Preferred Orientation Evolution of Hole Transport Materials for High Emitting Dipole Orientation Ratio of the Emitting Material

Abstract Control of the molecular orientation is an effective approach to enhance the performance of organic light‐emitting diodes. This study examines the molecular dipole orientation of hole transport materials depending on the deposition step and the consecutive effect on the transition dipole orientation of the emitting dopant. In contrast to other reference materials, 4,4′,4′′‐tris(carbazole‐9‐yl)triphenylamine (TCTA) shows a preferred orientation only in the continuously deposited films that is dependent on the film thickness. The horizontal dipole orientation ratio of the emitting dopant deposited on the TCTA is improved by the alignment of TCTA molecules. The power efficiency of the device produced from the above materials is increased from 28.9 to 34.4 lm W⁻¹ owing to the dual enhancement of the hole mobility of TCTA and the horizontal dipole orientation ratio of the emitting dopant. The molecular orientation of the charge transport layer contributes to charge transport in the layer and the molecular orientation of the adjacent emitting layer.