Perovskite Quantum Dots

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Perovskite quantum dots (PQDs) have been developed in the past few years and are now coming to commercial prominence due to their excellent photophysical properties, low heavy metal content and low cost fabrication. The development of PQDs dates back to the discovery in 2012 of enhanced PL emissions in nanosized perovskite embedded in meso-Al2O3.1 Research publications on these materials have grown significantly after highly luminescent colloidal perovskite QDs were first reported in 2015.2 3 4
They possess exceptional optical properties including:
• high brightness
• tuneable emission wavelength
• high colour purity
• high defect tolerance.
PQDs are being developed as alternative down-conversion materials in phosphor-converted light-emitting diodes (pc-LEDs) for lighting and next-generation of display technology.

Properties
The most common perovskite quantum dots are called lead halide pQDs. They have the chemical formula APbX 3, where:
• A can be either an organic component (e.g. methylammonium (MA) or formamidinium (FA)) or an inorganic atom, most commonly caesium (Cs).
• X is a halogen (either chlorine (Cl), bromine (Br) or iodine (I)).
The choice of halide affects the emission wavelength, as explained below:
• APbCl 3 dots give blue-like emission
• APbBr 3 dots give green-like emission
• APbI 3 dots give red-like emission
Just like MCQDs, the emission wavelength can be fine-tuned through the dot size, or through creating mixed-halide dots – where 2 different halogens are present in the same dot. Because they don’t contain volatile organic compounds within the dot itself, caesium pQDs tend to be more robust over time compared to MA or FA-based dots.

Figure 1. A pQLED device structure.
Right: A green pQLED in operation, based on CsPbBr 3 pQDs.
Source: Ossila.

Synthesis methods
Synthesis methods for PQDs include:
• Hot Injection Method.
• Ligand Assisted Reprecipitation (LARP).
• Mechanochemical Methods.
• In situ formation strategy within inorganic and polymer matrices.
These methods have numerous shortcomings that researchers have sought to rectify recently with new synthesis methods.5 6

Applications
Main applications are:

Electronics
• Light-emitting Diodes (LEDs).7
• MicroLEDs
• Single Photon Sources
• Photodetectors
• Quantum computing

Energy
• Solar Cells

Other
• Lasers

PQDs are attractive for display application due to their high optical properties including:
• high PLQY
• narrow FWHM
• tuneable emission wavelengths.

perovskite quantum dots under UV light (PRNewsfoto/Fuji Pigment Co., Ltd.)

Figure 2: Perovskite quantum dots under UV light.
Image credit: Fuji Pigment.

Even when compared to well-developed CdSe and InP QDs, prototype colour conversion LEDs based on perovskite QDs show superior colour performance and improved luminous efficiency (120% NTSC, 109 lm/W), enabling enhanced LCD panels. The performance of perovskite QD-based EL devices now approaches that of other conventional QDs, and demonstrates promise for use in flexible displays.

Challenges
PQDs usually show a poor crystallinity, relatively low photoluminescence quantum yield (PLQY), and poor stability based on synthesis methods such as injection. These limitations have hindered the use of PQDs in practical applications.
The structural and optical stability of PQDs is the main challenge. It has been reported that the disintegration of CsPbX3 QDs, accompanied by the photoluminescence (PL) quenching, is accelerated by polar organic solvents and water or under ultraviolet irradiation. To address this stability issues, surface ligand regulation is widely utilized to retain the colloidal and structural integrity of PQDs.

Recent commercial activity
In September 2018, Quantum Materials Corp., a manufacturer of Cadmium-free quantum dots, developed a continuous flow manufacturing process to produce stable, low cost, high purity perovskite quantum dots (PQDs). Quantum Materials Corp has developed a high-volume production process that produces extremely high purity PQDs with significantly improved stability.
China-based QD developer Zhijing Nanotech has demonstrated a perovskite-QD film (PQDF) for application in LCD-TVs. Avantama AG has developed a proprietary 1-step synthesis process for PQDs and has a production capacity of 1kg perovskite/day in suspension form.

Figure 3. Zhijing Nanotech Quantum Dot backlight units (QD-BLUs) using perovskite quantum-dot film (PQDF).

 

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