Progress in polymer solar cell - Springer Link

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Chinese Science Bulletin © 2007

Science in China Press Springer-Verlag

Progress in polymer solar cell LI LiGui, LU GuangHao, YANG XiaoNiu† & ZHOU EnLe State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

This review outlines current progresses in polymer solar cell. Compared to traditional silicon-based photovoltaic (PV) technology, the completely different principle of optoelectric response in the polymer cell results in a novel configuration of the device and more complicated photovoltaic generation process. The conception of bulk-heterojunction (BHJ) is introduced and its advantage in terms of morphology is addressed. The main aspects including the morphology of photoactive layer, which limit the efficiency and stability of polymer solar cell, are discussed in detail. The solutions to boosting up both the efficiency and stability (lifetime) of the polymer solar cell are highlighted at the end of this review.

1 Introduction Apart from nuclear power, all sources of energy consumed nowadays (such as fossil energy including coal, oil and natural gas, wind energy and waterpower) ultimately originate from the sun. It is estimated that one-day solar radiation reaching on the earth can supply enough energy to the currently existing 6 billion populations at present consumption rate for more than 27 years. With exhausting fossil energy and more and more serious environmental problem, we have to look for the other energy resources as the substitute, which should be clean and renewable. Solar energy, the so-called truly green energy, with almost unlimited supply capability, is widely distributed all over the earth. Having these prominent advantages, the worldwide researches on the solar energy have become one of the hottest scientific topics. Despite the fact that direct photovoltaic energy conversion is orders of magnitude more energy-efficient than any of those indirect sources, the global use of PV is only emerging at a slow pace. The issue behind is the cost of PV modules based on traditional PV technology is still too high to be afforded by common energy consumption. With the technology advancement during the past decades, the cost of the inorganic PV cell has alwww.scichina.com

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ready dropped a lot, but is still too expensive, and has come to a situation that further price decrease is very limited. Presently, the power conversion efficiency of a silicon-based PV cell has reached 24%, and the PV cell based on GaAs can even achieve an efficiency as high as 31%―32% (under AM 1.5 G condition)[1]. However, the numerous rigorous conditions required during manufacture processes yield expensive product, together with the environmental invasive chemicals formed during production, prevent its further development. Besides, the inflexibility of the modules makes it difficult to be applied to large scale installation. The PV cell based on conjugated polymer, the so-called polymer solar cell, has the same theoretical power conversion efficiency as silicon-based PV cell. However, the highest efficient polymer solar cell developed up to now is still less than 5%, and its stability in the ambient atmosphere is still a challenge for the scientists. The main issues behind the inefficient polymer solar cell are, for instance, the mismatched spectrum between the photo response of the device and the solar radiation, the low free charge carriReceived June 12, 2006; accepted October 13, 2006 doi: 10.1007/s11434-007-0001-y † Corresponding author (email: [email protected]) Supported by the Initiation Fund of “Hundreds of Talents Program” of Chinese Academy of Sciences and subsidized by the National Basic Research Program of China (Grant No. 2005CB623800)

Chinese Science Bulletin | January 2007 | vol. 52 | no. 2 | 145-158

POLYMER CHEMISTRY

polymer solar cell, bulk-heterojunction, optoelectronic thin film, morphology control, thin polymer film

ers’ mobility and the inefficient charge collection by the electrodes. However, due to its flexibility, low cost solvent-based thin film deposition technology (such as spin coating, ink-inject printing, screen printing) and lightweight as well, make polymer solar cell a very attractive ― solution of the energy resource in the future[2 4]. Additionally, it can be produced by using all polymer components, which can achieve a high light absorbing intensity[5,6]. The device performance can be easily manipulated via molecular design and synthesis of new polymer or organic semiconductors. Therefore, researches on polymer solar cell have recently become one of the hottest scientific topics. This review outlines the current progress in polymer solar cell.

