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What are perovskite quantum dots?

What are perovskite materials?

In material science and optoelectronics, perovskite materials gained a lot of attention due to their unique performance in solar cells, lighting, displays, catalysis and many other areas. Original term perovskite refers to calcium titanium oxide mineral, with the chemical formula CaTiO3. The terms “perovskite” and “perovskite structure” are often used interchangeably, but while true perovskite (the mineral) is composed of calcium, titanium, and oxygen in the form CaTiO3, a perovskite structure is anything that has the generic form ABX3 and the same crystallographic structure as perovskite (the mineral). In a perovskite structure, ‘A’ and ‘B’ are two cations of very different sizes, and X is an anion that bonds to both.Gustav Rose discovered perovskite in the Russian Ural Mountains in 1839 and named it after Russian mineralogist Lev Perovski (1792–1856). Many different cations can be embedded in this structure, facilitating the development of diverse engineered materials.

For optoelectronics, namely halide perovskite materials are of particular interest. Its perovskite structure ABX3 is formed by octahedral, with halogens on the corner (X: Cl, Br or I); with transition metals in the center (B: Pb or Sn); and cations in between octahedral (A: Cs, organic FA (formamidinium CH(NH2)2) or MA (methylammonium CH3NH3)). These semiconducting materials has the unique combination of optical properties like direct and tunable bandgap, high absorption coefficient, broad absorption spectrum, high charge carrier mobility, long charge diffusion lengths and remarkably low trap densities. These materials are considered as rising stars of the photovoltaic industry. They are cheap to manufacture, easy to make, and extremely effective. That is why they have been widely investigated for solar cells, lasing, light-emitting diodes, photodetectors.

Figure 1. Halide Perovskite crystal structure.

How do perovskite materials relate to the quantum dot industry?

A new class of quantum dots, based on perovskite semiconducting materials, is currently under development. As the size of the halide perovskite crystals going smaller to nanometers level, they start to exhibit quantum confinement and demonstrate photoluminescence. These nanocrystals (quantum dots) possess outstanding photoluminescence efficiency up to 100 % (the photoluminescence quantum yield, or PLQY, of a molecule or material is defined as the number of photons emitted as a fraction of the number of photons absorbed), high color purity (that is characterized with the emission full width at half maximum (FWHM) and usually is 20-35 nm that is the best among all quantum dots), and high absorption coefficient. These materials demonstrate superb light photo- and electrical- conversion and promising for applications in displays, UV and X-ray sensors, lighting devices. They are more resistant to defects than metal chalcogenide quantum dots, and they have excellent quantum yields of photoluminescence and high color purity that has already surpassed metal chalcogenide QDs.

While perovskite quantum dots behave similarly to traditional chalcogenide quantum dots, there are some differences. The main difference is that while in chalcogenide quantum dots the tuning of the bandgap (or color of the emitted light) is performed by changing quantum dots size (for example 6 nm CdSe QDs emit red light, 3 nm – green light and 2 nm – blue light), in perovskite QDs the tuning of the emitting light is performed by changing of perovskite QDs composition. Pure CsPbCl3 QDs emit the light in UV range, CsPbBr3 QDs emit green light and CsPbI3 emit red light. The most exciting thing is that you can get perovskite QDs emitting with any color by tuning their halide composition. For example, if you start to add bromide ions to the CsPbCl3 QDs that results on partial replacement of chloride ion to bromide ion CsPb(Cl/Br)3, its emission will gradually change from blue and sky blue light all the way to green light when bromide replaces chloride ions completely forming CsPbBr3 structure. By adding iodide ion to green emitting CsPbBr3 QDs, it is possible to go all the way through yellow and orange light for CsPb(Br/I)3 to deep red light CsPbI3  [1].

Figure 2. Tuning of the emitted light in cesium lead halide perovskite Quantum Dots [1]

But, is it possible to change the bandgap (in other words the color of the emitted light) by changing the perovskite nanocrystal size? Well, it is possible, but in some extend. For example, if the size of CsPbBr3 is reduced from 12 nm to 1 nm the bandgap changes just from 2.4 eV (515 nm) to 2.9 eV (430 nm) only or in average 0.05 eV/nm [2]. But for CdSe QDs, the size change from 6 nm to 2 nm results in bandgap change from 2.0 eV (630 nm) to 2.7 eV(460 nm) – in average 0.18 eV/nm [3]. In case of PbS QDs, the change of the size from 9 nm to 2.2 nm results in bandgap change from 0.6 eV (2050 nm) to 1.46 eV (850 nm) – in average 0.13 eV/nm. As seen, the bandgap in perovskite QDs is 3-5 times less sensitive to the nanocrystal size change than in traditional quantum dots. The big advantage of that is that the optical properties of perovskite QDs are more reproducible from batch to batch in their synthesis and it is possible to scale up the production in large volumes maintaining precise control of emission peaks (±1-3 nm) and achieving the best color purity (FWHM < 18-20 nm).

