Surface oxidation activates indium tin oxide for hole injection

Abstract

The relationship between the surface chemistry of in- dium tin oxide ~ITO! and the injection of carriers into or- ganic hole transport materials ~HTMs! is of paramount im- portance for the development of organic light-emitting devices ~OLEDs!. Although a multitude of explanations has been put forth to account for the behavior of variously ''ac- tivated'' ITO-based OLEDs, none has yet rationalized these ostensibly disparate treatments into a single, cohesive, sur- face chemical model. We now report an experimental insight regarding charge injection at the interface between ITO and typical HTMs used in high efficiency OLEDs, and we show that many of the reported experimental observations of en- hanced injection by chemically modified ITO can be ex- plained on the basis of a single surface modification reaction. Juxtaposing analysis of the chemistry and electronic structure of the ITO surface with direct interface transport measurements has been especially revealing. By direct mea- surement using ultraviolet and x-ray photoelectron spec- troscopies ~UPS; XPS!, we have shown that oxygen plasma treatment ~a common activation technique for ITO! generates a new type of oxygen species at the ITO surface. Further- more, we have shown that factors such as the In:Sn or @In, Sn#:O ratios at the surface, the presence of metallic Sn or In, 1 or the degree of surface hydroxylation, each of which has been suggested to account for differences in the measured work function of ITO prepared under various conditions, can be excluded as the rationale for the large increase in work function measured upon mild ITO oxygen plasma treatment; rather, this increase is ascribed to an increased surface layer dipole due to the formation of this new oxygen species.