An in-situ gas sealing annealing method for improving the quality of cadmium selenide films

Abstract

The present invention belongs to the field of photoelectric materials and thin-film solar cell preparation, and discloses an in-situ gas-sealed annealing method for improving the quality of cadmium selenide film. The method is to treat a cadmium selenide film prepared by a rapid thermal evaporation method as a treatment object, and in a protective gas atmosphere environment, and the pressure of the atmosphere environment is 50-500Torr, the cadmium selenide film is in-situ annealed at a temperature of 450-600°C, and the cadmium selenide film obtained after annealing has fewer holes and less roughness than the cadmium selenide film before treatment. The present invention can solve the technical problem of poor quality of the existing cadmium selenide film prepared by the rapid thermal evaporation method by performing in-situ thermal annealing treatment on the cadmium selenide film under specific temperature and specific environmental atmosphere conditions.

Description

Technical Field

[0001] The present invention belongs to the field of photoelectric materials and thin-film solar cell preparation, and more specifically, relates to an in-situ gas sealing annealing method for improving the quality of a cadmium selenide film, which can improve the quality of the cadmium selenide film in-situ. Background Art

[0002] Cadmium selenide (CdSe) is a II-VI binary inorganic semiconductor material with excellent optoelectronic properties and a high visible light absorption coefficient, making it an ideal candidate material for the absorption layer of solar cells. Photovoltaic power generation is one of the most promising methods to achieve "carbon neutrality", and the future development trend of photovoltaic devices will be to form a stack with crystalline silicon solar cells. Theoretical calculations show that the optimal bandgap width of a stacked solar cell formed with a mainstream crystalline silicon solar cell with a bandgap width of 1.25eV is 1.7eV, and the theoretical efficiency can reach more than 40%.

[0003] The bandgap width of CdSe material is 1.74eV, which is a direct bandgap semiconductor. The study further showed that CdSe, as a simple binary compound, is particularly easy to deposit using large-scale vapor deposition technology, and it is very easy to achieve stoichiometric CdSe thin films. At the same time, as a stable inorganic compound, CdSe bulk material is insoluble in water, not easy to thermally decompose, and has certain acid and alkali resistance. Therefore, it is feasible in principle to obtain high-quality CdSe thin films and realize thin-film solar cells, which has broad application prospects.

[0004] At present, there are literature reports that large-grained cadmium selenide films have been prepared on FTO substrates by rapid thermal evaporation, but the problems of large surface roughness and holes have not yet been solved, which will directly affect the efficiency of its thin-film solar cells. If the quality problem of cadmium selenide films can be solved, it will be a great boost to the development of cadmium selenide thin-film solar cells.

[0005] Chinese patent CN106868453A discloses a process for preparing CdSe thin film by thermal evaporation. Although it also discloses annealing the CdSe thin film to refine its surface grains, enhance its density, and improve the film quality, this annealing method is a commonly used post-annealing treatment method for thin films, which usually has limited improvement in film quality (especially when a polycrystalline thin film is obtained after the preparation process, ordinary post-annealing treatment usually has limited effect). In addition, the annealing method described in the patent is complicated to operate, the annealing conditions are unclear, and the annealing effect is not clearly demonstrated. Summary of the invention

[0006] In view of the above defects or improvement needs of the prior art, the purpose of the present invention is to provide an in-situ gas-sealed annealing method for improving the quality of cadmium selenide film, wherein the cadmium selenide film is subjected to in-situ thermal annealing treatment under specific temperature and specific environmental atmosphere conditions, thereby solving the technical problem of poor quality of cadmium selenide film prepared by the existing rapid thermal evaporation method. The annealing process of the present invention will not affect the film components and proportions (that is, the film components and proportions after annealing do not change), the film has good crystallinity, a smooth surface without holes, and can effectively improve the film quality reliability and high repetition rate.

[0007] To achieve the above-mentioned purpose, the present invention provides an in-situ gas-sealed annealing method for improving the quality of cadmium selenide film, characterized in that the method is to use a cadmium selenide film prepared by rapid thermal evaporation as a processing object, and to perform in-situ annealing on the cadmium selenide film at a temperature of 450 to 600° C. in an atmosphere of protective gas and a chamber pressure of 50 to 500 Torr. The cadmium selenide film after annealing has fewer holes and less roughness than the cadmium selenide film before treatment.

[0008] As a further preferred embodiment of the present invention, the cadmium selenide film to be treated is prepared by rapid thermal evaporation, specifically: in a deposition chamber, the conductive substrate is heated to 400-480°C and maintained for not less than 15 minutes, cadmium selenide powder is used as an evaporation source, the evaporation source is heated to 800-850°C, the evaporation time is 50-100 seconds, and the vacuum degree during the preparation process is not higher than 1Pa, so that a cadmium selenide film with a thickness of 1 to 3 microns can be obtained;

[0009] Preferably, the in-situ annealing is specifically as follows: after the deposition is completed, a protective gas is introduced into the deposition chamber, the chamber vacuum is adjusted to control the chamber pressure, and then the substrate temperature is adjusted to a target annealing temperature of 450 to 600° C., thereby performing an annealing treatment.

