Recent advances toward practical use of halide perovskite nanocrystals

Yuanyuan Dong , Yizhou Zhao , Siyu Zhang , Yi Dai , Lang Liu , Yujing Li * and Qi Chen *
Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail: qic@bit.edu.cn; yjli@bit.edu.cn

Received 3rd July 2018 , Accepted 31st July 2018

First published on 6th August 2018


Halide perovskite nanocrystals (NCs) and quantum dots (QDs) have received considerable attention, due to their superior photoluminescence quantum yields close to unity, variable morphologies, and tunable optical bandgaps achieved by modifying their composition, size and dimensionality. Their potential applications in solar cells, LEDs and photodetectors have driven research efforts to validate the feasibility of practical use of these materials in future optoelectronics and electronics. From the perspective of commercial applications, there are three issues of serious concern, namely, the toxicity of Pb, chemical instability, and the limited yield of NCs/QDs. In this review, we mainly focus on the recent research progress in the areas of relevance. First, we summarize the development of Pb-free perovskite NCs in the context of exploiting new materials. Second, we review feasible strategies to improve the stability of NCs and QDs during their preparation and incorporation. Third, batch synthesis methods are reviewed, which are focused on the reproducibility and yields towards mass production. Finally, a brief outlook is provided to forecast potential development to address the challenges in the future.


image file: c8ta06376a-p1.tif

Qi Chen

Qi Chen received both his B.S. and M.S. degrees from Tsinghua University, and received his Ph.D. degree from the University of California, Los Angeles (UCLA). From 2013 to 2016, he worked as a postdoc fellow at the California Nanosystem Institute (CNSI), UCLA, under Prof. Yang Yang's supervision. In the year 2016, he joined the Beijing Institute of Technology. His research focuses on hybrid materials design, processing and applications in opto-electronics and for energy harvesting and storage. He is now working on the commercialization of perovskite photovoltaics.


1. Introduction

Halide perovskites (HPs, with the chemical formula of ABX3), as light harvesting materials, received substantial attention from the research community because of their excellent optical and charge-transport properties such as adjustable bandgap, high absorption coefficient, long charge carrier diffusion length, excellent external quantum efficiency and high tolerance of chemical defects.1–5 The power conversion efficiency (PCE) of corresponding solar cells has rapidly increased from 3.81% to 23.3% within several years.6–9 Recent advances in large-area fabrication techniques have further encouraged perovskite photovoltaics marching toward practical use aggressively.10,11 In addition to solar cells,12–17 the applications of HPs mainly include light emitting diodes (LEDs),18,19 photodetectors (PDs),20,21 lasers22 and encryption devices.23

Along with the rapid development of bulk perovskite materials, the research in colloidal perovskite nanocrystals (NCs) has also emerged very recently. Lead halide perovskite (LHP) NCs, such as organic–inorganic CH3NH3PbX3 LHPs (in short MAPbX3) and all-inorganic CsPbX3 LHPs (X = Cl, Br, I), have been intensively investigated for various applications, such as LEDs and PDs, due to their colour tunability, narrow-band emission, ease of synthesis, by convenient solution-based processing, and low fabrication cost. As a young technology, the performance of perovskite-based optoelectronics is very impressive, but the commercialization process is hindered by several issues, mainly the toxicity of Pb, chemical instability, and the limited yield of NCs/QDs.

Regarding the toxicity of Pb, there has been a great deal of interest in replacing lead with nontoxic metals such as tin,24 bismuth,25,26 and antimony.27,28 However, these lead-free perovskite materials (mostly Sn2+ based materials) are rather unstable under ambient conditions (moisture, oxygen and temperature) and produce lower device performances compared to their lead-containing analogues, which could be ascribed to their higher defect densities.29,30 Employing double halide perovskites (A2M+M3+X6) is another promising approach to overcome the toxicity issue of lead.31–33 Interestingly, double halide perovskites (Cs4CuSb2Cl12,34 Cs2AgBiX6 (ref. 35)) show no signs of decomposition upon exposure to humidity and/or light.

The strong ionic bonding character of HPs leads to their extreme sensitivity to moisture, oxygen and light, imposing strict conditions for their storage.36,37 Over the past two years, many methods have been developed to improve their environmental stability, including surface modification with hydrophobic molecules,38 integrating/coating perovskites with polymers/inorganic oxides,39,40 doping perovskites with dopant ions (e.g. Mn2+)41–43 and the synthesis of HPs with low dimensionality.44–46 The stability of these colloidal nanocrystals and corresponding optoelectronic devices has thus been improved accordingly.

The synthesis methods of HP NCs mainly include hot-injection, ligand-assisted reprecipitation strategy (LARS), one-pot reaction, ultrasonic method, microwave assisted method, ball milling etc.47–52 By controlling the synthesis approaches, reaction temperature, capping ligands and other reaction conditions, various halide perovskite NCs can be obtained with variable components (inorganic, organic/inorganic hybrid), dimensions (0D, 1D, 2D, 3D), properties, sizes, and morphologies (QDs, nanowires and nanosheets),53 among which NCs open very promising vistas. However, there are a few issues that limit the application of NCs including low yield, high cost and energy consumption during mass production.

The rapid development of HPs in the field of optoelectronics has won worldwide attention. This mini-review would like to provide an update of the latest research on HPs, with emphasis on nanocrystals, but from the view of commercial application. In the first part, we will briefly introduce the crystal structure and photoelectric properties of HPs. The following parts cover the issues and solutions toward industrial applications. In the last section, we give a perspective on HP materials, including present challenges and possible development trends. We hope this review can provide valuable insights into the current status of HP materials and stimulate new ideas for further research on their potential applications.

1.1 Chemical structure and phase transition of HP nanocrystals

The crystal structure of LHPs is analogous to that of oxide perovskites. The parent motif is a cubic lattice consisting of corner-sharing [PbX6] octahedra connected in three dimensions (Fig. 1a).54,55 The large cavity between the octahedra (A-site) is occupied by one or a mixture of three large cations [Cs+, MA+, or CH(NH2)2+ (in short FA)], yielding an overall composition of APbX3. To maintain a high-symmetry cubic structure, the tolerance factor image file: c8ta06376a-t1.tif of the perovskite should be close to 1 and could be between 0.813 and 1.107, where RA, RB, and RX are the ionic radii of the corresponding ions. Fig. 1c lists the tolerance factors for the most popular lead or tin halide perovskites.54 Owing to the large Pb or Sn atom occupying the B sites of HPs, the A site must be large enough to satisfy the tolerance factor. Otherwise, the cubic structure will be distorted and crystal symmetry is reduced. In bulk LHPs, three 3D polymorphs are typically observed: cubic, tetragonal, and orthorhombic phases in the sequence of decreasing symmetry (Fig. 1a and b). The cubic phase is always stable at the highest temperature, and the phase transitions have well-defined temperatures. In the case of NCs, surface effects may adjust the relative stabilities of the various polymorphs, which receive limited attention. At room temperature (RT), all as-synthesized LHP NCs crystallize into 3D phases as follows: MAPbI3 NCs are tetragonal; FAPbBr3, MAPbBr3, and FAPbI3 NCs are pseudocubic; and CsPbBr3 and CsPbI3 NCs are orthorhombic.36 Recently, the 3D polymorphs of FAPbI3 and CsPbI3 NCs have gained primary interest for near-infrared emission. However, they are metastable at RT because of the suboptimal A site cations (FA+ being too large and Cs+ being too small). This problem, termed the “perovskite red wall,” has been addressed by synthesizing mixed-cation compounds such as Cs1−xFAxPbX3 NCs.56 In addition, when larger organic cations are employed in the A site, it is possible to form low-dimensional halide perovskite structures in which the connectivity of the inorganic network is reduced to 2D sheets, 1D chains, or 0D clusters (Fig. 1d).57
image file: c8ta06376a-f1.tif
Fig. 1 Crystal structure (a and b), tolerance factors (c),55 shape (d)57 and band structures (e)58 of halide perovskites; schematic of the halide anion-exchange process (f);59 survey PL spectra (g) and the corresponding photographs (h) (under mixed daylight and UV excitation) of colloids of composition-tuned APbX3 NCs.36

