New standard for electronics
LEDs revolutionised both lighting and display technology in the 1990s. Today, the LED luminaire is the industry standard for lighting and is increasingly displacing all other light generation technologies from the market. While organic LEDs (OLEDs) are currently still the technological leaders in displays, materials with the structure of perovskite could soon be used to produce screens.
Perovskite is not a new invention, but the production of hybrid perovskite semiconductor material by inkjet printing is revolutionary. The production of organic semiconductors by printer has been possible for a long time. Even solar cells made from the semiconductor material are already coming out of the printer. But with LEDs, of all things, there have been problems: The particularly efficient and luminous hybrid perovskite materials have so far eluded printability.
Because the perovskite structure only crystallises after deposition on the carrier material, the quality of the resulting crystal structure was difficult to control. "Up to now, it was therefore not possible to produce such semiconductor layers from a liquid solution of sufficient quality," says Prof. Dr. Emil List-Kratochvil, head of the joint research group of HZB and HU.
In June 2020, the HZB now announced the success: The research team has found out how to optimise the crystallisation of the perovskite material. The scientists achieved the breakthrough with the help of a trick: "inoculating" the surface with specific crystals.
After solar cells now also LEDs out of the printer
In recent years, scientists at HZB have already shown that solar cells can be printed from a solution of such semiconductor compounds. Now, the joint team of HZB and HU has succeeded for the first time in producing functional light-emitting diodes in this way. For this purpose, the research group used a metal halide perovskite: a material that promises particularly high efficiency in light generation but is difficult to process.
"The challenge was to use a kind of attractant to induce the salt-like precursor that we applied to the substrate with the printer to crystallise quickly and evenly," explains List-Kratochvil. The team chose a "seed crystal" for this purpose: an added salt that attaches itself to the substrate and serves as a scaffold for the growth of the perovskite structure.
Significantly better optical and electronic features
The researchers used this trick to create printed LEDs with far higher luminosity and significantly better electrical properties than could previously be achieved with additive manufacturing processes. However, at present, the luminous period that can be achieved with perovskite from the printer is more likely to be hours than months or years. But if you take into account that the first perovskite LED only lit up for a few seconds, the progress in development is certainly enormous.
Although the lifetime is not yet sufficient for commercial use, for List-Kratochvil this success is only an intermediate step on the way to future micro- and optoelectronics, which in his opinion will be based exclusively on hybrid perovskite semiconductors. "The advantages offered by a universally applicable class of materials, from which any components can be manufactured using a single simple and cost-effective process, are impressive," says the scientist. In the Berlin laboratory of HZB and HU he therefore intends to gradually produce all relevant electronic components in this way.
Author: Bernadette Trepte