Enhancement of the Luminance Efficiency due to an Embedded Self-Assembled Monolayer in Organic Light-Emitting Devices

Abstract

The electrical and optical properties of organic light-emitting devices (OLEDs) with and without a self-assembled monolayer (SAM) were investigated, and the effect of an embedded SAM was clarified. The contact angles of COOH-terminated alkanethiol SAMs were smaller than those of CH3-terminated alkanethiol SAMs. The operating voltage of OLEDs with COOH-terminated alkanethiol SAMs was 4.4 V lower than that with CH3-terminated alkanethiol SAMs. The luminance of OLEDs with COOH-terminated alkanethiol SAMs was higher than that with CH3-terminated alkanethiol SAMs. The enhancement of the luminance efficiency of OLEDs with COOH-terminated alkanethiol SAMs was attributed to the increase in hole injection due to a decrease in the chain length of the COOH-terminated alkanethiols, resulting in a better balance between the numbers of holes and electrons at an emitting layer in the OLEDs.