With the help of DOE funding, North Carolina State University is addressing the issue of extracting light trapped in the OLED device. For basic OLED devices on planar glass substrates, only 17% to 25% of the generated light is extracted from the panel. This inefficiency is due to trapping of photons in the electrodes, transparent substrate, and inner layers resulting from mismatches in the index of refraction along the photon path (i.e., organic materials, anode, substrate, encapsulation layers, and air). The project is exploring the development of a corrugated substrate coupled with a low-index-of-refraction buffer layer to minimize total internal reflection. Development has focused on optimal feature pitch and depth for maximum extraction without creating current leakage or pathways for shorting. The project is also investigating the use of a microlens array to extract light from the substrate mode. The overall goal is to achieve an extraction efficiency >70%.
By using a hemispherical lens to extract light from OLEDs made on corrugated substrates, the researchers are able to achieve more than a 2X enhancement in external quantum efficiency (EQE). The efficiency enhancement mainly comes from the diffraction of the surface plasmon polariton (SPP) mode into air mode, due to the corrugated substrate.
The corrugated device shows about 5% higher EQE compared to the planar device. With the application of the hemispherical lens, the planar device EQE is increased to 40% (an additional 18%), which is attributed to extraction of substrate mode, and the corrugated device EQE is increased to 60% (an additional 32%). Considering the simulated mode distribution, the enhancement comes from the extraction of the substrate mode and additional trapped modes. (July 2017)