Typically, the synthesis of QDs includes three main elements: precursors, ligands (or surfactants), and solvents. Precursors form highly active monomer during the reaction,^[57] which are used to form small seed nanocrystals (NCs) and then grow larger with the increase of reaction time, and QDs with desired size will be obtained when the reaction is stopped. Ligands provide sufficient space or electrostatic repulsion to stabilize QDs and prevent uncontrolled aggregation of monomers and nuclei in the solvent, but QDs are still allowed to grow at high temperature. Therefore, the size and shape of QDs could be controlled by ligands. Furthermore, ligands could passivate the surface of QDs by eliminating harmful surface defects and suspended bonds of QDs. However, the type and the amount of ligands can unpredictably affect the chemical composition of QDs surface, optical and electrical properties, functionality and processability. In addition, the procedure of purification is significant for the quality of QDs.

Solvents also play an important role. In the early days, most researchers synthesized colloidal QDs by traditional aqueous phase method, which has the deficiencies of low PL QY, wide particle size dispersions, and unwanted spectral tailing. Then the solvent for synthesis is gradually changed from water to organic solvents which have a better effect. For example, because many organic solvent precursors have a higher boiling point than water, QDs with better crystal quality can be obtained by increasing experimental temperature.^[58]

The PbX QDs formation process could be divided into three stages (Fig. 2).^[59,60] In the first stage, the metal and chalcogen precursors react to form monomers of PbX QDs under specific conditions. In the second stage, the supersaturated monomers aggregate fast into some small nuclei until the monomers concentration is lower than the “nucleation critical concentration”. In the third stage, additional monomers will coat on the nuclei to grow into PbX QDs until the monomers concentration is depleted. At this time, the PbX QDs are in the dynamic equilibrium process of growth and dissolution. Ostwald ripening process^[61,62] should not be overlooked at this stage that some smaller PbX QDs could dissolve into monomers and make some larger PbX QDs grow larger.

Fig. 2.

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	Fig. 2. The PbX QDs formation process: prenucleation stage (I), nucleation (II), and growth (III) phases.

In the synthesis of colloidal PbX QDs, reaction temperature, precursor types and concentrations, precursor reaction types (e.g., exothermic or endothermic), stabilizers on the surface of QDs (ligands and surfactants), solvents and so on are important parameters, which can affect both size and morphology of PbX QDs. Therefore, rational control of experimental parameters is necessary for yielding high quality PbX QDs with desired size and morphology. For better utilizing PbX QDs in their applications, it is of critical importance to ensure the quality of PbX QDs. The PL QY and stability in the ambient environment are two of the most vital technical indicators as the mass production of PbX QDs with high quality for commercialization is attracting more and more attention.

The PbX QDs are formed by the reaction of lead and chalcogenide precursors in one synthesis system with solvent and ligands. At present, the most commonly used solvents in the synthesis process are 1-octadecene (ODE) and oleic acid (OA).^[63,64] The OA is also the main choice for ligands of NIR QDs,^[65,66] which are used to stabilize QDs and passivate the dangling bonds on the surface of NIR QDs.

Lead oleates are one of the common lead precursors formed by reacting lead salt, e.g., PbCl₂,^[67–69] PbO,^[19,70–73] and Pb(CH₃COO)₂^{[63,64,74–76]} with OA^[72,73,75] as the coordinating ligand in the non-coordinating solvents, such as ODE and diphenyl ether. In some case, OLA is used as the coordinating ligands.^[68,69]

There are many selenium sources which could be used to make selenium precursors in the synthesis of PbSe QDs, such as n-trioctylphosphine selenide (TOP-Se)^[66,77–79] and bis(trimethylsilyl)selenium (TMS-Se),^[80,81] SeO₂,^[82] and selenourea.^[83] In 2001, Murray et al. first successfully synthesized high-crystallinity PbSe QDs by hot injection of lead oleate and TOP-Se precursors, and the exciton absorption peak also extended from 1200 nm to 2200 nm.^[65] Some researchers also synthesized a series of PbSe from 3 nm to 8 nm in diameter.^[84] Over the next few years, others made appropriate changes based on the method of Murray, and successfully synthesized other types of NIR PbX QDs. In 2010, SeO₂ with high chemical stability was used as a selenium sources in the synthesis of PbSe QDs^[82] conducted in air, without using air-free set up (e.g., glove box or Schlenk line); but SeO₂ material required a higher temperature for reaction, and the size of the synthesized QDs was also larger. Others like elemental selenium, selenourea in N, N-dimethylformamide (DMF), tris (diethylamino) phosphine selenide (TDP-Se), and oleylamine selenide (OLA-Se) are also used as selenium sources.^[79] However, some precursors like selenourea needed to use size-selection precipitation to achieve narrow nanocrystals size distribution^[85] because it was not easy to control the rate of nucleation and growth process.^[86]

