1 Introduction
Metal halide perovskite materials have attracted much attention due to their wide absorption spectra and high hole and electron mobility, which have potential applications in energy conversion, display lighting, information storage and other fields
[1]. Thanks to the introduction of solid hole transport layer and thin film device structure, the Power Conversion Efficiency (PCE) of organic-inorganic hybrid metal halide perovskite solar cells has exceeded 23%, which is comparable to that of traditional silicon-based solar cells
[2][3]. At the same time, all inorganic metal halide perovskite materials, represented by CsPbX
3(X=Cl,Br,I), also show excellent photoelectric properties. Especially when the size is gradually reduced to the nanometer order of magnitude, that is, CsPbX
3(X=Cl,Br,I) nanocrystals, the energy band structure is gradually changed from quasi-continuous energy levels to discrete energy levels due to the quantum confinement effect, showing novel photophysical and chemical properties
[4]. Compared with the traditional I-III and II-VI semiconductor nanocrystals, the CsPbX
3(X=Cl,Br,I) nanocrystals show better optical properties, such as high absorption coefficient, narrow FWHM, tunable emission spectrum in the whole visible range, and high PLQY. The optoelectronic devices based on CsPbX
3 nanocrystals also show the characteristics of easy control of structure, good flexibility, long carrier diffusion length (≈ 9.2 μm) and high mobility (≈4500 cm
2·V
-1·s
-1), which show great application prospects in solar cells, photodetectors, light-emitting diodes, lasers and other fields
[5,6][7,8][6,9][10,11].
In this paper, the research progress of CsPbX3 nanocrystals in recent years is reviewed.The crystal structure and composition control of CsPbX3 nanocrystals are analyzed, and the common preparation methods, morphology and size control strategies, as well as the current applications in the field of optoelectronics are summarized. Finally, the difficulties and challenges in this field are systematically analyzed, aiming to provide a reference for the preparation of CsPbX3 nanocrystals with better performance and wider application of optoelectronic devices.
2 Crystal Structure and Composition Control of CsPbX3;
The research on the crystal structure of CsPbX
3(X=Cl,Br,I) can be traced back to 1958. According to the environmental temperature and the total energy of the system, the crystal structure of CsPbX
3 can be divided into cubic phase, tetragonal phase and orthorhombic phase according to the difference of symmetry
[12][9]. The cubic phase is a high-temperature phase, the crystal structure of the cubic phase is similar to that of oxide perovskite, in the structural unit of the cubic phase, Cs
+ ions occupy eight vertex angles, Pb
2+ ions are positioned at the center of a body, and Cs
+ ions or Pb
2+ ions are used as nodes,Observed from the polyhedron of the Pb
2+ ion, the Pb
2+ ion and the X
- ion form a PbX
64- octahedron,The Cs
+ ions are filled in the gaps between the adjacent PbX
64- octahedra to form a three-dimensional network structure. The CsPbBr
3 nanocrystal has a cubic phase (alpha phase) structure (Pm$\bar{3}$m) at a high temperature, undergoes a first-order phase transition when cooled to 130 ° C, and the PbBr
64- octahedron rotates in the ab plane to generate a tetragonal phase (beta phase) structure (P4/mbm), and when cooled to 88 ° C, the PbBr
64- octahedron tilts along the C axis to generate an orthorhombic phase (gamma phase) structure as shown in fig. 1 (Pbnm)
[13]. Both the cubic phase and the partially twisted orthorhombic phase enable electrons to be confined in a three-dimensional grid and exhibit semiconducting properties. The double peaks at about 15 ° or 30 ° of the characteristic peak of powder X-ray diffraction are often used as the basis for judging the orthorhombic phase. With the change of the composition of the X
- ions, such as Cl
- ions or I
- ions, the lattice of CsPbX
3 nanocrystals will expand or contract, and the corresponding characteristic peaks of powder X-ray diffraction will shift slightly
[14,15]. In addition, due to the small size of CsPbX
3 nanocrystals, the characteristic peaks of X-ray diffraction patterns are often broadened.
The research of CsPbX
3 nanocrystals has made great progress due to their excellent optical properties and wide application prospects. Meanwhile, Cs
4PbX
6 and CsPb
2X
5 as perovskite-like nanocrystals have also attracted the attention of researchers. Despite having the same elements, these two kinds of materials are quite different from the CsPbX
3 in structure. For Cs
4PbX
6, the adjacent PbX
64- octahedra are separated by Cs
+ ions to form one-dimensional [CsPbX
6]
n3- chains, and the chains are further separated by Cs
+ ions to form dodecahedra
[16]. Compared with the Cs
4PbX
6, the CsPb
2X
5 shows a layered structure, and the Cs
+ ions are interspersed between the Pb
2X
5- layers
[17]. Zeng et al. Used oleic acid (OA) and (3-aminopropyl) triethoxysilane as surface ligands to achieve the transition from CsPbBr
3 to Cs
4PbBr
6 and CsPb
2Br
5 by adjusting the ratio of Cs
+ ions to Pb
2+ ions
[18].
3 Ynthesis of CsPbX3 nanocrystal
In recent years, a variety of methods have been developed to synthesize CsPbX3 nanocrystals, which can be divided into two categories according to the type of reaction, namely, chemical vapor deposition and liquid phase chemical synthesis. Chemical vapor deposition (CVD) is a method that sublimates or vaporizes the precursor at a certain pressure and temperature to transport the gas to the substrate for crystallization. This method usually requires a high reaction temperature and has certain limitations in controlling the shape and size of nanocrystals. The liquid phase chemical synthesis method can effectively control the morphology and size of nanocrystals, which has attracted wide attention. In this paper, we summarize the common methods of liquid phase chemical synthesis of CsPbX3 nanocrystals, such as high temperature thermal injection, room temperature reprecipitation, droplet microfluidics, solvothermal method, ion exchange method, microwave-assisted synthesis, ultrasonic synthesis and so on.
3.1 High temperature thermal injection
The high-temperature thermal injection method is a synthesis method for synthesizing nanomaterials by using an organic solvent with a high boiling point, the nucleation stage and the growth stage of crystals are separated, and organic ligand molecules can be bonded on the surface of the nanocrystals, so that the nanocrystals are separated from each other, the agglomeration of the nanocrystal can be effectively inhibited, and then the nanocrystalline with a narrow size distribution is obtained
[19⇓~21][4]. Different solvent systems should be selected for different reaction systems. Octadecene (ODE), OA and oleylamine (OAm) are commonly used as high boiling point non-coordinating low polarity solvents and organic ligand molecules. The reaction temperature is generally in the range of 100 ~ 330 ℃, and the nanoparticles prepared by high temperature thermal injection method generally have the characteristics of narrow particle size distribution, high crystallinity and good dispersion
[22,23]. In 2015, Protesescu et al took the lead in preparing CsPbX
3 nanocrystals by high temperature thermal injection method. Lead halide (PbX
2) was dissolved in ODE with a small amount of OA and OAm, and then cesium oleate (Cs-OA) precursor was rapidly injected into the reaction system under inert gas atmosphere and high temperature (140-200 ℃)
[4]. After a set period of time (generally 5 s), the system is rapidly cooled in an ice-water bath to terminate the reaction, as shown in Figure 2a. The reaction equation is:
2Cs-OA+3PbX2→2CsPbX3+Pb(OA)2
ODE is used as a reaction medium to dissolve reactants, so that the reaction is carried out at high temperature; and the mixture of OA and OAm is used as a surface ligand, so that on one hand, the dissolution of the PbX
2 is promoted, and on the other hand, the mixture is adsorbed on the surface of the CsPbX
3 nanocrystal to improve the stability of the CsPbX
3 nanocrystal. The oriented growth of CsPbX
3 nanocrystals can be realized by changing the equivalent weight of OA and OAm or using other ligands (such as dodecylamine, octylamine, butyric acid, hexanoic acid, etc.); Other cesium organometallic compounds such as cesium stearate and cesium acetate can be used as cesium sources. Cubic CsPbX
3 nanocrystals can be obtained when the Pb/Cs molar ratio is greater than 1.5
[4].
