A research team from the University of Cologne (Germany) and the University of St Andrews (Scotland) has shown in a new study how a basic physics concept can be used to enhance the color brilliance of smartphone, computer or TV screens without cutting power. efficiency. The results are published in Nature photonics.
Organic light-emitting diodes (OLEDs) have invaded the display market in recent years — from high-resolution smartphones to wall-sized TV screens. However, industry and science face many challenges in creating the next generation of devices with higher color saturation, brightness, and efficiency. The organic molecules from which OLEDs are made have intrinsically wide emission spectra — a property that limits the available color space and saturation of high-end displays. Color filters or optical resonators can be used to artificially narrow the emission spectra of OLEDs to circumvent this problem. However, this either comes at the expense of efficiency or leads to a strong dependence of perceived color on viewing angle.
Researchers at the two universities have now shown that a fundamental scientific principle — the strong coupling between light and matter — can be used to alter the emission spectra of OLEDs while avoiding color shift at oblique viewing angles. When photons (light) and excitons (matter) exhibit a large enough interaction with each other, they can couple strongly, producing what is called an exciton polariton. The principle can be compared to the energy transferred between two close pendulums, except that light and matter here are coupled with each other and exchange energy constantly. These polaritons eventually emit light again. By fully integrating the OLED layer stack between thin mirrors made of metallic materials, which are already widely used in the display industry, the coupling between light and organic materials can be greatly improved. So far, however, the strong coupling in OLEDs has inevitably led to lower electrical efficiency. To avoid this, the researchers added a separate thin film of highly light-absorbing particles similar to those already used in organic solar cells, but not in OLEDs. The additional layer increased the strong coupling effect, but without significantly reducing the efficiency of the light-emitting particles in the OLED.
“By generating polaritons, we can transfer some of the material’s beneficial properties to our OLEDs – including their significantly reduced angular dependence, so that the screen’s color impression remains bright and stable from any perspective,” said Dr. Andreas Mischuk. Study author.
Although polariton-based OLEDs have been reported in the past, their energy efficiency and brightness have been low. This precluded practical applications and essentially limited them to basic research. With the new strategy, the team has now succeeded in achieving Polariton-based OLEDs with efficiency and brightness levels suitable for practical application for the first time.
Professor Malte Gather, who led the study, believes that “With efficiency and brightness comparable to OLEDs used in commercial displays, but with greatly improved color saturation and color stability, Polariton-based OLEDs are of great interest to the display industry.”
Efficient on-demand production of large numbers of polaritons is not only suitable for the next generation of displays, but can also be used for a wide range of other applications – from lasers to quantum computing.
