The unique visual and electric qualities of halide perovskites make them a common choice for solar cells, LEDs, lasers, and photodetectors. Some of these qualities are a high absorption coefficient, a long carrier diffusion length, the ability to change the makeup and bandgap in a wide range of ways, and the ability to process solutions cheaply. Colloid halide perovskite quantum dots (HP-QDs) are a new area of study. Their electrical and visual qualities depend a lot on their crystal structure and particle size. Researchers have looked into HP-QDs for use in solar cells and LEDs. These include the mixed organic-inorganic MAPbX3, FAPbX3, and all-inorganic CsPbX3. This chapter gives an overview of the most recent progress in making HP-QDs, studying their photophysics, and using them in devices.
The Synthesis of Halide Perovskite QDs
Solution reaction methods have been used to make a group of highly solid and uniformly distributed HP-QDs with different particle sizes and chemical makes-ups. You can easily change these qualities by changing things like the chemical make-up, the concentration of the precursors, the reaction temperature, and the synthesis time. Several wet chemical methods are used to make HP-QDs, such as ligand-assisted reprecipitation (LARP), hot injection, and post-synthetic transformation. This makes it possible to finetune the visual and electric features of these materials.
Ligand-Assisted Reprecipitation Method
Schmidt et al. were the first to report on the LARP method in 2014. It was used to make MAPbBr3 QDs with a width of 6 nm. MABr, which is a long-chain alkyl ammonium bromide, was mixed with PbBr2 in the presence of oleic acid (OA) and octadecene (ODE). They made MAPbBr3 QDs, but they only had a 20% photoluminescence quantum yield (QY). However, their work proved that HP-QDs could be made using a solution-based method. Zhang et al. made the LARP method even better so they could make MAPbBr3 QDs with a high PLQY. To make a perovskite precursor solution, they mixed MABr, PbBr2, OA, n-octylamine (OAm), and polar dimethylformamide. They then added toluene while shaking very hard. Centrifugation was used to get the OA/OAm-capped colloidal MAPbBr3 QDs, which have an average width of 3.3 nm. It was thought that the high PLQY was because the exciton binding energy went up and the Br-rich surface was chemically properly protected. The LARP method was also used to make all-inorganic CsPbX3 QDs, which had a PLQY of up to 90%, a narrow full width at half maximum (FWHM) of the PL, and good photostability.
Hot Injection Method
The hot injection method can be used to make different kinds of standard semiconductor QDs, like CdSe, PbS, and PbSe QDs. In 2015, Protesescu et al. were the first to report a way to make colloidal all-inorganic halide perovskite CsPbX3 QDs. There were three steps to the method: first, a Cs+ cationic precursor solution was made; next, a solution with PbX2 salt, OA, OAm, and ODE was heated; and finally, the mixture was centrifuged.
The reaction starts with OA and Cs2CO3 dissolved in ODE. This is followed by heating and removing the gas from the hoover. When a solution with PbX2 salt, OA, OAm, and ODE is heated to 140–200 °C, another solution with PbX2 salt, OA, OAm, and ODE is made. After that, the solution is heated to between 140 and 200 °C, which turns it into a MAPbBr3 QD solution.
If you change the reaction temperature and halide elements, you can fine-tune the size and discharge of CsPbX3 QDs. A number of different types of colloidal organic–inorganic hybrid perovskite QDs and Pb-free all-inorganic HP-QDs have been made using this method. These include MAPbI3, FAPbI3, CsPb1−xSnxI3, and Cs2SnI6 QDs.
To sum up, the hot injection method is a flexible method that has been used to make many kinds of semiconductor QDs, including CsPbX3 QDs.
Ion Exchange Reactions
Hydrogen peroxide (H-QDs) has a lot of charged particles and is structurally unstable. This makes it possible to change their chemistry and optical qualities after they have been prepared, while keeping their original shape and size. We learnt about a halide ion exchange process for CsPbX3 QDs from Nedelcu et al. They used different halide intermediates, like PbX2 salts, oleylammonium halides (OAm-X), and methylmagnesium halides (MeMgX). You can change the halide ratios in the colloidal nanocrystal solution to change the PL spectra of CsPbX3 QDs over the whole visible range (410–700 nm) while keeping the high PLQYs (20–80%) and narrow emission line widths (10–40 nm) (from blue to red). Akkerman et al. used a similar halide ion exchange method to change the optical features of CsPbX3 QDs. They used tetrabutylammonium halides (TBA-X), octadecylammonium halides (ODA-X), OAm-X, and PbX2 salts as halide sources.
