Toward first-principles approaches for mechanistic study of self-trapped exciton luminescence

In recent years, broadband photo-luminescence phenomena arising from self-trapped exciton (STE) in metal halides, including perovskites and various low-dimensional derivatives and variants, have attracted increasing attention for their potential diverse optoelectronic applications like lighting, display, radiation detection, and sensing. Despite great success in experimental discovery of many efficient STE emitters, the current understanding of the STE emission mechanism in metal halides is still immature, and often controversial, which calls for help urgently from predictive first-principles theoretical calculation. Although density-functional theory (DFT) based calculations are routinely used to provide electronic band structure of materials and have contributed greatly to qualitative analysis of luminescence mechanism, more in-depth and quantitative information is highly needed to provide guidelines for rational design of new luminescent materials with desirable features. However, due to the complicated nature of STE emission, involving in particular electron–phonon coupling in both ground and excited states, the usage of DFT is no longer a routine job as for ground state properties. While more sophisticated methods formulated in the framework of many-body perturbation theory like GW-Bethe–Salpeter equation are available and provide theoretically rigorous and accurate description of electronic transitions in extended systems, their application to real STE systems is still severely limited due to highly demanding computational cost. In practice, approximated DFT methods are employed, which have their own strengths and limitations. In this review, we focus on the theoretical approaches that have been heavily used in interpreting STE luminescence mechanism, with a particular emphasis on theoretical methods for exciton self-trapping structural optimization. It is hoped that this review, by summarizing the current status and limitations of theoretical research in the STE emission, will motivate more methodological development efforts in this important field, and push forward the frontiers of excited state electronic structure theory of materials in general.