Fluorine is the most electronegative element and its small size and highly polar bonding enable it to play a unique and vital role in many materials. In this Research Update, the authors review the potential for fluorine, when incorporated at interfaces, to address fundamental materials challenges to the stability and photophysical properties of halide perovskites, a burgeoning class of photovoltaic absorber materials. While the halide perovskite electronic structure is considered to be defect-tolerant in the bulk, defects at surfaces and grain boundaries are sites of carrier recombination and phase degradation due to their greater reactivity towards moisture and oxygen. Fluoride and fluorocarbons can directly prevent defects by increasing the bonding strength at grain boundaries as well as by acting as a hydrophobic physicochemical barrier. Less direct benefits are also discussed, such as morphological and cell band alignment improvements. Our discussion covers partial incorporation in the perovskite lattice of fluoride as a halide substitution and substitution of the native cations for fluorocarbon cations, as well as the inclusion of molecular fluorocarbons.