Because the common source of the Pb
2+ ions and the X
- ions is the PbX
2, the actual feeding ratio (1:2) of the two ions deviates from the standard stoichiometric ratio (1:3) of the CsPbX
3, which leads to the formation of excessive Pb
2+ ions and the existence of by-products of lead oleate in the reaction system. To this end, Liu et al. Used a "three-precursor" strategy to synthesize CsPbX
3 nanocrystals, using Cs-OA, halogenated amine (NH
4X) and lead oxide (PbO) as precursors for cesium, halogen and lead sources, respectively. The excessive NH
4X provides an environment rich in X
- ions, so that the CsPbBr
0.9Cl
2.1, the CsPbBr
3 and the CsPbBr
0.9I
2.1 nanocrystals synthesized by the three-precursor strategy have good stability and optical properties
[25]. In order to further expand the universality of this method and reduce the production of by-products, Imran et al. Further used benzoyl halide with high activity as the precursor of halogen source, injected benzoyl halide at the required temperature,After implantation, the halide ions are rapidly released, and the prepared CsPbX
3 nanocrystals have high PLQY(CsPbCl
3 (the 65%,CsPbBr
3 is 55% of the 90%,CsPbI
3) and can stably exist in air for several weeks
[26]. During the nucleation process of CsPbX
3 nanocrystals in ice-water bath, the reaction thermodynamic energy of the system is always higher than the energy threshold required for nucleation due to the slow cooling rate, so that the quality of the formed crystals is poor and there are many defects on the surface. Luo et al. reported an ultrafast thermodynamic control strategy based on the "three-precursor" method, in which the cooling rate was increased by 33 times by using liquid nitrogen cooling instead of ice-water bath cooling, so that the prepared CsPbBr
xCl
3-x nanocrystals had fewer surface defects, uniform morphology, excellent crystallinity, PLQY up to 98% and good stability
[27]. The development of "three precursors" strategy improves the high temperature thermal injection method, which not only makes the synthesis reaction proceed according to the ideal stoichiometry, but also makes the prepared CsPbX
3 nanocrystals have excellent optical properties, reduces the by-products produced in the reaction process, and improves the crystallinity and stability of the CsPbX
3 nanocrystals.
Due to the existence of excessive halide, Cs-OA, OA, OAm, ODE and by-product lead oleate in the solution of CsPbX
3 nanocrystals prepared by the traditional high-temperature thermal injection method, the solution needs to be purified. The purification is generally divided into two steps: firstly, polar solvents such as acetone, ethyl acetate (EA), butanol, and the like are added into the stock solution, the stock solution is centrifuged, the precipitate is retained, and the supernatant containing impurities is removed; The precipitate is then dispersed in a weakly polar solvent such as toluene, n-hexane. Due to the strong ionicity of CsPbX
3 nanocrystals and the instability of surface ligands (such as OA, OAm), the ligands bound to the surface of CsPbX
3 nanocrystals are easy to separate during centrifugation and purification, resulting in agglomeration of CsPbX
3 nanoparticles, quenching of luminescence and even change of crystal phase
[15]. It was found by de Roo et al that a small amount of organic ligands OA and OAm could bind to the surface of the CsPbX
3 nanocrystals before the CsPbX
3 nanocrystals were centrifuged and purified, which effectively maintained the integrity of the CsPbX
3 Nanocrystals and prevented the agglomeration of CsPbX
3 Nanocrystals and the reduction of PLQY
[28]. Compared with the orthorhombic phase, the cubic phase of CsPbI
3 nanocrystals has a larger band gap (≈ 1.73 eV), which is more suitable for the active layer of solar cells. However, CsPbI
3 nanocrystals are unstable at room temperature in the atmosphere, and change from cubic phase to orthorhombic phase under the influence of air, moisture and other factors. Swarnkar et al. proposed to use methyl acetate with weak polarity to purify CsPbI
3 nanocrystals, which can not only remove excessive unreacted precursors, but also inhibit the structural phase transition and aggregation of nanocrystals, maintain the cubic phase structure, and be stable for several months in the atmospheric environment
[29].
3.2 Room temperature reprecipitation method
The room temperature reprecipitation method is a method for preparing CsPbX
3 nanocrystals, which is developed by using the different solubility of materials in good solvents and poor solvents
[19⇓⇓⇓~23]. In this method, the required raw materials are dissolved in a good solvent to reach an equilibrium concentration, and then the solution is transferred to a poor solvent to break the equilibrium state and form a supersaturated solution. In the supersaturated state, the nanomaterial will precipitate and crystallize until the equilibrium state is restored. This is a common method for preparing CsPbX
3 nanocrystals. This process requires the addition of organic ligands to regulate the crystallization process of nanocrystals, so that the formation and growth of crystals can be controlled at nanometer size. Polar solvents such as N, N-dimethylformamide (DMF), tetrahydrofuran, dimethyl sulfoxide (DMSO), isopropanol, ethanol, acetone and non-polar organic solvents such as chloroform and dichloromethane can be used as good solvents for dissolving raw materials and organic ligands
[30]; Non-polar solvents such as toluene and n-hexane are common poor solvents, which promote the precipitation of CsPbX
3 nanocrystals. Organic amine and organic carboxylic acid as ligands play an important role in the room temperature reprecipitation. Organic amine regulates the crystallization kinetics to achieve the control of the size of nanocrystals. Organic carboxylic acid as a stabilizer inhibits the agglomeration of nanocrystals
[31]. The method does not need to use inert gas and high temperature conditions in the preparation process, has low cost and simple and convenient experimental process, and is beneficial to the mass production of the CsPbX
3 nanocrystal.
In 2016, Li et al. First reported the preparation of CsPbX
3 nanocrystals by room temperature reprecipitation method. They dissolved PbX
2 and cesium halide (CsX) in polar solvents such as DMF or DMSO, added OA and OAm as ligands, and then quickly injected an appropriate amount of precursor solution into a vigorously stirred toluene solution. The color of the solution changed from colorless to yellow-green immediately, and emitted strong green light under ultraviolet light, as shown in Figure 2b. Through this strategy, CsPbX
3 nanocrystals can be produced in a few seconds, and are not limited by heating, inert gas and other operations
[24]. The PLQYs of the CsPbCl
3, the CsPbBr
3 and the CsPbI
3 nanocrystals prepared by the room temperature reprecipitation method respectively reach 70%, 95% and 80%, and the emission wavelength of the CsPbX
3 nanocrystals can be adjusted in the whole visible spectrum range by changing the type and proportion of X
- anions.
