Ion Migration in Organic–Inorganic Hybrid Perovskite Solar Cells: Current Understanding and Perspectives

Abstract

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...

Carbon monoxide induced self-doping in methylammonium lead iodide films and associated long-term degradation effects

“…The dark current reaches a minimum value of 1.01 nA at 120 s and then starts to increase in a monotonic fashion and reaches 1.12 nA after 275 s. This increase in dark current is attributed to the rapid degradation of the MAPbI 3 film under CO that leads to enhanced ion migration effects. 55 Under both light and dark conditions, the current increases with external bias (I-V in Fig. S6, ESI †).…”

Recent Criterion on Stability Enhancement of Perovskite Solar Cells

“…However, it was later corroborated by researchers that the hysteresis index of a solar cell somehow originates from its intrinsic properties, rather than the scan rates or direction of the applied bias. Amongst them, the ferroelectric and large dielectric properties of perovskite sensitizers at the nano-scale [34,134], the interfacial ferroelectric characteristics of the device [18], ion migration [35,36], mismatched energy band alignment of the device layers [38,39], slower trapping-detrapping of charges at the grain boundaries or the barriers of different layers [40,135], inefficient charge transportation or extraction by ETLs and HTLs [42,43], imbalance in the charge transport through HTLs and ETLs [44,45], various structural or chemical changes of the involved materials (e.g., perovskite absorber, ETL, and HTL) during operation or in response to various hostile environmental parameters such as temperature and humidity [134,136], undesirable charge trapping at the defect sites of different layers of the device created by such deteriorations [137,138], etc., are attributed to such anomalous PSC device behavior. Some researchers even mentioned it as a light-induced hysteresis in solar cell devices [35,129,139].…”

“…The entire mechanism of ion migration and its effects on device stability have been discussed elsewhere (Section 2.1.2). It is believed that ion migration from the perovskite sensitizer to its surrounding layers is one of the most critical parameters for influencing the anomalous hysteresis of PSC devices [35,298,299]. Therefore, the passivation of the interfaces between the perovskite sensitizer and charge transport layers (CTLs) with an inert passivating material may become an effective strategy for reducing hysteresis from the devices.…”

“…The reason behind such PV behavior is still unknown. However, researchers have attributed several factors to it, such as the large dielectric properties of perovskite sensitizers at the nano-scale [33,34], interfacial ferroelectric characteristics of the device [15], ion-migration [35][36][37], mismatched energy band alignment of the device layers [38,39], the slower trapping-detrapping of charges at the grain boundaries or the barriers of different layers [40,41], inefficient charge transportation or extraction by ETLs and HTLs [42,43], and imbalance in the charge transport through HTLs and ETLs [44,45], etc.…”

[PbX6]4−modulation and organic spacer construction for stable perovskite solar cells

“…Besides, the fragile lattice structure of metal halide perovskites leads to a low activation energy for ion migration, which is further facilitated by ion defects, especially A-and X-site-related defects. [20][21][22][23][24] Consequently, this results in the formation of more point defects. In addition, a large amount of lattice strain exists in polycrystalline perovskite films, which makes them mechanically brittle and prone to undergo phase transformation in response to external stimuli.…”