A method for preparing a perovskite thin film

Abstract

The present application provides a method for preparing a perovskite thin film. The method includes: dividing the surface of the perovskite thin film spin-coated on a substrate into corresponding several regions according to thickness, irradiating each of the regions with a first laser having a corresponding wavelength and a fixed spot size to generate a uniformly distributed seed crystal array, scanning each region with a second laser having a corresponding wavelength to make the surface temperature of each region reach a specified temperature range, ensuring that the temperature difference between each seed crystal region and the periphery is within a preset range to enlarge the crystallization size of the seed crystals and improve the crystallization uniformity of the perovskite thin film. Finally, the surface temperature of the perovskite thin film is slowly decreased through a temperature control system to further improve the crystallization uniformity and reduce the grain segregation caused by thermal stress and thermal gradient. Through the technical solution provided by the present invention, the crystallization uniformity and size suitability of the perovskite thin film can be taken into account, defects can be reduced, and the photoelectric conversion and transmission performance of the perovskite battery can be improved.

Description

Technical Field

[0001] This application relates to the field of production and manufacturing of perovskite batteries or perovskite thin films, and specifically relates to a method for preparing perovskite thin films. Background Art

[0002] A solar cell is a thin optoelectronic semiconductor sheet that directly converts sunlight into electricity. It directly converts light energy into electrical energy through the photovoltaic effect or the photochemical effect, and has the advantages of high reliability, long life, high conversion efficiency, etc. As a third-generation new concept solar cell, the perovskite solar cell has the advantages of high photoelectric conversion efficiency, low cost, and flexible processing. In recent years, it has developed rapidly, gradually comparable to silicon cells, and has approached the commercial application level.

[0003] Annealing treatment is a key step in the manufacture of perovskite batteries. It is used to improve the crystallinity and structural stability of perovskite thin films, thereby improving the optoelectronic properties of perovskite thin films and the efficiency of perovskite batteries. Currently, the annealing process of perovskite thin films mainly uses thermal annealing, that is, by placing the perovskite thin film in a high-temperature environment and undergoing heat treatment for a long time (about 1 hour) to promote the improvement of its crystallization quality. However, this thermal annealing method has the disadvantages of time-consuming, energy-consuming, difficult to precisely control the crystallization process (resulting in too large crystal sizes and difficult to control the uniformity of crystallization), unstable film performance, and not being suitable for the requirements of low-temperature processes for specific devices (such as flexible solar cells).

[0004] Laser annealing technology uses a high-energy density laser beam to anneal perovskite thin films in a short time. Under laser irradiation, the surface material of the perovskite thin film quickly melts, and during the subsequent cooling process, crystal thin films naturally grow epitaxially in the molten layer, thereby reconstructing the crystal structure of the molten layer. Compared with thermal annealing, the crystallization process of laser-annealed perovskite thin films is faster, but limited by the size of the laser spot, the crystal grain size of perovskite thin films is relatively small. In addition, how to achieve large-area uniform crystallization while maintaining rapid annealing is still a difficult point for laser annealing. Summary of the Invention

[0005] Aiming at the deficiencies of the above-mentioned perovskite thin film annealing methods, this application constructs a control process for laser annealing and combines the advantages of thermal annealing to achieve a technical solution for rapid annealing of perovskite thin films, large-area, large-size (crystal grain size) uniform crystallization, and few defects. The laser annealing in the previous stage provides a basis for the subsequent low-temperature thermal annealing, so that the technical solution provided by this application is suitable for preparing perovskite thin films required for flexible solar cells.

[0006] The technical solution provided by this application is implemented as a method for preparing a perovskite thin film. The method includes: dividing the surface of the perovskite thin film spin-coated on the substrate into several regions according to the thickness. For each of the regions, a first laser with a corresponding wavelength is used to irradiate according to its thickness to crystallize a uniformly distributed seed crystal array. Then, a second laser with a corresponding wavelength is selected to scan each region so that the surface temperature of each region reaches a specified temperature range, and the temperature difference between each seed crystal region and the periphery is within a preset range to expand the crystallization size of the seed crystals and improve the crystallization uniformity of the perovskite thin film; the spot size of the first laser corresponding to each of the regions is the same. The specified temperature range is greater than or equal to the crystallization temperature of the perovskite material and less than the thermal decomposition temperature of the crystallization of the perovskite material.

