utilize the perovskite (CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3 ) nanocrystal as absorbers in DSSC structure, achieving an efficiency of 3.8 % in 2009. 1 Later, in 2011, Park et al. got 6.5 % by optimizing the processing. 6 However, these devices showed fast degradation due to the decomposition of perovskite by liquid electrolyte. In 2012, Park and Gratzel et al. reported a solid state perovskite solar cell using the solid hole transport layer (Spiro-OMeTAD) to improve stability. 7 After that, several milestones in device performance have achieved using DSSCs structure. 6-16 However, these mesoporous devices need a high temperature sintering that could increase the processing time and cost of cell production.

It was found that Methylammonium based perovskites are characterized by large charge carrier diffusion lengths (around 100 nm for CH 3 NH 3 PbI 3 and around 1000 nm for CH 3 NH 3 PbI 3-x Cl x ). 17, 18 Further studies demonstrated that perovskites exhibit ambipolar behavior, indicating that the perovskite materials themselves can transport both electrons and holes between the cell terminals. 17 All of these results indicated that a simple planar structure was feasible. The first successful demonstration of the planar structure can be traced back to the perovskite/fullerene structure reported by Guo, 19 showing a 3.9 % efficiency. The breakthrough of the planar perovskite structure was obtained using a dual-source vapor deposition, providing dense and high-quality perovskite films that achieved 15.4 % efficiency. 20 Recently, the efficiency of the planar structure was pushed over 19 % through perovskite film morphology control and interface engineering. 12 These results showed that the planar structure could achieve similar device performance as the mesoporous structure.

The planar structure can be divided into regular (n-i-p) and inverted (p-i-n) structure depending on which selective contact is used on the bottom. The regular n-i-p structure has been extensively studied and was based on dye-sensitized solar cells; while removing the mesoporous layer the p-i-n structure is derived from the organic solar cell, and usually, several charge transport layers used in organic solar cells were successfully transferred into perovskite solar cells. 11

The p-i-n inverted planar of perovskite solar cells showed the advantages of high efficiencies, lower temperature processing and flexibility, and furthermore, negligible J-V hysteresis effects. Due to these various advantages, the only inverted planar structure of PSCs was used in our study. In this chapter, we will focus on the performance development of two kinds of PSCs (2-steps dipping, and 1-step spin coating processed PSCs), including optical and electrical characteristics of their perovskite thin films.

Solution processed perovskite solar cells (PSCs)

Dipping processed (2-steps) PSCs

Solution preparation for PSCs

In dipping processed (2-steps) PSCs, the perovskite solution and the [6,6]

Device fabrication

As explained in chapter 2, the substrates used in this study were prepared in two steps;

wet-etching patterning process and gold deposition process (figure 3.2). We did the UV ozone treatment in the substrates just before the deposition processes.

The layers excluding the electrodes were deposited layer by layer in N 2 condition by solution processes. The PEDOT:PSS used as hole transport layer (HTL) was deposited by spin-coating process. Firstly, we filtered AI 4083 PEDOT:PSS using a 0.45 µm PVDF filter.

Then we dispensed 35 µl of the filtered PEDOT:PSS solution using a pipette onto the ITO substrate spinning at 6000 rpm (so-called dynamic dispense) with total spin-time set to 40s.

Methanol was used for patterning the water based PEDOT:PSS. The perovskite deposited on top of gold was wiped for measuring current-voltage (J-V) characteristics of device. The substrate should now be placed onto a hotplate at 120°C during 20 minutes. This process creates a PEDOT:PSS film having a thickness of 50 nm. The complete drying of the PEDOT:PSS layer is important to the perovskite layer due to its poor water stability. The perovskite layer used as active layer was deposited in 2-steps dipping process. First, we used the spin-coating process for depositing the PbI2 layer on top of PEDOT:PSS layer.

After dropping 60µl by using a pipette, the thickness of the PbI 2 layer was controlled by the rate per minute (rpm) condition. As PEDOT:PSS depositing process, the perovskite layer was patterned before annealing process by DMF solvent. Lastly, we annealed the PbI 2 layer at 70℃ during 30 minutes.

For transforming the PbI 2 into the perovskite (CH 3 NH 3 PbI 3 ), methylammonium iodide The spin-coating process was used for depositing PC 60 BM layer (ETL) on top of perovskite thin film. We used a 0.45 µm PTFE filter for filtering the PCBM solution just before depositing.

We control the rpm conditions to determine the ideal thickness of PC60BM layer for performing the roles of hole blocking and electron transporting. This deposition technique is completed by an annealing process at 70℃ during 5 minutes.

The aluminum (Al) was deposited as the electrode on top of the PC 60 BM layer by thermal vacuum evaporation technique. The target thickness was 100nm, and the electrode was patterned by special mask for keeping the active area size in 0.28cm 2 . There was no encapsulation process in this study due to that most of the device characteristics can be measured in the glove box (N 2 condition). Figure 3.2c is a photo image of the full PSCs device just before measuring the device performances.

Characteristics of perovskite thin film

As we talked in the previous chapter, the dipping technique used for depositing the perovskite layer is the sensitive process. Therefore, the analysis study of perovskite thin film was essential for forwarding the cells performances and the reproducibility. In this study, we used optical (such as UV-Vis spectroscopy, SEM, XRD), and electrical (such as TLM, SSPG, J-V characteristic) techniques for analyzing the perovskite thin film.

In general, the thickness of the active layer is one of the critical factors in the photovoltaic device performance. The amount of light absorption and the electron-hole pair (EHP) are increased with thick active layer. However, we have to consider the carrier diffusion length for preventing the recombination effect. In this study, the thickness of the thin films was measured by the depth profiler system. The film has to be scratched before measuring this system. A sensitive tip scans the scratched area in contact with the surface during analysis process. The height difference between the scratched film and non-scratched film is defined as the thickness of the thin film. Figure 3.3 shows the result of the depth profiler system measurement, which shows the perovskite thickness difference depending on the IPA dip-cleaning process. This process reduces both the thickness of perovskite thin film from 300 nm to 270 nm and the surface roughness value. Considering the thin thickness of PCBM (~50 nm), which will be deposited on top of perovskite film, the roughness control technique is essential in fabricating the state of the art device. Considering the error range (~10 nm) of this measurement system, the variations by IPA dip-cleaning effect are significant and meaningful.

The light absorbance and transmittance of the perovskite thin film is studied by using the UV-vis spectroscopy system. Considering the figure 3.21 and figure 3.22, we investigated that the fast rpm and high annealing temperature induced the large grain size of perovskite thin film. However, XRD result was only changed when the grain size was controlled by annealing temperature. The link between this XRD results and J-V hysteresis will be discussed in chapter 5.

Solution deposition engineering of perovskite and electron transport layer (ETL)

From now on, the J-V performances of 1-step spin-coated (spin-casted) devices will be discussed. After checking the reproducibility limit of the 2-steps dipping processed PSCs device, the 1-step spin-coating (spin-casting) technique was studied for depositing the perovskite thin film. Figure 3.23 and table 3.4 show the J-V performances of the PSCs device, that we first fabricated by using 1-step spin-coating technique for depositing the perovskite layer. (8nm). The extra ETL layer, such as C60 and BCP layers were deposited by using the thermal evaporation process, onto the PCBM layer to avoid the short circuit. The thermal evaporation technique is less sensitive and better depositing process than the spin-coating technique onto the poor roughness film. With only adding 10nm of C60 and 8nm of BCP layers, the whole J-V performances were increased significantly. Among them, there was remarkable increment in V OC with great increment in R SH . The short circuit generated by the pinholes caused the R SH drop due to their leakage current. Therefore, preventing the short circuit by depositing C60 and BCP layer induced the R SH increment. The R SH is strongly linked with the V OC as explained in chapter 2. In conclusion, the PCE was significantly jumped from 0.3% to 10%.

There were no significant differences of J-V performance depending on the C60 thickness between 10nm to 60nm as below.

