Finally we investigated the correlation between the photo current (PC) and the effective absorption (Aeff) by the utilization of one of our photovoltaic devices with the following setup: dITO=140nm, dPEDOT=140nm, dCuPC=30nm, dBBP-perylene=70nm. The photocurrent action spectrum was taken by the use of a halogen lamp using a Keithley 237 source measure unit. The analysis of the PC and the Aeff curve in dependence on the incident wave length let us gain a better understanding of the diffusion lengths of excitons.

One finds evident correlation between PC and the Aeff which leads to an understanding about the energy conversion efficiency of a photovoltaic device based on their absorption spectra (see Fig. 10-11). The diffusion length of the excitons in the active layers becomes investigable by the analysis of the absorption spectra for different extends of the effective absorption region. The results of those investigations will be published in a further paper.

3.

Conclusion

Modeling of the spatial distribution of the light absorption in photovoltaic devices was carried out. To localize the light absorption density in the region of the pn-junction of a bilayer solar cell is of elementary importance for the improvement of efficiency. One

could achieve this by the proper adjustment of the optical parameters: thickness, refractive index and the coefficient of extinction of every device layer. Simulations show distinct maxima of absorption density for special layer thicknesses of the active layers. The top-views of the thickness variation graphs provide the optimal thickness for given solar cell materials like a map. Parameter studies were presented, showing where to go searching for materials with optimal absorption efficiencies for coming solar cell setups. Supplementary absorption density spectra show correlation with photo current curves which allows the prediction of the photo current and as a consequence the power conversion efficiency of a photovoltaic device.

Acknowledgements

We gratefully acknowledge the financial support by the “Friedrich Schiedel Stiftung fur Energietechnik Austria!’.

Part of the research work of this paper were performed within project S.14 at the Polymer Competence Center Leoben GmbH (PCCL, Austria) within framework of the Kplus-Program of the Austrian Ministry of Traffic, Innovation and Technology with the contributions by the University of Leoben, Graz University of Technology, Johannes Kepler University Linz, Joanneum Research ForschungsgesmbH and Upper Austrian Research GmbH. The PCCL is funded by the Austrian Governments of Styria and Upper Austria.