Opposing influence of hole blocking layer and a doped transport layer on the performance of heterostructure OLEDs

This paper reports on heterostructure small molecule organic light emitting devices (OLEDs), the design of which includes doped hole and electron transport layer (HTL and ETL) and a hole blocking layer (HBL) which can be either doped or not. Doped transport layers are expected to lower the operating voltage of devices. Insertion of a hole blocking layer increases the carrier and exciton confinement which consequently improves the recombination rate and the device efficiency. Nevertheless, an HBL tends to increase the threshold voltage. The opposing influence of doped transport layers and HBL is evidenced in this study and compromise structures are presented. The doped HTL material is N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ). A comparison with undoped TPD and poly(N-vinylcarbazole) (PVK) HTL is given. Devices with doped HTL show a lowering of the operating voltage from 6.5 (PVK) down to 4 V (these voltages refer to those necessary to achieve a luminance L = 10 Cd/m(2)). A constant current efficiency higher than 2 Cd/A is obtained in the voltage range 5-9 V with doped HTL. Insertion of a 5-20 nm thick HBL made of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (Bathocuproine BCP) between an 8-(hydroquinoline) aluminum (Alq(3)) electron transport layer (ETL) and a DCM doped Alq(3) emitting layer (EML) induces both a detrimental effect (increase of the operating voltage to 6 V attributed to a low electron mobility in BCP) and beneficial effect (strong increase of the luminance and doubling of the current efficiency, which reach to about 4.5 Cd/A thanks to improved carrier and exciton confinement). An optimization of the thickness cf the doped EML and of BCP is also reported. The doping of the HBL and of the ETL with 2-(4-biphenyl)-5-(4-ter-butylphenyl)-1,3,4-oxadiazole (PBD) leads to devices with luminance as high as 1000 Cd/m(2) at 5.3 V and a maximum efficiency of 1 Cd/A at 4 V. (C) 2003 Elsevier B.V. All rights reserved.