1 Introduction
Perovskite was discovered in 1839 and named after Russian mineralogist Perovski. In a broad sense, a material having a crystal structure similar to that of calcium titanate (CaTiO3). The ideal perovskite general formula is ABX3. Its structure is shown in Figure 1. A is cation such as CH(NH)2+(FA), CH3 (MA), cesium (Cs+), rubidium (Rb+),B is a divalent cation such as tin (Sn2+), bismuth (Bi2+), lead (Pb2+), etc.,X represents chlorine (Cl-), bromine (Br-), iodine (U I-) and mixtures thereof. Perovskites can be divided into organic-inorganic hybrid perovskites and all-inorganic perovskites according to the difference of A.
Although researchers have made great efforts to improve the performance and stability of organic-inorganic hybrid perovskites so far, the organic functional groups in these perovskites are still very sensitive to humidity, oxygen, light and heat, and are easily destroyed in such environments for a long time. Our group has also made good progress in enhancing the stability of hybrid perovskites, and the unpackaged solar cells prepared with gadolinium agarose composite as the light absorption layer are stable for 1392 H in humid environment
[5]. It is an effective way to improve these problems by replacing organic functional groups with more stable inorganic ions. In 2015, Protesescu et al. Successfully prepared inorganic CsPbX
3(X=I, Br, Cl) perovskite nanocrystals by thermal injection method for the first time, and reported their properties. The advantages of CsPbX
3 nanocrystals, such as adjustable band gap, high thermal stability and low defect density, have attracted the attention of researchers
[6]. In the following years, a lot of research work has been devoted to synthesis, morphology control and application of optoelectronic devices. Perovskite nanocrystals have a photoluminescence quantum yield (PLQY) close to 100%, a narrow emission bandwidth (< 20 nm), and a wide color gamut (close to 140%), and their optical properties are influenced by crystal size and composition
[7,8][9,10]. CsPbX
3 nanocrystals based on different halide compositions can be used as blue, green and red light sources. The inorganic perovskite with many advantages is a very suitable material for LEDs, photodetectors, solar cells and lasers.
Inorganic perovskites can be subdivided into four types according to their dimensions, namely, three-dimensional (3D), two-dimensional (2D), one-dimensional (1D) and zero-dimensional (0D) structures. Fig. 1 (B) is a schematic diagram of the classification of perovskite structures in which cations are omitted
[11]. Metal atoms (red spheres) and halide atoms (green spheres) form a zucheng octahedron, and the different dimensions of perovskite are determined by the connection of [PbX
6]
4- octahedrons. 3D perovskites are composed of corner-shared octahedrons extending in three dimensions, and cations fill the interstices of the octahedrons, while 2D/quasi-2D perovskites can be regarded as sheet-like or layered structures torn by 3D perovskites along specific directions. 1D perovskite grows in a certain direction through corner sharing, edge sharing and face sharing to form nanowires/nanorods; For 0 D perovskite, the octahedral [PbX
6]
4- is completely surrounded or isolated by inorganic cations. At present, a variety of synthesis methods have been developed to prepare perovskites with different dimensions, such as the most widely used thermal injection method, which can realize the synthesis of materials with different dimensions through regulation. A microwave method which can effectively slow down the reaction speed so as to prepare high-quality quantum dots; Through these methods, the particle size of quantum dots can be controlled in the range of 3 ~ 10 nm, the length of nanowires can be up to 10 μm, and the minimum thickness of nanosheets can be up to 2. 2 nm. The efficient and simple synthesis method provides convenience for the application of the perovskite, and the perovskite solar cell prepared by using the CsPbI
3 quantum dots as a light absorption layer achieves the photoelectric conversion efficiency of 14.4%,The efficiency of the cells fabricated with CsPbBr
3 nanowires remains at the initial level after 5500 H, the LEDs based on CsPb(Br/I)
3 have an external quantum efficiency of more than 20%, and the photodetectors fabricated with CsPbCl
3 nanowires have a high exchange ratio of 5.8×10
3[12][13][14][15]. The excellent performance of these devices confirms the great potential of inorganic perovskite.
In this paper, the research progress of perovskite materials with different dimensions is reviewed.The common synthesis methods, the methods to control the morphology and size of the materials, and the optical properties of the materials with different dimensions are introduced, and the application progress of perovskite materials in solar cells is also introduced in detail.
2 Zero-dimensional CsPbX3 quantum dot
All-inorganic CsPbX3(X=I, Br, Cl) quantum dots (QDs) have become a very promising material due to their various properties, including tunable band gap, high photoluminescence (PL), high quantum yield (PLQY), narrow emission peak width and excellent stability in air. Quantum dots (QDs) are more susceptible to the influence of quantum confinement effect due to their small three-dimensional size, which changes the photoluminescence properties of QDs and shows unique advantages in device applications.
2.1 Synthetic method
2.1.1 Hot injection method
The earliest CsPbX
3(X=I, Cl, Br) zero-dimensional quantum dots were synthesized by Protesescu et al. Through thermal injection method, and the size was 8 nm
[6]. The principle of the thermal injection method is shown in Figure 2 (a). In this method, CsCO
3, oleic acid (OA) and octadecene are used to synthesize the precursor cesium oleate in a N
2 atmosphere. Cesium oleate is selected as the precursor instead of CsX (X = I, Br, Cl) because CsX has low solubility in common solvents. The PbX
2 was then mixed with octadecene, oleic acid, and oleylamine (OAm) and heated to dissolve the PbX
2. Oleic acid and oleylamine are added to purify and maintain the photoluminescent properties of the CsPbX
3 nanostructures
[16]. After complete dissolution, the cesium oleate precursor solution is rapidly injected, and the reaction time is controlled to obtain CsPbX
3 quantum dots, as shown in fig. 2 (a). At present, this method has become a common method to prepare CsPbX
3 quantum dots. Suh et al prepared CsPbClBr
2, CsPbBr
3 and CsPbI
3 by replacing oleic acid with stearic acid
[17]. Zhang et al. Also used this method to prepare materials in the study of solar cells using CsPbBr
3 quantum dots as light absorption layers. The synthesis process only adjusted the temperature and solution concentration, and the particle size of the product was 10 nm
[18].
2.1.2 Solvothermal method
The advantages of solvothermal method are simple operation, high crystallinity and good repeatability. In the past few years, solvothermal synthesis has also been used to synthesize inorganic perovskites. Fig. 2 (B) shows the principle of the solvothermal method and the CsPbCl
3 quantum dots synthesized by this method. In this method, the precursor and the non-aqueous solvent are mixed in a stainless steel autoclave to react at a certain temperature, and then centrifuged to prepare the desired product. In 2017, Zhang et al. Synthesized inorganic perovskite quantum dots by this method. Cesium acetate and lead halide were mixed with oleic acid, oleylamine and octadecene in an autoclave, heated to 160 ℃ and held for 30 min. The particle size of the product was about 8.2 nm
[19]. Ye et al. Reported a water-bath heating method for the synthesis of zero-dimensional quantum dots, in which cesium carbonate, PbX
2, oleic acid, oleylamine, octadecene and TOP were mixed and heated in a water bath/oil bath at 80 ~ 120 ℃ for 15 ~ 30 min without inert gas protection to prepare perovskite quantum dots with uniform particle size of about 7 nm
[20]. Chen et al. Also synthesized Mn
2+ doped CsPbCl
3 quantum dots by solvothermal method, with a particle size of about 8 nm, and obtained CsPb(Cl/Br)
3 by ion exchange without changing the particle size
[21].
2.1.3 Microwave method and ion exchange
In order to solve the problems of fast crystallization and complex operation of inorganic perovskite, Li et al. Synthesized CsPbX
3(X=I, Br, Cl) nanocrystals by microwave heating, and the reaction time was extended to 4 min. The principle diagram and the corresponding quantum dot TEM diagram are shown in Figure 2 (C). The required solid-liquid reactants (oleic acid, oleylamine, octadecene, Cs
2CO
3, PbX
2) were directly added into a glass container, and then put into a household microwave oven for reaction
[22]. The CsPbX
3 with a particle size of about 10 nm can be prepared by this method. Li et al. optimized the microwave heating method by placing the reactant powder separately in the reaction vessel and immersing the solid powder with liquid reactant, which successfully extended the reaction time to 30 min. This method not only made it easier to observe the changes of the material during the reaction, but also controlled the particle size of the material at 8 ~ 10 nm
[23]. Based on experiments and related research results, they proposed the formation mechanism of CsPbBr
3, which starts with the initial framework of a common corner UK2 octahedron, and gradually develops into a complete crystal structure as the Cs
+ fills the gap. Kovalenko et al. selected appropriate precursor salts (PbX
2, MeMgX) to perform anion exchange on the synthesized CsPbX
3 quantum dots at 40 ℃, and prepared CsPbX
3 quantum dots with different halide compositions without changing the shape and particle size of the quantum dots
[10]. The principle is shown in Fig. 2 (d). Dry octadecene is used as the solvent in the reaction. A certain proportion of CsPbX
3 and halide salt are mixed, and the ion exchange can be realized at an appropriate temperature. For example, the large difference in ionic radius between Cl→Br,Br→Cl,I→Br,Br→I,Cl
- and I
- leads to the instability of Cl/I solid solution, so the exchange between Cl Cl I cannot be carried out. Although the temperature of anion exchange is very low, the CsPbX
3 quantum dots with different halide compositions still maintain their cubic phase structure well.
At present, quantum dots with good morphology can be obtained by various methods, and the detailed parameters are shown in Table 1. However, these methods still have certain defects: the hot injection method is widely used and has low requirements for equipment, but the operation is complex, and the crystallization process is rapid and difficult to control; Solvothermal method simplifies the operation process, but the reactants are mixed together in the reaction process, which makes it difficult to control accurately. The microwave method slows down the crystallization rate, but it requires high equipment. In addition, these methods can only prepare a small amount of quantum dots, and there is no example of large-scale production at present. Therefore, new synthesis methods need to be developed to achieve commercial application.
