Methods for analyzing perovskite films

Abstract

When perovskite films are etched using Ar-GCIB, artifacts are formed, making it difficult to obtain a depth profile that reflects the original distribution of perovskite components with high depth resolution. The present invention provides a component distribution analysis method that performs analysis under sample cooling, rather than at room temperature, to obtain a depth profile that is closer to the original distribution of perovskite components and that also obtains a depth profile with high depth resolution. [Solution] A method for analyzing a perovskite film, comprising: subjecting the perovskite film to a continuous or intermittent ion etching treatment while the perovskite film is cooled; and analyzing the components of the surface or the surface of the perovskite film exposed by the etching treatment at least once before, during, or at the end of the etching treatment.

Description

[Technical Field]

[0001] The present invention relates to a method for analyzing perovskite films. [Background technology]

[0002] Perovskite solar cells are solar cells that use a material with a crystalline structure called perovskite. They have attracted attention as a new next-generation solar cell material due to their high light energy conversion efficiency, light weight, and flexibility.

[0003] Perovskite films used in perovskite solar cells can be easily fabricated using techniques such as spin coating, but the component distribution in the film significantly affects cell performance. In recent years, treatment techniques for organically modified perovskite films have been actively developed to improve the performance of perovskite solar cells (see, for example, Patent Document 1). Investigating the correlation between the effects of such treatment techniques on the component distribution in the film and cell performance is important for designing solar cells with higher performance. Therefore, accurate analysis of the component distribution in perovskite films is required.

[0004] On the other hand, the perovskite crystals of the perovskite film may contain organic substances such as methylammonium (hereinafter sometimes abbreviated as MA) and formamidium (hereinafter sometimes abbreviated as FA). In addition, organic substances may also be contained in the upper and lower layers of the perovskite film, the organic modification layer, and additives in the perovskite film. Therefore, an analytical method that minimizes the destruction of organic substances during analysis is required.

[0005] To analyze the depth profile of components in perovskite films, GCIB-TOF-SIMS, which combines etching with an argon gas cluster ion beam (Ar-GCIB) and acquisition of mass spectra by time-of-flight secondary ion mass spectrometry (TOF-SIMS), has been used. Ar-GCIB is an ion beam that can etch organic materials without damaging them.

[0006] When performing depth profiling of perovskite films using GCIB-TOF-SIMS under commonly used conditions, damage occurs to the perovskite crystal, making it impossible to accurately obtain the depth profiles of each component. This damage is particularly severe on organic molecules such as MA and FA, which tend to lose intensity the deeper they are. Furthermore, the depth resolution is poor, and the depth profiles of each component at the interface are not sharp, but rather gentle.

[0007] In Non-Patent Document 1, a TOF-SIMS instrument was used to etch perovskite films from both the front and back sides with Ar-GCIB, and depth profile analysis was performed. Experiments showed that the MA intensity decreased with depth, i.e., artifacts were formed during measurement. Because the primary ions used were at a higher dose density than those used in conventional TOF-SIMS, it is believed that some of the artifacts were due to damage caused by the primary ions. To mitigate these artifacts, a method was devised in which the ions used for etching (hereinafter sometimes referred to as etching ions) were changed to 1 keV cesium ions. However, with this method, the intensities of Pb and I in the perovskite film decreased slightly toward the depth, making it impossible to fully capture the original distribution of the perovskite film.

[0008] In Non-Patent Document 2, etching ions are 500 eV cesium ions or Ar-GCIB (20 keV Ar 500 + It has been reported that etching with CN ions suppresses selective etching of organic materials, reduces artifacts compared to when Ar-GCIB is used under normal conditions, and improves the depth profile. However, the results only show the depth profile of negative secondary ions, and the behavior of positive secondary ions, which reflect the intensities of MA and FA, is unclear. -The depth profile of the perovskite film shows a constant intensity on the surface side, but the intensity increases near the interface with the lower layer, suggesting that artifacts remain in this method. Although the depth resolution is not mentioned, the TiO2 - Judging from the slope of the depth profile, this method was also insufficient in terms of depth resolution. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2018 / 043385 [Non-patent literature]

[0010] [Non-Patent Document 1] Steven P. Harvey, Fei Zhang, Axel Palmstrom, Joseph M. Luther, Kai Zhu, and Joseph J. Berry, ACS Appl. Mater. Interfaces 2019, 11, 30911. [Non-patent document 2] Celine Noel, Sara Pescetelli, Antonio Agresti, Alexis Franquet, Valentina Spampinato, Alexandre Felten, Aldo di Carlo, Laurent Houssiau, Yan Busby, Materials 2019, 12, 726. Summary of the Invention [Problem to be solved by the invention]

[0011] As mentioned above, it has been difficult to etch perovskite films without creating artifacts and to obtain a depth profile with high depth resolution that reflects the original component distribution of the perovskite film. Attempts to solve this problem have been made by changing the type of etching ion and finely adjusting the conditions, but the range of conditions is very narrow and a complete solution has not yet been achieved.