2 Principle and device configuration 2.1 Principles of photoelectric response In the traditional silicon-based PV cell, most of the incident light is directly converted into free charge carriers, i.e. electrons and holes, which are then transferred to the cathode and anode, respectively, with the presence of the intrinsic potential of p-n junction. Photovoltaic power will be released via undergoing an external circuit for completing a circle[7]. The photoelectric response in polymer solar cell is actually conducted by the photoactive layer therein. In contrast to traditional inorganic PV cell, optical absorption in organic molecular or polymer semiconductors mainly creates electron-hole pairs (excitons) which are bound at room temperature. These excitons must be dissociated into free charge carriers before they can be collected by the electrodes. Otherwise, these highly reversible excitons will decay to the ground state via nonor emissive mode, giving no contribution to the photo-

voltaic power. Therefore, the dissociation of excitons into free charge carriers without presence of external electric field is the prerequisite for the photovoltaic realization of polymer solar cell. 2.2 Exciton dissociation The electron donor/acceptor approach is an effective way to realize exciton dissociation in organic PV cell. Correspondingly, the photoactive layer in a polymer solar cell should at least consists of two constituents, electron donor (donor or D) and electron acceptor (acceptor or A), respectively. The widely used donor constituents nowadays are mainly the conjugated polymers such as polyphenylene vinylene (PPV), polythiophene (PT), polyfluorene (PF) or their derivatives. Unfortunately, all the polymers mentioned above do not have sufficiently low band-gap for ideal application in photovoltaic devices. Recently, donor materials with much lower band-gap are introduced, for instance the copolymers consisting of the segments of thiophene, fluorene, pyrizine and so on. The electron acceptor materials heavily used are usually C60 or its derivatives, the inorganic nanoparticle acceptor such as ZnO and CdSe, the conjugated polymers with cyano group having strong electron affinity. To ensure an efficient exciton dissociation process, the lowest unoccupied molecular orbital (LUMO) level of donor material should be a little higher than that of the acceptor, which paves the way for the electron transfer from donor to acceptor (Figure 1). This energy transfer has already been conducted successfully provided that the LUMO level of donor is 0.3―0.4 eV higher than that of the acceptor[8,9]. The ultrafast photo-induced charge transfer from donor onto acceptor, which is finished within 100 fs, even can compete with the trap time within PV cell[10]. This

Figure 1 Schematic of photovoltaic effect in an organic solar cell via donor/acceptor approach. 146

LI LiGui et al. Chinese Science Bulletin | January 2007 | vol. 52 | no. 2 | 145-158

2.2.1 Double-layer device. The first issue is how to realize the exciton dissociation through the D/A approach in a real device. The pioneer work was done by Tang[21] who introduced the double-layer D/A architecture to organic photovoltaic cell (Figure 2). Typically, the exciton diffusion length is limited to 10 nm in organic semiconductors (including small organic

2.2.2 Bulk-heterojunction (BHJ). To overcome the limitations occurring in double-layer PV cell, Yu et al.[23] dissolved the donor and acceptor (for example C60 or PCBM) together in an organic solvent to achieve a mixture solution for composite film preparation by applying a film deposition technique, for instance, spin-coating. A composite film with interpenetrating networks composing of both D and A constituents, the so-called bulkheterojunction, has thus been obtained (Figure 3). The area of D/A interface in the bulk-heterjunction PV device has been substantially increased so that the excitons can be efficiently dissociated. The performance of PV device based on BHJ made a breakthrough as the power conversion efficiency has reached 5.5% under a monochromic illumination with 10 μW/cm2 intensity at 430 nm. Recently, the bulk-heterojunction polymer solar cell

Figure 2 Schematics of a double-layer organic photovoltaic cell and exciton dissociation at the interface. LI LiGui et al. Chinese Science Bulletin | January 2007 | vol. 52 | no. 2 | 145-158

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molecules and polymer semiconductors)[22], which suggests that the excitons generated far away (> 10 nm) from the D/A interface do not have chances to be dissociated into free charge carriers and have no contribution to photo current. Therefore, the total thickness of a double-layer PV device should not exceed 20 nm in terms of exciton dissociation efficiency. A too thin film, however, is not able to harvest enough sunlight. As a consequence, the power conversion efficiency of an organic PV cell based on double-layer architecture can hardly exceed 1%.

