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 emittin g layer (EML) and the electron transport layer (ETL) to confine charge reco mbination within the EML. We employed an aluminum complex that has mixed li gands as an HBL and a family of aromatic diamines as an EML. Aromatic diami nes 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/m(2). To improve the performance of this blue-emitting device, novel blue dopants ar e investigated. Some of the dopants are effective to improve EL characteris tics of the PPD-based device. Furthermore, it was quite straightforward to modify the blue device with orange or yellow dopants, leading to a white-em itting device. (C) 2000 Elsevier Science S.A. All rights reserved.