We studied the hole mobility of molecularly doped hole transport layer (HTL), 4,4'-bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (α-NPD), as a function of the doping concentration of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) by employing the time-of-flight photoconductivity (TOF-PC) technique. The hole transport is non-dispersive for α-NPD and the hole mobility of pristine α-NPD is about 10-3 cm2/Vs at room temperature. However, the hole mobility decreases with the BCP doping concentration in α-NPD. We characterized the current-voltage-luminance dependence, the EL quantum efficiency, and transient EL response for the devices of ITO/doped α-NPD/Alq3/LiF/Al. The devices with the BCP doped α-NPD show higher EL efficiency compared with the device with pristine α-NPD. The reduced hole mobility in the BCP doped α-NPD enhances the electron-hole balance, resulting in an increased EL efficiency.