Antonio Luna Arriaga

Francis Bony

Thierry Bosch

Speckle-insensitive fringe detection method based on Hilbert transform for self-mixing interferometry

A robust phase measurement method to detect self-mixing signals from rough surface targets is addressed. Using the Hilbert transform to create the analytic signal of the monitored optical output power, an accurate instantaneous phase can be extracted without being affected by amplitude variations induced by speckle phenomenon. In case of fringe fading, a running average filter removes the local bias offset improving the calculations. This also allows to observe the phase at dynamic feedback levels within the same calculation, avoiding the need of adding external optical, mechanical or electronic components. This method provides confidence for fringe detection required in subsequent steps to reach sub-wavelength precision of a reconstructed displacement or for velocimetry applications.

Introduction

Self-mixing (SM) interferometry is nowadays an established instrumentation scheme mainly used to measure displacement, absolute distance and velocity of a remote target. The optical output power variations (OOP) obtained from this phenomenon notably related to the amount of backscattered light by the feedback coupling factor C, confer a supplemental challenge on the exploitation of these signals as compared to classical interferometers. When a laser diode (LD) is used as source, the poor signal stability caused by the inherent sensitivity of the emitted beam to the back-reflected light has been a limiting factor to spread this technique to perform in-situ accurate measurements. Even minor changes on the requirements specification usually imply major changes and often a complete redesign of this kind of instrumentation scheme [START_REF] Donati | Developing self-mixing interferometry for instrumentation and measurements[END_REF].

Speckle pattern statistics increase the complexity of exploitation by inducing random amplitude modulation to the already non-linear frequency modulated signal. As the laser spot has finite size, SM signals in this condition exhibit an envelope shape in time domain due to the fading which results of the incoherent superposition of different waveforms reflected by rough targets. Since each fringe contributes to half LD's wavelength of displacement amplitude, reconstruction of a target movement from SM signals relies on the proper extraction of interferometric features from an acquired signal. If fringes are lost during a measurement, the reconstruction will present proportional inaccuracies regardless the signal processing employed.

To overcome speckle effects, [START_REF] Norgia | Interferometric measurements of displacement on a diffusing target by a speckle tracking technique[END_REF] proposed a tracking technique which adjusts the spot on the target by means of piezo-actuators when the received signal amplitude decreases. In a less mechanical context, [START_REF] Giuliani | Selfmixing laser diode vibrometer[END_REF] proposed to use an electronic feedback loop as well as an optical arrangement, allowing to lock the signal measurement to the half of an interferometric fringe. However, the external arrangements used in these solutions dismiss the fully integrated nature of this sensing technique.

With the intention to relax this constraint, [START_REF] Zabit | Adaptive self-mixing vibrometer based on a liquid lens[END_REF] introduced the use of an adaptive liquid lens to adjust the beam spot preventing the LD to leave a moderate feedback regime. It is well known in SM literature that this operational mode of SM signals exhibits a characteristic sawtooth-like shape which is easier to exploit algorithmically. However, not all surfaces can provide the necessary amount of backscattered light for this working condition, limiting the scope of its applications. A liquid lens was also applied in [START_REF] Atashkhooei | Dealing with speckle effects in self-mixing interferometry measurements[END_REF], where signal fading due to speckle was avoided via dynamic focal length variations. Despite the satisfactory impact of this modern component, the reported response time of 80 ms is not suitable for real-time applications involving large speeds. [START_REF] Atashkhooei | Dealing with speckle effects in self-mixing interferometry measurements[END_REF], also proposed to use two LD's with different wavelength and spot size pointing the same target to provide different speckle patterns. Their algorithm alternates signal processing when signal-to-noise ratio decreases in one channel, however this still needs to be further detailed for a realtime implementation.

In a pure algorithmic context, [START_REF] Zabit | Adaptive transi-tion detection algorithm for a self-mixing displacement sensor[END_REF] proposed to settle a threshold in adaptive way to increase the scope of an often presented proposal in SM literature based on a phase unwrapping method (PUM from [START_REF] Bes | Displacement measurements using a self-mixing laser diode under moderate feedback[END_REF]). This convenient notion of having a single method to detect SM signals from a variety of conditions, has been recently improved by [START_REF] Huang | Piece-wise transition detection algorithm for a self-mixing displacement sensor[END_REF] to handle noisy conditions by using a predefined piecewise approach. However none of them have been demonstrated convenient to handle random amplitude modulations like the obtained by speckle phenomenon. Let us depict this drawback by referring to Fig. 1 where a simulated SM signal for a moderate feedback regime of C = 2 and amplitude 6λ 0 has been generated by the behavioral model described in [START_REF] Plantier | Behavioral model of a self-mixing laser diode sensor[END_REF], with λ 0 the LD's wavelength emission. The signal has been affected in amplitude with a sinusoidal wave as an attempt to illustrate degradations by speckle pattern found in real measurement environments. In the middle of the signal, it can be observed that those fringes whose amplitude became reduced were not detected by the adaptive threshold algorithm from [START_REF] Zabit | Adaptive transi-tion detection algorithm for a self-mixing displacement sensor[END_REF]. This is explained because this approach iterates a threshold over a set of acquired fringes which were assumed to be homogeneous in amplitude.

In order to observe the root issue of this adap-Fig. 1. The algorithm from [START_REF] Zabit | Adaptive transi-tion detection algorithm for a self-mixing displacement sensor[END_REF] applied to a simulated SM signal with amplitude modulation.

tive threshold proposal, let us outline the common rough phase estimation from [START_REF] Bes | Displacement measurements using a self-mixing laser diode under moderate feedback[END_REF]. When SM phenomenon occurs, the OOP variations monitored by a built-in photodiode in a LD package (or directly from the junction voltage), are described by:

P (t) = P 0 {1 + m cos[x f (t)]} (1)
where P 0 is the power emitted by the LD in free running state, m is a modulation index, and x f is the interferometric phase subject to back-reflections. The equation linking the original phase x o with x f is given by [START_REF]Unlocking Dynamical Diversity: Optical Feedback Effects on Semiconductors Lasers[END_REF]:

x o (t) = x f (t) + C sin [x f (t) + arctan (α)] (2)
where the parameter α is the LD's linewidth enhancement factor. As demonstrated in [START_REF] Bes | Displacement measurements using a self-mixing laser diode under moderate feedback[END_REF], by calculating α (usually comprised between 3 and 10) and C (the feedback coupling factor responsible for the varying shape of SM signals), x o can be approached by unwrapping x f . A rough estimation of x f can be done by normalizing the OOP from Eq.

(1) such as P N = P (t)/P 0 , and applying the inverse cosine function, i.e.:

xf (t) = arccos [P N (t)] (3)
Once calculated x o , the instantaneous distance D(t) between the LD and remote surface which leads to the target's movement can be retrieved from:

x o (t) = 4πD(t) λ o (t) . (4)
PUM was originally proposed to work for moderate feedback due to the simplicity of fringe detection and the unambiguity of target's direction retrieval. From Eq. (3) the usage of arccosine function is mathematically correct for rough phase estimation, however it seems to be more pertinent for the case of weak feedback when the fringe's shape is quasi-sinusoidal rather than for sawtooth-like SM signals.

Considering that the work in [START_REF] Fan | Improving the measurement performance for a self-mixing interferometry-based displacement sensing system[END_REF] reported improved precision in the reconstruction process, notably by obtaining an accurate feedback phase with a segmentation procedure, it can be guessed that further improvement on SM signal exploitation might come from a different phase estimation. Also, since identifying random amplitude variations to avoid missing fringes is a demanding task, we propose in this work a different approach consisting in the generation of orthogonal SM signals to obtain a robust phase estimation. From this phase signal, it is possible to detect SM fringes in a straightforward manner as this calculation remains homogeneous in amplitude along the time, being thus convenient to perform measurements from rough surface targets.

