Understanding Back Electron Transfer in Lead Halide Perovskites

Cesium lead halide nanocrystals have drawn immense interest because of their highemission quantum yield and composition-dependent tunability of emission responseover the entire visible range. Despite the extensive effort to probe photo-physical studies of these all-inorganic perovskite nanocrystals, their photo-catalytic properties are yet to be completely explored. We have systematically probed the key steps of photoinduced charge transfer betweencesium lead halide perovskite nanocrystals and an electron acceptor, methyl viologen(MV²⁺ ) molecule, using a combination of steady-state and time-resolvedspectroscopic techniques. The long-lived viologen radical peak observed inthetransient absorption spectrum indicates the electron transfer product between CsPbI3 nanocrystals and viologen acceptor. To establish the influence of driving force for charge transfer in nanocrystal-MV hybrids, we have controlled the donor energylevels and the bandgap by varying the halide ratio. Varying the compositionof different nanocrystal donors (CsPb(Br_xI_1-x)₃), we have studied the electron transfer rate remained the same which is consistent with the negligible changes intheconduction band energy levels. Interestingly, the changes in bandgap are mostlyreflected in the perovskite valence band which in turn changes the driving force for back electron transfer. The influence of the driving force on back electron transfer tothe donor nanocrystals was probed by tracking the decay of MV^+● radical, the electrontransfer product.