Halide perovskite materials have attracted considerable attentions owing to their high light absorption coefficient, long carrier diffusion length, remarkable optoelectronic properties and high efficiency in solar cells. However, due to the inevitable moisture instability for organic-inorganic hybrid perovskites, all inorganic perovskites with inherent stability have become research hotspots as promising candidates for commercial perovskite solar cells and advanced optoelectronics. In this thesis, all inorganic lead-free perovskites (Cs2SnIxCl6-x) have been synthesized by a solution-based method and the mechanisms governing their stability and degradation behavior are investigated. We were able to tune the optical properties and stability of the Cs2SnIxCl6-x all inorganic perovskites through controlling the incorporation ratios of iodide and chloride. The optical absorption can be systematically tuned into a wide range from UV to infrared. Meanwhile, the mixed halide perovskite with low chloride contents can significantly enhance the optical property with much higher PL intensity. The synthesized all inorganic mixed halide perovskites show greatly improved environmental stability without degradation in ambient conditions. The incorporation of Cl further enhances the phase stability of the binary system Cs2SnIxCl6-x with increased phase decomposition temperature. However, these materials can still decompose upon exposure to high humidity or aqueous condition and show different degradation behaviors as controlled by different compositions of the hybrid all inorganic perovskites (Cs2SnIxCl6-x). Several in-situ techniques have been adopted to monitor the interaction between perovskites (Cs2SnIxCl6-x) and water molecules to understand the mechanisms governing the environmental stability. The dissolution-precipitation processes of Cs2SnX6 perovskites (Cs2SnI6, Cs2SnCl6, and Cs2SnI0.9Cl5.1) have been investigated by direct exposure to water via in-situ measurements. The isostructural Cs2SnI6 and Cs2SnCl6 display different dissolution behaviors. Cs2SnI6 experiences direct dissolution. Through the application of in-situ synchrotron XRD and Raman spectroscopy, we determine Cs2SnI6 decomposes into CsI and SnI4 in water and a hydrolysis product Sn(OH)4 can be identified in the solution. A partially reversible reaction also occur with the formation of Cs2SnI6 with the dehydration process Cs2SnCl6 displays an enhanced stability and its crystallinity can be well maintained in water when exposure at the same condition as Cs2SnI6, except partial hydrolysis leading to amorphous precipitation during the dehydration process. As to the mixed halide lead free perovskite (Cs2SnI0.9Cl5.1), it shows two different dissolution stages while exposure to water. The iodide in the crystal structure dissolves fast with water addition and transforms to a more chloride-rich phase. The new phase displays a slower iodide dissolution rate and remains crystalline in water similar to Cs2SnCl6. Therefore, these results can help elucidate the fundamental decomposition pathways in Cs2SnIxCl6-x perovskite and understand their mechanisms of the dissolution-precipitation process, which is expected to in turn lead to new material preparation strategies and device optimization methods.
To further improve the quality of the all-inorganic perovskite and enhance their functionality for optoelectronic applications, we develop a new method to synthesize single crystalline mm-sized Cs2SnI6 perovskite at a liquid-liquid interface. By controlling solvent conditions and Cs2SnI6 supersaturation at the liquid-liquid interface, Cs2SnI6 crystals can be obtained from three-dimensional (3D) to two-dimensional growth (2D) with controlled geometries such as octahedron, pyramid, hexagon and triangular nanosheets. The formation mechanisms and kinetics of complex shapes/geometries of high quality Cs2SnI6 crystals were investigated. Free-standing single crystalline 2D nanosheets can be fabricated as thin as 25 nm, and the lateral size can be controlled up to sub-mm-regime. Electronic property of the high quality Cs2SnI6 2D-nanosheets was also characterized, featuring a n-type conduction with a high carrier mobility of 35 cm2V-1s-1.
Currently, the performance of Sn4+ based perovskite optoelectronics is still low. The lower performance can be attributed to nonoptimized device architecture and film preparation methods to obtain high quality crystals with reduced defect density (e.g. Sn vacancy defects) within the structure. A solution-based metal ion doping strategy is developed to incorporate Nd3+ into Cs2SnCl6 perovskite. By inducing a sub-band absorption, the Nd3+-doped Cs2SnCl6 perovskite exhibits strong photoluminescence intensity. By fabricating UV photodetectors with a design of interfacial charge-controlled hole-injection structure, high performance UV photodetectors could be achieved with a maximum detectivity of 6.3×1015 jones at 372 nm, fast photo-response speed with rise time and fall time on the order of milliseconds, and a large linear dynamic range of 118 dB.
In summary, based on a systematic study of the Cs2SnI6- Cs2SnCl6 binary system, new synthesis sciences have been achieved with the controlled chemical compositions and tunable bandgaps and optoelectronic properties. High quality and large-sized single crystal perovskites are demonstrated and the kinetics of crystal growth are investigated. The degradation mechanisms of the all-organic perovskites exposed to high moisture and aqueous environments are elucidated by various in-situ and ex-situ materials characterization, and their environmental stability is correlated with chemical compositions and key structure characteristics. To further improve their properties and functionalities, a new doping strategy is demonstrated to effectively increase the photoluminescence and device performance used as an UV-detector. The fundamental understanding of growth kinetics and degradation mechanisms and the combination of the new synthesis science will open up opportunities to design new types of perovskite materials with enhanced environmental stability, dimensionality and film quality, enabling their applications with improved functionalities and device performance.;
December 2020; School of Engineering
Dept. of Mechanical, Aerospace, and Nuclear Engineering;
Rensselaer Polytechnic Institute, Troy, NY
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