2.2 Particle size control
Since the first synthesis of perovskite CsPbX3(X=I, Br, Cl) quantum dots in 2015, researchers have done a lot of work to control the morphology and particle size. Perovskite has orthorhombic (δ), tetragonal and cubic (α) structures, among which the cubic structure can be prepared and exist stably at higher temperatures, and many excellent properties of perovskite are related to its cubic structure.At the same time, the morphology and particle size of the material also determine the overall and surface characteristics of the material, so it is of great significance to control the particle size reasonably to meet a variety of needs in different applications.
2.2.1 Effect of process temperature
Temperature is one of the important factors affecting the synthesis of quantum dots. Manna et al systematically explored the influence of temperature on the morphology of quantum dots, and summarized the results as shown in Figure 3 (a). It can be found that as the reaction temperature decreases from 190 ℃ to 125℃,CsPbBr
3, the particle size decreases from 12 nm to 4 nm
[24]. Ye et al. Also explored the effect of temperature on the particle size of quantum dots when preparing CsPbX
3(X=I, Br, Cl), and found that when using PbBr
2 and CsCO
3 to synthesize CsPbBr
3, when the temperature was lower than 70 ℃, the reaction almost did not occur. Between 80 ℃ and 120 ℃, with the increase of reaction temperature, the particle size of the material increased. When the reaction temperature increased from 80 ℃ to 120 ℃, the particle size increased from 10 nm to 20 nm
[20]. Dong et al. And Liu et al. Also concluded that the particle size of the quantum dots decreases as the temperature decreases
[25][26]. This shows that the appropriate temperature is a necessary condition for the reaction, and on this basis, higher temperature can accelerate the dissolution of the precursor, provide greater power for crystal formation, and accelerate crystallization.
2.2.2 Influence of the composition of precursor solution
In addition to exploring the influence of temperature, Manna et al also explored the proportion of oleic acid and oleylamine in the precursor solution. When the temperature is constant, with the increase of the proportion of oleic acid and oleylamine, the particle size of the material also increases. When the proportion is 1:1, the particle size of the synthesized CsPbBr
3 quantum dots is the smallest
[24]. In addition, the synthesis process can be carried out at a higher temperature by increasing the amount of ligand in the precursor solution. Liu et al. systematically studied the influence of ligands and solvents in the precursor solution on the morphology of the CsPbBr
3. Some TEM results are shown in Figure 3 (B). Under the conditions of oleic acid: oleylamine = 9:1, 8:2 and 6: 4.5, respectively, cubes with a particle size of 10.8 nm, nanorods with a length of 216 nm and nanosheets with a transverse size of 235.4 nm were prepared
[27]. The experimental results show that the 0 D-2D perovskite can be prepared by controlling the content of oleylamine in the precursor solution. The materials with different morphologies are prepared due to the anisotropic growth of crystals caused by the kinetic mechanism.In this process, oleylamine adheres to the crystal surface, resulting in different growth rates of the crystal surface.In the case of less oleylamine, the atomic surface will not be completely covered by ligands, which is more conducive to the growth of 0 D materials. Dong et al. Reported a method to control the morphology of quantum dots based on thermodynamic equilibrium.Because the X
- is the most unstable substance in the diffusion of CsPbX
3 and has a low kinetic barrier in the lattice, the particle size of quantum dots is easily affected by the change of Br
- equilibrium between quantum dots and solution
[25]. The control parameter is the ratio of Br/Pb in the reaction solution, and the particle size of the quantum dots decreases with the increase of the ratio of Br/Pb at a certain temperature. When Br/Pb = 10 ~ 25, 7 nm CsPbBr
3 quantum dots can be prepared. However, the Cs
+ and the concentration of the Pb
2+ have no effect on the size of the quantum dots. Liu et al added trioctylphosphine (TOP) as a ligand in the synthesis of acenaphthene. Due to the existence of lone pair electrons on the phosphorus atom, TOP has strong solubility for selenium and sulfur powder
[26]. Therefore, the oil-soluble anionic precursor TOP-X can be conveniently prepared. After TOP is used, the PbI
2 can be easily dissolved to form a stable and highly active PbI
2 precursor, and high-quality CsPbI
3 quantum dots with high luminescence intensity and enhanced stability are prepared, and the particle size is 11 to 16 nm. Subsequently, Liu et al. Synthesized CsPbI
3 quantum dots with GeI
2 and PbI
2 on this basis, and found that excessive Pb was not a necessary condition for the synthesis of quantum dots in this experimental process, and excessive PbI
2 was added to provide enough I
- for the reaction, and the particle size of quantum dots could be controlled at about 12 nm by reasonably adjusting the parameters
[28].
2.2.3 Effect of additives
As one of the effective means to improve the properties of materials, additives are widely used in the preparation of various materials. When Swarnkar et al prepared the α-CsPbI
3, methyl acetate (MeOAc) was added to purify the prepared quantum dots. This additive, as an anti-solvent, could remove the unreacted ligands without causing the material to agglomerate. The stability of the treated quantum dots in the air was also greatly improved, and no phase change occurred even after being stored in the air for one month
[29]. Liu et al. Obtained similar results by using MeOAc after the synthesis of CsPbI
3[26]. In general, the phase transformation of inorganic perovskite occurs in a high humidity environment, but a certain amount of moisture can promote the transformation of precursors and by-products into perovskite, thus improving the quality of the film
[30,31]. Mamgain et al. introduced moisture in the synthesis process, and studied the influence of humidity on the stability and optical properties of perovskite in detail. The CsPbX
3XRD results and corresponding CsPbBr
3TEM results synthesized under 0% and 30% relative humidity (RH) conditions are shown in Figure 3 (C). The peak positions of three different CsPbX
3 perovskite materials prepared under 0% RH and 30% RH are basically the same, indicating that 30% RH does not change the structure of CsPbX
3[32]. By comparing the CsPbBr
3TEM prepared under different humidity, it can be found that the morphology is basically unchanged. In addition, the perovskite prepared at 30% humidity has very good stability and can be stored for about one year.
For nanomaterials, doping different ions into the host lattice has been widely studied as an effective way to stabilize specific crystal phases and adjust the optical and electronic properties of different nanocrystals
[21,33]. van der Stam et al partially substituted Pb
2+ with van der Stam, Cd
2+ and Zn
2+, and found that a small amount of doping would not destroy the perovskite structure
[34]. Zou et al. Doped the CsPbCl
3 quantum dots with Mn
2+. The principle is shown in Figure 3 (d). After the salt containing the required doping element was added to the perovskite material, the Pb
2+ was gradually replaced by the doping element
[35]. Doping of Mn
2+ in the perovskite lattice of the CsPbX
3 quantum dots enhances the formation energy, which effectively enhances the thermal stability and phase stability of the CsPbCl
3. The quantum dots increase from 7.9 nm to 8.6 nm after doping. Chen et al. prepared Mn
2+ doped CsPbCl
3 quantum dots by using a solvothermal method. The XRD and TEM results are shown in Figure 3 (e). Compared with the TEM image, it can be found that the CsPbCl
3TEM image prepared after doping has a clearer outline
[21]. Compared with the tetragonal phase without doping, the XRD results of the product doped with Mn
2+ match the cubic phase CsPbCl
3 standard card, and the peak position shifts slightly after doping. Aft doping, that stability of the material is also improve, and even if the material is placed in the air for 30 days, no obvious change occurs. Fe
2+ is an environmentally friendly and inexpensive ion with good conductivity, so it is also used for doping
[36⇓~38]. Liu et al. Doped it into inorganic europium perovskite and found that it could reduce the defect density of the material and improve PLQY
[39]. Hu et al. Doped CsPbCl
3 and obtained CsPb
1-xFe
xCl
3(x=0.1, 0.2, 0.3) quantum dots with different Fe contents, and the particle size can be controlled in the range of 6. 7 ~ 7.8 nm. Doping can also reduce the toxicity of the material to a certain extent, which provides potential value for the application of perovskite in optoelectronic devices
[40].
2.3 Optical properties
Different from traditional materials, inorganic perovskites have attracted much attention due to their high photoluminescence quantum yield (PLQY) and narrow photoluminescence (PL) half-width. High PLQY can provide higher brightness under low voltage driving, and narrow PL half-peak width (12 ~ 30 nm) provides very high color purity for perovskite.The light absorption capability covering the visible light range and the high carrier mobility and diffusion length provide conditions for the application of the quantum dots in solar cells
[41]. The PL emission of traditional quantum dot materials such as metal sulfides is often affected by the particle size, while the PL emission of inorganic CsPbX
3 quantum dots can also be adjusted by the composition, and the PL peak position can be changed by changing the halogen composition. Protesescu et al. reported that the PL luminescence of this material can cover the entire visible light range (410 ~ 700 nm) when synthesizing CsPbX
3(X=I, Br, Cl) quantum dots. Fig. 4 (a) shows the colloidal solution of CsPbX
3 with different halogen components in toluene under ultraviolet lamp (λ = 365 nm). It can be clearly observed that perovskite with different halogen components shows different colors under ultraviolet excitation
[6]. Fig. 4 (B) is the PL spectrum and UV absorption spectrum of the perovskite product obtained by anion exchange. The PL peaks of the three pure halides CsPbX
3(X=I, Br, Cl) are located at 690, 510 and 410 nm, respectively
[10]. Fig. 4 (C) is the XRD result of the sample of anion exchange of CsPbBr
3 with PbCl
2 and PbI
2. It can be found that the PL spectrum of the material continuously shifts during the exchange process, indicating that the proportion of Cl
- and I
- is increasing. It is worth noting that although the ion exchange process changes the elemental composition of the material, it still maintains a cubic structure. As for the reports of PL and UV absorption spectra, the results of other articles are basically the same as those reported by Kovalenko et al. The influence of particle size on the optical properties of quantum dots is mainly caused by the quantum confinement effect. When the particle size of quantum dots is less than its corresponding Bohr radius, its PL and UV absorption spectra will shift. Protesescu et al reported the Bohr radius of CsPbCl
3(5 nm,75 emV), CsPbBr
3(7 nm,40 emV) and CsPbI
3(12 nm,20 emV). The PL spectra of CsPbBr
3 quantum dots with a particle size of about 3.8 nm can be observed a significant blue shift, and the PL peak moved to 460 nm
[6].