1.2 Defect tolerance and photoelectrical properties of HP NCs

One of the most striking features of LHPs is their high tolerance toward defects. The defect-tolerant nature of LHPs is mainly due to their electronic band structure, wherein the valence band maximum (VBM) is mainly composed of antibonding orbitals, and the conduction band minimum (CBM) gets stabilized by strong spin–orbit coupling.58,60,61 A schematic representation of the valence band structure of CsPbX3 with respect to isolated valence p and s atomic orbitals of Pb and X is shown in Fig. 1e.58 Such defect tolerance behaviour implies the preservation of a clean bandgap upon creation of typical defects, because their defect energy levels reside completely within either the CB or VB rather than within the bandgap itself. In this case, perovskite NCs are highly luminescent themselves without further electronic surface passivation. Protesescu et al. showed nearly ideal PL efficiency (∼100%) in the case of colloidal CsPbBr3 NCs.50

LHPs are multinary halide salts with substantial ionic bonding character that enables their facile formation at low temperature. The composition of LHP NCs can be conveniently adjusted through subsequent cation/anion exchange, as shown in Fig. 1f.59 The PL of LHP NCs spans the entire visible spectral range and the PL peak position (colour) is tunable by modulating the NC composition (Fig. 1g and h), size and shape.36,62 The PL full width at half maximum (fwhm) of perovskite NCs is rather narrow, not exceeding 100 meV — for example, 12 nm width in the blue limit of the visible range (CsPbCl3), 20 nm width in green at ∼520 nm (CsPbBr3), and 40–45 nm width in red at ∼690 nm (CsPbI3).63 Narrower line width emission is said to be more saturated, placing the fluorescence colour coordinates more toward the curved edge of the CIE chromaticity space (e.g., CIE 1931 standard).51,64 Combinations of the three emitters (red, green, and blue), which lie close to the fully saturated boundary curve, can then create the widest range of perceived colours, termed the colour gamut, by display and lighting manufacturers.

Tunable emission with high PLQY, low cost and easy synthesis of perovskite NCs make them attractive in optoelectrical devices. However, the toxicity of lead, extreme sensitivity to the environment (humidity, oxygen, light and temperature) and the limited yield of LHP NCs hinder their commercial application.

2. Nontoxicity

Despite the high PLQY (∼100%) and tunable emission of the LHPs, the manufacture, deployment and disposal of Pb-based optoelectronic devices are harmful to the environment. To address this toxicity issue, exploring environmental friendly semiconductors as substitutes for LHPs becomes an interesting topic, which mainly focuses on the substitution of a single non-toxic element at the Pb-site and the synthesis of double halide perovskite NCs.

2.1 Substitution of a single non-toxic element at the Pb site

Tin (Sn) was the first proposed candidate for the substitution of lead in the perovskite structure because both tin and lead belong to the same group in the periodic table. The similar electronic configuration to Pb enables Sn halide perovskite NCs to possess analogous optoelectronic properties. ASnX3 NCs (A = Cs, MA, FA; X = Cl, Br, I) were successfully synthesized by the hot injection or anion exchange method.66,70 Consider the example of CsSnX3, Fig. 2a and b show its cubic perovskite structure.65,66 The tunable optical absorption and PL spectra from the visible to the near-infrared (NIR) region in Fig. 2c were achieved through the control of its composition. It is noteworthy that CsSnI3 NCs could absorb a wider spectral range (from visible to NIR) than CsPbI3 NCs which absorb only the visible light. Therefore, CsSnI3 NCs could be a better light harvester. However, the PLQY of CsSnI3 NCs is extremely low. This lower PLQY was attributed to the higher defect density.71 Such trap states might arise from the undesirable and uncontrolled conversion of Sn2+ to Sn4+, as Sn4+ is a more stable state under ambient conditions. Improvement in reaction conditions along with suitable purification of the NCs can stop the oxidation of Sn2+ to Sn4+. NCs with strongly bound ligands and using excess of SnX2 in the reaction medium with reducing surface chemistry can possibly help to improve the long-term stability of Sn2+-based halide perovskite NCs.30,72 The Cs2SnI6 perovskite structure (Fig. 2d) was synthesized by replacing two Sn2+ sites in the CsSnI3 perovskite structure with one Sn4+ and one vacancy site.67 Density functional theory (DFT) calculations show that iodine vacancies exhibit a low enthalpy of formation, introducing shallow donor levels to the conduction band, rendering the defect-tolerant nature to Cs2SnI6.73 Unfortunately, approximately 90% of the bleach amplitude decays within 30 ps because of the non-radiative recombination of carriers on unpassivated surface defect sites.74
image file: c8ta06376a-f2.tif
Fig. 2 (a) Schematic of the cubic CsSnX3 perovskite structure.65 (b) Optical absorption and PL spectra of CsSnX3 NCs. (c) TEM image of CsSnI3 NCs.66 (d) Crystal structure of the vacancy-ordered Cs2SnI6 perovskites.67 (e) The bandgap of AGeX3 (A = Cs or MA) perovskites depending on X-site elements.68 (f) Photographs of as-obtained colloidal Cs3Bi2X9 (X = Cl, Cl0.5Br0.5, Br, Br0.5I0.5, I) NCs. (g) XRD patterns and (h) steady-state absorption and PL spectra of NCs containing pure and mixed halides.69

Germanium (Ge) is also considered to be an alternative to Pb. Owing to the same oxidation state as Pb, Ge-based halide perovskites possess similar optical and transport properties to those of LHPs. For most of the Ge-based halide perovskites, the bandgap is relatively larger than that of their Pb-analogues (Fig. 2e).68,75–77 These materials are thus utilized as top absorbers in tandem devices rather than as the principal absorbers. Additionally, the sturdy lone-pair effect in Ge halide perovskites leads to the poor conductivity of their low-dimensional structures. Therefore, despite the non-toxic and promising abundance of Ge, Ge halide perovskites have never received significant attention as light harvesters.

The vacancy-ordered bismuth halide perovskite A3Bi2X9, with A site of Cs and MA, is a promising alternative to LHPs due to its better stability against humidity and low toxicity.69,78,79 Han's group synthesized Cs3Bi2X9 (X = Cl, Br, I) NCs with emission wavelength ranging from 400 to 560 nm via a facile room temperature reaction (Fig. 2f–h).69 The photoluminescence quantum efficiency (PLQE) of Cs3Bi2Br9 NCs could be increased from 0.2% to 4.5% after treatment with an extra surfactant (oleic acid). This improvement stems from the passivation of the fast trapping process (2–20 ps). Interestingly, after exposure to humid conditions Cs3Bi2Br9 NCs exhibit high stability towards exposure to air exceeding 30 days. Such an observation is probably because the perovskite hydrate passivates surface trap-states.

