Organometal halide perovskites (OHPs, CH3NH3PbX3, X = Cl, Br, I) are receiving great attentions as one of the most promising semiconductor materials for low cost solution-processed photovoltaic devices [1-5]. Meanwhile, their excellent inherent optical properties also open up the potential application in the field of photonics and optoelectronics [6-12]. Bulk halide perovskites (films and single crystal) exhibit wide wavelength tunability (from green to infrared, 400-800nm) , narrow-band emission (full width at half maximum, FWHM ~ 20nm) , and long carrier diffusion lengths (up to hundreds of micrometers), which have been the desired characteristics for the use in laser and LEDs [4, 13]. However, the photoluminescence (PL) or electroluminescence (EL) efficiencies of these halide perovskites are very low at low power excitation or under low current density, which had been an obstacle for their exploration as functional emissive materials [8, 14, 15].
Quantum confinement occurs when the semiconductor has at least one dimension close to or smaller than the two times of its exciton Bohr radius [16-18]. It has been well-known to be powerful mean to tailor the physical properties of semiconductors toward new generation functional devices [19-22]. Based on this, a few efforts have been made to manipulate the optical properties of halide perovskite through artificial size control [23-28]. Especially, quantum dots (QDs) become one of the focuses in this research direction . Compared with bulk materials, the quantum yields (QYs) of halide perovskite QDs reach up to 90% , making them particularly attractive for display application.
In the past few years, our group has been working on new generation colloidal QDs for lighting and display applications, especially with a focus on I-III-VI CuInS2 and CuInSe2 QDs [31-36], and the emerging CH3NH3PbX3 QDs [12, 30, 37, 38]. In the following, we firstly introduce recent progress in colloidal synthesis of halide perovskites QDs and then discuss their optical properties. Finally we highlight earlier results of OHPs based light emitting devices. In addition, current challenges and important research tasks are also discussed.2. Synthesis of halide perovskite QDs 2.1. Template-based growth
CH3NH3PbX3 perovskite nanoparticles were firstly observed by using TiO2 mesoporous nanocrystalline films as template for photovoltaic cells . The porosity of TiO2 film allows the formation of 2-3 nm sized CH3NH3PbBr3 and CH3NH3PbI3 nanoparticles on the surface of TiO2 nanocrystalline. Besides TiO2, Al2O3 and ZrO2 mesoporous films were also used as templates [5, 39, 40]. These CH3NH3PbX3 perovskite nanoparticles prepared by spin-coating on mesoporous films is both light absorbers and hole transporter in solar cells . In 2012, Ikegami and co-workers observed an enhanced PL emission in CH3NH3PbBr3/Al2O3 films . Recently, Longo et al. further improved the PLQYs of CH3NH3PbBr3/Al2O3 composites to 40%, as a consequence of the precise size control .2.2. Hot-injection technique
In 2014, Perez-Prieto et al. reported the synthesis of freestanding nanometer-sized halide perovskite nanocrystals by using non-template method . The synthesis is very similar with the hot-injection method for inorganic nanocrystals. 6 nm sized CH3NH3PbBr3 nanocrystals were obtained by injecting a mixture of short methyl chain CH3NH3Br and PbBr2 into a longer chains alkyl ammonium bromide [e.g. CH3(CH2)17NH3Br], with oleic acid (OA) and octadecene (ODE) as the coordinating solvents. The longer chains ammonium bromide salts played a vital role in the success of small-sized nanocrystals. The resulting CH3NH3PbBr3 nanocrystals exhibited a considerable PLQY up to 20% as a colloidal solution and thin film, and could be kept stable for a few months. After that, an optimized route gives CH3NH3PbBr3 nanocrystals with PLQYs up to 83% . On the basis of this method, Tisdale's group obtained colloidal nanoplatelets with excitonic absorption feature by combining additional purification steps . In 2016, Kovalenko's group reported a novel synthesis method that without adding polar solvents . This method primarily based o n ionic mechanism, where methylamine was served as the CH3NH3+ source, PbX2 was served as both Pb2+ and X- source. Using this method, highly luminescent (15-50%) CH3NH3PbX3 nanocrystals with tunable shapes (nanocubes, nanowires, and nanoplatelets) can be prepared.2.3. Ligand-assisted reprecipitation technique