Photoluminescence quantum yield (PLQY) is also one of the important properties of perovskite. The higher the PLQY, the lower the defect density of the material. The trap state of quantum dots has an important impact on the optical and optoelectronic properties of quantum dots, and then affects the performance of devices. Kovalenko et al. Have also reported this, and their prepared CsPbX
3(X=I, Br, Cl) PLQY is 10% -80% (CsPbCl
3 is the lowest)
[10]. For CsPbCl
3, the PLQY of materials prepared by other researchers is also very low, but the researchers found that doping can effectively improve the PLQY. Liu et al. increased the PLQY of CsPbCl
3 from 1.85% to 4.32% by doping with Fe
2+, and Hu et al. increased the PLQY from 4.4% to 6.2% by doping with Fe
2+[39][40]. Ding et al. Applied doping to CsPbBr
3. After doping Zn
2+, the quantum efficiency and stability were improved while maintaining the fluorescence intensity of the material, and the PLQY increased from 32% to 36%
[42]. Mamgain et al. found that the PLQY was also affected by the humidity of the atmosphere during the synthesis process. Compared with 0% humidity, the PLQY of the CsPbBr
3 and the CsPbI
3 at 30% humidity was improved (74.2% ~ 92.3%, 69.8% ~ 91.5%)
[32]. Compared with other inorganic perovskites, CsPbI
3 has a very high PLQY, which is further improved to nearly 100% by using TOP to treat quantum dots, which is very beneficial to give full play to the optical properties of the material
[26].
Time-resolved photoluminescence spectroscopy is one of the important means to characterize perovskite. Tong et al. measured the synthesized 8-15 nm CsPbX
3(X=I, Br, Cl) quantum dots. As shown in Figure 4 (d), the decay time of CsPbX
3(X=I, Br, Cl) with different halogen compositions is between 2.5 and 34.5 ns, and the decay time of CsPbI
3 is the longest
[43]. Chen et al. Also measured the time-resolved photoluminescence decay curves of CsPbX
3(X=I, Br, Cl) nanocrystals with different halogen compositions at different times. The decay time of CsPbI
3 is the longest, about 7 ns, while that of CsPbBr
3 is the shortest, only 1.5 ns, which indicates that CsPbI
3 has a longer carrier lifetime
[44]. At the same time, it is found that the decay time of perovskites with the same halogen composition does not change much with time. Liu et al. Found that the decay rate of the CsPbI
3 after using TOP was significantly slower than that of the material synthesized in the oleic acid oleylamine system
[26]. The doping Fe
2+ adopted by Hu et al. Also has an effect on the decay rate, and the obtained CsPb
1-xFe
xCl
3 shows that when X = 0.2, the decay time is 14.6 ns, and when X = 0, the decay time is only 0.8 ns, which indicates that the treatment such as TOP and doping are effective for prolonging the carrier lifetime, and a longer carrier lifetime is favorable for exerting the excellent photoelectric properties of perovskite
[40].
2.4 Application in solar cell
At present, CsPbX3 inorganic perovskites have been widely used in photodetectors, LEDs, lasers and solar cells. Among them, inorganic perovskite quantum dots have achieved good results in solar cells, and the efficiency of the devices prepared has been continuously improved.
The CsPbI
3 has a very suitable bandgap (1.7 eV) for solar cells. In 2016, Swarnkar et al. First used α-CsPbI
3 quantum dots (QD) as light-absorbing materials to fabricate solar cells with a FTO/TiO
2/CsPbI
3QDs/spiro-OMeTAD/Al structure
[29]. CsPbI
3 quantum dots (QDs) are first spin-coated on a substrate, and then immersed in a saturated MeOAc solution for several times to prepare films with a thickness of 100 ~ 400 nm. The photocurrent conversion efficiency (PCE) of the device is 10. 77%, and the open circuit voltage is 1. 23 V. The optimized CsPbI
3QDs has good thermal stability and can maintain the cubic phase for a long time at room temperature, and the stability of the corresponding solar cell device is also very good. Bian et al. Prepared a solar cell with a double-layer perovskite structure of CsPbI
2Br and CsPbI
3QDs
[12]. The solar cell with double-layer perovskite structure can absorb light of 300 ~ 700 nm, and the photoelectric conversion efficiency of 14. 4% is obtained by optimizing the doping of the Mn
2+, the short-circuit current is 15.25 mA/cm
2, and the open-circuit voltage is 1. 204 V. Bian et al. Also used SCN
- to treat CsPbI
3 quantum dots to enhance stability, and no efficiency degradation was observed over a test time of 100 s. At the same time, the effects of the thickness of the CsPbI
3 quantum dots on the open circuit voltage (V
oc), current density (J
sc), fill factor (FF) and photoelectric conversion efficiency were also explored, and it was found that with the increase of the film thickness, the J
sc increased, but the V
oc and FF decreased, and the PCE increased first and then decreased. Liu et al. Synthesized CsPbI
3 quantum dots with TOP, GeI
2 and PbI
2 to prepare devices, and the quantum dots were prepared into films on FTO glass under the protection of N
2 atmosphere.An inorganic perovskite solar cell with an open circuit voltage of 1.11 V and a short circuit current of 14.80 mA/cm
2,12.15% photoelectric conversion efficiency was fabricated, and the quantum dots were effectively passivated by the use of GeI
2[28]. The device prepared by the method can maintain the initial efficiency for more than 60 days in an environment of 25 deg C and RH < 20%. Shivarudraiah et al. prepared a solar cell device with a trans-structure based on CsPbI
3 quantum dots. The device structure is a FTO/NiO
x/CsPbI
3-QDs/C60/ZnO/Ag, and the ligands on the surface of the quantum dots are removed by depositing layer by layer and performing FAI treatment on each layer. The principle is shown in Figure 5 (a) (Control QDs represents the traditional method for removing the ligands on the surface of the quantum dot film, and FAI QDs represents the improved method for removing the ligand on the surfaces of quantum dot films)
[46]. By using FAI layer by layer, the oleic acid and oleylamine ligands on the surface of the quantum dots are removed, and the binding between the layers and the carrier lifetime of the material are enhanced. The J-V curves of the devices prepared by the two ligand exchange methods are shown in Figure 5 (B), in which 75 mg/mL solution is used to prepare a film with a thickness of five layers, and 145 mg/mL corresponds to three layers, and the film thickness is 350 nm. It can be found that the device using FAI layer by layer shows better device performance, with the highest photoelectric conversion efficiency of 13.1% for the five-layer film.
CsPbBr
3 perovskite has attracted much attention because of its strong stability. Traditional methods usually use two-step method to prepare CsPbBr
3 thin films, and need to be annealed at a high temperature (250 ℃), which makes the preparation process complex and difficult to be widely used. In order to improve these problems, Akkerman et al. Used CsPbBr
3 quantum dots to prepare solar cells and achieved photoelectric conversion efficiency of more than 5%
[47]. Subsequently, Zhang et al. Designed and fabricated a solar cell with a mixture film of CsPbBr
3 and CsPb
2Br
5 using CsPbBr
3 quantum dots, and obtained a photoelectric conversion efficiency of 6.81%, an open circuit voltage of 1.43 V, and a short circuit current of 6.17 mA/cm
2[48]. The perovskite thin film is prepared by a spin coating method, and is treated by using a europium ethyl acetate solution, so that the CsPbBr
3 quantum dot thin film is converted into a CsPbBr
3-CsPb
2Br
5 thin film, and the existence of a small amount of CsPb
2Br
5 can realize surface passivation and prolong the service life of carriers, thereby improving the efficiency of the device. Zhang et al. introduced GASCN (guanidine thiocyanate) to treat the perovskite thin film when using CsPbBr
3QD to prepare solar cell devices. GASCN can replace the ligands on the surface of quantum dots and optimize the film at the same time, which has the effect of enhancing carrier transport. The open-circuit voltage of the device is as high as 1.65 V
[18].
In addition to the direct use of quantum dots to prepare solar cells, CsPbBr
3 quantum dots are also used to modify perovskite thin films. Duan et al. Used CsPbBrI
2 quantum dots to modify CsPbBr
3 perovskite thin films. The addition of quantum dots effectively reduced the radiation recombination between the perovskite layer and the electrode interface, which improved the efficiency of the device and increased the open circuit voltage and short circuit current
[49]. Chen et al. Added a 110 nm thick CsPbBr
3 quantum dot layer between the TiO
2 layer and the CH
3NH
3PbI
3 layer. The added quantum dot layer can produce a relatively rough interface on the dense TiO
2 layer, which can help the light absorption layer to better deposit, increase the light absorption capacity of the light absorption layer and help electrons to better transmit to the TiO
2 layer. After adding the modification layer, the device efficiency increases from 14.7% to 16.4%
[50]. In addition, Chen et al. prepared a CsPbBr
3@SiO
2 layer outside the device to reduce the influence of ultraviolet rays. The structure is shown in Fig. 5 (C). This structure can improve the PCE of the solar cell device prepared based on the CH
3NH
3PbI
3 from 19.7% to 20.8%, and the J-V curve is shown in Fig. 5 (d). In addition, the stability of the device is also improved, and 96% of the initial efficiency can be maintained after 100 H. Jin et al. reported a solar cell with a device structure of a CsPbBr
3/FTO/CeTiO
2/perovskite/Spiro-OMeTAD/Au/Al
2O
3, in which a CsPbBr
3 is used as an ultraviolet conversion layer and a Al
2O
3 is used as an encapsulation layer
[51]. When sunlight shines on the device, the CsPbBr
3 can convert part of the ultraviolet light into visible light, thereby optimizing the light absorption of the device and improving the efficiency of the device. When different layers of CsPbBr
3 quantum dots are used as UV conversion layers to prepare the device, 1, 3 and 5 layers can increase the efficiency of the device, and the effect is the best when 3 layers are used. When the thickness reaches 10 layers, the performance of the device is not as good as that of the unused device. In addition, compared with the device without CsPbBr
3 quantum dots, the efficiency of the device after 200 H is only 48% of the initial value, and the device with ultraviolet absorption layer can maintain 90% of the initial value.
