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.