About this Journal Submit a Manuscript Table of Contents
International Journal of Photoenergy
Volume 2010 (2010), Article ID 123534, 11 pages
http://dx.doi.org/10.1155/2010/123534
Review Article

Organic Solar Cells: Problems and Perspectives

Dipartimento di Chimica, Università degli Studi della Calabria, 87036 Arcavacata di Rende, Italy

Received 18 March 2010; Accepted 12 May 2010

Academic Editor: Leonardo Palmisano

Copyright © 2010 G. Chidichimo and L. Filippelli. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

For photovoltaic cells to convert solar into electric energy is probably the most interesting research challenge nowadays. A good efficiency of these devices has been obtained by using inorganic semiconductor materials. On the other hand, manufacture processes are very expensive in terms of both materials and techniques. For this reason organic-based photovoltaic (OPV) cells are attracting the general attention because of the possible realization of more economical devices. Organic materials are abundant and easily handling. Unfortunately OPV cells efficiency is significantly lower than that of inorganic-based devices, representing a big point of weakness at the present. This is mainly due to the fact that organic semiconductors have a much higher band gap with respect to inorganic semiconductors. In addition, OPV cells are very susceptible to oxygen and water. In this paper we will describe some of the different approaches to the understanding and improving of organic photovoltaic devices.

1. Introduction

Semiconducting conjugated polymers are the organic materials used in OPV cells, since they possess the base property required to activate the fundamental mechanisms to transform the radiative energy of light into an electric current. When a donor (D) and an acceptor (A) material are being in contact, the result is the so-called heterojunction and this is the basis for the operation of organic solar cells [19]. As a solar photon is absorbed, an exciton occurs which is a coulombically bound electron-hole pair, and it distributes to the D/A interface. At this stage the excitons are separated into free holes and electrons by the electric field. Two types of architectures are currently used to create a D/A heterojunction, that is, bilayer heterojunction and bulk heterojunction (BHJ) solar cells. The latter system is to date the most investigated way to produce solar devices. This is mainly due to the favorable combination of an easy manipulation in the production process and a higher conversion efficiency because of a markedly larger D/A interface. From a schematic point of view, organic solar cells operate the conversion of the incident solar irradiation to electrical current, through essentially four-steps process. Figure 1 depicts this process. In this view, the donor is termed the holes transporting material and it makes contact with the anode, while the electrons transporting material is the acceptor, which is in contact with the cathode.

123534.fig.001
Figure 1: Scheme of operative sequence of an OPV.

The creation of an exciton after the absorption of a photon is the first step. The exciton diffuses inside the material to reach the donor-acceptor interface where it will be separated. All of these four steps are potential targets for researchers to improve performances of the OPV. Absorption efficiency, that is how much light is harvested, mainly depends upon the absorption spectra of the organic materials. However the design of the devices, including their thickness, has not a minor role in capturing as much as incident wavelengths. The exciton diffusion length is the parameter that accounts for the efficiency of the second step. It comes out that longer diffusion length corresponds to a greater probability that the exciton reaches the D/A interface raising the holes-electrons production. Clearly the morphology of the D/A boundary also is of great importance. In fact the acceptor has possibly to be very close to the donor so that the D/A interface is always next to the point where the exciton is created. Indeed the third step is the exciton split-up into free charges and this step depends mainly on the properties of the donor and acceptor but also the overall device architecture plays an important role. The last step, that is the transport of the free charges through the sample and their collection at the electrodes, is the final step and it can be considered as central in a organic photovoltaic device. This crucial function derives from the fact that organic film structures are generally amorphous and disordered and, consequently, charge recombination is strongly favored. In addition, film resistance is large and reduces, thus, the current efficiency. The molecular structures of some of the materials are shown in Figures 2(a) and 2(b).

fig2
Figure 2: Examples of materials used as donors (a) and acceptors (b).

2. Absorption Efficiency

Conjugated π-systems are extensively used as both donor and acceptor materials (Figures 2(a) and 2(b)) and they are often referred as organic semiconductors. However they are intrinsically different from classic crystalline inorganic semiconductors which, for instance, can absorb a continuous spectrum of light and the three-dimensional rigid lattice provides large carrier mobility and broad conduction and valence bands. Typically conjugated materials exhibit intense and broad absorption, but since the thickness of organic semiconductors has to be tiny, because of short exciton diffusion lengths, the incident light is not captured efficiently. In addition organic semiconductors have well-defined electronic transitions that are typically quite narrows and have a propensity to be very susceptible to the background. Nevertheless when organic semiconductors can reach an ordered solid crystalline structure, their absorption band becomes broadened with evident benefits with respect to the solar cell efficiency. Hence the molecular structures and the eventual propensity to aggregate have a considerable influence on the absorption spectrum. For instance, many papers reported CuPc and SubPc (Figure 3) as materials used in organic solar cells. Phthalocyanines in general have been used as the only donor material within the system or as dye molecule in combination with a different donor [1014]. The absorption spectra of the above mentioned CuPc and SubPc have similar profiles when recorded in very dilute solutions, but they are very different in the solid state [15]. Furthermore the absorption spectrum of a thin film of SubPc is comparable to that of SubPc in solution while the CuPc spectrum recorded in solid state is quite different from the spectrum of CuPc in solution. This is mainly due to the fact that films made of CuPc show the presence of the cofacial aggregates of planar phthalocyanine molecules [1619] whereas SubPc forms a very smooth and amorphous film [20].

fig3
Figure 3: Molecular Structure of SubPC and CuPC.