Although non-template method can produce nano-sized halide perovskite nanocrystals, smaller sized QDs with diameter less than 4 nm (exciton Bohr radius of CH3NH3PbBr3 〜2 nm) are much urgent to demonstrate the quantum confinement effects. Reprecipitation synthesis is a very simple and available technique through mixing solvent and has been widely applied to prepare organic nanoparticles and polymer dots . By introducing long chain ligands into the precipitation process, our group developed a convenient and versatile method to fabricate highly luminescent and color tunable CH3NH3PbX3 QDs with controlled size and composition by simply mixing the perovskite precursor solution with insoluble nonpolar solvent, named ligand-assisted reprecipitation (LARP) technique . Fig. 1a schematically illustrates the detailed process of the LARP technique. A mixture of ion sources (CH3NH3X and PbX2) and surface ligands (OA and oleylamine) was dissolved into a good solvent (dimethylformamide, DMF) to form the precursor solution, after that, above precursor solution was dropped into vigorous stirring toluene. Almost immediately, a colloidal solution that has very strong fluorescence emission was formed, indicating the very rapid aggregation process of the precursor into QDs (Fig. 1b). The resulting CH3NH3PbX3 QDs synthesized by this method have an average diameter of 3.3 nm (Fig. 1c), and the corresponding absolute PLQY can reach up to 70%. By simple mixing the PbX2 in precursors, a series of colloidal CH3NH3PbX3 QDs with tunable composition could be fabricated. As shown in Fig. 1d, under a 365 nm UV lamp excitation, CH3NH3PbX3 QDs colloidal solution emit different light that cover entire visible spectrum colors from blue to red, with the PL peaks can be finely tuned by regulating the composition (Fig. 1e). This technique is probably the simplest and lowest cost method to produce highly emissive CH3NH3PbX3 QDs for potential lighting and display application.
|Figure 1. (a) Schematic of the LARP process. (b) The photo of colloidal CH3NH3PbBr3 QDs. (c) TEM image of colloidal CH3NH3PbBr3 QDs. (d) Photo images of CH3NH3PbX3 QDs under ambient light and 365 nm UV lamp. (e) PL spectra of CH3NH3PbX3 QDs. Reproduced with permission from ref. . Copyright 2015 American Chemical Society.|
Subsequently, Rogach's group founded a variation of the LARP method to synthesize CH3NH3PbBr3 QDs with different sizes and different emission colors . In their work, the diameters of QDs can be well controlled from 1.8 nm to 3.6 nm by varying the temperature (0, 30 and 60 ℃). The PL can be tuned between 470 nm and 520 nm with corresponding PLQYs between 74% and 93%, respectively. Meanwhile, Zeng's group and Deng's group extended the LARP synthesis to fabricate all inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite QDs [47, 48]. Recently, Zhang's group using the branched molecules as capping ligands synthesized CH3NH3PbBr3 QDs of different size (2.5-100 nm) with high PLQYs (15%-55%). Compared with straight-chain ligand, branched ligands-capped perovskite QDs show high stability in protic solvent, the authors attributed that to the strong steric hindrance and hydrolysis properties of these branched ligands .2.4. Nonaqueous emulsion synthesis
Colloidal perovskite QDs exhibit excellent PL properties, making them promising candidates for LEDs. However, in practical applications, excess precursors and surfactants usually limit the integration of QDs into devices . What is worse, the most commonly used purification techniques were failed to purify colloidal perovskite QDs, because the polar solvents usually induce PL quenching or even destroy the preformed perovskite QDs. To solve this problem, we further developed the nonaqueous emulsion synthesis . As shown in Fig. 2a, this method could divide into two steps. In the first step, immiscible polar (DMF) and nonpolar solvents (n-hexane), as well as a surfactant (OA) were mixed to form emulsion. Subsequently, the addition of demulsifier (tert-butanol, acetone) initialized the mixing of precursors and induced the crystallization process. After that, by adding a certain amount of tert-butanol into the colloidal solution, CH3NH3PbBr3 QDs can be precipitated, and then dried into solid-state powder (Fig. 2b). The size control in this method was achieved by varying the amount of demulsifier, resulting in size-tunable samples with diameter from 2 to 8 nm. The resulting purified CH3NH3PbBr3 QDs have an excellent PLQY up to 80%, and a modest yield (25 mg per bath, Fig. 2b). In addition, we also revealed that the resulting spherical dots could self-assembly into nanoplatelets due to the oriented alignment of the dipole moment of cationic methylamine (unpublished results).