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gives nearly all the excitons which have diffused to the D/A interface sufficient opportunity to be dissociated into free charge carriers[11]. The ultrafast photo-induced electron transfer at D/A interface has been verified by a ― number of experiments[12 15]. It has been shown that the photo-induced initial current in the PPV film is orders of magnitude increased after only 1 wt% C60 acceptor has been added. Both the amount and lifetime of photoinduced free charge carriers will be increased with more amount of C60 acceptor being added[16]. Consequently, the ultrafast photo-induced charge transfer from donor onto acceptor on one hand enhances the charge carrier generation capability of the photoactive layer, and on other hand decreases the recombination of the charge carriers via the stabilization for both types of charge carriers. These experiments also further confirm that the photo-induced dipoles in polymer/fullerene composite are independent of each other[11]. We would like to note that the free charge carriers in organic semiconductor ― are independent of temperature[17 20], which is in contrast to that described by Onsager model[10].

Figure 3 Schematic illustration of a typical polymer solar cell based on bulk-heterojunction structure.

with an efficiency up to 4.4% under AM 1.5 G illumination has been achieved[24]. 2.3 Charge carrier transportation After the excitons have been dissociated into electrons and holes, it is required that these free charge carriers to be transported to their corresponding electrodes via the networks composed of A phase and D phase, respectively. During this process, it is possible that these charge carriers will be hindered by the space traps resulting from, for examples, discontinuity of the networks, existence of traps and so on, leading to lower carrier mobility compared to the inorganic silicon semiconductor[25,26]. Taking account of that the thickness of the photoactive layer in a polymer solar cell is typically around 100 nm, the free charge carrier recombination rate will be lower than the Langevin recombination at several magnitudes[27] provided that the transportation paths for both electrons and holes have been well established. In this case, the charge carriers can pass the photoactive layer before being trapped, as product of n (carrier intensity) and μ (charge carrier mobility) is independent of temperature. With increased film thickness, the charge carriers need more time to get across the layer, which will be comparable to the averaged trap time, resulting in the dependence of (n μ) on temperature[10]. The transportation characteristics of the charge carriers in organic thin films can be well described by the spacelimited charge (SLC) model[28]. Surprisingly, in the case of conjugated polymer films doped by nano-rods, the observed conductivity shows temperature dependence, indicating that charge carrier transport mode in these thin film devices conforms to the thermal activation model[29]. When this hopping-like transportation takes place, the effect of space charge is very important because in that case the electron mobility is lower than that 148

in the bulk. In order to fully utilize the advantage of high mobility of the charge carriers in nano-rods, it is required that these nano-rods have high aspect-ratio themselves in shape and preferably are perpendicular to the substrates as their organization in the composite film, and additionally the surface of these nano rods should be modified to eliminate the traps on the interface between the rods and the matrix polymer. All these could help the electrons or holes to transport to the respective electrodes quickly and directly, giving a high charge carrier mobility[30,31]. Furthermore, it is believed that using the polymers whose side groups can mount to the surface of the nano-rods via the interaction of strong chemical bonds will enhance the molecular orbital overlap, which will pave the way for faster charge carrier transporta― tion[32 36]. The morphology of both the donor and acceptor components plays an important role in determining the transportation of free charge carriers in the composite film, which will be discussed in detail in section 3.

