A method for improving the quality of cadmium selenide thin films by annealing after hot pressing

Abstract

The present invention belongs to the technical field of semiconductor thin film annealing treatment, and discloses a method for improving the quality of cadmium selenide thin film by annealing after hot pressing. This method takes the cadmium selenide thin film as the treatment object, directly adheres the cadmium selenide thin film to a heat-conducting substrate in an atmosphere environment of a protective gas, applies pressure to the cadmium selenide thin film at the same time, and performs heating annealing under the condition of maintaining the pressure; the annealed cadmium selenide thin film obtained thereby has larger grain size and smaller film porosity compared with the cadmium selenide thin film before treatment. By applying additional pressure to the thin film during annealing and cooperating with heating to perform post-treatment on the cadmium selenide thin film, the present invention can improve the problems of porosity and high roughness of the current cadmium selenide thin film.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor thin film annealing treatment. More specifically, it relates to a method for improving the quality of cadmium selenide thin films by post-annealing under hot pressing. By using this post-annealing process, the quality of cadmium selenide thin films can be effectively improved, the grain size in the thin films can be increased, and the porosity of the thin films can be reduced. Background Art

[0002] Cadmium selenide (CdSe) is a II-VI group direct transition semiconductor material, which has an appreciable absorption coefficient (>10 5 cm -1 ) in the short-wave and visible light bands, and has excellent optoelectronic properties and extremely stable physical and chemical properties. And theoretical calculations show that cadmium selenide is very competitive in the application of silicon-based tandem solar cells.

[0003] Currently, cadmium selenide thin films are mostly prepared by vacuum methods or solution methods. Among them, the vacuum methods mainly include close space sublimation (CSS) and rapid thermal evaporation (RTE), etc. Through such high-speed vapor deposition techniques, large-area uniform cadmium selenide polycrystalline thin films can be prepared. However, due to problems such as temperature crosstalk and too fast deposition rate, the obtained polycrystalline thin films often have problems such as porosity and large roughness. These problems will seriously affect the preparation of subsequent high-performance solar cell devices. For example, the holes in the thin film will directly cause the short-circuit phenomenon of the solar cell device and it cannot work normally; the large roughness of the thin film will also cause the functional layer in contact with it to be difficult to completely cover the surface of the thin film, resulting in the device being unable to work normally.

[0004] Therefore, how to find a suitable post-treatment method to solve the problems of porosity and high roughness of existing cadmium selenide thin films has become the main problem in the preparation of high-efficiency cadmium selenide thin film solar cells.

[0005] Pressurized annealing, although it is already a known annealing means in existing thin film preparation technologies (such as Chinese patent document CN102969241), most of the related research is aimed at ceramic materials, III-V group semiconductors, perovskites, copper indium gallium selenide, etc.; for binary selenides, due to the high saturated vapor pressure characteristic of selenium (Se), selenium back-evaporation is likely to occur during the annealing process of selenide thin films, thereby introducing selenium vacancies and deteriorating the quality of the thin films. This makes the conventional pressurized annealing method not directly applicable to the post-treatment of selenide thin films. Secondly, for cadmium selenide thin films prepared by rapid thermal evaporation, there are problems of large roughness and porosity, and the crystallization morphology often needs to be further improved. Summary of the Invention

[0006] In view of the above deficiencies or improvement requirements of the prior art, the purpose of the present invention is to provide a method for improving the quality of cadmium selenide thin films by annealing after hot pressing. By applying additional pressure to the thin film during annealing and cooperating with heating to perform post-treatment on the cadmium selenide thin film, the problems of porosity and high roughness of the current cadmium selenide thin film can be improved. The cadmium selenide thin film annealed by the method of the present invention has a microscopic morphology with large grain size and low grain boundary density. The obtained cadmium selenide thin film can especially have the characteristics of being dense and pore-free, having a flat surface, high crystallinity, and single orientation. Moreover, the method of the present invention has good reliability and high repeatability in improving the film quality.