Conclusion

The optimized processes of the solution processed PSCs in two different types (2-steps dipping, and 1-step spin-casting techniques) were discussed by using various thin film analysis techniques, in this chapter. The electrical transport characteristics of the perovskite thin films were analyzed by using TLM and SSPG measurement systems. The optical characteristics of the perovskite thin film were studied by using SEM, UV-Vis spectroscopy, and XRD measurement techniques for understanding the J-V performance or optimizing the fabrication conditions.

With the 2-steps dipping process it was difficult to achieve great reproducibility by hand due to their various critical experimental conditions such as the dipping speed, the dipping angle, and the pulling speed. Achieving the great device performance or the great reliability was unavailable with poor reproducibility.

We switched the perovskite depositing technique from the 2-steps dipping process to the 1-step spin-coating (spin-casting) process for achieving the great reproducibility. The solution dropping moments (spin coating à spin casting) and the perovskite thin film patterning position were controlled for outstanding reproducibility of the device J-V performance. During the optimizing studying, we identified the seriousness of the DMF gas damage in the glove box. Consequently, the PCE of PSCs device reached around 10 % with settled reproducibility. The advanced researches were studied with this 1-step spin-casting processed PSCs for investigating the ionic migration, the J-V hysteresis, and the ageing characteristics. The detail results will be discussed in following chapters (chapter 4, 5, and 6)

Chapter 4. Ionic migration in CH 3 NH 3 PbI 3-x Cl x based perovskite solar cells using GD-OES analysis

Summary

The ionic migration of perovskite thin film is reported as a key factor for explaining the current-voltage (J-V) hysteresis and ageing characteristics. This chapter shows directly the ionic migration of halogen components (I -and Cl -) of CH 3 NH 3 PbI 3-x Cl x perovskite film under an applied bias using glow discharge optical emission spectrometry (GD-OES). Furthermore, no migration of lead and nitrogen ions has been observed. The ratio of fixed to mobile iodide ions is deduced from the evolution of the GD-OES profile lines as a function of the applied bias. The average length of iodide and chloride ion migration has been deduced from the experimental results.

Introduction

Charged ions, as well as charge carriers, are mobile under the applied electrical field in hybrid perovskite solar cells (PSCs). Although the phenomenon of the ion migration in halide-based perovskite materials has been reported over the last 30 years, 1 the corresponding ion migration in perovskite did not draw considerable attention until the broad observation of current-voltage (J-V) hysteresis problem in PSCs. The J-V hysteresis behavior on PSCs was first reported by Snaith et al. 2 and Hoke et al. 3 with mesoporous structure in 2013, and by Xiao et al. 4 with planar heterojunction structure in 2014. Various mechanisms have been proposed to explain the origins of the J-V hysteresis such as filaments, giant dielectric constant, unbalance between hole and electron mobility, trapping effect, ferroelectricity effect, and the ionic migration effect. [2][3][4][5][6] Among them, we intensively studied the ionic migration to explain the J-V hysteresis in PSCs device.

The possible mobile ions in MAPbI 3 crystal include MA + ions, Pb 2+ ions, I-ions [7][8][9] , and other impurities such as hydrogen-related impurities (H + and H -) 10 . Considering the activation energy of ion migration and the distance with its nearest neighbors, it is entirely reasonable to expect that both the MA + ions and I -ions are mobile in the MAPbI 3 films, while the Pb 2+ ions are difficult to move. 7, 11- 13 Furthermore, the I -ions are the most likely (majority) mobile ions in the MAPbI 3 . However, as the migration of MA + ions have been firmly proved, 14 more direct experimental shreds of evidence are needed to find out whether I -ions are mobile. While the I -ion migration under the operation or measurement condition of the perovskite devices at room temperature has not yet been revealed experimentally.

Here, we elucidate the mobile ions migrations in CH 3 NH 3 PbI 3-X Cl X based cells by direct measurement of glow discharge optical emission spectrometry (GD-OES).

In this study, we show experimentally the migration of ions in hybrid perovskites CH 3 NH 3 PbI 3-x Cl x based solar cells as a function of an applied bias using glow discharge optical emission spectrometry (GD-OES) (Figure 4.1a). Pulsed RF GDOES is a destructive technique, so one measurement per sample is only possible. This is why it was crucially important first to set up a stable process giving the possibility to generate a lot of samples in order to statistically validate the obtained results.

The size of the Ag top contact in our cells was slightly larger than the GD anode, therefore a direct analysis was possible. Figure 4.1a (right) shows the sample after GD measurement with the crater visible in the Ag contact.

The GDOES allows direct determination of major and trace elements. [17][18][19] The ratio of fixed ions versus mobile ions is deduced by applying electrical bias on the device. These results show directly that halogen ions (I -and Cl -) move through the device while lead and nitrogen ions are immobile. This migration of halogen ions influences the electrical characteristics of PSCs devices and may be responsible for the J-V hysteresis. 1. Table 1 shows a good reproducibility of the devices and we can thus consider that all the samples used for GD-OES experiments have the same characteristics. As shown in table 4.1, the power conversion efficiency (PCE) under 1 sun equivalent illumination is 12.6 % for the best cell (11.6 % in average) with an active area of 0.28 cm2. Figure 4.1b and table 1 represent the J-V characteristics of the best cell scanned in the forward and in the reverse directions. The hysteresis effect is small (less than 2.5 %) in our case. This is consistent with the results in the literature [20][21][22] reporting that the p-i-n architecture does not show hysteresis while the n-i-p architecture shows significant hysteresis. shows symmetry when the perovskite film was annealed at 100 ℃. There is a connection between the profile line of I and Cl, which are anions in perovskite thin film. As reported, Cl is totally removed from perovskite thin film at the end. In our study, Cl is totally removed when the perovskite thin film is annealed at 100 ℃. However, Cl remains when the annealing temperature is 80 ℃. Due to the Cl gas evaporation process, Cl atoms starts to disappear from the top surface (interface between PCBM and Perovskite). On this account, the profile line of Cl is not symmetric and it has a decisive effect on I profile line due to the both negative charges.

PSCs device characteristics

GD-OES result under applied bias

The GD-OES profile lines versus sputtering time under different applied biases (-2. without applied voltage. However, we observe the appearance of a second peaks when bias is applied on the device. The second peak is at 47 s of sputtering time under positive bias (+1.5

Iodide and chloride ion migration

V), and at 37 s of sputtering time under negative bias (-1.5 V). We attribute these second peaks to the iodide ionic migration due to the applied bias. These second peaks begin to shrink after removing the applied voltage, and disappear in 2 minutes when the device was under positive bias and in 3 minutes when the device was under negative bias. It signifies a reversibility (slow reaction) of iodide ionic migration. In addition, we observed the same phenomena in the blue solid line, GD-OES profile lines of chloride ions in the perovskite film.

(blue solid line in Fig4.15a and 4.15b, respectively) The initial peak is at 50 s of sputtering time before applied voltage. However, we observe the movement of the peaks when bias is applied on the device. The shifted peaks are at 54 s and 42 s of sputtering time under positive bias (+1.5 V) and negative bias (-1.5 V), respectively. We attribute these peaks movement to the chloride ionic migration due to the applied bias. These shifted peaks get back to initial position (sputtering time at 50 s) in 1 minute, which is shorter than that of iodide. It also means a reversibility of chloride ionic migration. These are consistent with the results in the

Conclusion

In conclusion, this GD-OES study has provided the direct experimental evidence of the ionic (I and Cl) migration in the CH 3 NH 3 PbI 3-x Cl x based perovskite films under applied bias.

We show that lead and MA ions are not migrating under the applied bias in the 2 minutes time-scale (Figure 4.16) .

(+1.5 V). Considering a short voltage scanning time (few ten seconds) in J-V measurement, the initial applied voltage is one of the critical conditions in halide ionic migration. The detailed discussion will be in chapter 5.

