A hole-blocking layer (HBL) of 4,7-diphenyl-1,10-phenanthroline (BPhen) is incorporated between the emitting layer (EML) and the electron transport layer (ETL) for a tris-(8-hydroxyqunoline)aluminum based organic light-emitting device (OLED). Such a structure helps to reduce the hole-leakage to the cathode, resulting in an improved current efficiency. The BPhen improves the balance of hole and electron injections. The current efficiency is improved compared with that of the device without the blocking layer. The highest luminous efficiency of the device with 6 nm BPhen acting as a blocking layer is 3.44 cd/A at 8 V, which is improved by nearly 1.5 times as compared with that of the device without it.