[0007] To achieve the above object, according to the present invention, a method for improving the quality of cadmium selenide thin films by annealing after hot pressing is provided, which is characterized in that the method takes the cadmium selenide thin film as the treatment object, directly adheres the cadmium selenide thin film to a heat-conducting substrate in an atmosphere environment of a protective gas, and simultaneously applies pressure to the cadmium selenide thin film, and performs heat annealing while maintaining the pressure; the annealed cadmium selenide thin film obtained thereby has a larger grain size and a smaller film porosity compared with the cadmium selenide thin film before treatment;

[0008] Among them, no chemical reaction occurs between the heat-conducting substrate and the cadmium selenide thin film.

[0009] As a further preference of the present invention, the application of pressure is specifically achieved by placing an object above the cadmium selenide thin film, and the pressure on the cadmium selenide thin film is achieved through the gravitational action of the object.

[0010] As a further preference of the present invention, the object can provide a pressure of (1.6 - 6.4)×10 5 Pa to the entire cadmium selenide thin film.

[0011] As a further preference of the present invention, the protective gas is nitrogen or an inert gas; preferably, the inert gas is argon.

[0012] As a further preference of the present invention, the heat annealing is carried out at a temperature of 480°C - 540°C.

[0013] As a further preference of the present invention, the annealing time of the heat annealing is 0.5 h - 2 h.

[0014] Through the above technical solution conceived by the present invention, compared with the prior art, by using pressure-assisted thermal annealing for post-treatment of cadmium selenide thin films, the grain size of the cadmium selenide thin film after annealing has been significantly improved compared to that of the cadmium selenide thin film before annealing, thereby reducing the crystal defects caused by grain boundaries. The grain size of the cadmium selenide thin film annealed by the method of the present invention is larger than that of the cadmium selenide thin film before annealing, and the porosity of the cadmium selenide thin film after annealing is smaller than that of the cadmium selenide thin film before annealing. At the same time, this method also has sufficient reliability and repeatability, and can effectively improve the quality of cadmium selenide thin films.

[0015] In the conventional annealing process, selenium vacancies are likely to appear in the cadmium selenide thin film due to the selenium vapor pressure characteristics during annealing. Moreover, the roughness of the cadmium selenide thin film often cannot be improved by annealing. In view of this, the present invention proposes a pressure-assisted annealing scheme for cadmium selenide thin films. In the method of the present invention, by using an inert heat-conducting substrate to fit with the cadmium selenide thin film to achieve hermetic sealing, and cooperating with a heavy object to apply pressure (especially capable of providing a pressure of (1.6 - 6.4)×10 5 Pa; taking a thin film with a surface area of 1 cm 2 as an example, the pressure applied by the heavy object to the whole thin film is 16 - 64 N), hermetic hot-pressing annealing is carried out to avoid the generation of selenium vacancies and solve the problems of film roughness and pores at the same time. Among them, the measure of hermetic sealing can inhibit the evaporation of selenium, thereby inhibiting the generation of selenium vacancies on the film surface; cooperating with pressure-assisted annealing, the cadmium selenide polycrystalline thin film can undergo grain fusion under the action of pressure and temperature, and a high-quality cadmium selenide thin film with a flat and pore-free surface can be effectively obtained. Compared with the conventional isostatic annealing method in the prior art (such as Chinese patent document CN102969241), the pressure-assisted annealing scheme proposed by the present invention can simultaneously achieve the inhibition of defect generation, the improvement of film roughness, and the improvement of film quality.

[0016] The present invention preferably performs heating annealing treatment at 480 - 540 °C, which can ensure that Cd and Se atoms have sufficient energy to overcome the migration barrier at this temperature, so that atoms migrate locally, and macroscopically, it is manifested as the elimination of pores; and it can avoid the situation of atoms escaping outward in the cadmium selenide thin film at a higher temperature.

[0017] The present invention is particularly applicable to cadmium selenide thin films prepared by rapid thermal evaporation or vapor phase transfer deposition (initial roughness ≤ 800 nm). The cadmium selenide thin films obtained by the post-annealing process of the present invention have a dense and pore-free microtopography, and the roughness of the annealed cadmium selenide thin films is less than 100 nm. Specifically, the number of grain boundaries in the annealed cadmium selenide thin films is less than that in the cadmium selenide thin films before annealing, and the grain size of the annealed cadmium selenide thin films is larger than that of the cadmium selenide thin films before annealing. Taking the CdSe thin films prepared by rapid thermal evaporation as an example, the CdSe thin films prepared by rapid thermal evaporation exhibit a (103) preferred orientation and there are weak signals of (002) and (102) orientations. After annealing treatment according to the present invention, the peak value of the (103) diffraction peak of the annealed cadmium selenide thin films is greater than that of the (103) diffraction peak of the cadmium selenide thin films before annealing, and the peak intensities of some other orientations are weakened or disappear, proving that there is a recrystallization process during the annealing stage, resulting in an orientation transformation. After annealing, the cadmium selenide thin films have a stronger (103) preferred orientation and better film quality (as shown in Figure 3 described later); the relative change value of the Se atomic ratio in the annealed cadmium selenide thin films and the cadmium selenide thin films before annealing is less than or equal to 2%.

