Design of low-cost non-fused ultranarrow-band-gap acceptors for versatile photovoltaic applications

Ultranarrow-band-gap organic semiconductors with fully non-fused conjugated structures display great potential for low-cost organic photoelectric devices. Here, we developed two fully non-fused acceptors, namely, A4T-7 and A4T-12, by introducing different alkoxyl side chains on the π-bridges of the non-fused acceptors. The resulting materials demonstrate ultranarrow optical band gaps of 1.15 and 1.21 eV, respectively. Compared with other ultranarrow-band-gap acceptors constructed with fully fused-ring or partially fused-ring structures, the synthetic complexity of the two acceptors is significantly reduced. Specifically, A4T-7, with symmetric alkoxy chains on the π-bridge, exhibits a more planar molecular configuration compared with A4T-12. Notably, the organic photovoltaic cells based on A4T-7 show a power conversion efficiency of 13.3%. Moreover, cells fabricated with a highly transparent active layer, characterized by an average visible transmittance value of approximately 62.7%, achieve an efficiency of 10.7%. These results represent the highest reported efficiencies for cells utilizing fully non-fused acceptors with ultranarrow band gaps.