Sometimes although a good combination of donor and acceptor is found, the efficiency of the cell is low unless a dye is introduced into the system to improve its performances. For example, one of the most investigated organic solar cell is based on the bulk heterojunction poly (3-hexylthiophene) (P3HT)  :   -phenyl-C(61)butyric acid methyl ester (PCBM) (Figures 2(a) and 2(b)) blend film, where relatively high values of efficiencies (4%–5%, see Table 1) have been reported [2125]. However, the major inconvenience of P3HT is the scarce matching of its absorption spectrum with the solar emission spectrum. The absorption of P3HT is limited to wavelengths below 650 nm, and as a consequence it is only able to collect little more than 20% of the solar photons [19, 26]. Introducing a dye in this system can improve performances. For example, 9,10-diphenylanthracene (DPA) has been used as a conjugated dye with different concentrations into a solar cell P3HT:PCBM system [27] and the photocurrent was improved by a factor 3,7. In addition all of the solar cell performance parameters and power conversion efficiency improved as a result of improvement in the light harvesting and charge carrier transfer taking place between P3HT and PCBM through the conjugated DPA molecules. The efficiency of the photogeneration can be also improved rising the exciton generation rate. We have already mentioned that the low charge mobility (high resistance) of organic semiconductors obliges the device thickness to stay thin. Hence the wavelength of the incident light is longer than film thickness causing optical interference. A standing wave occurs at each interface within the device and the complex refraction indices and the thickness of the different layers govern this wave pattern. The latter interference pattern can be precisely calculated and it corresponds to the exciton generation rate within the whole structure [2830]. Therefore the enhancement of photocurrent might be possible, at least for planar heterojunction cells, through the modulation of the thicknesses of the layers [31, 32]. In the next paragraphs we will resume the critical points of the other three steps of the working sequence of an OPV. Although each step can be viewed and treated separately from the others, the four different phases are strictly interrelated. Improving a single step might not lead to the overall improvement of the device or the enhancement might be less than expected. It is the case, for example, of the bulk heterojunction that greatly enlarged the interface area between donor and acceptor leading to a great production of separated charges, but the random nature of the phase separation did not help the charges to reach the electrodes and be collected.

tab1
Table 1: Best performances of different systems.

3. Exciton Diffusion

Once the exciton is generated, the electron and hole remain localized on a few polymer repetition units or a molecule and they are bound to each other by the electrostatic attraction. This is also called intrachain exciton to indicate that the constituent charges are situated on the same polymer chain. It is believed that intra-chain excitons represent the main species that are formed after photoexcitation in conjugated polymers [33, 34]. Upon adding a charge to a polymer chain, the chain will deform in order to reduce the energy of the carrier. This charge and deformation together constitute a polaron. The energy levels of a polaron are within the HOMO-LUMO gap. Polarons can drift along the conjugated chain. Once they reach the end of a conjugated segment, a hopping process to another conjugated chain can occur. It is known that in molecular crystals, excitons can diffuse efficiently by energy transfer and that the same mechanism takes place in thin layers of molecular semiconductors [35]. However the research efforts are currently not so much devoted to increase diffusion lengths by modeling new molecules, but most of the researchers focus their action.to decreasing the distance between donor and acceptor [3639].

4. Charge Dissociation

BHJ structures were designed for this purpose and they accomplished the requirement of reducing distances between D and A. In fact only when the exciton reaches the D/A boundary they will separate effectively. The energetic driving force for separating the positive and negative charges relies in the difference between the LUMO levels of the donor and acceptor. If this energy is larger than the binding energy of the exciton in the donor, then the dissociative process became more favorable. In this view donor and acceptor should stay very close each other as they do in the BHJ devices.

5. Charge Transport and Materials

However once separated, charges need to travel through the materials towards electrodes and the random oriented domains of donor can greatly reduce the cell efficiency not providing the necessary conductivity that charges require in order to be collected in the last step of the operating sequence. In this view, ordered structures are supposed to improve conductivity of the system thus cell efficiency. Unfortunately a real control on the morphology of these systems is not achieved yet.