[0018] In summary, the annealed cadmium selenide thin films obtained by the present invention are dense and pore-free, with an increased grain size, fewer grain boundaries and high quality. At the same time, the hot pressing and post-annealing method provided by the present invention has good repeatability. The high-quality cadmium selenide thin films obtained by the present invention are beneficial to the preparation of high-quality cadmium selenide thin film solar cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic diagram of the hot pressing and post-annealing device for cadmium selenide thin films used in each embodiment of the present invention.

[0020] Figure 2 is the SEM image of the cadmium selenide thin film before and after annealing in Example 1; among them, Figure 2 the (a) in Figure 2 corresponds to before annealing, and

[0021] Figure 3 the (b) in Figure 3 corresponds to after annealing. Figure 3 corresponds to after annealing.

[0022] Figure 4 is the X-ray diffraction pattern of the cadmium selenide thin film after annealing in Example 2; among them, Figure 4 the (a) in Figure 4 corresponds to before annealing, and

[0023] Figure 5It is the SEM images of the cadmium selenide thin films without pressure and with pressure annealing in Example 5; among them, Figure 5 in (a) is the cadmium selenide thin film without pressure annealing, Figure 5 in (b) is the cadmium selenide thin film with pressure annealing.

[0024] Figure 6 It is the current-voltage curve diagram of the solar cell devices corresponding to the cadmium selenide thin films without treatment, with pressure annealing and without pressure annealing in Example 5. Detailed implementation manners

[0025] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0026] In the method for improving the quality of cadmium selenide thin films by annealing after hot pressing in the present invention, hot annealing is performed on the cadmium selenide thin films under pressure conditions; the cadmium selenide thin films after annealing have a flat and dense microscopic morphology;

[0027] In principle, the present invention has no particular limitation on the change value of selenium atoms in the cadmium selenide thin films after annealing and before annealing. In order to improve the pore problem of the cadmium selenide thin films after annealing, reduce the grain boundary density of the cadmium selenide thin films, reduce the overall roughness of the cadmium selenide thin films, and improve the quality of the cadmium selenide thin films, the relative change value of the selenium atom proportion in the cadmium selenide thin films after annealing and before annealing is preferably less than or equal to 2%, more preferably less than or equal to 1%.

[0028] In principle, the present invention has no particular limitation on the thickness of the cadmium selenide thin films. Those skilled in the art can select and adjust according to the actual situation, raw material situation and product requirements. In order to improve the pore problem of the cadmium selenide thin films after annealing, reduce the grain boundary density of the cadmium selenide thin films, reduce the overall roughness of the cadmium selenide thin films, and improve the quality of the cadmium selenide thin films, the thickness of the cadmium selenide thin films is preferably 1000 nm to 3000 nm, more preferably 1800 nm to 2500 nm.

[0029] When the method of the present invention is specifically operated, as Figure 1 shown, in the atmosphere of a protective gas, the cadmium selenide thin film is placed upside down on an inert heat-conducting substrate, a heavy object is placed above the thin film to press the thin film, and heating annealing is performed through a heating device.

[0030] In principle, there is no special limitation on the protective gas atmosphere of the present invention, as long as it shows inertness with CdSe. Those skilled in the art can make selections and adjustments according to the actual situation, raw material situation, and product requirements. In order to improve the pore problem of the cadmium selenide thin film after annealing, reduce the grain boundary density of the cadmium selenide thin film, reduce the overall roughness of the cadmium selenide thin film, and improve the quality of the cadmium selenide thin film, the protective gas atmosphere preferably includes inert gases such as nitrogen or argon. Additionally, in the annealing reaction chamber, the pressure of the protective gas can be normal pressure.

