Compatibility between Solubility and Enhanced Crystallinity of Benzotriazole-Based Small Molecular Acceptors with Less Bulky Alkyl Chains for Organic Solar Cells

Optimizing the molecular structures of organic solar cell (OSC) materials and boosting the power conversion efficiencies are the eternal theme in the solar energy region. A series of fused benzotriazole (BTA)-based A-DA'D-A structures of nonfullerene acceptors (such as Y18) were developed for application in efficient OSCs, in which high quantum efficiencies and low voltage losses could be achieved because of the optimized electron-deficient core and specific molecular geometry. Here, based on the BTA core, the bulky alkyl chain on the BTA unit was further tailored to minimize the lateral alkyl chains and enhance the crystallinity while maintaining an adequate solubility. The resulting NFAs of BTA-C1, BTA-C5, and BTA-C6 are synthesized. Compared with the well-designed molecular Y18 (BTA-C8), we found that simply replacing the 2-ethylhexyl chain with a single methyl (BTA-C1) can easily improve the fill factor up to 77%, but its poor light absorption capacity and large domain size impeded further efficiency improvement. In particular, the BTA-C5, with a shortened branch alkyl chain of 2-methylbutyl, achieves suitable solubility and enhanced crystallinity. Significantly, owing to the balanced charge carrier mobility and suitable phase separation, the BTA-C5-based binary single-junction OSCs achieve a high efficiency of 17.11%, which is one of the top values in BTA-based OSCs.