Bis(trimethylsilyl)sulfide (TMS-S)^[19,71] was also commonly used as sulfur precursor in the synthesis of PbS QDs. In 2003, Hines et al. used a method of injecting TMS-S into a lead oleate precursor at a high temperature to obtain PbS QDs with a PL QY of 20%–30% and a narrow size dispersion (10%–15%).^[19] There were also other materials that could be used as sulfur sources for synthesizing PbS QDs, such as powdered sulfur,^[68,69] thioacetamide, sodium sulfide,^[75] hydrogen sulfide gas, thioureas,^[87] since many sulfur-containing compounds, the obtained PL QY was not very high and the tunable range was smaller than the QDs synthesized by TMS-S.

Trioctylphosphine telluride (TOP-Te) was usually used as tellurium precursor in the synthesis of PbTe QDs by dissolving powdered Te in TOP. In 2006, Murphy et al. synthesized PbTe QDs by injecting TOP-Te into the lead oleates that resulted from the reaction of PbO and OA in ODE.^[18] In 2012, Pan et al. synthesized PbTe QDs by injecting TOP-Te into PbCl₂-OLA.^[88] TeO₂ was also used as tellurium source by directly heating the mixture of TeO₂ and TOPO at elevated temperature, and Shen et al. used it to synthesize PbTe QDs in 2010.^[89]

Since the use of TOP-Se to synthesize PbSe QDs, the use of TMS-S to synthesize PbS QDs, and the use of TOP-Te to synthesize PbSe QDs are highly toxic and unstable in air, many researchers have been focusing on finding substitutable non-toxic precursors. Although other chalcogenide precursors mentioned above could also be used to synthesize high quality QDs, they still cannot completely replace the TOP-Se, TMS-S, and TOP-Te in synthesis due to either reaction requirement or QDs quality reasons.

Because the wavelength of emission of PbX QDs is closely related to their sizes, there is no doubt that a well control of the PbX QDs size is of great importance for practical application. The number of synthesized QDs is largely dependent on the quantity of nuclei and the rate of nucleation, and the size of prepared QDs is also related to the rate of growth, which are both decided by the monomer supply kinetics.^[87] It is well-known that the current PbX QDs synthesis is still very sensitive to reaction conditions, and thus it is necessary to control multiple parameters to obtain the desired QDs size, such as reaction temperature, reaction time, precursors, and the surface capping ligands involved in the reaction.

In general, higher temperature allows fast nucleation, but also greatly promotes the growth rate of PbX QDs, resulting in larger QDs at the same reaction time.^[63,71] For example, ultra-small PbSe QDs emitting visible light could be obtained at room temperature while mid-infrared emitting PbSe QDs could be achieved under high-temperature.^[77] Typically, as the reaction time prolongs, the total number of QDs decreases because of Ostwald ripening, and the QDs size would gradually become larger. For PbSe QDs, the longer the reaction time, the larger the size of synthesized QDs can be achieved. On the contrary, the PbS NC synthesis is unique since the particle size is not strongly determined by the growth time due to very fast QDs formation process which ends within about several seconds.^[71] Terminating the growth process of PbX QDs is an easy approach to control size, but with low chemical yield and batch-to-batch difference. In the synthesis system at full conversion, burst nucleation could conduct more uniform QDs nuclei,^[90] which is of great importance of synthesizing monodisperse and smaller PbX QDs. Therefore, it is necessary to separate the nucleation from the growth process in the synthesis of high quality and monodisperse QDs.^[91]

