Continuous modulation of electrode work function with mixed self-assembled monolayers and its effect in charge injection.

Self-assembled monolayers (SAMs) of binary mixtures of n-decanethiol and the fluorinated analogue (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol) were formed on silver surface. The film structure was characterized by reflection absorption IR and XPS to be a homogeneous mixture of the two components. The mixed monolayers serve to tune the work function of silver over a wide range by varying the surface composition of the mixed monolayer from 4.1 to 5.8 eV. The mixed SAM-modified Ag surfaces were used as the anode in the fabrication of hole-only devices with the device structure Ag/SAM/HTL/Ag, where HTL represents a hole-transporting layer. It is shown that depending on the HTL used and thus the HOMO level involved, the maximum current injection into the device occurred with differently modified Ag. Top-emitting organic light-emitting diodes fabricated with differently modified silver electrodes showed that the maximum current and maximum luminance efficiency occur at anodes of different modifications due to a change in the hole-electron charge balance.