A research team from imec has developed perovskite-based LEDs that emit light 1000× brighter than OLEDs. According to the researchers, the advancement is a pivotal milestone toward a perovskite injection laser, which promises advances in image projection, environmental sensing, and medical diagnostics, among other applications. Perovskite materials are well known for their potential in photovoltaic technology, though perovskites also carry potential as a light amplifier for thin-film laser diodes. Perovskites’ optoelectrical properties, low-cost processability, and efficient charge transport make it a strong candidate for light emission applications like LEDs. Researchers at imec developed perovskite-based LEDs capable of supporting stimulated emission of light. The transparent perovskite LEDs sit on a sapphire substrate with a scaled emission area for injection of ultrahigh current densities. Courtesy of imec. However, perovskite LEDs (PeLEDs) must be able to achieve electrically excited amplified spontaneous emission (ASE) before they can serve as a gain medium for perovskite laser diodes. In PeLEDs, the requirement for high conductivity conflicts with the need for high net modal gain of the device stack, hampering ASE. While perovskites can withstand very high current densities, they have yet to be used for laser operation with the emission of high-intensity coherent light. As the host institution for the European Research Council’s Ultra-Lux project, whose aim is to achieve injection lasing by increasing the brightness of thin-film light sources, imec developed a transparent PeLED architecture that combines low optical losses with excellent current-injection properties. When the imec team used 2.3-ns optical pulses at 77 K on the PeLED, the device demonstrated ASE with a threshold of 9.1 μJ/cm2. Upon sub-microsecond electrical excitation at 77 K of the same device, the PeLED realized current densities >3 kA/cm2, with irradiance values >40 W/cm2. Co-pumping the PeLED with optical pulses that were synchronized with the leading edge of an intense electrical pulse reduced the ASE threshold by 1.2 ± 0.2 μJ/cm2, demonstrating that electrically injected carriers contributed to optical gain. According to professor Paul Heremans, the Ultra-Lux project marked the first PeLED architecture with low optical losses as well as the first time that PeLEDs were pumped to current densities supporting the stimulated emission of light. “This novel architecture of transport layers, transparent electrodes, and perovskite as the semiconductor active material can operate at electrical current densities tens of thousands of times higher (3 kA/cm2) than conventional OLEDs can,” Heremans said. In contrast to III-V crystalline LEDs, OLEDs can be precisely designed and dimensioned, as single components or in arrays, into any target application without the need of hetero-assembly. However, their maximum brightness remains limited. The light power density of OLEDs remains about 300× smaller than that of III-V LEDs. Also, none of today’s thin-film light sources, including OLEDs, can be brought to lasing by electrical pumping. To demonstrate the feasibility of a perovskite semiconductor optical amplifier, the researchers probed the PeLED with 1 μs-long optical excitation. They observed continuous wave ASE at a threshold of 3.8 kW/cm2. The team also showed that intense electrical pulses can generate electroluminescence brightness levels close to half the irradiance produced by continuous wave optical pumping at the ASE threshold. “With this architecture, imec enhanced amplified spontaneous emission, with an electrical assist of the conventional optical pumping,” project manager Robert Gehlhaar said. “By doing so, imec demonstrated that electrical injection contributes 13% to the total amount of stimulated emission and thus approaches the threshold to achieve a thin-film injection laser.” The PeLED architecture developed by imec represents an important step toward achieving perovskite semiconductor optical amplifiers and perovskite injection lasers. “Reaching this landmark milestone towards high-power, thin-film laser diodes is paving the way to exciting new applications of thin-film perovskite lasers,” Gehlhaar said. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-023-01341-7).
