Organic light-emitting diodes (OLEDs) have received increasing interest as next-generation low-power display devices. They’ve even been touted as future white sources for illumination and lighting applications. But significant improvements are needed in OLED power efficiency and lifetimes, as well as lower-cost manufacturing, before they can achieve greater market success.
Using standard processing methods, Novaled has shown that OLEDs can be successful candidates for future white-light sources. The company has developed a high-efficiency long-lifetime white-light-output PIN OLED with what it calls the world’s most power-efficient fluorescent source, achieving output efficiency of 36 lm/W.
This development has increased OLED device emission by than 80%, compared with the 25% to 30% that’s typical of today’s devices, with a good color-rendering index (CRI) of 75. It also has improved the angular dependence of the emitted light. With it, white-light-output OLEDs are a step closer to becoming ideal for practical lamps and luminaires in general and design lighting.
The key to this breakthrough is the use of proprietary organic materials and a simple and new flat light-coupling method of light extraction. The technique was successfully tried on both bottom-emitting and top-emitting OLEDs, and Novaled says it is amenable to low-cost manufacturing. For both types of OLEDs, the technique used does not negatively impact electrical properties.
“Until now, typical outcoupling methods have been somewhat ineffective or have resulted in rising manufacturing costs. Novaled has overcome both challenges, pioneering a novel way to significantly improve light outcoupling results and boost external quantum efficiency by more than 80%, without the costly and time-consuming setup traditionally required for complex manufacturing,” says Novaled CEO Gildas Sorin.
Unlike other outcoupling enhancement approaches in bottom-emitting OLEDs such as depositing complicated structured layers between substrate glass and the indium-tin-oxide (ITO) anode, Novaled uses simple internal outcoupling methods with vacuum-evaporation processed organic materials to induce the scattering of the emitted light (Fig. 1). Such scattering does not negatively impact electrical properties.
NET61 material is used directly inside the electron transport layers (ETLs). The combination of the outcropping material and an external micro-lens array (MLA) film boosts power efficiency by more than 70% and quantum efficiency by more than 80% (Fig. 2). For top-emitting white-light OLEDs, scattering material NLE17 is used on top of the semi-transparent top electrode to extract light and improve the quality of emitted light.
In addition, Novaled’s new method improves the CRI value for top-emitting OLEDs on metal substrates. Although top-emitting OLEDs producing white light typically have stronger cavity effects than bottom-emitting types and show strong variations with the viewing angle of the light, Novaled increases white-light extraction from top-emitting structures and strongly reduces angular color dependence by using a scattering evaporation-processed organic layer on top of the semi-transparent top electrode. The demonstrated CRI value of 75 attained is ample for many commercial lighting applications, Novaled says.