2.2 Halide double perovskite nanocrystals (HDP NCs)

Use of halide double perovskite NCs (A2M+M3+X6) is another promising approach to overcome the toxicity issue of lead, in which one M+ cation and one M3+ cation replace two Pb2+ ions of ABX3. M+ ions mainly include Ag+, Au+ and Cu+, whereas M3+ ions are Bi3+, In3+, and Ga3+.32,80 Through first-principles calculations, Lijun Zhang's group identified 11 non-toxic and phase stable A2M+M3+X6 perovskites as LHP NC alternatives in perovskite solar cells.80 So far, only Cs2AgBiX6,33,35,81–83 Cs2AgInX6 (ref. 84) and Cs4CuSbCl12 (ref. 34) have been successfully synthesized. These materials possess robust intrinsic thermodynamic stability and photostability, and are tolerant to humidity. But the current reported double perovskites are not suitable light absorbers because of their larger bandgap. Some new strategies about bandgap engineering of HDPs were proposed. Through high-pressure treatment, the bandgap of Cs2AgBiBr6 could be narrowed by a considerable percentage of 22.3%.82 Moreover, the reduced bandgap is partially retained after releasing the pressure. By alloying with Sb(III), the bandgap of Cs2AgBiBr6 could be reduced by 0.41 eV.81

Cs2AgBiBr6 with an indirect bandgap of 1.95 eV is not ideal for application in photovoltaics. It is reported that the band structure depended on the dimension of LDPs. Hemamala I. Karunadasa's group synthesized direct-gap (BA)4AgBiBr8 and (BA)2CsAgBiBr7 perovskites through the insertion of organic molecules into 3D Cs2AgBiBr6.85 That is, a direct-gap LDP was obtained from indirect-gap hybrids when the dimension reduced. Diego Solis-Ibarra's group prepared a unique mixed metal 〈111〉-oriented layered perovskite Cs4CuIISbIII2X12 with a direct bandgap of 1.0 eV.34 The superior conductivity, being 1 order of magnitude greater than that of MAPbI3, makes this layered perovskite a promising material for photovoltaic applications.

3. Stability

Halide perovskites seem to be taking off toward viable commercial photovoltaic devices, while their counterpart NCs follow closely. However, the resistance to atmosphere (moisture, oxygen and temperature) and light remains the foremost challenge. To be comparable with the current existing photovoltaic technologies, understanding the degradation mechanism and further improvements in the stability of halide perovskite materials are the most principle issues at present. Recently, a large amount of literature has been reported aimed at improving the stability, which mainly includes surface passivation, encapsulation, doping or substitution at the B site and the synthesis of low dimension perovskites. This section will review them in detail with the emphasis on perovskite NCs.

3.1 Surface passivation

Owing to their electrovalent bond features, halide perovskite NCs are easily synthesized while it is just as easy to break them in the subsequent isolation and purification process (Fig. 3a). Surface passivation agents are thus necessary to maintain the halide perovskite NC structure intact, which further influence the PLQY and stability.
image file: c8ta06376a-f3.tif
Fig. 3 (a) Structural lability of CsPbX3 NCs due to the desorption of weakly bound ligands.1 (b) Precipitation quantities influenced by OA and APTES capping ligands.88 (c) UV-vis absorption and PL spectra of MAPbBr3 PNCs synthesized using different amino (APTES) and carboxylic capping ligands (oleic acid/benzoic acid/acetic acid). (d) A schematic diagram illustrating the surface passivation mechanism of Br and Pb surface defects.91 (e) Schematics for halide-poor and halide-rich circumstances for the synthesis of NCs.13 (f) PL spectra and PLQY of CsPbBr3 before and after self-passivation with ZnBr2.95

Generally, straight-chain capping agents with the –NH2/NH3+ terminal group, such as CnH2n+1NH3Br, oleylamine and octylamine, have been successfully used in the synthesis of LHPs in previous work. This implies the generality of amino capping ligands for the passivation of PNCs. However, binding of these straight-chain ligands is easily disintegrated during the purification process that requires repeated precipitation with an anti-solvent and subsequent redispersion in a pure solvent (Fig. 3a).1,86 The PLQY (∼20%) of halide perovskite NCs was also slightly enhanced because of their relatively weak passivation.87 Branched capping ligands ((3-aminopropyl) triethoxysilane, in short APTES) were successfully applied to the synthesis of MAPbBr3 NCs, resulting in the improved PLQY (ca. 15–55%) and stability.88 Compared to the different passivation effect with straight-chain ligands, Li's group proposed a mechanism based on the dissolution–precipitation model as shown in Fig. 3b. The strong steric hindrance and the formation of silica via the hydrolysis of APTES can hamper the dissolution of the as-formed perovskite NCs back in DMF, which is beneficial for preserving their original structural and optical properties.

Apart from alkyl amino and oleic acid ligands, other compounds containing a phosphonic group also play a similar role in the perovskite NC synthesis process. Jasieniak's group used an alkyl phosphinic acid during the synthesis of CsPbI3 nanocrystals. The NCs retain their cubic perovskite phase in solution.89 Shen's group demonstrated that trioctylphosphine (TOP) can coordinate with PbI2, forming a stable and highly reactive PbI2 precursor. The synthesized CsPbI3 QDs are of high quality with nearly 100% PLQY and enhanced chemical stability.90

Notably, the capping efficiency was improved in the presence of oleic acid and amine ligands, which demonstrates the synergistic effect between carboxylic and amine groups. Further, Li's group synthesized MAPbBr3 NCs using amino (APTES) and different carboxylic capping ligands (oleic acid/benzoic acid/acetic acid).91Fig. 3c shows the corresponding UV-vis absorption and PL spectra. Clearly, when both carboxylic and APTES capping agents were utilized in tandem, the perovskite NCs were well passivated and exhibited much higher PLQY (≈32–55%). The synergistic effect was confirmed (Fig. 3d), where amino and carboxyl groups passivate X and Pb2+ defects, respectively.91 The significant shift of the main emission peak in Fig. 3c was attributed to steric hindrance from carboxylic capping ligands.88,92 The 2,2′-iminodibenzoic acid (IDA)-treated CsPbI3 NCs display enhanced stability in the desired cubic phase with a PLQY of ∼95%, which further confirms the synergistic passivation between acid and amino ligands.93 Moreover, the perovskite NCs could be effectively passivated by zwitterionic ligands like sulfobetaines, phosphocholines, γ-amino acids as capping ligands, which have the cationic and anionic groups at the same end of the molecule. The zwitterionic ligands are capable of coordinating simultaneously to the surface cations and anions in NCs, resulting in an improved durability of NCs.94

The elemental ratios of halide/metal played an important role in the synthesis of high quality and stable halide perovskite NCs. Fig. 3e depicts the synthesis of CsPbBr3 NCs in halide-poor and halide-rich circumstances.13 The as-synthesized CsPbBr3 NCs are surrounded by oleylammonium bromide. However, the QY of NCs was usually significantly reduced due to the “peeling off” of the ligand when purified in the poor halide ambient. With the increasing amount of oleylammonium bromide capping around NCs in the halide-rich stoichiometry, the halide would remain largely on the surface of the NCs, which helps to retain the self-passivation conditions. The Br-rich capping could passivate the surface electron traps of NCs to ensure a high PLQY. Furthermore, the durability of perovskite NCs during the purification and device fabrication process could be improved. Self-passivation with inorganic halides can also enhance the structural stability and PLQY of CsPbBr3 NCs (Fig. 3f and g).95 However, the conductivity of commonly used long-chain surface ligands is poor. Excessive oleic acid and oleylamine would hamper the efficiency of charge injection. The LED used in the as-prepared CsPbBr3 NCs thus displays low brightness and efficiency. Wallace C. H. Choy's group introduced an additive POSS to improve the surface coverage and the morphological features of the films.96 The EQE and the luminance efficiency of LEDs with an additional POSS layer were significantly enhanced by 17-fold compared to the reference devices and the device stability was also improved.