Based on GD-OES, this study gives a way for observing directly ionic movements in hybrid perovskite films. It makes a step forward in the quest of elucidating electrical phenomena usually observed in perovskite based solar cells like J-V hysteresis, external electric field screening or interfacial effects with electrodes.

fast processes depending only on illumination (not on the voltage scan), any difference in initial collection must be influenced by time of transport and/or time of transfer at the interfaces. Carrier collection depends on the type of conductivity existing in perovskite and of the other layers and on the connectivity at their interfaces. Therefore, the diversity in device structures and fabrication methods together with changes in measurement conditions results in a wide variation of the hysteretic behavior 4 . As a result, the issue of hysteresis becomes too complex to be understood completely. [5][6][7] The reported parameters possibly affecting hysteresis are device structure [8][9][10][11][12][13][14] , process parameters [15][16][17] , measurement and priormeasurement conditions [18][19][20][21][22][23][24][25] .

In the recent years, a lot of effort has been made to understand the cause of J-V hysteresis in PSCs and different mechanisms have been proposed. Only few approaches have been successful in reducing/eliminating hysteresis in the devices. The anomalous hysteresis in J-V characteristics of PSCs could be due to ferroelectric polarization 14,19,[25][26][27] , ion migration 13,14 , carrier dynamics at different interfaces or deep trap states in the perovskite layer 14 . Although, at present, there is no single universally accepted mechanism that can explain the phenomenon coherently, the studies done so far have certainly provided deeper insights into the topic. What makes the complexity of the problem is that several factors such as device structure (planar, and mesoporous), perovskite film characteristics, electron collecting layer properties, etc., can possibly influence the J-V curves at the same time. The lack of complete understanding and the inadequacy of direct evidences demand further investigation.

Under illumination, not only the original film characteristics but also the effect of photo-generated carriers have to be considered when analyzing the J-V curve. In this work, we focused on the J-V hysteresis under dark conditions to take into account influence of the original film characteristics only. We explain here how the halide ionic migration we reported in chapter 4 influences the electrical characteristics of PSC devices and may be responsible for the J-V hysteresis.

In conventional semiconductors like Si, Ge, GaAs, or CdTe, the electrical conductivity concerns the electron and hole populations. When describing a semiconductor in thermal equilibrium, the Fermi level (chemical potential) of each carrier type is everywhere constant throughout the entire crystal, even across a p-n junction. From this requirement for which electron and hole current densities cancel, a relation can be derived between the diffusion constant or diffusivity (D n and D p ) and the mobility ( ) of the electrons and the holes, respectively. These relations are called the Einstein relations:

= and =

Electron mobility and hole mobility depend upon temperature and dopant concentrations through lattice scattering and impurity scattering.

In the ionic crystals comprising alkali halides and the metal oxides, the electrically charged particles are ions (cations and anions) and electrons. The transport of ionic and electronic charge carriers are exposed to chemical and electrical potential gradients, which correspond to electrochemical potential gradients.

The hybrid perovskite (for instance CH 3 NH 3 PbI 3 ) may be considered as an inhomogeneous mixture of a conventional semiconductor and an ionic crystal in which the ions are essentially associated to migration of vacancies (see chapter 4). The mobility of electrons and holes is several orders of magnitude larger than that of ions. When the perovskite is working under dark conditions, the intrinsic carrier concentration of electrons and holes defined by the band gap value (1.58 eV) is smaller than that of ionic carrier defects (around 10 17 cm -3 ). The point will be detailed thereafter in section 5.3.3. Nevertheless, the hybrid perovskite may still be considered as an electronic conductor depending upon temperature. In thermal equilibrium, the electrochemical potential of each carrier type through the entire crystal of the perovskite must be kept constant even across a heterojunction. From this requirement, more complex relations can be derived between the diffusivity of the particles and the mobility of the particles. These are the Nernst-Einstein relations that intervenes in the migration of species in crystalline solids, when species are subjected to a force.

Considering the electrical force ( = , where Z i is the atomic number, e is an electronic charge, and E is the electric field), we can define the electrical mobility as velocity per unit electric field: = = , where V i is the velocity, and k is the Boltzman constant. The electrical conductivity (S i ) is defined as charge flux per unit electric field with units (S/m). It can be expressed as = , whew C i is the concentration. For ionic species, we can apply Nernst-Einstein equation.

= =

thickness. In conclusion, the perovskite thickness variation doesn't influence its electrical transport characteristics. between 450 nm (Fig. 5. 5a and 5d), 390 nm (Fig. 5. 5b and 5e), and 350 nm (Fig. 5. 5c and 5f) by rpm conditions in the spin-casting process. The J-V performance is always higher when the applied voltage is scanned in reverse direction (+1 V à -1.5 V) than when the applied voltage is scanned from forward direction (-1.5 V à +1 V). As the perovskite thickness increased, the J-V hysteresis became stronger both in light and dark conditions. We can observe the increment of open circuit voltage (V OC ) and fill factor (FF), but no difference for the short circuit current density (J SC ) (Table 5.1). Reducing the thickness of the perovskite films because of short circuit current density reduction decreases VOC. Moreover, by reducing the perovskite thickness also decreases the VOC difference between forward and reverse bias.

In dark condition, the J-V hysteresis tendency in a is more observable way than under illumination. It is because the effect of the ion migration is less important compared with the applied electric field in thinner perovskite layer for which the total number of point defects is decreased.

As shown in Fig. 5.12c, V OC_dark is fixed to zero when the voltage scanning direction is forward (initial voltage = negative; -3, -2, -1.5, -1, and -0.5V). However, V 0_dark is increased to 0.16V when the initial voltage is positive (the voltage scanning direction = backward; 1, 1.5, 2, 2.5, and 3V). This shift of V 0_dark can be explained using our model discussed above in Fig. 5. We already experimentally evidenced the migration of halide ions (iodide and chloride ions) under an applied bias in this 'hysteresis free' p-i-n structures, using glow discharge optical emission spectrometry (GD-OES). When the bias is negative, we found as expected that the mobile iodide ions move toward the PCBM side, and that when the bias is positive, the mobile iodide ions are shifted toward the PEDOT:PSS side. The influence of iodide ions migration on energy bands of the perovskite thin film device is described in fig.

5.13. Considering the intrinsic characteristics of the perovskite layer, the increase and decrease of the iodide ions' concentration close to the interfaces can be viewed equivalent to N a -depleted region and N d + depleted region, respectively. The resulting band bending within the perovskite thin film (due to ionic migration) directly impact carrier injection as well as the leakage current. We suggest this model can explain the current -voltage (J-V) hysteresis observed under dark conditions. called leakage current density versus the initial applied voltage. We choose -1 V as a reference for the leakage current density. The leakage current density is almost fixed to 0.007 mA/cm 2 when the initial voltage is positive. A light increase of the leakage current is induced by the electron created on the PEDOT:PSS side. When the initial voltage is negative, the leakage current density is this time increased to 0.012 mA/cm 2 . The tilted band energy under negative initial applied voltage induces the increment of drifted leakage current potential as

J-V hysteresis versus measuring temperature

For concluding our study of electrical transport characteristics in hybrid perovskite thin film, we analyzed current-voltage (J-V) measurement at low temperature (135 ~ 370 K) by using photovoltaic (PV) device and transmission line measurement (TLM) device. Figure 5.14a and Figure 5.14b show a PV device and a TLM device onto the sample stage for measuring J-V at low temperature. The sample stage is located at the vacuum chamber. There are 4 mobile tips for measuring J-V at various positions keeping the vacuum condition (10 - 4 ~10 -2 Torr). All the J-V performances have been measured in dark condition due to studying electrical characteristics without considering photo-generated carriers.

migration-dominated conduction is 264 K, which agrees well with the value that we speculated as the ionic freezing temperature measured by PV device (Figure 5.17) and with the following article 28 .

Conclusion

In this chapter, we studied the J-V hysteresis versus the structure variation and the J-V measurement conditions. The perovskite layer thickness and the type of cathode (Al or Ag) are considered as the parameters of the structure variation. The voltage scanning rate, the initial applied voltage, and the measuring temperature are considered as the parameters of measurement conditions for understanding the J-V hysteresis.

