Molecular engineering of auxiliary acceptor for the development of efficient organic D-A-It-A sensitizers

Molecular structure is vitally important for the selective optimization of energy level and improving the interfacial charge transfer performance of dye-sensitized solar cells (DSSCs). With appropriate auxiliary acceptor, the organic sensitizer owing D-A-It-A structure can act as a promising efficient sensitizer for DSSCs. Herein, considering the conjugated It-system and electron-withdrawing capability, dyes CS-71 and CS-72 were designed and synthesized with naphthothiadiazole and naphthobisthiadiazole units as the auxiliary acceptors, respectively, while the benzothiadiazole unit was applied as the auxiliary acceptor for the reference dye CS-70. The structure-activity relationship between the molecular structure characteristics, including steric hindrance, It-conjugation system, and electron-withdrawing capability of the auxiliary acceptors, and the properties of the dye are systematically investigated. Ascribed to the extended auxiliary acceptor moieties, CS-71 and CS-72 obtained obvious downward shift LUMO levels accompanying with the HOMO levels remaining unchanged, with respect to the reference dye CS-70. Owing to the shrinked energy gap, their absorption spectra were also significantly bathochromic shifted, resulting in the effectively broadened light-harvesting region. Combining the improved charge injection process, cell devices sensitized by CS-72 achieves an optimal PCE of 7.17%, presenting us a promising way to obtain highly efficient organic sensitizer in the future.