Quantized Auger Recombination Of Self-trapped Excitons In α-Fe₂O₃ Single Crystalline Thin Film

As one of the important metal oxide for solar-to-fuel conversion via solar water splitting,α-Fe2O3 semiconductor has the advantages of suitable band gap,stable performance,cost-effective and easy to get.But α-Fe2O3 also presences the insufficient characteristics such as low mobility and short lifetime of photocarriers,the self-trapping phenomenon of photocarriers that arises from the ionic and polar nature of the metal oxides has been suggested to be responsible for the characteristics.Highly localized self-trapped excitons(the electron polaron can bind to the hole to form exciton,STE)in α-Fe2O3 has been experimentally confirmed.Although the STE dynamics determines metal oxides’ important parameters relevant to the energy conversion efficiency,the STE annihilation mechanism especially that involves multiple STE still remain unknown.Here,a single crystalline α-Fe2O3 thin film was employed as a platform to study the annihilation mechanism of highly localized STE.The dynamics of STE in αFe2O3 was measured by the ultrafast transient absorption(TA)spectrum.The purpose is to reveal STE annihilation mechanism and provide a certain theoretical basis for further improving the conversion efficiency of this type of light conversion energy material.In this article,through analyzing the transient absorption(TA)spectrum of α-Fe2O3 single crystal thin film,learning that the sub-bandgap absorption(SBA)feature is likely due to the self-trapped excitons.To explore the nature of the photocarriers in α-Fe2O3 single crystal thin film,we compared the SBA kinetics probed at different sub-bandgap regions(1.71 eV VIS、1.01 eV NIR、0.25 eV mid-IR)under the identical pump condition.By analyzing the dynamic of SBA feature can be divided into two part,before and after 1 ps were attributed to the STE formation(free carrier self-trapping)and STE annihilation,respectively.In the same time,the SBA spectral for different STE densities were also examined,the obtained STE annihilation curves for different STE densities exhibited a fast component sensitive to the STE density and a common slow component independent to the STE density.These STE kinetics traces could not be modeled accurately by the conventional charge recombination mechanisms.Inspired by the strong spatial localization of the coupled polaronic STE,we build the quantized Auger recombination model,which can well fit the fast STE annihilation kinetic process obtained from experiments.The quantized Auger recombination of STE is potentially a general phenomenon in the metal oxides.