UCLA

Metal-halide perovskite solar cells have emerged as promising photovoltaic technologies because of their high absorption coefficients, long carrier diffusion lengths, defect tolerance, and compatibility with low-temperature processing. Despite rapid improvements in power conversion efficiency, their commercialization remains limited by insufficient operational stability. Many degradation pathways originate from the nonequilibrium crystallization of solution-processed perovskite films, which can introduce unfavorable crystal orientations, residual phases, compositional heterogeneity, lattice disorder, and defects that promote nonradiative recombination and ion migration. This dissertation investigates how molecular interactions can be used to control perovskite crystallization across different stages of film formation, from facet-selective crystal growth to post-nucleation lattice reconstruction, thereby improving both photovoltaic performance and operational stability. Chapter One introduces the fundamental properties, crystallization behavior, and stability challenges of metal-halide perovskites. Particular emphasis is placed on the multistep and nonequilibrium nature of solution-based crystallization and on how precursor coordination, intermediate-phase formation, nucleation, crystal growth, and molecular additives determine the final film structure. In Chapter Two, a highly (111)-oriented FAPbI₃ film is developed using 3-aminopentane and propylammonium chloride as co-additives. Their cooperative effects regulate facet-selective growth and phase conversion, producing highly crystalline films with improved carrier transport and suppressed ion migration. The resulting devices exhibit enhanced photovoltaic performance and substantially improved stability under continuous illumination and elevated temperature. These results demonstrate that crystallographic orientation is a key design parameter linking film formation with electronic quality and long-term device reliability. Chapter Three establishes a molecular framework for understanding amine-directed facet growth. Systematic screening of structurally diverse amines reveals that facet preference cannot be predicted solely from their classification as primary, secondary, or tertiary amines. Instead, it is governed by facet-specific adsorption configurations and cooperative interactions. In addition to N–Pb coordination, secondary N–H···I hydrogen bonding can stabilize molecular anchoring on the (100) surface. Strong, persistent adsorption blocks the incorporation of new growth units and redirects crystallization toward the (111) orientation, whereas weaker or less uniform adsorption allows continued (100)-oriented growth. In Chapter Four, crystallization control is extended beyond early-stage nucleation through a thermally activated re-alloying pathway. The 3-APCl additive becomes active only after formation of the initial three-dimensional lattice, inducing a transient secondary intermediate and post-nucleation lattice reconstruction during annealing. This top-down reorganization heals persistent shallow defects, improves structural and optoelectronic homogeneity, suppresses ion migration, and enhances device efficiency and thermal stability. Together, these studies demonstrate that molecular structure, adsorption geometry, and activation timing can be rationally designed to control distinct stages of perovskite crystallization. The resulting framework connects molecular-scale interactions with facet selection, lattice reconstruction, defect evolution, and device stability, providing new strategies for the development of efficient and durable perovskite solar cells.