We report the enhancement of electron injection using a magnesium (Mg)-doped tris-(8-hydroxyquinoline) aluminum (Alq 3 ) layer between the Al and Alq 3 layer in flexible organic light emitting diodes (OLEDs). The turn-on voltage decreased from 13 to 11 V and the luminance increased from 800 to 3200 cd/m 2 at 15 V as Mg is doped by 5% to Alq 3 layer. In situ measurements of synchrotron radiation photoelectron spectroscopy revealed that the Fermi level moved about 0.45 eV toward the lowest unoccupied molecular orbital by doping Mg to Alq 3 , reducing the energy barrier for electrons via promotion of the electrical and optical properties.