Energy level alignment of vacancy-ordered halide double perovskites

TL;DR

This study uses first-principles calculations to analyze the electronic properties, surface stability, and energy level alignment of vacancy-ordered halide double perovskites, providing insights for optoelectronic device design.

Contribution

It offers the first comprehensive theoretical analysis of energy level alignment and surface stability in Cs₂MX₆ double perovskites using advanced hybrid functional methods.

Findings

Bulk band gaps agree with GW calculations.

Surface energy stability favors CsX terminations.

MX₄ terminations exhibit trap states that reduce carrier lifetime.

Abstract

Vacancy-ordered double perovskites have emerged as lead-free alternatives, offering remarkable stability and compositional tunability for optoelectronic applications. In this study, we provide first-principles insights into their electronic properties, surface stability, and energy level alignment using a non-empirical, dielectric-dependent hybrid functional. For a representative family of Cs$_2$MX$_6$ compounds, with M = Zr, Sn, Te, and X= Cl, Br, I, our calculations reveal that the predicted bulk electronic band gaps are in excellent agreement with those obtained using the state-of-the-art GW method, validating the accuracy of our approach. We investigate the stability of these materials under simulated experimental conditions, considering both the rich and poor chemical potentials of their precursor salts. Our results indicate distinct regions of surface energy stability that favor CsX terminations. In contrast, MX$_4$ terminations show in-gap surface states, which can act as trap states and reduce carrier lifetime. Finally, based solely on the intrinsic absolute energy levels, we identify promising candidates as charge transport and injection layers for typical photovoltaic and light-emitting applications. This study provides a detailed map of energy level alignment at Cs$_2$MX$_6$ surfaces, offering valuable design principles for the development of next-generation Cs$_2$MX$_6$-based optoelectronic devices.