3 Factors determining performance of polymer solar cell The efficiency of the present polymer solar cell is still too low to compare with well-developed silicon-based PV cell. However, the efficiency of a PV cell is the key point for considering the possibility of commercialization. The factors affecting the performance of polymer solar cell are outlined as follows. 3.1 Absorption spectrum of polymer solar cell It is well known that the efficiency of a PV cell heavily depends on the photo response range of the photoactive layer to the incident light. MDMO-PPV and P3HT are two widely used electron donor materials, with the absorption up to 550 nm (2.2 eV) and 630 nm (1.9 eV), respectively. However, a considerable amount of solar energy is located at near infrared or infrared region, which causes a non-negligible spectrum mismatch between the photo-response of active layer and solar emission spectrum. Correspondingly, the worse absorption of polymer solar cell to the sunlight is the main barrier preventing the substantial improvement of the efficiency. In contrast, the traditional silicon-based PV cell could be easily tuned to have a much better light absorption overlap with solar spectrum as shown in Figure 4, which


Figure 4 Energy distribution of ground solar spectrum at each wavelength and absorption coverage of a silicon-based photovoltaic cell.

The band-gap of a conjugated polymer is mainly determined by its effective conjugated length and functional groups on the main chains. For a given conjugated polymer, the most widely used and effective way to make the absorption spectrum red-shift is to resort to thermal annealing or choosing appropriate solvent to ― strengthen the inter-chain conjugation[37 39]. But the magnitude of red shift is quite limited and still far away from our destination if merely these methods can be chosen. Therefore, it is required to develop lower band-gap (Tg, thus treated device would show an even higher efficiency (Figure 7). Subsequently, more efforts are devoted to the study of the influence of thermal anneal on polymer PV cell perfor― mance[55 59]. Recently, Yang et al.[37] observed that the presence of fibrillar crystals of P3HT in P3HT/ PCBM composite films led to a restrict diffusion of PCBM molecules within the matrix, and thus the limited aggregation of PCBM into clusters during post-thermal annealing. PCBM only partly aggregated into nano-scale domains homogeneously distributed in the composite film, and the short P3HT fibrils extended into long fibrils upon thermal treatment. As a consequence, the interpenetrating networks composed of PCBM and P3HT constituents were formed, which provided both large interface area for exciton dissociation and continuous pathways for charge carrier transportation (Figure 8). In that system, the Jsc was twice compared to the pristine device[37].

Figure 6 UV/Vis absorption spectra. 1, [60]PCBM; 2, [60]PCBM: MDMO-PPV (4:1, w/w;); 3, [70]PCBM; 4, [70]PCBM:MDMO-PPV(4:1, w/w). All experiments used toluene as solvent[50].

3.2 Morphology of photoactive layer In polymer PV cell, the charge transport is greatly affected by the morphology of the photoactive layer. The morphological factors, for instance, the interface area between electron donor and acceptor constituents which provides the place for excitons dissociation, the quality of interpenetrating networks including the shape, continuity and orientation with respect to the film plane so as to provide enough highly-efficient pathways for both 150

Figure 7 Current-voltage (I-V) curves of P3HT-PCBM solar cell irradiated under white light with an illumination intensity of 800 Wm−2 for as-prepared solar cell (solid square), post-thermal annealed cell (open circle) and cell treated simultaneously by thermal annealing and applying an external electric field (open triangle)[55].

It has been demonstrated that the characteristics of solvent used and its evaporability play an important role in the morphology formation of the photoactive layer. In MDMO-PPV/PCBM composite system, the film made


REIVEW Figure 8 Bright-field transmission electron micrographs (BF-TEM) and schematics of pristine ((a) and (b)) and thermal annealed ((c) and (d)) P3HT:PCBM composite thin film in the photovoltaic cell[37].

from chlorobenzene exhibits small scale of phase separation and size distribution (Figure 9), and efficiency of the cell is much higher than that of the device by using toluene as the solvent[38]. It is believed that the poor solubility of PCBM in toluene should be responsible for this huge difference in both morphology and performance even the identical constituents were used for the devices fabrication[38,60]. Additionally, as shown in Figure 9(c), much coarser MDMO-PPV/PCBM film was obtained if a film deposition method with slow evaporation rate compared to spin coating is applied, which also hints that the fast solvent evaporation associated with spin coating will suppress phase separation, resulting in a smoother film as shown in Figure 9(b)[57,60]. By contrast, a better self-assembly organization of P3HT can be achieved in the P3HT/PCBM film at low solvent evaporation (such as solvent annealing)[55,61], giving improved device performance. Yang and van Duren et al.[57,62] have shown that the originally homogeneous MDMO-PPV/PCBM composite film at low PCBM ratio evolved into a morphology that the PCBM-rich domains distributed in the matrix composite as the ratio of acceptor PCBM increases up to a certain value, forming nano-scale phase separation observed in a direction normal to the film plane. It was noted that this morphology remained the same across the