Robust phase detection

Recently, [START_REF] Donati | Speckle pattern errors in self-mixing interferometry[END_REF] presented a detailed analysis on the errors induced by speckle phenomenon over SM signals. From this study, the notion of analytic SM signals obtained through a Hilbert transform has been introduced and enlighten as an intriguing hint to observe speckle phase errors over SM signals.

We have worked with analytic SM signals as a means to estimate a rough phase in a similar manner as the proposal in PUM, i.e. to detect fringe transitions aiming to obtain a practical exploitation of SM signals mainly in the context of displacement measurement.

2.A. Principle

We propose to calculate the Hilbert transform H from the OOP in Eq. ( 1) in order to produce an orthogonal 90 • phase-shifted SM signal as stated by its theoretical definition [START_REF] Bracewell | The Fourier transform and its applications[END_REF], i.e.:

P a (t) = P (t) + Im {H[P (t)]} (5)
where P a (t) is the resulting analytic SM signal. As a matter of fact, Hilbert transform is used in many fields to provide a frequency information over the time, being suitable for peak detection. In frequency domain it can be calculated by:

F{H[P (t)]} = j • sgn(f )F{P (t)} (6)
where sgn(f ), the signum function is defined as 1 for positive frequencies, -1 for negative frequencies and null at zero. From the analytic SM signal, the instantaneous phase can be calculated as:

φ(t) = arctan Im[P a (t)] Re[P a (t)] . (7)
To illustrate different calculations of analytic SM signals, a sinusoidal displacement of amplitude 4λ 0 has been used to simulate SM signals at increasing feedback levels. Fig. 2(a) corresponds to the calculation of a half period of displacement for weak feedback with C = 0.2. The bottom projection corresponds to the real axis of SM fringes describing the quasi-sinusoidal shape proper to this regime. Continuing to increase feedback values, in Fig. 2(b) it can be observed that C = 1.2 (thus in moderate regime), produces a discontinuity on the analytic signal. This allows to confirm the change of regime even if the real SM signal (bottom projection) does not show a so perceptible difference as compared with previous value of C = 0.2. Finally, a simulation for C = 3.6 [Fig. 2(c)] reveals a moderate regime by the discontinuity in the waveform, it also reflects hysteresis phenomenon as a more accentuated reduction in amplitude of the observed analytic signal.

From these representations, it can be observed that the calculated analytic form of the SM signal keeps coherent with the fringe's characteristics observed in real axis along the different feedback variations. This same behavior can be expected even at different amplitudes of the input SM signal. Therefore, the utility of such an elaboration shall be observed through its phase extraction.

The first observation that can be done from this proposal, is that the acquired SM signal does not need to be normalized as required by the arccosine function from PUM and still produces a similar estimated phase, thus xf (t) ≈ φ(t). It can also be seen that the range's phase can be extended to (-π,π] in order to match the fringe's periodicity of 2π by using a four-quadrant arctangent function (arctan2).

The major feature observed from this calculation is the robustness face to amplitude variations as illustrated in Fig. 3(a), where a simulated SM signal for a 10λ 0 displacement and C = 1 was combined with a sinusoidal signal of unitary amplitude and 0.25 Hz of frequency. By using the conventional calculation by arccosine function, a mirrored phase mapped version of the input signal was obtained [Fig. 3(b)]. This explains why the adaptive threshold algorithm failed to converge into a low value to properly detect all the SM fringes from the example in Fig. 1. Conversely, Fig. 3(c) demonstrates that the obtained analytic phase (in a range 0 -π to ease the comparative) from Eq. ( 7) was not affected by amplitude variations.

From the different feedback regimes previously simulated, it can be observed that hysteresis phenomenon produces a deformation on the analytic form of the signals. This implies that the homogeneous phase unwrap of SM fringes is still compromised for the case of fringes from moderate feedback whose zero-crossing has been affected, even if it resulted insensitive to amplitude variations.

We have found that by locally removing the fringe's offset introduced by hysteresis phenomenon, a homogeneous phase calculation can be retrieved like in the case of weak feedback. Fig. 4 presents on the left, a plane view of a small segment with few fringes of the previous analytic calculations for feedback values of C = 0.2, 1.2 and 3.6 from top to down respectively. On the right, from each obtained phase fringe unwrap it can be appreciated how a derivative threshold can be used to detect phase transitions and thus to distinguish the fringes from a signal disregarding their shape in time.

In particular, from the bottom case for C = 3.6 the zero-mean centered signal on the real axis is contrasted against a hysteresis affected signal to depict the advantage of this procedure. By doing so, the iterative procedure of the previously analyzed adaptive threshold algorithm can be avoided.

It can be stated that the combination of hysteresis-free SM signals with their analytic representation through the Hilbert transform provides a means to exploit their phase in a consistent basis even when amplitude variations affect the shape of a group of fringes along the time.

2.B. Method implementation

Based on previous analysis, we designed a fringe detection algorithm allowing to increase the scope of Hilbert transform usage in the context of sensors implementation. Since SM signals require a local zero-mean to properly provide an orthogonal calculation, one solution consists in calculating a filter bank for a range of measurements. However, due to the direct dependence of SM fringes to the amount of target's displacement in a period of time, such a filtering requires a self-adapting nature in the context of a generic procedure.

Let us represent in Fig. 5 a comparison between our proposal and the fringe detection basis from PUM algorithm. The analytic signal allows to re- main insensitive to amplitude modulation of the fringes, while the offset removal and direction assignation steps allow to process SM signals regardless their feedback regime.

First, we propose to calculate the derivative from an incoming OOP to obtain a rough observation of SM fringes as usually encountered in SM literature. From this set of points, a root-mean-square (RMS) value can be used to define an automatic threshold aiming to detect the sharpest fringes from a signal. Those detected fringes are then used as trigger points in a running average filter over the original acquired signal. By subtracting this averaged signal to the hysteresis affected signal, it is possible to automatically re-center affected segments (P z ), with the added value of being a noninvasive method for the case of weak feedback as the calculated running average already gives a zero value.

Performing these steps provides thus, a unique algorithm for an improved observation of SM signals' phase. From this calculation it is possible to detect the phase transitions and thus the fringes in a signal. Then, after segmenting the fringes, displacement reconstruction can be achieved by properly assigning the direction to each interferometric fringe (T r ). As a matter of fact, there are several proposals on literature for direction retrieval (e.g. [START_REF] Bes | Displacement measurements using a self-mixing laser diode under moderate feedback[END_REF][START_REF] Magnani | Selfmixing vibrometer with real-time digital signal elaboration[END_REF]). A simple calculation consists in assigning the sense directly from the observed differentiated OOP signal and multiplying this value by the detected phase transition instants from previous step (∆ xf ).

In order to validate the contribution of our generic fringe detection algorithm, it has been compared against the unfavorable conditions tested in Fig. 1 for the adaptive threshold algorithm from [START_REF] Zabit | Adaptive transi-tion detection algorithm for a self-mixing displacement sensor[END_REF]. In Fig. 6, the SM signal used to observe the issue of undetected fringes due to amplitude variations has been used as input to our algorithm (C = 2, displacement of 6λ 0 and with fringes modulated in amplitude). For convenience, it has been denoted HT since it relies on a Hilbert transform calculation to obtain the analytic form of the SM signal (which is then used for improved phase calculation).

As depicted in Fig. 6(a), our proposal has been able to successfully account all the fringes in the signal by exploiting its improved phase calculation scheme. Fig. 6(b) compares the impact of failing to detect all the fringes from a SM signal in the case of a simple fringe counting procedure. For this example, the 3 missed fringes from the adaptive threshold algorithm carry an error of 1.5λ 0 peak-to-peak. As depicted in Fig. 6(c), those undetected fringes directly affect a displacement reconstruction procedure like the case of PUM minimization algorithm which is able to reach precisions below 20 nm when all the fringes are properly detected.

