Excitonic Properties of Vacancy Ordered Double Perovskites—Electron-Hole Coupling and GW Quasi-Particles

Over the past decade ABX₃ halide perovskites based on Pb have emerged as a most-promising class of materials. To-date they have been employed for record-breaking solar-cells¹, highly-efficient light-emitters², new photo-catalysts³ and even as X-ray detectors⁴. A₂BX₆ vacancy ordered double perovskites (VODP) are air-stable and environmentally friendly (Pb-free) that have been proposed as alternatives⁵, to halide perovskites. Yet, VODP materials have not achieve the high performance of their Pb-based counterparts. In this work, we thoroughly analyze the properties of the VODP family of materials by employing state-of-art ab initio calculations to unveil the key details of the electronic structure and the effects of electron-hole coupling on the optical properties.<br/><br/>We sample the VODP structures by picking prototypes accordingly to the electronic configuration of the tetravalent metal at the B-site. That is, there are three possible closed-shell metal sites, with valences composed of either <i>s-</i>, <i>p-</i> or <i>d</i>-orbitals. In particular, we select known materials^5,6,7 Cs₂TeX_6,Cs₂SnX₆, and Cs₂ZrX₆ (with X=Br, I) for the groups with a valency comprised of an s, p and d closed-shell, respectively. The structural properties are first investigated, and show how the size of the vacancy, is in fact tuned solely by the size of the halogen. We assess the mechanical stability of each crystalline lattice by means of phonons calculations. The electronic structures are investigated within the GW many-body green’s function method, and we obtain band-gaps spanning a range of 1.5-5.0 eV at G₀W₀. More importantly, we discuss the effects of the substitutional engineering on the dispersion of the electronic bands, and relate our findings to the expected charge carrier mobilities of each type of VODP. The optical and excitonic properties of these compounds are investigated within the independent particle approach, and by solving the Bethe-Salpeter equation (BSE). We report the exciton binding energies and dark-bright exciton exchange splitting for each type of VODP. Finally, we address the exciton symmetries by performing a complete symmetry analysis of the compound’s band structure and excitonic wavefunctions, on which a direct link between these and the metal site species is established.<br/><br/>Overall, we explore comparatively the role of electron-hole coupling and GW quasi-particles in VODP materials, and explain its correlation with the choice of the B-metal and X-halide sites. These results shed light on the suitability and the prospect of VODP type might exhibit, due to their opto-electronic and excitonic properties. Finally, we identify the most and least promising materials that could act as photo-active materials or for selective charge transport layers in light-emitting and solar-cell applications.<br/><br/><b>Acknowledgments</b><br/>The research leading to these results has received funding from the Chaire de Recherche Rennes Metropole project, and from the European Union’s Horizon 2020 program, through a FET Open research and innovation action under the grant agreement No 862656 (DROP-IT).<br/><br/><b>References</b><br/>[1] Amran Al-A. <i>et al.</i>, Science <b>370</b>, 1300 (2020)<br/>[2] Yang D. <i>et al.</i>, Nat. Commun. <b>12</b>, 4295 (2021)<br/>[3] Pradhan S. <i>et al.</i>, Nanoscale Adv. <b>3</b>, 1464 (2021)<br/>[4] Wu. H. <i>et al.</i>, Matter <b>4</b>, 144 (2021)<br/>[5] Lee B. <i>et al.</i>, J. Am. Chem. Soc. <b>136</b>, 15379 (2014)<br/>[6] Xu Y. <i>et al.</i>, ACS Photonics <b>6</b>, 196 (2019)<br/>[7] Cucco B. <i>et al.</i>, Appl. Phys. Lett. <b>119</b>, 181903 (2021)