[0007] Uniformly distributed seed crystals are pre-generated by a laser with a first wavelength, and then each region is scanned with a second laser to increase the size of the crystallization region of each seed crystal. This can not only increase the size of the laser annealing crystallization of the perovskite thin film, but also ensure the crystallization uniformity. The increase in the grain size of the perovskite thin film is beneficial to the transport of electron-hole pairs, and the improvement of crystallization uniformity and the reduction of defects are beneficial to improving the photoelectric conversion efficiency of the perovskite thin film.

[0008] In some embodiments, the irradiating each of the regions with a first laser with a corresponding wavelength includes: for the regions with a thickness greater than a first threshold, irradiating with a first laser with a wavelength of 800 nm - 1200 nm, and for the regions with a thickness less than the first threshold, irradiating with a first laser with a wavelength of 400 nm - 700 nm. The short-wavelength laser of 400 nm - 700 nm can directly act on the material surface to improve the uniformity of the upper thin film and is suitable for a thinner perovskite layer; the long-wavelength laser of 800 nm - 1200 nm can penetrate deep into the thin film to achieve overall crystallization and is suitable for a thicker perovskite layer. Preferably, the first threshold is 200 nm.

[0009] For a thicker perovskite thin film region to crystallize seed crystals of the same size, more energy is required, which can be obtained by adjusting the energy density and pulse duration of the first laser. Further, the method further includes: adjusting the energy density and pulse duration of the first laser according to the thickness condition of the region so that the product of the energy density and pulse duration for irradiating each region is proportional to the thickness of the region, and the surface temperature of the region is lower than the thermal decomposition temperature of the perovskite material crystal used in the perovskite thin film.

[0010] Further, the wavelength of the second laser enables the absorption ratio of the perovskite material used in the perovskite thin film in the crystalline state and the amorphous state to the second laser to be within a first preset interval. This can be determined by analyzing the absorption rates of the same perovskite material in the crystalline state and the amorphous state for lasers with different wavelengths.

[0011] The scanning speed of the second laser corresponding to each of the regions and the scanning duration are adjusted according to the thickness of the region to ensure that the temperatures of various sub-crystals are approximately the same and the temperatures around various sub-crystals are approximately the same.

[0012] In order to eliminate defects between the perovskite thin film regions, further improve the uniformity of the perovskite thin film during the local area cooling process, and reduce grain segregation caused by thermal stress and thermal gradient, the preparation method further includes: after the second laser scanning is completed, the temperature of the surface of the perovskite thin film is gradually decreased to room temperature through a temperature control system to further improve the crystallization uniformity. Preferably, the temperature control system includes: an electric control hot plate and an electric control cooling system.

[0013] Preferably, the step of gradually decreasing the temperature of the surface of the perovskite thin film to room temperature through the temperature control system includes: dividing the temperature range from the highest temperature of the perovskite thin film after the second laser scanning to room temperature into several temperature intervals, and each temperature interval is cooled at a corresponding constant speed.

[0014] Using the technical solution provided by the present application to prepare the perovskite thin film can not only quickly realize the rapid annealing crystallization of the perovskite thin film, but also uniformly increase the size of the crystallization grains, improve the overall crystallization uniformity and reduce defects. In addition, the technical solution provided by the present application can also be applied to prepare the perovskite thin film required for high-performance flexible solar cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0016] Figure 1 It is a flowchart of the method for preparing the perovskite thin film provided by the present application in one embodiment.

[0017] Figure 2 It is a schematic diagram of generating seed crystallization on each region of the perovskite thin film by using the first laser in the method provided by the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the following will clearly and completely describe the technical solutions in the embodiments of this application with reference to the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are some, but not all, of the embodiments of this application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in this application without creative efforts shall fall within the scope of protection of this application.

