Performance Enhancement of Organic Light-Emitting Diodes by Electronic Doping

Abstract

Since the invention of organic light-emitting diodes (OLEDs) nearly 40 years ago, significant effort has been put into realizing their full potential. OLEDs exhibit several properties that make them ideal candidates for applications in displays as well as solid-state lighting: low power consumption, high contrast ratios, mechanical flexibility, and wide viewing angles. However, the low electrical conductivity and poor stability of organic materials remain critical factors that limit device performance. The focus of this work is OLED performance improvement through the introduction of novel inorganic dopants into organic charge transport layers. The results show a significant reduction in operating voltage and increase in reliability for devices with doped charge transport layers compared to those without. Further, with sufficiently high doping concentrations, it is demonstrated that device structure can be dramatically simplified. Electron-only devices (EODs) were fabricated using three different electron-transport materials: Alq3, BPhen, and TPBi to investigate the effects of Ca doping via co-evaporation. It was demonstrated that only the characteristics of the BPhen-based EOD were improved. The improvement suggests facile electron transfer from the Ca dopant to the BPhen matrix due to the low-lying LUMO level of BPhen. Despite the formation of gap states, increasing the Ca concentration up to 11.5 wt% shows a monotonic trend of decreasing operating voltage. It was also shown that above 4.1 wt% Ca the energy barrier between the cathode and electron-transport layer was sufficiently reduced to allow for the removal of the LiF electron injection layer (EIL) without any negative effect on device performance. Blue OLEDs with and without Ca in the BPhen electron-transport layer (ETL) were fabricated. The doped OLEDs showed lower operating voltage and higher luminance compared to the undoped OLEDs. While the best electrical characteristics were observed when the entire ETL was doped, it caused significant exciton quenching and reduced current efficiency. This effect was reduced by introducing an undoped BPhen spacer between the emissive layer (EML) and doped ETL. In both cases, current stressing showed that Ca is a stable dopant in BPhen. CBP homojunction devices were fabricated, with the ambipolar CBP matrix material doped p-type with MoO3 in one side, n-type with Ca in the other side, and with a BCzVBi blue emitter in the middle EML to produce blue light emission. Both hole-only devices (HODs) and EODs showed monotonic improvement as doping concentration increased, indicating that CBP was successfully doped p-type and n-type. Above 10 wt% MoO3