|Figure 2. (a) Scheme of the nonaqueous emulsion process. (b) Photos of CH3NH3PbBr3 emulsion, QDs solution and powder. (c) TEM image of CH3NH3PbBr3 QDs. Reproduced with permission from ref. . Copyright 2015 American Chemical Society.|
2.5. Other methods
Apart from above methods, Scholes's group reported an interesting two-step process to produce colloidal perovskite QDs . In this method, one of the most critical step is the preparation of nearly monodisperse spherical PbX2 QDs. After that, the PbX2 QDs were used as templates to react with long-chain alkyl ammonium halide to obtain corresponding perovskite QDs. By using different long-chain alkyl amine or the mixture of them, different layered perovskite QDs structure with n = 1, 2, 3 were produced. It is noted that the halide perovskite QDs prepared via this method has a different molecular formula with that prepared in the other reports.3. Optical properties 3.1. Excitonic properties
The special feature of colloidal QDs is the excitonic properties, which strongly correlated with the enhanced PL properties [37, 51]. Exciton binding energy is an important parameter to characterize the exctionic features. Fig. 3a and 3b show the temperature dependent PL spectra of CH3NH3PbBr3 QDs and corresponding micrometer-sized particles. The extracted exciton binding energy of bulk CH3NH3PbBr3 is about 65 meV, which is large enough to show excitonic effects. However, previous reports have suggested that the dominated recombination of bulk CH3NH3PbBr3 comes from free electrons and holes . Oppositely, the extracted exciton binding energy of CH3NH3PbBr3 QDs is about 375 meV, which is nearly six fold larger than bulk materials (Fig. 3c and 3d). Therefore, the PL emission of CH3NH3PbBr3 QDs mainly originates from exciton recombination due to the enhanced excitonic stability against thermal stability.
The dynamic of excitons in perovskite QDs has also been studied using transient absorption spectroscopy. Scholes's group concluded that hybrid perovskite QDs also exhibit similar excitonic dynamics in comparison to perovskite films . From the global analysis results, they attributed the short lived species to electroabsorption and/or a charge-induced "Stark-effect" [8, 52], and the long lived species to exciton recombination . Zhang's group studied the exciton dynamics of perovskite QDs with two different aliphatic ammonium capping ligands. They revealed that capping ligand selection plays a critical role in directly affecting the energy level as well as excitonic dynamics processes and PLQYs . Based on above results, it is concluded that the exciton binding energy in QDs increased with decreasing size. Unlike bulk materials, perovskite QDs are more likely inclined to generate stable excitons at room temperature, which account for the high PLQYs . Zhang et al. utilized time-resolved PL as well as transient absorption spectroscopy demonstrated the QDs dynamics was dominated by charge carrier recombination mode, the chemical passivation of QDs surface capping ligands can effectively weaken trap-assisted recombination .
|Figure 3. Temperature dependent PL spectra for (a) CH3NH3PbBr3 QDs and (b) CH3NH3PbBr3 bulk materials. Integrated PL emission intensity as a function of temperature of (c) QDs (273-393 K) and (d) bulk material (50-300 K). (e) PL and optical absorption spectra of CH3NH3PbBr3 QDs. (f) Time-resolved PL decay and corresponding fitting curve of CH3NH3PbBr3 QDs. Reproduced with permission from ref. . Copyright 2015 American Chemical Society.|
3.2. PL properties
For more information about the exciton recombination dynamic of perovskite QDs, steady and time-resolved PL spectra were further studied . Fig. 3e shows the PL and optical absorption spectra of CH3NH3PbBr3 QDs. An obvious excitonic absorption peak at about 505 nm was observed in absorption spectrum. The PL emission peaked at 515 nm with a FWHM value of only 21 nm. The observed small Stokes shift (〜49 nm) implied that the PL emission of QDs was originated from direct exciton recombination. As shown in Fig. 3f, the PL decay curve of CH3NH3PbBr3 QDs can be described by biexponential fitting, giving a short-lived PL lifetime of 6.6 ns (63.6%) and long-lived PL lifetime of 18.0 ns (36.4%). According to the study of highly luminescent CdSe QDs , the longer component was attributed to exciton recombination with involved surface states due to the stable excitons at room temperature, while the short-lived one could attribute to the recombination of initially generated excitons upon light absorption. The average PL decay lifetime of CH3NH3PbBr3 QDs was much smaller than thin film (〜100 ns) , which further confirmed the PL decay of CH3NH3PbBr3 QDs primary come from exciton radiative recombination.
