From Bulk Traps to Surface States: Thickness-Dependent Charge Transport and Recombination in CdS Thin Films

Understanding how film thickness governs the balance between trap-mediated transport and interfacial recombination is essential for the rational design of photoanodes. Here, ultrathin CdS films (22–66 nm) were synthesized with controlled thickness using a hybrid chemical bath deposition–spin coating method, and their electronic properties were characterized using time-of-flight photocurrent (TPC-ToF) transients and open-circuit voltage decay (OCVD) measurements. These techniques reveal a mechanistic transition from bulk trap-limited, highly dispersive transport in the thinnest films to faster diffusion coupled with intense surface recombination in thicker films. Electron diffusion coefficients increase by nearly an order of magnitude with increasing thickness, while recombination lifetimes concurrently decrease, evidencing a systematic strengthening of surface-controlled recombination pathways. Analysis of the lifetime–voltage behavior, supported by morphological characterization, shows that thinner films are dominated by deep bulk traps that transiently store charge, whereas thicker films exhibit a higher density of unsaturated surface states that act as efficient recombination centers. These results establish a quantitative framework linking thickness, trap localization (bulk vs surface), and charge-transport and recombination dynamics in CdS, providing mechanistic insight for the design of ultrathin semiconductor photoelectrodes.