The monomer supply kinetics are closely related to the precursor reactivity and the monomer concentration. Figure 3 illustrates the formation of PbX QDs with high and low reactive precursors.^[92] When the reactivity of precursors is high, the fast nucleation of a large number of monomers leads to the synthesis of particles with small size at a high concentration. Conversely, large particles with a lower concentration would be achieved by using low reactivity precursors. Controlled precursor reactivity and quantitative conversion could not only control the obtained PbX QDs size at industrial scales, but also improve the batch-to-batch consistency of PbX QDs.^[87] In 2015, Zhang et al.^[93] utilized Sn precursor to promote the nucleation of PbSe QDs, resulting in PbSe QDs with a size smaller than 3 nm. In 2015, Hendricks et al.^[87] reported a tunable library of substituted thiourea precursors with different monomer supply kinetics thereby the extent of nucleation would be adjusted and finally PbX QDs with a desired size at full conversion could be obtained.

Fig. 3.

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	Fig. 3. Illustration of colloidal nucleation and growth processes.

Surface capping ligands of PbX QDs would also affect the QDs size. In 2007, Baek et al.^[94] found that the sizes of PbSe QDs could be controlled by the chain length of organic ligands at a fixed reaction temperature and time. In 2009, Liu et al.^[71] reported that the higher OA-to-Pb feed molar ratios made larger PbS QDs because the larger amount of OA would provide a better reaction medium solubility for the resulting PbS QDs. They also mentioned that the Pb-to-S feed molar ratio should be larger than 1 in case of the fast and out-of-control growth. In 2011, Yue et al.^[75] reported that the average size of prepared PbS QDs decreased when increasing the concentration of OA; it was due to the reason that OA also influenced the reactivity of the supplied monomer and the higher concentration of OA would lead to a lower precursor reactivity, resulting in slower rate of QDs growth. In 2017, Ma et al.^[96] introduced tributylphosphine (TBP) to synthesize ultra-small PbS QDs (with diameter below 2.5 nm) because TBP slowed down growth process of PbS QDs.

The PL QY is the key factor to indicate the performance of PbX QDs. In the past few decades, the PL QY in core-only PbX QDs has been improved significantly.

PbSe QDs are usually synthesized by reaction of TOP-Se and lead precursor, and the improvement of PL QY in PbSe QDs is shown in Fig. 4. In 2001, Murray et al.^[65] synthesized the first PbSe QDs by rapidly injecting TOP-Se into lead oleate (lead acetate dissolved in OA), but PL QY of these PbSe QDs was not mentioned. In 2002, Krauss et al.^[20] reproduced Murray’s work, and the PL QY of PbSe QDs ranged from 12% to 81% with the emission wavelength ranging from 1200 nm to 1400 nm. The differences in the surface reconstruction during the reaction led to a wide range of changes in the PL QY. In 2004, Colvin et al.^[70] synthesized PbSe QDs with a very high PL QY of 85%. They dissolved PbO in OA and a noncoordinating solvent (ODE) as lead precursor instead of using lead acetate dissolved in OA, while the selenide precursor still used TOP-Se.

Fig. 4.

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	Fig. 4. Improvement of PL QY in PbSe QDs. The data are taken from Refs. [20], [65], [70], and [96]. Copyright 2002, American Chemical Society; Copyright 2004, American Chemical Society; Copyright 2009, American Chemical Society.

In 2009, Klimov et al.^[96] synthesized PbSe QDs with the highest PL QY of 100% using 1,2-hexadecanediol (HDD)-assisted synthesis. There were two possible mechanisms to form PbSe monomer;^[97] under the effect of TOP, Se⁰ and Pb²⁺ obtained two electrons to become Se²⁻ and Pb⁰⁻, then they reacted with Pb²⁺ and Se⁰ to form PbSe QDs, respectively, accompanied by the formation of trioctylphosphine oxide (TOPO) and anhydride. The reducing agents could reduce TOPO to TOP, which was helpful to form more PbSe QDs. It was notable that the suitable reduce agent was important for improving chemical yield of PbSe QDs, and maintaining PL QY at the same time by well controlling the growth rates.

