Effect of the crystallinity of indium tin oxide on the charge transfer at the interfaces and the performances of flexible organic light emitting diodes.

In this study, we investigated the factors that determine the efficiency of flexible organic light emitting diodes (f-OLEDs) in relation to transparent conductive electrodes (TCE). Indium tin oxide (ITO) was selected as the TCE and was deposited on polyimide substrates. Controlling the sputtering conditions, ITOs with various degrees of crystallinity were made. Flexible OLEDs (f-OLED) were fabricated on the ITOs, and the current density ( J)-luminescence ( L)-voltage ( V) characteristics of the f-OLEDs were evaluated. The experimental results revealed that the sheet carrier density and surface roughness of ITO are significantly increased and reduced, respectively, as the film crystallinity increased over a certain degree. Based on our X-ray photoelectron spectroscopy (XPS) analysis, chemical bonds formed between the mostly crystallized ITO and a hole transfer layer (HTL) in f-OLEDs, deposited on ITO. To the contrary, In and Sn of the less crystallized ITO diffused into HTL with no formation of interfacial bonds, which may become barriers to carrier flows through HTL. The f-OLED on the mostly crystalline ITO showed significantly better J- L- V characteristics than on the less crystallized ITO, which proved that the crystallinity of ITO is the critical factor that determines the performance of the flexible organic devices constructed on the ITO. [ABSTRACT FROM AUTHOR]