[0012] The present invention has been made in view of the above-mentioned current situation, and provides a method for analyzing component distribution that can obtain component information and a depth profile inside a perovskite film that is closer to the original component distribution of the film, and that can obtain a depth profile with high depth resolution. [Means for solving the problem]

[0013] The above-mentioned prior art studies were all conducted at room temperature (around 25°C), and there was no prior art that focused on the temperature during analysis. The present inventors came up with the idea of ​​the present invention by focusing on the temperature during analysis.

[0014] The present invention is a method for analyzing a perovskite film, which comprises continuously or intermittently subjecting the perovskite film to an ion etching treatment while the perovskite film is cooled, and analyzing the components of the surface or the surface of the perovskite film exposed by the etching treatment at least once before, during, or at the end of the etching treatment. [Effects of the Invention]

[0015] By using the analytical method of the present invention, it is possible to perform component analysis inside a perovskite film, as well as component analysis in the depth direction, which was difficult with conventional techniques, with high depth resolution while preserving the original component information, and it is possible to acquire high-quality data. The present invention is not limited to GCIB-TOF-SIMS, and can provide high-quality data for depth direction analysis methods that use ion etching, such as secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). [Brief explanation of the drawings]

[0016] [Figure 1] Positive secondary ion depth profile obtained in Example 1 according to the present invention [Figure 2] Negative secondary ion depth profile obtained in Example 1 according to the present invention [Figure 3] Positive secondary ion depth profile obtained in Comparative Example 1 using a conventional method [Figure 4] Negative secondary ion depth profile obtained in Comparative Example 1 using a conventional method [Figure 5] Positive secondary ion depth profile obtained in Example 2 according to the present invention [Figure 6] Positive secondary ion depth profile obtained in Comparative Example 2 using a conventional method DETAILED DESCRIPTION OF THE INVENTION

[0017] In the present invention, the perovskite film of the perovskite solar cell to be targeted has a crystalline structure known as a perovskite structure, and is preferably composed of metal elements such as lead, tin, cesium, etc., halogens such as bromine and iodine, and organic ions such as MA and FA, etc. However, applicable materials in the present invention are not limited to the above and also include organic-inorganic hybrid materials.

[0018] In the method for analyzing a perovskite film of the present invention, the perovskite film is cooled. By cooling the perovskite film, the formation of artifacts (selective etching or volatilization of organic substances) is suppressed, making it possible to obtain a depth profile that reflects the original component distribution. The cooling of the perovskite film in the present invention can be performed either inside or outside the analytical device. Preferred cooling methods include direct cooling by contacting the sample with a refrigerant such as ice, dry ice, or liquid nitrogen, or indirect cooling via heat transfer. From the perspectives of the temperature reached, ease of handling, and prevention of contamination, cooling by heat transfer from liquid nitrogen inside the analytical device is preferred.

[0019] In the present invention, when cooling is performed within the analytical instrument, it is preferable to introduce the sample stage into the instrument's pre-evacuation chamber, thoroughly evacuate the pre-evacuation chamber, then move the sample stage to the instrument's main chamber, and cool the sample stage by heat conduction from liquid nitrogen or the like. The sample is preferably cooled to a temperature of 0°C or below. A cooling temperature of 0°C or below, more preferably -140°C or below, can more effectively suppress the formation of artifacts. Furthermore, using liquid nitrogen, a commonly used refrigerant, allows for cooling down to -196°C. The sample may be cooled in the pre-evacuation chamber before being moved to the main chamber. However, because high water pressure can cause condensation and ice to form on the measurement sample, it is preferable to cool the sample after evacuating or replacing the atmosphere with an inert gas such as nitrogen gas or argon gas.

[0020] In the present invention, preferred examples of means for cooling a sample include heat conduction using liquid nitrogen, heat conduction using liquid helium, evacuation of liquid helium, a Peltier cooler, a dilution refrigerator, and nuclear adiabatic demagnetization refrigeration.