Wei et al. Dissolved cesium hydroxide (CsOH) or cesium carbonate (Cs
2CO
3) and PbO using fatty acids of different chain lengths (e.g., butyric acid, hexanoic acid, caprylic acid, capric acid, myristic acid, oleic acid) and then added it to toluene to prepare a No.1 precursor solution of Cs
+ ions and Pb
2+ ions; Quaternary ammonium bromide (such as tetrabutylammonium bromide, tetraoctylammonium bromide, hexadecyl trimethyl ammonium bromide, and hexacosyl dimethyl ammonium bromide) is dissolved in toluene as a second precursor solution of Br
- ions, and the second solution is added into the first solution to prepare the CsPbBr
3 nanocrystal. When myristic acid or oleic acid is used as a ligand, the fluorescence quantum efficiency of the prepared CsPbBr
3 nanocrystal is more than 80%; When butyric acid was used as the ligand, the PLQY was 20% ~ 30%, which was due to the weak binding ability between short-chain carboxylic acids and the surface of CsPbBr
3 nanocrystals
[32].
Although the room temperature reprecipitation method is easy to operate, the system using toluene as a poor solvent often has the problem of low yield (≈ 30%). Ng et al. Used EA to replace toluene as a poor solvent and octylamine to replace OAm as a surface ligand. EA effectively improved the precipitation rate, reduced the use of solvents, and increased the yield to 70%, which was 2. 3 times higher than the traditional room temperature reprecipitation method
[33].
3.3 Droplet microfluidic method
Droplet microfluidics is a technology based on a microchannel reactor with a size of tens to hundreds of microns, which uses the flow properties of fluids at the microscale to separate and detect substances, supplemented by active and passive control measures, and miniaturizes biochemical analysis techniques such as sample synthesis, separation, detection and other operational means into the microscale channel. Through the mutual combination of a plurality of single-stage microchannels, a microfluidic system with a complex structure is upgraded to realize the one-step preparation of microdroplets with narrow particle size distribution and target size.It provides a universal platform for the preparation of nano or micro particles, and has high flexibility in controlling the molar ratio of reactants, reaction temperature, reaction time and other parameters. Droplet microfluidics provides a new method for the synthesis of CsPbX3 nanocrystals, which promotes the further understanding of the nucleation and growth kinetics of nanocrystals.
Lignos et al. prepared CsPbX
3 nanocrystals based on droplet microfluidics technology. First, the precursors (Cs-OA, OA, OAm, PbX
2) loaded into a precision syringe pump were rapidly mixed through a cross mixing junction (R
1) and rapidly heated to the set reaction temperature to successfully prepare CsPbX
3 nanocrystals. For the mixed halogen CsPbX
3 system, they mixed the PbX
2 and PbY
2 precursors containing different halogens at the T-junction (R
2) in advance, and then transported the mixed solution to the cross-mixing junction. The fluorescence spectrum of the prepared CsPbX
3 nanocrystals covered the wavelength range of 470 ~ 690 nm, with a half-peak width of 20 ~ 45 nm, as shown in Figure 3A. The combination of the T-shaped junction and the cross-mixed junction can accurately, continuously and independently control the molar ratio of the lead, the cesium and the halide precursors, the reaction temperature, the reaction time and other parameters, so that the synthesis is more efficient, and ultrafast kinetic measurement and real-time characterization of fluorescence and absorption spectra are realized
[34].
3.4 Solvothermal method
The solvothermal method uses water or a non-aqueous organic matter as a solvent in a closed reaction vessel, such as a reaction tank or an autoclave, and the target product is obtained through a physical or chemical reaction of a certain proportion of reactants in the environment at a certain temperature and under the autogenous pressure of the solution. For liquid-phase reactions, solvents generally have two effects: first, they affect the kinetic behavior of the reaction by controlling the concentration of different types of systems, and second, they affect the morphology or macrostructure of the final product by coordinating some functional groups of the solvent with the reactants
[35]. Solvothermal method has the advantages of simple operation, low energy consumption, environmental friendliness, precise morphology control and high crystallinity. Under high pressure, the thermodynamics and kinetics of reactants and the phase state of products are different from those under normal pressure.
In 2017, Chen et al. First prepared high-quality CsPbX
3 nanocrystals by solvothermal method. They put cesium acetate (CH
3CH
2COOCs) or Cs
2CO
3, PbX
2, OA, OAm and ODE into an autoclave in a glove box, and reacted at 160 ℃ for 30 min. After the reaction, the autoclave was taken out and cooled to room temperature. After centrifugal purification, the CsPbX
3 nanocrystals were further dispersed in n-hexane, as shown in Figure 3 B. The CsPbX
3 nanocrystals prepared by the method have a cubic phase structure and a uniform size distribution.By changing the type and proportion of the X
- ions, the emission wavelength can be adjusted in the whole visible spectral range, and the nanocrystals have a narrow half-peak width (12-36 nm) and a high PLQY (≈ 80%)
[35]. Zhai et al. Put the mixture of Cs-OA and PbX
2 precursor solution into an autoclave, reacted at 100 ℃ for 72 H, cooled the solution to room temperature, centrifuged and purified, dispersed in n-hexane, and prepared CsPbX
3 nanowires with a width of about 10 nm and a length of 10 μm, which showed excellent optical properties. The PLQY of CsPbBr
3 nanowires was about 12% of that of 75%,CsPbCl
3 nanowires
[36]. In 2019, Chen et al. Used solvothermal method to change the morphology of CsPbX
3 nanocrystals by adjusting the amount of injected Cs
+ ion precursor solution. The results show that when the concentration of Cs
+ ions is high, the octylammonium ions do not compete with the Cs
+ ions for the surface sites of the nanocrystals, so the nanocrystals keep isotropic growth trend and the products are nanocubes. When the concentration of the Cs
+ ions is low, the octylammonium ions can occupy the sites of the Cs
+ ions and restrict the growth of the vertical crystal planes of the nanocrystals, so that the products tend to form nanosheets or nanorods
[37].
Liu et al. Prepared CsPbCl
3 nanocrystals doped with Mn
2+ ions by solvothermal method, which introduced d-d emission of Mn
2+ while maintaining the integrity of the nanocrystal structure. This work provides a new idea for the preparation of doped ionic nanocrystals
[38].
3.5 Anion exchange method
It is well known that metal halide perovskite is an ionic compound with certain defect tolerance, and the interaction between perovskite nanocrystals and surface ligands also shows ionic characteristics. Surface ligands do not bind tightly to the surface and switch between the bound and free States. The dynamic surface is favorable for the ion exchange reaction, and the ion exchange method usually adds an ion exchanger (metal or organic halide, etc.) To the CsPbX
3 nanocrystal solution, and utilizes the strategy that the cations or anions in the functional groups of the ion exchanger are reversibly exchanged with the like ions in the CsPbX
3 nanocrystal solution. The ion exchange method is divided into anion exchange (X
- ions) and cation exchange (organic ions, metal ions)
[39⇓⇓⇓~43]. The anion exchange method can change the halogen component in the CsPbX
3 through a simple operation at room temperature to obtain the required energy level structure and optical properties, and is also a commonly used post-treatment method for synthesizing the CsPbX
3 nanocrystals.
In 2015, Nedelcu et al. Prepared a series of CsPbX
3 nanocrystals by rapid anion exchange at room temperature. The PbX
2 was reacted with the ODE at 120 ° C until dissolved, then the temperature of the system was lowered to 40 ° C and the CsPbBr
3 nanocrystals dispersed in toluene were injected into the system. By adjusting the halide ratio in the nanocrystal solution, the emission spectra of the exchanged CsPbX
3 nanocrystals can cover 410-700 nm while maintaining high PLQY (20% -80%) and narrow FWHM (10-40 nm), as shown in Figure 3C
[14].