As shown in Fig. 5, the PL QY of PbS QDs has increased from 20% to 90% from 2003 to 2011. In 2003, Hines et al.^[19] synthesized the first PbS QDs using lead oleate (PbO dissolved in OA) and TMS-S diluted in ODE with the lead/sulfide molar ratio of 2:1. Their PbS QDs showed a good controllable wavelength from 800 nm to 1800 nm with a narrow size dispersion of 10%–15%. And the PL QY of PbS QDs could reach 20%, which would decrease as the particle size increased.

Fig. 5.

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	Fig. 5. Improvement of PL QY of PbS QDs. The data are taken from Refs. [19], [98], [99], and [100]. Copyright 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim; Copyright 2006, American Chemical Society; Copyright 2008, American Chemical Society; Copyright 2011, American Chemical Society.

In 2006, Cademartiri et al.^[98] presented another route to synthesize PbS QDs with wavelength from 1245 nm to 1625 nm, and higher PL QY of 40% was achieved. The elemental sulfur (S) dissolved in oleylamine (OLA) was quickly injected into PbCl₂ dissolved in OLA. In this unique system, amine served as the capping group and Cl⁻ ions balanced the charge on the surface of PbS QDs.

In 2008, Abel et al.^[99] introduced TOP in the synthesis to improve the PL QY of PbS QDs based on Hines’s synthesis, and the PL QY of 80% could be improved from 40%.

The TOP played an important role on the performance of the PbS QDs because OA ligands would well bind to lead atoms on the surface of PbS QDs, and sulfur atoms were not passivated and would trap hole, resulting in poor luminescence. The TOP introduced in the synthesis of PbS QDs could efficiently bond to the non-passivated sulfur atoms to decrease the hole defects, resulting in better luminescence. In 2011, Moreels et al.^[100] introduced TOP in the synthesis of PbS QDs based on Cademartiri’s synthesis, and the PL QY could be improved from 40% to 90%.

High stability of QDs in the ambient environment will enable mass production and wide range of applications of PbX QDs in the future. PbX QDs are air-sensitive and light-sensitive, which are easy to be oxidized under the light and air. The destructive oxidization of PbX QDs usually causes the blue shift of PL peak and the reduction of PL QY. There are three approaches to improve stability of PbX QDs illustrated in this review including the core/shell structure, halide treatments, and multiple injections.

2.5.1. Core/shell structure

In order to enhance the stability of PbX QDs, a shell material with larger bandgap and better chemical stability is grown over the PbX QDs core to passivate trap states, which could confine the holes and the electrons in the core and protect the core from being exposed to air. As shown in Fig. 6, there are two approaches to form core/shell structure including the cationic exchange and epitaxial shell growth.^[101,102] The coated shell may exist in three cases such as uniform shell, composition gradient shell or alloyed shell.

Fig. 6.

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	Fig. 6. Core/shell structure of QDs after cation exchange and epitaxial shell growth.

Cationic exchange is the most common method to form the shell for PbX QDs because the Pb²⁺ cations near the surface are susceptible to be exchanged with other metal ions in the solution such as Cd²⁺ cations, which could be proved by the blue-shifted emission of resulted core/shell QDs compared to that of core-only QDs due to the reduced size of PbX QD. In 2008, Pietryga et al.^[37] firstly produced PbSe/CdSe and PbS/CdS core/shell QDs using cationic exchange method, and those resulted QDs were stable against spectral attenuation and spectral shifting. Additional ZnS shell could also be coated on the surface of PbX/CdX core/shell QDs, which were likely to reduce toxic effects associated with exposure to heavy-metal ions. In 2010, Zegar et al.^[103] synthesized PbSe/CdSe core/shell QDs by Pietryga’s method. In 2011, Zhao et al. synthesized thick-shell PbS/CdS QDs with PL QY up to 67% and of much better photo- and thermal stability compared with those of core-only PbS QDs by two-step cationic exchange method. In the following years, there were still many other groups^{[50,64,104,105]} that reproduced PbX/CdX QDs based on Pietryga’s method. And in 2013, Ren et al. reported a microwave-assisted cationic exchange method for the synthesis of PbS/CdS core/shell QDs through a uniform growth method, and those obtained QDs showed a PL QY of as high as 57% which was about 1.4 folds higher than that achieved by the same PbS/CdS core/shell QDs prepared using traditional heating in an oil bath. In 2017, Yang et al. made NIR-QLEDs based on high-quality PbS/CdS QDs followed by Ren’s method.