[0021] In the present invention, while the perovskite film is cooled, the perovskite film is continuously or intermittently subjected to an ion etching treatment. The etching treatment is a means for digging the perovskite film in the depth direction by irradiating it with an ion beam in an analytical device. The etching ions are preferably one selected from the group consisting of an argon gas cluster ion beam, an argon ion beam, a cesium ion beam, an oxygen ion beam, a fullerene ion beam, a bismuth ion beam, a gallium ion beam, a water cluster ion beam, and a carbon dioxide gas cluster ion beam. The ion beam used for the ion etching is more preferably one selected from the group consisting of an argon gas cluster ion beam, a fullerene ion beam, a water cluster ion beam, and a carbon dioxide gas cluster ion beam. The ion beam used for the ion etching is even more preferably an argon gas cluster ion beam. The energy of the etching ions is not particularly limited.

[0022] In the present invention, the components of the surface or the surface of the perovskite film exposed by the etching treatment are analyzed at least once before, during, and at the end of the etching treatment while the perovskite film is cooled. Preferably, the components of the surface of the perovskite film exposed by the etching treatment are analyzed at least once while the perovskite film is cooled, during, and at the end of the etching treatment. Note that even in this preferred embodiment, further analysis of the surface components of the perovskite film before the etching treatment is not excluded. By performing such analysis, analytical results closer to the original components of the surface and interior of the perovskite film can be obtained. Furthermore, by performing the analysis multiple times, information on the component distribution in the depth direction of the perovskite film can be obtained, thereby obtaining a depth profile closer to the original component distribution of the perovskite film. Preferred analytical techniques include one selected from the group consisting of secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Of these, secondary ion mass spectrometry is preferably used. SIMS mass analyzers include time-of-flight, quadrupole, double focusing, and ion trap types. It is particularly preferable to use a time-of-flight type for secondary ion mass spectrometry. [Example]

[0023] The present invention will be described below with reference to examples, but the present invention is not limited thereto.

[0024] [Example 1, Comparative Example 1] (sample) The samples used in Example 1 and Comparative Example 1 were the same for comparison, and consisted of a perovskite film formed on an ITO (Indium-Tin Oxide) electrode. The perovskite film used had a thickness of 500 nm. This perovskite film contained cesium (Cs), lead (Pb), formamidinium (FA), methylammonium (MA), iodine (I), and bromine (Br) as its constituents, and had the composition formula Cs 0.05 FA 0.80 MA 0.15 PbI2.75 Br 0.25 This sample is used to confirm the accuracy of the analytical method of the present invention, and is expected to have a uniform component distribution, and does not contain any modifying layers or additives other than the perovskite film.

[0025] Example 1 The sample stage was cooled to -150°C, and a depth profile analysis was performed from the sample surface to the ITO electrode using GCIB-TOF-SIMS measurement.

[0026] The etching ions are 10 keV Ar-GCIB, the etching ion raster size is 500 μm square, and the primary ions are 60 keV Bi3 ++ The raster size of the primary ions was 200 μm square. Etching and analysis were performed intermittently by alternately repeating irradiation of etching ions and primary ions.

[0027] The depth resolution of the positive secondary ion depth profile is 113 In + The strength was calculated by the difference between the depth at which the strength was 84% ​​of the maximum strength and the depth at which the strength was 16%.

[0028] The obtained positive secondary ion depth profile is shown in Figure 1, and the negative secondary ion depth profile is shown in Figure 2. The vertical axis of the depth profile is the relative intensity (Pb + , PbI3 - The horizontal axis represents the depth (normalized so that the perovskite layer / ITO interface is at a depth of 500 nm).

[0029] Figure 1 shows the Pb ions, which are positive secondary ions derived from Pb in the perovskite film. + , Cs, a positive secondary ion derived from Cs in the perovskite film + , which are positive secondary ions originating from FA in the perovskite film. 13 CH5N2 +, CHN, a positive secondary ion derived from MA in the perovskite film + , which are positive secondary ions originating from the ITO electrode. 113 In + The depth profile of the

[0030] Figure 2 shows the PbI3 negative secondary ions derived from Pb and I in the perovskite film. - , CsI2, a negative secondary ion derived from Cs and I in the perovskite film - , I, a negative secondary ion derived from I in the perovskite film - , Br, a negative secondary ion derived from Br in the perovskite film - , InO2, a negative secondary ion originating from the ITO electrode - The depth profile of the

[0031] In Figures 1 and 2, the secondary ion intensity derived from the perovskite film remains roughly constant throughout the perovskite layer, reflecting the original component distribution. 113 In + The depth resolution calculated based on the depth profile was 40 nm.

[0032] (Comparative Example 1) Except for the fact that the temperature of the sample stage was kept at 25° C., the depth profile analysis was carried out in the same manner as in Example 1. The obtained positive secondary ion depth profile is shown in FIG. 3, and the negative secondary ion depth profile is shown in FIG. 4.

