An OLED has a doped emission layer (EML). The OLED comprises a first electrode, a second electrode, and an EML having a host material and a light-emitting dopant and interposed between the first electrode and the second electrode, wherein the light-emitting dopant has a concentration gradient between a first surface close to the first electrode of the EML and a second surface opposite to the first surface. As a result, luminous efficiency of the OLED can be enhanced and a driving voltage can also be reduced.