Identification of Exciton–Exciton Annihilation in Hematite Thin Films

Abstract

Hematite, a common type of iron oxide, is a promising material for solar technologies because of its small band gap that allows for solar radiation absorption in the visible region, low toxicity in aqueous solutions, easy synthesis, and natural abundance. However, fast electron–hole recombination has been hampering full applicability of hematite in solar technologies. In this study, we used visible femtosecond transient absorption spectroscopy to investigate the excited-state decay and electron–hole recombination dynamics of nanostructured hematite thin films. By varying the pump excitation fluence and performing global and target data analysis, we identified the presence of nonlinear decay processes during the initial picosecond after photoexcitation, which have a non-negligible contribution at pump fluences of >∼1 mJ/(pulse·cm2). Calculations show exciton–exciton annihilation to be the dominant nonlinear process, with an average rate constant of 7.09 × 10–9 cm3 s–1. Annihilation calculations also allowed us to estimate the annihilation radius to be 2.3 nm, thus explaining the rapid exciton–exciton annihilation in the immediate aftermath of photoexcitation. Probe wavelength-dependent decay dynamics points to excitation energy redistribution and involvement of traps in the recombination dynamics. We finally present a kinetic model, verified by performing target data analysis, showing the rates and channels of the dominant processes involved with electron–hole recombination upon relatively high excitation rates. The extremely fast electron–hole recombination process in hematite is one of the main reasons hindering the full applicability of the material in solar water splitting. Measures to limit these ultrafast recombination processes should, therefore, be incorporated into device fabrication and preparation to further improve the material.