White Organic Light-Emitting Diodes for Solid-State Lighting and Flat-Panel Display Applications

This thesis focuses on the development of novel white organic light-emitting diodes (WOLEDs) for solid-state lighting and flat-panel display applications. Firstly, non-doped type WOLEDs employing novel emitters featuring aggregation-induced emission are demonstrated. Due to elimination of the doping procedures, the fabrication process is much simplified, resulting in a reduced TACT time and an improved performance repeatability compared to conventional doped type WOLEDs. By employing emitters with good charge carrier mobility, bilayer non-doped WOLEDs are further demonstrated. The material cost is cut down significantly due to reduction of the number of materials/layers used to construct such bilayer WOLEDs.;Secondly, efficient WOLEDs with evenly separated red, green and blue peaks are demonstrated in a new three-emitter platform. In such system, WOLEDs based on a novel yellowish-green emitter demonstrate remarkably higher efficiency together with excellent color rendering capability as compared to the commercial benchmark green emitter Ir(ppy)3. The demonstrated WOLEDs with peak total efficiency up to 66 lm/W and high color rendering index up to 91 would be ideal candidates to bring WOLEDs into the next generation flat-panel display and solid-state lighting market.;To further enhance the efficiency of the WOLEDs, two simple, low cost and scalable substrate modification methods for effectively extracting the substrate waveguide light from OLEDs are proposed. By simply sandblasting the edges and the backside surface of glass, a 20% forward efficiency improvement has been achieved. To further increase the scattering probability, scattering films consisting of organic nanoparticles/nanowires are employed. OLEDs with scattering films demonstrated a forward efficiency improvement up to 31%. The proposed simple, low cost and scalable substrate modification techniques capable of simultaneously improving the luminous efficiency and uniformity are very attractive for large area WOLED lighting panel applications.;Finally, top-emitting WOLEDs for display applications are developed. In the first implementation, by taking advantage of the microcavity effect, efficient top-emitting blue OLEDs are firstly obtained and by capping a red color conversion layer on top of the blue OLEDs, a white emission resulting from the mixing of the converted red emission and the unabsorbed blue emission is achieved. The color converted top-emitting WOLEDs exhibiting pure white color and extremely high color stability can potentially be used to construct high-contrast white-black medical displays. In the second implementation, the microcavity effect is effectively suppressed by using a low reflection cathode. With an Yb (5 nm)/Au (15 nm)/MoO3 (30 nm) composite cathode, the demonstrated top-emitting WOLEDs exhibit negligible interference effect and consequently show broadband white emission with evenly separated red, green and blue peaks. The successful demonstration of the top-emitting WOLEDs on a SVGA+ microdisplay shows their great potential application in high-resolution full-color displays.