Organic Light Emitting Devices Containing High Glass Transition Temperature Hole Transport Layers

摘要

Since the demonstration of the first thin film organic light emitting device (OLED), there has been much interest in improving their performance. One of the key challenges in the further development of the technology is to meet flat panel display requirements by achieving a >10,000 hour device operational lifetime while maintaining a high (>1%) external quantum efficiency. An important device failure mechanism is the expansion of the hole transport layer (HTL) in a thermally stressed OLED . This expansion induces strain-driven failure, an effect which can be reduced by utilising a HTL with a high glass transition temperature (T_g). Here, we report the fabrication of a series of efficient OLEDs containing high T_g HTLs and their performance. Presently, a commonly used HTL is N,N'-diphenyl-N,N'-bis-α-napthylbenzidine (α- NPD)~2, with a T_g of 95℃. All of the HTLs used in this work had biphenyl backbones, like α-NPD, but utilised different amine based substituents. Both asymmetric and symmetric substitution of aryl groups on the nitrogen atom of each amine was studied, as shown in Fig.1. We incorporated the HTLs with T_g ranging from, 85℃ to 152℃, into two device structures: (i) ITO: HTL (120 nm): Au, and (ii) ITO: copper phthalocyanine (CuPc) (6 nm): HTL (35 nm): Alq_3 (40 nm): MgAg (100 nm): Ag (50 nm). Structure (i) was used to examine hole transport through the HTLs and the injection of holes from ITO since the large energy barrier at the HTL/Au interface (>2.5 eV) hindered electron injection. Structure (ii) enabled comparison of OLED performance incorporating the different HTLs with previous devices.