Thickness optimization for synergistic enhancement of efficiency and transmittance in Ca/Cr-Co-doped BiFeO3 semitransparent photovoltaic devices

The trade-off between power conversion efficiency (PCE) and average visible transmittance (AVT) in semi-transparent solar cells significantly restricts their application in building-integrated photovoltaics. Bismuth ferrite (BiFeO 3 , BFO)-based materials have shown great potential to address this trade-off due to their inherent ferroelectric properties and tunable optical bandgap. However, even with performance optimization through chemical doping, a considerable bottleneck remains. Using the Ca/Cr co-doped BFO system as an example—whose optimal composition still exhibits a PCE below 1.4 %—this study proposes a new strategy of physical thickness control to overcome the limitations of chemical doping. By systematically investigating the influence of the thickness (200–350 nm) of the Ca/Cr co-doped BFO layer with optimal composition on device performance, it was found that increased thickness significantly enhances light absorption and optimizes the distribution of the built-in electric field. The results demonstrate that at a thickness of 336 nm, the device achieves the best photovoltaic performance (PCE = 1.86 %, open-circuit voltage ( V oc ) = 1.57 V, short-circuit current density ( J sc ) = 2.30 mA/cm 2 ), representing an improvement of over 35 % compared to the reference thickness (210 nm). Meanwhile, at a thickness of 285 nm, the device maintains a relatively high AVT (33.70 %) and achieves the highest light utilization efficiency (LUE = 0.6100 %). Electrical impedance spectroscopy analysis indicates that increased thickness effectively reduces the carrier recombination probability. This study reveals the synergistic mechanism of thickness regulation through “light absorption–built-in electric field–carrier transport” in balancing PCE and AVT, providing a reliable optimization strategy for semi-transparent ferroelectric photovoltaic devices.