Improving organic light-emitting devices by modifying indium tin oxide anode with an ultrathin tetrahedral amorphous carbon film

The characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminum with an ultrathin tetrahedral amorphous carbon (ta-C) film on indium tin oxide have been investigated. The device with a 1.0-nm ta-C layer has the highest current and power efficiency. The current efficiency of a device with a ta-C layer thickness of 1.0 nm is 3.7 cdA at 20 mA cm2, however, the current efficiency of a standard device without a ta-C layer is 2.56 cdA at the same current density. The current efficiency is improved by 46% compared to the standard device. Although there is a 1.2-V increase in driving voltage for 100-cd m2 luminance, the power efficiency is still improved by 22% compared to that of the standard device. The improvement of the efficiency is due to smoothing indium tin oxide surface, blocking hole injection from anode and balancing hole and electron currents. The optimal thickness of ta-C layer for hole injection mechanics can be understood by tunneling.