An additional advantage of perovskite QDs over traditional quantum dots is that they are highly tolerant to defects, since they do not require any surface passivation (passivation is the process of treating or coating a metal in order to reduce the chemical reactivity of its surface) to retain their high PLQY. In the presence of defect and trap locations on the surface, their energies are positioned beyond the bandgap and are either located inside the conduction or valence bands and less (weakly) affect into the QDs optical properties. Such properties are highly desirable for electronic and optoelectronic applications. These perovskite material nanocrystals are easy to synthesize in a colloidal suspension and are also easily incorporated into optoelectronic devices using readily available processing techniques, making them a powerful potential contributor to future technologies in next generation LCD and OLED displays, photodetectors, and solar cells.

Perovskite quantum dot applications

Apparently, perovskite quantum dots are less well researched than other types of quantum dots as they were first reported in 2015 [1]; however, perovskite quantum dots proved extremely effectivein optoelectronics and nanotechnology for a variety of different applications. Potential applications for perovskite quantum dots include:

  • Displays
  • Light Emitting Diodes
  • X-Ray and UV Detectors
  • Lasers
  • Solar Cells
  • Single Photon Sources
  • Quantum Computing
  • Cell imaging

Displays

Green emitting perovskite QDs can be successfully used as a light down-converting material in LCD backlighting or OLED color filters where QDs convert the blue light into green light by photoluminescence. Due to Perovskite QDs high PLQY (up to 100%) and high color purity (FWHM <20-25 nm), it makes it as a better-quality alternative to the current green CdSe or InP based QDs. Perovskite QDs can enhance the color gamut in displays achieving > 90% of Rec.2020 color standard coverage, meaning that displays present more vibrant and vivid colors in TVs, laptops and tablets. First commercially available display with Perovskite QDs are expected to appear in the market in 2021.

Light-emitting Diodes 

Due to their high electroluminescent efficiency, perovskite materials may be good candidates for use in light-emitting diodes (LEDs). In this application, perovskite QDs convert the electric current into light with high color purity and brightness. It will be an ultimate solution for flexible and curved displays in TVs, mobile and wearable devices, virtual and augmented reality glasses, automotive displays and signage. Today, QDLED blue devices can achieve an external quantum efficiency of 12.3% and the green devices achieve an external quantum efficiency of 22 % with the brightness > 1000 Cd/m2 [7]. Since the durability of such perovskite QDLEDs is limited, this application of QDs are still under R&D stage.

X-Ray and UV Photodetectors

Perovskite QDs exhibit outstanding luminescence under exposure with high energy photons, such as UV light or X-rays. This can be utilized in image sensor devices where QDs are used as a scintillation material to convert high energy photons to the visible light that can be captured by commercially available silicon sensors. This technology is brand new [45] and research is still going on.

Lasers

Lead halide perovskite nanocrystals (NCs) have garnered recent attention because of their unique versatility as laser gain materials. The main advantage of Perovskite QDs is their high PLQY up to 100% and high perovskite QDs tolerance to defect states. Different structures of lasers with perovskite QDs, especially with CsPbBr3 QDs can be achieved, such as WGM, VCSEL, DBR, DFB, random cavity, and film only with ultralow lasing thresholds (as low as 0.39 μJ/cm2) [6].

Conclusion

Perovskite materials are extremely promising in the field of advanced optoelectronic industry. For instance, perovskite materials have demonstrated extensive potential in displays, photodetectors, lasing, photovoltaic devices, and even in other applications. The combination of their unique properties makes them useful in real-world applications for the benefit and service of humankind.

References:

https://pubs.acs.org/doi/10.1021/nl5048779

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5872950/

https://www.sciencedirect.com/science/article/pii/S2090997714000212

https://www.nature.com/articles/s41586-018-0451-1?proof=t

https://www.nature.com/articles/s41377-019-0204-4

https://onlinelibrary.wiley.com/doi/10.1002/inf2.12051

https://www.nature.com/articles/s41565-020-0714-5

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