The utilization of organic materials for photovoltaic devices has been investigated intensely during the last couple of decades (for a summary of the early reports see for example (1,2,3)). Because of the ultrafast photoinduced electron transfer (4) with long-lived charge separation, the conjugated polymer/C60 system offers the special opportunity to produce thin film photovoltaic devices from solution. The photoinduced charge separation happens with quantum efficiency near unity. The performance of such bulk heterojunction devices is remarkably enhanced compared to devices made from the single components (5).
The photovoltaic devices have been produced by spin casting from solution, yielding a typical film thickness around 100 - 200 nm. For the high work function electrode, transparent ITO substrates, either on glass or on polyester, have been used. The low work function electrode, Al, was evaporated onto the spin cast film.
The quality and homogeneity of the composite film as well as the choice of the substrate strongly influence the efficiency of the solar cell. We studied the current/voltage characteristics of the fullerene/conjugated polymer thin films on two different substrates and in two different geometries: (i) devices on ITO glass substrates with active areas around 15 mm2, and (ii) devices on ITO polyester substrates with typical areas of 6 cm by 6 cm and active areas of 4 times 360 mm2. Figure 1 shows the picture of a large area plastic solar cell, while figure 2 shows the characteristics of both devices.
Although dark negative currents are higher for the flexible substrate cell, both cells show comparable Voc and Isc. For both cells we calculated a filling factor FF around 0,35. The overall efficiency of the cells is calculated with app. 1,2 % under monochromatic illumination (488 nm) with 10 mW/cm2. These data show clearly, that upscaling to large area, flexible devices without significant loss of the efficiency is possible.
In plastic solar cells fullerenes act in a double role - as highly efficient e- acceptors as well as e- conductors. The power efficiency of plastic solar cells (> 1.2 %) is limited by charge transport. Encapsulation of plastic solar cells increases the shelf life time over 150 days. Further improvements in device efficiencies are expected by optimizing the composite composition, the network morphology and the charge transport properties of the single components.

FIGURE 1 Picture of a large area, flexible plastic solar cell.

FIGURE 2 (a) I/V characteristics of small area glass cell (( dark - n illuminated with 488 nm, 10 mW/cm2) and (b) large area polyester cell (( dark - n illuminated with 488 nm, 10 mW/cm2).

References

1.	J. Simon and J. J. Andre, Molecular Semiconductors, Springer, Berlin, (1985).
2.	J. B. Whitlock, P. Panayotatos, G. D. Sharma, M. D. Cox, R. R. Sauers, and G. R. Bird, Optical Engineering, 32, 1921 (1993) .
3.	C. W. Tang, Appl. Phys. Lett., 48, 183 (1986) .
4.	N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, Science, 258, 1474 (1992).
5.	G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, Science, 270, 1789 (1995).

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