Other than the feature of working with different fringe amplitudes, it is important to highlight from this use case that the iterative feature and normalization step from the adaptive threshold algorithm have been conveniently simplified by the analytic SM signal elaboration. Since Hilbert transform can be implemented as a filtering procedure, this proposed approach is convenient for real-time calculations by conventional signal processing hardware architectures, similarly to the implementation of fast Fourier transforms.

Experiments and discussion

The experimental setup is shown in Fig. 7. A Hitachi HL7851G LD, with wavelength λ 0 = 785 nm is powered in continuous wave at 20 mW. The emitted beam is collimated through a lens with focal length of 5 mm and focalized to a random point on The speaker is excited by a sinusoidal wave of 300 Hz and 10 Vpp from a Tektronix AFG3022 function generator. In a first experience, a surface area is coated with sandpaper P40 of 425 µm average roughness. In a second experience, the coating is a slow-fading phosphor material active in the IR (VRC2 from Thorlabs). This intentional coarse arrangement leads to speckle emergence over micrometric displacement as it would be on a real nonconditioned environment. The HL7851G embeds a photodiode which serves to monitor the LD's OOP variations. By means of a transimpedance amplifier (TA) and an oscilloscope (LeCroy HRO 66Zi) we observe and record the SM signals to apply our fringe detection algorithm and perform a displacement reconstruction as described hereafter.

3.A. Speckle affected SM displacement signal

Figure 8, shows the acquired SM signal for one period of the sandpaper-coated surface moving towards and away the LD. These fringes present in average a sawtooth-like shape, however it can be appreciated an abrupt amplitude degradation due to speckle phenomenon. As explained, this unpredictable variation is a main source of error in a measurement by this technique. For example, it can be observed in Fig. 9(a-b) that settling a threshold over a set of derived fringes is just an approach to partially extract some information from a SM signal, however it is not convenient as final solution due to the fringes that are not detected when the amplitude changes over a few of them. This is why we have considered this information just as a preliminary step, notice that the region of undetected pseudo-fringes actually shares a similar average value around zero for this experimental signal.

Performing the moving average over such detected pseudo-fringes in this signal allows to observe the non-intrusive nature of this approach, as the fringes were already zero-centered. i.e. the acquired SM signal was in moderate regime without hysteresis phenomenon [Fig. 9(c)]. In this condition, the analytic SM form was created by using the Hilbert transform allowing the improved phase observation instead of the arccosine function which would provide a mirrored amplitude modulated signal. From the homogeneous calculated phase, its derivative provided the transitions denoting each of the fringes. Their direction was assigned by multiplying them with the sign of the fluctuations of the original SM derivative [Fig. 9(d)]. Finally, target displacement reconstruction at 300 Hz has been Fig. 8. One period of a speckle affected signal for displacement retrieval. achieved properly as observed in Fig. 9(e). Let us highlight that the goal has been to validate the pertinence of the fringe detection scheme over the existing literature. In near future it is expected to propose another test with a countermeasure for a comparative validation. For convenience, let us summarize the performed steps over the acquired signal as follows: As previously explained, for this last point a precise displacement reconstruction can be done for example by the optimization algorithm from PUM, once that all the fringes have been properly detected.

3.B. Worst usage conditions

In the second experience, pointing the phosphorcoated area produced stronger amplitude degradations leading to a fringe fading [Fig. 10(a)].

Direction assignation by the simple differentiation approach provided a small segment of detected fringes in opposite sense to the rest of surrounding fringes within half period of target's displacement [Fig. 10(b)]. In fact, looking closely to the segments of fringe fading [Fig. 10(c)], it can be observed a weak feedback regime from the quasi-sinusoidal shape while the signal in general depicts sharpened sawtooth fringes. While this condition of dynamic feedback variation leads to a very complex identification, our approach which disregards the shape in time of fringes allows to observe their phase, even if direction assignation of a few of them still requires further improvement. It can also be observed that the single fringe of smallest amplitude (located at ≈3.2 ms) was not detected as it was almost equivalent to the signal noise amplitude. As mentioned in the introduction, the implication in both cases is an inaccuracy of the reconstruction. However those wrong detections can be corrected by improving this assignation step.

In Fig. 11(a), a statistical direction was assigned to those detected fringes by analyzing its average separation and accounting the surrounding detections in similar way as a visual inspection producing a smooth displacement reconstruction [Fig.

11(b)

]. As can be observed in a contemporary application for target speed with direction measurement from SM signals in frequency domain [START_REF] Magnani | Real-time self-mixing interferometer for long distances[END_REF], direction retrieval of single fringes degraded by noise and speckle still requires further signal processing research. While this ambiguity could be tolerated for several applications (e.g. displacements above the tens of µm), an increased processing complexity shall be expected to attain improved performance.

3.C. Application to velocimetry

In order to observe the viability of our proposal for real usage in extended applications, a test for fringe detection has been proposed with a basic velocimetry setup, where a distributed feedback (DFB) laser diode points the rough surface of a disk rotating at constant velocity in one direction [Fig. 12(a)].

An experimental signal segment [Fig. 12(b)] was acquired to observe the speckle phenomenon as a random amplitude modulation strongly affecting SM fringes. For clarity, just a part of this signal [Fig. 12(c)] has been passed to our algorithm and to the adaptive threshold algorithm for comparison. Looking closely to the fringes, it can be seen that those with bigger amplitude present a characteristic sawtooth-like shape allowing a simple target direction identification. On the other hand, those with smaller amplitude become more triangular and is almost impossible to observe a privileged sense (even if it is known that the target rotates only in one direction).

The obtained transitions from both compared algorithms are shown in Fig. 12(d). Those corresponding to the adaptive algorithm (scaled down for clearness) present few gaps for undetected values after a threshold settlement. Conversely, after denoising the input segment our algorithm detected all the fringes demonstrating thus its robustness.

Conclusion

The presented phase calculation approach allows to detect SM fringes regardless the dynamic variations in the feedback level even face to speckle degradations. The robustness of this detection scheme resides in the fact that it allies the benefits of frequency domain signal exploitation through the Hilbert transform, with the flexibility of dealing with SM fringes in time domain.

This method has allowed to derive a convenient algorithm for displacement reconstruction, avoiding the need of external arrangements and maintaining low the global cost of system development based on SM interferometry. Due to its versatility and capabilities, it shall be studied for displacements in the range of µm to mm, in unconditioned environments, and can be applied to related applications of SM interferometry like velocimetry.

The corresponding author would like to thank Clement Tronche for experimental setups and CONACYT for financial support.

Fig. 2 .

 2 Fig. 2. Analytic SM signals for a sinusoidal displacement of 4λ 0 amplitude, from weak to moderate feedback: (a) C = 0.2, (b) C = 1.2 and (c) C = 3.6.

Fig. 3 .

 3 Fig. 3. (a) Simulated SM signal with C = 1 and amplitude modulation. Comparison between (b) the common phase estimation by arccos, and (c) the proposed instantaneous phase via abs(arctan2).

Fig. 4 .

 4 Fig. 4. (on the left) Plane view of few fringes of the analytic SM signals from Fig. 2 and (on the right) the resulting phases for feedback values of C = 0.2, 1.2, 3.6 and C = 3.6 with hysteresis removal, from top to down respectively.

Fig. 5 .

 5 Fig.5. Fringe detection basis from PUM algorithm[START_REF] Bes | Displacement measurements using a self-mixing laser diode under moderate feedback[END_REF] compared to our proposal based on Hilbert transform.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Comparative between the adaptive threshold algorithm and our proposal: (a) Detected fringes, (b) Unwrapped fringe counting, and (c) Displacement reconstruction.

Fig. 9 .

