Insertion of hole-blocking layer (HBL) between the hole-transporting layer (HTL) and hole-injection layer for tris-(8-hydroxyquinoline) aluminum (Alq3)-based organic light-emitting device is shown to improve the current efficiency by 30%. The improvement was attributed to the better electron-hole balance in the device. Two different organic materials, 2,9-dimethyl-4, 7-diphenylphenanthroline (BCP) and Alq3 were used as HBL. Variation of HBL thickness was shown to adjust the current efficiency with the optimum thickness of 3 nm and 2 nm for BCP and Alq3, respectively. Reduction of HTL thickness was observed to reduce the operating voltage. For device with BCP HBL, the current efficiency is independent of the HTL thickness, optimum performance was observed for device with BCP and HTL thicknesses of 3 nm and 5 nm, respectively. However for device with Alq3 HBL the efficiency drops with reduction of HTL. The drop is due to electron leakage, as Alq3 HBL does not block the electron, whereby the inefficient exciplex emission was observed