First-Principles Investigation of Oxygen and Water Adsorption on (010), (100), and (111) Surfaces of BaZrS3 Chalcogenide Perovskite

BaZrS3 has recently attracted significant attention as a cost-effective, high-stability, and eco-friendly solar absorber. The characteristics of BaZrS3-based solar cells in terms of power conversion efficiency and durability can be critically influenced by surface and interface properties inherent in the design and manufacture of these devices under ambient conditions. Herein, we present first-principles density functional theory (DFT) insights into the adsorption chemistry of oxygen and water on the three most stable (010), (100), and (111) surfaces of BaZrS3. We studied the underlying changes in the surface electronic structure, band gap, and work function in contact with oxygen and water. The Zr sites are found to be generally more reactive than Ba sites toward the adsorbing molecules. It was demonstrated that water interacts weakly with the BaZrS3 surfaces, whereas molecular and dissociative oxygen interact strongly with the BaZrS3 (010), (100), and (111) surfaces at the Zr sites. Charge density difference isosurfaces and Bader charge analyses reveal that the adsorption of oxygen is characterized by a significant charge transfer from the interacting surface species to the O2 molecule, resulting in the elongation of the O-O bonds, which was confirmed by vibrational frequency analysis. Unlike the case of oxygen, the dissociation of water is shown to be unfavorable, suggesting that water would preferentially exist as molecular water instead of dissociating to form hydroxylized (-OH) and sulfhydrylized (-SH) BaZrS3 surfaces. Projected density of states (PDOS) analyses reveal that while the naked (010) surface retained the intrinsic semiconducting nature of the bulk BaZrS3 with a suitable bandgap for optical applications, the creation of the (100) and (111) surfaces renders them semimetallic as the Fermi level marginally crosses the top of their valence bands. The adsorption of dissociated O2 and H2O species was found to enhance the semimetallic features of the three surfaces. Despite the introduction of semimetallic features, the surfaces possess a clear band gap as their valence bands do not overlap with their conduction bands. Relative to the naked BaZrS3 surfaces, we observed only small changes in the band gap and work function upon O2 and H2O adsorption, suggesting that their exposure to ambient conditions may not have a significant impact on their PV device performance. These results present new exciting opportunities for optimizing the electronic properties of BaZrS3 nanostructures for the fabrication of efficient and stable solar cell devices.