Research On Surface Modification Of Electrodes For Organic Light-emitting Diodes And The Related Mechanisms

Highly efficient organic light-emitting diodes(OLEDs) have been greatlydeveloped in display and lighting. In my dissertation research has been focused onsurface modification of electrodes to reduce the charge-injection barrier, getting theefficient charge injection and high-efficiency OLEDs. The main research contents inmy dissertation are as follows:1. The approach of UV light irradiation was applied to modify the ITO anodewith o-DCB and H2O2. The maximum current efficiency of4.15cd/A was obtainedwith the device structure of ITO/CBP/Alq3/Mg:Ag/Ag when the ratio of H2O2too-DCB is1:5and the UV illumination time is10min. By UV radiation of ITO witho-DCB could obtain a monolayer of chlorinated ITO with the increased work function,and adding H2O2in the system can accelerate the rate of UV photolysis of o-DCB,thus reducing the UV illumination time, getting the high-efficiency OLED.2. Self-assembly of the organosiloxane materials on ITO surface was carried outto modify the ITO anode. The influence of the electronegativity on modification ofthe ITO substrate has been systematically investigated by attaching electron-withdrawing groups (Cl, Br, and I) and an electron-donating group (NH2) to theorganosiloxane material. The high electronegativity of the end group could result theimprovement of the work function of the ITO anode, leading to high-efficiencyOLEDs. The maximum current efficiency of3.67cd/A was obtained with the devicestructure of3Cl-Si-ITO/CBP/Alq3/Mg:Ag/Ag. A monolayer could be formed on theITO surface by self-assembly technique, which is equivalent to introducing a layer ofdipoles across the surface, increasing the work function of ITO, thus getting thehigh-efficiency OLED.3. KBH4was applied as the precursor of K metal with the stable metals Al andAg to develop the composite cathodes KBH4: Al/Al and KBH4: Ag/Ag for the firsttime, and the composite cathodes were investigated in the structure of ITO/NPB/Alq3/Cathode. After the optimizations of composition and thickness, the maximum current efficiency was obtained for the cathode structure of KBH4: Al(1:10,5nm)/Al(95nm) with4.6cd/A, and for KBH4: Ag(1:10,5nm)/Ag(95nm) with4.8cd/A,both higher than the device with2.8cd/A based on Mg: Ag/Ag cathode. Metal Kreleased from the thermal decomposition was responsible for the reduction in electroninjection barrier, and the introduction of the stable metal Al and Ag could not affectthe electron injection ability of KBH4, moreover enhance the stability of thecomposite cathode, getting the high-efficiency OLED.4. A cathode material PEO was applied in the PEO/Cs2CO3/Al compositecathode. Investigated in the small-molecule OLEDs, the current efficiency of theoptimized OLED with the PEO/Cs2CO3/Al cathode increases from18.8cd/A to32.1cd/A, and the turn-on voltage reduces from4.8V to3.4V, compared with the deviceof the Cs2CO3/Al cathode. The introduction of PEO layer could enhance theinjection of electrons, getting the high-efficiency OLED.