[0010] As a further preferred embodiment of the present invention, the heating rate used for heating the conductive substrate is 10-20°C / s;

[0011] When adjusting the substrate temperature to a target annealing temperature of 450-600° C. is achieved by heating the substrate, the heating rate used is 10-20° C. / s.

[0012] As a further preferred embodiment of the present invention, after the deposition is completed, a protective gas is introduced into the deposition chamber, and the specific gas introduction rate is 500-1000 sccm.

[0013] As a further preferred embodiment of the present invention, the annealing time of the in-situ annealing is not less than 10 minutes.

[0014] As a further preferred embodiment of the present invention, the protective gas is nitrogen or argon.

[0015] Through the above technical scheme conceived by the present invention, compared with the prior art, the cadmium selenide film is subjected to in-situ thermal annealing treatment under specific temperature and specific environmental atmosphere conditions, the annealing temperature is controlled to be 450-600°C, the atmosphere in which the annealing is performed is a protective gas atmosphere, and the pressure of the environmental atmosphere in which the annealing is performed is 50-500Torr, so that the film is recrystallized, which can effectively improve the quality of the cadmium selenide film. The number of holes in the cadmium selenide film after annealing is significantly less than that of the cadmium selenide film before annealing, and the roughness of the cadmium selenide film after annealing is significantly less than that of the cadmium selenide film before annealing. Taking the embodiments of the present invention as an example, the cadmium selenide film after annealing has a smooth and flat surface microstructure without holes.

[0016] The in-situ annealing method of the present invention can be started immediately after the film deposition is completed (at this time, the deposition chamber still maintains the vacuum condition during deposition). For example, the equipment for preparing cadmium selenide film can be used as an annealing device, and the annealing treatment can be directly performed after the film preparation is completed. Taking nitrogen as an example of a protective gas, the in-situ gas-sealed annealing method of the present invention is used to immediately introduce nitrogen after the evaporation process is completed to increase the air pressure in the chamber, thereby suppressing the back evaporation of the deposited film. At the same time, the CdSe vapor remaining in the evaporation chamber after the evaporation is completed is used to facilitate the film surface and the vapor in the chamber to fully exchange substances in a limited area at the set annealing temperature, so that the CdSe vapor and the film surface reach a balance between redeposition and back evaporation, thereby obtaining a flat and hole-free film.

[0017] The in-situ annealing method of the present invention has the following advantages: first, it is easy to operate, and annealing can be performed directly in the chamber where the film is deposited; second, it can inhibit back evaporation, which significantly improves the quality of the film. By high temperature and increasing the chamber pressure, the newly deposited non-steady-state film exchanges with the vapor in the chamber on the micro-surface, which promotes the filling of holes and makes the surface morphology smoother. In this method, the protective gas can use an inert protective gas such as nitrogen to prevent the film from being oxidized by oxygen. During the annealing process, the temperature should be controlled at 450-600°C. When the temperature is low, it cannot enhance the crystallinity. When the temperature is higher than 600°C, the film back evaporation is serious. The air pressure should be controlled at 50-500Torr. When the air pressure is too low, it cannot affect the back evaporation of the film surface. When the air pressure is too high, the film surface and the vapor in the chamber cannot fully collide and exchange, which is close to the effect of ordinary post-annealing, and the improvement of film quality is limited.

[0018] The invention is simple to operate, highly repeatable, highly efficient, has low requirements on experimental equipment and environment, and the prepared cadmium selenide thin film has regular and smooth morphology. The method of the invention can be particularly applied to the preparation of cadmium selenide thin film solar cells to obtain corresponding cadmium selenide thin film solar cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the in-situ annealing device of the cadmium selenide film adopted in the present invention.

[0020] Figure 2 The SEM images of the cadmium selenide films before and after annealing of Comparative Example 1 and Example 1 are shown; wherein, Figure 2 (a) corresponds to before annealing, Figure 2 (b) corresponds to the film surface after conventional post-annealing treatment. Figure 2 (c) corresponds to the film surface after in-situ annealing treatment in the present invention.

[0021] Figure 3 is an X-ray diffraction diagram of the cadmium selenide film after annealing in Example 1; wherein, Figure 3 (a) corresponds to before annealing, Figure 3 (b) in the figure corresponds to after annealing. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with 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 intended 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.

[0023] Comparative Example 1

[0024] Experimental process: (1) Cadmium selenide film was prepared by rapid thermal evaporation. The preparation process was as follows: the substrate was heated to 480°C and maintained for 15 minutes at a heating rate of 20°C / s. Cadmium selenide powder was used as the evaporation source. The evaporation source was heated to 820°C for 100 seconds. The vacuum degree during the preparation process was 1Pa, and a cadmium selenide film with a thickness of 2 microns was obtained. Then, referring to conventional treatment, the film was naturally cooled to room temperature, and the pressure was reduced to slightly less than 1atm before the sample was taken out. (2) Post-annealing treatment: the prepared sample was placed in a vacuum annealing chamber, and the chamber was evacuated to a vacuum degree of 1Pa. Then, nitrogen was introduced at a rate of 500sccm to raise the gas pressure to 50Torr. In a nitrogen-protected atmosphere, the chamber temperature was heated to an annealing temperature of 450°C at a heating rate of 20°C / s and maintained for 10 minutes. The film was then naturally cooled to room temperature and taken out. Before and after annealing, the SEM characterization images of the film samples were as follows: Figure 2 As shown in (a) and (b) in .

