Performance enhancement of organic light-emitting diodes using fluorinated self-assembled monolayer

Abstract

In order to enhance the performance of organic light-emitting diodes, fluorinated self-assembled monolayer (F-SAM) with long alkyl chain was introduced to the device between the anode and hole-transport layer. CF3-terminated F-SAM has a relatively high electric dipole moment, which may affect hole injection at the indium-tin-oxide (ITO) anode. Two kinds of F-SAM with different alkyl chain lengths (F3-SAM and F10-SAM) were formed on the ITO substrate through hydrolysis and dehydration process. Electric dipole moments of these F-SAMs were obtained from a simulation using Gauss View program. A device was made in a structure of ITO (170nm) / F-SAM / TPD (40nm) / Alq(3) (60nm) / Al (100nm). Formation of the SAM was confirmed by contact-angle measurements. It was found that the contact angles for the F3-SAM and F10-SAM are about 84 degrees and 112 degrees, respectively. A device with properly treated self-assembled monolayer at the anode gives an improvement in turn-on voltage, luminance, and efficiency compared to those of the device without SAM. Turn-on voltage was lowered from 4.25V to 3.50V in the device with SAM compared to the one without SAM, which indicates a reduction of hole-injection barrier height due to the electric dipole moment of the F-SAM. An external quantum efficiency of the device with F3-SAM was increased by a factor of about 3.5 compared to the one without SAM.