 9 Fig. 9. Steps of the proposed method for fringe detection for SM displacement reconstruction: (a) Derivative, (b) Pseudo-fringes, (c) Offset removal, (d) Phase transitions, and (e) Displacement reconstruction.

1 . 2 . 3 . 4 . 5 . 6 . 7 .

 1234567 Derive the OOP signal and calculate its RMS [Fig. 9(a)]. Define as trigger points those values above the RMS [Fig. 9(b)]. Perform signal average between each previous points and subtract it from OOP [Fig. 9(c)]. Create the analytic signal from Eq. (5) by using a Hilbert transform. Calculate the phase in (-π,π] range from Eq. (7) as shown in Fig. 4. Select the phase transitions in a mod[2π] basis and assign a positive or negative value according to target direction [Fig. 9(d)]. Reconstruct target's displacement from those assigned transitions [Fig. 9(e)].

Fig. 10 .

 10 Fig. 10. Strong speckle condition: (a) Input signal, (b) Detected fringes, and (c) zoom on input signal depicting dynamic variations of feedback regime.

Fig. 11 .

 11 Fig. 11. (a) Improved detection of fringes under strong speckle, and (b) Displacement reconstruction from them.

Fig. 12 .

 12 Fig. 12. Experimental velocimetry setup with a rotating disk, (b) Speckle affected fringes at constant frequency, (c) Algorithms' input signal for validation, and (d) Comparison of detected transitions.


Juan Luis Delgado

Pierre-Antoine Bouit

Salvatore Filippone

Mª Ángeles Herranz

Nazario Martín

Fernando Langa Afterwards

François Nierengarten

Salvatore Filippone Is A Ramón

Organic Photovoltaics: A Chemical Approach

The design of new organic functional molecules able to harvest sun light and efficiently undergo photon to current energy conversion processes is at the forefront of chemical challenges. In this review the fundamental contribution of chemistry to the multidisciplinary field of organic photovoltaics is presented in a systematic way through the wide variety of organic compounds synthesized to be successfully used in photovoltaic devices.

Introduction

Energy is currently the most important problem facing mankind. The "fire age" in which our civilization has been based from the very beginning is approaching to its end. Human being has been 75 burning a wide variety of materials since earlier times and with the advent of carbon-based fossil fuels in the last two centuries, its combustion has become nowadays a major problem due to the huge amounts of carbon dioxide emissions produced all over the world. Because of the resulting pollution, global warming and 80 degradation of the planet, a new era based on non contaminating renewable energies is currently a priority. In this regard, the Sun considered as a giant nuclear fussion reactor represents the most powerful source of energy available in our Solar system and, therefore, its use for providing energy to our planet is among the 85 most important challenges nowadays in science. Actually, the energy received from Sun, calculated in 120,000 TW (5 % ultraviolet; 43 % visible and 52 % infrared), surpass in several thousands that consumed in the planet along a year. 1 Photovoltaic (PV) solar cells is currently a hot topic in science and since the former silicon-based device prepared by Chapin in 1954 exhibiting an efficiency around 6 %, 2 different semiconducting materials (inorganic, organic, molecular, polymeric, hybrids, quantum dots, etc) have been used for transforming sun light into chemical energy. Among them, photo-and electro-active organic materials are promising due to key advantages such as the possibility of processing directly from solution, thus affording lighter, cheaper and flexible all-organic PV devices. Actually, these organic systems have received a lot of attention and a variety of excellent reviews and special issues have recently been published. [START_REF]For some very recent special issues, see: ([END_REF] They are mainly focussing on the race to increase efficiency as well as on those aspects mostly related with the understanding of those essential optical and electronic processes able to transform photon energy into electricity, thus providing a general overview on the state of the art in organic PV solar cells. [START_REF] Dennler | For some recent reviews on Organic Photovoltaics[END_REF] Therefore, the aim of this review is to emphasize the contribution of chemistry to this field by covering those aspects related with the nature and properties of those type of compounds used to construct "all-organic" PV devices. Furthermore, the systematic organization of those compounds used for different purposes and tasks in organic solar cells should help to the chemical community to think about and to rationalize further alternative compounds which, eventually, result in a better performance of the PV devices. Thus, this is not a comprehensive review on the chemistry of the so-called "all-organic" or "excitonic" (absorption of light results in the formation of excitons, basically a neutral electron-hole pair) solar cells but, on the contrary, a presentation of the most relevant chemical compounds which currently define the different classes of "all-organic" PV cells. Thus, we have organized this review in four main parts: i) Basic processes in organic solar cells; ii) Bulk-heterojunction plastic solar cells based on semiconducting π-conjugated polymers and fullerenes; iii) Molecular bulk-heterojunction solar cells based on π-conjugated oligomers or dyes and fullerenes; iv) Other carbon nanostructures, namely endohedral fullerenes and carbon nanotubes (CNTs). Actually, the presence of pristine fullerenes (C 60 and C 70 ), or most commonly a fullerene derivative (namely PCBM: 1- (3methoxycarbonyl)propyl-1-phenyl [6,6]C 61 ), is a common feature in sections ii) and iii). Therefore, it is not surprising the recent interest in other available related carbon nanostructures such as endofullerenes and CNTs, which is covered in iv).

i) Basic Processes in Organic Solar Cells

Organic solar cells are constituted by semiconducting organic materials formed by contacting electron donor and acceptor compounds (p/n type). A further improvement in the construction of organic photovoltaic devices (OPV) consists on the realization of interpenetrated networks of the donor and acceptor materials. In such bulk heterojunction (BHJ) solar cells, the dramatic increasing of contacting area between D/A materials leads to a significant increasing of the number of generated excitons as well as their dissociation into free charge carriers and hence on the power conversion efficiency (Figure 1).

60

Transformation of solar energy into electricity occurs through a series of optical and electronic processes which basically involves: i) optical absorption of sun light and formation of the exciton; ii) exciton migration to the donor-acceptor interface; iii) exciton dissociation into charges (electron and 65 holes), and iv) charge transport and collection at the electrodes. All these steps are not totally understood at present and a number of research groups are currently dedicated to unravel essential aspects in the search for better energy transformation efficiencies. [START_REF]functioning of organic solar cells[END_REF] In contrast to inorganic 70 semiconductors which upon light excitation form free electron and hole carriers, organic compounds form excitons whose dissociation into free carriers is not straightforward. Due to their low dielectric constants, it requires reaching the donoracceptor interface to dissociate into free charges. The driving 75 force for this exciton dissociation being provided by the energy difference between the molecular orbitals of the donor and acceptor. The transport of the thus generated free charges toward the electrodes represents another important issue. Organic 80 materials tipically show charge carrier mobilities significantly lower (around 10 -5 to 1 cm 2 V -1 s -1 ranging from amorphous to crystalline materials) than those of inorganic semiconductors (around 10 2 -10 3 cm 2 V -1 s -1 ). Since charge carriers mobility is strongly dependent on the molecular organization of the 85 material, in order to achieve better efficiencies, a good control on the morphology of the donor-acceptor materials at a nanometer scale is necessary. Furthermore, an efficient charge carriers mobility is essential to prevent charge recombination (geminal or bimolecular) processes which result in lower 90 energy conversion efficiencies. In summary, an appropriate choice of the donor and acceptor materials is critical to ensure a good match between them in terms of optical, electronic and morphological properties, which eventually determine the effective photocurrent and 95 performance of the PV device. Therefore, the rational design of new materials able to improve some specific demands within the PV device is critical for the successful development of competitive solar cells. In the following sections, we will discuss those materials mainly used in the preparation of "all-100 organic" solar cells according to their chemical nature and composition. ii) Bulk-heterojunction "plastic" solar cells

The term "plastic solar cells" has been coined for those photovoltaic devices using polymers in their constitution. Thus, although a variety of donor and acceptor polymers have previously been used for constructing PV devices, the highest efficiencies have been achieved by mixing electron donor polymers with fullerenes as acceptors. Polymer:fullerene (BHJ) based solar cells 6 , 4a are considered suitable candidates to obtain low-cost renewable energy from a large-area, flexible, plastic material.

