Perovskite Quantum Dots: Introduction, Theory, & Applications
The most well-studied semiconducting nanocrystals (NCs or ‘quantum dots - QDs’) are of the metal chalcogenide variety (e.g. CdSe, InP, PbS). They are single crystals, a few nanometers in diameter. When excited, their small size confines electrons and holes in an area smaller than the exciton Bohr radius, acting as a ‘quantum box’. The smaller the dot, the greater the confinement energy, and the higher the energy of photons which will be absorbed and emitted. The bandgap of metal chalcogenide quantum dots can be tuned through the entire visible spectrum simply by changing their size during synthesis.
For the highest photoluminescence quantum yields (PLQYs), a core/shell arrangement is usually required. With this, a second semiconductor encases the nanocrystal (e.g. CdSe/CdS, InP/ZnS) to passivate surface defects of the emissive core (which would otherwise act as non-radiative recombination sites for excitons). Due to their high PLQY, relative ease of fabrication, and wide emission-colour tunability, they are especially suitable for display and imaging technologies - already appearing in commercial products (e.g. televisions).
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A new class of nanocrystal is emerging based on perovskites, with properties rivalling or exceeding those of metal chalcogenide QDs. Due to their outstanding photovoltaic performance, perovskites have been receiving a lot of attention from the research community for several years already. Recently, it was shown that reducing the dimensions of the perovskite crystal down to a few nanometers results in a quantum dot with very high PLQY and color purity (i.e. narrow emission linewidths of ~10 nm for blue emitters and 40 nm for red emitters ).
These quantum dots are highly tolerant to defects, as they require no passivation of the surface to retain their high PLQY. Although defect and trap sites are present, the energies are fully within the conduction or valence bands, and not the bandgap itself . These perovskite nanocrystals are simple to synthesise in a colloidal suspension. They are also easily integrated into optoelectronic devices using readily available processing techniques, making them a strong contender for future technologies.
Halide perovskite nanocrystals have a cubic crystal structure with the chemical formula A+Pb2+X-3. They can be classed as hybrid organic-inorganic, 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 the hybrid organic-inorganic type . Mixed halide perovskites can be produced where X is a mixture of Cl/Br or Br/I.
For visible optoelectronic applications, the nanocrystals are generally synthesised with 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 ) by changing either the nanocrystal size or halide ratios (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 ). A mixture of methylamine bromide and lead bromide was injected into a long chain alkyl ammonium bromide and oleic acid in octadecene solution. 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% , and later to ~100% by changing the capping ligand .
Hot injection was again used for the colloidal synthesis of inorganic metal-halide perovskite quantum dots, first reported by Protesescu et al . That recipe is as follows:
- 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.
- 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 is dissolved, the temperature is increased to between 140-200°C (depending on the required nanocrystal size).
- The caesium precursor is injected. After 5 seconds, the mixture is rapidly cooled in an ice bath. The quantum dots can be isolated through centrifuging.
The resulting nanocrystals will have surface ligands comprised of OA and OLA . The nanocrystals were found to have PLQYs up to ~90%, and an emission linewidths of 12 nm for the smallest crystals (4 nm size, 410 nm emission wavelength) and 42 nm for the largest (15 nm size, 700 nm emission wavelength).
Mixed-halide Perovskite Quantum Dots
An advantage that perovskite quantum dots have over their metal chalcogenide counterparts is the simplicity with which their emission properties can be altered. 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]. Mixing a donor halide source such as octadecylammonium (ODA-Y), chloro-oleyalmine-oleylammonium chloride (OLAM-Y) or tetrabutylammonium (TBA-Y) halides (where Y is Cl, Br or I) with a solution of CsPbX3 nanocrystals allowed the chemical composition of the nanocrystals to be tuned continuously over the range CsPb(X1-Z:YZ), 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 the peaks of the constituent nanocrystals, thereby retaining the narrow linewidth needed for color purity. It is 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 can be easily accomplished simply by mixing different stock solutions of the nanocrystal constituents at different volume ratios (e.g. CsPbBr3 and CsPbI3 to get CsPb(Br1-Z:IZ)3 [7,9]). Both methods allow the nanocrystal emissions to be tuned over the entire visible range while retaining a high PLQY and color purity. The anion exchange can be suppressed by adding polyhedral oligomeric silsesquioxane (POSS) to the solutions. This acts as a protective cage around the nanocrystals, and allows mixing of different halide compositions while retaining the constituent nanocrystal's photoluminescent properties. It also has the added effect of protecting the nanocrystals from water .
Applications of Perovskite Quantum dots
Light-emitting Diodes (LEDs)
Metal chalcogenide quantum dots already play a role in consumer display products - so with their increased PLQY, ease of synthesis, excellent colour purity, and wide colour tunability, perovskite quantum dots should be well-suited to this application. However, charge injection and transport in nanocrystal films must be optimised in order to achieve high-efficiency devices.
First devices by Song et al. used ITO/PEDOT:PSS/PVK/ CsPbX3/TPBi/LiF/Al structure to demonstrate blue, green, and orange LEDs . While the emission linewidths were narrow, the brightness of the LEDs was modest (<1000 cdm-2), and the external quantum efficiencies (EQE) were limited to ~0.1%.
Li et al. showed the importance of nanocrystal surface chemistry when the EQE of CsPbBr3 nanocrystal LEDs was increased 50x (0.12% to 6.27%) through a mixture of charge-transport layer optimisation and surface ligand density control (through a washing procedure using hexane and ethyl acetate ). While the surface ligands are needed to passivate the surface and prevent aggregation (leading to high PLQY and greater stability), too many ligands inhibit electrical injection and transport. By controllably tuning the ligand density, a brightness of >15000 cdm-2 was obtained, with high colour purity (~20nm emission linewidth) for ~8nm nanocrystals.
