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

Brominated A-D-A-type small-molecule acceptor ITIC-2Br-gamma with certain molecular structure was designed and synthesized. Compared to the mixture of three isomers (ITIC-2Br-m), ITIC-2Br-gamma shows stronger absorption in the region of 600-800 nm, which is beneficial to afford higher J(SC). Furthermore, single-crystal structure analysis of ITIC-2Br-gamma indicates that although the bromine atom has a larger size, the end groups of adjacent molecules still exhibit strong interactions with short pi-pi distance of 3.28 angstrom. Because of the Br center dot center dot center dot S and O center dot center dot center dot 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-gamma 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.