Inorganic ligands could serve as capping ligands to maintain perovskite NCs stable. Kamat's group protected cesium lead halide NCs with PbSO4–oleate capping. This strategy of capping restrains the anion exchange reactions. The nanocrystal assemblies maintain their identity as either CsPbBr3 or CsPbI3 for several days.97

3.2 Encapsulation by an oxide/matrix

Coating is an efficient and effective method to stabilize moisture and solvent sensitive halide perovskites from harsh environments.39,98 Xiang's group prepared water resistant mesoporous silica incorporated CsPbBr3 NCs for white LEDs. They exhibit excellent optical performance and good thermal and photo-stability under illumination of UV light for 120 h (Fig. 4a).99 Cs4PbBr6 could also serve as a coating material in some cases.100 Zeng's group reported a CsPbBr3@Cs4PbBr6/SiO2 structure with simultaneous triple-modal fluorescence characteristic by the excitation of thermal, ultraviolet (UV) and infrared (IR) light. The diphasic structure CsPbBr3@Cs4PbBr6 NCs were first synthesized then encapsulated into silica microspheres. Cubic CsPbBr3 is responsible for the fluorescence, while Cs4PbBr6 crystalline and SiO2 mainly protect unstable CsPbBr3 NCs from being destroyed (Fig. 4b).101 Yamauchi's group embedded monodisperse CH3NH3PbBrxIx−3 perovskite nanocrystals inside mesoporous silica templates with controllable size depending on the pore size of the templates. Quantum confinement and photoluminescence were observed; the template improves their stability and enables tunable electronic properties via quantum confinement (Fig. 4c).102 Kovalenko's group achieved the formation of perovskite NCs by drying the perovskite precursor solutions on infiltrated mesoporous silica. NCs with very bright PL and quantum efficiencies exceeding 50% were obtained. The luminescence properties of the resulting templated NCs can be tuned by both quantum size effects as well as composition (Fig. 4d).63 Zeng's group used surface amine treated silica to create the nucleation platforms. During a one-pot reaction, the Pe-QDs were anchored onto the surfaces of monodisperse silica, creating highly luminescent, environmentally stable, and scalable Pe-QDs/silica sphere composites. Almost no PL degradation is observed after 40 d storage under ambient conditions, even 80% PL intensity can be maintained after persistently illuminating with UV lamps for 108 h.103 The silica coated perovskite has much improved stability and promising applications in harsh environments.
image file: c8ta06376a-f4.tif
Fig. 4 (a) Schematic illustration of the formation of the NC mesoporous silica nanocomposite.99 (b) Microstructure, TEM image and schematic diagram of CsPbBr3@Cs4PbBr6/SiO2 composites.101 (c) Photographs of the mesoporous silica powders after impregnation and crystallization.102 (d) Schematic of the template-assisted synthesis of APbX3 NCs (A = Cs+, CH3NH3+ (MA) or CH(NH2)2+ (FA), X = Br or I) in the pores of mesoporous silica.63

3.3 Doping and substitution

Intrinsic chemical instability, resulting from lower crystal lattice energy, enables halide perovskites to possess poor thermal stability. In this regard, the replacement of a B site cation with smaller cations such as Mn2+/Sn2+/Cd2+/Zn2+ ions has been confirmed to enhance the formation energies of perovskite lattices of CsPbX3 NCs resulting in improved thermal stability.104–107 Donega's group synthesized CsPb1−xMxBr3 NCs (M = Sn2+, Cd2+, and Zn2+; 0 < x ≤ 0.1) through a post-synthetic cation exchange method.104 This isovalent cation exchange caused a blue-shift of the PL bands with preservation of high PLQY and a narrow emission bandwidth. The blue-shift is attributed to the contraction of the NC lattice because of the incorporation of smaller guest cations (Fig. 5a), which lead to the shrinkage of B–X bonds and hence enhanced interaction between Br and Pb orbitals. Chen's work on CsPbCl3:Mn QDs further confirmed this contraction of NC lattice and the corresponding enhanced thermal stability.106 As shown in Fig. 5b, the integrated PL intensities of CsPbCl3:Mn were much higher than those of pure CsPbCl3 QDs after three heating/cooling cycles. Moreover, Chen coated CsPbBr3:Mn QDs with Mn2+ contents from 0 to 6.2 mol% on the surface of a glass slide and exposed them to ambient air conditions. The PL emission photographs of CsPbBr3:Mn QDs in Fig. 5c clearly indicate the improved air stability.
image file: c8ta06376a-f5.tif
Fig. 5 (a) Schematic illustration of the replacement of B sites in cubic perovskite NCs.104 (b) The room-temperature integrated PL intensities of CsPbCl3:Mn and pure CsPbCl3 QDs after the above three heating/cooling cycles. (c) PL emission photographs for CsPbBr3:Mn QDs coated on the surface of a glass slide with different Mn2+ contents from 0 to 6.2 mol% taken under UV irradiation at indicated time periods.106 (d) UV absorption and PL emission spectra of CsPb0.54Mn0.46Cl3 QDs. (e) XRD patterns of the CsPbxMn1−xCl3 QDs after preserving in the ambient environment for 1 and 3 months. (f) Schematic overview of the fluorescence tunability of CsPbxMn1−xCl3 QDs by altering the Mn-to-Pb molar feed ratio and RT.108 (g) Schematic structure of solar cells coated with a CsPbCl3:0.1Mn layer.41

Light induced degradation could also be significantly decreased by the substitution or doping of Mn. Yang reported tetragonal CsPbxMn1−xCl3 QDs with a Mn replacement ratio of 46%.108 The as-prepared QD solution exhibited good chemical stability. In ambient atmospheres and light for more than 3 months, their crystal structures and emission properties were still retained (Fig. 5d and e). Interestingly, the PLQY of CsPbCl3 QDs was improved from 5 to 54% after Mn substitution, indicating that the radiative recombination obviously increased. Doping Mn enables the energy of photoinduced excitons to be transferred from the CsPbCl3 host to the dopant Mn (Fig. 5d),108–112 which facilitates exciton recombination via a radiative pathway.108 The fluorescence properties could also be modulated by the Pb-to-Mn ratio and reaction temperature (Fig. 5f).108 Moreover, CsPbCl3:Mn QDs can also be used as energy-down-shift materials applied onto the front side of the perovskite solar cells (Fig. 5g).41 They effectively convert the harmful ultraviolet light into available visible light leading to an enhanced PCE. Meanwhile, conversion of the UV rays eliminated an obvious loss mechanism thereby maintaining the perovskite stability.