One proposal that bypasses the electrical properties of nanocrystal films is to use them as down-converters for inorganic blue or UV LEDs. Pathak et al. dissolved hybrid organic-inorganic perovskite quantum dots of various mixed halide compositions (representing green and red emission) into polymer solutions (polystyrene) and spincast them into thin films . The insulating polymer matrix prevented anion exchange, preserving the individual emission peaks of the nanocrystals, and allowing the generation of white light when illuminated with a commercial blue LED.
Amplified spontaneous emission (ASE) has been observed in dropcast films of CsPbBr3, and mixed CsPb(Br/I)3 and CsPb(Cl/Br)3 nanocrystals. Pump thresholds can be as low as 5 µJcm-2 , which compares very favourably with other colloidal QD systems (an order of magnitude lower than spectrally similar CdSe QDs). The ASE emission intensity is extremely stable in air, dropping only 10% after several hours of irradiation and ~107 shots in ambient conditions, which also compares extremely well to chalcogenide QDs . The stimulated emission is identified as coming 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 thresholds). The ASE wavelength can be tuned throughout the entire visible spectrum through mixed halide composition.
Lasing was observed in a whispering gallery mode configuration. It was later shown that stimulated emission could be observed in CsPbBr3 nanocrystal films following two-photon absorption . It was found that the two-photon absorption cross-section was 2 orders of magnitude larger than similar metal chalcogenide quantum dots, leading to a stimulated emission threshold of green-emitting CsPbBr3 nanocrystals of 2.5 mJcm-2. This is far lower than core-shell metal chalcogenide quantum dots. This non-linear stimulated emission could also be tuned across the visible wavelengths by varying mixed halide composition. Green stimulated emission from CsPbBr3 quantum dots (following three-photon absorption) was also observed – a first for any type of quantum dot. Perovskite quantum dots are therefore an exciting prospect for next-generation lasers.
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 the materials have been available. However, there are some very promising recent results that suggest perovskite quantum dots could play a role future photovoltaic devices.
The first use of perovskite quantum dots in solar cells was in 2011 by Im et al., where MaPbI3 nanocrystals acted as a light-sensitiser in a structure resembling a dye-sensitised solar cell . A power conversion efficiency of 6.5% was reported. This predated the synthesis of colloidal perovskite quantum dots, and the nanocrystals were instead formed through surface interactions when a mixture of methylammonium iodide and lead iodide was spincast onto a TiO2 surface.
At room temperature, bulk CsPbI3 forms an orthorhombic crystal lattice with a large bandgap of ~2.8 eV. The cubic phase is far more suitable for photovoltaic applications because of a smaller 1.73 eV bandgap. However, this only forms in bulk CsPbI3 at temperatures above 300°C. Due to the elevated temperature and the effect of reduced surface are, all CsPbX3 nanocrystals crystallise into the cubic phase during synthesis. Whereas CsPbCl3 and CsPbBr3 quantum dots are phase-stable in the cubic polymorph over long periods, CsPbI3 will convert back to an orthorhombic configuration over a few days in ambient conditions.
Swarnkar et al. showed that treating spincast CsPbI3 quantum dot films with methyl acetate stabilises the cubic structure . This is done by changing the surface energy via the removal of unreacted precursors - without causing aggregation of the dots. The resulting film was stable for months in ambient conditions, and had excellent electronic properties. When fabricated into solar cells, this film achieved a PCE of over 10% and a large open-circuit voltage of 1.23 V. Furthermore, LEDs with the stabilised CsPbI3 nanocrystals as the active layer displayed a low turn-on voltage of <2eV.
It was later demonstrated that coating the nanocrystals in A+X- (where A is formamidinium, methylammonium or Cs, and X is I or Br) further improves charge-carrier mobility of the nanocrystal films. This allowed solar cells of PCE ~13.4% to be fabricated – the highest efficiency photovoltaics based on quantum dots of any kind . This is also promising for perovskite tandem solar cells - where a bulk perovskite film performs the role of the small bandgap absorber, and the perovskite quantum dots act as the wide bandgap absorber .
Single Photon Sources
Single photon sources are required for new light-based quantum information systems. Current efforts are focused towards epitaxially-grown quantum dots, diamond colour centers and colloidal nanocrystals. Of these, colloidal NCs are the most promising for room-temperature visible operation .
Dilute CsPbX3 (X=Br, I or Br/I) NC solutions have been spincast to create spatially-separated individual QDs [20,21]. Imaging the photoluminescence from individual NCs showed the blinking behaviour that is characteristic of single emitters. Photon coincidence counting revealed low g(2) values of ~6%, demonstrating an efficient, anti-bunched single photon source at room temperature – all of which are desirable characteristics for quantum technologies.
In comparison with metal chalcogenide QDs, metal halide perovskite QDs display shorter fluorescence lifetimes and higher absorption coefficients - and therefore, faster and more efficient production of single photons.
The high absorption co-efficient of perovskite QDs over a wide spectral range may make them a suitable candidate for light-detection devices. Pan et al. have reported the fabrication of a phototransistor based on FAPbBr3 quantum dots and graphene . The QDs act as the light absorber, and are deposited onto a monolayer of graphene which carries photoexcited charges to the source/drain. The phototransistor shows a broad response spanning the visible spectrum, but with suppressed response to photons with energies below the bandgap of ~540nm. A photoresponsivity of 1.15×105 AW-1 was observed at 520nm, which is amongst the highest of any graphene-based photodetectors.
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