A method for preparing a low FA doped Cs-based perovskite film by using a lewis acid-base pair as raw materials

Abstract

The pure alpha-CsPbI2Br thin film has excellent photothermal stability, but has poor humidity stability, and will be converted from alpha phase to delta phase and lose optical activity in a low humidity environment. In view of the problem, the application designs a method for preparing alpha-Cs 1‑x FA x PbI2Br(0<x<0.5) perovskite thin film. The application takes PbI2·DMSO and PbBr2·DMSO adduct as raw materials, and overcomes the problem that the obtained perovskite thin film is phase-separated when the content of FA is less than 50% in the preparation process of a traditional perovskite thin film. The synthesis process of the application is simple, and does not need complex anti-solvent treatment or heating process. Finally, the alpha-Cs 1‑x FA x The PbI2Br(0<x<0.5) thin film has large grain size, good crystallinity and high density, and the doping of a small amount of FA ions greatly increases the tolerance factor of the perovskite, and the humidity stability is obviously increased compared with CsPbI2Br; meanwhile, the alpha-Cs 1‑x FA x PbI2Br(0

Description

Technical Field

[0001] The present invention specifically relates to a method for preparing a low-FA doped Cs-based perovskite thin film (α-Cs 1-x FA x PbI2Br, 0 < x < 0.5) using Lewis acid-base pairs (i.e., PbI2·DMSO and PbBr2·DMSO adducts) as raw materials, belonging to the field of perovskite solar cell materials. Technical Background

[0002] Among many perovskite solar cell materials, CsPbI2Br is very suitable as the top layer of tandem solar cells due to its excellent optical, thermal, and electric field stabilities and appropriate bandgap size.

[0003] Although CsPbI2Br has excellent stability in light, heat, and oxygen, its humidity stability is poor. When exposed to water vapor, CsPbI2Br will undergo a phase transition from the α-phase to the δ-phase, resulting in the failure of its solar cell. This is because the tolerance factor of CsPbI2Br is only 0.855. When water contacts the surface of CsPbI2Br, due to the large dissolution enthalpy of water for halide particles (such as I - )), it induces the movement of I - towards the water-containing area (surface), resulting in a lower vacancy formation energy for CsPbI2Br and promoting the appearance of vacancies inside the lattice. When there are vacancies in the lattice, the ionic bond constraint of the iodine-lead octahedron decreases, reducing the nucleation free energy of the vacancies, thus promoting the transformation from the cubic phase to the low-symmetry tetragonal or orthorhombic structure. Solving the humidity stability of CsPbI2Br can be controlled to a certain extent by doping a small amount at the B-site and X-site and encapsulation. However, doping a small amount at the B-site and X-site will cause changes in the properties of the [PbI6] 4- structure in CsPbI2Br, reducing the transport efficiency of effective carriers, and thus overall reducing the photoelectric conversion efficiency of CsPbI2Br. Currently, the organic materials used for encapsulation all have a certain water vapor permeability. During long-term use, a small amount of water vapor will also contact CsPbI2Br, causing the entire battery to lose activity. Moreover, strict control of water vapor is required during the encapsulation process, increasing the preparation cost of solar cells.

[0004] According to theoretical calculations, synthesizing an α-Cs 1-x FA x PbI2Br (0 < x < 0.5) perovskite thin film with a low FA content has a stable crystal structure, which can improve the humidity stability of the thin film while retaining good optical, electrical, and thermal stabilities. Currently, α-Cs 1-x FA xThere is a lack of reports on the preparation method of PbI2Br (0 < x < 0.5) perovskite thin films. The raw materials for preparing perovskite thin films are often CsI, FAI, PbI2, and PbBr2. Due to the high Cs content, perovskite thin films with a high Cs content require an annealing temperature above 200 °C to produce a pure perovskite phase, while FAI cannot withstand an annealing temperature above 200 °C. When using low-temperature heat treatment with CsI, FAI, PbI2, and PbBr2 as raw materials, impurities often occur in the thin films, resulting in impure phases. If antisolvent assistance or substrate preheating assistance is used, pure-phase α-Cs 1-x FA x PbI2Br (0 < x < 0.5) can be obtained, but the dripping process of the antisolvent and the process parameters of substrate preheating have a great impact on the quality of the thin films, which is not conducive to maintaining the quality control of the thin films. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing α-Cs 1-x FA x PbI2Br perovskite thin films using PbI2·DMSO and PbBr2·DMSO adducts as raw materials, with a low FAI doping content. The obtained α-Cs 1-x FA x PbI2Br (0 < x < 0.5) thin film has a pure phase, large, flat, and dense surface grains without pores. Compared with CsPbI2Br perovskite, its humidity stability is significantly improved, and it retains good light stability and thermal stability.

