Perovskite Quantum Dots

Perovskite quantum dots are semiconducting nanocrystals. Compared to metal chalcogenide quantum dots, perovskite quantum dots are more tolerant to defects and have excellent photoluminescence quantum yields and high colour purity. These properties are highly desirable for electronic and optoelectronic applications and hence perovskite quantum dots have huge potential for real world applications including LED displays and quantum dot solar cells.

Caesium lead bromide (CsPbBr₃) perovskite quantum dots powder is readily soluble in hexane, octane and toluene.

Applications of Perovskite Quantum Dots

Perovskite quantum dots have huge potential for a range of applications in electronics, optoelectronics and nanotechnology. Currently, the field is not well researched, but initial results are extremely promising. Details on a selection of the applications that have been investigated are given below.

Quantum Dot Solar Cells

Currently, reports of perovskite quantum dot solar cells are still limited, especially when compared to bulk and 2-dimensional perovskites. This is likely due to the limited time that such materials have been available. However, recent results  suggest that perovskite quantum dots could play a role in future photovoltaic devices.

Light-Emitting Diodes (LEDs)

Metal chalcogenide quantum dots already play a role in consumer display products - so the increased PLQY, ease of synthesis, excellent colour purity, and wide colour tunability of perovskite quantum dots suggest that they should be well-suited to such applications. However, charge injection and transport in nanocrystal films must be optimised in order to achieve high-efficiency devices.

Lasers

Amplified spontaneous emission (ASE) has been observed in drop cast films of CsPbBr₃, and mixed CsPb(Br/I)₃ and CsPb(Cl/Br)₃ nanocrystals. Pump thresholds can be as low as 5 µJ cm^-2 [13]; a value that compares very favourably with other colloidal QD systems (e.g. an order of magnitude lower than spectrally similar CdSe QDs). The ASE emission intensity is extremely stable in air, dropping by only 10% after several hours of irradiation and ~10⁷ shots in ambient conditions. This performance also compares extremely well to chalcogenide QDs [14]. The stimulated emission has been identified as resulting from the recombination of biexcitons (which are more stable at room temperature than excitons), with red-shifted emission leading to reduced self-absorption (and hence low lasing thresholds). The ASE wavelength can also be tuned throughout the entire visible spectrum via mixing the halide composition.

Singler Photo Sources

Single photon sources are required for new light-based quantum information systems. Here, current efforts mainly focus on the use of epitaxially-grown quantum dots, diamond colour centers and colloidal nanocrystals. Of these, colloidal NCs are the most promising for room-temperature visible operation [20].

Photodetectors

The high absorption coefficient of perovskite QDs over a wide spectral range may make them suitable candidates for use in light-detection devices. Pan et al. have reported the fabrication of a phototransistor based on FAPbBr₃ quantum dots and graphene [22]. The QDs which act as the light absorber, are deposited onto a monolayer of graphene that transports photoexcited charges to the source/drain. Such phototransistors have a broad response spanning the visible spectrum, although they have reduced response to photons having energies below the semiconductor bandgap (540 nm). Here, a photoresponsivity of 1.15×10⁵ AW^-1 was observed at 520 nm; a value that is amongst the highest of any graphene-based photodetectors.

Perovskite Quantum Dot Structure

Halide perovskite nanocrystals have a cubic crystal structure with the chemical formula A⁺Pb²⁺X^-₃. They can be classed as an organic-inorganic hybrid, where A is an organic cation such as methylammonium (MA) or formamidinium (FA), or fully inorganic (A=Cs), and where X is a halogen (Cl, Br or I). Due to the lack of volatile organics, fully-inorganic nanocrystals tend to have better stability and higher PLQY (>90%) than hybrid organic-inorganic materials [3]. Mixed halide perovskites can also be produced where X is a mixture of Cl/Br or Br/I.

For visible optoelectronic applications, the nanocrystals are generally synthesised to have a size of 4 - 15 nm (dependent on the halogen atom and the required optical properties). The emission wavelength can be tuned through the entire visible spectrum (400 - 700nm [4]) by changing either the nanocrystal size or halide ratio (for mixed halide systems).

