Intrinsic polaronic photocarrier dynamics in hematite

Abstract

Self-trapping of photocarriers due to the strong carrier-phonon interaction is ubiquitous in metal oxides, which exerts detrimental influence on the performance of the oxide-based optoelectronic devices. The self-trapped species could be isolated self-trapped charges (i.e., isolated polarons) or bound self-trapped electron-hole pairs (i.e., polaron-pairs, self-trapped excitons), and they may show distinct dynamic behaviors. Here, we find that the self-trapping in hematite ($\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$) primarily results in self-trapped excitons rather than spatially separated polaronic charges or electron-hole polaron-pairs based on both dynamic and spectral evidence. The formation and recombination of self-trapped excitons are found to be substantially independent of defect concentration varying from monocrystalline to polycrystalline $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ films. The spectral feature attributed to small electron polarons can only be resolved above the noise floor by raising excitation intensity so that the Auger process dominates the recombination. Based on the comparison of monocrystalline and polycrystalline sample data, we suggest that the isolated electron polaron formation is likely facilitated by defects. Our finding invokes reconsideration of the photocurrent generation mechanism in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ photoanodes.