Molecular Understanding of the Open‐Circuit Voltage of Polymer:Fullerene Solar Cells

The origin of open‐circuit voltage ( V OC ) was studied for polymer solar cells based on a blend of poly(3‐hexylthiophene) (P3HT) and seven fullerene derivatives with different LUMO energy levels and side chains. The temperature dependence of J – V characteristics was analyzed by an equivalent circuit model. As a result, V OC increased with the decrease in the saturation current density J 0 of the device. Furthermore, J 0 was dependent on the activation energy E A for J 0 , which is related to the HOMO–LUMO energy gap between P3HT and fullerene. Interestingly, the pre‐exponential term J 00 for J 0 was larger for pristine fullerenes than for substituted fullerene derivatives, suggesting that the electronic coupling between molecules also has substantial impact on V OC . This is probably because the recombination is non‐diffusion‐lmilited reaction depending on electron transfer at the P3HT/fullerene interface. In summary, the origin of V OC is ascribed not only to the relative HOMO–LUMO energy gap but also to the electronic couplings between fullerene/fullerene and polymer/fullerene.