Trying to accomplish the necessity of an ordered array of D/A, nanorods and nanotubes of donor and acceptor materials have been investigated. In particular organic solar cells were made of Single Walls carbon Nano Tubes (SWNTs) employed as acceptor, and the poly(3-octylthiophene) (P3OT) as donor [40]. The open circuit voltage of the device was found to be 0.75 V, quite larger than expected. It was proposed that the improvement in the photovoltaic properties is caused by the internal polymer/nanotube links within the polymer matrix. Once dissociating the exciton, there is a uninterrupted way for electron and hole transport towards electrodes. On the other hand, the photocurrent of such devices is relatively low due to the partial phase separation and scarce quantity of absorbed light. The difference between the optical absorption of the polymer and the solar spectrum causes the low photocurrent. In addition the nanotubes do not supply the photogeneration process. Thus the same authors incorporated a high absorbing organic dye at the polymer/nanotube junctions and the photogeneration process was improved, especially for the UV portion of the solar spectrum [41]. The short circuit current was found 5 times larger than the nondye system. However the open circuit voltage was lowered by 0,1 V and under solar illumination the overall performances did not improve significantly (current 1  Acm−2). A blend of two conjugated polymers as the photoactive film was reported in 1995 [42, 43], and since then, polymer-polymer solar cells have not attracted so much attention. However, regardless their modest performances (less than 2% efficiency) [44], these kinds of devices might be exploited for a series of advantages they can provide. As for solar absorption, for instance, two conjugated polymers could be tuned as harvesting a wider spectral wavelength. Moreover, the manipulation of polymers in terms of adjusting the donor-acceptor energy levels, is fairly easy. The main obstacle for the technology based on polymer-polymer solar cells resides in designing conducting acceptors polymers which should have properties similar to those showed by fullerenes. In this view the use of block copolimers for solar cells is an early stage research area. Block copolymers (BCPs) synthesis would be one potential approach because periodic, adjustable nanostructures can be obtained [45]. An ideal BCP can be described as a block containing a p-type donor and block containing an n-type acceptor (Figure 4).

123534.fig.004
Figure 4: Schematic representation of block copolimer as an OPV device.

Nevertheless as the use of block copolymer for solar cell devices was theoretically predicted [46], efficiencies beyond the 0.5% were not achieved [47, 48]. In the field of materials, when order and mobility are required, it is natural to think about liquid crystals. They are currently considered as the new generation of organic semiconductors. By using conjugated LCs one can, in principle, control order in the bulk and at interfaces, from molecular to macroscopic distances. Because of their liquid-like character they can self-repair structural defects. Large single domains can be obtained by simply thermal annealing [49, 50]. By easily tuning parameters such as concentration or temperature [51, 52] and irradiation with polarized light [53] or surface alignment layers [54, 55], one can orient molecules inside these large domains. In addition defect-free chemical structures and a high purity level can be obtained due mainly to the low molecular weight of conjugated LCs. In this frame discotic liquid crystals have been reported as semiconductors [5658]. Disk-like LCs molecules can self-assembly in a columnar array (Figure 5) [58].

123534.fig.005
Figure 5: Ideal architecture of discotic LC-based solar devices.

Columns of discotic mesogens display one-dimensional charge transport and within the columns adjacent disc-like molecules experience a large orbital overlap. The band width has been measured as 1.1 eV [60], and high values of the charge carrier mobility (m), that is, 0.2–1.3 cm2 V−1 s−1, in their liquid crystalline (LC) mesophases were reported [6164]. Furthermore the measurement of the exciton diffusion length in the same columnar mesophases was about 70 nm [65]. Probably the best discotic-based photovoltaic devices would be made by the columnar array of the donor, perpendicularly oriented to the substrate, which is embedded in an acceptor environment. Such a device has been proposed and an external efficiency of 5% with an external quantum efficiency (EQE, photon to current) of 34% at a monochromatic wavelength of 490 nm has been claimed [66]. The discotic liquid crystal hexa-peri-hexabenzocoronene (Figure 6(a)) as hole conductor was blended with a perylene dye (Figure 6(b)) to create thin films with separated perylene and hexabenzocoronene perpendicularly oriented.

fig6
Figure 6: (a) Hexa-peri-hexabenzocoronene. (b) perylene dye.

However the ideal morphology showed in Figure 5 that it is quite difficult to be obtained and the overall efficiency of about 0.5% reported is an evidence that the perfect blend discotic-perylene was not achieved. Very likely the high performance for the EQE is quenched by a scarce phase separation and high percentage of recombination phenomena due to the fact that perylene molecules infiltrate within columnar stacks of the discotic liquid crystal. Thus the optimal device would be composed of ordered structures of separate donor and acceptor lamellae as Figure 7(a) shows.

fig7
Figure 7: (a) The ideal device architecture. (b) BHJ architecture.

To get this goal the homeotropic alignment of liquid crystals can be a promising way to pursue. In fact uniformly oriented textures of oligomers and polymers each other connected can be obtained by polymerization processes (photo induced, as an example) performed in liquid crystalline systems that have been prepared, in a homeotropic alignment by application of electric fields or by standing surface interaction (Figure 8) [67]. In the sketch reported below is represented a possible way to overcome the problems caused by the bulk heterojunction architectures.

123534.fig.008
Figure 8: Possible device architecture for an ideal D/A organic solar cell.

The red drawn molecules are polymerizable liquid crystals while the blue ones are not polymerizable liquid crystals. The result of the polymerization of “red molecules” allows getting the textures as presented in Figure 9 [67].

fig9
Figure 9: Scanning Electronic Microscopy of a Liquid crystal based device.