The surface defects and poor crystallization quality of CsPbX
3 nanocrystals are often the important reasons for their instability. Chen et al. Reported a copper halide (CuX
2) assisted anion exchange reaction to prepare CsPbX
3 nanocrystals. The mixed solution of CuBr
2-OAm was added to the toluene solution of CsPbCl
3 nanocrystals and stirred at room temperature for 16 H. After the anion-exchange process, the surface defects of the CsPbCl
3 nanocrystals with poor crystal quality were reduced, and the corresponding PLQY was increased from 2. 7% to 92. 6%. After 30 weeks of storage, 90% of the initial luminous intensity was still maintained
[44]. Lv et al. Used non-toxic and cheap magnesium halide (MgX
2) metal inorganic salt as precursor solution to anion exchange CsPbBr
3 nanocrystals, and the product after reaction was cubic, with a uniform size distribution in the range of 8 ± 2 nm.The emission wavelength of the CsPbX
3 nanocrystals can be adjusted in the whole visible spectral range by changing the type and proportion of the X
- ions, and the product can be stable for 30 days at room temperature when exposed to air. Based on the fact that the bond energy of Pb-Br (236 kJ/mol) is close to that of Mg-I (229 kJ/mol) and Mg-Cl (323.8 kJ/mole), they selected a series of metal halides (MX
2) for experiments. The research shows that the difference between the M-X bond energy of the halide itself and the Pb-Br bond energy in the CsPbBr
3 is the key factor to determine whether anion exchange can occur. The smaller the difference is, the more favorable the anion exchange reaction is
[45].
3.6 Microwave-assisted synthesis
The microwave-assisted synthesis method is a method in which the microwave with the frequency of 300 MHz ~ 300 KMHz is used to penetrate the reaction vessel, and the polar solvent or ligand in the reaction medium is used to absorb the microwave radiation, and the energy is directly transferred to the substances participating in the chemical reaction. Compared with the conventional heat transfer, the reaction system heated by microwave is heated more uniformly and more efficiently.
Long et al. First reported the preparation of CsPbX
3 nanocrystals by microwave method. ODE, OA, OAm, Cs
2CO
3 and PbX
2 were added into a glass container and then put into a household microwave oven with 800 W power. After 4 minutes of reaction, the color change of the precipitate at the bottom was observed, which confirmed the synthesis of CsPbX
3 nanocrystals. The size of nanocrystals can be controlled in the range of 5 ~ 14 nm by changing the irradiation time (2 ~ 7 min)
[46]. Pan et al. Further studied the effects of microwave reaction temperature, heating rate and precursor solution pretreatment on the morphology of the reaction products. When the reaction temperature is 80 ℃, the anisotropic growth of the nanocrystals is promoted, and the nanosheets with a thickness of 3.3 nm and an edge length of 23.4 nm are produced. When the temperature is raised to 140 deg C, the nano-cube block with uniform appearance is produced; Continuing to raise the temperature to 160 ° C, a heterogeneous nanocrystalline dispersion was obtained. When the heating rate is slow (8 ℃/min), the size distribution of nanocrystals is wide. When the heating rate is increased (> 18 ℃/min), the size distribution of the nanocrystals is more uniform. This is due to the fact that a slow heating rate can prolong the nucleation time of the crystals, resulting in an inhomogeneous dispersion. The precursor solution was dissolved at 60 ℃ in advance, and then the temperature of the system was raised to 160 ℃ for reaction, resulting in nanowires with a length of 60 nm and a diameter of about 2. 7 nm
[47].
3.7 Ultrasonic synthesis method
The ultrasonic synthesis method is a simple, efficient, low-cost and environment-friendly preparation method, reactants are mixed in a glass container, a metal thin rod connected with an ultrasonic generator is placed in the container, and the reactants are gradually dissolved and reacted to form the CsPbX3 nanocrystal along with the progress of ultrasonic.
In 2016, Tong et al. First used ultrasonic method to prepare CsPbX
3 nanocrystals. The sizes of CsPbBr
3 and CsPbI
3 nanocrystals prepared by this method were 10 ~ 15 nm and 8 ~ 12 nm, respectively, with PLQY of 92% and 90%
[17]. Subsequently, they found that by increasing the concentration of reactants by 10 times (1 mmol Cs
2CO
3,3 mmol PbBr
2), super-large CsPbBr
3 nanocrystals with a size of 200-400 nm could be prepared. The large-size nanocrystals were formed by self-assembly of multiple small-size nanocrystals, which had a high PLQY (≈ 92%). Due to the electronic coupling between particles, the emission wavelength was relatively red-shifted by about 20 nm compared with the emission wavelength of small-size nanocrystalline (512 nm)
[48].
Li et al. Prepared Mn
2+ ion-doped CsPbCl
3 nanocrystals by ultrasonic synthesis, and the size and morphology of the nanocrystals were controlled by changing the ratio of Mn
2+ ions to Pb
2+ ions. The doped product shows double emission characteristics of 411 nm and 601 nm, which are attributed to the intrinsic emission of the nanocrystals and the d-d emission of the Mn
2+, respectively.The PLQY is improved from 17. 49% to 41. 15% by passivating the defects of the nanocrystals with triethylamine
[49].
4 Morphology and Size Control of CsPbX3 Nanocrystal
The size and morphology of CsPbX
3 nanocrystals not only affect their optical properties, but also play an important role in the performance of optoelectronic devices based on the materials.Precise control of the relevant parameters in the synthesis process, such as reaction temperature, concentration and type of ligands and precursors, is very important to control the morphology and size of CsPbX
3 nanocrystals. Taking high temperature thermal implantation as an example, different morphologies of CsPbX
3 nanocrystals (including nanospheres, nanowires, nanorods and nanosheets) can be obtained by changing the reaction temperature. Suitable organic ligand is an important prerequisite for obtaining CsPbX
3 nanocrystals with ideal morphology and size. In order to better understand the influence of these parameters on the morphology and size of CsPbX
3 nanocrystals, Figure 4 briefly summarizes the influence of reaction temperature and surface ligands on the morphology and size of CsPbX
3 nanocrystals
[31,50⇓ ~52].
4.1 Reaction Temperature
In the process of thermal injection synthesis, the reaction temperature is the key parameter to determine the morphology and size of nanocrystals
[50,53]. By changing the reaction temperature (140 ~ 200 ℃), the size of nanocrystals can be adjusted in the range of 4 ~ 15 nm in the thermal implantation method
[4]. Subsequently, Bekenstein et al studied in detail the control of reaction temperature on the morphology and size of CsPbX
3 nanocrystals. When the reaction temperature was as low as 70 ℃, an almost transparent suspension was formed, and the suspension was amorphous micron-sized flakes with almost no crystals, which was due to the unreacted precursor. When the reaction temperature is in the range of 90 ~ 130 ℃, the lamellar nanosheets with lower symmetry tend to be formed, and the nanosheets grow in the lamellar structure with a length of 200 ~ 300 nm, which indicates that the lamellar structure can be used as a soft template to guide the growth, thus destroying the inherent cubic symmetry of the crystal. When the reaction temperature is 150 ℃, the products are all nanocubes with side lengths ranging from 6.7 nm to 12.7 nm. When the reaction temperature is in the range of 170 ~ 200 ℃, the nanocubes with larger size are formed, and the side length is in the range of 10 ~ 15 nm
[53]. This size dependence on temperature is consistent with Ostwald ripening growth
[50]. Dutta et al. Used NH
4X as the only halogen source, accurately controlled the proportion of cesium source, lead source and halogen source in the reaction conditions, raised the reaction temperature to 220 ~ 260 ℃, obtained pure phase nanocubes, and prepared CsPbX
3(X=Cl,Br,I) nanocrystalline PLQY close to 100%
[54].
