Surface-Modified Zinc Oxide Nanoparticles for Electron Injection Layers in Organic Light-Emitting Devices

We report the application of two surface modifiers, 2-(2-hydroxyphenyl)benzothiazole (BTz) and 8-quinolinol (8Qn), to electron-transporting layers (ETLs) comprising zinc oxide nanoparticles (ZnO NPs) to improve the efficiency of electron injection into light-emitting layers in organic light-emitting devices (OLEDs). BTz and 8Qn were respectively added to ZnO NP dispersions; these surface modifiers then reacted with the ZnO NPs and changed their optical properties, proving the interaction between the ZnO NPs and modifiers. The modified ZnO NPs were employed as ETLs in fluorescent polymer-based OLEDs. The BTz-device showed a slightly lower driving voltage and a slightly higher luminous efficiency than the unmodified ZnO device, whereas the 8Qn-device showed a much lower driving voltage and a much higher luminous efficiency than the unmodified ZnO device. The device driving stability was evaluated, and both of the modified devices showed improved lifetimes compared with the unmodified device. In particular, a more than five-fold improvement in lifetime was obtained for the BTz-device.