perpendicular direction of the film. Although nearly all the photo-induced excitons have been dissociated into free carriers for the composite film with only 2wt% PCBM, the highest performing device, however, was achieved at 80wt% of PCBM (Figure 10). This is attributed to whether the continuous pathways are present or not for the electrons to be transported to the cathode. The phase separation has not yet reached at low PCBM ratio, and no continuous pathways composed of PCBM are obtained. As a result, the most electrons will recombine by space traps during their transportation to the cathode. Systematic studies by transmission electron microscopy (TEM) confirm that unless the weight content of PCBM reaches 70%, the nanoscale phase separa-

Figure 10 Short circuit current (Jsc) and power conversion efficiency (ηpc ) as a function of PCBM concentration in the composite film[62].


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Figure 9 Bright-field transmission electron micrographs (BF-TEM) and corresponding diffraction patterns of MDMO-PPV:PCBM (1:4, weight) films. (a) Spin-coated from toluene; (b) spin-coated from chlorobenzene; (c) drop-cast from chlorobenzene[57].

Figure 11 (a) External quantum efficiency (EQE) spectrums of 7-nm-diameter nanorods with lengths of 7, 30, and 60 nm with an illumination intensity of 0.84 W/m2 at 515 nm; (b) photocurrent spectrums for two devices using nanorods with 60 nm in length, 7 and 3 nm in diameter, respectively[31].

tion will not be observed[57,62]. At that ratio, the PCBM has been able to form large enough continuous pathways to help electrons to transport to the electrode, thus resulting in an improved efficiency[62]. The effect of spatial confinement also plays an important role in determining the final morphology of the photoactive layer during thermal annealing. Correspondingly, the device performance is also linked to the spatial confinement exerted to the composite film upon annealing. Yang et al.[63] observed that with the increased spatial confinement, the kinetics of PCBM diffusion decreased and large scale phase separation was inhibited in the MDMO-PPV/PCBM composite film during thermal treatment. With respect to the detailed morphology evolution of the thermal treated film, Zhong et al.[64] confirmed that the stacking rate of PCBM molecules in the vertical direction was higher than that of paralleling direction to substrates by performing quantitative analysis on the evolution of MDMO-PPV/PCBM composite film. In the case of inorganic CdSe nano-rod hybrid polymer solar cell, Huynh et al.[31] have achieved a homogeneous film by using bi-solvent approach (Figure 11). The optimal donor to acceptor ratio depends on the shape (aspect ratio) of nano-rod[65]. Due to the toxicity of CdSe, Beek et al.[66,67] developed ZnO nanoparticle served as electron acceptor. The performance of the cell based on ZnO is comparable with the CdSe-based polymer solar cell. In order to overcome the difficulty of morphology control on the photoactive layer composed of highly crystallizable constitutes, Ganesan et al.[68] intensively investigated a tetrahedron-like new electron acceptor which is not able to pack to form ordered clusters. It was 152

