Advanced diffuse reflectance spectroscopy for studies of photochromic/photoactive solids

The present article reports novel opportunities of diffuse reflectance (DR) spectroscopy extended through the use of a cryostat accessory for UV–vis–NIR spectrophotometers to investigate temperature dependences of DR spectra at the fundamental absorption edge of semiconductors at T  =  90–600 K. Examined are rutile TiO₂, a photochromic rutile TiO₂ with strong absorption in the visible region, and the halide double perovskite Cs₂AgBiBr₆ that exhibited two optical band-to-band transitions in low-temperature DR spectra. Also reported are DR spectral and kinetics measurements of the separation of photogenerated charge carriers in various trap sites, their thermostimulated detrapping and their recombination in two different photochromic materials. The similarity between absorption and temperature-programmed annealing (TPA) spectra induced in the UV and Vis regions yielded physical evidence of the photo-formation of charge carriers upon Vis-light excitation of intrinsic defects (F-type centers) in yellow rutile TiO₂. High-temperature oxidative/reductive treatments of samples, together with spectral and kinetics measurements were performed in situ with the accessory. Results led to assigning color centers in yellow TiO₂ to Ti³⁺ centers as deep electron traps, and to the establishment of several types of Ti³⁺-based color centers that include extra-negatively charged Ti^δ+ centers (3  >  δ  >  2). Photochromic occurrences are also elucidated in the Bi-doped perovskite CsPbBr₃ under illumination in the region of intrinsic absorption and annealing of photoinduced absorption at T  =  200–400 K. These phenomena are described in terms of the photogeneration of charge carriers followed by their trapping, which yielded Bi-related electron color centers responsible for the photoinduced absorption and for the thermostimulated detrapping of photoholes that ultimately recombine with the trapped electrons. The establishment of photochromism in the perovskites may lead to a further understanding of photoinduced and dark reversible phenomena in halide perovskites and halide perovskite-based solar cells. (paper)