[0006] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows:

[0007] A method for preparing α-Cs 1-x FA x PbI2Br perovskite thin films using PbI2·DMSO and PbBr2·DMSO adducts as raw materials, and the main steps are as follows:

[0008] (1) Mix PbI2·DMSO adduct, PbBr2·DMSO adduct, cesium iodide (CsI), and formamidinium iodide (FAI), and add them to a mixed solvent of N,N-dimethylformamide and dimethyl sulfoxide, and heat and stir until clear to obtain a precursor solution.

[0009] (2) Spin-coat the precursor solution obtained in step (1) onto a substrate. After spin-coating, a precursor thin film is obtained;

[0010] (3) Anneal the precursor thin film obtained in step (2). After annealing, α-Cs 1- x FAx PbI2Br, where 0 < x < 0.5.

[0011] According to the above scheme, in step (1), the molar ratio of PbI2·DMSO, PbBr2·DMSO, CsI, and FAI is 0.5:0.5:1 - x:x, where 0 < x < 0.5.

[0012] According to the above scheme, in step (1), the mixed solvent is obtained by mixing N,N - dimethylformamide and dimethyl sulfoxide in a volume ratio of 1:3 to 3:1.

[0013] According to the above scheme, in step (1), the total concentration of the four raw materials of the precursor solution is 0.6 - 1.2 mol / L.

[0014] According to the above scheme, in step (1), the heating temperature is 70 - 100 °C and the heating time is 4 - 8 hours.

[0015] According to the above scheme, in step (2), the precursor solution needs to be filtered through a 0.45 - μm filter head before spin - coating and then can be used. The spin - coating amount on the substrate is in the range of 60 - 100 μL / cm 2 range.

[0016] According to the above scheme, in step (2), the rotational speed of the substrate during spin - coating is 2000 - 4000 revolutions per minute, and the spin - coating time is 30 - 50 seconds; the substrate is transparent conductive glass such as FTO or ITO. It is required that the surface be cleaned thoroughly. The cleaning process is alcohol - acetone - ultrapure water - alcohol. When in use, the surface of the substrate is dry and free of contamination. In this invention, FTO is taken as an example.

[0017] According to the above scheme, in step (3), the annealing temperature is 100 - 160 °C and the annealing time is 0 - 0.6 h.

[0018] The α - Cs 1-x FA x PbI2Br perovskite thin film prepared by the above method has a chemical composition expressed as α - Cs 1-x FA x PbI2Br, 0 < x < 0.5, with a α - phase, a grain size of 0.1 - 1.0 μm, a thickness of 300 - 400 nm, and a flat and dense surface without holes.

[0019] Compared with the prior art, the beneficial effects of this invention are as follows:

[0020] Pure α-CsPbI2Br thin films have excellent photothermal stability but extremely poor humidity stability. They will lose their optical activity by transforming from the α-phase (1.90 eV) to the δ-phase (2.85 eV) in a low-humidity environment. To address this problem, the present invention designs a method for preparing α-Cs 1-x FA x PbI2Br (0 < x < 0.5) perovskite thin films. Benefiting from using PbI2·DMSO and PbBr2·DMSO adducts as raw materials, during the crystal nucleation and growth process, DMSO in the adducts will occupy more positions between the iodine-lead octahedra, increasing the spacing between the iodine-lead octahedra and promoting the entry of FA with a larger ionic radius into the lattice. Thus, it overcomes the problem of phase separation in the obtained perovskite thin films when the content of low-formamidinium (FA) is less than 50% during the preparation process of traditional perovskite thin films. Ultimately, the α-Cs 1-x FA x PbI2Br (0 < x < 0.5) thin films obtained in the present invention have large grain sizes, good crystallinity, high density, and the doping of a small amount of FA ions significantly increases the tolerance factor of this perovskite. The humidity stability is significantly increased compared to CsPbI2Br, and good thermal stability and optical stability are also retained. In addition, the preparation method of the present invention is simple to operate and does not require a complex heat treatment process, which is of great significance for the industrialization and practical application of future perovskite solar cells. Description of the Drawings

