Importance of interfacial crystallinity to reduce open-circuit voltage loss in organic solar cells

Reducing the energy loss in output voltage is critically important for further enhancing the efficiency of organic solar cells (OSCs). Here, we report that OSCs with high mobility and highly crystalline donor (D) and acceptor (A) materials were able to reduce an open-circuit voltage (V_OC) loss. The crystallinity of the acceptor layer could be altered by appropriate selection of the three molecules with different alkyl side chain lengths. The V_OC was found to increase as the crystallinity of the acceptor layer increased. The origin of the high V_OC was that the highly crystalline D/A interface reduced the energy loss in the output voltage by realizing ideal band-to-band recombination. Especially, the high crystallinity of the several molecular layers (less than 6 nm) in the vicinity of the D/A interface was important for realizing the high V_OC. Our results demonstrate that the careful design of the D/A interface enables high power conversion efficiencies to be achieved in OSCs by reducing open-circuit voltage loss.