4.2 Surface Ligand
The main function of surface ligands is to act as surfactants during the growth of CsPbX3 nanocrystals, selectively combine with the surface of CsPbX3 nanocrystals, affect the interaction (solubility, charge transfer, etc.) Between the CsPbX3 nanocrystal and the environment, control the growth kinetics of nanocrystal, improve its dispersion stability and prevent agglomeration, which is very important to obtain ideal morphology and size. Commonly used ligands are: hexadecylamine, stearic acid, trioctylphosphine, trioctylphosphine oxide, dodecylamine, myristic acid, OAm, OA and so on.
Changing the proportion and type of surface ligands can affect the growth rate of each crystal plane of CsPbX
3 nanocrystals, thus changing the size and morphology of nanocrystals. Liang et al. Regulated the morphology of CsPbBr
3 nanocrystals by changing the equivalent weight of OA and OAm at 90 ℃ by thermal implantation. When OA was 0.3 mL and OAm was 0.7 mL, nanowires with length of 400 nm and average diameter of 3.6 nm were synthesized. When OA was 0.5 mL and OAm was 0.5 mL, the nanosheets with a lateral dimension of about 16 nm and a thickness of about 2.3 nm were synthesized. When OA was 0.6 mL and OAm was 0.3 mL, the average diameter of the nanodots was about 2.4 nm. When OA was 0. 8 mL and OAm was 0. 2 mL, nanosheets with a transverse dimension of about 75 nm were synthesized
[55]. The relationship between the amount of OA, OAm and the morphology of the final nanocrystals is not obvious, which indicates that the morphology and size of CsPbBr
3 nanocrystals are not only affected by the equivalent change between OA and OAm, but also restricted by other factors.
Subsequently, the researchers further studied the effect of surface ligand chain length on the size and morphology of nanocrystals. Shamsi et al. Found that by adjusting the ratio of short chain (octanoic acid and octylamine) to long chain ligands (OA and OAm), the lateral dimension of the CsPbBr
3 nanocrystals can be adjusted from 300 nm to 5 μm, and the thickness can be maintained at 2.5 ~ 3 nm
[56]. Imran et al. introduced short-chain carboxylic acids, and the nanocrystals grew along the < 110 > plane, which made the products tend to form nanowires with a width in the range of 3 ~ 11 nm
[51]. The organic short-chain aliphatic amine ligands contribute to the formation of CsPbX
3 nanosheets because the short-chain amines diffuse faster than the long-chain amines, resulting in a faster exchange rate, which promotes the anisotropic growth of CsPbX
3 nanocrystals to form thinner nanosheets, while the organic long-chain fatty acid ligands contribute to the formation of cubic CsPbX
3 nanocrystals.
In addition, Sun et al studied the influence of different ligands on the morphology and size of nanocrystals, and found that the size and morphology of nanocrystals were also related to the electrostatic and hydrophobic interactions among ions, ligands and solvents
[31].
In Order to obtain large-sized nanocrystals with uniform morphology, Imran et al. Used OA as ligand and secondary amine instead of primary amine to prepare CsPbBr
3 nanocubes with uniform morphology and monodispersity in the transverse dimension of 50 μm.This result is not affected by the temperature of the bromide precursor, the chain length of the secondary amine, the concentration of OA and other factors, because the secondary amine can not effectively combine with the nanocrystal surface and does not compete with OA for binding sites on the nanocrystal surface, which inhibits the formation trend of low-dimensional structure and eliminates the possibility of forming non-cubic morphology
[52].
In the traditional thermal injection method, the stoichiometric ratio of Cs
2CO
3 and OA is used to prepare the cesium precursor, and the reaction is usually insufficient, resulting in some unreacted and insoluble Cs
2CO
3 in the product. In order to make the raw materials fully react, Lu et al changed the scheme of preparing Cs-OA precursor, increased the equivalent of OA (Cs ∶ OA = 1 ∶ 5), the reaction of preparing Cs-OA was more thorough, the Cs
2CO
3 in the system was completely consumed, and the synthesized CsPbBr
3 had higher PLQY
[57].
5 Strategies to improve the stability of CsPbX3 nanocrystal
CsPbX
3 nanocrystals are susceptible to polar solvents (especially water), high temperature, light and other factors due to their ionicity and large surface energy, resulting in the decrease of luminescence intensity or structural decomposition, and the poor stability of materials hinders their practical application. Therefore, improving the stability of nanocrystals is the basis for further application
[58].
In recent years, researchers have proposed a variety of strategies to improve the stability of CsPbX
3 nanocrystals, such as doping ions to stabilize nanocrystals, using ligands with strong coordination ability to protect nanocrystalline, and using polymers, inorganic salts and gold oxides to embed nanocrystalline. For example, Tang et al. Used 3-aminopropyltriethoxysilane molecules as ligands to stabilize γ-CsPbI
3 nanocrystals. The results showed that the crystal structure and morphology did not change in air or ethanol solution, showing excellent stability
[59]. Akkerman et al. prepared CsPbI
3 nanocrystals doped with Mn
2+ ions, the crystal structure of which is the same as that of α-CsPbI
3, and the solution and film samples can be stable for one month at room temperature. This is because the radius of Mn
2+ ions is slightly smaller than that of Pb
2+ ions, resulting in a decrease in lattice parameters, an increase in tolerance factor and binding energy, and an improvement in the stability of the material
[60].
Using inert matrix (such as polymer, inorganic salt, metal oxide, etc.) To coat nanocrystals to form core-shell structure and isolate them from the environment is also an effective strategy to improve stability. Wang et al. Reported a method of in-situ synthesis of CsPbX
3 nanocrystals in silicone oil, using the crosslinking reaction of tetramethyltetravinylcyclotetrasiloxane and tetramethylcyclotetrasiloxane, so that the nanocrystals are dispersed in the silicone resin network and exist stably. On the one hand, this strategy can effectively passivate the surface defects of nanocrystals,Its PLQY increased from 68% of the original solution (before the formation of the composite) to 82% of the composite powder. On the other hand, due to the protection of silicone resin, the CsPbBr
3- silicone resin composite showed excellent stability to water and high temperature in the environment, and still had good luminescent properties after being placed in air for one year or immersed in water for two months
[61]. Based on AlPO-5 zeolite crystals, Wang et al. Prepared perovskite-zeolite composites by in-situ growth at room temperature in air. Zeolite porous materials can guide the nanocrystals to grow within the pore size and isolate each other.AlPO-5 zeolite can be used as a protective layer to improve the stability of the nanocrystals.In addition, there is a hydrogen bond interaction between the nanocrystals and zeolite, which can passivate the defects of nanocrystals. Experiments have shown that the composite can be stored at 150 ° C for 15 months and in water for more than 2 weeks
[62]. Li et al. adsorbed superhydrophobic aerogel inorganic matrix (S-AIM) on the surface of nanocrystals. Due to its rough hydrophobic surface and lipophilic groups on the surface, the treated CsPbBr
3 nanocrystals maintained the initial PLQY after soaking in water for 11 days, and maintained 50. 5% of the initial PLQY after soaking for 3. 5 months, showing excellent water stability
[63]. Many researchers have also been working to enhance the stability of CsPbX
3 nanocrystals using metal oxides (e.g., TiO
2, Al
2O
3, SiO
2, ZrO
2) as protective layers to form core-shell structures. Duan et al. used a SiO
2/ZrO
2 bimetallic oxide as a protective coating for the CsPbBr
3 and fabricated a stable, highly efficient LED device based on this material, with a lumen efficiency of :75 lm·W
-1/200 h and a stable operating time of 70 lm·W
-1/700 h at 100 and 10 mA, respectively, at ambient conditions
[64].
