Role of ZnO nanoparticle-layers in enhancement of the performance of organic light-emitting diodes on plastics

We investigate roles of ZnO nanoparticle (NP)-layers in the enhancement of the emission from organic light-emitting diodes (OLEDs) with amorphous ZnO-doped In₂O₃ (a -IZO)/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/poly[(9,9-di-n -octylfluorenyl-2,7-diyl)-alt -(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT)/ZnO NP-layer/LiF/Al structure on plastic substrates. Compared to a Reference OLED without any ZnO NP-layer, the electroluminescence peak intensity of the OLED employing the thermally-annealed ZnO NP-layer spin-coated one time is remarkably enhanced by ∼283% at a wavelength of ∼535 nm owing to the effective hole blocking and good electron transporting properties of the ZnO NP-layer between the organic emissive layer (EML) and the electron injection layer (EIL). This enhancement is associated with the high junction potential barrier between the highest occupied molecular orbital level of the F8BT and the valence band level of the ZnO NP-layer, which contributes to the effective hole blocking. Our study clearly demonstrates that the ZnO NP-layer sandwiched between the EML and the EIL is a promising electron transport layer as well as an efficient hole blocking layer and enhances the light emission of OLEDs.