New charge-carrier blocking materials for high efficiency OLEDs

Three strategies for preparing high efficiency OLEDs are demonstrated, which involve the use of hole and electron blocking layers. The first of these strategies involves the use of a cyclometallated iridium compound (bis(2-(4,6-difluorophenyl)pyridyl-N,C2')iridium(III) picolinate, FIrpic) as a hole-blocking material for green and blue emissive OLEDs. Devices which utilized FIrpic as a combined hole blocking and electron transporting layer gave external quantum efficiencies > 14% (device structure: anode/HTL/EL/FIrpic/cathode, HTL = hole transport layer, EL = emissive layer). When the FIrpic layer of this device was replaced with bathocuproine (BCP), the device efficiency dropped to 12%. A host-guest approach to the formation of a hole blocking layer (HBL) has also been demonstrated. FIrpic was doped into two different wide energy band-gap organic matrix materials (i.e. octaphenyl-cyclooctatetraene, OPCOT, and 1,3,5-tris-phenyl-2-(4-biphneyl)benzene, SC5) forming a mixed HBL. Devices with doped OPCOT gave quantum efficiencies comparable to those with a BCP HBL, while the SC5 based devices gave higher efficiency than their BCP blocked counterparts. When blue electrophosphorescent devices are prepared in a conventional OLED structure (i.e. anode/HTL/EL/HBL/ETL/cathode), excessive HTL emission is often observed, resulting from electron leakage from the doped CBP layer into the HTL. This electron leakage can be eliminated by inserting an electron blocking layer (EBL) between the HTL and luminescent layers. Both fac-tris(1-phenylpyrazolato,N,C2')iridium(III) (Irppz) and Iridium(III) bis(1-phenylpyrazolato,N,C2')(2,2,6,6-tetramethyl-3,5-heptanedionato-O,O) have been used as efficient EBLs. The insertion of an EBL leads to both improved color purity and quantum efficiency, relative to devices without EBLs. For example, a white emitting device with the structure ITO/HTL/EL/HBL/ETL/LiF/Al gave an external efficiency of 1.9% and nearly exclusively HTL emission. Addition of a 100 Angstrom Irppz layer between the HTL and EL gave a device with an external quantum efficiency of 3.3% and electroluminescence from only the EL. (C) 2003 Elsevier B.V. All rights reserved.