UC Berkeley

Metal halide perovskite is a new generation of semiconducting materials that providing promise for the development of efficient optoelectronic devices such as solar cells, light-emitting diodes (LEDs), and lasers due to its outstanding optical and electronic properties. In particular, all-inorganic cesium lead iodide (CsPbI3) presents great promise for photovoltaic applications due to its suitable bandgap for single and multi-junction solar cells, and its enhanced thermal stability compared to organic-based metal halide perovskite. However, exposure to environmental stimuli such as ambient moisture tends to structurally transform CsPbI3 perovskite (high-T) into a nonperovskite (low-T) phase that has a larger bandgap. Thorough understanding of the influence of the environmental stimuli is key in predicting solar cell stability and in designing more stable devices. By directly monitoring the high-T to low-T CsPbI3 phase transformation under controlled relative humidity, I am able to quantify the dependence of phase transformation processes on relative humidity, extract the associated nucleation barriers, and uncover the rate-limiting process. Furthermore, I find heating under general solar cell operating temperature to be a potential method of mitigating moisture-induced phase transformation. In addition, I demonstrate that illumination using above band-gap continuous-wave laser transforms high-T phase CsPbI3 to low-T phase in ambient conditions. In the presence of moisture, laser induces structural phase transformation at rates order of magnitude much faster than moisture-induced phase transformation. In the absence of moisture, laser does not trigger phase transformation, but introduces long-lasting defects that lower the photoluminescence emission and accelerate phase transformation upon exposure to moisture. Finally, I demonstrate direct visualization of phase growth in individual single-crystals utilizing a continuous-wave laser via photoluminescence imaging, as well as in situ heating in cathodoluminescence microscopy coupled with scanning electron microscopy. I find that initial growth direction and shape show correlation to the interface migration speed. These studies all contribute to the fundamental understanding of phase transformation energetics of CsPbI3, which can serve as references for device applications of CsPbI3 and for future designs of stable photovoltaics systems.