Electro-optical data, performed in our laboratory, confirm the noticeable evidence: the polymer forms structures that can be assumed as small cylinders filled with the not polymerized liquid crystal. The cylinder diameter is less than 1  m, but it can be controlled through parameters such as temperature and relative concentrations of the components. The not polymerized component can experience a phase transition to crystalline solid without any effect for the preformed structures. This kind of ordered microscopic structure would represent a great promise for application in organic photovoltaic. Although they cannot be strictly classified as OPVs, DSSC (Dye Sensitizer Solar Cells) photovoltaic cells [68], because of their very good performances (11% conversion efficiency), are worthy to be mentioned. They are made with a dye which is absorbed into nanocrystalline TiO2, ruthenium complex sensitizers, electrolytes containing fluid redox couple, and Pt-coated counter electrode. The redox couple donates its own electron in order to regenerate the dye and it avoids the reduced form of the dye to take back the injected electron. However, redox system can substantially decrease long-term stability and can be incompatible with some metallic components. Furthermore iodine is an oxidizing agent and it can rust metals, particularly if water and oxygen are present. Additionally, photocurrent loss can be induced because of the electrolytes absorb visible light (  nm). In the view of the mentioned working principles, the titania layer can be considered as the electron-transporting and the redox couple as the hole-transporting layer. Thus redox couple can be substituted by a p-type semiconducting material as a hole-transporting material (HTM). A first reported alternative to was a new couple, cobalt(II)-bis[2, 6-bis( -butylbenzimidazol- -yl)pyridine] (Figure 10), and it was found to act as a redox couple in DSSC systems [69].

123534.fig.0010
Figure 10: Molecular structure of redox couple replacing iodine.

Other copper complexes were examined as substitutes of the redox couple [70] and the operative working of the cells was demonstrated. However, low efficiencies (under 2%) were obtained. In addition, the time decay of the original DSSC performances has been attributed to a solvent loss of the electrolyte mixture. Therefore gelled-electrolyte-based DSSC systems were prepared and studied [71], but none of the proposed nonvolatile solvent-based cells has reached the original performance of the liquid electrolyte system. Iodide/iodine-free DSSCs using conductive polymers (Polymeric Hole-Transport Materials pHTM) such as polythiophene derivatives (Figures 11 and 12) were attempted to construct using a spin-coating technique and related dip-coating methods.

123534.fig.0011
Figure 11: A L. C. polythiophene.
123534.fig.0012
Figure 12: Examples of hole transporting materials (P3TAA, P3TAA-PHT) and dyes.

Poly(3-butylthiophene) [72] and poly(octylthiophene) [73] were investigated using N3 dye as sensitizer, giving very poor results (efficiency 0.16%). Recent attempts using regioregular poly(3-hexylthiophene) (P3HT) for hole-transporting layer of N-719-sensitized DSSC gave a little improved performance.

The nanoporous structures of the titania layer represent a barrier for polymeric hole transporting material to infiltrate into the nanospace. A more complex way is to synthesize HTM polymers in situ; that is, the HTM is introduced as monomer in the system and then it is polymerized within the nanopore of the dyed nc-TiO2 electrode. The first reported system is a polypyrrole-based DSSC as a iodide/iodine-free DSSC. In order to plug polypyrrole into the pores of the dye-adsorbed titania layer, in situ photoelectrochemical polymerization (PEP) of pyrrole (Figure 13) [74] was used but a scarce conversion efficiency has been achieved (0.8%).

123534.fig.0013
Figure 13: Chemical structure of polypyrrole.

Among the attempts to reach the perfect structure, it is worth to mention the devices based onto ZnO nanorods [75]. Vertically aligned ZnO nanostructures coupled to a polythiophene (P3HT) through a fullerene mediator gave an efficiency of about 3%. In the end we cite another interesting experimental approach which has been carried out by Sicot et al. [76] that verified the improving of the performance of a polythiophene-based photovoltaic cell when a molecular orientation has been induced. The authors reported the so-called molecular rectification through dipole orientation. Layers of polythiophene, dye and small amount of a polar molecule (4-(dibutylamino)- -nitroazobenzene-DRPR) embedded in a polymer matrix, were used to make up the photovoltaic cell. By applying a DC-field under resonant illumination of DRPR, a photoinduced reorientation is achieved. Although the measured efficiencies were very low, the authors observed an increase of one order of magnitude of the power efficiency of the oriented as compared to the nonoriented cells.

6. Conclusion

As a concluding remark of this brief review concerning organic solar cell, we like to stress the fact that this field of research is just at the beginning. Efforts need to be done in order to get bulk-ordered separated structures of p and n organic semiconductors in order to improve contemporaneously both the charge separation processes and the transport of the free charge to the electrodes. In our opinion one of the most promising work directions is to investigate the possibility to use liquid crystal semiconductor molecules and to study phase separation strategies between these base components, in order to obtain fine cell bulk architectures.

