Research On Microstructured Electrodes In Organic Light-emitting Devices

Organic light-emitting devices(OLEDs) are being broadly investigated owing to their potential applications for highly efficient large-area solid-state lightings and full-color flat-panel displays, as well as their advantages of flexibility, light weight, low cost, and material abundance. However, there are several key issues hindering the commercial applications of OLEDs. Photons are trapped inside of O LEDs in surface plasmon–polariton(SPP) modes associated with the metallic electrode/organic interface, which leads to the significant power loss. Microscale organic light-emitting devices(micro-OLEDs) with fine designed pattern is required for the application of micro-display and three-dimensional(3D) display. So far, suitable fabrication technique for the arbitrarily patterned micro-OLEDs with high resolution is still a challenge, which is an obstacle for the applications of the micro-OLEDs. Indium–tin oxide(ITO), the commonly used transparent electrode material in OLEDs, faces several challenges due to its incompatible with the plastic substrates and high costs. An ultrathin and continuous metallic film with mechanical robustness, high electrical conductivity and high optical transparency has been considered as the ideal alternative of the ITO electrode. Although a high optica l transparency can be obtained for an ultrathin metallic film with a thickness of less than 10 nm, the thermally evaporated metallic layer follows the Volmer-Weber growth mode, and results in a non-continuous film in long-region with a rough surface morphology and poor conductivity. In this thesis, we have done much research on the electrodes of OLEDs. The electroluminescent(EL) performance s of O LEDs have been improved on the basis of physical design, process study and material choices for the microstructured electrodes. The research results include the following aspects:1 、 The power lost in SPP modes has been recovered by introducing one-dimensional(1D) periodic microstructured metallic electrodes in OLEDs. We fabricated 1D and two-dimensional(2D) periodic microstructured metallic electrodes by two beam interference-holographic lithography technique. A 1D periodic corrugation in wavelength-scale has been introduced into the bottom-emitting O LEDs based on Alq3, and the enhanced light extraction has been obtained by the outcoupling of the SPP modes, and results in a 30% enhancement in efficiency.2、The excitation and outcoupling of the SPP modes in the WOLEDs and O LEDs with dual metallic electrodes has been realized by introducing a 2D dual-periodic corrugated metallic electrode. The 2D corrugation consisted two sets of corrugations with different periods. The blue and orange emission are both efficiently extracted from the WOLEDs based on the two complementary color strategies by adjusting appropriate periods of the dual-periodic corrugation. The maximum luminance is increased from 21720 cd/m2 to 40680 cd/m2, and a 37% enhancement in current efficiency and 48% enhancement in the external quantum efficiency have been obtained. By engaging the 2D dual periodic microstructured metallic electrodes, we also fabricated color tunable WOLEDs associated with polarization. The emitting color of WO LEDs is tuned among standard-white, cold-white and warm-white by tuning the polarization. Recovering the power lost to SPPs at both cathode and anode interfaces in OLEDs with dual metallic electrodes, simultaneously, has been realized by introducing the 2D dual periodic microstructures, and the current efficiency has been enhanced from 17.98 cd/A to 23.22 cd/A. The 2D dual periodic microstructured metallic electrodes have also been engaged in organic solar cells. The SPP modes have been excited effectively and the light absorption has been enhanced in broadband owing to the effect of SPP-induced field enhancement. A 31% enhancement of power conversion efficiency has been obtained compared to the conventional planar devices.3、Based on the direct reduction and patterning of the reduced graphene oxide(R-GO) via femtosecond laser direct writing(Fs LDW) as the micro-electrodes, we fabricated micro-OLEDs with arbitrary shape customization. The graphene oxide was produced via modified Hummers method from natural graphite. Femtosecond laser was employed to fabricate the R-GO electrode by direct reduction and patterning of the GO films according to preprogrammed patterns. The patterned R-GO micro-electrode exhibits good conductivity, excellent surface morphology, and high resolution. Various complex patterned micro-OLEDs, such as ladder and bowknot, were successfully created through this simple Fs LDW fabrication process, and the micro-OLEDs exhibit well-defined sizes, shapes, edges and uniform electroluminescence characteristics.4、An ultrathin Au film with ultrasmooth and continuous morphology has been reported as an excellent alternative of traditional transparent electrode ITO in flexible OLEDs. The insertion of the SU-8 film modifies the glass substrate and fixes Au atoms via chemical bond interactions to form the ultrasmooth and continuous surface morphology of the deposited Au film. The 7 nm Au film shows exce llent characteristics with a root- mean-square roughness of 0.35 nm, a high transparency of 72% at the wavelength of 550 nm, and a sheet resistance of 23.75 ?/sq. By applying the ultrathin Au as the anode of the OLEDs, 17% enhanced current efficiency with Lambertian emission was realized. The flexible OLEDs with the ultrathin Au anodes also exhibit high flexibility and mechanical robustness.