Bromination of the Small-Molecule Acceptor with Fixed Position for High-Performance Solar Cells

Brominated A–D–A-type small-molecule acceptor ITIC-2Br-γ with certain molecular structure was designed and synthesized. Compared to the mixture of three isomers (ITIC-2Br-m), ITIC-2Br-γ shows stronger absorption in the region of 600–800 nm, which is beneficial to afford higher JSC. Furthermore, single-crystal structure analysis of ITIC-2Br-γ indicates that although the bromine atom has a larger size, the end groups of adjacent molecules still exhibit strong interactions with short π–π distance of 3.28 Å. Because of the Br···S and O···S interactions, all molecules form an interpenetrated three-dimensional network, which is beneficial for the charge to transport along multidirections. The organic solar cells (OSCs) based on the PBDB-T-2F:ITIC-2Br-γ blend film exhibit a higher power conversion efficiency (PCE) of 12.05% due to its better film morphology and higher charge mobilities, whereas a PBDB-T-2F:ITIC-2Br-m-based device only shows a moderate PCE of 10.88%. These results indicate that separation and purification of the brominated A–D–A-type small molecules are an effective way to further improve their photovoltaic performances. Furthermore, bromination is easily synthesized and is of low-cost, which exhibits great potential for the preparation of large-scale OSCs.