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KAIST Develops Integrated High Performance Light Outcoupling
November 05, 2018
 
OLEDs have had limited success on lighting because of high inorganic costs and low efficiency compared to LEDs. To address the issue of efficiency, a KAIST team provided a systematic way to yield OLEDs with an external quantum efficiency (EQE) greater than 50 percent by adopting an external scattering medium. Having properties suitable for thin and flexible devices, High efficiency is of the upmost importance for the successful deployment of light sources. Thanks to continuous research and the development of OLEDs, their efficiency is steadily on the rise, and a level equivalent to inorganic LEDs has been demonstrated in some reports. However, these highly efficient OLEDs were often achieved with a macroscopic lens or complex internal nanostructures, which undermines the key advantages of OLEDs as an affordable planar light sources and tends to hinder their stable operation, thus putting a limitation to their commercialization. Among various methods proven effective for OLED light extraction, a team led by Professor Seunghyup Yoo at the School of Electrical Engineering focused on the external scattering-based approach, as it can maintain planar geometry and compatibility with flexibility. It is also able to be fabricated on a large scale at a low cost and causes no interference with electrical properties of OLEDs. Conventionally, research on enhancing OLED light extraction using light scattering has been conducted empirically in many cases. However, KAIST developed comprehensive and analytical methodology to theoretically predict structures that maximize efficiency. Considering OLEDs with the external scattering layers as a whole rather than two separate entities, the researchers combined the mathematical description of the scattering phenomena with the optical model for light emission within an OLED to rapidly predict the characteristics of many devices with various structures. Based on this approach, the team theoretically predicted the optimal combination of scattering layers and OLED architectures that can lead to the maximum efficiency. Following this theoretical prediction, the team experimentally produced the optimal light scattering film and incorporated it to OLEDs with orange emitters having a high degree of horizontal dipole orientation. As a result, the team successfully realized OLEDs exhibiting EQE of 56 percent and power efficiency of 221 lm/W. This is one of the highest efficiencies ever realized for an OLED unit device without the help of a macroscopic lens or internal light extraction structures.


Read more at: https://phys.org/news/2018-10-lens-free-oleds-efficiency-inorganic.html#jCp