Journal of the Korean Physical Society

pISSN 0374-4884 eISSN 1976-8524


Condensed Matter

J. Korean Phy. Soc. 2011; 59(5): 3060-3063

Published online November 15, 2011

Copyright © The Korean Physical Society.

Interface Electronic Structure of a Strongly Electron Withdrawing Molecule on an Indium-tin-oxide Surface

Hyeseung Kang, Ji-Hoon Kim, Jeong-Kyu Kim, Jaewon Seo, Yongsup Park


Using in-situ X-ray and UV photoemission spectroscopy (XPS and UPS, respectively), we have investigated the electronic energy level alignment at the interface formed between a strongly electron withdrawing molecule 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) and indium-tin-oxide (ITO). The HAT-CN is an effective material in organic light emitting diodes for hole injection layer and ITO is widely used as an anode electrode. The deposition of a small amount of HAT-CN caused surface work function reduction from 4.46 eV to 4.12 eV at the coverage of about 0.02 nm. However, when more HAT-CN was deposited the work function started to increase and end up at 5.95 eV at the thickness of 90 nm. The UPS spectra shows that the highest occupied molecular orbital was at 3.80 eV below the Fermi level at this thickness. The XPS analysis of carbon C 1s core level peak implies that the HAT-CN molecular layer does not completely cover the ITO surface at the initial stages of deposition. The measured work function and energy level alignment indicate that the holes can not be injected from ITO to HAT-CN layer. Instead, it is suggested that electron-hole pairs are generated at the organic/HAT-CN interface.

Keywords: OLED, Interface, Electron energy level, Organic semiconductor, Photoemission spectroscopy