Colloidal quantum dots make LEDs shine bright in the infrared

December 4, 2018, ICFO
Dr. Santanu Pradhan, first author, explains the experiment results to ICREA Prof. at ICFO Gerasimos Konstantatos, group leader of the research group at ICFO. Credit: ICFO

The ideal optoelectronic semiconductor material would be a strong light emitter and an efficient charge conductor to allow for electrical injection in devices. These two conditions, when met, can lead to highly efficient LEDs as well as to solar cells that approach the Shockley-Queisser limit. Until now, the materials that have come closest to meeting these conditions have been based on costly, epitaxially-grown III-V semiconductors that cannot be monolithically integrated to CMOS electronics.

The ICFO team now reports a processed nanocomposite system comprising infrared colloidal quantum dots. It meets these criteria, and at the same time, offers low cost and facile CMOS integration. Colloidal quantum dots (CQDs) are semiconductor particles or crystals as small as a few nanometers in size, which therefore have unique optical and . They are excellent absorbers and emitters of light, and their properties change as a function of their size and shape: Smaller quantum dots emit in the blue range while larger quantum dots emit in the red.

The use of CQD LEDs could contribute to third-generation, solution-processed inorganic . The implementation of these nanocrystals in devices for optical sensing in the short-wave and mid-infrared have a vast number of applications, including surveillance, night vision, and environmental monitoring and spectroscopy.

In this recent study, published in Nature Nanotechnology, ICFO researchers Santanu Padhan, Francesco Di Stasio, Yu Bi, Shuchi Gupta, Sotirios Christodoulou, and Alexandros Stavrinadis, led by ICREA Prof. at ICFO Gerasimos Konstantatos, have developed CQD infrared-emitting LEDs with unprecedented values in the infrared range, an external quantum efficiency of 7.9 percent and a of 9.3 percent, a value never before attained with this type of device.

The key feature of this work has been the development of a CQD composite structure engineered at the suprananocrystalline level to reach unprecedentedly low electronic defect density. Prior efforts in suppressing electronic defects in CQD solids have been primarily been based on chemical passivation of the CQD surface, something that could not solve the problem in PbS QDs. The researchers at ICFO took an alternative path of creating the appropriate matrix in which they embedded the emitting QDs, to serve as a remote electronic passivant for the emitter CQDs. Moreover, the energetic landscape of the matrix was engineered in order to facilitate efficient charge funnelling into the QD emitters in order to achieve efficient electrical injection.

With these new hybrid devices, the researchers constructed solar cells to test their performance in the infrared range. They discovered that the effective passivation achieved in these nanocomposites, along with the modulation of the electronic density of states, results in solar cells that deliver open circuit voltage very close to the theoretical limit. The open circuit voltage (VOC), which is the maximum voltage available from a solar cell, increased from 0.4 V for a single QD configuration, up to ~0.7 V for the ternary blend configuration, an impressive value considering the lower bandgap of the cell at ~0.9 eV.

Researcher Gerasimos Konstantatos says, "The most surprising finding of this study is the extremely low electronic trap density that can be achieved in a conductive QD material system that is full of chemical defects arising on the surface of the dots. The very high quantum efficiency of those LEDs is the consequence of this passivation strategy. The other exciting outcome is the potential to reach such high VOC values for QD solar cells, thanks to the very low trap density, as well as to a novel engineering approach of the density of states in a semiconductor film."

Santanu Pradhan, the first author of the study, adds, "Next we will focus on how to further exploit this reduction of electronic density of states synergistically with other means to allow for simultaneous achievement of high Voc and current production, thereby targeting record power conversion efficiencies in solar cell devices."

The results obtained in this study prove that the engineering of QCD infrared-emitting LEDs at the nanoscale integrated in solar can significantly improve the performance efficiency of these devices in the infrared range. Such results open the pathway into a range of the spectra that is still to be fully exploited and offers amazing new applications, such as on-chip spectrometers for food inspection, environmental monitoring, manufacturing process monitoring as well as active imaging systems for biomedical or night vision applications.

Explore further: New efficiency record set for perovskite LEDs

More information: Santanu Pradhan et al, High-efficiency colloidal quantum dot infrared light-emitting diodes via engineering at the supra-nanocrystalline level, Nature Nanotechnology (2018). DOI: 10.1038/s41565-018-0312-y

Related Stories

Permanent, wireless self-charging system using NIR band

October 8, 2018

As wearable devices are emerging, there are numerous studies on wireless charging systems. Here, a KAIST research team has developed a permanent, wireless self-charging platform for low-power wearable electronics by converting ...

Gold changes photoluminescence of silicon quantum dots

April 9, 2018

A group of scientists from Russia and Sweden showed that applying gold stripes to a sample with silicon quantum dots modifies the dots' properties. Their study results were published in Scientific Reports.

Quantum dot white LEDs achieve record efficiency

July 12, 2018

Researchers have demonstrated nanomaterial-based white-light-emitting diodes (LEDs) that exhibit a record luminous efficiency of 105 lumens per watt. Luminous efficiency is a measure of how well a light source uses power ...

Recommended for you

New traffic rules in 'Graphene City'

December 6, 2018

In the drive to find new ways to extend electronics beyond the use of silicon, physicists are experimenting with other properties of electrons, beyond charge. In work published today (Dec 7) in the journal Science, a team ...

Artificial synapses made from nanowires

December 6, 2018

Scientists from Jülich together with colleagues from Aachen and Turin have produced a memristive element made from nanowires that functions in much the same way as a biological nerve cell. The component is able to save and ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

winthrom
not rated yet Dec 05, 2018
ICFO – The Institute of Photonic Sciences
Mediterranean Technology Park
Av. Carl Friedrich Gauss, 3
08860 Castelldefels (Barcelona), Spain
Tel.: (+34) 93 553 4002
Fax: (+34) 93 553 4000
E-mail: secretariat@icfo.eu

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.