3.4 2D perovskites

Two-dimensional (2D) halide perovskites, divided by long chain ligands from bulk 3D, have attracted intense research attention owing to their optical properties and promising applications. Much more interesting phenomena occur when diminished to the nanoscale.

Urban's group investigated quantum size effects in two-dimensional organometal halide perovskite nanoplatelets. By controlling the ratio of the organic cations used in the synthesis, the thickness of the obtained nanoplatelets can be controlled.113 Yang's group further investigated single-layered organic–inorganic hybrid perovskites with blue luminescence.114 In recent years, only few investigations have studied 2D structured nanoscale perovskite nanocrystals, and mainly focus on the physical and optical properties, and the applications in thin-film transistors, light-emitting diodes, and photodetectors.113–116 But the 2D structured nanoscale perovskite nanocrystals do show some improved stability in some cases. Peng's group developed a photodetector based on individual 2D (C4H9NH3)2PbBr4 perovskite crystals, assisted with the protection and top contact of graphene film. Both high responsivity (∼2100 A W−1) and extremely low dark current (∼10−10 A) are achieved with significantly improved stability.117

Zhang's group synthesized a series of single and few layered (PEA)2PbX4 (PEA = C8H9NH3, X = Cl, Br, I) perovskite nanosheets. The (PEA)2PbI4 nanosheets sustain about 86% of the emission intensity after they were exposed to moisture under ambient conditions for 96 h, while MAPbI3 quantum dots were quenched completely. Under irradiation with light of 400 nm wavelength, only 22% emission intensity loss was observed for (PEA)2PbI4 nanosheets, indicating enhanced photo-stability.118

The improved unique optical properties and stability showed promising applications in 2D perovskite nanocrystals. But the current research remains limited, further explorations in this topic are expected to yield fruitful results.

4. Batch production

In 2015, the hot injection method and LARS were developed by Kovalenko's group and Zhong's group independently to synthesise halide perovskite nanocrystals, which have been widely adopted by other researchers (Fig. 6a and b).50,51 Recently, many strategies, such as the one-pot reaction, ultrasonic method, microwave assisted method, ball milling etc., have been developed to synthesise halide perovskite nanocrystals.47–49,52 In order to achieve large scale industrial production, the production method must have the following features: high yield, low cost, low energy consumption and environment-friendliness. Among the numerous production methods, only a few can be applied to industrial production so far.
image file: c8ta06376a-f6.tif
Fig. 6 (a) Colloidal perovskite CsPbX3 NCs (X = Cl, Br, I) synthesized by the hot-injection method.50 (b) Schematic illustration of the reaction system and process involved in the LARS technique.51 (c) Scheme of a one-step reprecipitation procedure for the synthesis of CH3NH3PbBr3/NaNO3 nanocomposites.119 (d) Resultant low-dimensional CsPbBr3 by pristine and ultrasonic-treated OA2PbBr4 nanosheets.120 (e) Proposed growth process of CsPbBr3 NCs through the microwave-assisted synthesis approach. Without pre-dissolution of precursors, CsPbBr3 nanoplates are obtained at low temperature, and monodisperse nanocubes can be prepared at elevated temperature.121 (f) Photograph of the different as-prepared powders synthesized by ball milling.122 (g) Schematic diagram and SEM micrographs of the mechanism of scalable MAPbBr3 perovskite crystal formation and their fragmentation into nanoscale colloidal crystals.123

Halide perovskites are unstable ionic compounds, easily soluble in polar solvents and very sensitive to water and heat.1 In the hot injection method, oleic acid (OAc) and oleylamine (OAm) react with PbX2 (X = Cl, Br, I) to form PbOAc and OAmX. Apart from the cosolvent and capping ligands, OAc and OAm can enable dissolution of PbX2 in the nonpolar solvent 1-octadecylene(ODE). The mixture of ODE, PbOAc and OAmX is heated to 140–200 °C, followed by the injection of CsOAc dissolved in ODE. This method smartly evades the use of polar solvents, which lead to instability, producing highly luminescent halide perovskite nanocubes with monodisperse morphology. But the high temperature during processing costs time and energy, as the CsOAc solution has to be preheated before using owing to its poor solubility. These disadvantages limit the batch production of the hot injection method.

A one-pot synthesis method was developed by Yang's group, which involves heating all precursors in octadecene in air.124 It simplifies the injection part and scales up the yield to the gram scale. The obtained high-quality nanocrystals exhibit a quantum yield up to 87%, and the emission peak positions can be tuned from 360 to 700 nm. Recently Zeng's group have studied the heterogeneous nucleation mechanism of highly uniform halide perovskite quantum dots through a one-pot reaction. It could be easily scaled up to the gram scale (≈1.8 g) by adding uniformly heterogeneous nucleation agents (silica spheres) within a short reaction time.125

LARS operates at room temperature, so the energy used to heat the reactant is conserved. The precursors are firstly dissolved in polar solvents such as DMF or DMSO, then the solution is slowly dropped into a non-polar solvent like toluene. The nanocrystals are assembled immediately within seconds.51 High quality nanocrystals could be obtained easily and rapidly by this strategy, but the yield is limited by the ratio of polar and non-polar solvents. If a large amount of the precursor solvent is added, the obtained nanocrystals would re-dissolve in the non-polar solvent, hence normally no more than 5–10% of the precursor solvent is used. To address these issues, Zhong's group modified the LARS method.126 By embedding CH3NH3PbBr3 nanocrystals into a NaNO3 matrix, brightly luminescent CH3NH3PbBr3/NaNO3 nanocomposites are synthesized (Fig. 6c). This improved strategy is facile, reproducible, low-cost, and scalable, facilitating the bath synthesis. The nanocomposites have enhanced thermal, light, and solvent stability due to the protection of the NaNO3 matrix. Additionally, the perovskite NC morphology prepared by the LARS method is affected by several parameters, such as capping ligand, temperature, stirring speed, and dropping speed. These characteristics hinder the industrialization of the method.

As for the ultrasonic method, the precursors are added to a non-polar solvent, to form a precursor solution. The solution is either positioned in an ultrasonicator or treated by tip sonication. After the ultrasonic irradiation, the nanocrystals are obtained.48,128 Recently, Zeng's group synthesized solution-prepared OA2PbBr4 nanosheets that can be used as a template to construct low-dimensional CsPbBr3 with the inherited crystallinity, size and morphology after ultrasonic treatment (Fig. 6d). The photodetection performances are improved including prolonged charge-carrier lifetime, improved photo-stability, increased external quantum yield/responsivity, and faster response speed. This could be developed as a post-treatment method during the production of halide perovskite nanocrystals in the industrial scale.120

A microwave-assisted synthesis was reported by Na's group.49 Colloidal CsPbX3(X = Cl, Br, I) perovskite nanocrystals with tunable properties and morphologies are synthesized without precursor preparations. The same approach was also attempted by Yang and Zhang's group,121,129 almost at the same time. The microwave-assisted synthesis has the promising prospect of industrialization because of its simple, fast and polar solvent-free system (Fig. 6e). Recently Rogach's group used a slowed-down microwave-assisted synthesis to study the formation mechanism of CsPbBr3 nanoparticles, by examining the intermediate stages of their growth that revealing the formation of a bromoplumbate ionic scaffold with Cs-ion infilling lagging a little behind during the reaction.130 The ultrasonic method and the microwave-assisted method are both polar solvent-free methods, and show potential for large scale production.

