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

Abstract

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.

Electronic structure characterization by photoelectron spectroscopy of BaZrS3 perovskite powder and thin film

“…4: the powder with Al Kα (in blue), and the valence bands acquired by hard X-rays for the powder (in orange) and for the thin film (in purple). In the figure, the three valence bands were normalized to the main feature at predominantly from the 3p states of the S atoms and 4d states of the Zr atoms, in accordance with previous reports 6,16,42 . Moving to the energy range 10 -15 eV, the total DOS results from tilting of adjacent Zr-S6 (i.e.…”

“…DOS calculations on BaZrS3 in the GdFeO3-type perovskite structure reveal the valence band to be composed mainly by high-density states of S 3p and Zr 4d, while the main conduction band states are S 3d, Ba 5d and Zr 4d 6,15,16 . Comparison between direct mapping of experimentally measured DOS to theoretically calculated element-specific contributions is difficult to achieve and experimental values of the valence band maximum (VBM) are of interest for the evaluation of the materials' candidacy for forming a junction.…”

A Density Functional Theory Study of the Physico-Chemical Properties of Alkali Metal Titanate Perovskites for Solar Cell Applications

“…The energy loss function, 𝐿(𝜔), describes the energy that may be lost by fast electrons during motion [115,122], which is shown in Figure 7b, where 𝐿(𝜔) is calculated using Equation (S13). Table 4 provides the static values of reflectivity at zero energy, denoted as R(0), for each of the discussed materials.…”

“…The energy loss function, L(ω), describes the energy that may be lost by fast electrons during motion [115,122], which is shown in Figure 7b, where L(ω) is calculated using Equation (S13).…”

Electronic Structure and Surface Chemistry of BaZrS3 Perovskite Powder and Sputtered Thin Film

“…The theoretical study by Nishigaki et al indicates that the high absorption coefficient arises mainly from the electronic transition from the highly populated state of S 3p to the empty Zr 4d orbitals in the conduction band. Density of states (DOS) calculations on BaZrS 3 in the GdFeO 3 -type perovskite structure reveal the valence band to be composed mainly of high-density states of S 3p and Zr 4d, while the main conduction band states are S 3p, Ba 5d and Zr 4d orbitals. − …”