Although cationic exchange method could improve PL QY and stability compared to core-only PbX QDs, it was still difficult to control the interface between the core and the shell and to fine tune the thickness of the CdX shell. The successive ion layer adsorption and reaction (SILAR) method proposed by Peng et al.^[106] was used to coat epitaxial CdX shell on PbX QDs, increasing their stability over time which was attributed to the protective CdX layer against oxidation. In 2010, Zhang et al.^[107] obtained PbSe/CdSe core/shell QDs with high PL QY of 70% by SILAR method in solution, and a red-shifted emission could be observed with the growth of CdSe, which could be explained by the increased delocalization of carriers into the shell.^[101] They proved that the SILAR method could exactly control the thickness of CdSe shell and largely improved the stability of PbSe QDs, however, the SILAR method was also performed at temperature as high as that could be used in cationic method, which might lead to an ion exchange reaction, and it also needed to carefully avoid secondary nucleation of the excess shell precursors. In 2016, Sagar et al. demonstrated a colloidal atomic layer deposition growth (c-ALD) method for the synthesis of PbS/CdS core/shell QDs at room temperature, which largely preserved the pronounced spectral features of PbS QDs and fine control over the CdS shell growth. But they did not demonstrate PL QY enhancement by the epitaxial CdS shell growth on the PbS QDs, which might be caused by the nonradiative recombination by defects at the interface between PbS and CdS. In 2017, Bawendi et al. also used room-temperature c-ALD method to synthesize PbS/CdS core/shell QDs, and they found that the PL QY and the stability against photobleaching of resulted PbS/CdS QDs could be further tremendously improved by the Cd(OA)₂ treatment.

There were also some groups that combined cationic exchange and SILAR together. In 2016, Zhao et al. synthesized PbS/CdS/CdS core/shell/shell QDs with ultra large shells by using SILAR method to deposit CdS layer on the prepared PbS/CdS QDs by two-step cationic exchange method. In 2017, Hollingsworth et al. also sequentially applied partial-cation exchange method and SILAR method to reduce giant PbSe/CdSe/CdSe core/shell/shell QDs, which were highly stable for both photobleaching and blinking were suppressed.

2.5.2. Halide treatments

Halide treatments are attractive to improve the stability of PbX QDs because the resulting ligands are compact to avoid oxidation and the halide ions could balance excess charge resulting from nonstoichiometric surface termination, which could be realized by using halide precursors before the reaction or injecting halide salts during the reaction. In 2013, Zhang et al.^[30] introduced the halide in the synthesis of PbS and PbSe QDs. Sulfur precursor or selenide precursor was injected into lead precursor. PbY₂ (Y = Cl, Br, I) served as both the lead source and the capping ligand, as shown in Fig. 7; the halide atoms bonded to the surface of PbX QDs, which improved the photostability of both PbS and PbSe QDs stored under air. In 2014, Ju Young Woo et al.^[108] achieved air-stable PbSe QDs, by adding halide salts (PbCl₂ or PbI₂) after synthesizing PbSe QDs. In 2015, Yi Pan et al.^[109] used Zhang’s method^[30] to synthesize PbSe QDs and found that the PbSe QDs remained stable in air for 24 months.

Fig. 7.

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	Fig. 7. PbS QDs with halide treatments. Reproduced with permission from Ref. [30]. Copyright 2014, American Chemical Society.

2.5.3. Multiple injections

In 2016, Franky et al.^[110] synthesized PbS QDs based on Weidman’s synthesis, which used multiple injections of the sulfur precursor to grow the PbS QDs without initiating new nucleation during growth and Ostwald ripening.^[67] These QDs showed great stability and the resulting devices were stable even without encapsulation.

