Density Functional Theory Insights into the Structural, Electronic, Optical, Surface, and Band Alignment Properties of BaZrS3 Chalcogenide Perovskite for Photovoltaics

Chalcogenide perovskites (CPs) have recently emerged as attractive thermally and chemically stable candidates to overcome the inherent instability and toxicity issues of the conventional hybrid organic–inorganic halide perovskites (OIHPs). However, before further progress can be made in CP thin-film photovoltaic (PV) cells, there is a need to gain fundamental insights into their bulk, surface, and interfacial properties. Herein, we employed density functional theory (DFT) calculations to systematically characterize the bulk (structural, electronic, and optical), surface (compositions, relative stabilities, crystal morphology, and work function), and interfacial (energy band alignment) properties of BaZrS3, one of the most promising members of the CP family. BaZrS3 is found to exhibit a direct bandgap of 1.74 eV with small photocarrier effective masses, high absorption coefficient (∼105 cm–1), low reflectivity (22%), and low refractive index (2.75), all of which are desirable characteristics for efficient PV applications. Comprehensive analyses of the structures, compositions, and relative stabilities of the low-Miller-index surfaces of BaZrS3 revealed that the (010), (100), and (111) surfaces are the most stable surfaces, which are also largely expressed in the Wulff-constructed equilibrium crystal morphology of BaZrS3. Based on the computed ionization potential (IP) and electron affinity (EA), we have constructed a vacuum-aligned energy band diagram of BaZrS3 with commonly used hole- and electron-transport materials (HTMs and ETMs, respectively). CuI (HTM) and CdS (ETM) are predicted as the best heterojunction materials to form favorable alignment (staggered type II) with optimum band offsets with the BaZrS3(111) surface for efficient charge-carrier separation and improved solar cell performance. Our results show great promise for developing more efficient and stable BaZrS3-based chalcogenide perovskite solar cells.