Horizontal orientation of disk-like hole transport molecules and their application for organic light-emitting diodes requiring a lower driving voltage

Novel starburst type amorphous materials N ¹,N ¹,N ⁴,N ¹-tetra(biphenyl-4-yl)benzene-1,4-diamine, N ¹,N ¹,N ¹,N ¹-tetrakis(4-(thiophen-2- yl)phenyl)benzene-1,4-diamine, and N ¹,N ¹,N ¹,N ¹-tetrakis(4-(benzo[b]thiophen-2-yl)phenyl)benzene-1,4- diamine are synthesized and applied as hole injection layers (HILs) in organic light-emitting diodes (OLEDs). OLEDs containing these materials as a HIL demonstrated a significant reduction in the required driving voltage. We found that these molecules form a horizontally oriented amorphous thin film upon vacuum deposition. We observed a close correlation between the reduction of driving voltage in OLEDs and the orientation parameter of the organic thin films. We identified that the enhanced horizontal molecular orientation results in the lower driving voltage because of a decrease in the hole injection barrier at the anode/HIL interface.