Investigation of improving organic light-emitting diodes efficiency using an ultra-thin ultraviolet-ozone-treated Nb-doped ZnO film as anode buffer layer

Abstract

In this study, ultraviolet (UV)-ozone treated ultrathin Nb-doped ZnO (NZO) films are developed as an efficient anode buffer layer to improve the overall performance of organic light-emitting diodes (OLEDs). The results show that the UV-ozone treated NZO buffer layer containing 1 mol% of Nb2O5 possesses a higher oxygen content and a greater work function (5.22 eV) as compared to that of an indium tin oxide (ITO) film (~4.7 eV) signifying a reduction in hole injection barrier and thus an improvement in the injection efficiency. In addition, UV-ozone treatment can increase the surface energy of NZO films while reducing their surface roughness. Importantly, the UV-ozone treated 1 nm-thick NZO film with a Nb2O5 doping concentration of 1 mol% can help to lower the turn-on voltage from 3.2 V to 2.8 V, increase the luminance from 10,450 cd/m2 to 25370 cd/m2, and improve the current efficiency from 3.46 cd/A to 5.26 cd/A, (~52 % enhancement as compared to the standard OLED device). Moreover, OLEDs with the developed buffer layer reveal a significant improvement in the roll-off phenomenon under high current densities, indicating a key role of the optimized NZO film in enhancing the carrier balance of the devices. When applied to the p-i-n structure, the NZO film also lead to a superior device performance as compared to the conventional p-i-n structure using NPB:MoO3 as a hole injection layer, suggesting a widespread use of the developed thin film. These findings therefore show a promising use of the UV-ozone treated NZO ultrathin film as an effective anode buffer material for enhancing OLED overall performance.