[0031] In principle, there is no special limitation on the inert heat-conducting substrate of the present invention, as long as it does not react with the cadmium selenide thin film and has high thermal conductivity (similar to the requirements of conventional annealing, the substrate needs to have good thermal conductivity to ensure a uniform temperature field during the annealing process). Those skilled in the art can make selections and adjustments according to the actual situation, raw material situation, and product requirements. In order to improve the pore problem of the cadmium selenide thin film after annealing, reduce the grain boundary density of the cadmium selenide thin film, reduce the overall roughness of the cadmium selenide thin film, and improve the quality of the cadmium selenide thin film, the inert heat-conducting substrate is preferably aluminum nitride or a silicon wafer with high thermal conductivity, and more preferably a polished silicon wafer with high thermal conductivity.

[0032] In principle, there is no special limitation on the article applying gravity to the thin film of the present invention. Those skilled in the art can make selections and adjustments according to the actual situation, raw material situation, and product requirements. In order to improve the pore problem of the cadmium selenide thin film after annealing, reduce the grain boundary density of the cadmium selenide thin film, reduce the overall roughness of the cadmium selenide thin film, and improve the quality of the cadmium selenide thin film, the mass of the article applying gravity can preferably provide a pressure of (1.6 - 6.4)×10 5 Pa to the entire cadmium selenide thin film, and more preferably can provide a pressure of (3 - 5)×10 5 Pa.

[0033] In principle, there is no special limitation on the annealing equipment of the present invention. Those skilled in the art can make selections and adjustments according to the actual situation, raw material situation, and product requirements. In order to improve the pore problem of the cadmium selenide thin film after annealing, reduce the grain boundary density of the cadmium selenide thin film, reduce the overall roughness of the cadmium selenide thin film, and improve the quality of the cadmium selenide thin film, the annealing equipment is preferably a hot stage with a uniformly heat-conducting substrate.

[0034] In principle, there is no special limitation on the annealing time of the present invention. Those skilled in the art can make selections and adjustments according to the actual situation, raw material situation, and product requirements. In order to improve the pore problem of the cadmium selenide thin film after annealing, reduce the grain boundary density of the cadmium selenide thin film, reduce the overall roughness of the cadmium selenide thin film, and improve the quality of the cadmium selenide thin film, the annealing time is preferably 0.5 h - 2 h, and more preferably 1 h - 1.5 h.

[0035] In principle, there is no special limitation on the annealing temperature of the present invention, and those skilled in the art can select and adjust it according to the actual situation, raw material situation, and product requirements. In order to improve the problem of holes in the cadmium selenide thin film after annealing, reduce the grain boundary density of the cadmium selenide thin film, reduce the overall roughness of the cadmium selenide thin film, and improve the quality of the cadmium selenide thin film, the annealing temperature is preferably 480°C to 540°C, more preferably 520°C to 540°C.

[0036] The following are specific examples:

[0037] Example 1

[0038] 1) In this example, a cadmium selenide thin film prepared by rapid thermal evaporation (RTE) was used, and a 2.1-μm cadmium selenide thin film was deposited on an FTO (fluorine-doped tin oxide) substrate. The deposition process was that the deposition pressure was 1 Pa, the deposition time was 100 s, the deposition substrate temperature was 480°C, and the deposition evaporation source temperature was 820°C. After deposition, it was naturally cooled and the cadmium selenide thin film was placed in a dry environment for storage.

[0039] 2) The silicon wafer was ultrasonically cleaned successively with dishwashing liquid, acetone, isopropanol, ethanol, and deionized water. Each step required 30 minutes of ultrasonic cleaning. Finally, the cleaned silicon wafer was placed in clean ethanol for storage, and after drying the surface of the silicon wafer with nitrogen (N2), it was used to obtain a clean silicon wafer substrate.

[0040] 3) The cadmium selenide thin film to be annealed, the clean silicon wafer, and the hot stage were placed in a nitrogen glove box. The cadmium selenide thin film was cut into an annealing sample with a size of 1.25 * 2.5 cm, and then the cadmium selenide thin film was directly attached to the silicon wafer, that is, the cadmium selenide thin film was placed upside down on the silicon wafer. Subsequently, a heat-resistant weight of 5 kg was placed above the cadmium selenide thin film, and the cadmium selenide thin film was annealed using the hot stage at an annealing temperature of 540°C for 2 h, and then cooled naturally.

