A MoSe2 p-n diode with a van der Waals homojunction is demonstrated by stacking undoped (n-type) and Nb-doped (p-type) semiconducting MoSe2 synthesized by chemical vapor transport for Nb substitutional doping. The p-n diode reveals an ideality factor of approximate to 1.0 and a high external quantum efficiency (approximate to 52%), which increases in response to light intensity due to the negligible recombination rate at the clean homojunction interface.