Proceedings of nanoGe Fall Meeting 2021 (NFM21)
DOI: https://doi.org/10.29363/nanoge.nfm.2021.196
Publication date: 23rd September 2021
Along the past few years, the vacancy-ordered halide double perovskites (VOHDP) have been extensively investigated as promising candidates for various optoelectronic applications [1]. Among these compounds, Cs2TiBr6 and Cs2TiI6 are VOHDP that have been successfully synthesised, and are reported to be interesting photo-active materials with band-gaps well within the visible range (1.02-2.00 eV) and potentially good carrier transport properties [2–4]. Photovoltaic devices using Cs2TiBr6 achieved a 3.3% power-conversion efficiency [3], though their actual potential has been recently questioned [5]. In this work, we perform a thorough computational analysis of the electronic structure, the mechanical and chemical stability, and the optical properties, of the entire family of VOHDP that have the same B’-site electronic valence as Ti4+ (i.e., d10). By doing so, we probe the potential tuning limits of their opto-electronic properties, extend the existing understanding on the well-established Ti-based materials, and propose the less-known Zr-based compounds (i.e., Cs2ZrX6; X=Br, I) as potential prominent alternatives. In particular, we employ three levels of calculations: DFT-PBE, hybrid functionals, and state-of-the-art GW, to calculate their electronic structure, and perform a complete symmetry analysis of the electronic bands to reveal its consequences on the exhibited band-gaps and charge carrier effective masses. We also show the importance of spin-orbit coupling effects, highlight the limitations when calculating quasi-particle corrections, and demonstrate how a simple crystal field theory can be used to describe the observed electronic structure. Furthermore, we investigate all possible decomposition reaction pathways to competing compounds, and further assess their mechanical stability by means of phonon calculations. Overall, our findings unveil the fundamental electronic and optical properties of Cs2TiBr6, establish the stability and tunability limits of electronic band-gaps and effective mass within the family of VOHDP materials with a d10 electronic configuration, but most importantly propose a novel composition for the next generation of optoelectronic devices, namely Cs2ZrI6 that has been practically unexplored to date.
The research leading to these results has received funding from the European Unions Horizon 2020 program, through a FET Open research and innovation action under the grant agreement No 862656 (DROP-IT).
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