Optimizing Solid-State Ligand Exchange for Colloidal Quantum Dot Optoelectronics: How Much Is Enough?
Ahmad R. Kirmani, Grant Walters, Taesoo Kim, Edward H. Sargent, Aram Amassian
Abstract
Progress in chalcogenide and perovskite CQD optoelectronics has relied to a significant extent on solid-state ligand exchanges (SSEs): the replacement of initial insulating ligands with shorter conducting linkers on CQD surfaces. Herein we develop a mechanistic model of SSE employing 3-mercaptopropionic acid (MPA) and 1,2-ethanedithiol (EDT) as the linkers. The model suggests that optimal linker concentrations lead to efficient exchange, resulting in ca. 200–300 exchanged ligands per CQD, a 50% thickness reduction of the initial film, decreased interdot spacing, a 15 nm red-shift in the excitonic absorption peak, and a 10× reduction in carrier lifetime. It is the combined effect of these physicochemical changes that has traditionally made 1% MPA and 10–2% EDT (v:v) the concentrations of choice for efficient CQD optoelectronics.