Fullerenes and their derivatives possess important electronic properties like small reorganization energy, high electron affinity, ability to transport charge and stability, that makes them (see below) one of the best candidates to act as electron acceptor components in BHJ photovoltaic PV devices. [START_REF] Dennler | For some recent reviews on Organic Photovoltaics[END_REF][START_REF]The Chemistry of Fullerenes[END_REF] On the other hand, MDMO-PPV {poly [2-methoxy-5-(3',7'dimethyloctyloxy)]-1,4-phenylenevinylene} and P3HT [poly (3-hexylthiophene-2,5-diyl)] have been by far the most studied π-conjugated polymers as donor components in BHJ devices. Actually, both polymers have thoroughly been studied from a morphological and electronic standpoint to form BHJ devices by mixing with fullerene derivatives, namely PCBM (19, Figure 4). In fact, the best fullerene/polymer combination has been obtained with PCBM/P3HT(regioregular) mixtures, reaching energy conversion efficiencies in the range of 5%. 8

In the following, we will briefly describe the most relevant achievements both in the chemistry of polymers as well as on the fullerene chemical modifications for PV purposes.

a) Polymers for BHJ Photovoltaics

Polymers endowed with fullerenes is a broad interdisciplinary field of interest for a variety of purposes. [START_REF]Synthesis, Properties and Applications[END_REF] However, photovoltaics are among the most realistic applications for polymer/fullerene blends. The chemistry of π-conjugated polymers has been focused on tuning their energy (HOMO and LUMO) levels in order to have a good control on the band gap of the polymer. This is a key issue to modulate the light harvesting properties of the polymer [low band gap polymers (lower than 1.5 eV) absorb in the visible region] as well as the right energy match with the acceptor moiety which, eventually, controls the open circuit voltage (Voc) values, and hence the cell power conversion efficiency. Since π-conjugated polymers constitute an specific field by their own right, we will discuss the trends currently followed in polymer chemistry to improve the performance of those "classical" MDMO-PPV and P3HT. Some of the most relevant polymers synthesized so far are shown in Figure 2.

All of them involve the use of alternating donor and acceptor moieties within the polymer backbone since they allow a fine tune of the band gap of the polymer, as well as an improvement of planarity and hence of the mobility of the charge carriers. Thus, a variety of soluble electroactive carboand heterocycle derivatives such as fluorene, thiophene, cyclopentadithiophene and carbazole have been used as electron rich units, whereas benzothiadiazole and thienopyrazine have recently been used as the electron poor units. heptadecanyl-2,7-carbazole-alt-5,5-(4',7'di-2-thienyl-2',1',3'benzothiadiazole)], 12 4) poly [2,6-(4,4-bis-(2-ethylhexyl)

-4H- cyclopenta[2,1-b;3,4-b2]-dithiophene)-alt-4,7-(2,1,3- 65
benzothiadiazole)], 13 5) poly{5,7-di-2-thienyl-2,3-bis (3,5di(2-ethylhexyloxy)phenyl)thieno [3,4-b]pyrazine}, 14 6) platinum(II) polyyne polymer, 15 7)PDBTTT-C, 16 8), HXS1 17 9) PTB7 18 70 Among the different electron donor polymers prepared so far, we want to focus on a variety of polyfluorene copolymers prepared by Andersson et al. known as APFO (alternating polyfluorene copolymers) (2), which have allowed a good control on the band gap, covering the solar spectrum up to 75 1000 nm. Power conversion efficiencies over than 4 % have been demonstrated with APFOs in blends with PCBM. 19 Typically, π-conjugated polymers are endowed with flexible lateral chains to improve their solubility. However, in many cases the side chains prevent the polymer backbones from close 80 packing. Recently, a planar polymer conformation has been achieved in polymer poly 23452,carbazole, HXS-1) bearing two octyloxy chains on the benzothiazole ring and an octyl chain on the carbazole ring. The synthesis of HXS-1 is shown in Scheme 1 to illustrate the synthetic strategy followed for the preparation of this kind of polymers. Thus, the coupling reaction of 4,7-dibromo-5,6bis(octyloxy)benzo-2,1,3-thiadiazole (10) with 4,4,5,3,as the catalyst precursor and aqueous NaHCO 3 , and THF as solvent, led to compound 12 in moderate yield. Bromination of 12 with N-bromosuccinimide afforded 13 whose further Suzuki-Miyaura polycondensation with 2,4,5,3,-octyl-9H-carbazole ( 14) using Pd(PPh 3 ) 4 as the catalyst precursor in a biphasic mixture of THF-toluene (5:1)/aqueous NaHCO 3 formed polymer HXS1 as a black solid in good yield. 18 Semiconducting polymers based on alternating thieno [3,4b]thiophene and benzodithiophene units such as 7 and 9 show an excellent photovoltaic behaviour. The stabilization of the quinoid structure from the thieno [3,4-b]thiophene moiety leads to remarkable low band gap polymers (around 1.6 eV). A further optimization has been achieved with the new PTB7 in which the presence of the fluorine atom leads to a lower HOMO level, thus enhancing the V oc parameter. With an average molecular weight around 97.5 KDa and a polydispersity index of 2.1, this soluble polymer exhibits a strong absorption from 550 to 750 nm.

Interestingly, by blending this polymer with PC 71 BM, which nicely complements the absorption in the visible range, has allowed preparing a PV device exhibiting an energy conversion efficiency of 7.40 %. This value represents the highest efficiency reported so far for a polymeric solar cell. Since the development of new π-conjugated donor polymers for improving the efficiency in BHJ photovoltaic devices is currently a broad and very active field, out of the scope of this general presentation, for further details the reader is referred to some of the excellent monographs available in the literature. 20

b) Chemically Modified Fullerenes for BHJ Photovoltaics

The most widely used configuration of polymer solar cells is based on the use of a fullerene derivatives as acceptor component. Indeed, fullerenes have been demonstrated to be the ideal acceptor because of their singular electronic and geometrical properties and for the ability of their chemically 45 functionalized derivatives to form a bicontinous phase network with π-conjugated polymers acting as electron conducting (n type) material.

A variety of chemically modified fullerenes were initially synthesized for blending with semiconducting polymers 50 (namely PPV derivatives) and to prepare photovoltaic devices. These fullerene derivatives were covalently linked to different chemical species such as electron acceptors, 21 electron donors, 22 π-conjugated oligomers, 23 dendrimers, 24 etc. (Figure 3). However, in general, the obtained blends resulted in PV The best known and most widely used fullerene derivative as acceptor for PV devices is [6,6]-phenyl-C 61 butyric acid methyl ester 19 (PCBM), firstly prepared by Hummelen and 65 Wudl in 1995. [START_REF] Hummelen | [END_REF] Since its first reported application in solar cells, 27 This has led to the synthesis of many other PCBM analogues (20,21) 28 in an attempt to increase the efficiencies of the cells by improving the processability, the stability or PV parameters such as the open circuit voltage (Voc) by rising the LUMO energies of the fullerene acceptor (Figure 4). In this regard, only small shifts (< 100 meV) of the LUMO level have been obtained by attaching a single substituent on the fullerene sphere, even by using electron-donating groups.

