We quantify activation energy, concentration, and diffusion coefficient of mobile ions in MAPbI₃ perovskite solar cells using transient ion-drift measurements.

Solar cells based on metal halide perovskites often show excellent efficiency but poor stability. This degradation of perovskites devices has been associated with the migration of mobile ions. MAPbBr₃ perovskite materials are significantly more stable under ambient conditions than MAPbI₃. In this work we use Transient Ion Drift (TID) to quantify the key characteristics of ion migration in MAPbBr₃ perovskite solar cells. We then proceed to compare them with those of MAPbI₃. We find that in MAPbBr₃, bromide migration is the main process at play, and that contrary to the case of MAPbI₃, there is no evidence for methylammonium migration. Quantitatively, we find a reduced activation energy, reduced diffusion coefficient, and reduced concentration for halide ions in MAPbBr₃ compared to MAPbI₃. Understanding this difference in the mobile ion migration is a crucial step in understanding the enhanced stability of MAPbBr₃ versus MAPbI₃.

Halide double perovskites, A₂M^IM^IIIX₆, offer a vast chemical space for obtaining unexplored materials with exciting properties for a wide range of applications. The photovoltaic performance of halide double perovskites has been limited due to the large and/or indirect bandgap of the presently known materials. However, their applications extend beyond outdoor photovoltaics, as halide double perovskites exhibit properties suitable for memory devices, indoor photovoltaics, X-ray detectors, light-emitting diodes, temperature and humidity sensors, photocatalysts, and many more. This Perspective highlights challenges associated with the synthesis and characterization of halide double perovskites and offers an outlook on the potential use of some of the properties exhibited by this so far underexplored class of materials.

The bandgap tunability of mixed-halide perovskites makes them promising candidates for light-emitting diodes and tandem solar cells. However, illuminating mixed-halide perovskites results in the formation of segregated phases enriched in a single halide. This segregation occurs through ion migration, which is also observed in single-halide compositions, and whose control is thus essential to enhance the lifetime and stability. Using pressure-dependent transient absorption spectroscopy, we find that the formation rates of both iodide- and bromide-rich phases in MAPb(Br_xI_1–x)₃ reduce by 2 orders of magnitude on increasing the pressure to 0.3 GPa. We explain this reduction from a compression-induced increase of the activation energy for halide migration, which is supported by first-principle calculations. A similar mechanism occurs when the unit cell volume is reduced by incorporating a smaller cation. These findings reveal that stability with respect to halide segregation can be achieved either physically through compressive stress or chemically through compositional engineering.

Ion migration in perovskite layers can significantly reduce the long-term stability of the devices. While perovskite composition engineering has proven an interesting tool to mitigate ion migration, many optoelectronic devices require a specific bandgap and thus require a specific perovskite composition. Here, we look at the effect of grain size to mitigate ion migration. We find that in MAPbBr₃ solar cells prepared with grain sizes varying from 2 to 11 μm the activation energy for bromide ion migration increases from 0.17 to 0.28 eV. Moreover, we observe the appearance of a second bromide ion migration pathway for the devices with largest grain size, which we attribute to ion migration mediated by the bulk of the perovskite, as opposed to ion migration mediated by the grain boundaries. Together, these results suggest the beneficial nature of grain engineering for reduction of ion migration in perovskite solar cells.