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

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 cm² V^-1 s^-1) is also higher than that of P-BNBP-T (μ_e = 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 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.