Chemo-Strain Valence Engineering for Boosting Photovoltaic Response in Double Perovskite Epitaxial Films

Double perovskite films offer significant potential for multiferroic and ferroelectric photovoltaics due to their structural tunability. This study employs an aliovalent substitution strategy, partially replacing Bi with Pb in Bi₂FeMnO₆ (BFMO), to disrupt charge balance and local polarization while maintaining the host lattice. Pb incorporation simultaneously modulates the chemical states of all constituent elements, inducing pronounced lattice distortion and positive chemical strain. Unpoled Pb-BFMO films exhibit exceptional photovoltaic performance under 80 mW cm^-2 illumination, achieving a short-circuit current density (|J_SC|) of 192 μA cm^-2 and an open-circuit voltage (|V_OC|) of 0.525 V. This represents a 109-fold increase in intrinsic J_SC and a fourfold enhancement in V_OC compared to pure BFMO. The |J_SC| demonstrates electric field tunability via polarization switching, reaching 320 μA cm^-2 under negative polarization, the highest reported J_SC for sub-100 nm single-layer ferroelectric films under white light. High-resolution high-angle annular dark-field scanning transmission electron microscopy, synchrotron-based reciprocal space mapping and X-ray absorption spectroscopy analyses collectively confirm the coupling of crystal distortion, chemical strain, and valence state alterations. The synergy between chemical strain and ionic valence states effectively engineers the bandgap and enhances photovoltaic response, which unlocks new application pathways for perovskite materials in optical memory devices and sustainable energy systems.