Gao et al. Further expanded the use of CsPbBr
3 quantum dots, using quantum dots and chlorobenzene as anti-solvents to optimize the film formation and crystallization of MAPbI
3. The grain size of MAPbI
3 thin film was increased from 100 ~ 300 nm to 1 ~ 2 μm, which was very beneficial to the transmission and collection of carriers, and the device efficiency was increased from 18. 51% to 20. 46%
[52]. The CsPbBr
3 can play the role of a nucleation center in the process of preparing the thin film, and can also form a Cs
1-yMA
yPbI
3-xBr
x passivation layer on the surface of the perovskite layer to improve the stability. Table 2 summarizes the device parameters of the above perovskite quantum dot based cells.
3 One-dimensional CsPbX3 nanowire/nanorod
One-dimensional (1D) perovskite nanowires and nanorods have excellent transmission properties and are anisotropic structures. Their characteristics, such as significant light trapping, strong mechanical properties, and the ability to provide guiding channels for the transmission and propagation of carriers and photons in specific directions, make them potential materials for the manufacture of many optoelectronic devices.
3.1 Synthetic method
3.1.1 Hot injection method
Similar to the synthesis method of zero-dimensional quantum dots, the thermal injection method is also a common method for synthesizing one-dimensional CsPbX
3 nanowires, and the synthesis principle is the same as that in fig. 2 (a). Zhang et al. Modified the thermal injection method to synthesize CsPbX
3 nanowires, and prepared uniform single crystal nanowires by increasing the reaction temperature and prolonging the reaction time
[53]. Subsequently, they still used the optimized thermal injection method to synthesize ultra-thin CsPbBr
3 single crystal nanowires with a width of about 2.2 nm and a photoluminescence quantum yield of 30%, and used ethyl acetate as an anti-solvent to increase the purity of nanowires to more than 90% by stepwise purification
[54]. Chen et al. Directly synthesized cubic CsPbI
3 nanowires by hot injection method. Different from the conventional hot injection method, Chen et al. Injected the cesium precursor twice, as shown in Figure 6 (a). First, add oleic acid, oleylamine, PbI
2 and octadecene into a three-necked flask, heat and stir at 60 ℃ for 30 min, and then inject a certain amount of cesium precursor and heat and stir at 120 ℃ for 30 min.After the cesium precursor is injected, it can be observed that the yellow transparent solution turns red. After the cesium precursor is injected for the second time, wait for 30 minutes and then cool and centrifuge to obtain the desired CsPbI
3 nanowires. The TEM of the nanowires is shown in the right figure of Figure 6 (a). The α-CsPbI
3 nanowires have a width of 5 ~ 80 nm and a length of 10 ~ 20 μm
[55].
3.1.2 Solution Method
The room temperature recrystallization method is also one of the effective ways to synthesize the CsPbX
3 nanowires, and the method mainly utilizes the solubility of a substance in a solvent to realize the synthesis, and firstly needs to select a proper solvent, namely, the solubility of a halide precursor is high,But the solubility of the perovskite product is small, and the concentration of the lead halide precursor in the solvent is enough to form the perovskite by dissolving and recrystallizing with the lead precursor, and the lead precursor needs to dissolve the lead into the solvent at a proper speed
[56]. Zhai et al. Directly synthesized high-purity single-crystal CsPbX
3 nanowires by solvothermal method, with a width of 10 nm and a length of nearly 10 μm, showing ultra-high aspect ratio, and both CsPbBr
3 and CsPbCl
3 have photoluminescence properties and high PLQY(CsPbBr
3-75%,CsPbCl
3-12%)
[15]. He et al. Reported an in situ catalyst-free synthesis strategy for the synthesis of highly stable gadolinium nanorods in a polymer matrix, as shown in fig. 6 (B), in which a precursor solution is dissolved in the polymer matrix by a solvent-induced swelling process and distributed in the form of droplets, followed by annealing.Perovskite begins to nucleate after the solvent is evaporated. The evaporation of the solvent makes the precursor in the droplet supersaturated, and the nanorods grow under the modification of the surrounding polymer chain clusters. The fluorescence microscope image of the synthesized CsPbBr
3 nanorods is shown in the right figure of Figure 6 (B). It can be seen that the particle size of the material is relatively uniform, and the length of its nanorods is about 7.15 μm
[57].
3.1.3 Vapor deposition method
Vapor deposition is also used to synthesize CsPbX
3 nanowires, which has great advantages in synthesizing materials with adjustable composition and band gap
[58,59]. Park et al. Used this method to synthesize CsPbCl
3, CsPbBr
3, and CsPbI
3 nanowires of monohalides
[60]. CsX and PbX
2 pow are placed in a quartz tube and heat at 570-600 deg C, a silicon substrate is also placed in that same quartz tube and heated to 350-380 deg C at a distance of about 10 cm from the reactant powder, and argon with an appropriate flow rate is introduce in the synthesis process, so that the growth of the CsPbX
3 nanowires on the Si substrate is completed. Zhou et al. deposited on a SiO
2/Si substrate, and the preparation process was similar to that of Park et al. the cross section of the prepared nanorods was triangular. The reaction device and the corresponding CsPbBr
3TEM detection results were shown in Figure 6 (C). The growth direction was determined to be (100) by TEM, and according to the theoretical calculation, the free energy of (100) crystal face was less than that of (110) and (001)
[61][62]. The SiO
2 can reduce the surface energy during the growth of nanorods, and the nanocrystals grow along the (100) direction with the (011) plane parallel to the substrate to form nanorods with triangular cross sections. Meng et al. Reported a gas-solid-liquid method for the synthesis of inorganic perovskite nanowires, using Sn as a catalyst to synthesize CsPbX
3 nanowires with a diameter of 150 nm
[63]. The method comprises the following steps of: firstly, depositing Sn on a substrate, and adjusting the temperature to be between 290 and 330 deg C, wherein the Sn is in a liquid state due to a low melting point. The mixture of CsX and PbX
2 with a molar ratio of 2 ∶ 1 was annealed at 420-460 ℃ for 30 min, and then placed in the high temperature region of the tube furnace. The temperature was adjusted until the 430~470℃,CsPbX
3 powder was gasified and then carried to the low temperature region by argon/hydrogen to complete the growth of nanowires.
Wang et al. Proposed a synthesis route that combines the advantages of solution and vapor deposition
[64]. According to the method, a precursor solution is heated in an air environment, a substrate is placed right above the solution and kept at a sufficient distance, crystal growth is triggered by temperature difference, and uniform mass transfer is ensured by deposition in a vertical direction. The novel synthesis method can prepare the nanowires of the CsPbCl
3, the CsPbBr
3 and the CsPbI
3 under the condition of reasonably regulating and controlling parameters, and the length of the nanowires can be controlled within 10~10
3μm.
The above synthesis method can be classified into liquid phase synthesis method and gas phase synthesis method. Liquid phase synthesis methods, such as hot injection and solution methods, can mix precursors uniformly, but the crystallization process in solution is difficult to control. The reaction time of thermal injection method is longer than that of quantum dots, which can be extended to about 30 min. On the contrary, the gas phase method can control the crystallization process, and the crystal will grow along the crystal plane of the substrate, which is conducive to controlling the morphology, but the gas phase mass transfer process is carried out in the horizontal direction by means of gas flow, which is difficult to ensure uniform mass transfer
[9].
3.2 Topography control
In general, the properties of materials are determined by their composition and morphology, and the study of morphology control methods is helpful to understand the formation mechanism of materials, which is of great significance to the precise preparation of materials.
3.2.1 Effect of process temperature
Zhou et al. Found that temperature was an important factor affecting the synthesis when they used vapor deposition to prepare CsPbX
3(X=I, Br, Cl) nanowires with triangular cross sections. Fig. 7 (a) shows the SEM images of perovskite materials prepared at different temperatures, with the temperature increasing from left to right.When the synthesis temperature is low, the obtained CsPbBr
3 nanocrystals are small in diameter and irregular in morphology; when the temperature is increased, the synthesized material tends to be regular in morphology and square in cross section; and when the temperature is continuously increased, the obtained nanowire has a triangular cross section
[61]. Chen et al. Found that lower experimental temperature, the protection of end-capping ligands and the secondary growth of crystals are the key to the synthesis of α-CsPbI
3 nanowires
[55]. By controlling these factors reasonably, nanowires with a width of 5 ~ 80 nm and a length of 10 ~ 20 μm were obtained. The second injection temperature has a great influence on the morphology of the nanowires. The samples synthesized at 80 ~ 160 ℃ were observed, and it was found that short nanorods were formed at 80 ℃, and the length of the nanocrystals increased with the increase of temperature to 120 ℃, resulting in the formation of nanowires with high specific surface area. However, nanorods will be formed when the temperature exceeds 140 ℃. It is very important to choose the appropriate temperature to control the morphology.
3.2.2 Influence of the composition of precursor solution
He et al. Investigated the effects of precursor concentration and stirring speed on the size of CsPbBr
3 nanorods during the synthesis process, and found that reducing the precursor concentration or stirring speed could reduce the size of nanorods
[57]. Fig. 7 (B) shows the fluorescence micrographs of the CsPbBr
3 prepared at different stirring speeds. The length of the nanorods prepared at a stirring speed of 1000 rpm is the largest. With the increase of the stirring speed, it can be seen that the particle size of the CsPbBr
3 decreases continuously. The rod-like structure of the material prepared at 3000 rpm is basically not observed. In addition, the length of the nanorods was 1. 27 μm when the precursor concentration was 12 mg/mL, and the length increased to 7. 15 μm when the precursor concentration increased to 30 mg/mL. Combined with the distribution kinetics of the precursor in the polymer matrix, the lower the concentration of the precursor, the lower the viscosity of the solution, and the smaller the droplets and nanorods in the SDM (swelling-deswelling microencapsulation) process. Zhai et al believed that the formation of nanowires with high aspect ratio was mainly attributed to the anisotropy of the one-dimensional direction of the material under the action of oleic acid and oleylamine ligands, and a longer reaction time could make the ligands play a full role and make the material grow in a certain direction
[15]. Liu et al. Found that a higher concentration of PbBr
2 precursor is conducive to the anisotropic growth of 1D perovskite
[27]. This is mainly due to the fact that the shape of the NCs is strongly dependent on the concentration of the monomer in solution. In general, the higher the concentration of precursor and monomer, the more favorable the growth of one-dimensional nanorods. The nanorods with length of 274 nm were prepared under the condition of oleic acid and oleylamine 8 ∶ 2
[65].