[0041] The cadmium selenide thin film after annealing and the cadmium selenide thin film before annealing obtained in Example 1 were analyzed by scanning electron microscopy (SEM).

[0042] Figure 2 Shown are the SEM images of the surface microtopography changes of the cadmium selenide thin film after the annealing process in Example 1 and the cadmium selenide thin film before annealing. Among them, Figure 2 (a) in it is the surface microtopography of the cadmium selenide thin film before annealing, Figure 2 (b) in it is the surface microtopography of the cadmium selenide thin film after annealing.

[0043] According to Figure 2 the results shown, it can be seen that the cadmium selenide thin film processed in this example has significantly improved the hole phenomenon compared with the original sample (the cadmium selenide thin film before annealing), the grain boundary density of the thin film has decreased, and the surface flatness has been improved.

[0044] Example 2

[0045] 1) In this example, cadmium selenide thin films were prepared by rapid thermal evaporation (RTE). A 2.1-μm cadmium selenide thin film was deposited on an FTO substrate. The deposition process was carried out at a deposition pressure of 1 Pa, a deposition time of 100 s, a deposition substrate temperature of 480 °C, and a deposition evaporation source temperature of 820 °C. After deposition, it was naturally cooled and the cadmium selenide thin film was placed in a dry environment for storage.

[0046] 2) The silicon wafer was ultrasonically cleaned successively with dishwashing liquid, acetone, isopropanol, ethanol, and deionized water. Each step required 30 minutes of ultrasonic cleaning. Finally, the cleaned silicon wafer was placed in clean ethanol for storage. After drying the surface of the silicon wafer with nitrogen (N2), it was used to obtain a clean silicon wafer substrate.

[0047] 3) The cadmium selenide thin film to be annealed, the clean silicon wafer, and the hot stage were placed in a nitrogen glove box. The cadmium selenide thin film was cut into annealing samples with a size of 1.25 * 2.5 cm. Then, the cadmium selenide thin film was directly attached to the silicon wafer, that is, the cadmium selenide thin film was placed upside down on the silicon wafer. Subsequently, a heat-resistant weight of 5 kg was placed above the cadmium selenide thin film, and the cadmium selenide thin film was annealed using the hot stage. The annealing temperature was 480 °C, maintained for 2 h, and then naturally cooled.

[0048] X-ray diffraction (XRD) analysis was performed on the annealed cadmium selenide thin film and the cadmium selenide thin film before annealing obtained in Example 2. The XRD analysis results are as Figure 3 shown. According to Figure 3 the results shown, the cadmium selenide thin film prepared in this example has better crystallinity.

[0049] Example 3

[0050] 1) In this example, cadmium selenide thin films were prepared by rapid thermal evaporation (RTE). A 2.1-μm cadmium selenide thin film was deposited on an FTO substrate. The deposition process was carried out at a deposition pressure of 1 Pa, a deposition time of 100 s, a deposition substrate temperature of 480 °C, and a deposition evaporation source temperature of 820 °C. After deposition, it was naturally cooled and the cadmium selenide thin film was placed in a dry environment for storage.

[0051] 2) The silicon wafer was ultrasonically cleaned successively with dishwashing liquid, acetone, isopropanol, ethanol, and deionized water. Each step required 30 minutes of ultrasonic cleaning. Finally, the cleaned silicon wafer was placed in clean ethanol for storage. After drying the surface of the silicon wafer with nitrogen (N2), it was used to obtain a clean silicon wafer substrate.

[0052] 3) Place the cadmium selenide film to be hot-pressed and annealed, a clean silicon wafer, and a hot stage in a nitrogen glove box. Cut the cadmium selenide film into annealing samples with dimensions of 1.25 * 2.5 cm, and then directly attach the cadmium selenide film to the silicon wafer, that is, place the cadmium selenide film upside down on the silicon wafer. Subsequently, place a heat-resistant weight of 5 kg above the cadmium selenide film, and use the hot stage to anneal the cadmium selenide film. The annealing temperature is 480 °C, hold for 0.5 h, and cool naturally.

[0053] Perform scanning electron microscope (SEM) analysis on the film with pressure annealing and the unannealed film in Example 3.

