Effects of Cl₂ plasma treatment on stability, wettability, and electrical properties of ITO for OLEDs

We investigated various characteristics of chlorinated indium tin oxide (Cl-ITO)for highly efficient organic light-emitting diodes (OLEDs): the work function, surface morphology, wetting characteristic, and hole-injection property. Via the systematic analysis of Cl-ITO, we showed that the modification of ITO using the plasma treatment method has a simple process and offers higher stability than that achieved with a solution-based modification method. We fabricated Cl-ITO with a high work function of 6.04 eV, which is 1.04 eV higher than that of pristine ITO. We verified that Cl₂ plasma treatment does not affect the ITO surface morphology. The Cl halogenated surface showed a low surface polarity and influenced the growth of the organic material, especially for films as thin as ∼4 nm. Therefore, we found when using ultra-thin film on Cl-ITO for design devices, the wetting characteristic of Cl-ITO should be considered. In addition, using tris-(4-carbazoyl-9-ylphenyl)-amine resulted in increased current and power efficiencies compared with those obtained using 4,4′-N,N′-dicarbazolebiphenyl. When a carrier-only device was used, the carrier-injection characteristics of Cl-ITO were higher than those of other well-known hole-injection materials. Furthermore, the Cl-ITO OLEDs showed higher stability than OLEDs with other hole-injection materials.