Electrical Properties of Organic Light-Emitting Diodes with an Insertion of CMTS Self-Assembled Monolayers

Abstract

Electrical properties of organic light-emitting diodes were studied in a device with an insertion of chloromethyl-trichlorosilane self-assembled monolayer (SAM) between the ITO anode and organic layer. Self-assembled monolayer is generally used for a surface reformation of ITO substrate to enhance the charge injection at the anode/organic interface. For a formation of SAM onto the ITO anode, the ITO substrate was immersed in a 20 mM solution of SAM material. SAM-treatment time on the ITO substrate was varied to be 0, 10, 15, 20, and 25 minutes. And then, the device was made in a structure of ITO(170 nm)/SAM/TPD(50 nm)/Alq₃(70 nm)/LiF(0.5 nm)/Al(100 nm). A device with properly treated self-assembled monolayer at the anode/organic interface gives an improvement in turn-on voltage, luminance, and efficiency compared to the one without SAM layer. A current efficiency and an external quantum efficiency of the device with 15 minutes treated SAM layer were increased by 2.6 times compared to the ones without the SAM layer. Fowler-Nordheim tunneling conduction mechanism was applied to the current density-voltage characteristics to obtain a hole injection barrier height. It was found that there is a reduction in barrier height by 25% for the device with 15 minutes SAM-treated compared to the one without SAM-treated.

Keywords:

• Fowler-nordheim tunneling
• ITO
• organic light-emitting diodes
• self-assembled monolayer

Acknowledgment

This work was supported by 2009 Hongik University Research Fund.