Alkyl Chain End Group Engineering of Small Molecule Acceptors for Non-Fullerene Organic Solar Cells

Alkyl chain engineering is widely used to prepare high-performance donor materials. However, relatively few studies have been focused on the alkyl chain optimization of acceptor materials. Herein, a series of new A–D–A (acceptor–donor–acceptor) type small molecule acceptors (ITBTR-C2, ITBTR-C4, ITBTR-C6, and ITBTR-C8) with indacenodithieno[3,2-b]thiophene (IDTT) as the core, benzothiadiazole (BT) as the π bridge, and ethyl-, butyl-, hexyl-, and octyl-substituted 2-(1,1-dicyanomethylene) rhodanine as the end groups, respectively, are successfully synthesized to systematically investigate the alkyl substituent effects on the physical, chemical, and electronic properties of A–D–A type small molecule acceptors. All molecules exhibit a strong and broad absorption from 600 to 800 nm as well as similar HOMO and LUMO energy levels. ITBTR-C6 with hexyl substitution shows the highest electron mobility and better phase separation morphology after blending with a donor polymer (PBDB-T). Therefore, inverted bulk heterojunction organic solar cells based on ITBTR-C6:PBDB-T blends exhibit the highest power conversion efficiency (PCE) of 8.26% with an open-circuit voltage (VOC) of 0.89 V, a high short-circuit current density (JSC) of 15.80 mA/cm2, and a fill factor (FF) of 58.21%, while the PCEs of ITBTR-C2-, ITBTR-C4-, and ITBTR-C8-based devices are 7.04%, 7.43%, and 7.93%, respectively. After solvent vapor and thermal annealing, both the JSC and FF values of the ITBTR-C6-based device are further increased, leading to a PCE of 9.29%. The results demonstrate that the alkyl chain substitution of A–D–A type small molecule acceptors is critical, and an appropriate adjustment of the alkyl chains can effectively enhance device performance.