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

High-performance, lightweight solar cells with low-cost and non-toxic substituents are a major target in the field of solar photovoltaics.^1,2 Perovskite-inspired materials have emerged as promising candidates for this goal, with researchers employing materials design strategies including structural, dimensional and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain high optoelectronic performance. These strategies have recently been invoked to propose titanium-based vacancy-ordered halide perovskites (A₂TiX₆; A = CH₃NH₃, Cs, Rb, K; X = I, Br, Cl) for photovoltaic operation, following the initial promise of the Cs₂SnX₆ compounds.^3–5



Theoretical investigations of the electronic structure of this material, however, consistently overestimate the energy band gap of this material^4–6 — the most important property for photovoltaic application. In this work,⁷ 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.



These findings have vital implications for the optoelectronic application of these compounds, while also highlighting the crucial importance of frontier-orbital character for chemical substitution materials design strategies.



1. Huang, Y.-T., Kavanagh, S. R., Scanlon, D. O., Walsh, A. & Hoye, R. L. Z. Nanotechnology (2021)

2. Huang, Y.-T. & Kavanagh, S. R. et al. Nat Commun (2022)

3. Liga, S. M. & Konstantatos, G. J. Mater. Chem. C (2021)

4. Chen, M. et al. Joule (2018)

5. Euvrard, J., Wang, X., Li, T., Yan, Y. & Mitzi, D. B. J. Mater. Chem. A (2020)

6. Cucco, B. et al. Appl. Phys. Lett. (2021)