Anion-engineered chalcogenide perovskites Ba2HfCh4:from light emitters to photovoltaics

Abstract

We present a systematic first-principles study of Ruddlesden-Popper chalcogenide perovskites Ba₂HfS₄ and Ba₂HfSe₄ and their Te-alloyed derivatives. Stability analysis confirms that only the barium-based parent compounds are dynamically and thermally stable. These parent phases possess direct bandgaps (3.57 eV and 2.54 eV), large exciton binding energies (0.79 eV and 0.67 eV), and strong optical absorption (10⁵ cm^-1), making them promising candidates for solid-state lighting. Controlled Te substitution enables continuous bandgap tuning from the UV to the visible range. An anomalous bandgap reversal at intermediate doping levels (Ba₂HfS₂Te₂ and Ba₂HfSe₂Te₂) is explained by a p-p hybridization mechanism that modulates the valence band maximum. Notably, the high-Te-content compounds Ba₂HfSTe₃ and Ba₂HfSeTe₃ exhibit ideal photovoltaic bandgaps (1.6–1.7 eV), high absorption coefficients (10⁵ cm^-1), and balanced carrier transport, yielding predicted power conversion efficiencies of ∼24\% and ∼27\%. This work primarily establishes these Te-rich compounds as high-performance photovoltaic absorbers, while also revealing the parent compounds as promising light-emitting candidates, demonstrating anion alloying as an effective strategy for multi-functional design in layered chalcogenides.