Electroluminescence of organic light emitting diodes with a thick hole transport layer composed of a triphenylamine based polymer doped with an antimonium compound

We investigated the electroluminescence (EL) performance of organic light e mitting diodes having a thick doped hole transport layer [(DHTL):650 nm-1.5 mu m]. The basic cell structure is an anode/DHTL/hole transport layer [(HT L):50-60 nm]/emitter layer [(EML):50-60 nm]/cathode. We examined various co mbinations of host polymers and guest molecules as a component of DHTL in t his device structure. During the course of the materials' search, we found that the best combination of a hole transport polycarbonate polymer (PC-TPD -DEG) and a tris (4-bromophenyl) aminium hexachroloantimonate (TBAHA) as a dopant enabled us to form a uniform thick DHTL (typically 650 nm-1.5 mu m t hick), which resulted in excellent EL performance. The thick DHTL not only showed considerable reduction in cell resistance compared with a convention al anode/DHTL (without doping)/HTL/EML/cathode device with the same thickne sses of the organic layers, but also greatly contributed to the enhancement of the device stability, particularly to pinhole problems that can occur w ith conventional 100-nm-thick thin devices. Furthermore, the interposed HTL between DHTL and EML was confirmed to function not only as a HTL but also as electron and exciton blocking layers. Without the HTL, the EL quantum ef ficiency (Phi(EL)) was low, because of the serious exciton energy transfer and/or electron migration from EML to DHTL where the PC-TPD-DEG:TBAHA compl ex layer had absorption at around 485 nm based on a charge transfer complex between them. We could increase it by interposing a thin transparent N,N'- diphenyl-N,N'-bis(3-methyl phenyl)-1,1'-biphenyl-4,4'-diamine or 4,4'-bis[N -(1-naphthyl)-N-phenyl-amino] biphenyl (alpha-NPD) layer between DHTL and E ML, while keeping the driving voltage low. With the DHTL (650 nm, 10 wt % o f TBAHA) showed a luminance of 4004 cd/m(2) at 10.0 V and 220 mA/cm(2), of which the performance was comparable with that of typical thin film devices . Furthermore, we could expand the DHTL thickness up to 1.5 mu m. An indium tin oxide (ITO)/DHTL (10 wt %)(1.5 mu m)/alpha-NPD (60 nm)/Alq (60 nm)/MgA g device showed a luminance of 2600 cd/m(2) at 18.0 V and 210 mA/cm(2) with enhanced duration stability. In addition, the duration properties of the d evices were also examined in the device structure of an ITO/DHTL (650 nm)/a lpha-NPD (60 nm)/Alq(doped with rubrene 4.2 wt %) (60 nm)/MgAg. The half de cay of the initial luminance successively exceeded over 1000 h under a cons tant current density of 10 mA/cm(2). (C) 1999 American Institute of Physics . [S0021-8979(99)06020-X].