Improved charge balance in phosphorescent organic light-emitting diodes by different ultraviolet ozone treatments on indium tin oxide

Abstract

Ultraviolet ozone (UVO) treatment of indium tin oxide (ITO) is a well-known technique to realize improved performance in organic light-emitting diodes (OLEDs). Herein, we report the comparison of UVO treatments on ITO at different energies (187 and 254 nm) in varying the charge balance in green phosphorescent OLEDs (PhOLEDs). The amounts and types of removed organic contaminants (such as -OH, C-C, C = C, C = C, 0 = 0, etc.) were observed to differ for each UVO energy. The 254-nm UVO treatment could not remove all the contaminants from the ITO surface, causing a decrease in hole injection (observed by hole-only device trends), due to the decrease in work function(WF)compared to the 187-nm-treated ITO. Moreover, the presence of large negatively charged O = O groups on the surface of 254-nm-treated ITO induced electronic repulsion (observed by electrononly device trends). However, remarkably, a suitable charge balance was attained by ITO treated with 254-nm UVO source over un-treated ITO, as well as with samples exposed to the higher-energy source (187 nm). The devices utilizing ITO treated with 254-nm UVO demonstrated the enhanced current efficiency of 67 cd/A, while the 187-nm UVO-treated and untreated ITO anodes exhibit those of 47 cd/A and 57 cd/A, respectively, at 5000 cd/m(2). The variation in the WF of ITO with respect to different UVO sources and its role in the charge transport behaviors of the Ph-OLEDs were investigated in detail with contact angle measurements and interface barrier calculations through hole-only devices.