Kuznetsov, Glazkova, Mikhaylov, Murashkina, Serpone (2019) Advanced diffuse reflectance spectroscopy for studies of photochromic/photoactive solids Journal of physics. Condensed matter : an Institute of Physics journal ()


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 photo-chromic 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 treat-ments 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 Tiδ+ 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 thermo-stimulated 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. © 2019 IOP Publishing Ltd.