6 Optoelectronic Applications of CsPbX3 Nanocrystal
The optical properties of CsPbX3 nanocrystals mainly depend on their composition, structure and size.Benefiting from the special physical properties and excellent optoelectronic properties brought by the nano-size effect, CsPbX3 nanocrystals have shown great potential in optoelectronic fields such as solid-state luminescence, display, laser, photodetector and photovoltaic cells. In this section, we focus on the application status and development prospects of CsPbX3 nanocrystals in these fields.
6.1 White light emitting diode
As a new generation of solid-state lighting, WLED has the characteristics of energy saving, environmental protection and green, and has been commercialized in the field of lighting and display. WLEDs are fabricated from ultraviolet or blue chips and a single color phosphor or a dual color phosphor or a combination of blue, red, and green phosphors. A gallium nitride (GaN) or indium gallium nitride (InGaN) semiconductor chip is used as a light source for emitting ultraviolet light and blue light, and a fluorescent material absorbs part of the energy emitted by the chip and emits white light covering the whole spectrum through down-conversion. In view of the excellent photoluminescence properties of CsPbX3 nanocrystals, such as narrow full width at half maximum (FWHM), high PLQY and tunable emission color, CsPbX3 nanocrystals are gradually developed as potential materials for the preparation of WLED devices.
Huang et al. Coated CsPbX
3 nanocrystals with polyhedral oligomeric silsesquioxane (POSS), and combined green light POSS-CsPbBr
3, red light POSS-CsPbBr
xI
3-x nanocrystals and blue light InGaN chips to prepare WLEDs, with color coordinates of (0.349, 0.383), color rendering index value of 81, and lumen efficiency of 14.1 lm·W
-1, which are close to those of traditional incandescent lamps (17 lm·W
-1)
[65]. Wang et al. fabricated WLEDs based on mesoporous silica (MPS) coated green light MPS-CsPbBr
3, red light MPS-CsPb(Br
0.4I
0.6)
3 nanocrystal mixture and blue light chip combination, with white light color coordinates of (0.24, 0.28) and lumen efficiency of 30 lm·W
-1, covering 113% of the color gamut of the National Television Standards Committee (NTSC)
[66]. Li et al. Used ZnX
2/ hexane solution to post-treat CsPbX
3 nanocrystals, and the PLQY of ”(PbX
2),CsPbCl
3, CsPbBr
3 and CsPbI
3 nanocrystals that eliminated "black spots" adhered to the surface increased from 4%, 58% and 63% to 86%, 93% and 95%, respectively. They used polydimethylsiloxane (PDMS) to coat the post-treated CsPbBr
3 nanocrystals and made them into thin films. Compared with the original CsPbBr
3 nanocrystals, the coating did not change the luminescence intensity and peak position of the nanocrystals.The WLED with a "sandwich" structure prepared by combining the red light KSF fluorescent material and the blue light GaN chip achieves a luminous spectrum covering the entire visible light region, and the lumen efficiency is 98 lm·W
-1, covering 130% of NTSC, as shown in fig. 5
[67].
Ye et al. embedded CsPbX
3 nanocrystals into Y-type zeolite to prepare a composite material (CsPbX
3-Y), and further prepared WLEDs by combining a blue CsPb(Cl
0.5,Br
0.5)
3-Y composite material, an orange CsPb
0.75Mn
0.25Cl
3-Y composite material and a UV chip, with color coordinates of (0.34, 0.36), color temperature of 5336 K and color rendering index of 81
[68]. Based on the combination of green light CsPbBr
3- kaolin, red light CsPb(Br
0.5I
0.5)
3- kaolin composite and blue light chip (λ = 460 nm), Lv et al. Prepared WLEDs with white light color coordinates of (0.34,0.31), color temperature of 5048 K and lumen efficiency of 68.2 lm·W
-1[69].
White light emission obtained by mixing three primary colors of light (red, green, and blue) has a poor color rendering index due to a narrow emission spectrum distribution. Yoon et al. combined five monochromatic CsPbBrxClyI1-x-y of cyan, green, yellow-green, orange and red with blue chips to prepare WLEDs, which showed a super-high color rendering index of 96, covering 145% of NTSC, and a lumen efficiency of 62 lm·W-1[70].
6.2 Lectroluminescent diode
Electroluminescence (EL) is a solid luminescence phenomenon in which electric energy is converted into light energy. Like other luminescence processes, EL includes three main processes: excitation, energy transport and recombination luminescence.Through a certain electric field distribution, the velocity and energy distribution of carriers in the luminescent material are changed, and the luminescent center is collided or ionized to produce recombination luminescence. The electroluminescent diode has the advantages of low energy consumption, long service life, environmental protection and the like. CsPbX3 nanocrystals show great potential in the field of electroluminescent devices due to their excellent optical properties.
In 2015, Song et al. prepared blue, green and orange LEDs based on CsPbX
3 nanocrystals for the first time, and the device structure is shown in the following :ITO/PEDOT:PSS/PVK/CsPbX
3/TPBi/LiF/Al. The maximum brightness of blue, green and orange LEDs is 742 cd·m
-2, 946 cd·m
-2 and 528 cd·m
-2, respectively, and the maximum external quantum efficiency (EQE) is 0. 07%, 0.12% and 0. 09%, respectively
[6].
Li et al. Used trimethylaluminum (TMA) vapor crosslinking method to treat CsPbX
3 nanocrystals. Aluminum oxide formed a dense crosslinking network on the surface of the nanocrystals, which prevented the CsPbX
3 nanocrystalline film from dissolving in organic solvents. Under the condition of not using organic solvents, it promoted the deposition of charge injection layer and formed high-quality nanocrystalline films
[71]. The structure is as follows: :ITO/ZnO/CsPbX
3/TFB/MoO
3/Ag, the CsPbI
3 nanocrystal is synthesized based on the cross-linking technology, the EQE of the prepared red light emitting diode device is 5.7%, and the maximum luminous brightness of the green light emitting diode prepared based on the green light CsPbBr
3 is 2335 cd·m
-2.
Zhang et al added a perfluorinated ionomer (PFI) interlayer between the hole transport layer and the CsPbBr3 nanocrystalline light-emitting layer, which increased the valence band top of the hole transport layer by 0. 34 eV and improved the hole charge injection efficiency.
The introduction of the PFI interlayer is beneficial to the charge balance of the CsPbBr
3 nanocrystals, and the excellent optical properties of the nanocrystals are retained. Based on this scheme, the green LEDs were fabricated with the emission peak at 516 nm, the full width at half maximum (FWHM) of 18 nm, the maximum brightness of 0. 06% of the 1377 cd·m
-2,EQE, and the color coordinates of (0. 09, 0. 76)
[72]. Zhang et al. added polyethyleneimine (PEI) between the charge transport layer and the perovskite layer to passivate the surface defects of the CsPbBr
xI
3-x nanocrystals, which increased the PLQY from 15% to 70%, and the fluorescence lifetime from 26. 2 ns to 147. 5 ns. The maximum brightness of the corresponding red LED device reached 2015 cd·m
-2, and the current efficiency reached 6. 3% 3.4 cd·A
-1,EQE
[73]. Pan et al. Proposed to passivate the surface defects of CsPbX
3 nanocrystals by using short-chain ligands such as didodecyldimethylammonium bromide (DDAB) instead of long-chain ligands such as OA and OAm, which improved the crystalline quality of CsPbX
3 nanocrystals, the carrier mobility of thin films and the EQE of devices
[74]. The corresponding device structures are as follows: :ITO/PEDOT:PSS/PVK/CsPbX
3/TPBi/LiF/Al, the EQE of the green LEDs is 3.0% and the maximum brightness is 330 cd·m
-2, and the EQE of the red LEDs is 1.9% and the maximum brightness is 35 cd·m
-2. Pan et al. Doped Ni
2+ ions into the CsPbCl
xBr
3-x, effectively passivating the surface defects of the nanocrystals. When 2.5% of Ni
2+ ions were doped, the PLQY of the CsPbCl
0.99Br
2.01 was up to 89%. Based on this material, they fabricated a blue light emitting diode device. The device structure is as follows: the :Al/LiF/TPBi/CsPbCl
xBr
3x:Ni/TFB/PFI/PEDOT:PSS/ITO/Glass,EQE is 2.4%, and the maximum brightness is 612 cd·m
-2, which is the highest performance of the current blue light CsPbX
3 nanocrystal LEDs, as shown in Figure 6
[75].
