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A key parameter for polymer electron acceptors is the lowest unoccupied molecular orbital (LUMO) energy level ( E  LUMO ). For state-of-the-art polymer electron acceptors based on the naphthalene diimide (NDI) unit, their E  LUMO are low-lying and cannot be tuned, leading to a low open-circuit voltage ( V  oc ) of the resulting all-polymer solar cells (all-PSCs). We report that polymer electron acceptors based on the double B←N bridged bipyridine (BNBP) unit exhibit tunable E  LUMO because of their delocalized LUMOs over polymer backbones. The E  LUMO of the copolymer of the BNBP unit and selenophene unit ( P-BNBP-Se ) is lower by 0.16 eV than that of the copolymer of the BNBP unit and thiophene unit ( P-BNBP-T ). As a result, the energy levels of P-BNBP-Se match well with the widely-used polymer donor, poly[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene] ( PTB7-Th ). The electron mobility of P-BNBP-Se ( μ  e = 2.07 × 10 -4 cm 2 V -1 s -1 ) is also higher than that of P-BNBP-T ( μ  e = 7.16 × 10 -5 cm 2 V -1 s -1 ). While the all-PSC device based on the PTB7-Th : P-BNBP-T blend shows a moderate power conversion efficiency (PCE) of 2.27%, the corresponding device with P-BNBP-Se as the acceptor exhibits a PCE as high as 4.26%. Moreover, owing to the suitable E  LUMO of P-BNBP-Se , the all-PSC device of P-BNBP-Se shows a V  oc up to 1.03 V, which is higher by 0.22 V than that with the conventional NDI-based polymer acceptor. These results indicate that BNBP-based polymers can give all-PSCs with high PCEs, remarkably high V  oc values and small photon energy losses.

A polymer acceptor with an optimal LUMO energy level for all-polymer solar cells