Colloidal quantum dots (CQDs) are semiconductor particles just a few nanometres in size and could be used as the light-absorbing or emitting component in optoelectronics devices. They can be synthesized in solution, which means that films of the particles can be deposited quickly and easily onto a wide range of flexible or rigid substrates – just like paint or ink. Another advantage of CQDs is that they absorb light over a wide spectrum of wavelengths thanks to the fact that their bandgap can be tuned over a large energy range by simply changing the size of the nanoparticles.

Most CQD films available today, however, suffer from low luminescence efficiencies, especially in the IR because of an effect known as self-quenching. Here, defects in the material trap charge carriers (electrons and holes) so that they recombine before they have had a chance to produce useful photocurrent. Researchers have developed various strategies to prevent such quenching in CQD films – for example, by growing a protective shell around the CQDs, capping them with insulating organic ligands and incorporating them into polymer matrices. However, devices made using these techniques invariably require more power to run because a higher turn-on voltage is needed to make them produce bright light.

This trade-off between non-radiative recombination, as it is called, and efficient charge transport goes a long way in explaining the low power conversion efficiencies (PCEs) of CQD-based LEDs. The PCE is defined as the ratio of the output optical power to the input electrical power.

Embedding CQDs in a high-mobility mixed-halide perovskite matrix solves the problem

Researchers, led by Edward Sargent, say that they may now have found a way to overcome this problem – by embedding CQDs in a high-mobility mixed-halide perovskite matrix. Mixed halide perovskites have the chemical formula MAPbX3 (where X = iodine or bromine).

The new composite allows for radiative recombination in the quantum dots by preventing charge carriers from becoming trapped in defects as they travel through the material, and this without increasing the turn-on voltage in a device. By carefully engineering the composition of the mixed halide matrix, the researchers made bright NIR CQD LEDs with electroluminescence PCEs of 4.9%. “This value is more than twice that of previously reported values for devices made from these materials,” team member Kevin Yang tells nanotechweb.org. “This means that with same amount of electricity we can get twice as much NIR light power out.”

Our new material platform combines the advantages of strong CQD luminescence and low loss of electrons and holes, adds team member Xiwen Gong.

The researchers, reporting their work in Nature Photonics doi:10.1038/nphoton.2016.11, say that they are now busy developing applications for their quantum-dot-in-perovskite hybrids as near-infrared laser and telecommunications emitters.

For more information on perovskites, visit the Nanotechnology focus collection at Focus on Perovskite Solar Cells, Topical Review and Lab Talk.