The ion exchange method was also used for mixed MAPbX3 QDs. The MAPbBr3 QDs were added to a solution of MACl or MAI-dissolved isopropyl alcohol (IPA). Ion exchange was used to change the “A-set” or “B-set” of the original HP-QDs after they were synthesised. Stam et al. made Pb-free CsPb1−xMxBr3 QDs using a “B-set” cation exchange method. In this method, the Pb2+ cations in the CsPbBr3 QDs are switched out for Sn2+, Cd2+, and Zn2+ cations. It’s true that the unit cells of the CsPb1−xMxBr3 QDs get a little smaller than those of the CsPbBr3 QDs, but their shape and size stay the same.
Protesescu et al. described a “A-site” bidirectional cation exchange reaction between FAPbI3 and CsPbI3 QDs. This reaction took place in a toluene solution with FAPbI3 QDs and a Cs-oleate precursor, creating CsxFA1−xPbI3 QDs. You can finetune the peak PL bands of CsxFA1−xPbI3 QDs by using cation exchange processes after the QDs are made.
The Photophysics of Halide Perovskite QDs
Tunable Bandgap
Halide perovskite is made up of corner-sharing BX6 octahedra with A-site cations and a three-dimensional structure with the formula ABX3. The highest and lowest points of the valence band and conduction band are set by hybrid states that are antibonding between the B-s and X-p orbitals and nonbonding between the B-p and X-p orbitals. The bandgap of halide perovskite depends on the size and type of ions. You can change this by mixing and swapping elements at each site. Due to the quantum size effect in QDs, the size of HP-QDs can also change their bandgaps. This can be seen when the temperature of synthesis goes from 110 to 170 °C.
Multiple Exciton Generation
Quantum dots (QDs) have an interesting feature called multiple exciton generation (MEG). MEG has been seen in both direct and indirect bandgap materials. According to Vogel et al., MEG in MAPbI3 QDs lasts for a few femtoseconds, which is less time than it takes for exciton cooling and recombination. It took Li et al. 90 fs to find MEG in FAPbI3 QDs because energetic hot carriers cool down slowly. As the size of the FAPbI3 QDs got bigger, the MEG QY got bigger too. We saw that 7.5 nm FAPbI3 QDs had a MEG threshold of 2.25 Eg and a MEG slope efficiency of 75%. Weerd et al. used TA spectroscopy to find MEG that worked well in all-inorganic HP-CsPbI3 QDs. You can find out that the MEG threshold and efficiency for 12 nm CsPbI3 QDs with an Eg of 1.77 eV were 2 Eg and 98%, respectively, by looking at how the TA spectrum changes when light hits it.
Hot Electron Extraction
For many years, people have been studying how to get hot electrons out of quantum dots (QDs). Molecular adsorbates are often used as electron acceptors to get the hot electrons out. Ultrafast spectroscopy with time resolution is a good way to watch the hot and cold electron extraction process from QDs. Wu et al. used benzoquinone (BQ) as an electron acceptor to study for the first time how dynamic electron extraction works in CsPbBr3 QDs using TA spectroscopy. The time it takes for an electron to move from CsPbBr3 QDs to BQ has been found to be 65±5 ps. Sarkar et al. used BQ as an electron acceptor and used time-resolved terahertz spectroscopy to show efficient hot electron transfer from CsPbBr3 QDs to BQ on a timescale of less than 300 fs.
Hot electrons were also seen moving, and the time constants were found to be 30–50 ps. Organic–inorganic MAPbBr3 QDs have been used to get hot electrons. Researchers by Li et al. used a molecule called Bphen, which accepts electrons, to get about 83% of the photogenerated hot electrons out of MAPbBr3 QDs in just 1 ps at room temperature. The researchers discovered that the MAPbBr3 QDs had hot-carrier cooling times about twice as slow (∼18 ps) and temperatures about four times higher (∼1800 K) than the MAPbBr3 bulk film. This is because the QDs have an inherent phonon bottleneck effect and an Auger heating effect.
