Rational design of a main chain conjugated copolymer having donor-acceptor heterojunctions and its application in indoor photovoltaic cells

We successfully synthesized a crystalline main chain conjugated copolymer, P(BDBT-co-NDI2T), having BDBT donor-NDI2T acceptor heterojunctions in the film state. Interestingly, the polymer exhibited a strong absorption band from 500 to 650 nm, which overlapped well with the emission spectrum of white light-emitting diodes (LEDs) used as an indoor light source. The P(BDBT-co-NDI2T) films had a relatively smooth surface and internal morphology compared to the blend films of P(BDBT) and P(NDI2T). In particular, the Kelvin probe force microscopy data explained the increased uniformity in the distribution of the surface charge in the annealed P(BDBT-co-NDI2T) films as compared to those of the annealed blend films of P(BDBT) and P(NDI2T). Indoor photovoltaics (IPVs) produced by using the P(BDBT-co-NDI2T) synthesized in this study as an active layer showed a high power conversion efficiency of 12.70% (@500 lux) with a short circuit current density of 49.89 μA cm⁻². The PCE of the P(BDBT-co-NDI2T) film-based IPVs was twice as high as that of the blend film-based IPVs and showed excellent operational stability. This study is the first time that the P(BDBT-co-NDI2T) has been applied in IPVs, which suggests the potential for their widespread use in the development of indoor organic PVs in the future.