Insights into Electron Dynamics in Mn-Doped Ag–In–S Quantum Dots for Efficient Photoelectrocatalytic Hydrogen Evolution

The electron transfer mechanism involving multiple trapping states has great implications for dictating the photoelectrochemical (PEC) hydrogen evolution reaction (HER). Uncovering the ultrafast dynamics of multiple trapping states in defect-rich catalysts to suppress undesirable Shockley-Read-Hall (SRH) recombination and boost electron extraction remains a crucial yet challenging task. Herein, we bypass defect-induced SRH recombination sites and redirect more excited electrons toward the HER by engineering an additional electron transfer channel within Ag-In-S quantum dots (AIS QDs) via Mn doping. Photoluminescence spectroscopic studies indicate effective charge carrier transfer from the band edge to the doping states, inducing a 5.3-fold enhancement in the electron transfer rate. Using ultrafast transient absorption spectroscopy, we reveal that host photoexcitation leads to subpicosecond electron transfer from the conduction band to the empty Mn²⁺ d-orbitals in Mn-doped QDs, and the resultant charge transfer state shortens the excited-state electron transfer lifetime from 3.73 to 1.19 ns, corresponding to a 300% acceleration of carrier depopulation and enhancing excited electron availability. Moreover, the evolution of the product composition indicates the improved interfacial utilization of holes, as the applied PEC bias relieves kinetic constraints on hole transfer. Consequently, a saturated photocurrent density of 8.6 mA cm^-2 at 0.5 V vs RHE was achieved for the as-fabricated PEC device under one-sun irradiation, comparable to the latest bare heavy-metal-free QD-based PEC systems. These results underscore that modulating electron dynamics, previously dominated by intrinsic defects, through intermediate energy levels is a promising strategy to enable high-efficiency PEC solar energy conversion.