Especially, the dark J-V curves study together with complimentary GD-OES measurements provides a deeper understanding of the relation between halide (iodide and chloride) migration and J-V performance in CH 3 NH 3 PbI 3-x Cl x based PSCs. We verified that the halide migration is slow (1 -3 minutes) and reversible. Even with a 'quasi-hysteresis free' structure (p-i-n), we were able to evidence a J-V hysteresis under dark conditions, versus initial applied voltage, voltage scanning direction, and measuring temperature. The effect of halide migration on the J-V performance is more visible with the absence of photo-generated carriers. It is due to that the ion migration-related phenomena, including photocurrent hysteresis, such as switchable photovoltaics, photo-induced poling effect, light induced phase separation, giant dielectric constant, and self-healing, will be unlikely to occur with excess carriers. The V 0_dark value shifts only under reverse scanning direction due to the electron barrier created by the halide migrations at the interfaces. The leakage current density under forward scanning direction is always higher than that under backward scanning direction. The maximum leakage current density, is obtained at 343 K, which is consistent with the phase transition temperature. The 263 K, the transition temperature for electronic-to ion migrationdominated conduction, is the J-V hysteresis generating point in dark condition. The activation energy for ionic conduction is 0.253 eV and for electronic conduction is 0.112 eV.

On the basis of GD-OES, this study gives a framework for observing directly ionic movement in hybrid perovskite films. And the dark J-V curve study provides a step forward in the quest of elucidating a link between halide migration and J-V hysteresis performance.

This behaviour has major repercussions for understanding PSCs device performance and for the design of future architectures.

of the other layers of the solar cell stack. For instance, the organic hole transporting material (HTM) is unstable when in contact with water. This can be partially limited by proper device encapsulation [12][13][14] using buffer layers between perovskite and HTM 15 or moisture-blocking HTM 16 such as NiO X delivering in this case, up to 1,000h stability at room temperature 17 .

However, this approach increases the device complexity, and the cost of materials and processing. It is also worth to mention that most of the device stability measurements reported in literature are often done under arbitrary conditions far from the required standards 18 such as not performed under continuous light illumination 17 , measured at an undefined temperature, or leaving the device under uncontrolled light and humidity conditions 19 . This makes a proper comparison among the different strategies used challenging.

Thin film analysis of CH 3 NH 3 PbI 3-x Cl x based perovskite

In this study, optical, electrical thin film analyzing techniques and surface analyzing techniques are used for verifying the ageing mechanisms of the CH 3 NH 3 PbI 3-x Cl x based perovskite thin film. UV-Vis spectroscopy and XRD are used for investigating the energy band gap and crystallinity variation during ageing progress, respectively. The TLM is used for verifying the variation of contact resistance (R C ) and sheet resistance (R Sheet ). Finally, AFM is used for analyzing the surface variation versus ageing effects. indicates that the MAI is removed as gas and the PbI 2 remains in the perovskite thin film.

Optical thin film analysis

observed at the surface is considered as the proof of ageing effect. This indicates that the significant variation occurred at the surface of the perovskite film.

Figure 6.7 shows the SEM image of the fresh perovskite film. We can observe that the surface exhibits a dense-grained uniform morphology with grain sizes in the range 100-700 nm. The entire film is composed of a homogeneous, well-crystallized perovskite layer.

Among AFM topographies (figure 6.6a, 6.6c, and 6.6e), the observed grain size become similar to that of fresh sample measured by SEM by ageing process. The grain size observed in figure 6.6 e is around 500 -700 nm. It means that the grain bulk is well fixed during ageing process.

In summary, we concluded that the interface variation is more critical than the bulk variation in the perovskite thin film during the first 1 week of ageing. The following three points can be drawn. First, we observed that the absorbance variation is significant only at low wavelength part (between 440 nm and 520 nm) in UV-Vis spectroscopy results. Second, we observed that the contact resistance variation is more significant than the sheet resistance variation in TLM results. Third, the RMS values in AFM results show remarkable differences in first week of perovskite ageing.

the reason why we observe more dynamic variation of J SC and R S than the variation of V OC and R Sh during ageing process.

Of course, there are various process and reasons for poor stability of the PSCs.

However, the iodide ionic diffusion must be one of them. Specially, on the basis of GD-OES, this study gives a framework of observing directly the iodide ionic diffusion in hybrid perovskite film. And this study provides a step forward in the quest of elucidating a link between the interface variation and J-V ageing performance. This behavior will be the great guideline for understanding PSCs device ageing performance and for the design of future stable PSCs device.

Chapter7. Conclusion and Outlook

In this thesis, results on physics-based thin film analyses of CH 3 NH 3 PbI 3-x Clx based perovskite thin film and solar cells have been presented. The current-voltage (J-V) hysteresis and the cause for the ageing process have been investigated with multiple approaches. Here, the major findings and related conclusions are summarized and some ideas on further works are suggested.

At first, we optimized the fabricating processes of the perovskite solar cells. The perovskite thin film deposition is sensitive process and critical techniques of cells performances. Among various materials and depositing techniques, we decided to study with Finally, the power conversion efficiency was reached at 12.7 %, indicating the state of the art device considering the structure and active area (0.28 cm 2 ). Our optimized deposition technique is very simple, so even those who are new to this method can produce the perovskite solar cells with efficiencies of more than 9 %.

At second, the GDOES analysis technique was first tried for getting the direct experimental evidence of the ionic (I and Cl) migration in the CH 3 NH 3 PbI 3-X Cl X based perovskite films under applied bias. We verified that lead and MA ions are not migrating under the applied bias. We found that the ratio of fixed to mobile iodine saturates at 35 % and the average length of iodine migration is around 120 nm. In addition, we observed that the halide ionic migration is reversible and slow reaction both in positive and in negative applied bias.It takes 1 min (chloride ions) and 3 min (iodide ions) to come back from migrated position after stopping the negative voltage applied (-1.5 V). On the other hand, it takes 1 min (chloride ions) and 2 min (iodide ions) after applied positive voltage (+1.5 V). Based on GDOES, this study gives a way for observing directly ionic movements in hybrid perovskite films. It makes a step forward in the quest of elucidating electrical phenomena usually observed in perovskite based solar cells like J-V hysteresis, external electric field screening or interfacial effects with electrode.

At third, the J-V hysteresis, which is a special characteristic of the perovskite solar cells, has been studied in this work versus the structure variation and the J-V measurement conditions. The perovskite layer thickness and the type of cathode (Al or Ag) are considered as the parameters of the structure variation. The voltage scanning rate, the initial applied voltage, and the measuring temperature are considered as the parameters of measurement conditions. Especially, the dark J-V curves study together with complimentary GDOES measurements provides a deeper understanding of the relation between halide (iodide and chloride) migration and J-V performance. The effect of halide migration on the J-V performance is more visible with the absence of photo-generated carrier. The V 0_dark value shifts only under reverse scanning direction due to the electron barrier created by the halide migrations. The leakage current density under forward scanning direction is always higher than that under backward scanning direction. In addition, we studied the J-V performance at low temperature with PSCs and TLM devices for checking the J-V hysteresis versus the measuring temperature. The maximum leakage current density, is obtained at 343 K, which is consistent with the phase transition temperature. The 263 K, the transition temperature for electronic-to ion migration-dominated conduction, is the J-V hysteresis generating point in dark condition. The activation energy for ionic conduction is 0.253 eV and for electronic conduction is 0.112 eV. On the basis of the dark J-V analysis, this study provides a step forward in the quest of elucidating a link between halide migration and J-V hysteresis performance. This behavior has major repercussions for understanding PSCs device performance and for the design of future architectures. In addition, studying the effect of electron or hole affinity difference at the both interface between HTL-perovskite layer and between ETL-perovskite layer on the J-V hysteresis can be an informative topic for the future work.