In contrast, significantly higher Voc values have been achieved through the polyaddition of organic addends to the fullerene cage ( 100 mV raising the LUMO per saturated double bond). Recently, an externally verified powerconversion efficiency of 4.5% has been reported by Hummelen et al. employing a regioisomeric mixture of PCBM bisadducts as result of an enhanced open-circuit voltage, while maintaining a high short-circuit current (J sc ) and fill factor (FF) values. 29 The cyclopropanation reaction to superior fullerenes to form PCBM analogues is more complex than for C 60 . Indeed, the less symmetry and the presence of more than one reactive double bond is often responsible for the formation of regioisomeric mixtures. Nevertheless, the loss of symmetry of C 70 induces a stronger absorption, even in the visible region. As a result, PC 71 BM 30 is considered a suitable candidate for more efficient polymer solar devices. Moreover, such devices performed the highest verified efficiency determined so far in a BHJ solar cell, with an internal quantum efficiency approaching 100%. 31 Analogously, PC 84 BM, 32 has been obtained as a mixture of three major isomers. The stronger electron affinity and the diminished solubility gave rise, however, to poor power conversion efficiencies.

Although the PCBMs are the acceptor that best performances guarantees to the moment, it does not mean that they are necessarily the optimum fullerene derivatives. Therefore, a variety of other fullerene derivatives 33 have been synthesized in order to improve the device efficiency or to achieve a better understanding on the dependence of the cell parameters from the structure of the acceptor. Among the different modified fullerenes prepared so far, diphenylmethanofullerene 22 (DPM12) prepared by Martín et al. is another successful type of methanofullerene endowed with two alkyl chains to drastically improve the solubility of the acceptor in the blend and reaching efficiencies in the range of 3 % (Figure 5). Although the LUMO energy level for DPM12 is the same as that for PCBM, an increase for the Voc of 100 mV for the DPM12 over PCBM has been observed. 34,35 This is currently an important issue for improving the design An alternative molecular approach to control both the length scale of polymer/fullerene phase segregation and the electron mobility in the fullerene network is based on the employ of a self assembly fullerene shuttlecock derivative (24), able to form one-dimensional wirelike domain within the active 65 layer. 37 less-known acceptors for PV devices. 39 The synthesis of these pyrrolidino-pyrazolino fullerene dimers has been carried out in a straightforward manner from the formyl-containing pyrazolino [60] or [70]fullerene, 40 which is used as the aldehyde component in the subsequent pyrrolidine formation 80 by 1,3-dipolar cycloaddition of the respective azomethyne ylide with the fullerene. 41 As expected, [70]fullerene dimers showed better absorption in the visible spectrum than the analogues of [60]fullerene. Interestingly, non optimized BHJ solar cells formed by blending with P3HT exhibited quantum 85 conversion efficiencies of 37 %, with a PCE of 1,0 % (Figure 6). (25-27) and bis-cycloadduct 28 bearing two indene units.

A remarkable bis-adduct fullerene derivative formed by two indene units covalently connected to the fullerene sphere of 5 C 60 (28) has recently been reported by Hou and Li. 42 Interestingly, the presence of two aryl groups improves the visible absorption compared to the parent PCBM, as well as its solubility (> 90 mg/ml in chloroform) and the LUMO energy level, which is 0.17 eV higher than PCBM.

10 Surprisingly, PV devices formed with P3HT as semiconducting polymer revealed PCE values of 5.44 % under illumination of AM1.5, 100 mW/cm 2 , thus surpassing to PCBM which afforded an efficiency of 3.88 % under the same experimental conditions.

A major drawback in the synthesis of bis-adducts of fullerenes is that the products formed are constituted by a mixture of regioisomers which are not separated because of the experimental difficulties. This fact, however, does not seem to have a strong influence on the PV parameters and power 20 conversion efficiencies. Nevertheless, from a chemical viewpoint, the synthesis of isomerically pure bis-adducts and multiadducts of fullerenes is still an open question which should be properly addressed by the chemical community.

Although some of the fullerene derivatives prepared so far 25 exhibit good performances in PV devices, the synthesis of new fullerene derivatives with stronger visible absorption and higher LUMO energy levels than PCBM is currently a challenge for all those chemists engaged in the chemical modification of fullerenes for PV applications.

c) Double-Cable Approach for BHJ Photovoltaics

This strategy is based on the appendage of the acceptor moiety to the π-conjugated polymeric skeleton with the aim to maximize the interface between the two active components, thus preventing the phase separation between them. At the same time, such approach is expected to promote an efficient charge transport to the electrods through a nanoscale homogeneous distrubution of the domains of the donor and the acceptor in the blend.

The first example of a PV cell prepared by using the double-40 cable approach was carried out by spin coating from poly(pphenylenevinylene) and poly(p-phenyleneethynylene) covalently connected to methanofullerene units. Photovoltaic parameters (Voc = 0.83 V; Jsc = 0.42 mAcm -2 ; FF = 0.29 under AM1.5 conditions) resulted to be quite promising. 43 45 Interestingly, the realization of effective double-cable polymers brings the p/n heterojunction at the molecular level, thus allowing a better control from a chemical viewpoint. 44 However, to date, and despite the beauty of this chemical design, double-cable polymers 45,9b,11c have yielded poor 50 energy conversion efficiencies. One of the reasons to account for this finding has been the low fullerene loading of the material. Therefore, new polymers have been synthesized such as PTF (29) or PBTC-F (30) with a higher fullerene content (up to 63 wt %) (Figure 7). Other studies involving 55 polythiophenes followed to this one with slightly better performances and efficiencies around 0.60 %. 46 An interesting alternative approach is based on the use of welldefined rod-coil block copolymers consisting of P3HT donor and C 60 acceptor chromophores (P3HT-b-P(S x A y )-C 60 ) (31) as 60 "surfactant". The control of the interface morphology of the P3HT:PCBM donor-acceptor phase domains within the composite resulted in an improvement of the efficiency with respect to the analogous solar cell fabricated without the "surfactant". Despite the outstanding results obtained from BHJ solar cells formed from fullerenes and polymers, the use of commercially available P3HT displays a number of troubles associated to its synthesis, purification, and inherent electronic properties. 48 75 Factors such as polydispersity, regioregularity and molecular weight have a strong influence on the final performance of the photovoltaic device, which indicates that the control over P3HT structure and purity is an important issue for controling organic photovoltaics. Concerning its electronic properties, it 80 has been calculated that P3HT can only absorb 46% of the available solar photons, 49 owing to a bandgap of 1.90 eV and a relatively narrow absorption band. A promising solution to overcome these limitations could be the use of soluble small conjugated molecules instead of polymers. This strategy would avoid the above mentioned structural problems inherent to polymers, and allow the use of several well-defined small molecules whose electronic properties can be tuned by chemical synthesis, in order to obtain a better photovoltaic performance. Following this approach, in the last years, a number of molecules 48 such as oligomers, dendrimers, dyes and porphyrines have been used as molecular donors blended with pristine [60]fullerene or [60]fullerene derivatives, in order to prepare new and efficient photovoltaic devices.

a) Conjugated oligomers and dendrimers as donors in BHJ-PV devices

The synthesis and study of new organic monodisperse πconjugated oligomers is a hot topic in photovoltaics. 50 Oligothiophenes are among the most promising oligomers, due to their exceptional electronic properties and easy tuning of their properties by introducing functional substituents.

Several substitued oligothiophenes have been described and studied for organic photovoltaics. However, most of them show a planar geometry, which tends to aggregate due to π-π stacking interactions. Sun et al, 51 described recently the preparation of new X-shaped oligothipohenes (32-35) as electron donors in BHJ-PV devices blended with PCBM (Figure 8). The authors observed that both the open circuit voltage (V OC ) and the short circuit current (J SC ) are highly dependent on the oligothiophene chain length. While increasing the number of oligothiophene rings, the absorption was red-shifted and broadened, therefore the light harvesting properties and the PCE increase from 0.008 % to 0.8 %. Other interesting oligothiophene-based compound, was recently described by Tamayo et al, 52 namely oligothiophene diketopyrrolo-pyrrole (36) (SMDPPEH, Figure 9). This appealing oligothiophene is a low bandgap well-defined molecule that displays intense absorption bands at 650 and 720 nm. Blends of this material and PC 71 BM were prepared in different ratios and the results indicated the highest efficiency described so far for a molecular bulk heterojunction (PCE of 3 %) being 0.75 V the V OC and 9.2 the J SC. Interestingly, the presence of the ethyl-hexyl groups increment the solubility and procesability and, as a consequence, no microscale phase segregation was observed in the films

b) Organic Dyes as donors in BHJ-PV devices

Following the use of monodisperse and well defined molecules for the preparation of BHJ-PV devices, in the last 70 years a new and interesting approach has emerged in the photovoltaic field. Organic dyes are single molecules which display high absorption extinction coeficients and electronic properties which can be easily tuned by chemical synthesis.