Except the exciton binding energy, surface properties also play an important role to obtain highly luminescence QDs [59, 60]. In our study, we revealed that the surfaces of CH3NH3PbX3 QDs were halogen-rich  and the surface defects are well capped to confine the excited electrons to achieve improved PLQYs. As shown in Fig. 4a, the richness of Br atoms at surface play an important role in inhibiting the trapping of electrons by surface defects and resulting in a high PLQYs. The energy dispersive spectroscopy (EDS) result of Br/Pb radio up to 3.5 further evidenced a Br-rich surface of QDs. This conclusion is also supported by the X-ray photoelectron spectroscopy (XPS) measurements (Fig. 4d). This inherent halogen-rich surface was denoted as self-passiviation effects, although the deep insight is needed. Recently, Yan et al. investigated the effect of CH3NH3Br and Cl concentrations on the PL properties of CH3NH3PbBr3_xClx, they found both the CH3NH3Br and Cl concentrations have significant influence over the PL emission intensities of CH3NH3PbBr3 and CH3NH3PbBr3-xClx .
|Figure 4. (a) Structural model for CH3NH3PbBr3 QDs. (b) EDS spectrum for CH3NH3PbBr3 QDs. (c) Relationship between the percentage of the surface atoms and Br/Pb ratio with particle size. (d) XPS spectra of Br-3d of CH3NH3PbBr3 QDs and bulk material. Reproduced with permission from ref. . Copyright 2015 American Chemical Society.|
3.3. Nonlinear absorption properties
Recent reports also illustrate that both of organic-inorganic and all inorganic perovskite nanocrystals exhibit attractive nonlinear optical properties, especially for low-threshold laser . We recently investigated the nonlinear optical properties of CH3NH3PbBr3 QDs and CsPbBr3 QDs through Z-scan techniques . The results show that both of CH3NH3PbBr3 and CsPbBr3 QDs exhibit nonlinear refraction coefficient of approximately 10-11 esu. However, compared with CsPbBr3 QDs, the two-photon absorption cross-section of CH3NH3PbBr3 QDs is 5.23 × 106 GM, which is one order higher than that of CsPbBr3 QDs (1.2 × 105 GM).4. Lighting and display applications 4.1. White LED
Because of the easily tuned emission color from blue to red, narrow emission band (FWHM 〜20 nm), and low-cost solution processed properties, OHPs have attracted great attentions as emissive source in lighting and display devices [8, 25, 50, 63-66]. Compare with perovskite films, halide perovskite QDs exhibit greatly improved PLQYs. Along with the inherent optical properties, OHPs QDs have become the perfect choice for display technology. Wide color-tunable emission of halide perovskites QDs make it possible to get high color rendering index , narrow emission band of halide perovskites QDs provide the possibility to achieve wide color gamut .
To demonstrate the potential use of halide perovskite QDs in display technology, our group fabricated wide color gamut prototype white-light-emitting-diodes (WLED) . In this case, a layer of silicone gel with red emissive rare-earth phosphor K2SiF6:Mn4+ (KSF) was painted on the surface of the blue chip, followed by casting a layer of green emissive CH3NH3PbBr3 QDs in poly(methyl methacrylate). The configuration of the device is shown in Fig. 5a, and the corresponding electroluminescence (EL) spectrum is shown in Fig. 5b. As shown in CIE color coordinates, the CH3NH3PbX3 QDs showed quite a wide color gamut, and the color coordinates of obtained WLED was (0.33, 0.27) (Fig. 5c). The enlarged color-gamut show great potential of organometal halide perovskites QDs in liquid crystal display (LCD) display application. Soon after that, Snaith's group demonstrated their potential use in lighting . By blending perovskite QDs with different PL wavelengths in a polymer host, different emitting perovskite QDs thin films were fabricated. When positioned the green and red emitting crystal/polymer stack in front of the commercial blue GaN LED, desired white light that covering cool white, natural white, and warm white were generated, demonstrating the perovskite QDs a potential candidate for further lighting technology.
