Bandgap Engineering of Halide Perovskite Nanocrystals for Maximizing Hole Transfer: Accessing the Marcus Inverted Region

Controlling charge transfer at the semiconductor-acceptor molecule interface is important for improving the performance of semiconductor assisted photocatalytic processes. The difference between the band energies of the semiconductor and the redox potential of the acceptor is known to control the kinetics of charge transfer. By employing p-phenylenediamine (PPD) and m-phenylenediamine (MPD) as probe molecules, we have systematically probed the hole transfer from excited perovskite nanocrystals of different bandgaps. The valence band energy of the donor mixed halide perovskite nanocrystal, which varied from 1.74 to 0.94 V vs NHE through halide composition, viz., varying Cl:Br and Br:I ratio, allowed us to change the driving force (−ΔG) of hole transfer. The rate constant of hole transfer as determined from the transient absorption and photoluminescence decay measurements showed a nonlinear dependence on −ΔG. Analysis of this dependence followed Marcus-electron transfer theory with a reorganization energy of ∼1 eV. Relatively higher reorganization energy as compared to pure solvent showed the ligand shell (oleylamine) and charged nanocrystal lattice playing a major role in the interfacial charge transfer processes. The energy dependence of the charge transfer rate constant provides new insights into the photocatalytic properties of perovskite nanocrystals and ways to maximize the charge transfer yield through bandgap engineering of the semiconductor.