[0021] Figure 1(a) is an XRD pattern comparing Comparative Example 1 (CsPbI2Br thin film) and Example 1 (Cs 0.7 FA 0.3 PbI2Br thin film); Figure 1(b) is an XRD pattern comparing Comparative Example 2 (α-Cs 0.7 FA 0.3 PbI2Br thin film) and Example 1 (Cs 0.7 FA 0.3 PbI2Br thin film).

[0022] Figure 2 is the UV absorption spectrum comparing Comparative Example 1 (CsPbI2Br thin film) and Example 1 (Cs 0.7 FA 0.3 PbI2Br thin film).

[0023] Figure 3 From left to right are the SEM surface morphologies comparing Comparative Example 1 (CsPbI2Br thin film) and Example 1 (Cs 0.7 FA 0.3 PbI2Br thin film) and the SEM surface morphology of Example 1 (Cs 0.7 FA0.3 SEM cross-sectional image of PbI2Br thin film.

[0024] Figure 4 Comparative Example 1 (CsPbI2Br thin film) and Example 1 (Cs 0.7 FA 0.3 Humidity stability of PbI2Br thin film.

[0025] Figure 5 This is Example 1 (Cs) 0.7 FA 0.3 The photothermal stability test results of the PbI2Br thin film are shown in the left figure as the XRD pattern and the right figure as the UV absorption pattern. Detailed Implementation

[0026] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the present invention is not limited to the following embodiments.

[0027] In the following examples, the molar ratio of PbI2 to DMSO in the lead iodide-dimethyl sulfoxide adduct PbI2·DMSO powder is 1:1, and the molar ratio of PbBr2 to DMSO in the lead bromide-dimethyl sulfoxide adduct PbBr2·DMSO powder is 1:1. PbI2·DMSO or PbBr2·DMSO powders can be obtained by antisolvent precipitation from a PbI2 DMSO solution or a PbBr2 DMSO solution, respectively. For example, the preparation method of PbI2·DMSO and PbBr2·DMSO powders can be found in the literature High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, SCIENCE, 21 May 2015, Vol 348, Issue 6240, 1234-1237.

[0028] Example 1

[0029] A method for preparing α-Cs using PbI2·DMSO and PbBr2·DMSO adducts as raw materials 0.7 FA 0.3 The method for producing PbI2Br perovskite thin films is as follows:

[0030] (1) PbI2·DMSO, PbBr2·DMSO, cesium iodide (CsI), and formamidine hydroiodide (FAI) were added to a mixed solvent of DMSO and DMF in a molar ratio of 0.5:0.5:0.7:0.3 (the volume ratio of DMSO to DMF was 2:3, and the total concentration of the four raw materials in the precursor solution was 1 mol / L). The mixture was then heated and stirred at 70°C for 10 hours to obtain the precursor solution.

[0031] (2) The above precursor solution was filtered using a 0.45 μm microporous filter head. 250 μL was immediately spin-coated onto a 1.5 cm × 2 cm FTO conductive glass without preheating treatment. The spin-coating speed was 3000 r / min and the spin-coating time was 35 s to obtain the precursor film.

[0032] (3) The precursor film was annealed on a heating table at 160°C for 15 min, and then the film was cooled to room temperature with the equipment to obtain FA-doped CsPbI2Br-based perovskite thin film material. The chemical composition can be expressed as Cs 0.7 FA 0.3 PbI₂Br, denoted as Cs 0.7 FA 0.3 PbI2Br, with a thickness of 340 nm.