[0054] Figure 4 The SEM images showing the cross-sectional microtopography changes of the cadmium selenide film after the annealing process in Example 3 and before annealing are shown. Among them, Figure 4 (a) in it is the cross-sectional microtopography of the cadmium selenide film before annealing, Figure 4 (b) in it is the cross-sectional microtopography of the cadmium selenide film after annealing.

[0055] According to Figure 4 the results shown, the roughness of the cadmium selenide film processed in this example is significantly reduced compared to the original sample (the cadmium selenide film before annealing).

[0056] Example 4

[0057] 1) In this example, a cadmium selenide film prepared by rapid thermal evaporation (RTE) is used, and a 2.1-μm cadmium selenide film is deposited on an FTO substrate. The deposition process is that the deposition pressure is 1 Pa, the deposition time is 100 s, the deposition substrate temperature is 480 °C, and the deposition evaporation source temperature is 820 °C. After deposition, cool naturally and store the cadmium selenide film in a dry environment.

[0058] 2) Ultrasonically clean the silicon wafer successively with dishwashing liquid, acetone, isopropyl alcohol, ethanol, and deionized water. Each step requires ultrasonic cleaning for 30 minutes. Finally, place the cleaned silicon wafer in clean ethanol for storage, and blow dry the surface of the silicon wafer with nitrogen (N2) before use to obtain a clean silicon wafer substrate.

[0059] 3) Place the cadmium selenide film to be hot-pressed and annealed, a clean silicon wafer, and a hot stage in a nitrogen glove box. Cut the cadmium selenide film into annealing samples with dimensions of 1.25 * 2.5 cm, and then directly attach the cadmium selenide film to the silicon wafer, that is, place the cadmium selenide film upside down on the silicon wafer. Subsequently, place a heat-resistant weight of 20 kg above the cadmium selenide film, and use the hot stage to anneal the cadmium selenide film. The annealing temperature is 480 °C, hold for 0.5 h, and cool naturally.

[0060] In this embodiment, due to the adoption of a greater external pressure, it is more conducive to improving the surface roughness of the thin film and enhancing the thin film quality.

[0061] Example 5

[0062] 1) In this embodiment, cadmium selenide thin films prepared by rapid thermal evaporation (RTE) are used. A 2.1-μm cadmium selenide thin film is deposited on an FTO substrate. The deposition process is as follows: the deposition pressure is 1 Pa, the deposition time is 100 s, the deposition substrate temperature is 450 °C, and the deposition evaporation source temperature is 800 °C. After deposition, it is naturally cooled and the cadmium selenide thin film is placed in a dry environment for storage. Considering that this embodiment is for comparing cadmium selenide thin films after unpressurized annealing, cadmium selenide thin films after pressurized annealing, and cadmium selenide thin films without treatment, therefore, 3 thin film samples of the same preparation batch are obtained in this step.

[0063] 2) The silicon wafer is ultrasonically cleaned successively with dishwashing liquid, acetone, isopropanol, ethanol, and deionized water. Each step requires 30 minutes of ultrasonic cleaning. Finally, the cleaned silicon wafer is placed in clean ethanol for storage. After drying the surface of the silicon wafer with nitrogen (N2), it is used, so as to obtain a clean silicon wafer substrate.

[0064] 3) Annealing treatment: In order to compare and analyze the influence of annealing conditions on the thin film, 2 thin film samples are taken, and the following two parallel steps 3-1) and 3-2) are set respectively:

[0065] 3-1) Place the unpressurized annealed cadmium selenide thin film, the clean silicon wafer, and the hot stage in a nitrogen glove box. Then directly attach the cadmium selenide thin film to the silicon wafer, that is, place the cadmium selenide thin film upside down on the silicon wafer. Subsequently, without using a heavy object, directly anneal the cadmium selenide thin film using the hot stage. The annealing temperature is 480 °C, keep it for 2 h, and cool it naturally. The corresponding sample is denoted as the cadmium selenide thin film after unpressurized annealing.

[0066] 3-2) Place the cadmium selenide thin film to be pressurized annealed, the clean silicon wafer, and the hot stage in a nitrogen glove box. Then directly attach the cadmium selenide thin film to the silicon wafer, that is, place the cadmium selenide thin film upside down on the silicon wafer. Subsequently, place a heat-resistant heavy object with a mass of 5 kg above the cadmium selenide thin film, and anneal the cadmium selenide thin film using the hot stage. The annealing temperature is 480 °C, keep it for 2 h, and cool it naturally. The corresponding sample is denoted as the cadmium selenide thin film after pressurized annealing.