|Figure 5. (a) Diagram and (b) EL spectrum ofWLED. (c) CIE color coordinates corresponding to the CH3NH3PbX3 QDs and NTSC standard. (d) Photos ofWLED without and with biased current. Reproduced with permission from ref. . Copyright 2015 American Chemical Society.|
4.2. EL devices
As we all know, EL in LEDs is determined by the radiative recombination of electrons and holes in emissive layer. However, to prepare uniform and pinhole-free perovskite films is still a challenge. Thus encourage the formation of electrical shunting paths when injected charge carriers get through the emissive perovskite layer, which means the EL yields are losing. Very recently, Lee's group achieved the brightness and efficient green organometal halide perovskite LEDs with maximum current efficiency of 42.9 cd/A and EQE of 8.53% by decreasing the grain size of thin film down to 〜90 nm . The available highly luminescent QDs in nonpolar solvent provide an alternative to improve the EL efficiency. In the earlier report, Greenham' group fabricated LEDs using nanocrystalline perovskite in dielectric polymer matrix as emitting layer . Perovskite QDs provide excellent light emission, while the surrounding dielectric polymers effectively block electrical shunts. The best external quantum efficiency (EQE) reached 1%, which is 10-fold enhancement over previous reports. To tackle the EL yields problem at its source, Zhong and Pei et al.using purified organometal perovskites QDs as emissive layer to fabricated efficient EL devices . As shown in Fig. 6a, in their devices, poly(3, 4-ethylenedioxythio-phene):po- ly(styrenesulfonate) (PEDOT:PSS) was used as hole injection layer, CH3NH3PbBr3 QDs film was used as emissive layer, 1, 3, 5-tris (2-N- phenylbenzimidazolyl) benzene (TPBi) is applied as electron transport layer, whereas CsF as the cathode butter layer. The EL device exhibited a maximum luminance brightness of 2503 cd/m2 with a current density of 120 mA/cm2 at 5.5 V (Fig. 6c). The achieved best devices show a maximum currency efficiency of 4.5 cd/A and power efficiency of 3.5 lm/W, with champion EQE of 1.1% at the brightness of 410 cd/m2 (Fig. 6d). The device performance is much higher than most reported halide perovskite bulk materials. Furthermore, we recently achieved high-brightness flexible organometal perovskite LEDs with efficient luminance performance of 10.4 cd/A, 8.1 lm/W and EQE of 2.6% at 1000 cd/m2, demonstrating prospect of halide perovskite QDs for flexible display (unpublished results).
|Figure 6. (a) Energy-level diagram of different layers in QD-LED. (b) Normalized EL and PL spectra of CH3NH3PbBr3 NCs. (c) Current density and brightness versus voltage plots. (d) Current, power efficiency, and EQE versus brightness for the devices. Reproduced with permission from ref. . Copyright 2015 American Chemical Society.|
Soon after that, Jie's group reported a simple dip-coating way to prepare high-quality OHPs QDs films . Based on this method, the CH3NH3PbBr3 QDs based device shown a maximum current efficiency of 3.72 cd/A and EQE of 1.06% at a luminance of 2398 cd/m2, at the same time, multicolor OHPs QDs LEDs were also achieved. Chen et al. synthesized highly luminescent (PLQY = 67.3%) and air stable layered perovskite as the emitter for LEDs, and a maximum power efficiency and the EQE of 1.43 cd/A, 0.89 lm/W and 0.53 were demonstrated . Xiong' group fabricated high- efficiency green LED using amorphous CH3NH3PbBr3 QDs with efficient luminance performance of 11.49 cd/A, 7.84 lm/W and EQE of 3.8% .5. Conclusions and outlook
In summary, the important advances in colloidal synthesis, optical properties and LED application of halide perovskite QDs are presented in this review. As an emerging new star material, halide perovskite QDs have demonstrated their wide color-tunable and narrow-band emissions with extremely high PLQYs, as well as their ultra-facile colloidal synthesis. All of these properties make halide perovskite QDs a compelling choice as emissive materials for lighting and display.
By the way, along with the rapid development of OHPs QDs, all inorganic cesium lead halide perovakite QDs (CsPbX3, X = Cl, Br, I) have also been flourished. Many groups made contributions to the development of CsPbX3 QDs, such as Kovalenko's group [73, 74], Manna's group [75, 76], Rogach's group [77, 78], Zeng's group [47, 79], Deng's group , Xiao's group , etc.
Although halide perovskite QDs have been developed at a fast speed in the past two years, there are still many vital hinders on the way toward real applications. In the following, we list a few key issues that should receive greater attentions in further works. Firstly, the photo-, thermal- and solvent-stability of halide perovskite QDs are not as good as well-known metal-chalcogenide QDs, more researches on this area are urgent needed. We have noticed that the strategy of SiO2 coating is successfully applied to improve the stability of preformed perovskite QDs . Secondly, although the synthesis methods of halide perovskite QDs are facile and low-cost, the productivity is very low, the lack of high quality solid-state QDs powder have severely hinder the wide application of perovskite QDs. Thereby, it is highly desirable for one efficient scale up route to produce QDs powders. Thirdly, to better understand the luminescent mechanism of this new emerging QD, more works are needed to build physical model to describe the exciton recombination and carrier relaxation process. Finally, the LED devices based on EL and PL are attracting a great of attentions.
However, compare with traditional QDs based LEDs, the device performance ofhalide perovskite QDs is still very poor, new device structures or novel transport layers are required to achieve a qualitative leap.Acknowledgments This work was supported by the Major State Basic Research Development Program of China (973 Program, No. 2013CB328804), National Natural Science Foundation of China (No. 21573018), and Beijing Nova Program (No. xx2014B040).
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