High-Efficiency As-Cast Organic Solar Cells Based on Acceptors with Steric Hindrance Induced Planar Terminal Group

A series of alkyl, alkoxyl, and alkylthio substituted A–π–D–π–A type nonfullerene acceptors (NFAs) IDTCN-C, IDTCN-O, and IDTCN-S are designed and synthesized. The introduction of a lateral side chain at the outer position of the π bridge unit can endow the terminal moiety with a confined planar conformation due to the steric hindrance. Thus, compared with nonsubstituted NFA (IDTT2F), these acceptors tend to form favorable face-on orientation and exhibit strong crystallinity as verified with grazing-incidence wide-angle X-ray scattering measurement. Moreover, the variation of side chain can significantly change the lowest unoccupied molecular orbital (LUMO) energy level of acceptors. As state-of-the-art NFAs, a power conversion efficiency of 13.28% (V_oc = 0.91 V, J_sc = 19.96 mA cm⁻², and FF = 73.2%) is obtained for the as-cast devices based on IDTCN-O, which is among the highest value reported in literature. The excellent photovoltaic performance for IDTCN-O can be attributed to its slightly up-shifted LUMO level and more balanced charge transport. This research demonstrates side chain engineering is an effective way to achieve high efficiency organic solar cells.