Enabling electron conduction in anisotropic hole transport materials for superior optical properties in organic light emitting diodes

Abstract Alignment of the emitter molecules in organic light emitting diodes (OLEDs) has attracted growing attention in recent years as it can significantly increase the device efficiency. There is continuous progress in tailoring dyes towards a higher degree of horizontal orientation with respect to the substrate plane. While most of the studies focus on the emitting layer, this report will discuss orientation in the transport layers of the diode. Alignment of the molecules within a neat film can lead to optical anisotropy. From a theoretical point of view it was shown, that birefringence can increase OLED efficiency. In this study we demonstrate this effect for actual OLED materials. We will present a strategy to modify the charge transport in a birefringent hole conductor. Doping with cesium carbonate ( Cs 2 CO 3 ) resulted in good electron conduction in the usually hole transporting material. Efficient devices were realized utilizing the doped material as electron transport layer. Optical simulations confirm the benefit for OLEDs through superior optical properties of the modified transport layer.