The partially description of the folded planar device with geometric optics is only valid for structure dimensions up from several tenths of microns [3]. The optical modeling of the system can be divided into two parts. As the thickness of thin film systems of sev­eral hundred nanometers is much smaller than the coherence length of the incident light, wave optics has to be considered. The calculations of the absorbtance in distinct lay­ers were performed by near field simulations [4], [5],[6]. Optical constants are taken for the MDMO-PPV/PCBM system [7] which is comparable to the P3HT/PCBM system with a slight shift in complex refractive index towards shorter wavelengths. The thickness of the photoactive layer and the PEDOT layer was 100nm. Both polarisations of the incident light — transversal electric (TE) and transversal magnetic (TM) have to be considered.

The absorptance in the distinct layers and the fraction of reflected light is calculated.

The propagation of the reflected light inside the prismstructure is described by geomet­ric optics. Calculations are presented for the simple setup of normal incidence on the substrate leading to a twofold reflection on the thin film system under 45°. The spec­tral absorptance for TE- and TM-polarisation is shown in comparison to the case of nor­mal incidence on a planar thin film system (figure 3). From convolution of the spectral absorption with the AM1.5 solar spectrum, an increase in absorptance of 55 % in TE — polarisation and of 33 % in TM-polarisation was calculated. This leads to an average in­crease of 44 % for unpolarised light. Optical losses due to shadowing by the microgrid are not taken into account.