Simultaneous engineering of the substrate temperature and mixing ratio to improve the performance of organic photovoltaic cells

In this study, we investigated the effect of the donor/acceptor mixing ratio and the substrate temperature (T_SUB) during the co-deposition process on the performance of bulk heterojunction organic photovoltaic cells. We found that the ratio of dispersed donor islands (less than 10 nm), which hinders charge carrier transport, increased as the donor concentration (C_D) increased in the film processed at room temperature. By contrast, the donor cluster (larger than 10 nm), providing percolation paths for the carriers, was enlarged in the film containing a high C_D fabricated at high T_SUB (70 °C). This enhanced phase separation in the mixed layer led to an improved fill factor and a decreased activation energy of the short-circuit current (J_SC). Therefore, we demonstrated a 23% improvement in the device performance by employing an elevated T_SUB and optimized mixing ratio in comparison with the device fabricated at room temperature.