Finally, we studied the ageing process of the CH 3 NH 3 PbI 3-X Cl X based perovskite thin film by studying optical (XRD, UV-Vis spectroscopy, and AFM) and electrical (TLM, J-V performance) thin film analysis techniques. And we could speculate that the perovskite interface variation is more critical than the variation in bulk. The GD-OES analysis gave the direct experimental evidence of iodide ionic diffusion toward silver electrode during ageing process. Finally, we understood the reason why we observe more dynamic variation of J SC and R S than the variation of V OC and R Sh during ageing process. Of course, there are various process and reasons for poor stability of the PSCs. The iodide ionic diffusion, which we

Chapter 3 .Chapter 5 .Chapter 7 .

 357 Development of cells performance by studying film characteristics ................................................................................................... Summary .............................................................................................................................. 3.1. Introduction ................................................................................................................... 3.2. Solution processed perovskite solar cells (PSCs) ......................................................... 3.2.1. Dipping processed (2-steps) PSCs .......................................................................... 3.2.2. Spin-casting processed (1-Step) PSCs .................................................................... 3.3. Conclusion ..................................................................................................................... Reference .............................................................................................................................. Chapter 4. Ionic migration in CH 3 NH 3 PbI 3-x Cl x based perovskite solar cells using GD-OES analysis ................................................................................... Summary .............................................................................................................................. 4.1. Introduction ................................................................................................................... 4.2. PSCs device characteristics ........................................................................................... 4.3. GDOES optimization process for perovskite thin film ............................................... 4.4. GD-OES analysis without applied bias ....................................................................... 4.5. GD-OES result under applied bias .............................................................................. 4.5.1. Iodide and chloride ion migration ........................................................................ 4.5.2. Lead and MA ion migration ................................................................................. 4.6. Conclusion ................................................................................................................... Reference ............................................................................................................................ Hysteresis characteristics in CH 3 NH 3 PbI 3-x Cl x based inverted solar cells ......................................................................................................... Summary ............................................................................................................................ 5.1. Introduction ................................................................................................................. 5.2. J-V Hysteresis depending on device structure ............................................................ 5.2.1. J-V Hysteresis versus perovskite thickness and Al as cathode ............................ 5.2.2. J-V Hysteresis versus perovskite thickness and Ag as cathode ............................ 5.3. J-V Hysteresis depending on measurement conditions ............................................... 5.3.1. J-V hysteresis versus voltage scanning rate ......................................................... 5.3.2. J-V hysteresis versus initial applied voltage ......................................................... 5.3.3. J-V hysteresis versus measuring temperature ....................................................... 5.4. Conclusion ................................................................................................................... Reference ............................................................................................................................ Chapter 6. Ageing study in CH 3 NH 3 PbI 3-x Cl x based inverted perovskite solar cells ......................................................................................................... Summary ............................................................................................................................ 6.1. Introduction ................................................................................................................. 6.2. Thin film analysis of CH 3 NH 3 PbI 3-x Cl x based perovskite .......................................... 6.2.1. Optical thin film analysis ...................................................................................... 6.2.2. Electrical thin film analysis .................................................................................. 6.2.3. Surface analysis .................................................................................................... 6.3. J-V characteristics of CH 3 NH 3 PbI 3-X Cl X based PSCs ................................................. 6.4. Ionic diffusion ............................................................................................................. 6.5. Conclusion ................................................................................................................... Reference ............................................................................................................................ Conclusion and outlook 오류! 책갈피가 정의되지 않았습니다.

Figure 1 .

 1 Figure 1.2 shows the summary of the best research cell efficiencies from different types of photovoltaic devices throughout the timeline. The first practical photovoltaic devices were demonstrated in the 1950s. Research and development of photovoltaics received its first major boost from the space industry in the 1970s which required a power supply separate from "grid" power for satellite applications. And there was the oil crisis in the 1970s to focus world attention on the desirability of alternate energy sources for terrestrial use, which in turn promoted the investigation of photovoltaics as a means of generating terrestrial power. In the 1980s research into silicon solar cells paid off and solar cells began to increase their efficiency. In 1985 silicon solar cells achieved the milestone of 20% efficiency. Over the next decade, the photovoltaic industry experienced steady growth rates of between 15% and 20%, largely promoted by the remote power supply market. Furthermore, researchers began studying the various solar cells such as dye-sensitized cell in 1991, organic photovoltaics devices (OPV) in 2001, and quantum dot cells in 2010.

  occurs at zero current. The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated current. The open-circuit voltage is shown in figure 1.3.b. The equation for V OC is found by setting the net current equal to zero in the solar cell equation to give:

4 .Figure 1 . 4 .

 414 Figure 1. 4. J-V curve showing 2 different resistances

Figure 1 .

 1 Figure 1. 16. J-V characteristics of (a) planar heterojunction PbI2 and (b) CH 3 NH 3 PbI 3-X Cl X perovskite solar cells.[START_REF] Jena | The interface between FTO and the TiO2 compact layer can be one of the origins to hysteresis in planar heterojunction perovskite solar cells[END_REF]

  have recently confirmed iodide migration to positive electrode, leaving iodide vacancies at the negative electrode through DC dependent electroabsorption (EA) spectra, temperature dependent electrical measurement and XPS characterization. According to the authors, accumulation of iodide ions at one interface and the corresponding vacancies at the other creates barriers for carrier extraction. Modulation of such interfacial barriers at CH 3 NH 3 PbI 3-x Cl x / Spiro-OMeTAD and TiO2/ CH 3 NH 3 PbI 3-x Cl x , caused by the migration of iodide ions/interstitials driven by an external electrical bias leads to J-V hysteresis in planar (FTO/TiO2 CL/ CH 3 NH 3 PbI 3-x Cl x /Spiro-OMeTAD/Au) perovskite solar cells. Based on temperature dependence of hysteretic change in current density, Grätzel et al. 70 estimated activation energy for diffusion of different ions in MAPbI3 and found that the iodide ions have the highest mobility with lowest activation energy. Hence, it is the halide anions (I-) not the MA+ ions which migrate more easily in perovskite, causing polarization and charge accumulation at interfaces under voltage scans and eventually creates hysteresis 70 .

Figure 1 .

 1 Figure 1. 18. (a) device structure and (b) forward and backward J-V curves of perovskite cells without and with PCBM (thermally annealed for 15 and 45min)[START_REF] Shao | Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells[END_REF]

Figure 1 . 19 .

 119 Figure 1. 19. Schematics showing shallow and deep trap states, and their role in causing hysteresis.[START_REF] Li | Hystersis mechanism in perovskite photovoltaic devices and its potential application for multi-bit memory devices[END_REF]

Figure 2 .

 2 1(b) shows the ITO substrate after etching process.

  graph versus wavelength. Absorption may be presented as transmittance (T=I/I 0 ) or absorbance (A=logI 0 /I). If no absorption has occurred, T=1.0 and A=0. Most spectrometers display absorbance on the vertical axis, and the commonly observed range is from 0 (100% transmittance) to 2 (1% transmittance). The wavelength of maximum absorbance is a characteristic value, designated as λ . The optical band gap (E opt ), expressed in electronvolt, depends on the incident photon wavelength by means of the Planck relation

2 )

 2 All elements have different sized nuclei and as the size of the atom the light incident upon the sample may be decomposed into s and p component (the scomponent of the electric field is oscillating parallel to the sample surface and vertical to plane of incidence, the p-component is of the electric field oscillating parallel to the plane of incidence). The reflection coefficients of the s and p component, after reflection are denoted by Rs and Rp. The fundamental equation of ellipsometry is then written: Thus, tanΨ is the amplitude ratio upon reflection, and Δ is the phase shift. Since ellipsometry is measuring the ratio of two values (rather than the absolute value of either), it is very robust, accurate (can achieve angstrom resolution) and reproducible. For instance, it is insensitive to scatter and fluctuations, and requires no standard or calibration. Experimental, the advantages of ellipsometry measurement are -Non-destructive and non-contact technique -No sample preparation -Solid and liquid samples -Fast thin film thickness mapping -Single and multi layer samples -Accurate measurement of ultra-thin films of thickness < 10nm

9 )

 9 Probes are applied to a pair of contacts and the resistance is measured by applying a voltage across these contacts and measuring the resulting current. The current flows from the first probe into the metal contact, across the metal-semiconductor junction, through the sheet of semiconductor, across the metal-semiconductor junction again (except probe lithography and local stimulation of cells. Simultaneous with the acquisition of topographical images, other properties of the sample can be measured locally and displayed as an image, ofen with similarly high resolution. Examples of such properties are mechanical properties like stiffness or adhesion strength and electrical properties such as conductivity or surface potential. In fact, the majority of SPM techniques are extensions of AFM that use this modality.

