Mid-Infrared Cascade Intraband Electroluminescence with HgSe/CdSe Core/Shell Colloidal Quantum Dots

Efficient infrared light sources are needed for applications like gas sensing, environmental monitoring, and spectroscopy. In the visible, electroluminescence from colloidal quantum dots is highly efficient, wavelength-tunable, and cost-effective, which inspires using the same approach in the infrared. Despite the promising performances of colloidal quantum dots light-emitting diodes in the near-infrared, mid-infrared devices show quantum efficiencies of about 0.1% due to the much weaker emission. Moreover, these devices exclusively relied on the interband transition, restricting the possible materials.<br/><br/>Here, we show electroluminescence at 5 microns using the intraband transition between 1Se and 1Pe states within the conduction band of core/shell HgSe/CdSe colloidal quantum dots. With a three-monolayer CdSe shell thickness, the electroluminescence efficiency is about 50-fold larger than in a device that uses the interband transition of similar-sized core-only mid-infrared HgTe colloidal quantum dots. The device showed an EQE of 4.5% at 2 A/cm² while the power efficiency was 0.05%. The quantum efficiency approaches that of commercial epitaxial cascade quantum well light-emitting diodes. The high emission efficiency and the electrical characteristics support a similar cascade process where the electrons, driven by the bias across the device, repeatedly tunnel into 1P_e and relax to 1S_e as they hop from quantum dot to quantum dot.<br/><br/>The efficiency of the device could be further enhanced through the improvement of the colloidal quantum dots synthesis and film processing. Moreover, similar intraband transitions exist in many other materials, and the cascade mid-infrared electroluminescence design could then be extended to lower toxicity systems.