3.2.3 Effects of additives and other factors
Waleed et al. designed and reported a high-density CsPbI
3 nanowire array with stable cubic phase at room temperature. By growing the CsPbI
3 nanowires in the nano-holes of the pre-deposited anodic aluminum film, the size of the nanowires can be accurately controlled while ensuring the stability. The synthesized nanowire array has a band gap of 1.74 eV, which matches the cubic phase
[66]. The anodic aluminum film plays a role in passivating the surface of the product and can effectively reduce the influence of moisture on the perovskite. Chen et al. Used ethyl acetate as an anti-solvent to purify the synthesized nanowires. As a weak polar organic solvent, ethyl acetate can not only remove amine impurities in the purification process, but also retain the necessary ligands on the surface of nanowires to enhance the stability of nanowires
[55].
Zhai et al proposed that the reaction time also plays an important role in the synthesis of nanowires. In the reaction process, the product is a nanosheet in a short time (0.5-24 H). With the reaction time extended to 48 H, the intermediate product becomes a mixture of nanowires and nanosheets. If the reaction time is further extended, high-purity nanowires will be obtained
[15]. Zhang et al. recorded the morphological changes of the material at different reaction times, and the corresponding TEM results are shown in Figure 7 (C). When the reaction time is less than 10 min, quantum dots with a particle size of 3 ~ 7 nm are mainly formed. When the reaction time reaches 10 min,When the reaction time reached 40 min, the product was mainly composed of nanowires, and the quantum dots almost disappeared.When the reaction time continued to 90 min, the product was composed of nanowires with uniform particle size
[53]. When the reaction time reached 180 min, it was observed that the nanowires had completely disappeared and the product became perovskite crystals with large particle size.
Generally speaking, temperature, reaction time, ligand and precursor concentration are the main factors affecting the morphology of the material in the synthesis process. By reasonably regulating these factors, the precise control of the morphology can be achieved. In the thermal injection method, the influence of temperature on the morphology is very obvious. In addition, the reaction speed of this method is very fast, so it is difficult to control the morphology. However, the vapor deposition and room temperature recrystallization methods are more sensitive to other factors, for example, in the room temperature recrystallization method, the products with different morphologies can be obtained in 0. 5 ~ 72 H.
3.3 Optical properties
Similar to zero-dimensional quantum dots, one-dimensional CsPbX
3 nanowires also have many excellent optical properties, such as fluorescence, photoluminescence and so on. The CsPbX
3 nanowires also have a PL spectrum covering almost the entire visible light range (410 ~ 691 nm), and the PL peak can be changed by adjusting the halide composition. These results are very similar to those of the previously described quantum dots. At the same time, the peak positions of nanorods synthesized by different methods and reaction conditions are approximately the same, which is due to the fact that the size of nanorods is in the micron level, and the quantum confinement effect can not play a significant role at this time. However, when the nanowire size is reduced below the Bohr radius, the PL spectra will shift due to the quantum confinement effect, which is different from the optical properties of perovskite prepared in general. Fig. 8 (a) shows the PL spectrum and absorption spectrum of the CsPbBr
3 nanowire synthesized by He et al., in which the UV absorption peak is at 520 nm and the PL peak is at 526 nm
[57]. The absorption and PL spectra of CsPbI
3 nanowires were reported in the study of Zhang et al. The absorption and PL peaks were located at 457 nm and 523 nm, respectively
[53]. Zhai et al. Reported the optical properties of CsPbBr
3 and CsPbCl
3, in which the PL peak of CsPbBr
3 is basically the same as that of He et al., while CsPbCl
3 shows a peak at 410 nm
[15]. Chen et al. tested the PL spectrum of the CsPbBr
3 with a diameter smaller than the Bohr radius. Fig. 8 (B) shows the ultraviolet absorption spectrum and PL spectrum of the ultra-thin nanowire. It can be found that the absorption peak of the ultra-thin nanowire moves from 510 nm to 445 nm and the emission peak moves from 520 nm to 460 nm under the effect of quantum confinement effect
[19]. Wang et al. And Zhou et al. Recorded the PL peak and the corresponding halide composition information, resulting in Figure 8 (C), from which the linear relationship can be clearly observed, indicating that the vapor deposition method proposed by Wang et al can accurately control the PL peak
[64][61]. Compared with other vapor deposition methods, the FWHM of the CsPbX
3 nanowires synthesized by Wang et al. Is 6.4 nm(CsPbCl
3),17.7 nm(CsPbBr
3),27.6 nm(CsPbI
3)
[60,61,63].
PLQY has also been widely reported as one of the important properties, and Zhai et al. Obtained CsPbCl
3 and CsPbBr
3 with PLQY of 12% and 75%, respectively, through the method they used
[15]. Similar to quantum dots, the PLQY of CsPbCl
3 nanowires is also very low. The PLQY of CsPbBr
3 ultra-thin nanowires reported by Zhang et al. Is also relatively low, although it is only 30% after PbBr
2 treatment, but this surface treatment improves the PLQY of the material and is also conducive to enhancing stability
[54]. He et al. Also reported the PLQY of the synthesized CsPbBr
3 nanowires, and the PLQY was between 23% and 30%
[57]. In contrast, the PLQY of the CsPbBr
3 prepared by Park et al. By vapor deposition is much higher, about 80%
[60].
Chen et al. Measured the time-resolved PL decay curve of CsPbI
3 nanowires and obtained a longer effective PL lifetime (22.8 ns), which is equivalent to that reported by Liu et al. This indicates that the synthesis method adopted by Chen et al can effectively suppress the non-radiative recombination on the surface of CsPbI
3 nanowires
[55][26]. In contrast, CsPbBr
3 and CsPbCl
3 decay faster than CsPbI
3.The decay times of the ultra-long CsPbX
3(X=Cl and Br) nanowires synthesized by Zhai et al are 4.3 ns(CsPbCl
3) and the decay time of the CsPbBr
3 reported by 13.6 ns(CsPbBr
3),Park et al is less than 10 ns, respectively
[15][60]. Comparing the decay time of quantum dots, it can be found that both quantum dots and nanowires have longer decay time for CsPbI
3, which brings higher carrier lifetime and lower recombination rate for CsPbI
3, making it more suitable for optoelectronic materials.
3.4 Application in solar cell
Although one-dimensional nanowires/nanorods have shown great potential in solar cells, the use of inorganic perovskite nanowires as light-absorbing layers in solar cells has not yet been demonstrated. Even in hybrid perovskites, the application of one-dimensional nanowires in solar cells has not achieved high photoelectric conversion efficiency
[67].
The researchers found that using nanowires to modify the perovskite film can make the film more uniform and form an ideal gradient heterojunction. Zai's research is the most representative. They doped CsPbBr
3 nanowires into MAPbI
3 films to form a graded heterojunction, which improved the interface energy band and the photoelectric conversion efficiency of the cell
[68]. Zhang et al. reproducibly prepared uniform nanowire heterojunctions to improve the PCE and operational stability of the cell
[69]. Perovskite solar cells with PCE as high as 21.4% can be obtained by doping CsPbX
3 nanowires into MA
0.3FA
0.7PbI
3 bulk thin films. In addition to the application of inorganic perovskite nanowires to improve other solar cells, they can also be used to directly prepare devices. Liao et al. Reported devices prepared using CsPbBr
3 and CsPbI
3 nanowires.Because too much oleylamine oleate ligand on the surface of the prepared nanowires will affect the performance of the device, they used a simple solution method and ion exchange method to synthesize CsPbBr
3 and CsPbI
3 nanowires directly on FTO glass
[13]. Although the efficiencies are 1.21% and 0.11% respectively, the devices fabricated by CsPbBr
3 nanowires still have 99% of the original efficiency over 5500 H. The prepared device structure is a FTO/TiO
2/CsPbI
3(CsPbBr
3)/Spiro-OMeTAD/Au. The device prepared by the CsPbI
3 is J
sc=0.415 mA/cm
2,V
oc=0.643 V, and the filling factor FF is 0.429. The device prepared by the CsPbBr
3 is J
sc=2.96 mA/cm
2,V
oc=0.851 V, and the filling factor FF is 0.44. Table 3 summarizes the synthesis and preparation parameters, optical performance parameters and corresponding cell device parameters of one-dimensional nanowires.
4 Two-dimensional CsPbX3 nanosheet
Like other dimensional materials, two-dimensional nanosheets also have many excellent optoelectronic properties, such as adjustable band gap, high carrier mobility and so on. In addition, the unique planar geometry of nanosheets enables them to transfer charge rapidly in the plane, which makes them have certain advantages in the design and preparation of traditional thin film devices.
4.1 Synthetic method
4.1.1 Hot injection method
A reliable synthesis method is the prerequisite for the study of optical properties and applications of 2D perovskite. Up to now, there are many methods that can be used to synthesize CsPbX
3(X=I, Br, Cl) nanosheets. Bekenstein et al. Synthesized the CsPbX
3 nanosheets by the adjusted thermal injection method. The reaction temperature was reduced to 90 ~ 130 ℃, and the synthesis was carried out in an air-isolated environment. The nanosheets with a thickness of about 1 ~ 5 times the cell number of perovskite were prepared
[70]. Akkerman et al. Synthesized inorganic perovskite nanosheets with a thickness of 3 to 5 unit cells by using cesium oleate instead of MA-Br as a precursor and only oleic acid and oleylamine as ligands, drawing on the method of Schmidt et al
[71][72]. This method will not react until acetone is injected, which can achieve precise control of thickness. In addition, Akkerman et al. Prepared CsPbI
3 and CsPbCl
3 nanosheets by ion exchange method after synthesizing CsPbBr
3, and kept the size and morphology of the material unchanged.
