Solution Chemistry Affects Processing and Properties of Perovskite Active Layers for Solar Energy Conversion

Abstract

Hybrid organic-inorganic perovskites (HOIPs) have recently emerged as a promising class of photoactive materials for use as active layers in efficacious photovoltaic (PV) devices. Yet, numerous obstacles must be overcome before PV devices comprising HOIP active layers are ready to be integrated into the renewable energy market. In this thesis, we systematically studied interactions and reactions between HOIP precursors and processing solvents/additives to elucidate how structuring in solution affects crystallization kinetics, morphology, composition, structure, and optoelectronic properties of the solid-state HOIP active layer. We demonstrate that the strength of interactions between processing solvent and the lead halide HOIP precursor impacts coordination in solution and the crystallization kinetics of HOIP materials. We identified a correlation between the Lewis basicity of the processing solvent, quantified by Gutmann’s donor number (DN), and the strength of interactions between common oxygen-donor solvents and lead halide precursor. Subsequently, we extended this analysis to include sulfur-donor solvents, demonstrating that hard and soft acid base theory is a reliable predictor of coordination strength between Pb2+ and solvents and/or additives in HOIP precursor solutions. Our studies provide a comprehensive design rule for solvent selection for HOIP processing and identifies acid-base interactions as a primary driver of structuring in the precursor solution. Separately, we demonstrated that reactions between processing solvents/additives and organoammonium halide precursors results in the formation of organic products, which when incorporated in the solid state, act as substitutional defects in HOIP thin films. The incorporation of these products in HOIP films directly influences the composition and crystal structure of the films and alters the optoelectronic properties of HOIP active layers for device applications. This study represents a vital first step in understanding the origins of compositional drifts, structural heterogeneity, and electronic defects in HOIP active layers, and will guide future work to identify precursor formulations that eliminate such deleterious reactions. Collectively, this thesis underscores the importance of considering the interactions and reactions between perovskite precursors in solution and their effects on solid-state structure and properties and serves as a fundamental undergirding for improved understanding of processing-structure-function relationships of HOIP active layers for optoelectronic and PV applications.