Mixed halide hybrid perovskites: a paradigm shift in photovoltaics

Perovskite solar cells (PSCs) have improved at an unprecedented rate, reaching over 22% power conversion efficiency in less than a decade, and have generated a wealth of high impact work in the literature studying the properties of these unique materials. While the highest performance and best progress have been observed with methylammonium lead triiodide perovskites, their low tolerance to moisture and rapid degradation when exposed to UV light and high temperatures have posed a major setback to their commercialization. Recently, mixed halide hybrid perovskites (MHHPs) where l(-) is substituted with Br- and Cl- have driven a paradigm shift in PSCs, synergistically combining high performance with high ambient stability. In this comprehensive review, we summarize the progress made in the past 5 years in methylammonium-based organic-inorganic MHHPs and cesium-based all inorganic MHHPs. We discuss the fabrication approaches for halide substitution and the resulting crystal structure, long-term stability and durability, and device performance. Unique from other reviews, we focus this review on the optical properties of MHHPs, including light-induced phase segregation and alterations in the optical band gap and photoluminescence with halide substitution. We also summarize trends in carrier dynamics, including carrier thermalization, recombination, and charge transfer probed with time-resolved optical spectroscopies. Furthermore, we provide a short overview on mixed cation hybrid perovskites, where methylammonium is substituted with formamidinium/cesium or Pb is substituted with less toxic and benign metals to improve both the stability and the environmental impact of PSCs. We discuss the optical characteristics of mixed cation perovskites and approaches to lower nonradiative recombination with cation substitution. We conclude this review with a discussion on innovative future directions that will propel research on MHHPs as a game changer in perovskite-based optoelectronics.