shown that ion migration is an inherent property of perovskite. [8][9][10][11] When discussing perovskites, due to their soft lattice feature, weak chemical bonds and low defect formation energy are generally considered. For the ABX 3 perovskites, A-site ions (methylamium ions (MA + ) and formamidium ions (FA + )), B-site ions (lead ions (Pb 2+ ) and tin ions (Sn 2+ )), and X-site ions (iodide ions (I − ), bromide ions (Br − ), chloride ions (Cl − ), and other halogen ions) have low activation barrier and high diffusion coefficient. [3] These ionic defects within the lattice are easily activated to cause severe ion migration in the perovskite films under external factors. In fact, in addition to the internal ions of perovskite, ions introduced from the ambient atmosphere also have the potential to migrate. [12] To detect the effects of ion migration, abnormal phenomenon in perovskite were given attention. Snaith et al. first reported the abnormal photocurrent-voltage hysteresis phenomenon, which was that forward and reverse scan J-V curves can't overlap in PSCs. [13] Electric field-driven ion migration was considered to be the key factor to affect photocurrent hysteresis. Xiao et al. [14] reported the switchable photovoltaic effect in a plane heterojunction structure with symmetric electrodes, in which the current direction could be completely overturned. Ion migration was speculated to be the main cause. The reason for this inference is that the migration as well as the accumulation of ions can change the intrinsic electric field of the perovskite films, even causing local crystal structure changes, which in turn lead to further degradation of the PSCs and severely affect the operating stability. [15,16] Apart from these, ion migration can also cause slow photoconductivity response, halide redistribution, and segregation. [17] In order to push the commercialization step of PSCs, a clear understanding of the ion migration in OIHPs is meaningful and highly desired. Although some reports have reviewed the ion migration in OIHPs, [18][19][20] the explanation of the mechanisms behind ion migration is still incomplete and is usually introduced directly from theoretical calculations. Meanwhile, more reviews only focused on the ion migration in PSCs; ion migration in fact has profound implications for many other applications of perovskite materials, like photodetector, light emitting diodes, and random access memories. The use of low-dimensional materials to inhibit ion migration is an effective strategy, but the various species involved are not discussed in detail. What's more, the study of ion migration is quickly Organic-inorganic hybrid perovskite (OIHPs) solar cells are the most promising alternatives to traditional silicon solar cells, with a certified power conversion efficiency beyond 25%. However, the poor stability of OHIPs is one of the thorniest obstacles that hinder its commercial development. Among all the factors affecting stability, ion migration is prominent because it is unavoidable and intrinsic in OH...

Exploring stable and efficient lead‐free perovskite solar cells (PSCs) is critical to solving the environmental concerns caused by lead. Recently, tin halide perovskites (THPs) have become a promising candidate due to its low toxicity and similar electronic configuration to lead counterparts. Currently, the power conversion efficiency of tin‐based PSCs (TPSCs) has been pushed over 14%. However, there is still a considerable gap compared to lead PSCs due to the non‐negligible open‐circuit voltage loss (Vloss). Therefore, understanding the origins and regulation strategies of Vloss for TPSCs is of great importance. Herein, the nature of THPs is first reviewed from the crystal structure, electronic structure, and phase transition. Subsequently, the origins and determinants of Vloss are discussed in TPSCs. Besides the intrinsic low bandgap, the bulk recombination of tin perovskite, and the non‐radiative recombination of the associated interfaces induce the Vloss of the TPSCs devices. Then, some recently emerged strategies to suppress the Vloss in TPSCs are introduced. Finally, a perspective on the further suppression of Vloss in TPSCs including purifying the precursor solution, suppressing the oxidation Sn2+, and optimizing the device structure is outlined.

Tin halide lead‐free perovskite solar cells (TPSCs) have received tremendous research interest recently due to their nearly ideal bandgap, broad light absorption, non‐toxicity, and environmental friendliness. However, the uncontrollable crystallization process and the facile oxidation of Sn2+ limit the further increase of power conversion efficiency (PCE). To solve these problems, a series of acetates are introduced into the perovskite precursor solution to regulate the crystallization process. It is revealed that formamidine acetate (FAAc) has strong COSn coordination with Sn2+ compared with acetic acid (HAc) and methylammonium acetate (MAAc), which can stabilize the lattice structure, minimize defect states and suppress the oxidation of Sn2+. Meanwhile, benefiting from this coordination ability, it not only leads to large‐size colloidal clusters in precursor but also slows down the crystallization process and improves the crystallinity of tin halide perovskite films. The device with FAAc achieved an increased PCE from initially 9.84% to 12.43%, and it could maintain 94% of its initial value for 2000 h in N2 atmosphere. This work provides a feasible strategy for depositing high‐quality tin perovskite films with low defect density and lattice distortion, which will be crucial for related photovoltaics and other optoelectronic devices.

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