The challenge of creating next-gen organic light-emitting diode (OLED) displays has been finding a way to improve color brilliance without lowering electrical efficiency. Now, researchers have found a way to achieve this by applying a fundamental scientific principle.
OLED displays are everywhere, from high-resolution smartphones to computer monitors and huge television screens. An OLED consists of a thin, carbon-based semiconductor layer that emits light when electricity is applied by adjacent electrodes. OLEDs work similarly to conventional LEDs, but instead of using layers of n- and p-type semiconductors, they use organic molecules to produce electrons.
A simple OLED is made up of six layers. On the top (the seal) and bottom (the substrate) are layers of protective glass or plastic. In between, there’s a negative terminal (cathode) and a positive terminal (anode), and between these are two layers of organic molecules: the emissive layer, which is next to the cathode and from which light is produced, and the conductive layer, next to the anode.
The organic molecules used to create these layers intrinsically have a broad emission spectrum, which impacts their illumination characteristics, limiting the range of available colors (color space) and saturation on high-end displays. While color filters or optical resonators can artificially narrow the emission spectrum, this can reduce energy efficiency.
Researchers at the University of Cologne in Germany and the University of St Andrews in Scotland collaborated to tackle this issue head-on, applying a fundamental scientific principle: the strong coupling of light and matter.
“When photons (light) and excitons (matter) exhibit sufficiently large interaction with each other, they can strongly couple, creating so-called exciton polaritons,” the researchers said. “The principle can be compared to energy transferred between two coupled pendulums, except here it is both light and matter that are coupling with each other and continuously exchanging energy.”
The researchers found that by embedding OLEDs between thin mirrors made of a metallic material that is already widely used in the display industry, the coupling between light and organic material could be improved significantly.
To avoid the decreased electrical efficiency that usually results, researchers added a separate thin film of strongly light-absorbing molecules like the ones used in organic solar cells. They found that the additional layer amplified the effect of the strong light-matter coupling without significantly lowering the efficiency of the light-emitting molecules in the OLED.
“With efficiency and brightness comparable to OLEDs that are 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,” said Malte Gather, the study’s lead author.
While polariton-based OLEDs (POLEDs) are already known in the scientific world, their practical application has been hampered by poor energy efficiency and low brightness.
With these issues now addressed, the researchers hope their work will not only produce the next generation of OLED displays but have broader applications for lasers and quantum computing.
The study was published in the journal Nature Photonics.
Source: University of Cologne
