Method for preparing perovskite thin film

Abstract

The invention provides a method for preparing a perovskite thin film, and the method comprises the steps: dividing the surface of a perovskite thin film spin-coated on a substrate into a plurality of corresponding regions according to the thickness, carrying out the irradiation of each region through employing a first laser with a corresponding wavelength and a fixed light spot size, so as to generate a uniformly-distributed seed crystal array, and scanning each region by adopting second laser with corresponding wavelength to enable the surface temperature of each region to reach a specified temperature range, and ensuring that the temperature difference between each seed crystal region and the periphery is within a preset range to enlarge the crystal size of the seed crystal and improve the crystal uniformity of the perovskite thin film. And finally, the surface temperature of the perovskite thin film is slowly reduced through a temperature control system, so that the crystallization uniformity is further improved, and grain segregation caused by thermal stress and thermal gradient is reduced. According to the technical scheme provided by the invention, the uniformity and the size moderation of the perovskite thin film crystal can be considered, the defects are reduced, and the photoelectric conversion and transmission performance of the perovskite cell is improved.

Description

Technical Field

[0001] The present application relates to the field of production and manufacturing of perovskite cells or perovskite films, and specifically to a method for preparing a perovskite film. Background Art

[0002] Solar cells are photoelectric semiconductor sheets that use sunlight to generate electricity directly. They convert light energy into electrical energy directly through the photoelectric effect or photochemical effect. They have the advantages of high reliability, long life, and high conversion efficiency. As the third generation of new concept solar cells, perovskite solar cells have the advantages of high photoelectric conversion efficiency, low cost, and flexible processing. They have developed rapidly in recent years and are gradually comparable to silicon cells, approaching the level of commercial application.

[0003] Annealing is a key process in the manufacture of perovskite cells. It is used to improve the crystallinity and structural stability of perovskite films, thereby improving the photoelectric properties of perovskite films and the efficiency of perovskite cells. At present, the annealing process of perovskite films mainly adopts thermal annealing, that is, by placing the perovskite film in a high temperature environment and undergoing a long period of heat treatment (about 1 hour) to promote the improvement of its crystallization quality. However, this thermal annealing method has the disadvantages of being time-consuming and energy-consuming, difficult to accurately control the crystallization process (resulting in excessively large crystal size and difficulty in controlling the uniformity of crystallization), leading to unstable film performance, and not 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 films in a short time. Under laser irradiation, the surface material of the perovskite film melts rapidly, and in the subsequent cooling process, a crystalline film naturally grows in the liquid phase epitaxy of the molten layer, thereby reconstructing the crystal structure of the molten layer. Compared with thermal annealing, the crystallization process of laser annealed perovskite films is faster, but due to the size of the laser spot, the crystal grain size of the perovskite film is relatively small. In addition, how to achieve uniform crystallization over a large area while maintaining rapid annealing is still a difficulty in laser annealing. Summary of the invention

[0005] In view of the shortcomings of the above-mentioned perovskite film annealing method, this application constructs a laser annealing control process and combines the advantages of thermal annealing to achieve a technical solution for rapid annealing of perovskite films, uniform crystallization of large areas and large sizes (grain sizes) and few defects. The previous stage of laser annealing provides a basis for subsequent low-temperature thermal annealing, so that the technical solution provided by this application is suitable for preparing perovskite films required for flexible solar cells.

[0006] The technical solution provided in the present application is implemented as a method for preparing a perovskite film. The method comprises: dividing the surface of the perovskite film spin-coated on the substrate into a plurality of regions according to thickness. For each of the regions, a first laser of a corresponding wavelength is used to irradiate according to its thickness to crystallize a uniformly distributed array of seed crystals. 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 crystal size of the seed crystal and improve the crystallization uniformity of the perovskite 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, but less than the thermal decomposition temperature of the crystallization of the perovskite material.

[0007] The first wavelength laser is used in advance to produce uniformly distributed seed crystals, and then the second laser is used to scan each area to increase the size of each seed crystal crystal area, which can not only increase the size of the laser annealing crystal of the perovskite film, but also ensure the uniformity of the crystallization. The increase in the grain size of the perovskite film is conducive to the transmission of electron-hole pairs, and the improvement of the uniformity of crystallization and the reduction of defects are conducive to improving the photoelectric conversion efficiency of the perovskite film.

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

[0009] The thicker the perovskite film area, the more energy is needed to crystallize the seed crystal of the same size, which can be obtained by adjusting the energy density and pulse duration of the first laser. Furthermore, 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 of each area is proportional to the thickness of 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.

[0010] Furthermore, the wavelength of the second laser makes the absorption ratio of the second laser by the perovskite material used in the perovskite film in the crystalline state and the amorphous state within a first preset range. This can be determined by analyzing the absorption rate of the crystalline and amorphous states of the same perovskite material to lasers of different wavelengths.

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

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

[0013] Preferably, gradually lowering the surface temperature of the perovskite film to room temperature by a temperature control system comprises: dividing the temperature range from the highest temperature of the perovskite film after the second laser scan to room temperature into several temperature intervals, and cooling each temperature interval at a corresponding constant speed.

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

[0015] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0016] Figure 1 A flow chart of a method for preparing a perovskite film provided in the present application in one embodiment.

[0017] Figure 2 This is a schematic diagram of using a first laser to generate seed crystals in various regions on a perovskite film in the method provided in the present application. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0019] like Figure 1 As shown, the method for preparing a perovskite film provided in the present application comprises the following steps: S1. Pre-deposit a perovskite film on a substrate. This step is usually achieved by using a conductive glass or silicon wafer as a substrate, cleaning it, and then spin-coating a layer of perovskite precursor solution on it and leaving it for a period of time for deposition.

