Organic solar cells (OSCs) combine the advantages of low-cost and large-area fabrication with the use of nonhazardous and environmentally friendly materials. Over the last few years, power conversion efficiencies improved continuously, now exceeding 10% in single-junction solar cells. Improvements often originated from the synthesis of new absorber materials that allowed for an enhanced spectral coverage of the solar spectrum or enabled higher internal quantum efficiencies. In the lab, screening and optimization of new materials and cell architectures often follow a trial and error approach, although only very little amounts of material are available.An alternative and material saving route to optimized OSCs is a comprehensive optoelectronic device simulation that reduces the experimental parameter space. This becomes particularly important for sophisticated device architectures, such as tandem solar cells. However, in order to carry out meaningful optical simulations, a profound knowledge of the refractive indices is mandatory.