Surface modification of indium tin oxide anode with self-assembled monolayer modified Ag film for improved OLED device characteristics

Modification of electrodes has attracted much attention in the study of organic semiconductor devices. A self-assembled monolayer (SAM) of 4-fluorothiophenol is employed to modify the Ag film on the surface of indium tin oxide (ITO) to improve the hole injection and the surface morphology. The modified anode was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and UV-vis transmittance spectra. To investigate the effect of the modification on the device characteristics, typical double layer devices with the structure of anode/α-naphthylphenylbiphenyl diamine (NPB, 60 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3, 60 nm)/LiF(0.7 nm)/Al(100 nm) were fabricated using the modified anode and the bare ITO. The effect of Ag layer thickness on the device performance is also investigated. The results revealed that SAM modified ultra-thin Ag film is an effective buffer layer for organic light emitting diode. The device using the ITO/Ag (5 nm)/SAM as anode show improved device characteristics than that of using bare ITO as anode. The enhancements in luminance and efficiency are attributed to enhanced hole injection and smooth surface between anode and the organic material. The Ag thickness of 5 nm is chosen as an acceptable compromise between substrate transparency and the device performance.