He and others coated silica gel between the GaN chip and the perovskite layer, and then used atomic layer deposition technology to grow a layer of Al
2O
3 on the surface of CsPbX
3 nanocrystals as a thermal insulation layer to improve its stability. Yellow LEDs based on CsPbBr
xI
3-x nanocrystals were fabricated, with color coordinates of (0. 49, 0.49) and luminous efficiency of 13. 51 lm/W at 6 mA, and the device still maintained 60% performance after 60 min at 50 mA current, showing excellent thermal stability
[76].
6.3 Laser
Laser is a device that uses semiconductor material as medium to generate stimulated radiation. The population inversion is realized in the band gap of the semiconductor material through a certain excitation mode, and when the electrons in the population inversion recombine with the holes, the stimulated emission phenomenon, namely laser, is generated. CsPbX3 nanocrystals are ideal semiconductor laser materials due to their high absorption coefficient, long carrier lifetime, low nonradiative recombination rate and high optical gain.
In 2007, Kondo et al. First studied the stimulated emission of CsPbBr
3 nanocrystalline thin films. The results show that the CsPbBr
3 nanocrystals have high optical gain and can achieve strong stimulated emission at low temperature (77 K) and room temperature (295 K), and the threshold intensity is 50 kW·cm
-2 and 100 kW·cm
-2, respectively, which is a potential new laser medium
[77]. In 2015, based on CsPbX
3 nanocrystals, Protesescu et al. realized low threshold amplified self-emission and laser emission in the whole visible spectral range (440 ~ 770 nm) by controlling the composition and proportion of X
- ions, with a pumping threshold of 5±1μJ·cm
-2 and a gain coefficient of 450±30 cm
-1. Two laser modes were successfully observed: whisper-gallery mode laser with :CsPbX
3 nanocrystal-coated silica microspheres as a high fineness resonator and random laser with CsPbX
3 nanocrystal films
[78]. Xu et al. Observed efficient two-photon absorption (2.7×10
6GM in cross section) in toluene solution of CsPbBr
3 nanocrystals, and excited large optical gain (>500 cm
-1) in the film. CsPbBr
3 nanocrystals are coupled with a microtube resonator to realize stable two-photon pumping laser at a low threshold
[79].
Eaton et al. Synthesized surfactant-free CsPbX
3 nanowires using a low-temperature solid solution method, with lengths of 2 to 40 μm and widths of 0.2 to 2.3 μm, which are in the ideal nanocrystalline size range for laser emission. The femtosecond pulse laser excites the CsPbBr
3 and the CsPbCl
3 nanowire to realize laser emission
[10]. Fu et al. Prepared CsPbBr
xCl
3-x nanowires by low-temperature gas-phase anion exchange method, with cross-sectional dimensions of hundreds of nanometers and lengths of several microns to tens of microns, and realized wavelength-tunable laser emission in the whole visible light range (420 ~ 710 nm). When the pump intensity was lower than the lasing threshold, the luminescence spectrum of CsPbBr
3 nanowires showed a broad peak emission centered at 530 nm.When the pump intensity is higher than the lasing threshold, a narrow luminescence peak is generated and the luminescence intensity is rapidly increased with the increase of the pump energy. Under continuous illumination for 8 H or 7.2×10
9 of pulsed laser irradiation, stable laser emission is shown, as shown in fig. 7
[11].
In 2020, Li et al. Designed and prepared CsPbBr
3 nanocrystals with an optical gain coefficient of 954 cm
-1. In addition, the use of micro-hydrophobic zeolites improves their stability, and the strong scattering introduced by them is conducive to three-dimensional random laser. Under the extreme conditions of high humidity (85% relative humidity) and high temperature (85 ℃), 80% of the optical performance can be maintained after 15 hours of operation. Due to the high sensitivity of the laser to humidity, the laser intensity shows a sharp decrease with increasing relative humidity when the relative humidity is between 72% and 82%, and then gradually increases as the relative humidity gradually decreases, which makes the perovskite laser applicable to humidity detection
[80].
6.4 Photoelectric detector
Photodetector is a device that converts the incident light signal (ultraviolet light, visible light or infrared light, etc.) into an electrical signal. When the light shines on the photodetector, due to the photoelectric conversion characteristics of the material, the electrons in the valence band jump to the conduction band to form freely moving carriers, thus causing the change of current. CsPbX3 nanocrystals have shown great application prospects in the field of photodetection due to their easily adjustable band gap, high absorption coefficient and long carrier transport distance.
In 2015, Ramasamy et al. First prepared a photodetector based on CsPbI
3 nanocrystals, which consists of a silicon or ITO/PET substrate, an active layer of CsPbI
3 nanocrystals and a pretreated gold electrode. The on/off light ratio of the photodetector based on the CsPbI
3 nanocrystalline film is 10
5, and the rise time and the decay time are 24 ms and 29 ms, respectively
[81]. Subsequently, Song et al. fabricated photodetectors based on CsPbI
3 and CsPbBr
3 nanosheets, with maximum response wavelengths of 630 and 517 nm, respectively. Thanks to the two-dimensional planar structure and high crystallinity, the photodetectors fabricated based on the CsPbBr
3 nanosheets have 10
3 on/off light ratio, rise and decay times of 0. 019 ms and 0. 025 ms, and response speed of 0.25 A·W
-1 at a bias of 5 V, which is comparable to that of commercial silicon photodetectors (0.2 A·W
-1)
[7].
The quality of CsPbX
3 nanocrystalline thin films determines the performance of devices, but the residual solvents and ligands in the preparation process will lead to pinholes and cracks in the films, which greatly limits the improvement of device performance. Li et al. Reported the size change and self-repair phenomenon of CsPbX
3 nanocrystals at room temperature. Under the action of organic ligands and polar solvents, the surface dissolution and recrystallization of CsPbX
3 nanocrystals occur continuously. They used this crack-free, low-roughness CsPbBr
3 nanocrystalline film to prepare photodetectors, which had a responsivity of 0.176 A·W
-1 and an external quantum efficiency of 41% at a bias of 10 V. It could be stable for more than two months at an air humidity of 40% -50%, with a rise and decay time of 1 ms and 1.8 ms
[8].
Tong et al. Used the two-dimensional structure of the CsPb
2Br
5 as a nucleation site to induce the initial growth of CsPbBr
3 nanowires. The phase transition (CsPb
2Br
5→CsPbBr
3+PbBr
2) from the CsPb
2Br
5 to the CsPbBr
3 is controlled, and the crystals are rearranged and slowly grown to have a higher crystallinity. They grew CsPbBr
3 nanowires on a SiO
2/Si substrate and deposited a gold layer on the surface of the nanowires. Under different laser intensities at 400 nm wavelength, the photocurrent-time curves of the devices showed significant differences, with 6.44 A·W
-1 responsivity and about 10
12J detectivity. The temperature existed for more than two months under the condition of 45% air humidity, as shown in Figure 8
[82].
