n-Type Conjugated Polymers Based on Double B←N Bridged Bipyridine Unit

ConspectusBoth p-type conjugated polymers and n-type conjugated polymers are required for organic optoelectronic devices, such as organic solar cells (OSCs), organic field-effect transistors (OFETs), organic thermoelectrics (OTEs), etc. The development of n-type conjugated polymers lags far behind that of the p-type counterparts in view of material diversity and optoelectronic device performance. This is mainly due to the lack of strong electron-withdrawing building blocks, which are always based on the imide unit. Double boron-nitrogen coordination bond (B←N) bridged bipyridine (BNBP), which was first developed in 2016, is an alternative kind of electron-withdrawing building block based on a B←N unit. BNBP itself possesses a planar and fixed configuration, low-lying electronic energy levels, strong fluorescence, and facile functionalization. A family of BNBP-based conjugated polymers has been developed. They show excellent and tunable optoelectronic properties, such as high electron mobility, low-lying and tunable lowest unoccupied molecular orbital (LUMO) energy level (E_LUMO), medium bandgap, narrow absorption spectra in the visible range, high fluorescence quantum efficiency, etc. With rational molecular design of BNBP-based conjugated polymers, they have been widely used in organic optoelectronic devices with high performance, including OSCs, OFETs, and OTEs, indoor photovoltaics (IPVs), organic light-emitting diodes (OLEDs), organic photodetectors (OPDs), etc. Therefore, BNBP-based conjugated polymers have become an important class of optoelectronic materials. In this Account, we summarize the research progress on BNBP-based conjugated polymers.At first, we discuss BNBP itself, including its molecular design, synthesis, chemistry, and optoelectronic properties. Then we introduce the optoelectronic properties of BNBP-based conjugated polymers, including their light absorption property, fluorescence, electron mobility, and frontier electronic energy levels. We have systematically elucidated the relationship between the chemical structures, optoelectronic properties, and optoelectronic device performance of BNBP-based n-type conjugated polymers. The unique property of BNBP-based conjugated polymers is the high electron mobility in the amorphous state. Other noteworthy properties of these polymers are the medium bandgap and absorption spectra in the visible range. Next, we discuss the applications of BNBP-based conjugated polymers in OSCs, IPVs, OFETs, OTEs, OPDs, and OLEDs. The excellent optoelectronic device performance is noteworthy, such as power conversion efficiency (PCE) of 10% in OSCs, PCE of 26% in IPVs, electron mobility of 0.3 cm² V^-1 s^-1 in OFETs, power factor of 25 μW m^-1 K^-2 in OTEs, specific detectivity of 1.79 × 10¹³ cm Hz^1/2 W^-1 in OPDs. Finally, we propose that great attention should be paid to the deep understanding of the electronic structures of BNBP itself and BNBP-based conjugated polymers as well as the new applications of BNBP-based conjugated polymers.