Figure 3 . 2 .

 32 Figure 3. 2. The photo images of the perovskite dipping-process (a) dipping in MAI solution, (b) Just after dipping process, (c) full device after electrode deposition, and (d) annealing process

(

  MAI) has to be penetrated into PbI 2 film during dipping process in MAI solution. First, we dipped the PbI 2 film into isopropyl alcohol (IPA) solvent during 10-15 seconds for cleaning.Thereafter, we made progress the dipping process in MAI solution during 30-40 seconds as shown in figure3.2a. We could see the change of the color of the film from yellow to dark brown upon dipping time. The IPA dip-cleaning progressed again during 10-15 seconds, immediately after the MAI dipping process. Finally, we did the spin-coating process in 4000 rpm during 30 seconds and annealing at 70℃ during 45 minutes for removing IPA solvent remaining in the perovskite film. The dipping technique is a sensitive process. There are numerous experimental conditions effecting the crystallinity of the perovskite thin films such as the dipping angle, dipping speed, pulling speed of dipped samples. However, we decided to study the dipping process due to the high performance reported among the solution processed PSCs at that time.

Figure 3 . 5 and

 35 Figure 3.6 show the light absorbance and transmittance characteristics of PbI 2 and perovskite film in the range of 300 to 800 nm wavelengths, respectively. First, the PEDOT:PSS layer, well-known as transparent hole transport layer (HTL), has zero absorption in a range of 300 to 800 nm wavelengths (red solid line in figure3.5 and 3.6). As depicted in figure3.5, the slower PbI 2 solution spun, the more light were absorbed due to the thicker thickness. However, the energy band-gaps of the PbI 2 films were fixed around 2 eV in spite of different thickness. In general, the energy band gap of thin film can be defined by the maximum wavelength of absorption range.

Figure 3 . 5 .

 35 Figure 3. 5. The (a) transmittance, and (b) absorbance graphs of the PbI 2 film in different thickness controlled by rpm conditions.

Figure 3 . 3 . 7 .

 337 Figure 3.6 shows the absorbance and transmittance characteristics of perovskite thin films depending on different dipping times in MAI solution. The absorbance of the film advanced by increasing the dipping time until 50 seconds and dropped over than 50 seconds. The crystallinity of the perovskite thin film collapsed due to the MAI solution damage from overtime-dipping process. We can confirm the damage by SEM measurement as shown in figure 3.7. The perovskite thin film dipped in ideal time has outstanding crystalline quality with 100 nm of grain size (figure 3.7a). However, the film crystallinity collapsed immediately over than 50 seconds dipping time (figure 3.6b).

Figure 3 .

 3 figure 3.23a, the color of perovskite thin film was completely dark brown with nice uniformity comparing to the photos in figure3.12 (the perovskite thin film deposited by using 2-step dipping technique).

Figure 4 .

 4 Figure 4.1a shows the inverted (or p-i-n) CH 3 NH 3 PbI 3-x Cl x based planar structure PSCs devices used in this work (ITO = anode, Ag = cathode). The photovoltaic performance of a series of 10 samples is reported in table1. Table1shows a good reproducibility of the

Figure 4 . 1 .

 41 Figure 4. 1. (a) Detailed scheme of the perovskite planar solar cell architecture facing the plasma of GD-OES, (b) Measured J-V curves of the best CH 3 NH 3 PbI 3-x Cl x solar cell under 1 sun illumination scanned in the forward (blue line) and reverse (red line) directions, (c) pictures of the perovskite solar cells before and after GD-OES measurement with GD crater visible.

  5 V to +2.5 V vs. anode) are shown in figure 4.7 and figure 4.8 for iodide, chloride, lead and nitrogen ions. For clarity, the intensity values in figure 4.8 are shifted vertically as compared to intensity values in figure 4.5 in order to highlight the ionic migration. In the GD-OES line profiles, the initial sputtering time (30 s) corresponds to the PCBM-perovskite interface and the final sputtering time (60 s) to the PEDOT:PSS-perovskite interface. As shown in figure 4.7 and figure 4.8, two different behaviors are obtained: on one hand, the iodide and chloride ions (Figure 4.8a and 4.8b respectively) move according to the sign of the voltage and on the other hand lead and nitrogen do not move at all (Figure 4.8c and 4.8d respectively).

Figures 4 .

 4 Figures 4.7aand 4.8a display GD-OES profile lines of iodide ion versus sputtering time

  Figures 4.7aand 4.8a display GD-OES profile lines of iodide ion versus sputtering time for different biases showing directly the iodide ion movements. For a negative bias, I -ions move towards PCBM layer and for the positive bias I -ions move towards PEDOT:PSS layer.

Figure 4 .

 4 Figure 4.15 shows GD-OES profile lines of iodide and chloride ions versus sputtering time for different recovery time (from 1 to 3 minutes after stopping the applied voltage) showing directly the reversibility of ionic migration. By considering the plasma etching direction (from silver to ITO), the shorter sputtering time is silver cathode side and the longer sputtering time is the ITO anode side. For generating the ionic migration, 1.5 V or -1.5 V were applied in the PSCs device during 30 seconds (Figure 4.15). In comparison with the GD-OES profile lines measured before applying bias, both iodide and chloride ions are shifted towards the PEDOT:PSS side (longer sputtering time) under the positive bias and both iodide and chloride ions are shifted toward the silver cathode side (shorter sputtering time) under the negative bias. These results confirm that the iodide and chloride ions are negatively charged species. The GD-OES profile lines of iodide ions in the perovskite film (red solid lines in Fig 4.15a and 4.15b) show only one peak (sputtering time around 40 s)

Figure 5 . 4 .

 54 Figure 5. 4. Transmission line measurement (TLM) versus perovskite thickness on top of glass; (a) 450 nm, (b) 390 nm, and (c) 350 nm

Fig. 5 .

 5 Fig. 5.13b, while the slope generated under positive initial applied voltage induces the decrement of leakage current potential as Fig. 5.13d. Both V 0 and leakage current density versus initial applied voltage represent an interpretation of the energy band model considering the halide ionic migration.

Fig. 6 .

 6 Fig.6.1 represents the photos of the perovskite sample showing the color variation during ageing progress. The sample structure is glass/ITO/PEDOT:PSS/perovskite for confirming the identical conditions of bottom layers as PSCs device. The sample is stored in the glove box (N 2 condition) without illumination at room temperature. It is for removing the ageing effects due to thermal, water, moisture and illumination. During the 1 st week, the color of the perovskite thin film fixed in dark brown as initial color. However, the film color becomes light brown between the 2 nd week and the 4 th week. Finally, it takes over 6 weeks to change color to full yellow. As shown in Figure6.1 for the 5 th week aged sample, the perovskite thin film turns yellow from the edge of the sample due to the exposed area difference to air. The color variation of the perovskite thin film (dark brown à yellow)

CH 3 NH 3

 33 PbI 3-x Clx based perovskite thin film deposited by 1-step spin-casting process. The PEDOT:PSS and PC 60 BM layers are deposited by spin-coating process as HTL and ETL, respectively. The electrical transport characteristics (conductivity, resistivity, contact resistance, and carrier diffusion length) of the film are measured by TLM and SSPG techniques. The crystallinity of the film is optimized with the results of SEM and XRD.