[0020] S2. Divide the perovskite film into several regions according to thickness. Since the thickness of the pre-deposited perovskite film is uneven, it is difficult to ensure the uniformity of crystallization using uniform process parameters in subsequent processing. Therefore, it is necessary to divide different operating areas according to the thickness of the pre-deposited perovskite film so that the same area can be processed with the same process parameters in the subsequent processing.

[0021] S3. For each of the regions, a first laser of a corresponding wavelength is used for irradiation according to its thickness to crystallize a uniformly distributed array of seed crystals. Since lasers of different wavelengths effectively act on the perovskite film at different depths, it is necessary to select a first laser of a corresponding wavelength for irradiation according to the thickness of different perovskite film regions. For example, a short-wavelength laser of 400nm-700nm can act directly on the surface of the material to improve the uniformity of the upper film, which is suitable for thinner perovskite layers; a long-wavelength laser of 800nm-1200nm can penetrate deep into the film to achieve overall crystallization, which is suitable for thicker perovskite layers.

[0022] In some embodiments, the first laser irradiation of each region with a corresponding wavelength includes: for the region with a thickness greater than a first threshold, the first laser irradiation with a wavelength of 800nm-1200nm, and for the region with a thickness less than the first threshold, the first laser irradiation with a wavelength of 400nm-700nm. Preferably, the first threshold is 200nm.

[0023] like Figure 2As shown, for the perovskite film area A with a thickness greater than 200nm, a first laser with a wavelength of 900nm is used for irradiation, and for the perovskite film area B with a thickness less than 200nm, a first laser with a wavelength of 600nm is used for irradiation (the size of the first laser spot is the same); by controlling the power density and irradiation time of the first laser irradiating A and B, an array of uniformly distributed seed crystals 1 is generated.

[0024] Furthermore, the method further includes: adjusting the energy density and pulse duration of the first laser according to the thickness of the region, so that the product of the energy density and pulse duration of 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 film. Since the thicker the perovskite film region is, the more energy is required to crystallize the seed crystal of the same size, which is obtained by adjusting the energy density and pulse duration of the first laser.

[0025] S4. Scan each of the regions using a second laser 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 is within a preset range.

[0026] When the temperature of the crystalline zone of the seed crystal is higher than that of the peripheral non-crystalline zone, the grains in the crystalline zone will grow outward; by controlling the temperature difference between the crystalline zone and the non-crystalline zone of the perovskite film within a certain range (the surface temperature of the crystalline zone is lower than the thermal decomposition temperature of the perovskite material crystal used in the perovskite film), the epitaxial growth process of the grains can be accelerated, cracks and other defects can be reduced, and the crystallized parts will not be destroyed.

[0027] Since the absorption efficiency of the perovskite material to the same wavelength laser is different in the crystalline state and the amorphous state, when the second laser of the same wavelength is used to scan the region, the temperature rise of the crystalline region is faster than that of the amorphous region. The optimal wavelength of the second laser can be selected by analyzing the absorption rate of the crystalline and amorphous states of the same perovskite material to lasers of different wavelengths.

[0028] That is, the wavelength of the second laser is selected so that the absorption ratio of the second laser by the perovskite material used in the perovskite film in the crystalline state and the amorphous state is within a first preset range.

[0029] Furthermore, 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 temperature of various sub-crystals in different regions is roughly the same and the temperature of the periphery of various sub-crystals is roughly the same, thereby ensuring the uniformity of the overall crystallization of the perovskite film.

[0030] S5. Use a temperature control system to gradually reduce the temperature of the surface of the perovskite film to room temperature.

[0031] In order to eliminate defects between the regions of the perovskite film, further improve the uniformity of the cooling process of the perovskite film in the local region, and reduce the grain segregation caused by thermal stress and thermal gradient, the preparation method also includes: after the second laser scan is completed, the temperature of the surface of the perovskite film is gradually reduced to room temperature through a temperature control system to further improve the uniformity of crystallization. The temperature control system may include: a temperature control system of an electrically controlled hot plate and an electrically controlled cooling system. The temperature control system is used to finely adjust the substrate temperature drop to ensure that the perovskite film experiences different cooling rates in different regions to avoid thermal stress caused by excessive cooling.

[0032] Preferably, gradually lowering the surface temperature of the perovskite film to room temperature by a temperature control system comprises: dividing the temperature range from the highest temperature of the perovskite film after the second laser scan to room temperature into several temperature intervals, and cooling each temperature interval at a corresponding constant speed.

[0033] The above description is only an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the technical solutions provided by the present application may be subject to various modifications and changes. 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: dividing the surface of the perovskite film spin-coated on the substrate into a number of corresponding regions according to thickness, irradiating each region with a first laser of a corresponding wavelength according to its thickness 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 region is the same.

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 The first laser irradiation with a corresponding wavelength is used for each of the regions, including: for the regions with a thickness greater than the first threshold, the first laser irradiation with a wavelength of 800nm-1200nm; for the regions with a thickness less than the first threshold, the first laser irradiation with a wavelength of 400nm-700nm.

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 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 6, characterized in that 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 fall within a first preset range.

8. The method according to claim 7, characterized in that The scanning speed and duration of the second laser corresponding to each area are adjusted according to the thickness of the area.

9. 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.

10. 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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