Surface Modification On Anode Of Organic Light-emitting Diodes

Organic light emitting diodes (OLEDs) have gained great interest due to their potential application in future flat panel display technology. OLED performance, operating voltage and efficiency, is critically affected by carrier injection. For efficient hole injection into OLED devices, the work function of the anode must be aligned with the highest energy occupied molecular orbital (HOMO) of the adjacent organic film. Indium tin oxide (ITO) is generally used as anode materials in OLEDs. However, its work function is not aligned with the typical hole transport layer (HTL) materials. More attentions have been paid to the modification of ITO anode to achieve an effective and balanced injection.Organic light emitting device fabricated on flexible substrate have the ability to be light weight, extremely rugged, conformable and flexible thus enabling new display product design. However, a number of important issues must first be resolved. However, the use of plastic substrates is restricted by its low processing temperature and high heat-induced shrinkage and high gas permeability. In particular, Indium tin oxide fabricated on plastics substrates tends to be crash when bending or mechanical strain. So, there is a great need for finding a more mechanically robust flexible tans parent conductor to replace the Indium tin oxide as anode of flexible device. Some research work was conduct to resolve these problems.In this work, a self-assembled monolayer (SAM) of 4-fluorothiophenol is employed to modify the Ag film on the surface of ITO surface. X-ray photoelectron spectroscopy (XPS), Four Point Probe, atomic force microscope (AFM), and UV-vis transmittance spectra were used to characterize the modified anode. Organic light emitting device with the structure of ITO/Ag(x nm)/SAM/α-naphthylphenylbiphenyl diamine (NPB, 60 nm)/ tris-(8-hydroxyquinoline) aluminum (Alq3, 60 nm) /LiF(0.7nm) /Al(100nm) was fabricated. Current density-voltage-luminance characteristics of the devices were investigated using the modified anode and the bare ITO. The effect of Ag layer thickness on the device performance is also investigated. The result revealed that SAM modified ultra-thin Ag film is an effective buffer layer to improve OLED performance. The enhancements are attributed to enhanced hole injection and smooth surface between anode and the organic material, which lead to the more balance of the carriers in emitting zone, then increases the current efficiency. The Ag thickness of 5 nm is chosen as an acceptable compromise between substrate transparency and the device performance. Based on this research, Organic light emitting device with the structure of ITO/Ag(5nm)/SAM/4, 4', 4''-tris(3-methylphenylphenylamino) triphenylamine (m-MTDATA)/α-naphthylphenylbiphenyl diamine (NPB)/ tris-(8-hydroxyquinoline) aluminum (Alq3) /LiF) /Al was fabricated. With the presence of the self-assembled monolayer-modified Ag films, the luminance of the device reaches 34680 cd/m2 at 12 V at the current density of 550 mA/cm2, which corresponds to an efficiency of 6.9 cd/A. However, the control device using bare ITO as anode was 25300 cd/m2 with current density of 433 mA/cm2 at the same bias.Organic light emitting device fabricated on flexible PET substrate with a self-assembled monolayer of 4-fluorothiophenol modified the Ag film as anode is also investigated. The thickness of Ag film on the transmittance and Sheet resistance Rs was investigated by UV-vis transmittance spectra and Four Point Probe. Atomic force microscope was used to measure the surface parameters. Flexible bottom-emitting and top-emitting devices with a self-assembled monolayer of 4-fluorothiophenol modified the Ag film as anode were fabricated. A luminance of 27000 cd/m2 at 8 V, which corresponds to an efficiency of 5.6 cd/A was achieved for Flexible bottom-emitting device. However, the luminance of flexible top-emitting devices was 27760 cd/m2 at the bias of 6.5V, corresponding to an efficiency of 7.38 cd/A.