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

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 (Ph-OLEDs). The amounts and types of removed organic contaminants (such as -OH, C-C, C≡C, C=C, O=O, 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 electron-only 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/m². 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.