Perovskite Quantum Dots: A Rapidly Expanding Area of Research

Over the last two decades, quantum dots have elicited a considerable amount of excitement and attention from both research scientists [1] and the media [2]. When Sony launched their XBR line of televisions in 2013 [3], quantum dots successfully moved from pure research into the commercial sphere. Despite this, there are still some barriers to overcome before we can expect to see widespread adoption of quantum dot-based products.

What are Quantum Dots?

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Quantum dots are semiconductor nanocrystals, usually only a few nanometres in diameter [4]. Irradiation by UV light promotes electrons to a higher energy state, creating an electron-hole pair. Due to quantum confinement effects, this electron-hole pair recombines to emit light of a very specific wavelength. By tuning the size of the nanoparticles, the wavelength of light emitted can be tuned across the whole of the visible spectrum.

The number of photons emitted relative to those absorbed is known as the photoluminescent quantum yield. This metric largely dictates the quality and usefulness of the quantum dot.

Achieving a High Photoluminescent Quantum Yield

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Early quantum dots were typically made from materials such as cadmium selenide or lead sulphide/sulphide [5]. These materials alone are unable to achieve high photoluminescent quantum yields, however, as some electrons do not recombine with their paired hole. Instead, they are trapped in a non-radiative way by dangling bonds on the nanoparticle surface.

In order to overcome this problem, traditional quantum dots are often coated in another material to form a core-shell structure. This process is known as surface passivation [6]. Although surface passivation has proved successful, the extra synthetic step introduces another layer of complexity to the manufacturing process and makes the resultant quantum dots more prone to defects.

Combined with concerns around the toxicity of cadmium metal [7], this has led to the search for other quantum dot architectures.

Are Perovskite Quantum Dots the Answer?

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As the name suggests, perovskite quantum dots are based on perovskites (any material which has the same crystal structure as calcium titanate) rather than metal chalcogenides.

Perovskite quantum dots are synthesised either at room temperature and in air, or by ‘hot-injection’ [8] in an inert atmosphere. In either case, capping ligands such as oleic acid and oleylamine are used to control nanoparticle size and enhance colloidal stability. Importantly, unlike other quantum dots, perovskite quantum dots do not require passivation in core-shell structures. Perovskite quantum dots are therefore more defect resistant, easier to mass-produce, and can have photoluminescent quantum yields of up to 100% [9].

Commercial Applications of Perovskite Quantum Dots

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The ease of synthesis of perovskite quantum dots, combined with properties that rival those of metal chalcogenide quantum dots, make them an attractive prospect for commercial applications.

LED fabrication and display devices

The excellent colour purity and tunability of perovskite quantum dots, as well as their high photoluminescent quantum yields, in theory makes them ideal for LED fabrication. Research is ongoing to achieve high-efficiency devices, but we may well see perovskite quantum dot-based displays become a reality in the not-too-distant future.

Perovskite quantum dot solar cells

Perovskites are best known for their use in perovskite solar cells, so it is no surprise that perovskite quantum dots have shown a great deal of promise in this area.

The first perovskite quantum dots in solar cells were generated in situ by Im et al. [10] by spin casting salts on to a titanium dioxide substrate. The quantum dots acted as a light-sensitiser and a power conversion efficiency of 6.5% was reported.

Cesium lead iodide perovskite quantum dots are now more commonly used for photovoltaic devices as the nanocrystals have a cubic structure with a small bandgap of 1.73 eV. Researchers have optimised these devices to achieve power conversion efficiencies of up to 15.5% [11].

Photodetection devices

The high absorption co-efficient of perovskite quantum dots over a wide spectral range may also mean that they are suitable for the production of photodetection devices.

Phototransistors based on formamidinium lead bromide quantum dots and graphene have been reported by Pan et al. [12] The quantum dots act as the light absorber and are deposited onto a monolayer of graphene, which carries photoexcited charges to the source/drain.

Using this method, a photoresponsivity of 1.15 × 105 AW-1 has been observed at 520 nm. This is among the highest of any graphene-based photodetectors.

Conclusion

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Perovskite quantum dots are an exciting and rapidly expanding area of research. Their remarkable optical properties and low cost make them appealing candidates as semiconductor materials in a wide range of devices, from LEDs to photovoltaics. In addition, when used in conjunction with other materials such a graphene or PEDOT:PSS, they have shown to form novel device architectures with superlative results.

To facilitate research in the field, we have recently launched a range of low price perovskite quantum dots. We synthesise these in-house using the hot-injection method and are currently able to supply toluene and octane dispersed CsPbBr3 quantum dots types coming soon.

There is a bright future ahead for perovskite quantum dots!