References

  1. R. N. Marks, J. J. M. Halls, D. D. C. Bradley, R. H. Friend, and A. B. Holmes, “The photovoltaic response in poly(p-phenylene vinylene) thin-film devices,” Journal of Physics: Condensed Matter, vol. 6, no. 7, pp. 1379–1394, 1994. View at Publisher · View at Google Scholar · View at Scopus
  2. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, “Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions,” Science, vol. 270, no. 5243, pp. 1789–1791, 1995. View at Scopus
  3. C. J. Brabec, N. S. Sariciftci, and J. C. Hummelen, “Plastic solar cells,” Advanced Funtional Materials, vol. 11, no. 1, pp. 15–26, 2001. View at Publisher · View at Google Scholar · View at Scopus
  4. W. Brutting, Ed., Physics of Organic Semiconductors, Wiley-VCH, Weinheim, Germany, 2005.
  5. C. Brabec, V. Dyakonov, J. Parisi, and N. S. Sariciftci, Eds., Organic Photovoltaics: Concepts and Realization, Springer, New York, NY, USA, 2003.
  6. S. S. Sun and N. S. Sariciftci, Eds., Organic Photovoltaics: Mechanisms, Materials, and Devices, Taylor < Francis, New York, NY, USA, 2005.
  7. S. Günes, H. Neugebauer, and N. S. Sariciftci, “Conjugated polymer-based organic solar cells,” Chemical Reviews, vol. 107, no. 4, pp. 1324–1338, 2007. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  8. J. Xue, B. P. Rand, S. Uchida, and S. R. Forrest, “A hybrid planar-mixed molecular heterojunction photovoltaic cell,” Advanced Materials, vol. 17, no. 1, pp. 66–71, 2005. View at Publisher · View at Google Scholar · View at Scopus
  9. B. Kippelen and J.-L. Brédas, “Organic photovoltaics,” Energy and Environmental Science, vol. 2, no. 3, pp. 251–261, 2009. View at Publisher · View at Google Scholar · View at Scopus
  10. C. W. Tang, “Two-layer organic photovoltaic cell,” Applied Physics Letters, vol. 48, no. 2, pp. 183–185, 1986. View at Publisher · View at Google Scholar · View at Scopus
  11. B. P. Rand, J. Genoe, P. Heremans, and J. Poortmans, “Solar cells utilizing small molecular weight organic semiconductors,” Progress in Photovoltaics: Research and Applications, vol. 15, no. 8, pp. 659–676, 2007. View at Publisher · View at Google Scholar · View at Scopus
  12. K. L. Mutolo, E. I. Mayo, B. P. Rand, S. R. Forrest, and M. E. Thompson, “Enhanced open-circuit voltage in subphthalocyanine/C60 organic photovoltaic cells,” Journal of the American Chemical Society, vol. 128, no. 25, pp. 8108–8109, 2006. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  13. H. Gommans, D. Cheyns, T. Aernouts, C. Girotto, J. Poortmans, and P. Heremans, “Electro-optical study of subphthalocyanine in a bilayer organic solar cell,” Advanced Functional Materials, vol. 17, no. 15, pp. 2653–2658, 2007. View at Publisher · View at Google Scholar · View at Scopus
  14. P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” Journal of Applied Physics, vol. 93, no. 7, pp. 3693–3723, 2003. View at Publisher · View at Google Scholar · View at Scopus
  15. H. Paul, C. David, and B. P. Rand, “Strategies for increasing the efficiency of heterojunction organic solar cells: material selection and device architecture,” Accounts of Chemical Research, vol. 42, no. 11, pp. 1740–1747, 2009. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  16. H. Ogata, R. Higashi, and N. Kobayashi, “Electronic absorption spectra of substituted phthalocyanines in solution and as films,” Journal of Porphyrins and Phthalocyanines, vol. 7, no. 8, pp. 551–557, 2003. View at Scopus
  17. R. D. George, A. W. Snow, J. S. Shirk, and W. R. Barger, “The alpha substitution effect on phthalocyanine aggregation,” Journal of Porphyrins and Phthalocyanines, vol. 2, no. 1, pp. 1–7, 1998. View at Scopus
  18. E. Bundgaard and F. C. Krebs, “Low band gap polymers for organic photovoltaics,” Solar Energy Materials and Solar Cells, vol. 91, no. 11, pp. 954–985, 2007. View at Publisher · View at Google Scholar · View at Scopus
  19. G. P. Smestad, F. C. Krebs, C. M. Lampert, C. G. Granqvist, K. L. Chopra, X. Mathew, and H. Takakura, “Reporting solar cell efficiencies in Solar Energy Materials and Solar Cells,” Solar Energy Materials and Solar Cells, vol. 92, no. 4, pp. 371–373, 2008. View at Publisher · View at Google Scholar · View at Scopus
  20. R. F. Bailey-Salzman, B. P. Rand, and S. R. Forrest, “Near-infrared sensitive small molecule organic photovoltaic cells based on chloroaluminum phthalocyanine,” Applied Physics Letters, vol. 91, no. 1, Article ID 013508, 3 pages, 2007. View at Publisher · View at Google Scholar · View at Scopus
  21. C.-J. Ko, Y.-K. Lin, F.-C. Chen, and C.-W. Chu, “Modified buffer layers for polymer photovoltaic devices,” Applied Physics Letters, vol. 90, no. 6, Article ID 063509, 3 pages, 2007. View at Publisher · View at Google Scholar · View at Scopus