These results show that taking hot electrons out of HP-QDs makes the photocurrent of a solar device based on HP-QDs stronger. It has been seen that Halide Perovskite QDs have surface passivation with time constants of 30–50 ps. The study gives us useful information about how hot electron extraction from QDs might be used to make solar devices more current dense.
Surface Passivation of Halide Perovskite QDs
QDs have a higher ratio of surface area to volume than bulk materials, and their main source of charge comes from the unbonded orbitals of their surface atoms. It is easy to break HP-QDs when they are being isolated and cleaned. Surface passivation using ligand engineering or the core/shell approach can lower the QD surface trap-state density and stop phase changes, deterioration, and oxidation.
Surface Ligand Engineering
When high-performance liquid crystals (HP-QDs) are being made, long-chain organic acids and amines are often used as ligands to control their growth. Oleylamine and OAm have been used successfully as surface ligands in HP-QD synthesis, but it is easy for them to come off during the purification process. Long-chain organic acids, such as OA and OAm, can be added to the production system to help ligands stick to the HP-QD surface better.
Luo et al. made MAPbBr3 QDs by mixing APTES with various acidic ligands, including OA, benzoic acid, and acetic acid. The MAPbBr3 QDs that were made had higher PLQY (32–55%) than QDs that were made with only OA and APTES ligands or without any ligands. This proves that the acidic and amino groups work together to make something stronger.
To make α-CsPbI3 QDs, phosphinic acid ligands, like bis-(2,2,4-trimethylpentyl)phosphinic acid (TMPPA), were used instead of OA ligands. These included acidic ligands and OAm. It is shown that TMPPA-based QDs have higher PLQY and better phase stability in solution compared to OA-based QDs.
Using trioctylphosphine (TOP) as the ligand and solvent, a new way was found to make a PbI2 precursor that is stable and highly volatile. The α-CsPbI3 QDs that were synthesised have a PLQY of about 100% and are more chemically stable than QDs that were made using the OA/OAm approach. Using the TOP method, they were also able to make Pb-less halide perovskite CsPb1−xSnxI3 QDs.
A study by Krieg et al. described a way to make CsPbX3 perovskite QDs using a zwitterionic molecule that has both cationic and anionic groups in one molecule. When this kind of ligand was used to make CsPbI3 perovskite QDs, they showed a PLQY of up to 90%, even after four rounds of precipitation and redispersion.
Post-Synthetic Treatment
Passivating quantum dots (QDs) after they are synthesised can make them more stable in dispersion and lower the number of surface defects in high-performance liquid-phase semiconductors (HP-QDs). In their study, Pan et al. used IDA to passivate the surface of CsPbI3 QDs, which led to a 95% PLQY and better stability. The amount of metal to halogen in HP-QDs has a big effect on their stability and qualities. Surface traps on HP-QDs are mostly caused by sites that are “B”-site-element-rich or “X”-site-element-poor. Koscher et al. created a post-synthetic thiocyanate salt treatment method to fix the lead-filled surfaces of CsPbBr3 QDs. This reduced surface trap states and increased the PLQY of both newly synthesised and old QDs. Ahmed et al. described using a post-synthetic tetrafluoroborate salt treatment to improve the PLQY of CsPbBr3 and CsPbClxBr3−x QDs. This treatment led to a 50-fold improvement in PLQY, even for blue-emitting QDs. To get rid of surface traps on CsPbX3 QDs, Woo et al. used metal bromide salts and metal iodide salts in a simple but successful in situ passivation method. The PLQYs of CsPbX3 QDs went up a lot after passivation because the surface trap density went down. Lead halides made the surface of CsPbX3 QDs better, which made HP-QDs more stable.
Surface Coating
Putting a polymer or artificial safety shell on top of halide perovskite QDs is a good way to keep them from reacting with the surroundings. Long-chain polymers like polystyrene (PS), polymethyl methacrylate, ethyl cellulose, poly(styrene-butadiene-styrene), polyacrylonitrile, and polyvinyl chloride can be mixed with HP-QDs to make perovskite–polymer composites that glow brightly and are very resistant to moisture. These composites can be found in film or bulk form.
