Bandgap Engineering and Efficient Conversion of a Ternary Perovskite (Cs<sub>3</sub>Bi<sub>2</sub>I<sub>9</sub>) to a Double Perovskite (Cs<sub>2</sub>NaBiI<sub>6</sub>) with the Aid of Alkali Metal Sulfide
Lekha Peedikakkandy, Soumyo Chatterjee, Amlan J. Pal
Abstract
The success of iodobismuth ternary perovskites, such as Cs3Bi2I9, in solar cell applications has so far been limited due to their wide bandgaps and low structural dimensionalities. In order to address these prevailing issues, we report the bifunctional role of an alkali metal sulfide (Na2S) treatment on the Cs3Bi2I9 perovskite. We found that the incorporation of Na2S in such a zero-dimensional Cs3Bi2I9 structure not only lowers its optical bandgap but also leads to an efficient conversion to a three-dimensional Cs2NaBiI6 double perovskite structure with a closer-to-optimal bandgap for solar energy conversion. The structural characterization and optical properties of Cs3Bi2I9 with a varying contents of Na2S show that each monovalent sodium cation replaces a trivalent bismuth ion in the Cs3Bi2I9 structure and enforces its transformation to a double perovskite framework (Cs2NaBiI6); the divalent sulfur ions occupy the interstitial sites and in turn tune the optical bandgap. The conduction and valence band energies, and hence the transport gap, of this series of materials have been estimated from scanning tunneling spectroscopy studies and the density of states spectra obtained thereof. The evolution of the band edges following the incorporation of Na2S has been explained by considering the contributions of the atomic orbitals. As evidenced from atomic force microscopy studies, such metal sulfide-aided conversion also leads to a superior surface morphology in the converted double perovskites. This work hence demonstrates an effective process for converting a large bandgap low-dimensional ternary perovskite to a low-bandgap three-dimensional double perovskite.