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

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 indiumtin-oxide (ITO). The HAT-CN is an effective material for a hole injection layer in organic light emitting diodes, and ITO is widely used as an anode electrode. The deposition of a small amount of HAT-CN caused a surface work function reduction from 4.46 eV to 4.12 eV at a coverage of about 0.02 nm. However, when more HAT-CN was deposited, the work function started to increase and ends up at 5.95 eV at a thickness of 90 nm. The UPS spectra show that the highest occupied molecular orbital was at 3.80 eV below the Fermi level at this thickness. The XPS analysis of the carbon C 1s core level peak implied that the HAT-CN molecular layer did not completely cover the ITO surface at the initial stages of deposition. The measured work function and energy level alignment indicated that the holes could not be injected from the ITO to the HAT-CN layer. Instead, we suggest that electron-hole pairs were generated at the organic/HAT-CN interface.