[0033] Comparative Example 1

[0034] To facilitate comparison of Cs after FA doping 0.7 FA 0.3 To assess the humidity stability of PbI₂Br, a comparative example was prepared using CsPbI₂Br perovskite thin film materials. The specific process is as follows:

[0035] (1) PbI2·DMSO, PbBr2·DMSO, and cesium iodide (CsI) were added to a mixed solvent of DMSO and DMF in a molar ratio of 0.5:0.5:1.0 (the volume ratio of DMSO to DMF was 2:3, and the total concentration of the four raw materials in the precursor solution was 1 mol / L). The mixture was then heated and stirred at 70°C for 10 hours to obtain the precursor solution.

[0036] (2) The above precursor solution was filtered using a 0.45 μm microporous filter head. 250 μL was immediately spin-coated onto a 1.5 cm × 2 cm FTO conductive glass without preheating treatment. The spin-coating speed was 3000 r / min and the spin-coating time was 35 s to obtain the precursor film.

[0037] (3) The precursor film was placed on a heating table at 160°C for annealing for 15 minutes, and then the film was cooled to room temperature with the equipment.

[0038] Comparative Example 2

[0039] To compare the effect of the DMSO adduct raw material, α-Cs was prepared using a non-adduct as the raw material 0.7 FA 0.3 PbI2Br perovskite thin film, and the specific process is as follows:

[0040] (1) Lead bromide (PbBr2), lead iodide (PbI2), cesium iodide (CsI), and formamidinium hydroiodide (FAI) were added to a mixed solvent of DMSO and DMF at a molar ratio of 0.5:0.5:0.7:0.3 (the volume ratio of DMSO to DMF was 2:3, and the total concentration of the four raw materials in the precursor solution was 1 mol / L). Then, it was heated and stirred at 70 °C for 10 hours to obtain a precursor solution;

[0041] (2) The above precursor solution was filtered using a 0.45 μm microporous filter head. 250 μL was immediately spin-coated on an unpreheated 1.5 cm × 2 cm FTO conductive glass at a spin-coating speed of 3000 r / min and a spin-coating time of 35 s to obtain a precursor thin film;

[0042] (3) The precursor thin film was annealed on a 160 °C heating stage for 15 min, and then the thin film was cooled to room temperature with the equipment.

[0043] As shown in Figure 1, the α-Cs 0.7 FA 0.3 PbI2Br perovskite thin film prepared in Example 1 and the CsPbI2Br perovskite thin film prepared in Comparative Example 1 are both in the α phase, and no other impurity phases were found in the XRD results, indicating that the obtained thin film has a single and pure phase. In Comparative Example 2 where no adduct was used as the raw material, when the FA content was 30%, an obvious non-perovskite δ phase appeared, and the thin film could not maintain a pure phase and could no longer be used as the absorption layer of a solar cell thin film. This precisely shows the promoting effect of the PbI2·DMSO and PbBr2·DMSO adducts on the formation of a pure-phase low-FA-content α-Cs 1-x FA x PbI2Br (0 < x < 0.5) thin film.

[0044] As Figure 2 shown, compared with Comparative Example 1, due to the doping of FA in Example 1, the band gap of the Cs 0.7 FA 0.3 PbI2Br thin film also decreased from 1.9 eV to 1.8 eV.

[0045] As Figure 3 shown, the Cs 0.7 FA 0.3 PbI2Br thin film obtained in Example 1 has a dense and hole-free surface (see details in Figure 3(See the middle image in the image), the grain size is 0.1–1.0 μm, and the thickness is 300–400 nm (see details). Figure 3 (See right image in the image); The surface of the CsPbI2Br film obtained in Comparative Example 1 is uneven and has no obvious grain boundaries (see the image in ... Figure 4 (Left image in the image).

[0046] To test the humidity stability of the perovskite films obtained in Example 1 and Comparative Example 1, the two film samples were placed in an air environment with a relative humidity of 30% and the phase changes of the films over time were observed. The results are as follows: Figure 5 As shown. Therefore, it can be seen that in an air environment with a relative humidity of 30%, the Cs prepared in Example 1... 0.7 FA 0.3 The band gap of the PbI₂Br titanium dioxide film remained unchanged, indicating that its phase remained the α phase. In contrast, the CsPbI₂Br film prepared in Comparative Example 1 showed a complete change in band gap and phase transformation within 1 hour. Therefore, the α-CsPbI₂Br film of this invention... 1-x FA x PbI2Br perovskite thin film material exhibits significantly better humidity characteristics than CsPbI2Br perovskite thin film.

