This study presents the creation of an atomically sharp heterojunction p-n diode using vertically stacked monolayer tungsten diselenide (WSe₂) and few-layer molybdenum disulfide (MoS₂). The diode exhibits excellent current rectification with an ideality factor of 1.2 and a high external quantum efficiency of up to 12%. Photocurrent mapping shows rapid photoresponse across the entire overlapping region, while electroluminescence studies reveal prominent band edge excitonic emission and enhanced hot electron luminescence. Systematic investigations show distinct layer-number dependent emission characteristics, providing insights into the origin of hot-electron luminescence and electron-orbital interactions in TMDs. These heterojunction p-n diodes represent an interesting system for studying fundamental electro-optical properties in TMDs and can enable novel optoelectronic devices such as atomically thin photodetectors, photovoltaics, and spin- or valley-polarized light emitting diodes. The diode's performance is attributed to the atomically sharp interface and the unique electronic properties of the TMDs. The study also demonstrates the potential of these heterojunctions for exploring electron-orbital interactions and the nature of charge transport in TMDs. The results highlight the importance of the vertical heterojunction structure in achieving efficient charge separation and high optoelectronic performance. The study also shows that the heterojunction's performance is influenced by temperature, with the hot electron luminescence peaks showing a relative enhancement under certain conditions. The findings suggest that these heterojunctions could be a promising platform for fundamental research on the microscopic nature of carrier generation, recombination, and electro-optical properties in TMDs.This study presents the creation of an atomically sharp heterojunction p-n diode using vertically stacked monolayer tungsten diselenide (WSe₂) and few-layer molybdenum disulfide (MoS₂). The diode exhibits excellent current rectification with an ideality factor of 1.2 and a high external quantum efficiency of up to 12%. Photocurrent mapping shows rapid photoresponse across the entire overlapping region, while electroluminescence studies reveal prominent band edge excitonic emission and enhanced hot electron luminescence. Systematic investigations show distinct layer-number dependent emission characteristics, providing insights into the origin of hot-electron luminescence and electron-orbital interactions in TMDs. These heterojunction p-n diodes represent an interesting system for studying fundamental electro-optical properties in TMDs and can enable novel optoelectronic devices such as atomically thin photodetectors, photovoltaics, and spin- or valley-polarized light emitting diodes. The diode's performance is attributed to the atomically sharp interface and the unique electronic properties of the TMDs. The study also demonstrates the potential of these heterojunctions for exploring electron-orbital interactions and the nature of charge transport in TMDs. The results highlight the importance of the vertical heterojunction structure in achieving efficient charge separation and high optoelectronic performance. The study also shows that the heterojunction's performance is influenced by temperature, with the hot electron luminescence peaks showing a relative enhancement under certain conditions. The findings suggest that these heterojunctions could be a promising platform for fundamental research on the microscopic nature of carrier generation, recombination, and electro-optical properties in TMDs.