A few months ago, Wong et al. showed how photoactivated polymerisation of vinyl monomers could create a scattered CsPbBr3 QD–polymer hybrid. The polymer chains grew and firmly attached to the surface of the CsPbBr3 QDs by interacting with the olefin’s filled π-orbitals and the electron-deficient Pb of the CsPbBr3 QDs. This created nanocomposites of QDs and polymers that were each individually sealed. These were very stable against wetness and had glowing qualities.
A group of scientists led by Zhang used polyvinyl pyrrolidone (PVP) as a binding ligand to protect the surface of CsPbX3 QDs during the solution self-assembly synthesis at room temperature. They then put the QDs inside PS microhemispheres (MHSs) to make “water-resistant” CsPbX3 QDs@MHSs nanocomposites. The PVP protective layer not only made it possible to change the makeup of the CsPbX3 QDs, but it also served as an interface layer, allowing the CsPbX3 QDs to be compatible with PS polymers. The “water-resistant” CsPbX3 QDs@MHSs nanocomposites that were made showed high moisture and photostability and can be used as bioassay displays probes.
Putting HP-QDs inside an artificial material is another good way to keep perovskite QDs safe. Wang et al. mixed CsPbBr3 QDs with mesoporous silica in a hexane solution. The mesoporous silica has pores that are about 12–15 nm in size. This made the material more stable at high temperatures and in light. Scientists Dirin and others mixed perovskite precursor solutions with mesoporous silica. This created high-quality halide perovskite APbX3 QDs in the silica pores after they dried. These QDs had very bright PL and quantum efficiencies above 50%. Malgras et al. made hybrid MAPbX3 QDs that are uniformly distributed inside mesoporous silica templates. The templates’ pore sizes can be changed to make the QDs fit different needs.
The anchoring-induced separation keeps the HP-QDs from touching each other and also stops the growth and breakdown caused by light. The HP-QD-SiO2 nanocomposites that were made are very stable when exposed to light and air. Inorganic oxides like AlOx and TiO2 can also be used to treat HP-QDs to make them more stable in air and humidity.
Putting a polymer or artificial safety shell on top of halide perovskite QDs is a good way to keep them from reacting with the surroundings. Long-chain plastics such as polystyrene, polymethyl methacrylate, ethyl cellulose, poly(styrene-butadiene-styrene), polyacrylonitrile, and polyvinyl chloride can be mixed with HP-QDs to make composites that glow brightly and don’t get wet easily.
Applications of Halide Perovskite QDs
Light-Emitting Diode (LED)
Halide Perovskite QDs (HP-QDs) have been looked into for use in LEDs because they have special optical qualities, including a high PLQY close to unity, highly saturated colours, a narrow emission bandwidth, and a wavelength that can be changed to cover the whole visible spectrum. The external quantum efficiency (EQE) of HP-QD-based LEDs has gone from 0.12% to 21.3% in just four years. This is because the quality of the HP-QDs has gotten better and the structure of the device has been optimised.
Schmidt et al. were the first to use ODs of the organic hybrid perovskite MAPbBr3 in LEDs with a structure made of ITO, PEDOT:PSS, Ploy-TPD, MAPbBr3 QDs, Ba, and Ag. Huang and others used MAPbBr3 QDs of high quality (PLQY∼80%) to make green LEDs with an ITO/PEDOT:PSS/MAPbBr3 QDs/TPBi/CsF/Al structure. These LEDs had a maximum current efficiency of 4.5 cd/A, a power efficiency of 3.5 lm/W, and an EQE of 1.1%. A group of scientists led by Xing created a very good LED using an ITO/PEDOT:PSS/MAPbBr3 QDs/TPBi/Cs2CO3/Al structure and amorphous MAPbBr3 QDs as the emissive layer. The LED had a maximum current efficiency of 11.49 cd/A, a power efficiency of 7.84 lm/W, and an EQE of 3.8%.
In their study, Yan et al. discovered that Auger non-radiative recombination in HP-QDs is a key part of stopping emission. To fix this problem, they added high PLQY MAPbBr3 QDs that gave off one-order excitonic emission into the LED device. The device was made up of ITO/PEDOT:PSS/QDs/B3PYMPM:TPBi/B3PYMPM:Cs2CO3/Al. LEDs got a high EQE of 12.9% and a high power efficiency of 30.3 lm/W at high brightness levels above 1000 cd/m2. This was possible by stopping Auger non-radiative recombination and balancing the charge in the low driving current density range.
