With rapid progress in the deployment of metal halide perovskites in various device applications such as solar cells, light-emitting devices, field-effect transistors, photodetectors, etc., the next eminent focus is on the single crystals of these materials. With a lack of grain boundaries and low trap densities, remarkably long charge carrier diffusion lengths, and high ambient and operational stabilities, this class of materials seems greatly promising. Yet, the growing concern for lead toxicity in commercial semiconductor devices has entailed a thrust in the research of alternative lead-free perovskites, including their single crystalline forms. However, there is still no consolidated account of the state-of-the-art in this domain and accordingly, countless feasible systems still remain unexplored. To bridge this gap, we attempt to provide here, an up-to-date overview of lead-free perovskite single crystals with respect to their synthesis methods, structural diversity, stability, photophysical and electrical properties, and device applications. We discuss various approaches to designing, modeling, fabricating, and characterizing new single-crystal systems and conclude with some critical insights for further investigating this field of research.

Lead (Pb) in conventional perovskite solar cells (PSCs) is toxic and has to be replaced. Situated in one group of the periodic table of elements, tin (Sn) has the same valence electrons' configuration as Pb (ns 2 np 2 ), promising analogous chemical properties. Hence, Sn is considered a suitable replacement to Pb. However, because of the lack of lanthanide shrinkage, Sn behaves differently: Pb is stable in Pb 2+ form, an oxidation state needed for perovskite structure, while Sn tends to lose all its valence electrons forming Sn 4+ . As a result, PSCs based on Sn are not efficient. Traces of oxygen have been conventionally discussed as a source of Sn oxidation. But recent findings point to the oxidation of Sn-based perovskites even in the absence of oxygen. This perspective summarizes recentlydiscovered unconventional oxidation pathways of Sn perovskites, including reaction with solvent molecules and disproportionation. We explain these phenomena by a Frost−Ebsworth diagram and argue that a deeper understanding of this diagram is a key toward stable and efficient Pb-free Sn-based PSCs.

To feed a rapidly growing population of 250 million, the Indus river basin in South Asia is one of the most intensively cultivated regions on Earth, highly water stressed and lacking energy security. Yet, most studies advising sustainable development policy have lacked multi-sectoral and cross-country perspectives. Here we show how the Indus countries could lower costs for development and reduce soil pollution and water stress, by cooperating on water resources and electricity and food production. According to this analysis, Indus basin countries need to ramp up investments to 10 billion USD/year to mitigate water scarcity issues and ensure improved access to resources by 2050. These costs could shrink to 2 billion USD/year, with economic gains for all, if countries pursued more collaborative policies. Downstream regions would benefit the most, with reduced food and energy costs and better water access, while upstream regions would benefit from new energy investments. Using integrated water-energy-land analysis, this study quantifies the potential benefits for novel avenues to sustainable development arising from greater international cooperation.