The Emerging Role of Halogen Bonding in Hybrid Perovskite Photovoltaics

The ongoing effort toward stabilizing hybrid perovskite solar cells and enhancing their performance has stimulated the community to pursue a number of strategies. Over the recent years, these efforts have focused on perovskite materials design, which increasingly relies on molecular modulators that engage in halogen bonding, a uniquely directional noncovalent (supramolecular) interaction. Halogen bonding in hybrid perovskites is reported to drive perovskite assembly, increase its stability against moisture and ion migration, passivate defects, and tune interfacial energetics. The resulting perovskites are shown to exhibit superior mechanical properties, and devices incorporating these materials have seen drastic improvements in their performance, all of which have been ascribed to halogen bonding. While most of these developments have so far relied on the incorporation of off-the-shelf molecular modulators that interact with the perovskite surface to effect interfacial properties, further advancements will require careful consideration of the nature of halogen bonding and rigorous structural assessments in order to aid development of next-generation materials. Here, we provide a critical overview of the recent developments in the use of halogen bonding agents in hybrid perovskite photovoltaics with a perspective on their utility in the future.