However, the PbX QDs could not be upscale because of the long-time synthesis process, the complicated operation, and high cost. Adjusting the temperature and reaction time is the simplest operation of synthesis. In 2006, Cademartiri et al.^[98] synthesized 1.5 g PbS QDs from a 50 mL reaction by a solventless technique. In 2013, Zhang et al.^[30] batched up PbS QDs based on Hines and Cademartiri approach, and PbS QDs of 47 g were achieved. It was also a challenge to add large volumes of the precursors in current hot-injection protocols for mass production. In 2017, Yarema et al.^[111] reported an under-pressure-assisted method to upscale QDs for the large-batch synthesis (Fig. 8), which allowed fast addition of large injection volumes, and 17.2 g PbS QDs were finally achieved. In addition, those creative additives (such as the secondary phosphine sulfide precursors^[72,112,113] or the thiourea precursors^[87]) could not only improve nucleation of QDs, but also upscale the QD quantity.

Fig. 8.

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	Fig. 8. Schematic illustration of setups and obtained products for (a) conventional syringe-assisted hot-injection and (b) underpressure-governed hot-injection methods.[111]

Long alkyl chains insulating molecules, such as OA and OLA, are usually used as ligands in the synthesis of PbX-based QDs. However, those insulating ligands will greatly affect the performance of NIR-QLEDs caused by blocking inter-dot carrier transfer. Therefore, it is necessary to replace the original long alkyl chains with shorter ligands to enhance the diffusion of carriers for high-efficiency NIR-QLEDs. In 2012, Frank W. Wise et al.^[6] in Cornell University studied the effects of different length ligands on PbS-based NIR-QLEDs. They used ligands with different carbon chain lengths (such as MPA, MHA, MOA, MUA) to control the inter-dot spacing in the emitting layer by solid-state ligand exchange. They found that the inter-dot spacing became smaller as the length of the surface ligands decreased. On the one hand, the diffusion of carriers in the emission layer was increased, thereby increasing the exciton recombination; on the other hand, too short inter-dot spacing would enhance the probability of exciton separation and reduce the exciton recombination. In addition, surface ligands of different lengths also affected the light extraction efficiency and carrier balance of NIR-QLEDs. Their experimental results (Fig. 11(a)) showed that the 8-carbon chain (8-mercaptooctanoic acid, MOA) ligand was the best choice for NIR-QLEDs (the energy-level diagram of the device is shown in Fig. 11(b), which first achieved 2% peak EQE and 6.4 W · Sr⁻¹ · m⁻² radiance at 1054 nm.

Fig. 11.

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	Fig. 11. (a) EQE dependence on inter-dot spacing; (b) energy-level diagram of the device. Reproduced with permission from Ref. [6]. Copyright 2012, Springer Nature.

The low PL QY of NIR QDs films was another reason for the poor EQE of NIR QLEDs, which was mainly caused by self-quenching, wherein inter-dot transport facilitated transport-assisted trapping, leading to non-radiative recombination even at relatively rare defect sites. Four methods (“core/shell QDs”, “perovskite matrix”, “iodide capped QDs”, “ternary blend of QDs film”) were used to passivate NIR QDs film to improve device performance in recent years (as shown in Fig. 12).

Fig. 12.

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	Fig. 12. Passivation of traps in PbX based NIR-QLEDs.

In 2015, Vladimir Bulovic et al.^[105] in Massachusetts Institute of Technology improved the performance of NIR-QLEDs by replacing the PbS QDs to PbS/CdS core/shell QDs by cationic exchange. On one hand, the CdS shell could significantly passivate PbS QDs against in situ nonradiative pathways in NIR-QLEDs (as shown in Fig. 13(a)). On the other hand, most of the carrier would be confined in the core because the band gap of the shell was larger than the core’s, and the CdS shell could protect PbS QDs from the damage of water and oxygen. Therefore, the PL QY and stability of the QDs were both significantly improved. The PL QY of PbS based QDs films could be increased by 100 times, and efficient PbS/CdS QDs based NIR-QLEDs of 4.3 ± 0.3% EQE at 1242 nm were achieved (the energy-level diagram of the device is shown in Fig. 13(b).

Fig. 13.