found that not only the excitons could be dissociated completely at its interface with conjugated polymer, but also the lifetime of charge carriers was increased to the microseconds order. 3.3 Charge carriers mobility Besides the mismatch of photo response range of photoactive layer to the solar spectrum and the morphology of photoactive layer, another factor which also heavily limits the efficiency of polymer solar cell is the low mobility of charge carrier in the organic semiconductors. The electron mobility of the materials widely used in the present polymer solar cell is mainly at an order of 10−3 cm2 V−1 s−1 (the hole mobility is even much lower), which is quite lower compared to the traditional inorganic semiconductor based on, for instances, silicon or germanium, which typically possesses a mobility at an order of 104 cm2 V−1 s−1. Low charge carrier mobility will increase the probability of recombination, and consequently results in the low efficiency of the polymer PV device. Generally, the intrinsic mobility of free charge carriers depends on the perfection of conjugated bonds along the chain and the ordered π-π stacking length[69]. As mentioned above, the charge carrier mobility in the composite film will be improved by several magnitudes upon thermal annealing due mainly to the increased phase separation, which results in improved order for both electron donor and acceptor constituents[26,37,57]. In the screening of new organic semiconductor having high charge carrier mobility, lately Drolet et al.[69] developed a 2,7-carbazolenevinylene-based oligomer, whose charge carrier mobility reaches 0.3 cm2 V−1 s−1. Kim et al.[70] achieved the 50%―70% enhancement in


3.4 The electrodes and their interfaces It has been demonstrated that better ohmic contact can be achieved between cathode and photoactive layer if an extremely thin LiF layer (around 0.6 nm) was additionally inserted in between, which is in favor of increasing fill factor (FF) and stabilizing open circuit voltage (Voc), subsequently leading to the improvement of device performance[72]. If a too thick LiF layer was inserted, however, the device performance would rapidly drop down. The efficiency of the PV cell always significantly decreases provided that the thin LiF layer was replaced by an insulating SiOx layer. Therefore, it is safe to conclude that the efficiency enhancement in the devices upon LiF layer insertion has nothing to do with the insulating buffer effect[72]. Shaheen et al.[73] have intensively investigated the effect of a number of alkali salts with high dipole moment on the performance of the PV cells. They observed that only the Li compounds could enhance the performance of PV device, and the insertion of Cs or K compounds, in contrast, led to severe efficiency drop. They ultimately attributed this efficiency enhancement upon LiF layer insertion to the fact that the Li+ preferentially adhered to the surface of photoactive layer, forming a bulk dipole moment or chemical reaction induced electrons cross the interface, resulting in an interpenetrated dipole moment between the organic layer and the cathode[72]. As a consequence, a vacuum level offset is formed between the organic layer and the cathode[74], which lowers the work function of the cathode metal and is in favor of the electron injection to the cathode[75,76]. If the LiF was replaced by Cs or K compounds, neither preferential alignment nor the same alignment direction as LiF could be achieved[74]. Correspondingly, no improved device performance can be observed. Built-in potential is supposed to be the prerequisite for realizing the photovoltaic function of polymer solar


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cell. Almost all the necessary processes for photovoltaic effect including charge carriers generation, transportation and then collection by the corresponding electrodes rely on this potential. The built-in potential within a PV cell is usually estimated from Voc. Brabec et al.[74,75] observed that the Voc of bulk-heterojunction solar sell based on fullerene or its directives was directly related to the acceptor’s electron affinity, i.e. the first reduction potential. They confirmed that the Voc of such device was determined by the different values between the acceptor’s LUMO level and the donor’s HOMO level, which is independent of the work function of cathode material[74,75] and device geometrical factors such as shape and thickness[75]. The fact that the Voc is insensitive to the cathode materials can be explained by the formation of quasi-ohmic contact between metals and fullerenes through Fermi level pinning[74], i.e. pinning the work function of cathode metals to that of fullerenes[74,75]. In the case of high work function metals, such as Au electrode, the quasi-ohmic contact can also be realized by the formation of interface dipoles through charge transfer between the first monomolecular layer of cathode metals and fullerenes[74]. The exact formation of ohimic contact has been intensively studied by Mihailetchi et al.[76]. They observed that if the work function of cathode metals was lower than the LUMO level of the donor, the ohmic contact between cathode and acceptor phase would easily form. In this case, the value of Voc was determined by the difference between the LUMO level of the acceptor and HOMO level of the donor (Figure 12). It should be noted that although the influence of the electrode work function on Voc is negligible, the charges accumulated on the interface will lead to band bending. As a result, the electric field intensity decreases, leading to a dropped Voc down around 0.4 V. In the case that the work function of the cathode is higher than the LUMO level of the acceptor, a non-ohmic contact is formed on the interface between cathode and the acceptor. The formation of non-ohmic contact will hinder the electron from injecting into cathode, exhibiting an interfacial barrier potential ϕb. And the value of Voc is obtained by the different work functions of anode and cathode (Figure 12(b)). Therefore, in order to enhance the efficiency of charge carreirs’ collection, we should ensure an ohmic contact to be formed between electrodes and photoactive layer.