Perovskite Quantum Dot Synthesis

The first hybrid organic-inorganic perovskite quantum dot colloidal synthesis of MAPbBr₃ was reported by Schmidt et al. using a hot injection method (similar to that used to synthesise metal chalcogenide QDs [4]). A mixture of methylamine bromide and lead bromide was injected into an octadecene solution containing oleic acid and a long chain alkyl ammonium bromide. The PLQY of the resulting QDs was ~20%, and was stable for several months due to the stabilising and capping effects of the ammonium bromide and oleic acid. By optimisation of the reactant molar ratios, the PLQY was increased to over 80% [5], and later to ~100% by changing the capping ligand [6].

Hot injection was again used for the colloidal synthesis of inorganic metal-halide perovskite quantum dots, first reported by Protesescu et al [1]. That recipe developed was as follows:

1. The caesium precursor Cs-oleate is first prepared by mixing caesium carbonate (Cs₂CO₃) and oleic acid (OA) in octadecene (ODE), and heating under nitrogen until the Cs₂CO₃ has reacted with the OA. This solution must be kept above 100°C to prevent precipitation of the Cs-oleate.
2. A lead halide precursor is prepared by mixing a lead halide (PbCl₂, PbI₂, PbBr₂ or a mixture of these) in ODE at 120°C under nitrogen, along with OA and oleylamine (OLA) that act as stabilising agents. Once the lead halide has dissolved, the temperature is increased to between 140-200°C (depending on the required nanocrystal size).
3. The caesium precursor is then injected. After 5 seconds, the mixture is rapidly cooled in an ice bath, with the quantum dots being isolated through centrifuging.

The resulting nanocrystals have surface ligands comprised of OA and OLA [3]. Such nanocrystals were found to have PLQYs up to ~90%, with the smallest crystals (4 nm diameter) having an emission linewidth (full width half maximum) of 12 nm at an emission wavelength of 410 nm, with the largest quantum dots (15 nm diameter) having a linewidth of 42 nm at 700 nm.

Mixed-Halide Perovskite Quantum Dots

An advantage that perovskite quantum dots have over their metal chalcogenide counterparts is the simplicity by which their emission properties can be modified. In addition to tuning the emission wavelength during synthesis through reaction temperature (and ultimately, nanocrystal size), it can also be changed post-synthesis through an anion-exchange reaction [7,8]. By mixing a donor halide source such as octadecylammonium (ODA-Y), chloro-oleyalmine-oleylammonium chloride (OLAM-Y) or tetrabutylammonium (TBA-Y) halide (where Y is Cl, Br or I) with a solution of CsPbX₃ nanocrystals, the chemical composition of the nanocrystals can be tuned continuously over the range CsPb(X_1-Z:Y_Z), where 0≤Z≤1.

Anion exchange is followed by lattice reconfiguration, giving a mixed halide structure. This results in a single emission peak at an energy somewhere in between those of the constituent nanocrystals, thereby retaining the narrow linewidth needed for color purity. However it has been found that direct conversion between CsPbI₃ and CsPbCl₃ is not possible because of the large mismatch in the size of the halide ions.

It has also been demonstrated that this anion exchange process can be easily accomplished by simply mixing different stock solutions of the nanocrystal constituents at different volume ratios (e.g. CsPbBr₃ and CsPbI₃ to obtain CsPb(Br_1-Z:I_Z)₃ [7,9]). Both methods allow the nanocrystal emission to be tuned over the entire visible range while retaining a high PLQY and color purity. The anion exchange process can however be suppressed by adding polyhedral oligomeric silsesquioxane (POSS) to the solution. This creates a protective cage around the nanocrystals, and allows mixing of different halide compositions while retaining the photoluminescent properties of the constituent nanocrystals. It also has the added effect of protecting the nanocrystals from water [10].

Literature

References

• Nanocrystals of Cesium Lead Halide Perovskites (CsPbX₃, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut, L. Protesescu et al., Nano Lett., 15 (6), 3692–3696 (2015)
• Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance, Huang et al., ACS Energy Lett., 2 (9), 2071–2083 (2017)