Blue organic electroluminescent devices with a novel structure by using a new hole transporting material have been fabricated. The structure is ITO/CuPc/J0503/JBEM:perylene/TPBi/Alq

3

/LiF/Al

in which a (hole-blocking) layer was inserted between electron-transporting layer and emitting layer. For comparison

devices with a conventional structure have also been made

and the structure is ITO/CuPc/J0503/JBEM:perylene/Alq

3

/LiF/Al. Here CuPc(Copper phthalocyanine)and LiF are hole and electron injection layers

respectively

J0503 hole-transporting layer(HTL)

JBEM(9

10-bis(3’5’-diaryl)phenyl anthracene)emitting-layer(EML)

TPBi(1

3

5-tri(pheny1-2-benzimidazole)-benzene)hole-blocking layer(HBL)

and Alq

3

(tris(8-quinolinolato)aluminium complex)electron-transporting layer(ETL). Devices with different EML thickness of each structure have also been prepared in order to investigate the effect of EML thickness on the performance. The results indicate that the devices with the novel structure show improved efficiency and luminance as compared to the conventional ones. However

the improvements depend on the thickness of EML. Small differences are found in both structures with the EML thickness larger than the diffusion length of excitons because the blocking-effect from HBL has little effect on the increasing density of excitons.While the novel devices with thickness approximate to the diffusion length of excitons show much higher efficiency and luminance than that of the conventional ones having the same EML thickness because of the good confinement to both charge carriers and excitons. When the thickness of EML is too small

probably

strong interface effect leads to break the performances of the novel devices.