Low-voltage organic light-emitting diodes by doped amorphous hole transport layers

Abstract

Organic light emitting diodes generally suffer from higher operating voltages compared to inorganic ones. This limits their application in passive or active driven displays based on OLED-technology. As was previously shown by our group, p-type doping of the hole injection and transport layer of an organic light emitting diode (OLED) by co-evaporation of a matrix and an acceptor molecule leads to lower operating voltages of the device. In OLEDs using doped transport layers, the use of a proper buffer layer between the doped layer and the light emission layer is essential to yield a high current efficiency and a low operating voltage at the same time. In order to further enhance the device efficiency, we apply here the doping concept to OLEDs with a light emission layer which is doped with a fluorescent dye. This approach proves that doping of the transport layer is able to improve the optoelectronic properties of already highly efficient OLEDs. The doped hole injection and hole transport layer is a Starburst layer p-type-doped with tetrafluoro-tetracyano-quinodimethane (F 4 -TCNQ). As blocking layer, a diamine (TPD) is used. The emitter layer consists of quinacridone (QAD) doped aluminum-tris-(8-hydroxy-quinolate) (Alq 3 ). Holes are injected from untreated ITO, electrons via a lithium-fluoride (LiF)/aluminum cathode combination. For this OLED layer sequence, we achieved a luminance of 100cd/m 2 in forward direction at the lowest operating voltage reported for completely non-polymeric OLEDs (3.2-3.4V) with a current efficiency of around 10cd/A.