Mid-infrared Electroluminescence of HgTe Colloidal Quantum Dots

HgTe colloidal quantum dots show great potential in many infrared regions because of the zero band gap of bulk HgTe. They have been explored for infrared detection as photoconductors, photovoltaics, and phototransistors, and for electroluminescence in the short-wave region (&lt; 2.5 µm). Here, we report mid-infrared electroluminescence using HgTe CQD photovoltaic devices previously designed for mid-IR detection.<br/><br/>Mid-infrared electroluminescence is challenging because of the low photoluminescence efficiency. To reach a certain radiance, a much higher operation current is required, compared to visible devices. This could relax the requirement of band alignment between layers, but gives extra challenges on material stability and device efficiency. Strong thermal signal from Joule heating also becomes a consideration.<br/><br/>Using a p-n HgTe homojunction sandwiched between ITO and a gold electrode, we demonstrate band-edge electroluminescence at 4 µm (and at 2.5 µm with smaller HgTe quantum dots) when operated at forward bias modulated at 100 kHz. Fast modulation helps reduce the contribution from thermal emission. By comparing the photoluminescence and electroluminescence of the same device, we found that the electroluminescence efficiency at low current was limited by the photoluminescence efficiency of the quantum dots while the diode structure provided efficient electron-hole recombination. The power efficiency is limited by the resistance of transparent electrode, and we show that the efficiency can be improved through the incorporation of a metal conductive grid.