Roquet et al 54 have described the synthesis of triphenylamine (TPA) based compounds (39-41) (Figure 11) bearing several units of dicyanovinylene. Photovoltaic studies on these compounds blended with C 60 in bulk and bilayers heterojunctions reveal a high value of V OC in the BHJ-PV devices due to their high oxidation potential. Owing to an intramolecular charge transfer band from the TPA core to the dicyanovinilene acceptor groups, and to the above mentioned high V OC , an improvement of the power conversion efficiency was observed, while the number of dicyanovinylene acceptor Kronemberg et al, 55 have recently described the photovoltaic 15 study of blends of merocyanines and PCBM. These dyes exhibit intense absorption bands (ε up to 1.4 x 10 -5 M -1 cm -1 ) at different wavelengths in he visible spectral region. For several merocyanine dyes (42) power conversion efficiencies around 1% were obtained under standard AM 1.5 conditions.

20

Although the best performance was obtained for MD 304 due to a higher J SC value (6.3 mAcm -2 ) and a V OC of 0.76 V which lead to a PCE of 1.74%. In order to harvest light in the NIR region, where almost 50 % of the sunlight power is displayed, Silvestri et al 56 described the synthesis of two new squaraine derivatives (43,44) (Figure 12) displaying intense absorption from 700 nm to 800 nm in solution. Photovoltaic studies of blends of these dyes with PCBM revealed a power conversion efficiency up to 1.3%. To the best of our knowledge this is the first efficient device prepared using NIR light harvesting molecular dyes. In order to improve the efficiciency of squaraine/PCBM based BHJ, Mayerhöffer et al 57 studied the charge carrier mobility of a series of squaraine dyes bearing a dicyanovinyl unit, preparing OFET devices. The authors showed that the most efficient photovoltaic device (PCE= 1.79%, J SC = 12.6mA• cm -2 ) presented good hole-carrier mobility. This remarkable PCE value was attributed to well packed domain of squaraine and PCBM. Besides the introduction of the dicyanovinyl moiety favorably changes the absorption properties of these squaraine dyes, and therefore Rousseau et al 58 took advantage of the ease of chemical modification on the bodipy core to tune the optical properties of the dyes and reached 1.3% of efficiency in bodipy:PCBM BHJ devices (Figure 13). Using the same family of dyes, these 55 authors 59 built BHJ devices with both Bodipy donors which display complementary absorption properties in the visible range (λ max1 = 590 nm, λ max2 = 675 nm). This strategy has allowed to solve the problem of the narrow absorption of these organic molecular dyes. An efficiency of 1.7% has been 60

described with this multidonor BHJ, which represents an increase of 0.4% compared to the best efficiency described with a single Bodipy donor. This significant increase nicely illustrate the importance of harvesting sunlight in the whole solar spectrum in order to obtain more efficient photovoltaic 65 systems. Using a similar strategy, some of us have described a new 70 organic salt ( 49) by association of two NIR absorbing cyanine dyes (47,48), which exhibit a good solubility in organic solvents (Figure 14). 60 As a result of the combination of the two cyanines, the complex is able to absorb light in a broad range of the solar spectrum (650-950 nm) displaying outstanding light harvesting 75 properties in the NIR region with giant extinction coefficients. In preliminary experiments, the sun to power conversion efficiency obtained for the cyanine-cyanine complex: [60]PCBM (PCE = 0.4%) was higher than the efficiency observed for blends of PCBM:cyanine-47 or PCBM:cyanine-48. This result indicates 80 that the new dye is an appealing system for further studies as light harvester for photovoltaics

N B N

I R 2 R 1 O O O O O O O Z 45 R 1 =CH 3 , R 2 =Z 46 R 1 =R 2 =Z

iv) Organic Solar Cells with Innovative Carbon Nanostructures

As mentioned above, one of the fundamental limitations of OPV device PCEs has been the molecular orbital offset between the available donor and acceptor materials. [START_REF] Dennler | For some recent reviews on Organic Photovoltaics[END_REF] Toward this goal, new acceptor materials systems are being designed and synthesized to optimize the donor/acceptor energy levels 10 in orden to increase the device photovoltage. From the variety of nanoscale size shapes with fascinating properties that carbon is able to provide, 61 carbon nanotubes (CNTs) 62 and endohedral fullerenes [START_REF]For recent reviews on endohedral fullerenes[END_REF] are of current interest because of their promising applications in next-generation organic photovoltaics, due to their outstanding intrinsic electronic properties.

a) Endohedral fullerenes

Novel acceptor materials with LUMO energies closer to that of the donor, such as the derivatives of metallo endohedral 20 fullerenes M 3 N@C 80 , where the choice of metal (M = Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Lu) allows the fine tunning of the LUMO levels, as well as, the absorptive coefficients of the fullerene species, offer the possibility of considerably increase the PCE of OPV devices.

In preliminary investigations, Sc 3 N@C 80 and Y 3 N@C 80 shown to be applicable as part of electron-donor/acceptor systems in combination with powerful donors such as ferrocene 64 or π-extended tetrathiafulvalene 65 derivatives (exTTFs) (Figure 15) (50 and 51 respectively). The photophysical investigations revealed a significant stabilization of the radical ion pair state of Sc 3 N@C 80ferrocene when compared to an analogous C 60 -ferrocene dyad. 50) (left) and the down isomer of an exTTF derivative of [6,6]-I h -Y 3 N@C 80 (51) (Courtesy of Prof. J. M. Poblet).

These positive results have been recently complemented with 40 the preparation of BHJ based solar cells considering a Lu 3 N@C 80 fulleroid modified with a phenyl butyric acid hexyl ester (52, PCBH) solubilizing group and, blended with P3HT. 66 In photophysical experiments, the efficient charge transfer between P3HT and the Lu 3 N@C 80 -PCBH were confirmed. The optimized P3HT/Lu 3 N@C 80 -PCBH active layer morphology has resulted in OPV devices with V oc values as high as 890 mV (260 mV above reference devices made with C 60 -PCBM) and a similar photocurrent and fill factor compared to P3HT/C 60 -PCBM reference devices (Figure 16). 50 The higher overall PCE (4.2%) of the devices fabricated is attributed to a better positioned LUMO level that captures more of the energy associated with each absorbed photon. 46 Further investigations, considering comparative analysis of the film absoption and X-ray diffraction, indicate that the 55 optimized blend ratio in a P3HT matrix differs from a molecular equivalent of an optimized P3HT/C 60 -PCBM active layer, and this is related to the physical differences of the C 80 fullerene. 67 Although additional investigations are necessary in the near 60 future, the potential of endohedral metallofullerene species to enable optimized OPV devices is certainly very promising. Since their pioneering work, many groups have exploited NTs in bulk heterojunction solar cells. 48,73c,74 However, the power conversion efficiencies of polymer-CNT solar cells are as high as ca. 1%, which is much lower than typical values of high-performance bulk heterojunction solar cells (3-6%) with a combination of π-conjugated polymers and fullerene derivatives including PCBM [START_REF] Hummelen | [END_REF] or DPMs. 34,35 When blending CNTs with different polymers, one of the major issues is to properly disperse the CNTs whithin the polymer matrix. In this sense, the addition of appropriate functional groups to the CNTs may result in a form compatible with the conjugated polymer. To this end, a C 60 -SWCNT complex has recently been synthesized via a microwave induced functionalization approach and used as component of the photoactive layer in a bulk heterojunction photovoltaic cell (Figure 17). 75 When compared to a control device with only C 60 and P3HT, the addition of SWCNTs resulted in an improvement of both J sc and FF, with power conversion efficiency, by as much as 78%. 76 In recent years, several groups have investigate the possibility of employ CNTs as a flexible replacement for ITO, 77 since 65 high quality ITO is expensive due to the resource constraint of indium. In addition, ITO is not compatible with roll-to-roll fabrication processing in contrast with today's market requirements for a full printable solar cell. For example, Rowell et al. demonstrated that SWCNts based hole-collecting 70 electrodes show well above 80% of the performance of classical ITO electrodes, and allow a significant higher bending stress than ITO based plastic substrates. 77a Thus, corrected power conversion efficiencies of 2.5% were presented and the slight power loss in comparison to test 75 devices using ITO contacts was mainly attributed to the increase serial resistance. In order to improve the practical applications of CNTs in OPV devices, several aspects need to be consider: charge recombination pathways, exciton dissociation, solubility and, 80 which is probably the major challenge, the selective preparation of semiconducting SWCNTs. In this sense, in the past year important improvements have been reported towards the selective separation of semiconducting vs metallic CNTs by means of chemical functionalization 78 or microwave 85 irradiation. 79

