Frenkel Excitons in Vacancy-Ordered Titanium Halide Perovskites (Cs₂TiX₆)

Low-cost, non-toxic and earth-abundant materials are a long-sought after target in the thermoelectrics research community. "Perovskite-inspired" materials have emerged as promising candidates for thermoelectric applications due to their semiconducting nature and low lattice thermal conductivities.^1,2 Researchers commonly employ materials design strategies including structural, dimensional and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain the excellent electronic structure features of their Pb-containing counterparts. These strategies have recently been invoked to propose vacancy-ordered halide perovskites (A₂BX₆; <b>A</b> = Cs, Rb, K; <b>B</b> = Sn, Ti; <b>X</b> = I, Br, Cl) for thermoelectric applications. ^3,4<br/><br/>Theoretical investigations of the electronic structure of the Ti based systems, however, consistently overestimate the energy band gaps, which makes understanding dopability very difficult. In this work, we reveal strong excitonic effects as the origin of this discrepancy between theory and experiment; a consequence of both low structural dimensionality and band localization.⁵<br/><br/>These findings have vital implications for the optoelectronic and thermoelectric application of these compounds, while also highlighting the crucial importance of frontier-orbital character for chemical substitution materials design strategies.<br/><br/>1. Huang, Y.-T., Kavanagh, S. R., Scanlon, D. O., Walsh, A. & Hoye, R. L. Z. Perovskite-inspired materials for photovoltaics and beyond—from design to devices. <i>Nanotechnology</i> <b>32</b>, 132004 (2021).<br/>2. Wang, X. <i>et al.</i> Cubic halide perovskites as potential low thermal conductivity materials: A combined approach of machine learning and first-principles calculations. <i>Phys. Rev. B</i> <b>105</b>, 014310 (2022).<br/>3. Mahmood, Q. <i>et al.</i> Study of lead-free double perovskites halides Cs₂TiCl₆, and Cs₂TiBr₆ for optoelectronics, and thermoelectric applications. <i>Materials Science in Semiconductor Processing</i> <b>137</b>, 106180 (2022).<br/>4. Bhumla, P., Jain, M., Sheoran, S. & Bhattacharya, S. Vacancy-ordered Double Perovskites A₂BX₆ (A = Cs, B = Pt, Pd, Te, Sn, X = I): An Emerging Class of Thermoelectric Materials. Preprint at https://doi.org/10.48550/arXiv.2209.08559 (2022).<br/>5. Kavanagh, S. R. <i>et al.</i> Frenkel Excitons in Vacancy-ordered Titanium Halide Perovskites (Cs₂TiX₆). (2022) doi:10.26434/chemrxiv-2022-0zg7r.