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 31(42) 424001


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.

本文报道了通过使用低温恒温器附件用于UV-Vis-NIR分光光度计扩展的漫反射(DR)光谱的新机会,以研究在T = 90-600K时半导体的基本吸收边缘处的DR光谱的温度依赖性。研究了金红石TiO2,一种在可见光区域具有强吸收的光致变色金红石TiO2,以及卤化双钙钛矿Cs2AgBiBr6,在低温DR光谱中表现出两种光学带 - 带转变。还报道了在各种捕获位点分离光生电荷载体的DR光谱和动力学测量,它们的热刺激去除和它们在两种不同的光致变色材料中的重组。在UV和Vis区域中诱导的吸收和程序升温退火(TPA)光谱之间的相似性产生了在黄色金红石TiO2中的本征缺陷(F型中心)的可见光激发下电荷载体的光致形成的物理证据。样品的高温氧化/还原处理以及光谱和动力学测量用附件原位进行。结果导致将黄色TiO 2中的色心分配为Ti 3+中心作为深电子陷阱,并且建立了几种类型的基于Ti 3+的色心,其包括带负电荷的Tiδ+中心(3>δ> 2)。在T = 200-400K的光致吸收和光诱导吸收退火的区域中,在Bi掺杂的钙钛矿CsPbBr3中也阐明了光致变色的发生。这些现象用电荷载体的光生成然后它们的捕获来描述,这产生了与Bi相关的电子色心,其负责光诱导的吸收和热刺激的光孔的去除,最终与被捕获的电子重新结合。在钙钛矿中建立光致变色可以导致对卤化钙钛矿和卤化物钙钛矿基太阳能电池中光致和暗可逆现象的进一步理解。 ©2019 IOP Publishing Ltd.