Dynamic Crystallization and Stability Engineering in Hybrid Organic-Inorganic Perovskites: Towards Water-Stable and Thermochromic Materials

Abstract

Dynamic Crystallization and Stability Engineering in Hybrid Organic-Inorganic Perovskites: Towards Water-Stable and Thermochromic Materials Jiyoon Kim ABSTRACT Hybrid organic-inorganic perovskites (HOIPs) have emerged as promising semiconductors for optoelectronic applications. But their intrinsic instability limits practical applications. This dissertation explores dynamic crystallization as a design principle to enhance stability in halide perovskite materials through three studies. In Chapter 2, we developed a pH-controlled synthetic strategy for water-stable perovskitoid semiconductors using the zwitterionic molecule cysteamine (CYS). By varying pH conditions (acidic, neutral, and basic), we tuned the coordination environment to make Pb-S and Pb-N covalent bonds. Crystals synthesized under basic conditions maintainedtheir structure for over one month in direct contact with water. In contrast, crystals synthesized under acidic condition decomposed within one day. This approach represents a new synthetic method for making ionic perovskite structures. In Chapter 3, we investigated composition and passivation effects on thermochromic MAPbI3 (MA = methyl ammonium) thin films. Systematic variation of MAI:PbI2 ratios revealed that excess MAI is critical for complete reversible transitions between colored and colorless states. Chlorine incorporation from MACl accelerated structure transition kinetics by approximately 50%, with transition times of 52 seconds to 27 seconds. However, there was a trade-off between transition speed and long-term durability with chlorine incorporation. Passivation with 2-amino-4-methylpyridine improved thin film stability but extended transition times. This bifunctional additive coordinates with undercoordinated Pb2+ and Sn2+ sites at surfaces and grain boundaries. In Chapter 4, we developed color-neutral thermochromic perovskites through Pb-Sn compositional engineering and multi-layer encapsulation. Mixed Pb:Sn = 1:1 ratio produced neutral gray coloration by broadening absorption across the visible spectrum. We discovered that potassium halide additives (10% KF + 10% KI) showed better stability in Sn-mixed perovskites than with sodium halide additives. A tri-layer encapsulation architecture comprising hygroscopic polymers, the perovskite layer, and hydrophobic PMMA enabled reversible thermochromic cycling through moisture in the encapsulated system. Optimized films maintained stable optical property through 10 cycles. These works establish the methods to overcome intrinsic instability of halide perovskites into stable semiconductors for optical materials.