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	Fig. 13. The CdS shell could significantly passivate PbS QDs against in situ nonradiative pathways in NIR-QLEDs. Reproduced with permission from Ref. [105]. Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In 2017, based on the work mentioned-above, Xuyong Yang et al.^[7] in Shanghai University improved PL QY of core/shell PbS/CdS QDs film by iodine treatment in order to further improve the performance of NIR-QLEDs. As shown in Fig. 14(a), the PL emission intensity of the I⁻ ligand capped PbS/CdS QDs film was 1.56 times higher than that of untreated PbS/CdS QDs film because the capped I⁻ could passivate the surface defects of PbS/CdS QDs. In addition, the capped I⁻ on PbS/CdS QDs played a significant positive role on the charge transport process that the current injection efficiency of the NIR-QLEDs was higher than that of without I⁻ treated based NIR-QLEDs. These significant improvement in the surface characteristics of PbS/CdS QDs contributed to the performance enhancement of NIR-QLEDs, and a maximum EQE of 4.12% and a maximum radiance of 6.04 W · Sr⁻¹ · m⁻² at 1510 nm was achieved (the energy-level diagram of the device is shown in Fig. 14(b).

Fig. 14.

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	Fig. 14. (a) The PL intensity of solution-phase iodine treated QDs films vs. the untreated QDs films; (b) energy-level diagram of the device. Reproduced with permission from Ref. [7]. Copyright 2017, Springer Nature.

In 2016, Edward H. Sargent et al.^[33] in University of Toronto selected perovskite as matrix of PbS QDs to improve the performance of NIR-QLEDs because perovskite had higher carrier mobility and fewer defect states than other matrix materials such as polymers and inorganic crystals. On one hand, the perovskite matrix enabled efficient transportation photo-generated carriers to PbS QDs, and the diffusion length of carriers could be largely enhanced. On the other hand, MAPbI_xBr_{3 − x} perovskite matrix could naturally passivate the surface of PbS QDs, and the strain at the interface between perovskite and PbS QDs and the density of defects were reduced because the lattices of PbS QDs and MAPbI_xBr_{3 − x} perovskite matrix matched well. These reasons mentioned above improved the PLQY of the QDs. As shown in Fig. 15(a), the PL QY of QDs was enhanced and reached the peak at 50% iodine content when increasing the bromine concentration. And efficient NIR-QLEDs of 4.7 ± 0.4% EQE at 1391 nm were finally achieved (the device structure is shown in Fig. 15 (b).

Fig. 15.

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	Fig. 15. (a) PL QY of PbS QDs in MAPbIxBr3 − x; (b) the SEM image of the device. Reproduced with permission from Ref. [33]. Copyright 2016, Springer Nature.

In 2018, Gerasimos Konstantatos et al.^[56] in Institució Catalana de Recercai Estudis Avançats (ICREA) fabricated high-efficiency NIR-QLEDs by blending ZnO nanoparticles (NPs) and two different sizes of PbS QDs. The defects in PbS QDs film were passivated and the carrier supply to the emitting layer could be balanced by the ternary blend method. As shown in Fig. 16(a), PL QY of PbS QDs films was increased by introducing smaller PbS QDs matrix into defective PbS QDs and reached the peak of 60% while the PL QY of single PbS QDs films was only 2.3%. It was explained that the small PbS QDs matrix could absorb excited light and possessed long carrier diffusion lengths to supply the emitting QD species with carriers, therefore, the number of proportionally defective large PbS QDs existence in the same volume was lower. The n-type ZnO NPs would passivate electron traps and enhance radiative recombination in the PbS QDs film. The PL QY of ternary blend films was increased to 80% at 20% ZnO NPs content of the emitting layer. Mix ligands exchange (MPA and ZnI₂) was used to further improve the efficiency of NIR-QLEDs because mixed ligands could improve carrier’s mobility and passivate defects, and mixed ligand exchange based NIR-QLEDs showed lower turn-on voltage and higher radiance than that of single MPA ligand. Finally, efficient NIR-QLEDs of 7.9% EQE and radiance of 9 W · m⁻² · Sr⁻¹ at 1400 nm were achieved, which was the highest performance of NIR-QLEDs up-to-date (the energy-level diagram of the device is shown in Fig. 16(b).

Fig. 16.

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	Fig. 16. (a) PL QY of binary and ternary QD blends with different emitter QD loadings in the donor QD matrix; (b) energy-level diagram of the device. Reproduced with permission from Ref. [56]. Copyright 2018, Springer Nature.