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

Abstract

We investigated the electroluminescence (EL) performance of organic light emitting diodes having a thick doped hole transport layer [(DHTL):650 nm–1.5 μm]. The basic cell structure is an anode/DHTL/hole transport layer [(HTL):50–60 nm]/emitter layer [(EML):50–60 nm]/cathode. We examined various combinations of host polymers and guest molecules as a component of DHTL in this 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 μm thick), which resulted in excellent EL performance. The thick DHTL not only showed considerable reduction in cell resistance compared with a conventional anode/DHTL (without doping)/HTL/EML/cathode device with the same thicknesses of the organic layers, but also greatly contributed to the enhancement of the device stability, particularly to pinhole problems that can occur with 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 efficiency (ΦEL) was low, because of the serious exciton energy transfer and/or electron migration from EML to DHTL where the PC–TPD–DEG:TBAHA complex 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 (α-NPD) layer between DHTL and EML, while keeping the driving voltage low. With the DHTL (650 nm, 10 wt % of TBAHA) showed a luminance of 4004 cd/m2 at 10.0 V and 220 mA/cm2, of which the performance was comparable with that of typical thin film devices. Furthermore, we could expand the DHTL thickness up to 1.5 μm. An indium tin oxide (ITO)/DHTL (10 wt %)(1.5 μm)/α-NPD (60 nm)/Alq (60 nm)/MgAg device showed a luminance of 2600 cd/m2 at 18.0 V and 210 mA/cm2 with enhanced duration stability. In addition, the duration properties of the devices were also examined in the device structure of an ITO/DHTL (650 nm)/α-NPD (60 nm)/Alq(doped with rubrene 4.2 wt %) (60 nm)/MgAg. The half decay of the initial luminance successively exceeded over 1000 h under a constant current density of 10 mA/cm2.