Dominating recombination mechanisms in organic solar cells based on ZnPc and C60

We investigate the dominating recombination mechanisms in bulk heterojunction solar cells, using a blend of ZnPc and C60 as model system. Analyzing the open-circuit voltage (Voc) as a function of illumination intensity, we find that trap-assisted recombination dominates for low light intensities, whereas at 1 sun, direct/bimolecular recombination becomes important. The recombination parameters are not significantly influenced by the blend mixing ratio and are also valid for injected charges. By changing the hole transport layer, recombination at the contact is separately identified as further mechanism reducing Voc at higher light intensities.