Molecular-Shape-Induced Efficiency Enhancement in PC61BM and PC71BM Based Ternary Blend Organic Solar Cells

Evolution of a self-organized molecular packing at the microscopic scale in a bulk heterojunction active layer is critical for improving the performance of organic solar cells. We demonstrate that the molecular-shape-induced effects improve the morphology of the ternary thin films as PC61BM molecules were added to the host binary blend, PTB7-Th:PC71BM. Compared to the binary PTB7-Th:PC71BM devices, the ternary devices with 20% PC61BM content (relative to PC71BM) exhibited an enhanced efficiency, from 6.4% to 8.5%. In particular, we find that the spherical PC61BM molecules with better precipitation kinetics than ellipsoidal PC71BM alter the morphology of ternary thin film and modulate interfaces to expedite charge transport and collection by reducing various recombination losses.