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

Low-cost, non-toxic and earth-abundant photovoltaic materials are a long-sought target in the solar cell research community. 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 Ti-based vacancy-ordered halide perovskites (A₂TiX₆; A = CH₃NH₃, Cs, Rb, K; X = I, Br, Cl) for photovoltaic operation, following the initial promise of Cs₂SnX₆ compounds. Theoretical investigations of these materials, however, consistently overestimate their band gaps – a fundamental property for photovoltaic applications. Here, 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 importance of frontier-orbital character for chemical substitution in materials design strategies.