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

Abstract
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.

Citation
Zhang Y, Yin J, Parida MR, Ahmed GH, Pan J, et al. (2017) Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals. The Journal of Physical Chemistry Letters 8: 3173–3177. Available: http://dx.doi.org/10.1021/acs.jpclett.7b01381.

Acknowledgements
This work was supported by King Abdullah University of Science and Technology (KAUST).

Publisher
American Chemical Society (ACS)

Journal
The Journal of Physical Chemistry Letters

DOI
10.1021/acs.jpclett.7b01381

PubMed ID
28644033

Additional Links
http://pubs.acs.org/doi/full/10.1021/acs.jpclett.7b01381

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