Optimal PbS quantum-dot loading in composition-graded Bi1-xCaxFeO3 stacks for improved photovoltaic performance

A PbS quantum-dot (QD) interfacial layer was integrated with a composition-graded Bi _{1- x} Ca ₓ FeO ₃ (BCFO) oxide stack to enhance photovoltaic output via coupled optical sensitization and interfacial field regulation. By tuning the PbS precursor concentration, an optimal QD loading window was identified. Increasing PbS loading enhanced optical absorption and interfacial band bending at the PbS/BCFO junction, whereas excessive loading induced aggregation and non-uniform thickening, leading to transport bottlenecks and trap-assisted recombination. Consequently, the photocurrent increased with PbS loading while the power conversion efficiency(PCE) exhibited a volcano-type dependence, peaking at 0.0075 mol L ^-1 with short circuit current density ( J _sc ) = 2431 μA cm ^-2 , open circuit voltage ( V _oc ) = 0.872 V, fill factor ( FF ) = 41.5%, and a maximum PCE of 1.235%, together with stable light on/off switching. These results highlight that controlling QD loading, rather than simply maximizing sensitizer content, is critical for achieving reproducible performance enhancement in QD/oxide composite photovoltaics.