In the past few years, a lot of work has been made on making all-inorganic HP-QD-based LEDs. In 2015, Zeng et al. were the first to report all-inorganic CsPbX3 QD-based LEDs. The blue, green, and orange LEDs had EQEs of 0.07%, 0.12%, and 0.09%, respectively. In their study, Zeng et al. showed an OIHL passivation method for treating HP-QDs. This method used organic ligands on the surface of HP-QDs to keep the safety of HP-QD films.
By using aryl-based aniline hydroiodide (An-HI) and long alkyl-based oleylammonium iodide (OAM-I), Chiba et al. made deep-red CsPbBr3−xIx QDs from CsPbBr3 QDs in an anion-exchange method. The matching CsPbBr3−xIx QDs were used in LEDs with a structure of ITO/PEDOT:PSS/poly-TPD/CsPbBr3−xIx QDs/TPBi/Liq/Al, which showed EQEs of up to 21.3% and 14.1%, respectively. Shi et al. created a solution-processed method to make all-inorganic CsPbBr3 QD-based LEDs with n-ZnO and n-ZnO-based ligands. This shows that HP-QDs can be used in LEDs.
Solar Cells
A lot of people are interested in halide perovskites because they have the right bandgap, a high extinction coefficient, a high carrier mobility, a low trap density, and a long diffusion length. In only 10 years, the power conversion efficiency (PCE) of halide perovskite solar cells has gone from 3.8% to over 25%. One of the first times that MAPbBr3 and MAPbI3 were used as photosensitisers for QD-sensitized solar cells was by Kojima et al. in 2009. They got PCEs of 3.13 and 3.81%, respectively. The PCE of MAPbI3-based QD-sensitized solar cells was raised by Im et al. to 6.5% in 2011. A big step forward was made by Luther et al. when they reported stable cubic CsPbI3 QD-based planar heterojunction solar cells that worked well and had a high PCE of 10.77%.
Luther et al. came up with an A-sitecation halide salt (AXE) treatment method to change the surface chemistry of QDs and make the charge carriers move around more freely in α-CsPbI3 QD films. The coupling effect between HP-QDs got stronger after AXE treatment, and the photocurrent of the AX-treated CsPbI3 QD solar cells went up. This led to a high PCE of up to 13.43%. Wang et al. used μ-graphene (μGR) sheets with high mobility to connect the α-CsPbI3 QDs and make μGR/CsPbI3 QD films. They discovered that the μGR/CsPbI3 QD films they made not only work well as a route for carrier transfer, but they also stay stable at high temperatures and in wet conditions better. The solar cells had a high PCE of 11.40 percent and stayed stable over time.
Bian et al. recently made a CsPbBrI2 perovskite solar cell structure by combining a CsPbBrI2 bulk film with CsPbI3 QDs. The solar cells had a high PCE of 11.40 percent and stayed stable over time. The μGR/CsPbI3 QD film-based solar cells have shown good results in terms of their energy level, the amount of charge carriers they create, and their ability to catalyse reactions.
In conclusion, halide perovskites have a lot of promise in the solar cell business because they have the right bandgap, a high extinction coefficient, a high carrier mobility, a low trap density, and a long diffusion length. New discoveries in the field have led to the creation of stable and effective planar heterojunction solar cells made of cubic CsPbI3 QDs. These cells have a high PCE and stay stable over time. These solar cells work even better now that MAPbBr3 and MAPbI3 are used as photosensitisers and high-mobility β-graphene sheets are added.
Conclusion and Outlook
People are becoming more interested in HP-QDs because they have great optical and electrical qualities that make them potential options for the next wave of optoelectronics. They have a PL spectrum that can be changed, a high PLQY, and are easy to process as solutions, which makes them appealing for use in optoelectronic fields. This part talks about how to make HP-QDs, how they work, how to passivate their surfaces, and how they can be used. Even though manufacturing and surface posttreatment have gotten better, flaws still exist. This means that effective surface passivation or posttreatment technologies need to be created. Long-term safety is a big problem that needs to be solved by creating new designs and device packing methods.
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