Quantum Dots - Page 8

Switzerland-based Avantama demonstrated its perovskite quantum dots at Displayweek 2018. QDs are currently used as color down-conversion films to turn the emission of blue LEDs to white light.

Currently used QDs are either Cadmium-based or Indium-based, and Avantama claims that its pQDs outperform both technologies by a wide margin (3X CdSe, 12x InP), which means that using these will enable much more efficient QD-LCDs. Of course pQDs contain lead, but the amount is very small and it is way below the thresholds required by the EU and other countries.

Researchers from Fudan University in China have developed a high-bandwidth white-light based system made from a blue gallium nitride (GaN) micro-LED with yellow-emitting perovskite quantum dots. This system could open the door to high-speed real-time visible light communication (VLC).

The researchers used a 80 x 80 um blue-emitting micro-LED that has a modulation bandwidth of about 160 MHz and a peak emission wavelength of ~445 nm. The white-light system (following the perovskite QD conversion) achieves 85 Mhz - which means a maximum data rate of 300 Mbps.

Emberion researchers have shown that colloidal quantum dots (QDs) combined with a graphene charge transducer can provide a photoconducting platform with high quantum efficiency and large intrinsic gain, yet compatible with cost-efficient polymer substrates. The team demonstrated methods to couple large QDs (>6 nm in diameter) with organometal halide perovskites, enabling hybrid graphene photo-transistor arrays on plastic foils.

The resulting arrays simultaneously exhibited a specific detectivity of 5 × 1012 Jones and high video-frame-rate performance. PbI2 and CH3NH3I co-mediated ligand exchange in PbS QDs improved surface passivation and facilitated electronic transport, yielding faster charge recovery, whereas PbS QDs embedded into a CH3NH3PbI3 matrix produce spatially separated photocarriers leading to large gain.

Researchers at the National Renewable Energy Laboratory (NREL) and the University of Washington have designed an interesting strategy for driving down the cost of solar cells while ramping up efficiency: the team developed a high cost, high efficiency quantum dot solar cell for space applications, and provided the expensive solar cell up with a cheaper perovskite layer. The combined solar cell would be aimed at terrestrial applications with a more moderate price point. Note that in the proposed lower cost solar cell, the cheap layer is not the only role for perovskite. The expensive quantum dot layer would also be made of perovskite.

The NREL team explains that colloidal quantum dots are electronic materials and because of their astonishingly small size (typically 3-20 nanometers in dimension) they possess fascinating optical properties. That first quantum dot solar cell had a conversion efficiency of just 2.9% and was based on a lead sulfide formula. Things moved along quickly after that, and NREL noted a record of 12% for lead sulfide achieved by the University of Toronto just last year.

Fuji Pigment recently reported that it is researching and developing a new type of perovskite quantum dots. Fuji stated that the half width of their emission spectra is substantially narrower than that of InP; this property could very beneficial to the application of the dots in display materials, LED, bio-imaging and more.

emission spectra of perovskite quantum dots under 420 nm of irradiation light

The chemical composition of perovskite quantum dots are either CsPbX3 or CH3NH3PbX3 (X= Cl, Br, I). Their quantum efficiency is 50'80 % and their half width is 15'39 nm. Their base solvent is either hexane or toluene. However, finding alternative solvents is a challenge that is now being addressed.

EPFL researchers have designed a new type of inorganic nanocomposite that makes perovskite quantum dots (nanometer-sized semiconducting materials with unique optical properties) exceptionally stable against exposure to air, sunlight, heat, and water.

Quantum dots made from perovskites have already been shown to hold potential for solar panels, LEDs and laser technologies. However, perovskite quantum dots have major issues with stability when exposed to air, heat, light, and water. The EPFL team has now succeeded in building perovskite quantum dot films with a technique that helps them overcome these weaknesses.

Researchers from Sun Yat-Sen University in China have created a composite of perovskite quantum dots and graphene oxide that can reduce CO₂ when stimulated with light. It is referred to as the first known example of artificial photosynthesis based on perovskite quantum dots.

The team prepared quantum dots ' semiconductor nanoparticles ' of a highly stable cesium'lead halide perovskite, as well as a composite material made of these quantum dots and graphene oxide. Both materials showed an efficient absorption of visible light and strong luminescence. The team used these products to achieve a fundamental step in artificial photosynthesis ' the reduction of CO₂. To simulate sunlight, they used a xenon lamp with an appropriate filter.

Dr. Lioz Etgar obtained his Ph.D. at the Technion'Israel Institute of Technology, and completed his post-doctoral research with Prof. Michael Grätzel at EPFL, Switzerland. Dr. Etgar was the first to demonstrate the possibility to work with the perovskite as light harvester and hole conductor in the solar cell which result in one of the pioneer publication in this field.

Dr. Etgar is now leading the Perovskite solar lab in the Hebrew University in Jerusalem, Israel. Etgar's research group focuses on the development of innovative solar cells. Etgar is searching for new excitonic solar cells architectures while designing and controlling the inorganic light harvester structure and properties to improve the photovoltaic parameters. Etgar was kind enough to answer a few questions we had for him.

Researchers at the University of Toronto in Canada and ShangaiTech University in China have succeeded in using colloidal quantum dots in a high-mobility perovskite matrix to make a near-infrared (NIR) light-emitting diode (LED) with a record electroluminescence power conversion efficiency of nearly 5% for this type of device. The NIR LED could find use in applications such as night vision devices, biomedical imaging, optical communications and computing.

The researchers say that they may have found a way to overcome the known problem of low power conversion efficiencies (PCEs) of CQD-based LEDs, by embedding CQDs in a high-mobility mixed-halide perovskite matrix. 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 said to be more than twice that of previously reported values for devices made from these materials, which means that with same amount of electricity it is possible to get twice as much NIR light power out.

Researchers from the Universitat Jaume I and the Universitat de València have studied the interaction of two materials, halide perovskite and quantum dots, revealing significant potential for the development of advanced LEDs and more efficient solar cells.

The researchers quantified the "exciplex state" resulting from the coupling of halide perovskites and colloidal quantum dots. Both known separately for their optoelectronic properties, but when combined, these materials yield longer wavelengths than can be achieved by either material alone, plus easy tuning properties that together have the potential to introduce important changes in LED and solar technologies.