A report on organic solar cells [1] was given by us in our annual workshops only once in 1993 [2]. Continuing research led to a considerable increase in understanding this type of solar cell. So we will report here on our latest results obtained on measurements and modelling of the optical and photoelectrical behavior of p/n-type structures made from zinc phthalocyanine and N,N,-dimethyle perylene tetracarboxylic diamide. Although the theory of electronical and optoelectronic phenomena is well understood in the case of organic crystals [3, 4] there is a lack of unique theories concerning organic thin film systems. A transfer of the theories developed for inorganic semiconductor thin films is seldom possible and useful.
In our investigations photocurrent spectroscopy has proven to be the best tool to determine the width and effectiveness of the charge generation layer formed at Schottky or p/n-type junctions [5, 6]. However, the understanding of the spectra obtained critically depends on the exact determination of a light absorption profiles in our devices.
We reexamined the optical and electrical response of organic p/n-junction solar cell consisting of the dyes zinc phthalocyanine and the methyl perylene pigment (MPP) N,N,-dimethyle perylene tetracarboxylic diamide as well as ITO and gold as electrical contact materials. The dye layers and the gold contacts were prepared by evaporation techniques in a high and an ultra high vacuum system on ITO coated glass substrates.
We present measured I/U characteristics, optical transmission and reflectivity spectra and also short circuit photocurrent and open circuit photovoltage spectra. The photocurrent and photovoltage spectra were measured under constant photon density conditions in a self build action spectrometer.
We compare the experimental with the simulated photocurrent spectra. The simulated photocurrent spectra are based on a simple model considering the optics of a thin film system including interference phenomena (compare fig. 1), the influence of the serial resistances as well as the photoconductivity of the layers. Figure 1 shows a calculation of the normalized intensity and the absorbance at any position inside of dye layers of a multi layer system assuming a coherent superposition of the light waves. Within the scope of the simple model it will be shown that only charge carriers generate in a small area nearby the contact of the two organic materials contribute to the photocurrent. The width of this charge generation area is typically 10 nm - 20 nm and hence most parts of the dye layers only acts as serial resistances. A detailed electrical equivalent circuit for the organic solar cell considers the influence of these serial resistances on the short circuit photocurrent.
In further investigations we investigated the photoconductivity of organic thin films and came to the conclusion that also the photoconductivity decreasing the serial resistances has to be taken into account too.

REFERENCES

[1]	Wöhrle, D., Meissner, D., Adv. Mat. 3 (1991) 129.
[2]	D. Meissner in: G. Calzaferri, L. Forss, M. Lux-Steiner, H. Ries (Eds.): Proc. 5th Workshop on Quantum Solar Energy Conversion, Brigels, Switzerland, 1993, p. 13.
[3]	Simon, J; Andre, J .J: Molecular Semiconductors, Springer Verlag Berlin, Heidelberg, 1985.
[4]	Pope, M., Swenberg, C. E.: "Electronic processes in organic semiconductors", Clarendon Press, Oxford 1982.
[5]	S. Siebentritt, S. Günster, D. Meissner, Synthetic Metals 41-43 (1991) 1173.
[6]	S. Günster, S. Siebentritt, J. Elbe, L. Kreienhoop, B. Tennigkeit, D. Wöhrle, R. Memming, D. Meissner, Molecular Cryst. Liquid Cryst. 218 (1992) 117.

ACKNOWLEDGEMENT
We gratefully acknowledge financial support of the European Commission under contract number JOR 3-CT 96-0106.

Figure 1: Light intensity and light absorbance profiles inside the dye layers for the organic solar cell (glass / ITO (27 nm) / MPP (40 nm) / ZnPc (80 nm) / Au (40 nm)). The cell is illuminated through the semitransparent ITO electrode.

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