Non-polymeric OLEDs with a doped amorphous hole transport layer and operating voltages down to 3.2 V to achieve 100 cd/m2

We have previously shown that p-type doping of the hole injection and transport layer of an organic light emitting diode (OLED) by co-evaporation of two molecules leads to lower operating voltages of the device. In such OLEDs, the use of a proper buffer layer between the doped layer and the light emission layer is essential to yield a high efficiency. Here, we apply the doping concept to OLEDs with a light emission layer doped with a fluorescent dye in order to prove that doping of the transport layer is able to improve the optoelectronic properties of highly efficient OLEDs. The emitter layer consists of quinacridone (QAD) doped aluminum-tris-(8-hydroxy-quinolate) (Alq3). The hole injection and transport layer is a Starburst layer p-type-doped with tetrafluoro-tetracyano-quinodimethane (F4-TCNQ). As buffer layer, a diamine layer (TPD) is used. 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 100 cd/m2 in forward direction at the lowest operating voltage ever reported for non-polymeric OLEDs (3.2–3.4 V) with a current efficiency of around 10 cd/A.