Preserving the work function of Ultra-Violet-ozone treated indium tin oxide by triarylamine-based small molecule modification for solution-processed organic light-emitting diodes with increased external quantum efficiency

Abstract

UV-ozone treatment is one of the most common ways to increase the work function of indium tin oxide (ITO), which is used as the transparent conducting anode in organic light-emitting diodes (OLEDs). However, the work function increase is time sensitive when the samples are left or processed in air, often returning to a similar value to that measured before UV-ozone treatment. We found that for OLEDs formed by solution processing and containing a poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole injection layer that there was no advantage in terms of device efficiency of using UV-ozone treated ITO. However, if a triarylamine-based small molecule [N1,N3,N5-tris(4-n-butylphenyl)-N1,N3,N5-triphenylbenzene-1,3,5-triamine, 4-n-BTDAB] was introduced onto the UV-ozone treated ITO then the work function was stabilised. When 4-n-BTDAB was introduced between UV-ozone treated ITO and PEDOT:PSS, the resultant solution processed OLEDs were found to have a maximum external quantum efficiency ≈20% higher (corresponding to an absolute increase of ≈2% to around 12%) compared to devices with the same structure but without the triarylamine layer.