With the tunability of perovskites through compositional alloying being one of their most attractive properties, the photoinstability arising from the mixing is a very serious drawback. Specifically, in mixed halide perovskites a light induced halide segregation is observed, presumably of thermodynamic origin, as suggested by its reversibility and the existence of an illumination threshold. This instability leads to domains of different band gaps within the perovskite, with the low-band gap domains acting as photocarrier traps.

Although many explanations have been proposed for the light-induced halide segregation, none of them addresses all aspects of the phenomenon. For instance, the existence of an illumination threshold is often not accounted for. This leads to strategies for suppressing the segregation that are based on trial and error and not carefully targeted design. To effectively address the issue a comprehensive understanding is needed that can only come from a transferable and flexible unified theory.

Such a thermodynamic unified theory was recently developed by Zehua and published in Nature Communications and is highlighted at TU/e Home page and several media. The work is a result of collbaoration with colleages from our own university Geert Brocks and Peter Bobbert and is an elegant example of the application of multiscale methods in this exciting field.

This theory is based on minimization of the sum of a compositional free energy, obtained from binary alloying theory, and an electronic free energy of photocarriers, which distribute thermally over a nucleated phase and a parent phase with different band gaps due to different I-Br compositions. The theory was applied to the most common mixed perovskites and lead to predictions that are in very good agreement with experimental observations. Most importantly, the theory is flexible and transferable not only to other perovskites but also other semiconductors.

Another joint publication with exprimental researchers that made use of (part of) this theory can be found at Joule and was featured previously at TU/e as understanding the love-hate-relationship of halide perovskites with the sun.