Lens-free OLEDs with efficiency comparable to that of inorganic LEDs

A comprehensive and analytical methodology to theoretically predict structures that maximize efficiency. 

(from left: Professor Seunghyup Yoo and PhD candidate Jinouk Song)
(from left: Professor Seunghyup Yoo and PhD candidate Jinouk Song)

In a new research by the KAIST team, scientists have found a new way to yield OLEDs with an external quantum efficiency (EQE) greater than 50% with an external scattering medium. Scientists mainly 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.

Having properties reasonable for thin and adaptable gadgets, OLEDs are famous light sources for displays, for example, cell phones and brilliant TVs. As of late, various endeavors have been made to apply OLEDs in lighting and light sources for vehicles.

For such applications, high effectiveness is of the highest significance for the fruitful organization of light sources. On account of consistent research and the advancement of OLEDs, their effectiveness is relentlessly on the ascent, and a level equal to inorganic LEDs has been shown in a few reports.

Figure 1.Photographs of OLEDs with SiO₂ -embedded scattering layers according to scatterance
Figure 1.Photographs of OLEDs with SiO₂ -embedded scattering layers according to scatterance

Though, these efficient OLEDs were frequently accomplished with a microscopic lens or complex internal nanostructures, which undermines the key preferences of OLEDs as a reasonable planar light source and tends to hinder their stable operation, thus putting a limitation to their commercialization.

Ordinarily, research on improving OLED light extraction utilizing light dispersing has been led experimentally much of the time. This time, the group created a far-reaching and analytical method to hypothetically foresee 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 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% 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.

Professor Yoo said, “There are various technologies developed for improving OLED light extraction efficiency; nevertheless, most of them have not reached a level of practical use. This research mainly provides a systematic way to attain an EQE of 50% or higher in OLEDs while keeping in mind the constraints for commercialization. The approach shown here can readily be applied to lighting devices or sensors of wearable devices.”.

The study is published Nature Communications.