Large light absorption is fundament

Large light absorption is fundamentally important for OSCs toproduce electricity from sunlight. Complementary absorptionbetween donors and acceptors are normally expected for maximizing sunlight harvesting and the Jsc, which is critical forhigh-performance BHJ OSCs. As an example, Fig. 13a showsthe absorption spectra of PCBM, N2200 and PTB7–Th. Due tothe weak absorption of PCBM and poor spectral overlap betweenN2200 and PTB7–Th, the BHJ active layers based on PTB7–Th/PCBM and PTB7–Th/N2200 can’t effectively harvest the solarenergy. When PTB7–Th was replaced with another widebandgap donor J51 to pair with N2200, it exhibits complementary absorption with narrow-bandgap acceptor N2200(Fig. 13b) [76]. The resulting OSCs show an impressive improvement in PCE, since both donor and acceptor made great contribution to the photo-current generation. Thus, thecomplementary absorption between donors and acceptors inthe wide wavelength range, and large absorption coefficientscovering the range of 400–800 nm or beyond are critical forhigh-performance fullerene-free OSCs. Either the combinationof wide-bandgap donor /narrow-bandgap acceptor or narrowbandgap donor /wide-bandgap acceptor is applicable for goodlight harvesting in OSCs.