This paper presents the first demonstration of a gate-tunable p-n diode based on a black phosphorus-monolayer MoS₂ van der Waals heterojunction. The device exhibits strong current-rectifying characteristics, with a maximum photodetection responsivity of 418 mA/W at 633 nm and a photovoltaic energy conversion efficiency of 0.3%. The p-n diode's performance is attributed to the high hole mobility and thickness-dependent direct bandgap of black phosphorus. The study also explores the optoelectronic properties of the device, showing its potential for broadband photodetection and solar energy harvesting. Photocurrent mapping reveals uniform photocurrent generation across the junction, suggesting the feasibility for large-area solar cells and photodetectors. The findings highlight the promise of black phosphorus-MoS₂ heterojunctions in advanced electronic and optoelectronic applications.This paper presents the first demonstration of a gate-tunable p-n diode based on a black phosphorus-monolayer MoS₂ van der Waals heterojunction. The device exhibits strong current-rectifying characteristics, with a maximum photodetection responsivity of 418 mA/W at 633 nm and a photovoltaic energy conversion efficiency of 0.3%. The p-n diode's performance is attributed to the high hole mobility and thickness-dependent direct bandgap of black phosphorus. The study also explores the optoelectronic properties of the device, showing its potential for broadband photodetection and solar energy harvesting. Photocurrent mapping reveals uniform photocurrent generation across the junction, suggesting the feasibility for large-area solar cells and photodetectors. The findings highlight the promise of black phosphorus-MoS₂ heterojunctions in advanced electronic and optoelectronic applications.