In polymer solar cells (PSCs), molecular design and modulation of chemical composition of the active layer materials are of critical importance for enhancing power conversion efficiencies (PCEs). In our previous work, a PCE of 11.2% was demonstrated by using a new combination of a polymer donor named PBDB-T and a small molecular acceptor (SMA) named ITIC. In order to optimized photovoltaic performance of the PBDB-T:ITIC-based PSCs, subtle chemical modifications of end-groups (EGs) were applied to precisely optimize the molecular energy level of ITIC. By successively introducing one- and two- methyl substituents, the lowest unoccupied molecular orbital (LUMO) levels of the two new acceptors (IT-M and IT-DM) were raised by 0.04 eV and 0.08 eV, respectively, compared to ITIC. Hence, improved open-circuit voltages (VOC) of 0.94 V and 0.97 V were obtained in the corresponding PSCs. Benefiting from the optimized molecular energy levels in the PBDB-T:IT-M-based blend, a PCE of 12.05% was obtained. Furthermore, we also introduced the third component Bis-PCBM[70] to enhance light absorption of the PBDB-T:IT-M blend in the short wavelength region. In the ternary PSCs based on PBDB-T:IT-M:Bis-PCBM[70], a higher PCE of 12.2% was achieved.
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