Impact of enhanced ferroelectric polarization through La doping on photovoltaic properties of BiFeO₃ thin films on HOPG

BiFeO₃ (BFO) thin films are well-known for their multiferroic and photovoltaic properties, driving extensive research into potential applications. Highly ordered pyrolytic graphite (HOPG) substrates, with a graphene-like surface structure, provide an ideal platform for assessing the impact of graphene electrodes. This study investigates the photovoltaic properties of ITO/BFO/HOPG devices with La-doped BFO thin films at concentrations of 5, 10, and 15 mol%. The polycrystalline BFO thin films exhibited a preferential (111) orientation, with the 10 mol% La-doped thin films demonstrating optimal crystallinity and the highest remanent polarization of 50.8 µC/cm². Photovoltaically, the 5 mol% La-doped BFO thin film exhibited an open-circuit voltage (V_oc) of 0.46 V and the highest short-circuit current (J_sc) of 0.38 mA/cm². The 10 mol% La-doped BFO thin film achieved the highest V_oc of 0.57 V with a J_sc of 0.35 mA/cm², likely because of enhanced ferroelectric polarization. In contrast, the 15 mol% La-doped BFO thin film showed a reduced band gap but diminished photovoltaic performance. The minimal variation in the band gap (within 0.1 eV) suggests that the improved photovoltaic performance is primarily driven by increased polarization resulting from enhanced tetragonality.