Urban's group prepared perovskite powders by grinding the precursors (MABr and PbBr2) together in a mortar, then treating with oleylamine followed by tip sonication. The obtained microcrystals are converted to 2D nanocrystals with controllable layers (BnX3n+1n = 1, 2, 3,…) and tunable bandgap by ligand-assisted liquid-phase exfoliation. By enlarging the scale of the grinding, the ball milling is an efficient mechanochemical approach for the synthesis of halide perovskites. The precursors of perovskite including PbI2, methylammonium iodide (MAI), formamidinium iodide (FAI), and guanidinium iodide (GAI) are mixed in a ball mill to produce large amounts of polycrystalline powders (Fig. 6f).122 Choi's group improved the ball milling by adding capping ligands (OA: n-octylamine or ODA: octadecylamine) during ball milling (Fig. 6g).123 In the presence of capping ligands, the perovskite microcrystals are peeled off and reduced to nanoscale crystals with much enhanced PL quantum yields. The advantages of ball milling are it is solvent-free, easy to scale up to the kilogram scale and can be performed at room temperature.

A brief summary of the reported studies for batch production of perovskite NCs is shown in Table 1, only few studies focus on this issue. Techniques to achieve both high yields and low costs need to be further studied.

Table 1 Summary of reported studies for batch production of perovskite NCs
Perovskite/structure Production methods Total production Ref.
CsPbX3, X = Cl, Br, and I or Cl/Br and I/Br One-pot Gram scale 124
CsPbBr3 One-pot ≈1.8 g 125
CsPbBr3 One-pot <1 g 47
MAPbBr3/NaNO3 LARS 2 g 126
CsPbBr3 LARS Gram scale 127
CsPbX3, X = Cl, Br, and I Ultrasonic Not mentioned 128
MAPbI3, FAPbI3, GUA2PbI4 Ball milling 5–10 g 122
MAPbBr3 Ball milling 10 g 123


5. Applications

5.1 Solar cell

In the past years, organic–inorganic hybrid lead halide perovskite solar cells (PSCs) have progressed in an impressive manner approaching commercialization. However, their chemical instability stemming from the volatility of the organic cation has become a major bottleneck for long-term practical deployment. All-inorganic Pb-halide perovskite absorbers have thus been much desired. Relatively large +1 A-site cations are required to adjust the geometrical constraints of the perovskite structure. Cubic (α) CsPbI3 with the bandgap of 1.73 eV has thus been regarded as the most appropriate PV material.131 However, films of α-CsPbI3 can be easily converted to the nonactive orthorhombic phase in an ambient atmosphere, which severely affects the efficiency of the corresponding solar cell. Previous studies indicated that the phase stability of α-CsPbI3 was closely related to the crystallite size, which was obviously improved when the α-CsPbI3 grains were trimmed towards the nanoregime.50,132 Therefore, it becomes imperative to synthesize stable α-CsPbI3 QDs/NCs and fabricate the corresponding photovoltaic cells. Joseph M. Luther's group reported an α-CsPbI3 QD solar cell with an open-circuit voltage of 1.23 V and efficiency of 10.77% (Fig. 7a–c).133 The use of methyl acetate (MeOAc) as the antisolvent played a crucial role in the purification stage, which removed excess unreacted precursors restraining the agglomeration. Using this extraction procedure, both α-CsPbI3 QDs and QD films are stable for several months under ambient storage conditions. During the synthesis process, the addition of a surfactant, such as a polymer, can also stabilize CsPbI3 in the cubic phase. Eiichi Nakamura's group found that PVP mixing significantly decreased the perovskite crystal size from ≈40 to ≈15 nm.134 The small-grain perovskite crystals of 15 nm retained the cubic structure of the crystal lattice between 100 and 5 °C. When such a small crystal is used to fabricate a semi-transparent solar cell device, not only efficiency but also thermal stability and transparency have been improved in air (Fig. 7d–f).
image file: c8ta06376a-f7.tif
Fig. 7 (a) Schematic (with the TEM image of QDs) and (b) SEM cross-section of the CsPbI3 PV cell. (c) Current density–voltage curves of a device measured in air over the course of 15 days. The black diamond represents the stabilized power output of the device at 0.92 V.133 (d) Cross-sectional images of the SCs with a 150 nm Ag electrode using 3.0 6.0 wt% PVP as an additive to PV. (e) JV curves of PV SCs with 0–6 wt% PVP as additives. (f) Transparent solar cells with humidity-thermometers under sunlight from the ITO side or Au side.134 (g) Comparison of the performance of CsPbI3 and μGR/CsPbI3 devices. (h) Comparison of the stability of CsPbI3 and μGR/CsPbI3 devices under different atmospheric conditions.135

In addition to the stable cubic CsPbI3, an increase in the carrier transport efficiency can also enhance the performance of the all-inorganic perovskite CsPbI3 QD photovoltaic cells. A-site cation halide salt (AX) treatment was reported as an effective tool. Joseph M. Luther's group considered that AX treatment could tune the coupling between perovskite QDs, which facilitated the enhanced charge transport for fabricating high-quality QD films and devices.131 The AX treatment introduced here doubles the film mobility, thereby increasing the photocurrent and resulting in a record certified QD solar cell efficiency of 13.43%. The usage of conductive μ-graphene (μGR) to form μGR/CsPbI3 film was found to provide an effective channel for carrier transport (Fig. 7g).135 Owing to the conductivity and hydrophobicity of μGR, the μGR/CsPbI3 based solar cell exhibited improved device performance and better stability under thermal stress and in an ambient atmosphere (Fig. 7h). Moreover, Edward H. Sargent's group has devised mixed-quantum-dot solar cells on the basis of enhanced charge carrier extraction in QD solar light harvesting layers.136 Efficient charge separation could be achieved at the nanoscale interfaces between two classes of QDs. The PCE of 10.4% surpassed the PCE of previously reported bulk heterojunction QD devices two-fold, which indicated the potential of the mixed-QD approach.

Moreover, cubic CsPbI3 could be employed as an interface engineering material because of its high valence band position (VBP) and high moisture stability. Chun Cheng's group proposed an interface engineering strategy that controls the VBP of perovskite and the interface (CsPbI3) layer to increase the transfer efficiency of holes, leading to an increased PCE from 15.17 to 18.56%.137 Similarly, Liu Shengzhong's group designed a graded bandgap structure-based solar cell, comprising a CsPbI2Br bottom cell and a CsPbI3 QD top cell.138 The corresponding well-matched energy levels, extended photoresponse and high carrier mobility afford a PCE of 14.45% and JSC of 15.25 mA cm−2. The optical and energy-band manipulation is thus an effective approach to improve the performance of inorganic perovskite solar cells.

