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. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.