In recent years, organic light-emitting diodes (OLEDs) have dominated the display market, from high-resolution smartphones to TVs the size of walls. However, industry and science must overcome several obstacles to develop the next generation of devices with even higher color saturation, brightness and efficiency.

The organic compounds used to make OLEDs naturally have broad emission spectra; this property limits the range of colors and saturation used in high-end displays. To get around this problem, color filters or optical resonators can be used to artificially limit the emission spectra of OLEDs. Unfortunately, this reduces efficiency or the perceived color is greatly affected by the viewing angle.

Researchers at the two universities – the University of Cologne (Germany) and the University of St Andrews (Scotland) – have now shown that a fundamental scientific principle can be used to improve the color brilliance of smartphones, computers or TV screens without having to cut down on energy efficiency.

According to them, the principle – the strong coupling of light and matter – can change the emission spectra of OLEDs, while avoiding the color change at oblique viewing angles.

Exciton polaritons are produced when photons (light) and excitons (matter) show strong enough contact with each other. The idea is similar to how energy is transferred between two connected pendulums, except that matter and light are couplers and in this case transfer energy continuously.

These polaritons eventually emit light again. The coupling between light and organic material can be significantly improved by embedding the entire layer stack of the OLED between thin mirrors of metallic materials, which are already widely used in the display industry. However, until now, strong coupling in OLEDs has inevitably led to low electrical efficiency.

Researchers avoided this by adding a separate thin film of highly light-absorbing molecules, similar to those already used in organic solar cells, but not in OLEDs. This addition enhanced the effect of the strong coupling without reducing the efficiency of the light-emitting molecules in the OLED.

Dr. Andreas Mishok, the study’s first author, said: “Generating polaritons allows us to transfer some of the beneficial properties of matter to our OLEDs – including their significantly lower angular dependence, so that a screen’s color impression remains brilliant and stable from any perspective.”

Polariton OLED with green emission
Polariton OLED with green emission. The underlying concept of light-matter interaction through an absorbing layer can be transferred to different components in different spectral ranges and thus can be used in different ways. Credits: University of Cologne

“While polariton-based OLEDs have been reported in the past, their energy efficiency and brightness have been low. This has prevented practical applications and limited them to basic research. With the new strategy, the team has now for the first time successfully realized polariton-based OLEDs with efficiency and brightness levels suitable for practical application.”

Professor Malte Gather, who led the study, believes: “With efficiency and brightness comparable to OLEDs used in commercial displays, but with significantly improved color saturation and color stability, our polariton-based OLEDs are of great interest to the display industry.”

“The on-demand and efficient production of a large number of polaritons is not only relevant for next-generation displays, but can also be used for a wide variety of other applications – from lasers to quantum computing.”

Magazine reference:

  1. Mischok, A., Hillebrandt, S., Kwon, S. et al. Highly efficient polaritonic light emitting diodes with angle-independent narrowband emission. Wet. Photon. (2023). DOI: 10.1038/s41566-023-01164-6