[0019] As Figure 1 shown, the method for preparing a perovskite thin film provided in this application includes the following steps:

[0020] S1. Pre-deposit a perovskite thin film on a substrate. This step generally uses conductive glass or a silicon wafer as the substrate. After cleaning it, spin-coat a layer of perovskite precursor solution on it and let it stand for a period of time for deposition to achieve this.

[0021] S2. Divide the perovskite thin film into several regions according to its thickness. Since the thickness of the pre-deposited perovskite thin film is uneven, it is difficult to ensure the uniformity of crystallization when using uniform process parameters in subsequent processing. For this reason, it is necessary to divide different working regions according to the thickness of the pre-deposited perovskite thin film so that the same process parameters can be used for processing in the subsequent same region.

[0022] S3. For each of the regions, irradiate with a first laser of a corresponding wavelength according to its thickness to crystallize a uniformly distributed array of seed crystals. Since lasers of different wavelengths act effectively at different depths in the perovskite thin film, it is necessary to select a first laser of a corresponding wavelength for irradiation according to the thickness of different perovskite thin film regions. For example, short-wavelength lasers of 400 nm–700 nm can directly act on the material surface to improve the uniformity of the upper-layer thin film and are suitable for thinner perovskite layers; long-wavelength lasers of 800 nm–1200 nm can penetrate deep into the thin film to achieve overall crystallization and are suitable for thicker perovskite layers.

[0023] In some embodiments, the irradiating each of the regions with a first laser of a corresponding wavelength includes: for the regions with a thickness greater than a first threshold, irradiating with a first laser having a wavelength of 800 nm–1200 nm; for the regions with a thickness less than the first threshold, irradiating with a first laser having a wavelength of 400 nm–700 nm. Preferably, the first threshold is 200 nm.

[0024] As Figure 2As shown, for the perovskite thin film region A with a thickness greater than 200 nm, it is irradiated with a first laser with a wavelength of 900 nm. For the perovskite thin film region B with a thickness less than 200 nm, it is irradiated with a first laser with a wavelength of 600 nm (the sizes of the spots of the first laser are the same); by controlling the power density and irradiation duration of the first laser irradiating A and B, an array of uniformly distributed seed crystals 1 is generated.

[0025] Further, the method further includes: adjusting the energy density and pulse duration of the first laser according to the thickness condition of the region, so that the product of the energy density and pulse duration of irradiating each region is proportional to the thickness of the region, and the temperature on the surface of the region is lower than the thermal decomposition temperature of the perovskite material crystal used in the perovskite thin film. Since a thicker perovskite thin film region requires more energy to crystallize seed crystals of the same size, it is obtained by adjusting the energy density and pulse duration of the first laser.

[0026] S4. Use a second laser to scan each of the regions so that the surface temperature of each region is within a specified temperature range, and the temperature difference between the seed crystal region and the peripheral region therein is within a preset range.

[0027] When the temperature of the crystallization region of the seed crystal is higher than that of the peripheral non-crystallization region, the grains in the crystallization region will grow outward; by controlling the temperature difference between the crystallization region and the non-crystallization region of the perovskite thin film within a certain range (the surface temperature of the crystallization region is lower than the thermal decomposition temperature of the perovskite material crystal used in the perovskite thin film), the epitaxial growth process of the grains can be accelerated, cracks and other defects can be reduced, and at the same time, the crystallized part is not damaged.

[0028] Since the perovskite material has different absorption efficiencies for the same wavelength laser in the crystalline state and the amorphous state, when using the second laser with the same wavelength to scan the region, the temperature rise in the crystallization region is faster than that in the non-crystallization region. By analyzing the absorption rates of the crystalline state and the amorphous state of the same perovskite material for different wavelength lasers, the optimal wavelength of the second laser can be selected.

[0029] That is, select the wavelength of the second laser so that the absorption ratio of the perovskite material used in the perovskite thin film in the crystalline state and the amorphous state for the second laser is within a first preset interval.