5.2 Photocatalysis

Halide perovskite materials have obtained much attention in photocatalysis owing to their wide absorption spectra covering the entire visible range, tunable bandgaps, and superior charge transport properties. It was firstly reported by Nam et al. that halide perovskites could effectively catalyse photocatalysis reactions (Fig. 8c).139 Considering that MAPbI3 is a water-soluble ionic compound, the dynamic equilibrium of dissolution and precipitation of MAPbI3 in saturated aqueous solutions are exploited (Fig. 8a–b). The I and H+ ions enable the tetragonal MAPbI3 phase to be stable in aqueous solution, which is a prerequisite for the effective catalysis of perovskites. The solar HI splitting yield of MAPbI3 was ∼300 μmol g−1 when using Pt as a cocatalyst (Fig. 8d). To further improve the photocatalytic performance, Huang mixed MAPbI3 with rGO to promote the separation of charge carriers.140 As an electron acceptor, rGO helps the transfer of photogenerated electrons from MAPbBr3 through the Pb–O–C bonds and inhibits recombination with holes. The H2 evolution rate, durability and stability of the catalyst have been thus enhanced. Later, inspired by the relationship between the structure and energy levels of perovskite solar cells, Li's group employed nano-TiO2 to construct a heterojunction photocatalyst. The photo-generated electrons can transfer from the conduction band of MAPbI3 to that of TiO2, and the hybrid catalyst shows much higher hydrogen evolution efficiency. Additionally, Goddard's group came up with a two-step Pb-activated amine-assisted (PbAAA) mechanism of hydrogen evolution.141 Firstly, H of –NH3 migrates to the nearest Pb atom to form the intermediate product Pb–H, meanwhile the H from water combines with CH3NH2 to form a new MA+ species; secondly, the H of MA+ migrates to PbH to generate H2, reproducing CH3NH2 and Pb2+. Both MA+ and Pb2+ play important roles in the reaction. These results indicate that it is very important to design the perovskites towards their catalyst functionality apart from the photoexcitation properties.
image file: c8ta06376a-f8.tif
Fig. 8 (a) Solutions of MAPbI3 in aqueous HI solution at different concentrations. (b) Constructed phase map as a function of [I] and [H+]. (c) Schematic band diagram of the MAPbI3 powder for the HI splitting photocatalytic reaction. (d) Photocatalytic H2 evolution from MAPbI3 powder that was thermally annealed in a polar solvent atmosphere with and without a Pt cocatalyst.139 (e) Schematic diagram of CO2 photoreduction over the CsPbBr3 QD/GO photocatalyst. (f) Photocatalytic performance: yield of the CO2 reduction products after 12 h of photochemical reaction.143 (g) Schematic diagram of alcohol oxidation over FAPbBr3/TiO2 hybrid photocatalysts.145 (h) Correlation between ket and kPNP for different composition ratios of MAPbBr3: p-g-C3N4 in MAPbBr3/p-g-C3N4 NCs.146

It is reported that CsPbBr3 QDs can also effectively catalyse the photocatalytic reduction of CO2 (Fig. 8e). Sun's group reported that high selectivity over 99% and an efficient yield of 20.9 mmol g−1 could be achieved for solar CO2 reduction catalyzed by CsPbBr3 QDs.142 In order to solve the instability problem of CsPbBr3 QDs in water, Kuang used ethyl acetate, a non-aqueous organic solvent with medium polarity, as the reaction medium.143 The results show that the photocatalytic reaction can be stably performed under continuous light for 12 hours, and the photoelectron yield involved in the reaction reaches 23.7 μmol g−1 h−1. It is noteworthy that methane and carbon monoxide dominated the products (Fig. 8f), implying the efficient suppression of the H2 evolution reaction in the nonaqueous solvent. Additionally, Cs2AgBiBr6 double perovskite NCs, which present superior stability in humidity, light, and temperature, are applied to the photocatalytic CO2 reduction.144 Uder AM 1.5G illumination for 6 h, the total electron consumption was 105 μmol g−1.

Halide perovskites can also be applied in the photo-oxidation or photo-reduction of organic compounds through constructing a complex with electron conductors. Benzyl alcohol could be oxidized to benzaldehyde over an FAPbBr3/TiO2 hybrid photocatalyst (Fig. 8g).145 The formation of ·O2 species, which originate from the transfer of photo-generated electrons from FAPbBr3, plays an crucial role. Similarly, photoexcited electrons in the CB of MAPbBr3 could be transferred to the CB of C3N4, and a significant reduction was observed in the photocatalytic reduction of p-nitrophenol (Fig. 8h).146

5.3 Other applications

5.3.1 LED. Owing to the narrow emission band widths, LHP NCs provide high PLQY and highly saturated colours. Moreover, the primary colours (red, green and blue) stemming from LHP NCs achieve an impressive gamut, being up to ∼140% of the North American National Television Standard Committee (NTSC) specification (Fig. 9a) and even up to 100% of the International Telecommunication Union Rec. 2020 standard. With these features of excellent optoelectronic properties and ease of fabrication, organic–inorganic halide perovskites have become one of the most promising LED materials.87,147,148Fig. 9b describes the three-color LED pixel with LHP NCs as the emissive layer. The electrons and holes from electrodes recombine via a radiative path and inject into a thin LHP NC layer. Electroluminescence (EL) is thus observed, but the lower luminescence efficiency is the critical drawback. A simple bilayer perovskite structure to fabricate was reported by Min-Ho Park and the current efficiency (CE) achieved is 42.9 cd A−1, being 2 orders of magnitude greater than that with LEDs with a single perovskite structure.149 Since then a series of strategies to obtain high-efficiency LEDs have been developed. Exciton confinement was first proposed.150 It is well known that the quantum well structure could confine the electrons and the holes which would be beneficial to radiative recombination. On the basis of multiple quantum wells, Wang's group demonstrated the preparation of a red perovskite LED with the highest EQE of 11.7%. Additionally, the matching degree of energy levels between the perovskite and charge transfer layers directly determines the charge injection efficiency of LEDs. The modifications of charge-injection and transport layers were considered as an effective method.151–154 Zhang's group introduced a thin film of perfluorinated ionomer sandwiched between the hole transporting layer and perovskite emissive layer.154 This elegant engineering increased the valance band maximum of poly-TPD, significantly enhanced hole injection efficiency, resulting in a threefold increase of the peak brightness. Moreover, because of the low interaction energy between metal cations and halide anions, ionic defects were generated inevitably which would cause blinking behaviour of LEDs.155–158 To address this issue, Lee's group used amine-based materials to passivate the defect sites. This treatment significantly reduced undercoordinated Pb, which greatly enhanced the LED's efficiency and device stability.159 Other strategies such as construction of mixed ion perovskites and light extraction techniques also proved to be efficient to improve device performance.56,160,161 Apart from EL efficiency, the stability stemming from the perovskite itself,162–164 ion migration,165 metal electrode diffusion166 and p–i–n junction formation167 remain big challenges.168 One of the methods to improve the stability was introduction of large organic ammonium cations to form a Ruddlesden–Popper layered perovskite, which could significantly suppress ion migration and enhance perovskite material stability.150 With in-depth understanding of the instability mechanism, we believe the stability issue will be overcome eventually.
image file: c8ta06376a-f9.tif
Fig. 9 (a) PL spectra of CsPbX3 NCs plotted on CIE chromaticity coordinates (black dots) compared with common colour standards (LCD television, dashed white line, and NTSC television, solid white line), reaching 140% of the NTSC colour standard (solid black line). (b) Schematic of a three-color LED pixel with LHP NCs as the emissive layer.36 (c) CsPbI3 NCs (d) HRTEM image of a single CsPbI3 NC. (e) Schematic diagram of the photodetectors based on CsPbI3 nanocrystals/QDs.169
5.3.2 Photodetectors. Photodetectors based on halide perovskites have been on the rise in the recent years because of their outstanding photoelectrical properties.42,170–172 The key photodetection performance of a perovskite-based photodetector is determined by its responsivity, detectivity, noise equivalent power, linear dynamic range, response speed and so on, which are closely related to the composition, structure and morphology of the photoactive material. Compared with its bulk crystals, the halide perovskite QDs/NCs applied in photodetectors usually show more excellent detective performance due to the high excitonic binding energy and quantum confinement.173 Ramasamy's group demonstrated, for the first time, photodetector devices based on CsPbI3 nanocrystals (Fig. 9c–e).169 Later, they developed a highly sensitive hybrid photodetector based on graphene–CsPbBr3–xIx nanocrystals.174 1D perovskite nanostructures (nanowires, nanobelts, and nanorods) have attracted a great deal of interest in the application of high-performance photodetectors, due to their defect-free single crystals,175 high crystalline quality,176 and large specific surface area.177 Moreover, the conductive channel of 1D nanostructures could confine the active area of charge carriers and shorten the carrier transit time,178 making them beneficial to the exploration of high-performance devices. A single CsPbI3 nanorod photodetector showed high stability and excellent performance with a responsivity of 2.92 × 103 A W−1 and an ultrafast response time of 0.05 ms, respectively, which were both comparable to the best ones ever reported for all-inorganic perovskite photodetectors.179 2D perovskite crystals, such as nanosheets and nanoplates, show splendid photoresponsive properties.180 To provide fast carrier tracks, CsPbBr3 nanosheet/CNT composite films were fabricated and the performance of the corresponding photodetectors was enhanced.181 There is a developing trend that combines 2D perovskite nanosheets or nanoplates with 2D materials such as graphene, WSe2etc. to construct van der Waals heterojunction photodetectors.182 The exploration of high-performance photodetectors based on perovskite nanocrystals with different compositions is still a promising study.
5.3.3 Lasers. Owing to their absorption coefficients, slow Auger recombination and high optical gain, halide perovskites are considered as an attractive class of materials for lasing.60,183 Tze Chien Sum's group reported ultra-stable amplified spontaneous emission (ASE) at strikingly low thresholds based on MAPbX3 perovskite thin films.183 Similar to the regular PL emission, the visible spectral tunability of lasers is directly demonstrated by varying the halide ion content of perovskites.183,184 Interestingly, single-crystalline organic–inorganic hybrid lead perovskite NWs emit in a nearly uniform manner across their whole length at low pump fluences. And the emission predominantly stems from the ends of NWs when excited above the lasing threshold.184 Other than NWs, hybrid perovskite microdisks have also shown efficient optical gain.185,186 Compared with hybrid perovskites, all-inorganic perovskite NCs possess higher PLQYs and greater stability and are thus considered as promising materials for low-threshold laser devices.60,187,188