  22. G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, “High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends,” Nature Materials, vol. 4, no. 11, pp. 864–868, 2005. View at Publisher · View at Google Scholar · View at Scopus
  23. W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger, “Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology,” Advanced Functional Materials, vol. 15, no. 10, pp. 1617–1622, 2005. View at Publisher · View at Google Scholar · View at Scopus
  24. M. Reyes-Reyes, K. Kim, and D. L. Carroll, “High-efficiency photovoltaic devices based on annealed poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1- phenyl- (6,6) C61 blends,” Applied Physics Letters, vol. 87, no. 8, Article ID 083506, 3 pages, 2005. View at Publisher · View at Google Scholar · View at Scopus
  25. M. Reyes-Reyes, K. Kim, J. Dewald, R. López-Sandoval, A. Avadhanula, S. Curran, and D. L. Carroll, “Meso-structure formation for enhanced organic photovoltaic cells,” Organic Letters, vol. 7, no. 26, pp. 5749–5752, 2005. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  26. R. Kroon, M. Lenes, J. C. Hummelen, P. W. M. Blom, and B. De Boer, “Small bandgap polymers for organic solar cells (polymer material development in the last 5 years),” Polymer Reviews, vol. 48, no. 3, pp. 531–582, 2008. View at Publisher · View at Google Scholar · View at Scopus
  27. Y. A.M. Ismail, T. Soga, and T. Jimbo, “Improvement in light harvesting and performance of P3HT:PCBM solar cell by using 9,10-diphenylanthracene,” Solar Energy Materials and Solar Cells, vol. 93, no. 9, pp. 1582–1586, 2009. View at Publisher · View at Google Scholar · View at Scopus
  28. P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” Journal of Applied Physics, vol. 93, no. 7, pp. 3693–3723, 2003. View at Publisher · View at Google Scholar · View at Scopus
  29. L. A. A. Pettersson, L. S. Roman, and O. Inganäs, “Modeling photocurrent action spectra of photovoltaic devices based on organic thin films,” Journal of Applied Physics, vol. 86, no. 1, pp. 487–496, 1999. View at Scopus
  30. D. Cheyns, B. P. Rand, B. Verreet, J. Genoe, J. Poortmans, and P. Heremans, “The angular response of ultrathin film organic solar cells,” Applied Physics Letters, vol. 92, no. 24, Article ID 243310, 2008. View at Publisher · View at Google Scholar · View at Scopus
  31. H. Gommans, D. Cheyns, T. Aernouts, C. Girotto, J. Poortmans, and P. Heremans, “Electro-optical study of subphthalocyanine in a bilayer organic solar cell,” Advanced Functional Materials, vol. 17, no. 15, pp. 2653–2658, 2007. View at Publisher · View at Google Scholar · View at Scopus
  32. B. Verreet, S. Schols, and S. Schols, “The characterization of chloroboron (iii) subnaphthalocyanine thin films and their application as a donor material for organic solar cells,” Journal of Materials Chemistry, vol. 19, no. 30, pp. 5295–5297, 2009. View at Publisher · View at Google Scholar · View at Scopus
  33. R. H. Friend, G. J. Denton, and G. J. Denton, “Electronic excitations in luminescent conjugated polymers,” Solid State Communications, vol. 102, no. 2-3, pp. 249–258, 1997. View at Scopus
  34. R. H. Friend, G. J. Denton, and G. J. Denton, “Electronic processes of conjugated polymers in semiconductor device structures,” Synthetic Metals, vol. 84, no. 1–3, pp. 463–470, 1997. View at Scopus
  35. J. Simon and J.J. André, Molecular Semiconductors, Springer, Berlin, Germany, 1985.
  36. M. Riede, T. Mueller, W. Tress, R. Schueppel, and K. Leo, “Small-molecule solar cells—status and perspectives,” Nanotechnology, vol. 19, no. 42, Article ID 424001, 2008. View at Publisher · View at Google Scholar · View at Scopus
  37. B. P. Rand, J. Genoe, P. Heremans, and J. Poortmans, “Solar cells utilizing small molecular weight organic semiconductors,” Progress in Photovoltaics: Research and Applications, vol. 15, no. 8, pp. 659–676, 2007. View at Publisher · View at Google Scholar · View at Scopus
  38. B. C. Thompson and J. M. J. Fréchet, “Polymer-fullerene composite solar cells,” Angewandte Chemie - International Edition, vol. 47, no. 1, pp. 58–77, 2008. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  39. G. Dennler, M. C. Scharber, and C. J. Brabec, “Polymer-fullerene bulk-heterojunction solar cells,” Advanced Materials, vol. 21, no. 13, pp. 1323–1338, 2009. View at Publisher · View at Google Scholar · View at Scopus
  40. E. Kymakis, I. Alexandrou, and G. A. J. Amaratunga, “High open-circuit voltage photovoltaic devices from carbon-nanotube-polymer composites,” Journal of Applied Physics, vol. 93, no. 3, pp. 1764–1768, 2003. View at Publisher · View at Google Scholar · View at Scopus
