n‑Type\nConjugated Polymers Based on Double\nB←N Bridged Bipyridine Unit

ConspectusBoth p-type conjugated polymers\nand n-type conjugated polymers\nare required for organic optoelectronic devices, such as organic solar\ncells (OSCs), organic field-effect transistors (OFETs), organic thermoelectrics\n(OTEs), etc. The development of n-type conjugated polymers lags far\nbehind that of the p-type counterparts in view of material diversity\nand optoelectronic device performance. This is mainly due to the lack\nof strong electron-withdrawing building blocks, which are always based\non the imide unit. Double boron–nitrogen coordination bond\n(B←N) bridged bipyridine (BNBP), which was first developed\nin 2016, is an alternative kind of electron-withdrawing building block\nbased on a B←N unit. BNBP itself possesses a planar and fixed\nconfiguration, low-lying electronic energy levels, strong fluorescence,\nand facile functionalization. A family of BNBP-based conjugated polymers\nhas been developed. They show excellent and tunable optoelectronic\nproperties, such as high electron mobility, low-lying and tunable\nlowest unoccupied molecular orbital (LUMO) energy level (<i>E</i><sub>LUMO</sub>), medium bandgap, narrow absorption spectra in the\nvisible range, high fluorescence quantum efficiency, etc. With rational\nmolecular design of BNBP-based conjugated polymers, they have been\nwidely used in organic optoelectronic devices with high performance,\nincluding OSCs, OFETs, and OTEs, indoor photovoltaics (IPVs), organic\nlight-emitting diodes (OLEDs), organic photodetectors (OPDs), etc.\nTherefore, BNBP-based conjugated polymers have become an important\nclass of optoelectronic materials. In this Account, we summarize the\nresearch progress on BNBP-based conjugated polymers.At first,\nwe discuss BNBP itself, including its molecular design,\nsynthesis, chemistry, and optoelectronic properties. Then we introduce\nthe optoelectronic properties of BNBP-based conjugated polymers, including\ntheir light absorption property, fluorescence, electron mobility,\nand frontier electronic energy levels. We have systematically elucidated\nthe relationship between the chemical structures, optoelectronic properties,\nand optoelectronic device performance of BNBP-based n-type conjugated\npolymers. The unique property of BNBP-based conjugated polymers is\nthe high electron mobility in the amorphous state. Other noteworthy\nproperties of these polymers are the medium bandgap and absorption\nspectra in the visible range. Next, we discuss the applications of\nBNBP-based conjugated polymers in OSCs, IPVs, OFETs, OTEs, OPDs, and\nOLEDs. The excellent optoelectronic device performance is noteworthy,\nsuch as power conversion efficiency (PCE) of 10% in OSCs, PCE of 26%\nin IPVs, electron mobility of 0.3 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> in OFETs, power factor of 25 μW m<sup>–1</sup> K<sup>–2</sup> in OTEs, specific detectivity\nof 1.79 × 10<sup>13</sup> cm Hz<sup>1/2</sup> W<sup>–1</sup> in OPDs. Finally, we propose that great attention should be paid\nto the deep understanding of the electronic structures of BNBP itself\nand BNBP-based conjugated polymers as well as the new applications\nof BNBP-based conjugated polymers.