Probing longitudinal carrier transport in perovskite thin films via modified transient reflection spectroscopy

Accurate characterization of the longitudinal (along the thickness direction) carrier transport property is of significant importance for evaluating the quality and performance of perovskite thin films. Herein, we report the development of a modified transient reflection (TR) spectroscopy method to realize the direct observation and determination of the longitudinal carrier transport process in MAPbI(3) polycrystalline thin films. Unlike the traditional TR spectroscopy, the carrier transport dynamics along the film thickness is resolved by making the pump (excitation) and probe beams spatially separated on each side of the film, so that the carrier transport from the excitation side to the probe side is directly captured. Utilizing this method, the longitudinal carrier diffusion coefficients (D) in various perovskite films with different thicknesses and grain sizes (extracted from SEM images) are determined, showing D values of similar to 1.5 to 1.8 cm(2) s(-1) (similar to 0.5 to 0.8 cm(2) s(-1)) for films with grain size larger (smaller) than the thickness. This empirical correlation between the longitudinal D and film thickness/grain size provides a reference for quick quality screening and evaluation of perovskite polycrystalline thin films.

Automated summary

In this study, a modified transient reflection (TR) spectroscopy method was developed to directly observe and determine the longitudinal carrier transport process in MAPbI(3) polycrystalline thin films. By spatially separating the pump and probe beams on each side of the film, the carrier transport dynamics along the film thickness were resolved. The longitudinal carrier diffusion coefficients (D) in perovskite films with different thicknesses and grain sizes were determined, providing a reference for quick quality screening and evaluation of perovskite polycrystalline thin films.