[0033] In Figures 3 and 4, the secondary ions Pb from the perovskite film are + , 13 CH5N2 + , CH6N + , PbI3 - , CsI2 - The intensity of the polycrystalline film decreases from 0 to 100 nm, and the depth profile reflecting the uniform composition distribution of the polycrystalline film is not obtained due to the formation of artifacts. 113 In +The depth resolution determined based on the depth profile was 186 nm. 113 In + Since the ion intensity did not reach the plateau region (region where the intensity is constant), the end point of the measurement was taken as the maximum intensity.

[0034] [Example 2, Comparative Example 2] (sample) The samples used in Example 2 and Comparative Example 2 were the same for comparison, and consisted of a perovskite film formed on an ITO electrode. The perovskite film had a thickness of 500 nm. This perovskite film had the composition formula Cs 0.05 FA 0.80 MA 0.15 PbI 2.75 Br 0.25 The polycrystalline film represented by the formula (1) was prepared by adding an organic additive.

[0035] Example 2 The sample stage was cooled to -150°C, and a depth profile analysis was performed from the sample surface to the ITO electrode using GCIB-TOF-SIMS measurement.

[0036] The etching ions are 10 keV Ar-GCIB, the etching ion raster size is 600 μm square, and the primary ions are 60 keV Bi3 ++ The raster size of the primary ions was 200 μm square. Etching and analysis were performed intermittently by alternately repeating irradiation of etching ions and primary ions.

[0037] The depth resolution of the positive secondary ion depth profile is 113 This was determined by the difference between the depth where the In+ intensity was 84% ​​of the maximum intensity and the depth where it was 16%.

[0038] The obtained positive secondary ion depth profile is shown in Figure 5. The vertical axis of the depth profile is the relative intensity (Pb +The horizontal axis represents the depth (normalized so that the perovskite layer / ITO interface is at a depth of 500 nm).

[0039] In addition to the ion species shown in Figure 1, Figure 5 also shows the depth profile of secondary ions derived from organic additives. A uneven distribution of organic additives is observed on the perovskite surface and the perovskite layer / ITO interface, with both the rise and fall being steep. 113 In + The depth resolution calculated based on the depth profile was 31 nm.

[0040] (Comparative Example 2) Except for the fact that the measurement was carried out while the temperature of the sample stage was maintained at 25° C., the depth profile analysis was carried out in the same manner as in Example 2. The obtained positive secondary ion depth profile is shown in FIG.

[0041] In Figure 6, the secondary ions Pb from the perovskite film are + , CH6N + The intensity decreases from 0 to 100 nm, and the depth profile does not reflect the original composition distribution due to the formation of artifacts. Compared to Example 2, the fall of secondary ions derived from the organic additive near the perovskite surface is steeper due to less artifact formation, but a peak due to the influence of artifact formation is observed around 50 nm, and the original distribution is not obtained. Furthermore, artifact formation progresses at the perovskite layer / ITO interface, and the gradient of the depth profile of secondary ions derived from the organic additive and ITO is smaller compared to Example 2. In Figure 6, 113 In + The depth resolution calculated based on the depth profile was 173 nm.

Claims

1. A method for analyzing a perovskite film, comprising: subjecting the perovskite film to a continuous or intermittent ion etching treatment while the perovskite film is cooled; analyzing the components of the surface or a surface of the perovskite film exposed by the etching treatment at least once before, during, or at the end of the etching treatment; and cooling the perovskite film to a temperature of 0°C or lower.

2. 2. The method for analyzing a perovskite film according to claim 1, wherein the temperature to which the perovskite film is cooled is −196° C. or higher and −140° C. or lower.

3. 3. The method for analyzing a perovskite film according to claim 1, wherein the analysis is performed multiple times to obtain information on the component distribution in the depth direction of the perovskite film.

4. The method for analyzing a perovskite film according to claim 1 or 2, wherein the analysis is carried out using secondary ion mass spectrometry.

5. The method for analyzing a perovskite film according to claim 4, wherein the secondary ion mass spectrometry is time-of-flight.

6. 3. The method for analyzing a perovskite film according to claim 1, wherein the ion beam used in the etching treatment is selected from the group consisting of an argon gas cluster ion beam, an argon ion beam, a cesium ion beam, an oxygen ion beam, a fullerene ion beam, a water cluster ion beam, and a carbon dioxide gas cluster ion beam.

7. 3. The method for analyzing a perovskite film according to claim 1, wherein the ion beam used in the etching treatment is selected from the group consisting of an argon gas cluster ion beam, a fullerene ion beam, a bismuth ion beam, a gallium ion beam, a water cluster ion beam, and a carbon dioxide gas cluster ion beam.

8. 3. The method for analyzing a perovskite film according to claim 1, wherein the ion beam used in the etching treatment is an argon gas cluster ion beam.

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