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device efficiency by doping the photoactive layer with Ag and Au nanoparticles whose Fermi levels are between the HOMO level and the LUMO level of the donor, which was attributed to the decreased serial resistance and the improvement of hole mobility in the photoactive layer. A similar experiment has also been conducted by Li and his co-workers[71]. They doped the photoactive layer with another organic molecule having high hole mobility and achieved substantial enhancement in the device performance.

Figure 12 Schematic illustration of the variation of Voc with acceptor strength and the work function (Φ1 and Φ2) of electrodes for a bulk-heterojunction photovoltaic cell. The bended arrows in the figure represent the photo-induced charge transfer in D/A interface.

4 Stability of polymer solar cell Compared to a 25-year lifetime of inorganic PV cell, the durability of polymer solar cell is still rather short. A number of factors can damage the performance of the polymer cell, for example, oxidation of the involved materials in the cell induced by moisture and oxygen, the photo-induced reduction between conjugated polymer and the cathode, the photo-induced polymer degradation and so on. Thus, it is inevitable that the chemical reactions will take place as conjugated polymers are exposed to intensive ultraviolet light, elevated temperature, undergoing strong current, highly reactive electrode, oxygen or humidity environment. In order to find the appropriate solutions to increase the lifetime of polymer solar cell, it is necessary to understand the physical or chemical details involved in the device degradation, so as to take actions to hinder this degradation process. 4.1 Chemical structure and stability The prerequisite for a safe fabrication of polymer solar cell device is that the materials should be stable in air. As far as a certain donor polymer is concerned, the polymer whose HOMO level is close to 5.2 eV or lower can maintain its electrochemical stability in air[77]. The donor materials widely used in the present polymer solar cell are the conjugated polymers MEH-PPV, P3HT, and MDMO-PPV, whose HOMO levels are 5.4, 5.2, and 5.1 eV, respectively. Based on this result, we could assume 154

that they are all chemically stable in air. Nevertheless, one cannot come to the conclusion that the devices made out of them are highly stable. A HOMO level as low as 5.2 eV can only ensure that the materials involved in the photoactive layer are relatively stable in air. Long-term stability of the device performance, however, can only be evaluated after the rigorous examinations. The polymers with rigid backbone architecture are usually more stable because of their chemical structure. Therefore, if the donor polymers with rigid structure or functional groups were applied to PV cell fabrication, the devices with enhanced stability and elongated lifetime could be anticipated. Studies confirm that stability and durability will be greatly enhanced when the as-prepared polythiophene-based photoactive layer with thermal cleaved side chains is converted into an insoluble rigid structure by thermal cleavage[78,79]. Moreover, attention should also be paid to the stability of photoactive layers fabricated under the ambient condition with dust, oxygen, moisture and so on. Figure 13 clearly shows that the protection against air and oxygen can considerably extend the lifetime of polymer PV devices. The conjugated polymer components involved in the photoactive layer degrade quickly under ambient condition with the presence of photo-irradiation and oxygen. However, the kinetics of this degradation could be significantly decreased upon addition of electron acceptor such as fullerenes and their derivates. This is usually


attributed to the photo-stabilizing effect of electron acceptors due to the presence of ultrafast electron transfer from conjugated polymers to the acceptors. This electron transfer quenches the triplet state associated with the excited conjugated polymer[81,82], reducing the quantity of highly reactive singlet oxygen results from triplet-triplet annihilation. However, under the condition of long-term photo irradiation and in the presence of oxygen, it is no doubt that the conjugated polymer will react with the residual oxygen introduced during device fabrication or inleaked oxygen resulting from poor encapsulation, forming more and more defects in the photoactive layer. As a result, the device performance will gradually decrease[83]. Additionally, traps that capture free charge carriers will form in the photoactive layer once the humidity in air has penetrated in, resulting in low current density. Therefore, precautions towards dust, moisture and oxygen free are necessary for enhanced stability and elongated lifetime of polymer PV cells. 4.2