  Real space carrier distribution measurement with the help of Kelvin Probe Force Microscopy (KPFM) also shows unbalanced hole and electron extraction rate in perovskite devices using TiO2 and spiro-OMeTAD as electron collector and HTM, respectively[START_REF] Tao | 17.6 % steady state efficiency in low temperature processed planar perovskite solar cells[END_REF] . It is known that structural defects and/or mismatching at any heterogeneous interface can develop a potential barrier for carrier extraction and thus, results in accumulation of these carries at the interfaces. Therefore, such interfacial defects lead to unbalanced carrier extraction.

. Unbalanced carrier extraction (hole extraction rate ≠ electron extraction rate) caused by imperfect matching of the properties of layers is believed to result in J-V hysteresis. Heo et al.

[START_REF] Im | 18.1 % hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells[END_REF]

found that perovskite cells of inverted architecture using PCBM as electron collector do not show hysteresis. PCBM being more conductive (0.16 mS/cm) than the widely used TiO2 (6 × 10-6 mS/cm) collects/separates the electron more efficiently from perovskite (CH 3 NH 3 PbI 3 ), resulting in balanced carrier extraction (hole extraction rate = electron extraction rate), which in consequence, eliminated hysteresis.

Table 1 . 1 .

 11 Several representative devices performances of inverted planar structured PSCs

	Perovskite Processing	HTL	ETL	V OC (V)	J SC (mA/cm 2 )	FF (%)	PCE (%)	Stability	#
	One-Step	PEDOT	PC 61 BM/BCP 0.60	10.32	63	3.9	-
	Two-Step	PEDOT	PC 61 BM	0.91	10.8	76	7.4	-
	One-Step (Cl)	PEDOT	PC 61 BM	0.87	18.5	72	11.5	-
	One-Step (Cl)	PEDOT	PC 61 BM/TiO X 0.94	15.8	66	9.8	-
	Solvent Engineering	PEDOT	PC 61 BM/LiF	0.87	20.7	78.3	14.1	-
	One-Step (Moisture, Cl)	PEDOT	PC 61 BM/PFN 1.05	20.3	80.2	17.1	-
	One-Step
	(Hot-casting,	PEDOT	PC 61 BM	0.94	22.4	83	17.4	-
	Cl)
	One-Step (HI additive)	PEDOT	PC 61 BM	1.1	20.9	79	18.2	-
	Co-Evap	PEDOT/Poly-TPD	PC 61 BM	1.05	16.12	67	12.04	-
	Co-Evap	PEDOT /PCDTBT	PC 61 BM/LiF	1.05	21.9	72	16.5	-
	Two-Step Spin-Coating	PTAA	PC 61 BM/ C 60 /BCP	1.07	22.0	76.8	18.1	-
	One-Step Solvent	PEDOT	C 60	0.92	21.07	80	15.44	-
	One-Step(Cl)	PEDOT	PC 61 BM/ZnO 0.97	20.5	80.1	15.9	140 h
	One-Step(Cl)	PEDOT	PC 61 BM/ZnO 1.02	22.0	74.2	16.8	60 days
	One-Step	NiO X	PC 61 BM/BCP 0.92	12.43	68	7.8	-
	One-Step	NiO X	PC 61 BM/C 60	1.11	19.01	73	15.4	244 h
	Solvent Engineering	NiO X	PC 61 BM/LiF	1.06	20.2	81.3	17.3	-
	Two-Step	NiO X	ZnO	1.01	21.0	76	16.1	> 60days
	Solvent Engineering	NiLiMgO	PC 61 BM /TiO 2 :Nb	1.07	20.62	74.8	16.2	1000 h (Sealed)

  1,2 Its owns ABX 3 crystal structures, where A,B, and X are organic cation, metal cation, and halide anion, respectively. The bandgap can be tuned from the ultraviolet to infrared region through varying these components.

[2][3][4]

This family of materials exhibits a myriad of properties ideal for PV such as high dual electron and hole mobility, large absorption coefficients resulting from s-p antibonding coupling, a favorable band gap, a strong defect tolerance and shallow point defects, benign grain boundary recombination effects and reduced surface recombination.

5

After 7 years efforts, the power conversion efficiency (PCE) of perovskite solar cells has risen from about 3% to 22%. 1,6-16

  The PC 60 BM was used as electron transport layer (ETL). The highest occupied molecular orbital (HOMO) level of PC 60 BM (-6.3eV) is enough to block the hole generated in the perovskite film (the valence band of perovskite is at -5.4eV). PC 60 BM has same lowest unoccupied molecular orbital (LUMO) level as perovskite film (-3.9eV). It makes that electron can pass through well from perovskite to Al electrode (-4.2eV). Therefore, PC 60 BM is suitable material as ETL. The PC 60 BM is diluted in Chlorobenzene (CB) in 2wt% and then it was stored in glove box (N 2 condition) at room temperature.

	-phenyl-C60-
	butyric acid methyl ester (PC 60 BM) solution have to be prepared one day before the
	deposition process for dissolving sufficiently. First, the CH 3 NH 3 PbI 3 film was used as the
	active layer in perovskite solar cells. PbI 2 solution and CH 3 NH 3 I solution have to be
	dissolved at least 16 hours before deposition process. For PbI 2 solution, PbI 2 was diluted in
	N,N-Dimethylformamide (DMF) solvent in 33wt%. It was stored in N 2 condition with
	annealing at 80°C. For CH 3 NH

3 I (MAI) solution, MAI was diluted in 2-propanol (IPA) solvent in 10mg/ml. The solution was stored in glove box at room temperature.

Table 4 . 1 .

 41 Photovoltaic performance of perovskite solar cells used in this study.

		J SC (mA/cm 2 )	V OC (V)	FF (%)	PCE (%)
	Average performance (10 samples)	19.9 (±2)	0.92 (±0.02) 64.2 (±4)	11.7 (±0.8)
	Best performance
	Forward scan	19.9	0.92	67.0	12.3
	Reverse scan	20.2	0.93	67.0	12.6

Table 4 . 2 .

 42 The performance parameters of the perovskite solar cells fabricated for direct measurement of ion migration using GD-OES analysis. Figure 4.6 is the GD-OES results of I and Cl in the perovskite thin film without applied voltage. As shown in figure 4.6a, the initial I profile line (before applied voltage) is changed depending on annealing temperature of perovskite thin film. It is not symmetric (little shifted toward PCBM) when the perovskite film was annealed at 80 ℃ in N 2 conditions. However, it

	Device (#)	V OC (V)	J SC (mA/cm 2 )	FF (%)	PCE (%)
	1	0.94	20.2	60	11.5
	2	0.92	19.8	63	11.5
	3	0.91	20.0	63	11.5
	4	0.92	19.1	67	11.7
	5	0.92	19.9	64	11.7
	6	0.91	18.7	67	11.3
	7	0.93	18.4	67	11.3
	8	0.94	21.9	62	12.6
	9	0.91	20.8	62	11.6
	10	0.93	20.2	67	12.6
	Average	0.92	19.9	64.2	11.7

4.5.2. Lead and MA ion migration

	GD-OES profile lines of Pb and N show no ionic migration under an applied bias (Figure
	4.7c, 4.7d, 4.8c, and 4.8d). These results are in accordance with the fact that the migration
	activation energies of Pb (2.31 eV) and MA (0.84 eV) ions vacancies are higher than the
	value of I ions vacancies (0.58 eV) as reported by Eames et al. 7

Reference ..............................................................................................................................

Acknowledgements

Chapter 5. Hysteresis characteristics in CH 3 NH 3 PbI 3-x Cl x based inverted solar cells

Summary

The current voltage (J-V) hysteresis observed in the perovskite solar cells (PSCs) is reported as a key issue. This chapter shows the J-V hysteresis versus structure variation (thickness of perovskite layer and type of cathode) and versus measurement conditions (scanning rate, initially applied voltage, and measuring temperature). Not only the J-V curves under illumination but also the J-V curves in dark condition were analyzed for understanding the ionic migration effect on J-V performance. It's because that we need to consider the J-V hysteresis without photo-generated excess carriers.