The addition of other metal halides can also synthesize perovskite nanosheets with high quality and uniform size. Mir et al. Further modified the method of Akkerman et al., adding acetone to the reaction solution as an anti-solvent to carry out the synthesis reaction at room temperature, and adding MnCl
2·4H
2O to the precursor solution to prepare CsPbCl
3 nanosheets with a thickness of 2.2 nm
[73]. Wang et al. synthesized uniform nanosheets after adding CuCl
2 in the thermal injection method. Fig. 9 (a) shows the perovskite synthesized under the conditions of [CuCl
2]∶[PbBr
2] of 0:1 and 2:1, respectively. When no CuCl
2 was added, the product was a quantum dot
[74]. When the ratio was increased to 2:1, the product was completely transformed into nanosheets. The addition of AlCl
3, ZnCl
2 and HAuCl
4 can also achieve the same effect, but the thickness and size of nanosheets obtained by adding different halides are different.
4.1.2 Microwave method and others
The microwave method has the advantages of high heating rate and uniform particle size. Pan et al. Synthesized CsPbX
3 quantum dots with uniform particle size and 75% PLQY by microwave method, and on this basis, synthesized nanosheets by adjusting reaction parameters. Compared with the synthesis of quantum dots, the reaction temperature of nanosheets is lower.Fig. 9 (B) shows the CsPbBr
3 nanosheet synthesized at 80 ℃. The edge length of the product is 23.4 nm and the thickness is 3.3 nm. When the thickness of the nanosheet is less than the Bohr radius (3.5 nm) of the CsPbBr
3, the quantum confinement effect can be observed
[75][76,77].
Zhao et al. reported the synthesis of CsPbBr
3 nanosheets mediated by tryptophan, and nanosheets with different thicknesses can be obtained by adjusting the concentration of tryptophan. The principle is shown in Figure 9 (C). Tryptophan plays a decisive role in the formation of CsPbBr
3 nanosheets.Tryptophan can significantly reduce the growth kinetics of CsPbBr
3 nanosheets during the reaction, and play a key role in breaking the inherent cubic symmetry of CsPbBr
3 nanocrystals, thus determining their two-dimensional growth
[78]. Until now, there has been no report on the combination of biomolecules with inorganic perovskites, which provides an efficient and environmentally friendly synthesis route for inorganic perovskites. Li et al. Reported a two-step process for the preparation of CsPbX
3(X=Cl, Br, I) nanosheets. In this method, nanorods were prepared by thermal injection, and then the nanorods were added to a stainless steel autoclave and cultured at 160 ℃ for 1 ~ 5 H. The thickness of the prepared nanosheets was about 3 ~ 6 nm
[79].
Uddin et al. Reported a ligand-assisted ion-exchange method for the preparation of CsPbX
3 nanosheets, in which DDT (dodecanethiol), AlX
3(X=Cl, Br, I) and synthesized 8-10 nm CsPbCl
3 quantum dots were mixed to prepare CsPbX
3 nanosheets with a thickness of 4-15 monolayers
[80]. In that reaction proces, the AlX
3 acts as a halide source, and the Al(OH)
nX
3-n,DDT generate by the hydrolysis reaction acts as a surface ligand. At the same time, by-products such as Al(OH)
nX
3-n and thiolate can also play a role in passivation and enhance the stability of nanosheets.
At present, most of the methods used to prepare CsPbX3 nanosheets are based on the hot injection method, which is a widely used synthesis method and can realize the synthesis of 0D-2D materials by adjusting the temperature and reaction time.Researchers have taken a variety of optimization measures, but the crystallization process is still difficult to control, the material gradually grows from 0D to 2D, this process is difficult to ensure that all CsPbX3 are converted into nanosheets. The ion exchange method can adjust the halide composition based on the synthesized material and does not greatly affect the thickness, but this kind of method also needs to rely on the hot injection method to prepare the precursor. The microwave method has more advantages in the uniformity of the product. The existing synthesis methods are far from enough to promote the wide application of 2D perovskite, so it is of great significance to continue to optimize the existing methods and develop new methods.
4.2 Thickness control
Based on the successful synthesis and preparation of two-dimensional nanosheets, it can be envisaged that a large number of studies will report their various optoelectronic applications in the future. Further understanding and controlling the influence of various factors on materials in the preparation process and preparing materials with morphologies that meet the actual needs for different applications will help to promote their rapid development, and it can be predicted that more and more research will focus on morphology control in the future.
4.2.1 Effect of process temperature
The physical and chemical properties of perovskite nanosheets are mainly affected by their morphology and size, and the reaction temperature has been explored in various studies. Bekenstein et al. Found that temperature has an important influence on the morphology and thickness of nanosheets in the process of thermal injection synthesis of materials. For example, at 130 ℃, low-symmetry nanosheets with blue-green light will be formed, while at 90-100 ℃, extremely thin nanosheets and 200-300 nm layered structures can be formed
[70]. Pan et al. Found that the precursor concentration and reaction temperature are two important factors affecting the final morphology of the material
[75]. At lower temperature, the precursor dissolves slowly, which leads to lower reaction power and tends to form nanosheets, while at higher temperature, the precursor dissolves quickly and the reaction power is strong, which tends to form nanowires/nanorods, and quantum dots can be synthesized by further increasing the temperature. Yang et al. Proposed that it is easier to generate thin nanosheets at low temperature, and nanosheets with a thickness of 1 to 5 unit cells can be synthesized by mixing precursors at room temperature and then heating to a higher temperature
[81]. The CsPbBr
3 nanosheets with the thickness of 1.3 nm, 1.9 nm, 2.5 nm and 3.1 nm were obtained by controlling the temperature. In addition, they also found that the heating rate is also an important factor affecting the morphology of nanosheets, for example, when the heating rate is 10 ℃/min and the reaction temperature is 80 ℃, the product with a thickness of 2.4 nm and a side length of 32 nm can be obtained. Liu et al. Also concluded that lower temperatures are more suitable for the synthesis of nanosheets
[82].
4.2.2 Influence of the composition of precursor solution
Yuan et al. explored the effects of different cesium salts on the synthesis of nanosheets, using CsCO
3 and CsOAc to carry out comparative experiments, and found that CsOAc had higher solubility in octadecene (ODE) at room temperature, and the nanosheets synthesized by the two cesium salts at 140 ℃ had similar morphologies, of which the thickness of the nanosheet synthesized by CsOAc was 2.5 nm
[83]. This indicates that different cesium salts have no significant effect on the crystallization and nucleation process, but after using CsOAc, other ligands can be replaced in the synthesis process without using oleylamine oleate, which provides convenience for the systematic study of the effects of ligands.Li et al. studied the influence of ligands with 2-18 carbon chain length on the thickness of nanosheets, and found that the influence of amine chain length on nanosheets was more significant, and short-chain amine ligands were more conducive to the growth of thin nanosheets. When the carboxylic acid chain length was 18 and the amine chain length was 6, the thickness of nanosheets prepared was only 1.8 nm
[84]. Wang et al found that different metal halides used in the synthesis process had an impact on the thickness of the nanosheets, and the thickness of the perovskite prepared by CuCl
2 under the same other conditions was 2.9 nm,AlCl
3 4.6 nm,HAuCl
4 3.5 nm,ZnCl
2 2.3 nm.This phenomenon occurs because the adsorption capacity of metal ions on nanocrystals is determined by the surface, shape and ligands of the nanocrystals, which in turn changes the morphology of the nanocrystals
[74].
4.2.3 Effects of additives and other factors
Akkerman et al. Found that the addition of HBr in the synthesis process had an effect on the thickness of the nanoplates.With the increase of the amount of HBr added, the prepared nanoplatelets became thinner, and the thickness of the nanoplatelets decreased from 2. 5 nm to 1. 8 nm when 0 to 7. 5 μL HBr was added
[71]. This control can be explained by the fact that the addition of HBr increases the acidity of the solution during the protonation of the oleylamine, and the protonated oleylamine can compete with the Cs
+ ions on the surface of the growing nanosheet, essentially slowing down the growth in the vertical direction. Yuan et al. Reached the same conclusion about the effect of HBr
[83]. Yang et al. Proposed that the reaction time is also one of the factors affecting the growth of nanosheets. For example, when the reaction time is controlled at 180 ℃ for 5 s, the thickness of the prepared CsPbBr
3 nanosheets is 3.1 nm, and the horizontal size is 22 nm. When the reaction time extends to 30 min, the horizontal size increases to 32 nm, and the thickness remains unchanged
[81]. Li et al. Also studied the effect of time, and found that the longer the nanorods were cultured in the autoclave, the thicker the nanosheets were. The thickness of nanosheets increased from 3 nm to 6 nm when the culture time increased from 3 H to 5 h,CsPbBr
3 and CsPbCl
3[79]. The culture time of CsPbI
3 nanosheets should be controlled below 1. 5 H, otherwise the material will find a serious phase change.
4.3 Optical properties
There are many similarities in optical properties between nanosheets and perovskites of other dimensions, such as fluorescence and PL spectra covering the whole visible light range. There are also some differences, such as the nanosheet has a sharper exciton absorption/emission peak and a larger absorption cross-section
[85]. Unlike bulk CsPbBr
3,Pan, CsPbBr
3 nanosheets synthesized by microwave method show bright blue-green color
[75]. As shown in Fig. 9 (d), the influence of the quantum confinement effect can be confirmed in the UV absorption spectrum, where the absorption and emission peaks are located at 482 nm and 495 nm, respectively, while the absorption and emission peaks of the bulk CsPbBr
3 are located at 516 nm. The ultrathin nanosheets synthesized at lower temperature have tunable blue light under the influence of quantum confinement effect. As shown in Figure 9 (e), Akkerman et al. compared the absorption and PL spectra of different thicknesses of CsPbBr
3 nanosheets, cubic nanocrystals, thin films composed of crystallites within a few hundred nanometers in size, and cubic perovskite 8.5 nm CsPbBr
3 quantum dots
[71]. It can be seen from the results that the absorption and emission spectra of the thin film are very similar to those of the cubic perovskite, indicating that the two samples do not have strong quantum confinement effect. For the nanosheets with a thickness of less than 3 nm, a significant blue shift was observed.
