Flexoelectric Modulation of Photovoltaic Performance in Two-Dimensional CuInPS: Enhanced Shift Current, Carrier Separation, and Power Conversion Efficiency

Abstract

Two-dimensional ferroelectrics hold exceptional promise for photovoltaic energy conversion owing to their intrinsic bulk photovoltaic effect, yet effective strategies for continuously tuning their photovoltaic performance remain limited. Here, we demonstrate that mechanical bending through the flexoelectric coupling between strain gradients and polarization provides a continuous and reversible strategy for enhancing the photovoltaic functionality of monolayer CuInP2S6 (CIPS). We develop an analytical model for the flexoelectric polarization and quantitatively determine the dominant flexoelectric coefficient. Using density functional theory and Wannier interpolation, we show that uniaxial bending generates an in-plane flexoelectric polarization that amplifies the shift-current conductivity σ^yyy by factors of 1.66–2.23 across the visible-to-ultraviolet spectrum, associated with bending-induced modifications of the Bloch wavefunctions and momentum-resolved interband optical response, accompanied by a pronounced redistribution of the Berry curvature. Meanwhile, the flexoelectric field generates an electrostatic asymmetry between the compressive and tensile surfaces that compresses the conduction band offset from 0.20 to 0.14 eV and establishes a type-II band alignment, promoting spatial separation of photogenerated carriers, while the strain-gradient-modulated band dispersion enhances carrier mobility, particularly for holes (by up to 32%). Separately, a modified Scharber-type estimate based on the PBE bandgap and effective surface-to-surface conduction-band offset yields a comparative PCE proxy that increases to 12.2% with bending. This estimate does not explicitly incorporate the intrinsic shift-current response. This work identifies flexoelectricity as a versatile tuning strategy for the photovoltaic performance of two-dimensional ferroelectrics and provides quantitative materials design guidance for strain-gradient-engineered optoelectronic devices.