Surface treatments of indium-tin-oxide substrates: Comprehensive investigation of mechanical, chemical, thermal, and plasma treatments

Abstract

Various surface treatments significantly affect the work function and surface roughness of indium tin oxide (ITO), and thusly influence charge injection and overall performance of organic light emitting diodes (OLEDs). Large number of treatments, most commonly oxygen plasma treatment and UV-ozone treatment, have been proposed to improve characteristics of ITO. In this work, we have investigated a) mechanical treatments (mechanical rubbing, followed by ultrasonic bath), b) chemical treatments (dipping into aqueous solutions of various acids, including acids which have not been investigated previously) c) thermal treatments (thermal annealing in different atmospheres) d) plasma treatments e) UV ozone treatment f) different combinations of the above. We have measured surface sheet resistance of the samples and investigated surface morphology of the treated samples and compared them to "as-received" samples. We have selected several treatments giving best results. Then we have fabricated OLEDs using ITO substrates treated with treatments selected, as well as a control OLED fabricated on "as-received" ITO. The impact of ITO treatments on the performance of OLEDs have been investigated on two types of devices, OLEDs with and without transport layer, having the structures glass/ITO/Alq 3/Al and glass/ITOFrPD/Alq 3/Al, respectively, where Alq 3 (tris-(8-hydroxyquinoline) aluminum) is emitting layer and TPD (N,N′-diphenyl-N,N′-bis(3-methyl-phenyl)-1, 1′ biphenil-4,4′diamine) is a hole transport layer.