Introduction

The continuous and skyrocketing rise in power conversion efficiency (PCE) of hybrid organic-inorganic perovskite materials (HOIPs) based solar cells [1][2][3] has attracted enormous attention among the photovoltaics community. These materials have become an utmost interest to all working on photovoltaic technologies because of its high absorption coefficient and long range carrier diffusion with minimal recombination, which are the main factors usually used to explain the large current density, high open circuit voltage, and thus high PCE of perovskite solar cells (PSCs). However, there are several unusual issues necessary to tackle in order to further improve their efficiency. Among them are the hysteresis observed in current-voltage curves, side distribution in performance, difficulties in reproducing the results, etc. These issues require deeper scientific understanding and demand serious attention. Among all the above issues, hysteresis has been considered by the community as crucial. It has been widely observed that perovskite solar cells show substantial mismatch between the current density -voltage curve (J-V curve) measured on forward scan (from negative to positive bias) and backward scan (from positive to negative bias). Hysteretic J-V curves imply that there is a clear difference in transient carrier collection at a given voltage whether it is measured during a forward or a backward scan. In general, carrier collection in the device depends on carrier generation, separation, and its transport from the bulk across the different interfaces of the device. As carrier generation and separation are considered as We must notice that the temperature dependence of diffusivity shows two distinct regions: extrinsic region (low temperature) and intrinsic region (high temperature). For the same reasons, temperature dependence of ionic conductivity also exhibit intrinsic and extrinsic regions. Now, we can develop a theory for ionic conduction using Boltzmann statistics ( = -

, where is the accommodation coefficient, is the vibrational frequency of the ions, and E a can be considered as activation energy or free energy change per atom.)

and see whether we arrive at the same formalism as the ionic conductivity versus diffusivity.

We can write an expression for conductivity as

This expression has similar form, which was shown above by using diffusivity and mobility, and it shows the similar dependence on temperature as well as activation energy for migration. In conclusion, the activation energy of ionic conductivity can quantitatively characterize the rate of ion migration, which can be extracted from the temperaturedependent electrical conductivity by the Nernst-Einstein relation. UV exposed ITO was used as the anode. Al or Ag was deposited as the cathode on top of PCBM layer. Two square (dashed line) represents the parameters of the device structure to control the J-V hysteresis; the thickness of the active layer (perovskite thin film) and cathode (Al / Ag). The thickness of the active layer was controlled by rpm conditions in the spincasting process and the cathode layer was deposited by thermal vacuum evaporation process.

J-V Hysteresis depending on device structure

J-V Hysteresis depending on measurement conditions

J-V hysteresis versus voltage scanning rate

The measurement condition for studying J-V hysteresis is the voltage scanning rate in the range between 25 mV/s and 25,000 mV/s. We already discussed reversibility of the halide ionic migration and its recovering time (around 2 -3 minute) by using GD-OES analysis as figure 4.15. This recovering time is longer than the voltage scanning time in J-V curve measurement (less than 1 minute). Therefore the halide ionic migration can be sustained during J-V measurement. And the amount of halide ionic migration can be changed by the voltage scanning rate. Fig. 5.7 and Fig. 5.8 show the J-V performances under illumination versus voltage scanning rate. As the scanning rate increased, the J-V hysteresis became stronger. It is due to that the ionic migration, induced by initial applied voltage, is kept well when the voltage-scanning rate is high. The short circuit current (J SC ) in reverse bias measurement is higher than that in forward bias measurement in fast voltage scanning rate.

As the scanning rate is reduced, not only J SC but also V OC , FF, and PCE approach a similar value (0.912 V, 65%, and 11.4%), indicating that the J-V hysteresis becomes weaker.

When the voltage scanning direction is forward, the V OC value is constant at 0.913 V in the whole range of scanning rates (25 -25,000 mV/s). However, the V OC value increased as a function of voltage scanning rate decrement in reverse voltage scanning direction. Both in forward and reverse scanning direction, fill factor (FF) is increased by decreasing the voltagescanning rate.

Chapter 6. Ageing study in CH 3 NH 3 PbI 3-x Cl x based inverted perovskite solar cells

Summary

The poor stability is reported as one of the key issues in the perovskite solar cells (PSCs). In this study, we used various thin film analysis techniques for investigating the ageing process of the CH 3 NH 3 PbI 3-x Cl x based perovskite thin film such as optical thin film analysis (UV-Vis spectroscopy and X-ray diffraction (XRD)), electrical thin film analysis (transmission line measurement (TLM)), surface thin film analysis (atomic force microscopy (AFM)). We investigated that the surface variation is more critical than the bulk variation during ageing process. The direct experimental evidence of iodide ionic diffusion toward silver electrode during ageing process is observed by studying the glow discharge optical emission spectroscopy (GD-OES) analysis.

Introduction

With power conversion efficiencies (PCE) beyond 22%, organic-lead-halide perovskite solar cells (PSCs) are stimulating the photovoltaic research scene. However, despite the big excitement, the unacceptably low-device stability under operative conditions currently represents an apparently unbearable barrier for their market uptake. [1][2][3][4][5] Notably, a marketable product requires a warranty for 20-25 years with <10% drop in performance. This corresponds, on standard accelerated ageing tests, to having <10% drop in PCE for at least 1,000h. Hybrid perovskite solar cells are still struggling to reach this goal. Perovskite are sensitive to water and moisture, ultraviolet light and thermal stress. [6][7][8] When exposed to moisture, the perovskite structure tend to hydrolyse, 6 undergoing irreversible degradation and decomposing back into the precursors, for example, the highly hygroscopic CH 3 NH 3 X and CH(NH 2 ) 2 X salts and PbX 2 , with X=halide, a process that can be dramatically accelerated by heat, electric field and ultraviolet exposure. 7,8 Material instability can be controlled to a certain extent using cross-linking additives 9 or by compositional engineering 10 , that is, adding a combination of Pb(CH 3 CO 2 ) 2 •3H 2 O and PbCl 2 in the precursors 11 or using cation cascade, including Cs and Rb cations, as recently demonstrated, 2,3 to reduce the material photoinstability and/or optimize the film morphology. However, solar cell degradation is not only due to the poor stability of the perovskite layers, but can be also accelerated by the instability figure 3.19 (chapter 3). Though the contact resistance is high, we get great reproducibility of TLM results. After 3 days, the contact resistance is increased to 6E+10 Ω, which is around 130 times higher than the initial value. The resistivity is 3.4 E+6 Ω•cm after 3 days. This value is 3 times higher than the initial resistivity. We can conclude that the variation of contact resistance is more significant than the variation of resistivity. The ageing effect of the perovskite thin film is more critical from surface than from bulk of the perovskite thin film.

These TLM results are in agreement with the results that we speculated with the UV-Vis spectroscopy analysis (figure 6.2).

However, after 3 days, the TLM method is no more available due to the too low current resulting from the ageing process. The total resistance becomes over than 5E+12 Ω and linear increment versus electrode length is no longer observed.

Surface analysis

As we already discussed with the UV-Vis spectroscopy results (figure 6.2) and TLM analysis (figure 6.5), we observed that the ageing effect is more critical from surface than from bulk of the perovskite thin film. In this paragraph, we used AFM and SEM analysis for investigating the surface variation during ageing process. For preparing the AFM and SEM samples, the CH 3 NH 3 PbI 3-x Clx based perovskite thin film is deposited onto the glass/ITO/PEDOT:PSS by using spin-casting process in the glove box (N 2 condition). We tried to fabricate the perovskite thin film at the same conditions as the PSCs. The AFM and SEM samples are stored in the N 2 condition and in dark. found, must be one of them. This behavior will be the great guideline for understanding PSCs device ageing performance and for the design of future stable PSCs device.