[0067] Perform scanning electron microscope (SEM) analysis on the cadmium selenide thin film after unpressurized annealing and the cadmium selenide thin film after pressurized annealing obtained in Example 5. The results are as Figure 5 shown. It is not difficult to see from this that the surface roughness of the unpressurized annealed thin film is large and there are holes.

[0068] In addition, solar cell devices were prepared using the cadmium selenide thin films obtained after non-pressure annealing, pressure-annealed cadmium selenide thin films, and cadmium selenide thin films without treatment (one of the three samples obtained from the "cadmium selenide thin films without treatment", i.e., step 1) in Example 5. The device structure was FTO / cadmium selenide (CdSe) / PEDOT (poly(3,4-ethylenedioxythiophene) monomer):PSS (polystyrene sulfonate) / copper(I) iodide (CuI) / indium tin oxide (ITO). Taking the solar cell device with the pressure-annealed cadmium selenide thin film as an example, the preparation steps can be as follows: 1) First, prepare a cadmium selenide thin film on an FTO substrate through RTE; 2) Perform pressure annealing on the cadmium selenide thin film on the FTO substrate; 3) Spin-coat PEDOT and CuI on the film after the pressure annealing treatment; 4) Prepare ITO by magnetron sputtering as the top electrode of the device. For the preparation processes of other layers in the device except for the cadmium selenide layer, please refer to the Chinese patent document CN112490298A. Correspondingly, three types of devices can be obtained: devices with cadmium selenide thin films without treatment, devices with pressure-annealed cadmium selenide thin films, and devices with non-pressure-annealed cadmium selenide thin films. The performance of these devices is as Figure 6 shown in Table 1. It is not difficult to see from this that the performance of the device corresponding to the pressure-annealed thin film has been greatly improved compared to the device corresponding to the thin film without treatment. Specifically, in terms of the open-circuit voltage, it shows that the quality of the thin film has been improved after pressure annealing. The performance of the device corresponding to the non-pressure-annealed thin film is much lower than that of the device corresponding to the thin film without treatment, manifested by the weakening of rectification and a significant decrease in the open-circuit voltage, indicating that the introduction of selenium vacancy defects due to non-pressure annealing has deteriorated the quality of the thin film.

[0069] Table 1: Performance parameter table of solar cell devices corresponding to cadmium selenide thin films without treatment, pressure-annealed, and non-pressure-annealed

[0070]

[0071] Those skilled in the art can easily understand that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for improving the quality of cadmium selenide thin films by annealing after hot pressing, characterized in that, The method consists of the following steps: (a) Using a cadmium selenide thin film as the object to be treated, and the cadmium selenide thin film is deposited on a substrate; (b) Under the atmosphere of a protective gas, placing the cadmium selenide thin film on the substrate upside down on a heat-conducting substrate, making the cadmium selenide thin film directly contact the heat-conducting substrate, and at the same time applying pressure to the cadmium selenide thin film from above the cadmium selenide thin film, and performing heat annealing under the condition of maintaining the pressure; The annealed cadmium selenide thin film obtained in this way has a larger grain size and a smaller film porosity compared with the cadmium selenide thin film before treatment, and at the same time can avoid the generation of selenium vacancies; Among them, no chemical reaction occurs between the heat-conducting substrate and the cadmium selenide thin film; The application of pressure is specifically achieved by placing an object above the cadmium selenide thin film, and the pressure on the cadmium selenide thin film is realized through the gravitational action of the object; the object can provide a pressure of (1.6 - 6.4)×10 5 Pa for the entire cadmium selenide thin film; the annealing time for the heat annealing is 0.5 h to 2 h; The protective gas is nitrogen or an inert gas.

2. The method for improving the quality of cadmium selenide thin films by hot pressing and subsequent annealing as described in claim 1, characterized in that, The inert gas is argon.

3. The method for improving the quality of cadmium selenide thin films by annealing after hot pressing according to claim 1, wherein, The heat annealing is carried out at a temperature of 480°C to 540°C.

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