Simulation-based experimental design for investigating the structural and physical properties of novel layered ferroelectric Bi₂O₂S

As materials science advances into the atomic scale, computational simulation has become a powerful tool for bridging microscopic structures and macroscopic properties. This study employs first-principles calculations to investigate the layered bismuth oxysulfide Bi₂O₂S, systematically comparing the structural and physical properties of its orthorhombic (antiferroelectric) and tetragonal (ferroelectric) phases. By elucidating the mechanisms through which crystal phases regulate ferroelectric and photovoltaic properties, this study provides a theoretical foundation for the rational design of low-power ferroelectric photovoltaic devices. Furthermore, by integrating computational simulation into experimental teaching, this study aims to help students develop a systematic framework for understanding structure–performance correlations, thereby enhancing their ability to design materials and predict their properties.

All first-principles calculations were performed using the Vienna Ab Initio Simulation Package. Electron–ion interactions were described via the projector augmented-wave method, whereas the exchange–correlation effects were treated within the generalized gradient approximation using the Perdew–Burke–Ernzerhof functional. Ferroelectric polarization was calculated using the Berry–phase method, a modern theoretical approach for quantifying polarization in periodic systems. Photovoltaic performance was evaluated using the spectroscopic limited maximum efficiency (SLME) model, which incorporates the detailed balance principle and accounts for radiative and nonradiative recombination losses under the AM1.5G solar spectrum. Data post-processing and analysis were conducted with VASPKIT, whereas crystal structures and electron density distributions were visualized using VESTA software.

Structurally, the orthorhombic phase of Bi₂O₂S (space group Pnnm) was identified as the ground state. Adjacent [Bi₂O₂]²⁺ layers exhibited antiparallel dipole moment arrangements, resulting in zero net macroscopic polarization, which is a characteristic of antiferroelectricity. By contrast, the tetragonal phase (space group I4/mmm) displayed significant ferroelectric polarization with a low polarization–switching energy barrier. In the unstrained state, the tetragonal phase showed an in–plane polarization of 89.10 μC/cm² and a polarization switching barrier of merely 5 meV, indicating that only minimal energy is required for polarization reversal, while its out–of–plane polarization was only 2.10 μC/cm². Mechanical strain effectively regulated the ferroelectric behavior: a 5% tensile strain enhanced the in-plane polarization to 312.41 μC/cm², whereas a –2% compressive strain completely suppressed ferroelectricity. Photovoltaic performance was evaluated using the SLME model. At a film thickness of 0.5 μm, the tetragonal ferroelectric phase achieved a maximum power conversion efficiency (PCE) of 20.66%, with a short-circuit current density (J_sc) of 28.52 mA/cm² and an open-circuit voltage (V_oc) of 0.84 V. By contrast, the orthorhombic antiferroelectric phase exhibited a slightly lower maximum PCE of 19.53% (J_sc = 27.74 mA/cm² and V_oc = 0.82 V). Both phases showed similar fill factors (~86.50%), indicating that the phase transition had a negligible effect on optical absorption.

This study employed first-principles calculations to explore the ferroelectric and photovoltaic properties of the orthorhombic and tetragonal phases of Bi₂O₂S. The mechanisms through which crystal phase and mechanical strain regulate material performance were clarified, providing theoretical support for the design of low-power ferroelectric photovoltaic devices. From an educational perspective, this study integrates computational simulation with experimental teaching, using structure–performance correlations as the core framework to guide students through a systematic research workflow, from crystal structure analysis to property prediction. This approach enhances students' capabilities in computational materials design and performance prediction and lays a solid foundation for future research on functional materials.