PLQY is one of the important properties of perovskite materials, and its size corresponds to the defect density of the material. At present, inorganic perovskite emitting red light (CsPbI
3) and green light (CsPbBr
3) have PLQY of more than 90%, but that PLQY of CsPbX
3 emitting blue light is difficult to be as high as that of the red light and the green light, the band gap of the blue light perovskite is wider, and high-energy exciton are easy to be captured by interband traps, so that the PLQY is reduced
[84]. Generally, blue perovskites are synthesized by different Br
- and Cl
-, but PLQY is difficult to exceed 40%. Bekenstein et al. Found that blue luminescence can also be achieved by controlling the size and shape of CsPbBr
3 nanocrystals, that is, by synthesizing ultra-thin nanosheets
[70]. Based on the former idea, Bi et al. Reported a CsPbBr
3 perovskite nanosheet that could spontaneously assemble into a larger cuboid (50 nm × 50 nm × 20 nm), which increased PLQY to 91% while maintaining blue luminescence
[86]. This self-assembled cuboid can effectively reduce the defects of perovskite nanosheets and eliminate most non-radiative recombination, so PLQY will rise. The preparation parameters and properties of the nanosheets are summarized in detail in Table 4.
4.4 Application in solar cell
Inorganic perovskite nanosheets have excellent photoelectric properties, which make them show great potential in solar cells, LEDs and photodetectors. At present, there are many studies on the application of nanosheets in photodetectors, lasers and LEDs, but few in solar cells
[87⇓~89]. The photoelectric conversion efficiency of inorganic perovskites prepared by solution method is still lower than that of hybrid perovskites in solar cells, which is mainly due to the insufficient film thickness and the phase transition of perovskites.To overcome this obstacle, Jiang et al. Used cesium acetate (CsAc) and hydrogen trihalide (HPbX
3) as precursors to prepare high-quality perovskite films with a thickness of 500 nm
[90]. At the same time, the introduction of phenylethylammonium iodide (PEA) results in the formation of quasi-2D perovskite, which effectively inhibits the phase transition and reduces the defect density of the film. The PCE of the prepared α-CsPbI
3 perovskite solar cell is 12.4%. Generally, CsX is used as the precursor to synthesize perovskite CsPbX
3, but the solubility of CsX in polar solvents is low, which will affect the quality of perovskite films. After using the new precursor, the black cubic perovskite thin film was prepared at 100 ℃, which began to absorb light at 718 nm and had a band gap of 1. 73 eV, which was the same as that reported in other literatures
[91]. The introduction of PEA by Jiang et al. Was inspired by the PEA
2(CH
3NH
3)
n-1Pb
nI
3n+1 perovskite, which shows better moisture stability while maintaining the high performance of the traditional UK2 perovskite
[92⇓~94]. Cesium perovskites with different n (10-100) values were prepared by adjusting the molar ratio of PEA to Cs. These perovskites with different n values were used to fabricate perovskite solar cells, where n = 60 corresponds to a perovskite thin film thickness of about 500 nm. The photoelectric conversion efficiency of the device with different n values increases first and then decreases with the increase of n values.When n = 60, the device has the highest efficiency of PCE = 12. 4%, corresponding to the V
oc=1.07 V,J
sc=16.59 mA/cm
2, and the fill factor FF is 70%.
5 Three-dimensional CsPbX3 perovskite
Three-dimensional all-inorganic perovskite is proposed based on the instability of optimized organic-inorganic hybrid perovskite, which has good photothermal stability and humidity stability. At present, the highest photoelectric conversion efficiency of inorganic perovskite is also achieved by this dimension material. Unlike low-dimensional perovskites, which are prepared first and then applied, three-dimensional CsPbX3 are usually synthesized directly on the corresponding devices. The synthesis methods and morphology control of three-dimensional CsPbX3 will be introduced in the following.
5.1 Synthetic method
5.1.1 Solution Method
Solution method is widely used in the preparation of three-dimensional perovskite thin films, which has the advantages of simple operation and rapid preparation of high-quality thin films. In the one-step solution method, the precursors of CsX, PbX
2(X=Cl, Br and I) are mixed and dissolved in a specific solvent according to a certain proportion, and the perovskite thin film can be directly prepared on the substrate by spin coating and annealing. The principle is shown in Figure 10 (a). Zhang et al. Used cesium acetate (CsAc) and methyl ammonium halide (MAX, X = Br, I) instead of CsI as precursors to prepare perovskite thin films. After spin-coating, the precursor thin films were transformed into black phase CsPbI
3 perovskite after annealing at 100 ℃
[95]. In addition, MAX does not remain in the perovskite, and the composition is still completely inorganic perovskite. At present, the photoelectric conversion efficiency of 18.4% has been achieved in CsPbI
3 solar cells based on solution method
[96].
One-step method can work well with soluble precursor salts, but it is difficult to prepare high-quality films with insoluble salts such as CsBr, so researchers have developed a multi-step method to improve this problem. Multi-step method is usually used to prepare bromine-containing three-dimensional perovskite thin films. After depositing a PbBr
2 layer on the substrate in advance, CsX (X = I, Br) is introduced by spin coating or liquid immersion for many times, and the two are diffused and reacted by annealing, thus preparing three-dimensional perovskite thin films. On the basis of optimizing the spin coating times (n), Duan et al. Summarized the conversion process of perovskite, that is, CsPb
2Br
5(n≤3)→CsPbBr
3(n=4)→Cs
4PbBr
6(n≥5)
[97]. When n = 4, the grain size of the film is about 1.5 μm. In the traditional liquid immersion method, the PbBr
2 layer is placed upward in the process of preparing perovskite, while Teng et al. Turned it over and proposed a downward liquid immersion method. The preparation process is shown in Fig. 10 (B). The downward liquid immersion process occurs in a limited space, which inhibits excessive CsBr from entering the PbBr
2 layer
[98]. Fig. 10 (C) is the TEM image of the product under different immersion time. It can be found that with the increase of immersion time, the grain size of the three-dimensional perovskite thin film increases, and the pores of the thin film also decrease. The optimized immersion method increases the grain size of the thin film to 860 nm while ensuring the stability of perovskite in the CsPbBr
3. Liu et al. combined the liquid immersion method with the multi-step spin coating method. After depositing a PbBr
2 layer on the substrate, the substrate was first soaked in a methanol solution containing CsBr for a period of time to form a CsPb
2Br
5.The solution of CsBr was added to the film for several times, and the film was transformed into a CsPbBr
3 by spin-coating annealing.This method effectively eliminated the holes on the surface of the three-dimensional perovskite film and increased the grain size to about 1 μm
[99].
5.2 Other methods
In addition to the use of solution method to prepare three-dimensional perovskite thin films, spraying method is also one of the important methods. In this method, the solution is atomized and sprayed out by pressurizing the spray gun to prepare perovskite thin films. Compared with the multi-step method, the process is simpler and more convenient. The CsPbI
2Br film prepared by Ho-Baillie et al. In air atmosphere by this method has good stability
[100]. Zhou et al. Applied the spray method to the preparation of CsPbBr
3, improved the low solubility of precursors and prepared thin films with grain size of 1 ~ 2 μm
[101].
In addition, dual-source co-evaporation can also be used to prepare all-inorganic perovskite. Ma et al. Prepared CsPbIBr
2 perovskite by heating and evaporating CsI and PbBr
2 powder together, keeping the ratio of 1:1, and prepared non-porous solar cell devices based on this method
[102]. Chen et al. Reported three-dimensional CsPbBr
3 nanocrystals with multiple branches
[103]. The synthesis method adopts a seed-mediated solvothermal method. The method utilizes the different dissolution rates of a cesium precursor and a lead precursor to synthesize a CsX-rich Cs
4PbBr
6 seed, and the CsX-rich structure is transformed into a CsPbBr
3 with the increasing concentration of the lead precursor. The product could be observed 12 branches from different angles, and the longest diagonal length was 57. 7 nm, with an obvious flower-like three-dimensional structure.
Among the existing synthesis methods, the one-step solution method and the multi-step solution method have been widely used because of their convenience, rapidity and low equipment requirements.Through optimization and improvement, they have been able to prepare high-quality perovskite thin films, but the one-step method is difficult to deal with insoluble precursor salts, and the multi-step method is not suitable for large-scale use because of its cumbersome operation. Spraying method and dual-source co-evaporation method are still in the stage of development, and the latter requires high equipment and high preparation cost. Therefore, it is still necessary to further study the preparation methods, improve the existing methods and develop new methods to achieve commercial application.
5.2 Topography control
The continuous development and optimization of the preparation process can effectively improve the quality of the prepared three-dimensional perovskite thin films, but this approach can not completely solve the problems of film holes and material phase transition, so it is necessary to further study the morphology control methods.
5.2.1 Effects of solvents and additives
The choice of appropriate solvent is an important prerequisite for the preparation of perovskite materials. Ramadan et al. explored the effects of different solvent systems on the CsPbI
3, and tested four systems: DMF, DMSO, DMF + HI, DMF + DMSO
[104]. Both DMF and DMSO can prepare three-dimensional perovskite thin films with hundreds of nanometers of grain size, but the thin film prepared by DMF has some pinholes and can detect the PbI
2,DMSO which is not completely converted into perovskite, resulting in poor coverage of the thin film. The combination of DMF and HI can effectively reduce the phase transition temperature, but has no significant effect on the film. The mixed solvent of DMF + DMSO can reduce the evaporation rate of the solvent and enhance the dispersion ability of the solute, and form a uniform and complete film-forming CsPbI
3. CsPbBr
3 perovskite is usually prepared by a two-step method. Its precursor salt CsBr is dissolved in methanol, but the solvent is volatile, which has a negative impact on the preparation of thin films
[97⇓~99]. Cao et al. Used water as a solvent for CsBr to prepare three-dimensional perovskite thin films
[105]. Because CsBr has extremely high solubility in water and is almost insoluble in water (5.8 mol/L),PbBr
2, the damage to the PbB
2 is avoided when the CsBr/H
2O solution is added dropwise, and meanwhile, the solution with high concentration can ensure that the reaction is fully carried out. This new study also breaks the inherent understanding that water is harmful to perovskite structure and provides a green solvent for the preparation of CsPbBr
3.
