Ultrafast microscopy and image segmentation of spatially heterogeneous excited state and trap passivation in Cu₂BaSnSSe₃

Earth-abundant Cu2BaSnS4-xSex (CBTSSe) represents a recent alternative for Cu2ZnSn(S,Se)4 for solar energy conversion with a lower level of disorder and band tailing. We report the heterogeneous excited-state and trap-state pattern in different solution-processed CBTSSe films using ultrafast two-color pump-probe diffuse reflectance microscopic imaging. The spectroscopy/microscopy method can visualize and correlate the microscopic compositional and electronic variations (i.e., trap states) in real space with time-resolved photophysics. Heterogeneity patterns in TAM images show that some grains exhibit a positive excited-state absorption (ESA) signal, while others show negative ground-state bleaching (GSB). Our results visualize that film processing, such as air annealing and Na addition, has a clear influence on the heterogeneity of the excited state pattern. Importantly, we report stable charge carrier over 100 ps. We applied the image principal component and histogram for quantitative analysis of TAM images to deconvolute and visualize the contribution and fingerprints of minority free carriers and sub-band-gap trapped carriers.