Understanding the Stability of MAPbBr₃ versus MAPbI₃: Suppression of Methylammonium Migration and Reduction of Halide Migration

Abstract

Solar cells based on metal halide perovskites often show excellent efficiency but poor stability. This degradation of perovskite devices has been associated with the migration of mobile ions. MAPbBr 3 perovskite materials are significantly more stable under ambient conditions than MAPbI 3 perovskite materials. In this work, we use transient ion drift to quantify the key characteristics of ion migration in MAPbBr 3 perovskite solar cells. We then proceed to compare them with those of MAPbI 3 perovskite solar cells. We find that in MAPbBr 3 , bromide migration is the main process at play and that contrary to the case of MAPbI 3 , there is no evidence for methylammonium migration. Quantitatively, we find a reduced activation energy, a reduced diffusion coefficient, and a reduced concentration for halide ions in MAPbBr 3 compared to MAPbI 3 . Understanding this difference in mobile ion migration is a crucial step in understanding the enhanced stability of MAPbBr 3 versus MAPbI 3 .

Mixed Conductivity of Hybrid Halide Perovskites: Emerging Opportunities and Challenges

“…Photothermal-induced resonance microscopy, however, revealed migration of MA + in addition to I − , evidencing that both cations and halides are able to migrate through the perovskite bulk (Yuan et al, 2015). These differences in the obtained characteristics might be the result of variations in sample fabrication, which could also explain why various studies report different results on whether I − or Br − has a higher conductivity in MAPbX 3 McGovern et al, 2020). Nevertheless, it is generally accepted that the halide ions are the fastest mobile ion species.…”

“…Moreover, most models assume only a single mobile ion species, which is not always the case. As a result, there is an ongoing debate about the mobile ion density and its influence on the operation of perovskite-based devices (Moia et al, 2019;Caram et al, 2020;McGovern et al, 2020).…”

Grain Size Influences Activation Energy and Migration Pathways in MAPbBr3 Perovskite Solar Cells

“…We choose MAPbBr 3 , a well-characterized perovskite in terms of ion migration. 19 − 21 The major advantage of using this perovskite material is the possibility to synthesize films of varying grain size, without altering key physical or chemical properties of the film. To characterize and quantify ion migration, we use transient ion drift (TID), a capacitance-based technique which allows for determination of the nature of the mobile ions and quantification of their migration activation energy, diffusion coefficient, and number density.…”

Photocorrosion at Irradiated Perovskite/Electrolyte Interfaces

“… 64 The anion in the perovskite lattice affects the activation energy of ion migration, because of the difference in the Pb–X bond strength and the vacancy density (which are the active participants in ion migration). 65 For example, halide ion migration is more pronounced in MAPbI 3 compared to MAPbBr 3 . Interestingly, the migration of MA + is also faster as a result of the expanded lattice of MAPbI 3 .…”

“…Interestingly, the migration of MA + is also faster as a result of the expanded lattice of MAPbI 3 . 65 While these mobile ions in the “soft” perovskite lattice are the main reasons behind the instability, they also allow the synthesis of perovskites with defect-free bulk structure using solution phase processes. 9 Furthermore, these mobile ions are found to be the key reason for the self-healing properties of perovskites.…”