Laser focus on perovskites
Organic light-emitting diodes hold immense value in modern society, yet they are relatively limited in terms of brightness. The team behind the Ultra-Lux project are developing a new type of thin-film light-emitting diode, targeting extreme brightness, using a class of materials called perovskites, as Professor Paul Heremans and Dr. Karim Elkhouly explain.
A device which essentially converts electricity into light, OLEDs hold immense value in modern society, as light-emitting pixels in smart phone displays, televisions and Virtual Reality glasses. However, OLEDs are relatively limited in their brightness. As a leader of the Ultra-Lux project team, Professor Heremans has been working to develop a new type of thin-film light emitting devices using a class of materials called perovskites. “The hope in the project is to achieve superbright thin-film LEDs, 10,000 times brighter than an OLED, as a stepping stone to build laser diodes out of thin-film light emitting devices,” he outlines.
From light emitting diodes to laser diodes
This research encompasses all aspects of the design of a light-emitting device, with the aim of going well beyond the capabilities of current OLEDs. The brightness of a diode is a product of its efficiency in converting electricity to light, and also its current density, in which OLEDs are relatively limited. “The layers in OLEDs are organic semiconductors with a very low charge carrier mobility. OLEDs are extremely efficient at low current densities, but they aren’t at super-high current densities,” says Professor Heremans. This may be perfectly sufficient for some applications,
but not for others like laser diodes, which have much higher demands in terms of high current-density operation, an issue that the project team are working to address. “We want to find a new thin-film device system, compatible with high current densities,” continues Professor Heremans.
The project team are working with metalhalide perovskites, which will form the active layer of these new LEDs, where light is created. The advantage of this material class is that they have a high mobility for charge carriers and hence should in principle allow high current density through the device. Beyond the active perovskite
layer, researchers are investigating the entire architecture of the LED, with special attention to heat management. Designs become even more complex when targeting laser diodes. “There are three main components in a laser. First you need a pump source - we want to make an electrically-pumped laser diode, so we pump electrically. Then you need the gain medium, in this case the perovskite, which we can fine-tune chemically. The third thing is a resonator, which is a cavity, a typical example is a pair of mirrors,” explains Karim Elkhouly, a researcher at imec & KU Leuven who is working on the Ultra-Lux project in the imec laboratories. One major priority in this part of the project work is to reduce the lasing threshold. “In a laser diode, you always have absorption of light, and you always have generation of light by stimulated emission. The lasing threshold is the point at which there is more
can achieve both high efficiency and low threshold, but there are compromises to make,” explains Elkhouly. “For instance, in order to improve the efficiency of the device, it may sometimes be necessary to do things that increase the threshold to a higher level.”
A type of perovskite with large threedimensional grains is being used, with indium tin oxide for the electrodes, to mitigate optical losses. “Indium tin oxide is a transparent conductor, meaning that it can conduct current pretty well, but at the same time, it’s also transparent, so it allows you to control the amount of electrical conductivity versus the optical losses,” he explains. “We optimize indium tin oxide material properties to the best possible compromise between low optical losses and high electrical conductivity. We are also integrating an optical cavity, by designing a cavity that fits underneath the entire lightemitting device stack.”
“The aim in the project is to achieve super-bright thin-film LEDs with a brightness 10,000 times higher than that which an OLED can deliver.”
stimulated emission than absorption, and from then on you can actually see lasing,” says Professor Heremans. Researchers aim to reduce this lasing threshold to the lowest possible level, limiting the need to increase the current, as reaching very high current densities is particularly hard in thin-film light-emitting devices. “We need to reach a certain threshold or carrier density to actually see lasing. We are trying to reduce that threshold as much as possible by reducing the absorption and photon losses through various modifications, such as changes to the architecture and design, and improvements to the resonator,” outlines Professor Heremans. The project team are also using quantum confinement features to create conditions where lasing exceeds absorption in a smaller, more enclosed space. “If you manage to narrowly confine the space where you reach that lasing, then the density means there is an absolute smaller number of carriers. That means you can put in a lower absolute current,” continues Professor Heremans.
Perovskite layer
The nature of the perovskite active layer is an important factor in terms of the goal of reducing the lasing threshold, but it’s also important to consider the layers around it, which can also contribute to losses. “We aim to design a device stack in which we
ULTRA-LUX
Ultra-Bright Thin-Film Light Emitting Devices and Lasers
Project Objectives
ULTRA-LUX aims to overcome brightness limitations of thin-film light sources and achieve injection lasing via electrical pumping. By developing a high-brightness thin-film light-emitting device using metal halide perovskite semiconductors and creating a thin-film injection laser with low lasing thresholds, it will enable new applications in photonic integrated circuits, sensing and ICT.
Project Funding
This project has received funding from European Union’s Horizon 2020 research and innovation programme under grant agreement No 835133.
Project Partners
https://erc-ultralux.eu/index.php/our-team.html
Contact Details
Senior Researcher, Karim Elkhouly IMEC
Kapeldreef 75 3001 Leuven
Belgium
T: +32471633783
E: Karim.Elkhouly@imec.be
There is a trade-off here between improving a device’s capability to emit light at very high current densities while at the same time minimising optical losses, and significant progress has been made on both fronts, with researchers now closer to making a laser diode from these perovskites. The long-term goal is not to simply increase the brightness of perovskite LEDs, but rather to develop laser diodes that can be electrically pumped, which Professor Heremans says represents a radical shift.
“We want to develop laser diodes that can be integrated on virtually any substrate by what I call monolithic integration,” he outlines. Ultra-Lux itself has recently concluded, but Professor Heremans is optimistic that full laser diodes will be developed in the next few years, while he is also interested in building on the project’s findings in future.
“We’ve been working on the near-infrared, but we could extend our research towards visible wavelengths in future,” he says. “It’s fairly easy to find laser pointers for blue and for red, but they are pretty rare in green.”
This is sometimes referred to as the ‘green gap’, referring to the limited green laser options currently available. Perovskites could be a possible option here, says Professor Heremans. “Perovskites can address a wide wavelength range, and they could hold potential in terms of making green lasers,” he says.
W: https://www.imec-int.com/en/whatwe-offer/research/government-fundedresearch/public-funded-projects/ultra-lux
Heremans
Paul Heremans is Senior Fellow and Vice President for Future CMOS, Sensors and Energy at imec, and a Professor in the Electrical Engineering Department at KU Leuven. He started the organic semiconductor activities at imec in 1998 and the perovskite activities in 2014.
Karim Elkhouly is a Senior Researcher at imec, Leuven, Belgium, within the exploratory materials and devices group, having gained his PhD in 2024. His research focuses on pioneering novel device concepts for thin-film gain media, aiming to push the boundaries of photonic and optoelectronic technologies.
