This study demonstrates the creation of an atomically thin and sharp p-n diode by vertically stacking monolayer tungsten diselenide (WSe₂) and few-layer molybdenum disulfide (MoS₂). The diode exhibits excellent current rectification behavior with an ideality factor of 1.2, and shows rapid photoresponse over the entire overlapping region with a maximum external quantum efficiency (EQE) of 12%. Electroluminescence (EL) studies reveal prominent band edge excitonic emission and enhanced hot electron luminescence. The device's performance is attributed to the atomically sharp heterojunction, which allows for efficient charge separation and current rectification. The study also investigates the origin of hot-electron luminescence and the nature of electron-orbital interaction in transition metal dichalcogenides (TMDs), providing insights into the fundamental electro-optical properties of TMDs. This work opens up new avenues for the development of novel optoelectronic devices, including atomically thin photodetectors, photovoltaics, and spin- and valley-polarized light-emitting diodes.This study demonstrates the creation of an atomically thin and sharp p-n diode by vertically stacking monolayer tungsten diselenide (WSe₂) and few-layer molybdenum disulfide (MoS₂). The diode exhibits excellent current rectification behavior with an ideality factor of 1.2, and shows rapid photoresponse over the entire overlapping region with a maximum external quantum efficiency (EQE) of 12%. Electroluminescence (EL) studies reveal prominent band edge excitonic emission and enhanced hot electron luminescence. The device's performance is attributed to the atomically sharp heterojunction, which allows for efficient charge separation and current rectification. The study also investigates the origin of hot-electron luminescence and the nature of electron-orbital interaction in transition metal dichalcogenides (TMDs), providing insights into the fundamental electro-optical properties of TMDs. This work opens up new avenues for the development of novel optoelectronic devices, including atomically thin photodetectors, photovoltaics, and spin- and valley-polarized light-emitting diodes.