Polymer Electron Acceptors with Conjugated Side Chains for Improved Photovoltaic Performance

The development of polymer electron acceptors lags far behind that of polymer electron donors. A general approach to improve photovoltaic performance of polymer electron donors is to incorporate conjugated side chains to the electron-rich unit. In this article, we introduce the “conjugated side chain” strategy to molecular design of polymer electron acceptors by incorporating conjugated side chains to the electron-deficient unit. The polymer backbones consist of alternating electron-deficient double B←N bridged bipyridine (BNBP) unit and electron-rich thiophene or selenophene unit. Polymer electron acceptors are developed by incorporating conjugated alkoxyphenyl side chains to the BNBP unit. Compared with conventional alkyl side chains, the conjugated alkoxyphenyl side chains endow the polymer electron acceptors with low-lying LUMO energy levels, enhanced π-π stacking, and high electron mobilities, which are very desirable for electron acceptors. The resulting all-PSCs exhibit an enhanced power conversion efficiency (PCE) of 4.46% with a small photon energy loss (E_loss) of 0.51 eV or a PCE of 3.77% with an extremely small E_loss of 0.47 eV. This E_loss is among the smallest values reported for organic solar cells. These results demonstrate that the “conjugated side chain” strategy can be used not only for high-efficiency polymer electron donors but also for high-performance polymer electron acceptors.