Photo-effect on ion transport in mixed cation and halide perovskites and implications for photo de-mixing

TL;DR

This study investigates how light influences ionic and electronic charge transport in mixed cation and halide perovskites, revealing that halide choice critically affects photo-ionic effects and photo-de-mixing, with implications for stability and degradation.

Contribution

It provides a quantitative analysis of ionic and electronic transport in various mixed perovskites under illumination, highlighting the halide's role in photo-ionic effects and demixing.

Findings

Iodide perovskites show significant ionic conductivity enhancement under light.

Bromide substitution reduces the photo-ionic effect, making it negligible in pure bromide perovskites.

The results explain the mechanisms behind light-induced demixing in mixed perovskites.

Abstract

Organic-inorganic hybrid perovskites are considered to be most promising photovoltaic materials. Highest efficiencies of perovskite solar cells have been achieved by using appropriate cation and anion mixtures. Mixed perovskite solar cells also show an improved stability. For both performance as well as stability, experimental information on electronic and ionic charge carriers is key, an information that so far has only been provided for methylammonium lead iodide; there we also found that light can enhance not only electronic but also ionic conductivities by more than one order of magnitude. We also proposed a mechanism for this surprising photo-ionic effect and explained its impact on photo-decomposition. Here we quantitatively deconvolute ionic and electronic transport properties for the practically relevant substitutions and mixtures. Specifically, we investigate various cation and anion substitutions (Cs; FA; Br) with a special eye on their photo-ionic effect. The results are not only of importance for light-induced degradation but also for light-induced demixing. As far as the photo-ionic effect is concerned, we find that the choice of the halide is of crucial importance, while the cationic substitutions are less relevant. The huge ionic conductivity enhancement found for iodide perovskites, is weakened by bromide substitution and eventually becomes insignificant for the pure bromide. Based on these experimental results, we provide a rationale for the experimentally observed photo-demixing.