Sequential Photoluminescence Measurements to Explore Electron-Trapping Dynamics in Slow Photoresponse Materials: Zinc Oxide Thin Films as a Model System

Spectroscopic probing of electron de-excitation is a matter of debate in many materials that exhibit slow photoresponse. Sequential photoluminescence spectra with varying delay times between scans (SPSVDT) is explored as a strategy to investigate the dynamics of electron de-excitations in such systems with zinc oxide ($\mathrm{Zn}\mathrm{O}$) thin films, a prototypical material exhibiting slow photoresponse, as a model. The selection of model system is based on its wide use in various optoelectronic applications as an electron transport layer as well as an active layer. The peak intensities of PL spectra of $\mathrm{Zn}\mathrm{O}$ thin film exhibited an exponential decrease while performing continuous PL measurements and is attributed to the prolonged lifetime of electrons in the conduction band. The time constant (${\ensuremath{\tau}}_{\mathrm{cap}}$) associated with the slow electron relaxations to the trap states is estimated to be 1.95 minutes by performing SPSVDT. The very slow electron relaxations to the trap states in $\mathrm{Zn}\mathrm{O}$ after UV exposure is found to be affecting the Fermi level of $\mathrm{Zn}\mathrm{O}$ thin film, reverse saturation current in ${\text{ITO/ZnO/MoO}}_{3}/\mathrm{Ag}$ heterojunction and photocurrent in ITO/$\mathrm{Zn}\mathrm{O}$/$\mathrm{Ag}$ device. These modest variations are successfully simulated using the time constant ${\ensuremath{\tau}}_{\mathrm{cap}}$ obtained from the analysis. The proposed strategy can be adopted to quantitatively interpret the electron trapping dynamics in materials exhibiting slow photoresponse and can be used to tune their photoresponse.