  41. E. Kymakis and G. A. J. Amaratunga, “Photovoltaic cells based on dye-sensitisation of single-wall carbon nanotubes in a polymer matrix,” Solar Energy Materials and Solar Cells, vol. 80, no. 4, pp. 465–472, 2003. View at Publisher · View at Google Scholar · View at Scopus
  42. J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglla, R. H. Friend, S. C. Moratti, and A. B. Holmes, “Efficient photodiodes from interpenetrating polymer networks,” Nature, vol. 376, no. 6540, pp. 498–500, 1995. View at Scopus
  43. G. Yu and A. J. Heeger, “Charge separation and photovoltaic conversion in polymer composites with internal donor/acceptor heterojunctions,” Journal of Applied Physics, vol. 78, no. 7, pp. 4510–4515, 1995. View at Publisher · View at Google Scholar · View at Scopus
  44. C. R. McNeill, A. Abrusci, and A. Abrusci, “Dual electron donor/electron acceptor character of a conjugated polymer in efficient photovoltaic diodes,” Applied Physics Letters, vol. 90, no. 19, Article ID 193506, 2007. View at Publisher · View at Google Scholar · View at Scopus
  45. F. S. Bates and G. H. Fredrickson, “Block copolymers-designer soft materials,” Physics Today, vol. 52, no. 2, pp. 32–38, 1999. View at Scopus
  46. F. Meyers, A. J. Heeger, and J. L. Brédas, “Fine tuning of the band gap in conjugated polymers via control of block copolymer sequences,” The Journal of Chemical Physics, vol. 97, no. 4, pp. 2750–2758, 1992. View at Scopus
  47. M. Sommer, A. S. Lang, and M. Thelakkat, “Crystalline-crystalline donor-acceptor block copolymers,” Angewandte Chemie. International Edition, vol. 47, no. 41, pp. 7901–7904, 2008. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  48. Q. Zhang, A. Cirpan, T. P. Russell, and T. Emrick, “Donor-acceptor poly(thiophene-block-perylene diimide) copolymers: Synthesis and solar cell fabrication,” Macromolecules, vol. 42, no. 4, pp. 1079–1082, 2009. View at Publisher · View at Google Scholar · View at Scopus
  49. V. de Cupere, J. Tant, P. Viville, R. Lazzaroni, W. Osikowicz, W. R. Salaneck, and Y. H. Geerts, “Effect of interfaces on the alignment of a discotic liquid-crystalline phthalocyanine,” Langmuir, vol. 22, no. 18, pp. 7798–7806, 2006. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  50. A. J. J. M. van Breemen, P. T. Herwig, and P. T. Herwig, “Large area liquid crystal monodomain field-effect transistors,” Journal of the American Chemical Society, vol. 128, no. 7, pp. 2336–2345, 2006. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  51. A. Tracz, J. K. Jeszka, M. D. Watson, W. Pisula, K. Müllen, and T. Pakula, “Uniaxial alignment of the columnar super-structure of a hexa (alkyl) hexa-peri-hexabenzocoronene on untreated glass by simple solution processing,” Journal of the American Chemical Society, vol. 125, no. 7, pp. 1682–1683, 2003. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  52. W. Pisula, A. Menon, and A. Menon, “A zone-casting technique for device fabrication of field-effect transistors based on discotic hexa-peri-hexabenzocoronene,” Advanced Materials, vol. 17, no. 6, pp. 684–689, 2005. View at Publisher · View at Google Scholar · View at Scopus
  53. H. Monobe, K. Awazu, and Y. Shimizu, “Change of liquid-crystal domains by vibrational excitation for a columnar mesophase,” Advanced Materials, vol. 12, no. 20, pp. 1495–1499, 2000. View at Publisher · View at Google Scholar · View at Scopus
  54. S. Zimmermann, J. H. Wendorff, and C. Weder, “Uniaxial orientation of columnar discotic liquid crystals,” Chemistry of Materials, vol. 14, no. 5, pp. 2218–2223, 2002. View at Publisher · View at Google Scholar · View at Scopus
  55. O. Bunk, M. M. Nielsen, T. I. Sølling, A. M. Van de Craats, and N. Stutzmann, “Induced alignment of a solution-cast discotic hexabenzocoronene derivative for electronic devices investigated by surface X-ray diffraction,” Journal of the American Chemical Society, vol. 125, no. 8, pp. 2252–2258, 2003. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  56. F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer, and A. P. H. J. Schenning, “About supramolecular assemblies of π-conjugated systems,” Chemical Reviews, vol. 105, no. 4, pp. 1491–1546, 2005. View at Publisher · View at Google Scholar · View at PubMed
  57. J. A. A. W. Elemans, A. E. Rowan, and R. J. M. Nolte, “Mastering molecular matter. Supramolecular architectures by hierarchical self-assembly,” Journal of Materials Chemistry, vol. 13, no. 11, pp. 2661–2670, 2003. View at Publisher · View at Google Scholar
  58. C. D. Simpson, J. Wu, M. D. Watson, and K. Müllen, “From graphite molecules to columnar superstructures—an exercise in nanoscience,” Journal of Materials Chemistry, vol. 14, no. 4, pp. 494–504, 2004.
  59. D. Adam, P. Schuhmacher, and P. Schuhmacher, “Fast photoconduction in the highly ordered columnar phase of a discotic liquid crystal,” Nature, vol. 371, no. 6493, pp. 141–143, 1994. View at Publisher · View at Google Scholar