6.5 Solar Cell
Solar cell is a hot spot in the current renewable energy research. The third generation solar cell technology (including dye-sensitized solar cell, polymer solar cell and perovskite solar cell) shows great application potential because of its low cost, low energy consumption, wide range of raw materials and environmental friendliness. Perovskite materials have become a research hotspot in the field of solar cells due to their excellent optical properties, suitable band gap and strong carrier transport ability. Although organic-inorganic hybrid perovskite solar cells have made significant progress, organic materials are less stable than inorganic materials, resulting in more sensitive to water vapor and high temperature.And that organic component are easy to volatilize, so that the perovskite thin film is degraded, the long-term stability of the device is threaten, and the practical application of the battery is greatly limited
[83,84][85].
The α-CsPbI
3 has a suitable band gap (1.73 eV) and a strong absorption ability in the whole visible light region, so it is a suitable active layer. However, in air, the distortion of the PbI
64- of the octahedral skeleton of the CsPbI
3 angle sharing will lead to phase transition, which will be transformed into δ-CsPbI
3 that are not suitable for photovoltaic applications, so the preparation of stable α-CsPbI
3 is the key to the stability of devices. In 2015, Eperon et al. First prepared a perovskite solar cell based on α-CsPbI
3 by replacing the methyl ammonium cation in the organic-inorganic hybrid perovskite with Cs
+ ions. The device structure is shown in the :ITO/PEDOT:PSS/CsPbI
3/PCBM/Ca/Al below. Although the PCE is only 2.9%, which is much lower than that of the organic-inorganic hybrid perovskite solar cell (≈ 20%), the light stability and thermal stability of the device have been improved
[86]. Luo et al. Reported a continuous solvent addition process, adding isopropanol after adding hydriodic acid to prepare stable and highly crystalline α-CsPbI
3 nanocrystalline thin films in air. The intermediate Cs
4PbI
6 has a protective effect on the α-CsPbI
3 nanocrystals at room temperature. Under reverse scanning, the PCE of the prepared devices reached 4.13%
[87]. Wang et al. Used methyl acetate to remove excess unreacted precursors, and the prepared nanocrystals could exist stably in the air for several months. The photovoltaic devices based on the treated α-CsPbI
3 nanocrystal films showed high stability and water resistance, and the PCE reached 10.77%
[88].
The lattice defects and surface vacancies in the CsPbI
3 nanocrystals are the main reasons that hinder the further improvement of the device performance. Simple and effective surface passivation using inorganic cesium salts such as cesium acetate can not only fill the vacancies on the surface of CsPbI
3 nanocrystals, but also improve the electronic coupling between CsPbI
3 nanocrystals. Ling et al. Used cesium acetate to treat the surface of CsPbI
3 nanocrystals, which increased the PCE to 14.10% and greatly improved the stability to water
[90]. Chen et al. Treated the surface of mesoporous titanium dioxide (m-TiO
2) with methyl acetate solution rich in electrons and europium ions, which reduced the solid-liquid interfacial tension of the TiO
2 surface and improved the wettability.It is beneficial for the CsPbI
3 nanocrystals to migrate to the m-TiO
2, while the Cs
+ ions passivate the surface of the nanocrystals and promote the charge transfer at the m-TiO
2/ nanocrystal interface, thus increasing the electron injection rate by three times. The device structure is as follows: The :Au/spiro-OMeTAD/QDs/S-m-TiO
2/QDs/C-TiO
2/FTO/glass,J
sc value is 17.77 mA·cm
-2, the PCE of the reverse scan is 14.32%, and the EQE is 92%, as shown in Figure 9
[89].
Ion doping is also an effective method to passivate perovskite defects and improve the performance of solar cells. Yao et al. Added SCN
- to the CsPbI
3 film to improve the crystallinity of the perovskite film. With the addition of Pb(SCN)
2, the crystallinity and thermal stability of the CsPbI
3 nanocrystalline films were improved significantly, and the PCE was up to 17. 04%
[91]. Shi et al. Proposed a strategy of in-situ ytterbium (Yb) doping during the synthesis of CsPbI
3 nanocrystals. The experimental results show that Yb doping can effectively reduce the density of defects and trap States caused by surface vacancies and lattice vacancies, which is helpful to improve PLQY, crystalline quality, thermal stability and carrier transport capacity. The highest PCE of the device is 13. 12%, and the stability in air is significantly improved
[92].
Although β-CsPbI
3 nanocrystals have a bandgap suitable for tandem solar cells, the preparation of stable and high-performance β-CsPbI
3 nanocrystals remains a challenge. Wang et al. treated the surface with choline iodide (CHI) to inhibit the generation of cracks and pinholes in the perovskite layer, increase the lifetime of carriers, and improve the energy level matching between the β-CsPbI
3 absorption layer and the carrier contact layer. The treated perovskite solar cell has high repeatability and stability under the environmental condition of (45 ± 5) ℃, and the PCE can reach 18.4%
[88]. In order to further reduce the influence of cracks and pinholes in perovskite layers and prepare europium perovskite films with desired crystal phases and morphologies, they studied the role of dimethyl ammonium iodide (DMAI) in the fabrication of CsPbI
3 perovskites.It is demonstrated that DMAI is an effective volatile additive that does not alloy with the lattice of CsPbI
3 nanocrystals and can control the crystallization process of CsPbI
3 perovskite thin films with different crystal phases and morphologies. The PCE of perovskite thin film passivated by PTACl is 19.03%, which is the highest value at present
[93].
7 Conclusion and prospect
The CsPbX3 nanocrystals with excellent photoelectric properties such as high absorption coefficient, narrow half-peak width, adjustable emission wavelength, high fluorescence quantum yield, strong stability and the like can be prepared by selecting a proper preparation method, regulating and controlling factors such as reaction temperature, surface ligands and the like, and subsequently passivating surface defects of the CsPbX3 nanocrystals, doping ions, introducing organic or inorganic substances and the like. At the same time, remarkable progress and achievements have been made in the application of CsPbX3 nanocrystals in solar cells, photodetectors, LEDs and lasers. However, the research on CsPbX3 nanocrystals still faces many challenges:
(1) At present, the preparation methods of efficient CsPbX3 nanocrystals, the effects of reaction temperature and surface ligands on the morphology have been gradually explored. However, the nucleation and growth process of CsPbX3 nanocrystals, the evolution of precursors and the interaction between nanocrystals and surfaces are not clear enough, and need to be further deepened.
The stability of (2)CsPbX3 nanocrystals in polar solvents and high temperature conditions, as well as the design of heterojunction structures based on CsPbX3 nanocrystals and their applications in photocatalysis need to be further explored.
(3) In addition, due to the toxicity of lead, low-lead perovskites such as binary perovskites, lead-free perovskites such as tin-based perovskite and other non-heavy metal perovskite have attracted the attention of researchers and achieved some development, but these materials often have the problems of low luminous efficiency and poor stability, which need further research.
National Natural Science Foundation of China(21304049)
National Natural Science Foundation of China(21674049)
National Natural Science Foundation of China(21001065)
National Natural Science Foundation of China(21274065)
National Natural Science Foundation of China(21601091)
Natural Science Foundation of Jiangsu Province of China(BK20160891)
1311 Talents Program of Nanjing University of Posts and Telecommunications
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