Role of fluoride and fluorocarbons in enhanced stability and performance of halide perovskites for photovoltaics

Abstract

Halide perovskites are promising absorber materials for photovoltaics due to their high efficiency and low-temperature solution processing, though the instabilities inherent to these systems remain a primary obstacle to their utilization. In this research update, we review the chemical, structural, and functional effects of fluoride and fluorinated organic additives in halide perovskites, especially on stability and photovoltaic performance. We detail the evidence that, when incorporated as its anion, fluoride is typically localized to perovskite surfaces and grain boundaries. Due to its high electronegativity, fluoride and fluorinated species can impart strong ionic and intermolecular bonding with perovskite materials, passivating surfaces and diminishing their susceptibility to further chemical reactivity. Additionally, fluorocarbon-based additives impart hydrophobicity to perovskite films and serve as physiochemical barriers to reactive infiltrating species. To understand how these effects operate in different contexts, herein we summarize the dual role surface defects play in both diminishing photovoltaic efficiency and introducing structural instabilities in various ABX3 perovskite halide materials: hybrid organic/inorganic lead halide, all-inorganic cesium lead halide, and tin halide perovskites. We review three avenues of fluorine inclusion in perovskites: fluoride substitution at the anion site, partial incorporation of fluorinated organic cations at the A site, and large- and small-molecule fluorocarbon-based additives. We conclude with a perspective on the prospects for fluorine-based enhancement of halide perovskites and suggestions for new analytical and synthetic chemistries that will deepen our understanding and control of these systems.