Mechanism of band gap persistent photoconductivity (PPC) in SnO₂ nanoscrystalline films: Nature of local states, simulation of PPC and comparison with experiment

Highlights: • RT photoconductivity (PC) model of response and decay in SnO₂ film was proposed. • Surface BG electronic states are a source for excitation by photons with hv < E_g. • BG electron LDOS determines the magnitude and time of PC response. • Intergrain barrier height related to oxygen ionosorption determines PC decay time. • Structural disordering results in stretched exponential behavior of PC decay. - Abstract: A phenomenological model of room temperature photoconductivity in nanocrystalline SnO₂ under photon excitation below the fundamental bandgap based on electronic states located at the bottom part of the band gap was proposed. Nature of these states is related to the surface oxygen vacancies and Sn-derived electronic states. Appropriate distribution of these states was considered. Numerical simulation of the photoconductivity response and decay on the basis of balance rate equation for excited electrons and immobile holes was done. Analysis revealed that response time is determined by the photoionization cross section of these states and intensity of illumination. Stationary photoresponse is saturated due to the limited number of these states. Intergrain potential barrier that originated due to the ionosorbed oxygen is the main factor limiting the reverse annihilation process and determining the photoconductivity decay time. Stretched exponential behavior of the photoconductivity decay was interpreted in terms of structural and electronic film disordering that results in asymmetric probability distribution of intergrain barrier heights and corresponding distribution of time constants.