Additives, as one of the important means to control materials, are also used in three-dimensional perovskite thin films. In the preparation of CsPbI
3 perovskite, researchers have found that the addition of HI can distort the lattice of CsPbI
3 and reduce the phase transition temperature
[106]. Zhang et al. Added MAX to the precursor to obtain a similar effect, and the black CsPbI
3 can be prepared by annealing at 100 ℃
[95]. Li et al. Introduced polyvinylpyrrolidone (PVP) to stabilize the chemical bond between the amido group in the CsPbI
3,PVP which is easy to phase transition and the CsPbI
3, so that the electron cloud density on the surface of the CsPbI
3 is enhanced and the surface tension is reduced, which is helpful for the CsPbI
3 to maintain the cubic phase in the large-scale film of micron scale
[107]. In addition, Zhao et al and Huang et al studied water as an additive and found that it could reduce the surface energy of perovskite, promote the formation of α-phase film and affect the octahedral structure of [PbI
6]
4-[108][109]. With the increase of water content, the CsPbI
3 crystal shrinks first and then increases, while the roughness of the film decreases.
5.2.2 Doping
The change of composition of inorganic perovskite CsPbX
3 has an important impact on its morphology and photoelectric properties. Alkali metals such as Li, Na, Ka and Rb are used to partially replace Cs, which can regulate the band gap of perovskite and reduce the radiation recombination of corresponding photovoltaic devices
[110]. In addition, doping the B site can also stabilize the crystal lattice and adjust the band gap,Duan et al. Systematically explored lanthanide ions (Ln
3+=Sm
3+, Tb
3+,Effects of Ho
3+, Er
3+ and Yb
3+ on CsPbBr
3 perovskite
[111]. The grain size of the undoped CsPbBr
3 perovskite thin films is only 300 nm,Ln
3+ that of the doped ones. Due to the strong interaction between the dopants and the Br
-, larger colloidal particles can be formed in the precursor solution, resulting in a significant increase in the grain size of the thin films, and the Sm
3+ has the best effect (~ 1 μm). However, due to the high valence, the Ln
3+stays at the grain boundary of the perovskite film and can not enter the crystal lattice. Based on this study, Zhao et al. Doped CsPbBr
3 with divalent hard Lewis acids (Mg
2+, Ca
2+, Sr
2+, Ba
2+).Fig. 10 (d) is the XRD structure of each product after doping. Compared with undoped CsPbBr
3,Mg
2+, Ca
2+ and Sr
2+, the ion radius is smaller than that of Pb
2+, which causes the characteristic peak to shift to a large angle
[112]. In addition, unlike Ln
3+, these ions can enter the perovskite lattice, which is confirmed by the increased binding energies of Cs 3D, Pb 4F and Br 3D detected by XPS spectra. At the same time, doping can inhibit the formation of crystal nuclei, enhance the formation energy, optimize the crystal structure, and increase the grain size of three-dimensional perovskite thin films.
5.3 Optical properties
Unlike low-dimensional materials, three-dimensional perovskites are not affected by quantum confinement effects because of their large grain size. Sutton et al. adjusted the halide composition by adding CsI and CsBr in different proportions during the liquid leaching process, and measured the PL spectra and UV absorption spectra of different halide compositions. The results are shown in Fig. 10 (e). It can be seen that with the increase of bromine content in perovskite, both the UV absorption peak and PL peak move to the short wavelength direction
[113]. Chen et al. Measured the PL spectrum of the three-dimensional perovskite nanoflowers prepared by Chen et al., and the peak position is between 415 and 685 nm from the CsPbCl
3 to the CsPbI
3[103].
5.4 Application in solar cell
In recent years, inorganic perovskites have developed rapidly. Unlike low-dimensional perovskites, which are rarely used in solar cell devices, three-dimensional inorganic perovskites have been studied extensively and achieved fruitful results.
The photoelectric conversion efficiency of Zha's solar cell devices based on three-dimensional CsPbI
3 perovskite is as high as 18.4%, which refreshes the record of inorganic perovskite devices
[96]. On the basis of the preparation of highly stable β-CsPbI
3 films, choline iodide (CHI) was introduced to passivate the surface defects and optimize the perovskite energy level to better match the electron transport layer and hole transport layer materials.
The device fabricated by Duan et al. With the three-dimensional perovskite thin film as the light absorption layer has an ultra-high open circuit voltage of 1.594 V
[111]. The grain size of the perovskite thin film is effectively increased by doping lanthanide ions, and the grain size can be increased to 1 μm by doping 3% Sm
3+. Larger grain size is conducive to charge transfer and eliminates charge recombination reaction, thus optimizing the photovoltaic performance of PSC devices. After doping treatment, the photoelectric parameters of the devices are significantly improved. In addition, the prepared device has excellent stability, and can still maintain 90% of the initial efficiency after 110 days under the condition of 80% RH and 25 deg C.
Fang et al. prepared a device with a structure of ITO/ETL/CsPbI
2.25Br
0.75/HTL/MoO
3/Ag (ETL: electron transport layer, HTL: hole transport layer) by using a three-dimensional CsPbI
2.25Br
0.75 as a light absorption layer, and modified ZnO with 3,3-diphenylpropylamine to passivate the surface defects and form Zn-N bonds at the same time. The V
oc, J
sc and FF of the device were all enhanced, and the photoelectric conversion efficiency of the device was 15.98%
[114]. Then they doped and optimized the hole transport layer, adding polymer PBD2T to PTAA to optimize the surface defects of perovskite and inhibit the recombination of interface charges. When PBD2: PTAA = 6:1 (wt%), the photoelectric conversion efficiency was as high as 17.37%, which is the highest record of bromine-containing inorganic perovskite at present
[115][116].
Zhang et al. carried out research on CsPbIBr
2. Usually, when preparing three-dimensional CsPbIBr
2 perovskite thin films by one-step method, CsI and PbBr
21∶1 will be mixed. However, due to the difference in solubility between the two, the deposition process can not be synchronized, which will lead to defects in the composition of perovskite and affect its performance
[117]. Based on the above considerations, they explored the ratio of precursors, tested 0.9∶1(PbBr
2-rich), 1:1 (stoichiometric), 1.1: 1 (CsI-rich) respectively, and found that when the ratio of CsI/PbBr
2 precursors was 1.1: 1.0, it could effectively improve the self-doping phenomenon in the film process, while reducing charge recombination, and the grain size of the perovskite film prepared was more than 500 nm. The cross-sectional SEM image and the corresponding J-V curve of the device prepared with the CsPbIBr
2 as the light absorption layer are shown in Figure 10 (f) and (G). The device has an excellent open circuit voltage of 1.32 V and a photoelectric conversion efficiency of 10.48% at a ratio of 1.1: 1.0, setting a new record for perovskite solar cells based on the CsPbIBr
2.
6 Conclusions and outlook
All-inorganic CsPbX
3(X=I, Br, Cl) -type perovskites have high thermal stability and excellent optical and electrical properties (nearly 100% PLQY, covering the entire visible PL spectrum), making them very promising materials. At present, reliable synthesis methods have been developed, and precise control of material morphology has been achieved, and rapid development has been made in the application of various optoelectronic devices. In this paper, the synthesis methods, morphology control, optical properties and applications in solar cells of perovskites with different dimensions are summarized in detail.Some parameters in the synthesis process are listed in the form of tables, and the device parameters of solar cells prepared with perovskites with different dimensions are also summarized in detail. Unlike perovskite thin films prepared directly on glass substrates, materials with reduced size to nano-scale show higher stability, even CsPbI
3 which are prone to phase transition can be stable for more than 30 days at room temperature
[29,106].
Despite the rapid development of all-inorganic perovskites in recent years, there are still many problems to be solved. First of all, the toxicity of Pb element in the all-inorganic europium perovskite is one of the obstacles to its wide application. At present, there are studies to replace Pb with other non-toxic elements, such as Sn, but the stability and photoelectric properties of the new lead-free perovskite are unsatisfactory. Secondly, the stability of CsPbX3 inorganic perovskite is better than other perovskites, but it still can not reach the level required for commercial application, and it will be seriously degraded when exposed to water and air for a long time. At present, the existing synthesis methods are suitable for small batch synthesis, but there is no example of large-scale preparation, which is also a problem to be considered for the commercialization of materials. Based on these problems, it is still necessary to improve the synthesis method, stability and performance of perovskite in the future, as well as to conduct in-depth research on harmless and large-scale production, so as to truly explore the great potential of all-inorganic CsPbX3 perovskite in solar cells.
In solar cell devices, the research of all-inorganic CsPbX3 perovskite mainly focuses on CsPbI3, CsPbBr3 and CsPb(I/Br)3. Inorganic perovskite containing iodine can prepare devices with high PCE, while CsPbBr3 is good at stability and can prepare devices in air environment. However, the PCE of devices made of any material is far behind the commercial requirements, so improving the PCE of devices while maintaining their stability will be the main research direction in the future. The phase transition mechanism of CsPbI3 and CsPb(I/Br)3 is deeply explored, and the PCE of CsPbBr3 is improved. In addition, new device structures are designed by combining the adjustable bandgap characteristics of inorganic perovskite, so that it can play its potential in single cell and tandem cell.
National Natural Science Foundation of China(61774169)
Qingyuan Innovation and Entrepreneurship Research Team Project(2018001)
Science and Technology Program of Guangdong Province(2018B030323010)
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