6. Summary, outlook and challenges

In summary, HPs have been regarded as the next generation semiconductors with excellent optoelectronic properties. HP NCs have tunable photoluminescence with high quantum efficiency and narrow emission width, and solution-processability, which hold great promise for future optoelectronic applications. Their fascinating optical properties have inspired the use of perovskite NCs in LEDs, solar cells and photodetectors.

We review the research advances and challenges for the practical use of HP NCs in three aspects: non-toxicity, stability and mass production. The toxicity of HP NCs can be overcome by replacing Pb with atoxic elements such as Sn and Ge. Compared with Ge-based HPs with poor conductivity, Sn-based HP NCs have superior photoelectronic properties. They absorb a wider spectrum (from the visible to the NIR range) than CsPbI3 NCs, serving as better light harvesters. Unfortunately, they exhibit fairly low PLQY due to higher defect density. Furthermore, they suffer from the instability originating from the oxidative Sn2+, which requires further study from the perspective of both materials and devices. Furthermore, lead-free double halide perovskites may provide an alternative solution. They possess robust intrinsic thermal stability, photo-stability, and are tolerant to humidity, but they often exhibit large bandgaps that lead to poor light absorption. Some strategies have been suggested to narrow the bandgaps, like high pressure treatment and alloying. Additionally, double perovskite NCs with a direct bandgap are of great interest, which would be a promising material for photovoltaic applications.

Stability determines the lifetime of devices, which eventually correlates with the success of commercialization. Popular strategies to enhance stability include surface passivation, encapsulation, doping, and dimensionality reduction. It is revealed that surface capping ligands influence the stability and the PLQY of the as-prepared NCs in opposite directions, wherein a trade-off has to be achieved to obtain robust and high performance NCs. Coating is proven to be an efficient method to protect sensitive HPs from moisture and solvents. The coating materials are subcategorized into inorganic materials and organic materials. The recently reported Cs4PbBr6 perovskite suggests a new direction of future research in this area. In addition, doping can also enhance the formation energies of perovskite lattices, which has been successfully adopted in CsPbX3 NCs for improved thermal stability.

In the context of mass production, the hot injection method, LARS, one-pot reaction, ultrasonic method, microwave assisted synthesis, and ball milling are reported. Each method has its own merits, wherein ball milling is documented to achieve the best yield, and hot injection leads to the best morphology and properties of the as-prepared NCs.

Due to the various advantages of HP NCs, potential applications like solar cells, photocatalysts, LEDs, and photodetectors are reviewed. In the case of solar cells, the all-inorganic HP QDs possess much better stability under environmental stress. They provide potential solutions to the commercialization of HP photovoltaic materials, which is not trivial in the current prevailing thin film devices. Additionally, HPs may serve as promising candidates in the visible light photocatalytic field, which further widens the palette of commercially viable photocatalytic semiconductor systems. Compared to their bulk material counterparts, HP NCs exhibit several advantages that should be further exploited to find their niche market. For LEDs, LHP NCs provide high PLQY and highly saturated colours owing to the narrow emission band widths, which makes LHP NCs a competitive material for LED applications. In the case of photodetectors, the photodetection performances are related to the composition, structure and morphologies of photoactive materials, which exactly evidence the superiority of HP NCs and is highly tunable for commercial applications.

Despite the broad prospects of perovskite materials, they face several great challenges in practical applications. Due to the unsteadiness of LHP NCs, it is highly desirable to establish some effective methods to stabilize their optically active phases and structures with respect to light, temperature, and the environment. The improvement in the carrier transport and separation efficiency is worthy of concern for the all-inorganic perovskite PV cells. Some other issues, including the poor conductivity of perovskite NC layers and low electroluminescence efficiency, need to be addressed for their applications as LEDs. Furthermore, there is an urgent need to explore alternative metal halide compounds that contain environmentally friendly elements rather than Pb. Additionally, development of some high-throughput continuous-flow or segmented-flow microfluidic methods is essential to meet the requirements of commercial production. As promising new generation semiconductors, HP NCs deserve further study to benefit human race in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21706080, 51673025), China Postdoctoral Science Foundation (2018M631359), The start-up funding of BIT, National Key Research and Development Program of China Grant (2016YFB0700700), and Young Talent Thousand Program. Dr Yuanyuan Dong and Yizhou Zhao contributed equally to this work.

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Footnote

Yuanyuan Dong and Yizhou Zhao contributed equally to this work.

This journal is © The Royal Society of Chemistry 2018