Sulfuric acid treatment of indium tin oxide for application of organic light-emitting diodes

摘要

In this paper, indium tin oxide (ITO) substrates were treated by ethanol, sulfuric acid (98%) and oxygen plasma respectively, based on which organic light-emitting diodes (OLEDs) with the same double-layer structure: ITO/N,N'-bis-(1-naphyl)-N,N'-diphenyl-1,1'- biphenyl-4,4'-diamine(NPB)/tris-(8-hydroxyquinoline) aluminum(Alq_3)/ Mg:Ag/Al were fabricated. The morphology and compositions of differently treated ITO films were analyzed by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). AFM results showed that all these treatments didn't change much the average roughness of ITO films, while ethanol has little influence on the large "peak-valley" (P-V) distance of ITO film; ITO film treated by sulfuric acid has suitable surface morphology with low P-V distance, low average roughness and small particle radius. XPS analysis proved that ethanol has the weakest ability to remove carbon contamination on the surface of ITO film which resulted in unevenly distributed current and high barriers for hole injection; sulfuric-acid-treated ITO film has the least contamination of carbon and higher concentration of oxygen vacancies, both of which are helpful for the improvement of devices. In the condition of room temperature and atmosphere and without encapsulation, measurements of the current-voltage, brightness-voltage and lifetime characteristics of these devices were conducted and showed good agreement with what expected from the results of AFM and XPS analysis: the ethanol-treated ITO provides very unstable performance with low efficiency for the device based on it, while the device based on sulfuric-acid-treated ITO anode had even better performance at higher driven voltage ( > 7V) comparing with the device based on oxygen-plasma-treated ITO substrate and had the highest current-luminance efficiency (3.2 cd/A) and the longest lifetime (2 hours).