Strain Control of Photovoltaic Performance for Ferroelectric Oxynitrides: Theoretical Investigation

ABSTRACT Achieving high‐efficiency ferroelectric photovoltaic (FE‐PV) devices requires materials that combine a narrow bandgap with strong spontaneous polarization. To address this challenge, polar oxynitrides have emerged as promising candidates for enhanced photovoltaic performance. Their behavior under both uniaxial and triaxial strain has been systematically investigated using Density Functional Theory (DFT) calculations. In their relaxed (strain‐free) configurations, most of these materials exhibit direct bandgaps ranging from approximately 2.24 to 4.46 eV (calculated using the Tran–Blaha modified Becke–Johnson (TB‐mBJ) method), with the exception of LaSiO 2 N. Notably, YGeO 2 N and YSiO 2 N display substantial spontaneous polarization, with values reaching up to ∼160 µC/cm 2 in the case of YGeO 2 N. The application of mechanical strain was found to significantly influence their electronic and optical properties. Optical absorption spectra reveal that these oxynitrides can absorb light in the visible range and exhibit strong absorption in the ultraviolet region, supporting their suitability for solar energy harvesting. Among them, LaGeO 2 N stands out due to its strain‐tunable bandgap, particularly under tensile strain. Using the Spectroscopic Limited Maximum Efficiency (SLME) model, the power conversion efficiency of LaGeO 2 N was predicted to reach up to 29% under 10% strain, underscoring its potential as a high‐performance absorber material for next‐generation solar cell technologies.