Understanding charge transport in non-doped pristine and surface passivated hematite (Fe₂O₃) nanorods under front and backside illumination in the context of light induced water splitting

Hematite (Fe₂O₃) nanorods on FTO substrates have been proven to be promising photoanodes for solar fuel production but only with high temperature thermal activation which allows diffusion of tin (Sn) ions from FTO, eventually enhancing their conductivity. Hence, there is a trade-off between the conductivity of Fe₂O₃, and the degradation of FTO occurring at high annealing temperatures (>750°C). Here, we present a comprehensive study on undoped Fe₂O₃ nanorods under front and back illumination to find the optimum annealing temperature. Bulk/surface charge transport efficiency analysis demonstrates minimum bulk recombination indicating overall high quality crystalline Fe₂O₃ and the preservation of FTO conductivity. Surface recombination is further improved by growing a TiO_x overlayer, which improves the photocurrent density from 0.2 mA cm^-2 (backside) to 1.2 mA cm^-2 under front side and 0.8 mA cm^-2 under backside illumination. It is evident from this study that the performance of undoped and unpassivated hematite nanorods is limited by electron transport, whereas that of doped/passivated hematite nanorods is limited by hole transport.