[0030] Further, according to the thickness of different regions, controlling the scanning speed and scanning duration of the second laser corresponding to each region can ensure that the temperatures of various seed crystals in different regions are approximately the same, and the temperatures around various seed crystals are approximately the same. Thus, the overall crystallization uniformity of the perovskite thin film is ensured.

[0031] S5. Use a temperature control system to gradually lower the temperature on the surface of the perovskite thin film to room temperature.

[0032] In order to eliminate the defects between the perovskite thin film regions, further improve the uniformity of the perovskite thin film during the local area cooling process, and reduce the grain segregation caused by thermal stress and thermal gradient, the preparation method further includes: after the second laser scanning is completed, gradually lowering the temperature of the surface of the perovskite thin film to room temperature through a temperature control system to further improve the crystallization uniformity. The temperature control system may include: a temperature control system of an electric control hot plate and an electric control cooling system. The temperature drop of the substrate is finely adjusted through the temperature control system to ensure that the perovskite thin film experiences different cooling rates in different regions, and to avoid the thermal stress caused by too fast cooling.

[0033] Preferably, gradually lowering the temperature of the surface of the perovskite thin film to room temperature through the temperature control system includes: dividing the temperature range from the highest temperature of the perovskite thin film after the second laser scanning to room temperature into several temperature intervals, and each temperature interval is cooled at a corresponding constant speed.

[0034] The above are only the embodiments of the present application and are not used to limit the present application. For those skilled in the art, the technical solutions provided by the present application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a perovskite film, characterized in that: The method comprises: spin coating a perovskite film on a substrate, dividing the upper surface of the perovskite film into a number of corresponding regions according to the thickness of the perovskite film; for each of the regions, irradiating the region with a first laser of a corresponding wavelength according to the thickness of the corresponding perovskite film to crystallize a uniformly distributed seed crystal array, and then selecting a second laser with a corresponding wavelength to scan each region so that the surface temperature of each region reaches a specified temperature range, and the temperature difference between each seed crystal region and the periphery is within a preset range to expand the crystal size of the seed crystal and improve the crystal uniformity of the perovskite film; the spot size of the first laser corresponding to each of the regions is the same, and the wavelength of the second laser makes the absorption ratio of the perovskite material used in the perovskite film to the second laser in the crystalline state and the amorphous state within a first preset range.

2. The method according to claim 1, characterized in that The method further comprises: after the second laser scanning is completed, gradually lowering the temperature of the surface of the perovskite film to room temperature through a temperature control system to further improve the uniformity of crystallization.

3. The method according to claim 1, characterized in that For each of the regions, a first laser with a corresponding wavelength is used for irradiation according to the thickness of the corresponding perovskite film, including: for a region where the thickness of the perovskite film is greater than a first threshold, a first laser with a wavelength of 800nm-1200nm is used for irradiation; for a region where the thickness of the perovskite film is less than the first threshold, a first laser with a wavelength of 400nm-700nm is used for irradiation.

4. The method according to claim 3, characterized in that The first threshold is 200 nm.

5. The method according to claim 3, characterized in that The method also includes: adjusting the energy density and pulse duration of the first laser according to the thickness of the area, so that the product of the energy density and pulse duration irradiating each area is proportional to the thickness of the perovskite film corresponding to the area, and the temperature of the surface of the area is lower than the thermal decomposition temperature of the perovskite material crystal used in the perovskite film.

6. The method according to claim 1, characterized in that The specified temperature range is greater than or equal to the crystallization temperature of the perovskite material, and less than the thermal decomposition temperature of the perovskite material crystal.

7. The method according to claim 1, characterized in that The scanning speed and scanning duration of the second laser corresponding to each of the regions are adjusted according to the thickness of the perovskite film corresponding to the region.

8. The method according to claim 2, characterized in that The temperature control system includes an electrically controlled hot plate subsystem and an electrically controlled cooling subsystem.

9. The method according to claim 2, characterized in that The temperature control system is used to gradually reduce the surface temperature of the perovskite film to room temperature, including: dividing the temperature range from the highest temperature of the perovskite film after the second laser scanning to room temperature into several temperature intervals, and each temperature interval is cooled at a corresponding constant speed.

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