[0047] To test the α-Cs described in Example 1 1-x FA x The photothermal stability of the PbI₂Br perovskite thin film was assessed by treating the film obtained in Example 1 with ultraviolet irradiation in nitrogen atmosphere, ultraviolet light in air atmosphere, and heating at 140°C in nitrogen atmosphere, respectively. The phase composition and band gap changes were then measured using XRD and ultraviolet absorption spectroscopy. The results are as follows: ​ As shown, after 12 hours of irradiation with a 100W ultraviolet lamp under nitrogen and air atmospheres, the phase and band gap of the film did not change, indicating that the film has excellent light stability; after heating at 140℃ for one hour, the phase and band gap of the film also did not change, indicating that the film has excellent thermal stability.

[0048] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for preparing α-Cs using PbI2·DMSO and PbBr2·DMSO adducts as raw materials. 1-x FA x The method for producing PbI2Br perovskite thin films, characterized in that... The main steps are as follows: (1) PbI2·DMSO, PbBr2·DMSO and CsI, FAI are mixed and added to a mixed solvent of N,N-dimethylformamide and dimethyl sulfoxide. The mixture is heated and stirred until clear to obtain a precursor solution. (2) Spin-coat the precursor solution described in step (1) onto the substrate. After spin-coating, a precursor film is obtained. (3) Anneal the precursor film described in step (2). After the annealing is completed, α-Cs 1-x FA x PbI2Br perovskite thin film is obtained, where 0 < x < 0.5, the phase is the α phase, the grain size is 0.1 - 1.0 μm, the thickness is 300 - 400 nm, and the surface is flat, dense, and free of holes.

2. The method for preparing α-Cs using PbI2·DMSO and PbBr2·DMSO adducts as raw materials according to claim 1. 1- x FA x The method for producing PbI2Br perovskite thin films, characterized in that... In step (1), the molar ratio of PbI₂·DMSO, PbBr₂·DMSO, CsI, and FAI is 0.5:0.5:1-x:x, where 0 <x<0.5。 3. The method for preparing α-Cs using PbI2·DMSO and PbBr2·DMSO adducts as raw materials according to claim 1. 1- x FA x The method for producing PbI2Br perovskite thin films, characterized in that... In step (1), the mixed solvent is obtained by mixing N,N-dimethylformamide and dimethyl sulfoxide in a volume ratio of 1:3 to 3:

1.

4. The method for preparing α-Cs using PbI2·DMSO and PbBr2·DMSO adducts as raw materials according to claim 1. 1- x FA x The method for producing PbI2Br perovskite thin films, characterized in that... In step (1), the total concentration of the four raw materials in the precursor solution is 0.6 to 1.2 mol / L.

5. The method for preparing α-Cs using PbI2·DMSO and PbBr2·DMSO adducts as raw materials according to claim 1. 1- x FA x The method for producing PbI2Br perovskite thin films, characterized in that... In step (1), the heating temperature is 70-100℃ and the heating time is 4-8 hours.

6. The method for preparing α-Cs using PbI2·DMSO and PbBr2·DMSO adducts as raw materials according to claim 1. 1- x FA x The method for producing PbI2Br perovskite thin films, characterized in that... In step (2), the precursor solution is filtered before spin-coating, and the spin-coating volume is 60–100 μL / cm. 2 Within the range.

7. The method for preparing α-Cs using PbI2·DMSO and PbBr2·DMSO adducts as raw materials according to claim 1. 1- x FA x The method for producing PbI2Br perovskite thin films, characterized in that... In step (2), the substrate rotation speed is 2000-4000 rpm and the spin coating time is 30-50 seconds; the substrate is transparent conductive glass with a clean and dry surface.

8. The method for preparing α-Cs using PbI2·DMSO and PbBr2·DMSO adducts as raw materials according to claim 1. 1- x FA x The method for producing PbI2Br perovskite thin films, characterized in that... In step (3), the annealing temperature is 100-160℃ and the annealing time is 0-0.6h.

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