Multifunctional energy conversion and storage by selective La doping in flexible BiFeO3 thin films

Abstract

Flexible ferroelectric oxide thin films that simultaneously exhibit energy storage, piezoelectric, and photovoltaic functionalities are highly attractive for next-generation wearable and multifunctional energy devices. Here, we systematically investigate doping-dependent multifunctional responses in flexible La-doped BiFeO₃ (BFO) thin films deposited on Pt/mica substrates by pulsed laser deposition. La incorporation modulates crystallinity, domain configuration, and lattice distortion, enabling selective control of multifunctional energy properties. Moderate La doping (10 mol%) maximizes remanent polarization (P_r ∼ 68.7 μC/cm²) and delivers superior photovoltaic performance with an open-circuit voltage of 0.56 V and a short-circuit current density of 0.45 mA/cm². At 20 mol% La, the thin film exhibits the highest piezoelectric response, generating an AC output voltage of ∼2.3 V under periodic mechanical excitation. At higher La concentrations (≥25 mol%), P_r is suppressed and slimmer P–E hysteresis loops are obtained; in particular, the 30 mol% La-doped film exhibits enhanced energy-storage performance with a recoverable energy density of 32.6 J/cm³, an energy storage efficiency of 78 %, and stable cycling behavior. These results demonstrate that La doping serves as an effective design parameter to selectively tailor the multifunctional properties of flexible BFO thin films. By controlling the doping concentration, a single oxide material platform can be engineered to favor photovoltaic energy conversion, piezoelectric power generation, or high-performance energy storage, indicating the applicability of flexible La-doped BiFeO₃ thin films on mica substrates to integrated flexible energy systems.