Visualization and deconvolution of carrier kinetics within grains of Cu2ZnSnS4−xSex using ultrafast diffuse reflectance microscopy

Abstract

Cu₂ZnSnS_4−xSe_x (x = 0–4, CZTSSe) materials are gaining interest for their solar energy conversion applications. We investigated the use of state-of-the-art ultrafast pump–probe microscopy and ultrafast image segmentation to visualize and spatially deconvolute photoexcited dynamics with CZTSSe materials at grain-size level. Band-tailing and electronic trap states are limitations in achieving high photovoltaic performance. We tested a combination of ultrafast pump–probe diffuse reflectance transient absorption microscopy (DR-TAM), and broadband femtosecond transient absorption spectroscopy in diffuse reflectance mode (DR-TAS) to understand the dynamic behaviors of charge carriers and sub-grain-resolved charge carrier kinetics in CZTSSe thin film-based devices. Broadband UV-NIR pulses have been used to monitor the photoexcited dynamics within grains of CZTSSe films under different UV and NIR photoexcitation wavelengths. Pump–probe microscopy showed spatially heterogeneous photoexcited patterns across different grains of CZTSSe films. Image segmentation analysis helped identify the type of photophysical processes at each grain and to determine whether the TAS signal therein consists of only one type of photophysical process or a combination of different transient absorption features including excited-state absorption (ESA), ground-state bleaching (GSB), or ESA/GSB spectral reshaping. UV- and NIR-photo-excitations compared the charge carrier dynamics (trapping and de-trapping or free carriers) under high and low energy (compared to the bandgap) photoexcitation conditions. We observed features of localizing states in both DR-TAM and DR-TAS at even 200 nm above the bandgap implying that the bandgap may be better defined as a mobility gap (similar to amorphous silicon). Our findings present a modern approach and toolbox for the visualization and detailed photophysical analysis of semiconductor materials that are important for energy conversion applications.