Conclusions and outlook

The most significant organic compounds prepared so far to be used in all-organic solar cells have been presented in a systematic way. Bulk heterojunction plastic solar cells 90 constituted by a semiconducting electron donor π-conjugated polymer and a fullerene as the acceptor moiety are currently the most promising devices for practical applications. The most remarkable achievements in π-conjugated polymers have been presented as well as those modified fullerenes 95 which, by blending with the appropriate polymers, have afforded the best energy conversion efficiencies reported so far, reaching values as high as 7.4 %. Covalently connected π-conjugated polymers-fullerene forming the so-called double-cable approach represents an 100 elegant methodology to prepare new materials which, up to now, has afforded comparatively very low energy conversion efficiencies.

Molecular bulk heterojunction represents an interesting alternative to the use of polymers for the preparation of PV devices. These compounds have been classified in those systems formed by π-conjugated oligomers or dendrimers and fullerenes, and the most recent dyes blended with fullerenes.

Other carbon nanostructures such as endohedral fullerenes and carbon nanotubes are promising materials for PV devices.

Although so far only a couple of examples are known on PV devices involving endofullerenes, the high energy conversion efficiency (4.2 %), similar to that found for the efficient [60]PCBM make these materials good candidates for further studies. Obviously, the main problem of these endofullerenes is currently related with their availability in larger scale.

Regarding the use of CNTs, it is important to note that the fate of these carbon nanoforms for PV devices depends primarily on the separation of the metallic and semiconducting types of CNTs which typically are mixed in the commercially available materials. Although still low efficiencies have been reported with CNTs, the huge potencial of these carbon allotropes makes them very appealing in this field.

A most important matter in all the above materials for 20 improving the cell performance is the precise control of the energy levels of donor and acceptor components as well as the network morphology to maximize the mobility of the charge carriers. In this regard the realization of nano-structured materials by means of a bottom-up supramolecular organization is currently in its infancy 80 and, therefore, more work is need for improving the materials used for PV applications. In summary, organic solar cells are call to play an important role for satisfying the increasingly energy demanding of our society. Because of their features of low cost, flexibility and ligthness, these new PV cellswhich should nicely complement the commercially available silicon cellsare very appealing for a variety of new practical purposes. The outstanding achievements obtained so far put these solar cells 35 closer to the market.

Figure 1 .

 1 Figure 1. Basic chemical components for a typical Organic105

Figure 2 .

 2 Figure 2. Representative polymers for PV devices: 60

Figure 3 .

 3 Figure 3. Some examples of modified fullerenes bearing different organic addends (15-18) used to prepare photovoltaic 60

Figure 4 .

 4 Figure 4. PCBM and other representative PCBM analogues involving a thienyl group ([60]ThCBM) or a [70]fullerene (PC 71 BM).

Figure 5 .

 5 Figure 5. Different modified fullerenes used as successful acceptors for PV devices 70

47

 47

Figure 7 .

 7 Figure 7. Some representative examples of fullerenecontaining polymers used fo the double-cable approach.70

Figure 8 .

 8 Figure 8. Chemical structures of four X-shaped oligothiophenes.

Figure 9 : 55 ( 65 Figure 10 .

 9556510 Figure 9: Chemical structures of SMDPPEH and C 71 -PCBM On the other hand, conjugated dendrimers constitute a new kind of well-defined molecular materials with monodisperse 50

Figure 11 .

 11 Figure 11. TPA derivatives bearing one or more dicyanovinylene groups.

40 theFigure 12 . 42 )

 401242 Figure 12. 42) Merocyanine MD 304; 43 and 44) Squaraine derivatives used for preparing PV devices by blending with PCBM. Borondipyrromethene (BODIPY) dyes (45, 46) are another kind of organic dye widely used for their outsdanding 50

Figure 13 .

 13 Figure 13. Bodipy dyes used for PV devices by blending with PCBM.

Figure 14 .

 14 Figure 14. UV-vis spectra and structures of the cyanines blended with PCBM for PV devices.

  photophysical investigations revealed a significant stabilization of the radical ion pair state of Sc 3 N@C 80ferrocene when compared to an analogous C 60 -ferrocene dyad.44

35

 35

Figure 15 .

 15 Figure 15. Optimized structures of a ferrocenene-I h -[5,6]-Sc 3 N@C 80 fulleropyrrolidine (50) (left) and the down isomer of an exTTF derivative of[6,6]-I h -Y 3 N@C 80(51) (Courtesy of Prof. J. M. Poblet).

52

 52

Figure 16 .

 16 Figure 16. Lu 3 N@C 80 -PCBH (52) and current-voltaje characteristics of P3HT/Lu 3 N@C 80 -PCBH (black) and65

Figure 17 .

 17 Figure 17. SEM image of the C 60 -complex prepared by microwave irradiation. The photoinduced charge separation at the polymer-C 60 interface is followed by electron transfer from C 60 to SWCNTs. The SWCNT netwok provides a direct 60

C 8 H 17 O OC 8 H 17

  18

	C 8 H 17 O OC 8 H 17 Br Br	+	S	O O B	C 8 H 17 O OC 8 H 17 S S
	N S	N				N S	N
					Br	S	S	Br
	N S N				N S	N
						+
					O	B	B	O
					O	N	O
						C 8 H 17

C 8 H 17 O OC 8 H 17 S S * N C 8 H 17 * n 10 11 12 13 14 8 HXS1

Scheme 1. Synthesis of polymer HXS1.

  Homo-and hetero-dimers of C 60 and C 70

	N	N	N	NO 2 C 8 H 17	N	N	N	NO 2	N	N	N	NO 2
	C 8 H 17								C 8 H 17
			25			26				27
							28
	Figure 6.

Acknowledgements

This work has been supported by the European Science Foundation (SOHYD, MAT2006-28170-E), the MEC of Spain (CT2008-00795/BQU, and Consolider-Ingenio 2010C-07-40 25200, Nanociencia Molecular) and Comunidad de Madrid (MADRISOLAR-2, S2009/PPQ-1533). S.F. and J.L.D. thanks the MICINN of Spain for a Ramón y Cajal Fellowship, cofinanced by the EU Social Funds. P.A.B. thanks IMDEA-Nanociencia for a postdoctoral research grant.

Notes and References