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

摘要

A key parameter for polymer electron acceptors is the lowest unoccupied molecular orbital (LUMO) energy level (ELUMO). For state-of-the-art polymer electron acceptors based on the naphthalene diimide (NDI) unit, their ELUMO are low-lying and cannot be tuned, leading to a low open-circuit voltage (Voc) 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 ELUMO because of their delocalized LUMOs over polymer backbones. The ELUMO 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 (μe = 2.07 × 10^–4 cm² V^–1 s^–1) is also higher than that of >P-BNBP-T (μe = 7.16 × 10^–5 cm² 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 ELUMO of >P-BNBP-Se, the all-PSC device of >P-BNBP-Se shows a Voc 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 Voc values and small photon energy losses.