Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals

With record efficiencies achieved in lead halide perovskite-based photovoltaics, urgency has shifted toward finding alternative materials that are stable and less toxic. Bismuth-based perovskite materials are currently one of the most promising candidates among those alternatives. However, the band structures of these materials, including the nature of the bandgaps, remain elusive due to extremely low photoluminescence quantum yield (PLQY) and scattering issues in their thin-film form. Here, we reveal the specific nature of the material's electronic transitions by realizing monodisperse colloidal nanocrystals (NCs) of hexagonal-phase Cs₃Bi₂X₉ perovskites, which afford well-resolved PL features. Interestingly, the PL profile exhibits a dual-spectral feature at room temperature with comparable intensities, based on which we propose an exciton recombination process involving both indirect and direct transitions simultaneously - an observation further supported by temperature-dependent and density functional theory (DFT) calculations. Our findings provide experimental and theoretical insights into the nature of the bandgaps in bismuth halide materials - essential information for assessing their viability in solar cells and optoelectronics.