Lead-Free Cs2BB′X6 (B: Ag/Au/Cu, B′: Bi/Sb/Tl, and X: Br/Cl/I) Double Perovskites and Their Potential in Energy Conversion Applications

Abstract

A mere decade-long research focused on perovskites for photovoltaics (PVs) has unearthed their exciting promise. However, as an alternative to the inaugural class of single perovskites, which suffer from instability and toxicity, an interesting class of halide double perovskites (DPs) have recently been in the hot list of photovolatic (PV) researchers. This article presents 27 Cs2BB′X6 halide DPs using different exchange–correlation approximations and reports investigations based on factors such as the Goldschmidt tolerance factor (t) and modified tolerance factor (t′) and thermodynamical investigations considering mixed decomposition pathways for assessing their stability. The spin–orbit coupling (SOC) is explored to understand the role of the heavy element as an alternative to Pb. A wide range of band gaps (0.2 eV to 2.35 eV) are noticed for the 27 DPs. As a single exception, a halide DP with a trivalent thallium cation exhibits metallic characteristics. Further, an insight into the mechanism behind band gap variation is correlated with different B/B′/X combinations and understood from molecular orbital alignment analysis. Such a broad band gap spectrum has the potential to provide the possibility of integrating them in single- or multiple-junction solar cells or thermoelectric applications. This article, on the whole, hopefully provides insights into the potential of DPs for various applications befitting their particular material properties.