  60. X. Crispin, J. Cornil, and J. Cornil, “Electronic delocalization in discotic liquid crystals: a joint experimental and theoretical study,” Journal of the American Chemical Society, vol. 126, no. 38, pp. 11889–11899, 2004. View at Publisher · View at Google Scholar · View at PubMed
  61. J. M. Warman, M. P. De Haas, G. Dicker, F. C. Grozema, J. Piris, and M. G. Debije, “Charge mobilities in organic semiconducting materials determined by pulse-radiolysis time-resolved microwave conductivity: π-Bond-conjugated polymers versus π-π-stacked discotics,” Chemistry of Materials, vol. 16, no. 23, pp. 4600–4609, 2004. View at Publisher · View at Google Scholar
  62. Z. An, J. Yu, and J. Yu, “High electron mobility in room-temperature discotic liquid-crystalline perylene diimides,” Advanced Materials, vol. 17, no. 21, pp. 2580–2583, 2005. View at Publisher · View at Google Scholar
  63. B. A. Jones, M. J. Ahrens, M.-H. Yoon, A. Facchetti, T. J. Marks, and M. R. Wasielewski, “High-mobility air-stable n-type semiconductors with processing versatility: dicyanoperylene-3,4:9,10-bis(dicarboximides),” Angewandte Chemie - International Edition, vol. 43, no. 46, pp. 6363–6366, 2004. View at Publisher · View at Google Scholar · View at PubMed
  64. H. Iino, Y. Takayashiki, J.-I. Hanna, R. J. Bushby, and D. Haarer, “High electron mobility of 0.1 cm2V-1s-1 in the highly ordered columnar phase of hexahexylthiotriphenylene,” Applied Physics Letters, vol. 87, no. 19, Article ID 192105, 3 pages, 2005. View at Publisher · View at Google Scholar
  65. D. Markovitsi, S. Marguet, J. Bondkowski, and S. Kumar, “Triplet excitation transfer in triphenylene columnar phases,” Journal of Physical Chemistry B, vol. 105, no. 7, pp. 1299–1306, 2001.
  66. L. Schmidt-Mende, A. Fechtenkötter, K. Müllen, E. Moons, R. H. Friend, and J. D. MacKenzie, “Self-organized discotic liquid crystals for high-efficiency organic photovoltaics,” Science, vol. 293, no. 5532, pp. 1119–1122, 2001. View at Publisher · View at Google Scholar · View at PubMed
  67. G. Chidichimo, G. De Filpo, S. Manfredi, S. Mormile, L. Tortora, C. Gallucci, and R. Cassano, “High contrast reverse mode PDLC films: A morphologic and electro-Optical analysis,” Molecular Crystals and Liquid Crystals, vol. 500, pp. 10–22, 2009. View at Publisher · View at Google Scholar
  68. M. Grätzel, “Photoelectrochemical cells,” Nature, vol. 414, no. 6861, pp. 338–344, 2001. View at Publisher · View at Google Scholar · View at PubMed
  69. H. Nusbaumer, J.-E. Moser, S. M. Zakeeruddin, M. K. Nazeeruddin, and M. Grätzel, “CoII(dbbip)22+ complex rivals tri-iodide/iodide redox mediator in dye-sensitized photovoltaic cells,” Journal of Physical Chemistry B, vol. 105, no. 43, pp. 10461–10464, 2001. View at Publisher · View at Google Scholar
  70. S. Hattori, Y. Wada, S. Yanagida, and S. Fukuzumi, “Blue copper model complexes with distorted tetragonal geometry acting as effective electron-transfer mediators in dye-sensitized solar cells,” Journal of the American Chemical Society, vol. 127, no. 26, pp. 9648–9654, 2005. View at Publisher · View at Google Scholar · View at PubMed
  71. W. Kubo, K. Murakoshi, and K. Murakoshi, “Quasi-solid-state dye-sensitized TiO2 solar cells: effective charge transport in mesoporous space filled with gel electrolytes containing iodide and iodine,” Journal of Physical Chemistry B, vol. 105, no. 51, pp. 12809–12815, 2001. View at Publisher · View at Google Scholar
  72. L. Sicot, C. Fiorini, A. Lorin, J.-M. Nunzi, P. Raimond, and C. Sentein, “Dye sensitized polythiophene solar cells,” Synthetic Metals, vol. 102, no. 1–3, pp. 991–992, 1999. View at Publisher · View at Google Scholar
  73. D. Gebeyehu, C. J. Brabec, and C. J. Brabec, “Hybrid solar cells based on dye-sensitized nanoporous TiO2 electrodes and conjugated polymers as hole transport materials,” Synthetic Metals, vol. 125, no. 3, pp. 279–287, 2002. View at Publisher · View at Google Scholar
  74. K. Murakoshi, R. Kogure, Y. Wada, and S. Yanagida, “Fabrication of solid-state dye-sensitized TiO2 solar cells combined with polypyrrole,” Solar Energy Materials and Solar Cells, vol. 55, no. 1-2, pp. 113–125, 1998.
  75. I. Gonzalez-Valls and M. Lira-Cantu, “Vertically-aligned nanostructures of ZnO for excitonic solar cells: a review,” Energy and Environmental Science, vol. 2, no. 1, pp. 19–34, 2009. View at Publisher · View at Google Scholar
  76. L. Sicot, C. Fiorini, A. Lorin, P. Raimond, C. Sentein, and J.-M. Nunzi, “Improvement of the photovoltaic properties of polythiophene-based cells,” Solar Energy Materials and Solar Cells, vol. 63, no. 1, pp. 49–60, 2000. View at Publisher · View at Google Scholar