Polymer Solar Cells Exceeding 10% Efficiency Enabled via a Facile Star-Shaped Molecular Cathode Interlayer with Variable Counterions

A series of star-shaped small molecular cathode interlayer materials are synthesized for PTB7:PC71BM based polymer solar cells (PSCs), comprising neutral amino groups, quaternary ammonium ions, amino N-oxides, and sulfobetaine ions as pendant polar functionalities, respectively. For the first time, the effect of these different pendant functional groups with or without mobile counterions on the cells' photovoltaic properties is investigated in detail. A large improvement in device performance is observed by inserting these cathode interfacial layers (CILs) between the PTB7:PC71BM active layer and the Al electrode. The CILs could effectively lower the work function of the Al cathode, increase the built-in potential, and decrease the series resistance of the related PSCs. poly(9,9-dioctylfluorene-co-N-[4-(3-methyl-propyl)]-diphenylamine) with pendant quaternary ammonium ions shows the best cathode modification ability, giving rise to the highest power conversion efficiency of 10.1%, even better than that of the typical poly[(9,9-bis(3'-(N, N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] based device. The design strategy and structure-property relationships concluded in this work will be helpful to develop more efficient cathode interface materials for high-performance PSCs in the future.