Blue-emitting organic EL devices with a hole blocking layer

We found that a hole blocking layer (HBL) is essentially needed to develop a new type of blue-emitting device. The HBL is inserted between the emitting layer (EML) and the electron transport layer (ETL) to confine charge recombination within the EML. We employed an aluminum complex that has mixed ligands as an HBL and a family of aromatic diamines as an EML. Aromatic diamines such as PPD exhibit strong fluorescence in the blue region. The EL peak maximum was at 455 nm with a CIE coordinate of (x=0.176, y=0.195). The luminous efficiency of the undoped device was 0.8 lm/W at 100 cd/m2. To improve the performance of this blue-emitting device, novel blue dopants are investigated. Some of the dopants are effective to improve EL characteristics of the PPD-based device. Furthermore, it was quite straightforward to modify the blue device with orange or yellow dopants, leading to a white-emitting device.