Fused or unfused? Two-dimensional non-fullerene acceptors for efficient organic solar cells

Abstract

Two benzotrithiophene-based two-dimensional non-fullerene acceptors (NFAs), namely B3T-BT-6F with a fused-ring structure and B3T-TT-6F with an unfused-ring structure, were designed and synthesized to investigate the effects of a fused or unfused NFA molecular backbone on the performance of organic solar cells. Compared to the fused B3T-BT-6F, the unfused B3T-TT-6F shows a 34 nm red-shifted absorption with an optical bandgap reduced from 1.54 to 1.44 eV and it exhibits almost unchanged lowest unoccupied molecular orbital (LUMO) levels. Importantly, the blended photoactive film based on the unfused B3T-TT-6F as an NFA and PBDB-T as a donor exhibits a higher and more balanced mobility in comparison with the fused B3T-BT-6F based film. As a result, the optimized photovoltaic device based on the unfused B3T-TT-6F acceptor shows a high efficiency of 9.94%, which is superior to that of the device using fused B3T-BT-6F (8.40%) and is among the highest efficiencies for 2D NFA based devices. The success of this molecular design strategy should have important implications for developing efficient NFAs for high performance organic solar cells.