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 ELUMO 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[(ethylhexylthiophenyl)- benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7-Th). The electron mobility of PBNBP- Se (mu(e) = 2.07 x 10(-4) cm(2) V-1 s(-1)) is also higher than that of P-BNBP-T ((mu(e) = 7.16 x 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 ELUMO of P-BNBP-Se, the all-PSC device of PBNBP- 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.