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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 (<i>E</i><sub>loss</sub>) of
0.51 eV or a PCE of 3.77% with an extremely small <i>E</i><sub>loss</sub> of 0.47 eV. This <i>E</i><sub>loss</sub> 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