The demand for inexpensive, renewable energy sources is driving new approaches to the production of low-cost photovoltaic devices. In the last couple of years, a large amount of effort has been put in the development of solar cells based on organic molecules [1] and conjugated polymers [2,3]. Especially polymer-based solar cells would offer tremendous advantages for the fabrication, like low cost roll-to-roll production of large area, flexible solar cells. Because of these advantages, the development of polymer solar cells would have a major impact, even if the efficiencies of these types of photovoltaic devices up to now are smaller than the efficiencies achieved in inorganic solar cells.
For the generation of electrical power by light conversion it is necessary to spatially separate the electron-hole (e-h) pair generated in the absorption process before recombination can take place. In conjugated polymers, the stabilization of the photoexcited e-h pair can be achieved by blending the polymer with an acceptor molecule having an electron affinity that is larger than the electron affinity of the polymer, but still smaller than the ionization potential of the polymer donor. Under these conditions it is energetically favorable for the excited conjugated polymer to transfer an electron to the acceptor molecule, while a hole remains in the polymer valence band as the lowest energy level.
In our work this alignment of the energy levels is realized by blending of poly [2-methoxy, 5-(3',7'-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) or poly-3-octyl-thiophene (P3OT) with derivatives of C₆₀. The polymers act as electron donor when blended with the acceptor molecule C₆₀ or the soluble methanofullerene [6,6]-Phenyl C₆₁-butyric acid methyl ester (PCBM) or a mixture of methanofullerene =91higher adducts=92 (multi-PCBM) in which the fullerene cage bears two or more identical pending groups on various positions. It has been shown that in these systems photoinduced electron transfer from the polymer onto the fullerenes occurs within approximately 200 fs after the excitation [4]. Since all other known competing relaxation processes in conjugated polymers occur on time scales that are orders of magnitudes larger than 200 fs, this ultra fast charge transfer has a quantum efficiency of approximately 1. Thus nearly all photons absorbed by the polymer are converted to electrons on the C₆₀. In addition, the lifetime of the charge separated excited state is in microsecond time scale at room temperature [5], allowing the generation of high concentrations of non-equilibrium electrons and holes.
By sandwiching the blend of conjugated polymers and C₆₀ between electrodes that have different work functions F _m (for example ITO (indium tin oxide, F ito = 4.7 eV) and Al (F _Al = 4.3 eV)) an electrical field is established across the polymer layer. At acceptor concentrations higher than the percolation threshold of 17 mol % [6] this electric field separates the non-equilibrium carriers generated by the incident light and a current is delivered to an electrical circuit connected to the electrodes. However, in these polymer blends the mobilities of electrons and holes are rather small due to the hopping transport, thereby limiting the efficiency of the photovoltaic cells.
We compare the I-V characteristics of devices made from MDMO-PPV and P3OT blended with various C₆₀ derivatives. Large area photovoltaic devices have been fabricated showing power conversion efficiencies up to 1.5 % under monochromatic illumination at 488 nm. It is shown that among the various combinations of materials both the open circuit voltage and the short circuit current is maximal for a blend of MDMO-PPV and a highly soluble fullerene derivative (PCBM). In addition, it is shown that by decreasing the device thickness to an optimum value, the efficiency can be increased significantly.
The practical application of the conjugated polymer fullerene devices is limited by their stability under ambient conditions (i. e. in the presence of water or oxygen). Recent FTIR degradation studies on the polymer composites showed rapid bleaching of the conjugated polymer and somewhat slower degradation of C₆₀ under oxygen and light [7]. Stabilization of these composites was realized by the exclusion of oxygen, either by sealing or by handling the devices under inert conditions. The embedding of the photoactive conjugated polymer fulle rene blend in a conventional polymer matrix (guest host approach) is a sound and promising method to improve the photoactive sample quality for the following reasons [8]:

• The conjugated polymer diluted by the convention al polymer host matrix shows less interaction compared to pure films.
• Macroscopic ordering of the conjugated polymer can be performed by mechanical stretching of the host polymer.
• In the guest host approach the conjugated polymers are better encapsulated against environmental influences.
• By the choice of a proper host matrix the charge transfer between the conjugated polymer and the fullerene may be advantageously tuned, either by changing the intermolecular distances due to morphology control or by the overall dielectric constant of the system.

Polymeric photovoltaic devices consisting of a mixture of a MDMO-PPV, PCBM and a conventional polymer (as conventional polymers polystyrene (PS), polyvinylcarbazole (PVK) and polyvinylbenzylchloride (PVBC) were investigated) were fabricated and characterized under monochromatic light and by wavelength dependent photocurrent measurements. From these data power efficiencies were calculated and compared to the electron-photon conversion efficiencies.

[1]	C. W. Tang, Appl.Phys. Lett., 48, 183 (1986).
[2]	J. H. Burroughes, D. D. C. Bradley, A R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature, 347 (1990) 183.
[3]	G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science, 270, 1789 (1995).
[4]	N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science, 258, 1474 (1992).
[5]	N. S. Sariciftci and A. J. Heeger, Handbook of Organic Conductive Molecules and Polymers, H. S. Nalwa (ed), p. 413, John Wiley & Sons 1997
[6]	C. J. Brabec, F. Padinger, V. Dyakonov, J. C. Hummelen, R. A. J. Jansen and N. S. Sarciftci, in "Molecular Nanostructures", Proceedings of the International Winterschool on Electronic Properties of Novel Materials, Kirchberg 1998.
[7]	H. Neugebauer, C. J. Brabec, and N. S. Sariciftci, Synth. Met. ICSM 98, in print.
[8]	C. J. Brabec, H .Johannson, F. Padinger, H. Neugebauer, J. C. Hummelen and N. S. Sariciftci, submitted to Solar Energy Materials and Solar Cells

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