Tin Halide Perovskites Going Forward: Frost Diagrams Offer Hints

Abstract

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.

Crystallization Dynamics of Sn‐Based Perovskite Thin Films: Toward Efficient and Stable Photovoltaic Devices

“…Due to the higher orbital energy of 5p orbital than 6p orbital, it makes outermost 5p orbital of Sn 2+ show stronger electrophilic ability than 6p of Pb 2+ , leading to the strong Lewis acidity of Sn 2+ . [26] This strong Lewis acidity causes the fast reaction between Sn 2+ in Sn halides and Iin organic halides (e. g., MAI and FAI) to rapidly form perovskite crystal, resulting in the fast nucleation rate of the crystal during solution crystallization process. For example, MASnI 3 perovskite is shown to crystallize much more rapidly than MAPbI 3 , which is proved by the rapid formation of black-phase MASnI 3 perovskite film right after the anti-solvent dripping, whereas the crystallization of MAPbI 3 is much slower and is only complete after thermal treatment.…”

“…[21][22][23][24][25] For Sn-based perovskite, the uncontrollable crystallization process mainly originates from the unique property of Sn 2+ , including high Lewis acidity and easy oxidation of Sn 2+ . [26,27] On the one hand, due to the higher energy of 5p orbital compared to 6p orbital, the Lewis acidity of Sn 2+ is higher than that of Pb 2+ , resulting in the fast reaction rate of SnI 2 with MAI or FAI. On the other hand, because the two electrons on 5s orbital of Sn 2+ are active and easy to lose, giving rise to the easy oxidation of Sn 2+ , the formation of Sn vacancy Tin-based perovskites show great potential in photovoltaic applications, and the development of the corresponding solar cells (PSCs) has made exciting progress during the past few years.…”

Optoelectronic Properties of Low‐Bandgap Halide Perovskites for Solar Cell Applications

“…[67] A more detailed account of various stability enhancement strategies of Sn-based perovskite solar cells can be found elsewhere. [35,98,99]…”

Enhancing Performance and Stability of Tin Halide Perovskite Light Emitting Diodes via Coordination Engineering of Lewis Acid–Base Adducts

“…[15][16][17][18][19] However, because Sn lacks lanthanide shrinkage, Sn 2+ ions are easily oxidized to Sn 4+ ions to cause detrimental Sn vacancies and impurities in the perovskite films. [20][21][22] Thus, regardless of the stringency in fabrication environment controls, an inevitable oxidation of the precursors and films result in much inferior device performance of Sn perovskite-based PeLEDs. More seriously, recent reports unraveled that dimethyl sulfoxide (DMSO), ubiquitously used Lewis base for fabrication of high-quality Pb-based perovskite thin films via adduct approach, oxidizes the Sn 2+ in the perovskite precursor solution.…”