Morphology stability

As described above, exciton separation only takes place at the interface between the donor and acceptor. Thus, the D/A interface plays a significant role in determining the power conversion efficiency of the PV devices. The morphology of photoactive layer, which usually gives the length scale of phase separation and the area of the interface between the components, plays a key role in determining the device performance of the PV cells. The morphological stability of the photoactive layer certainly influences the stability of device performance. However, the morphological stability, which nearly has nothing to do with the chemistry of materials, is determined by the physical characteristics of both the components involved

4.3 Possible solutions The stability of the polymer solar cell is still an urgent problem to be solved in the near future before considering the possibility of commercialization. We propose the following methods as feasible solutions for this problem: 1) Choosing donors materials whose HOMO levels are higher than 5.2 eV[43,77]. 2) Stabilizing the morphology of photoactive layer via utilization of the conjugated polymers with high ― Tg[55,78 80]. 3) Introducing some methods widely used in the fabrication of light emitting diodes, for instance, chemical reaction or nuclear irradiation to induce chains cross― linking[84 86]. 4) Utilizing p-n block copolymers[87 89], in which the phase separation will be restricted by covalent bonds. ―

5 Outlook and strategies As the energy crisis and environmental problem are becoming more serious all over the world, the utilization of this truly green energy, solar energy, has become the hottest topics for both the governments and scientific


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Figure 13 The absorption spectrums of MDMO-PPV film as a function of time under illumination of 1506 cm−1 with different protections[80].

and the thin film formed. The instability of the photoactive layer associated with its morphology is usually impossible to be solved via device encapsulation. The nano-scale phase segregation in as-prepared composite film is in a thermodynamically non-equilibrium state at elevated temperature, and the morphology of photoactive layer has great tendency to evolve towards its equilibrium state under the driving force from thermodynamics, forming larger scale phase separation. In MDMO-PPV: PCBM composite, Yang et al.[57] demonstrated that the diffusion of PCBM molecules, which always leads to large scale crystallization of PCBM, even occurred when a thermal annealing was carried out at a temperature as low as 60℃. Large scale phase separation will consequentially result in poor performance of PV cells as the interface area between the donor and acceptor decreases. In conclusion, to maintain the morphological stability of the as-prepared photoactive layer is a challenge for the scientists since the environmental temperature at which a solar cell device really works is quite high, which provides driving force for thermodynamically unstable composite film to evolve towards more stable state via large scale phase separation, ruining the performance of PV devices.

communities. The substantial investments from both the scientific realm and human resources to this field will certainly boost the performance of polymer solar cell. With a 4.4% power conversion efficiency already achieved, together with a large room left for enhancement, it is believed that polymer solar cell has tremendous potential to achieve great success in the future. By utilizing solvent-based thin film deposition technology, which is capable of large-area fabrication with low-cost, the energy crisis problem will hopefully be solved. Attention should be paid to the following aspects to achieve further improvements in polymer solar cell: 1. Screening low band-gap donor materials, whose 1

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absorption range covers better solar spectrum and correspondingly could harvest more sunlight. 2. Optimizing the electrodes to achieve better ohmic contacts at both interfaces with the electrodes for the better collections of the charge carriers. 3. Improving morphology of the composite films to achieve more continuous and ordered pathways for charge carrier transportation to the electrodes directly and quickly. 4. Introducing new device architectures, for instance, the devices with multi-junction and multi-layer configurations are more suitable for the efficient utilization of incident sunlight. 13

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