[0025] Example 1

[0026] Experimental process: (1) The cadmium selenide film was prepared by rapid thermal evaporation. The preparation process was as follows: the substrate was heated to 480°C and maintained for 15 minutes. The heating rate was 20°C / s. Cadmium selenide powder was used as the evaporation source. The evaporation source was heated to 820°C. The evaporation time was 100 seconds. The vacuum degree during the preparation process was 1 Pa. A cadmium selenide film with a thickness of 2 microns was obtained. (2) In-situ atmosphere sealed annealing. After the cadmium selenide film was prepared, nitrogen was introduced into the sealed evaporation chamber. The nitrogen introduction rate was controlled to 500 sccm, and the gas pressure was raised to 50 Torr. Under the nitrogen protection atmosphere, the chamber temperature was naturally cooled to the annealing temperature of 450°C and maintained for 10 minutes. Then it was naturally cooled to room temperature and taken out. The obtained annealed film was flat and free of holes. The SEM characterization of the film after annealing is shown in the figure below. Figure 2 As shown in (c) in .

[0027] Example 2

[0028] The gas pressure in Example 1 was changed to 500 Torr, and other conditions were the same as in Example 1. The sample in Example 2 was analyzed by SEM and XRD patterns. Compared with the unannealed film, the surface was smooth without holes, the orientation tended to be quasi-single, and the crystal quality was improved.

[0029] Example 3

[0030] The gas atmosphere in Example 1 was changed to argon, and other conditions were the same as in Example 1. The sample in Example 3 was analyzed by SEM and XRD patterns. Compared with the unannealed film, the surface was smooth without holes, the orientation tended to be quasi-single, and the crystal quality was improved.

[0031] Example 4

[0032] The heating rate of 20°C / s in Example 1 was changed to 10°C / s, and other conditions were the same as in Example 1. The sample in Example 4 was analyzed by SEM and XRD patterns. Compared with the unannealed film, the surface was smooth without holes, the orientation tended to be quasi-single, and the crystal quality was improved.

[0033] Example 5

[0034] The annealing temperature in Example 1 was changed to 600° C., and other conditions were the same as in Example 1. The sample in Example 5 was analyzed by SEM and XRD patterns. Compared with the unannealed film, the surface was smooth without holes, the orientation tended to be quasi-single, and the crystal quality was improved.

[0035] Example 6

[0036] The annealing time in Example 1 was changed to 30 min, and other conditions were the same as in Example 1. The sample in Example 6 was analyzed by SEM and XRD patterns. Compared with the unannealed film, the surface was smooth without holes, the orientation tended to be quasi-single, and the crystal quality was improved.

[0037] Example 7

[0038] The rate of nitrogen gas flow in Example 1 was adjusted to 1000 sccm, and other conditions were the same as in Example 1. SEM and XRD pattern analysis of the sample in Example 7 showed that compared with the unannealed film, the surface was smooth without holes, the orientation tended to be quasi-single, and the crystal quality was improved.

[0039] Figure 2 The SEM images of the CdSe thin films before and after annealing of Comparative Example 1 and Example 1 are shown (Comparative Example 1 corresponds to conventional post-annealing treatment, and Example 1 corresponds to the in-situ annealing treatment in the present invention), wherein: Figure 2 (b) is the surface of the film after conventional post-annealing treatment. It can be found that for the prepared polycrystalline film (i.e. the film before annealing, such as Figure 2 (a) in the figure), the improvement of ordinary post-annealing is limited, and the grain size and the number of holes do not change significantly (as shown in Figure 2 (b) in the figure). Figure 2 (c) is the surface of the film after in-situ annealing of the present invention, from which it can be seen that the SEM image of the polycrystalline cadmium selenide film after annealing has a smoother surface and no holes on the surface. It can be seen that the method of the present invention can improve the surface flatness of the cadmium selenide film after annealing, better eliminate holes, and better improve the quality of the cadmium selenide film. The cadmium selenide film after annealing has a smooth surface microscopic morphology and no cracks.

[0040] Figure 3 The XRD diagram of the CdSe film before and after annealing in Example 1 is shown. It can be seen from the figure that the XRD diffraction peaks of the cadmium selenide film after annealing are consistent in orientation and have an obvious (103) preferred orientation. At the same time, the peak intensity is higher and the film crystallinity is better.

[0041] In the above embodiments, the rapid thermal evaporation method is performed using a rapid thermal evaporation tube furnace (MTI, Hefei, China).

[0042] There is no particular restriction on the source of the raw materials required for preparing the thin films in the above embodiments. They can be purchased on the market or prepared according to conventional methods well known to those skilled in the art. In addition, there is no particular restriction on the purity of the raw materials. The present invention preferably uses analytical grade or conventional purity in the field of thermal evaporation treatment.

[0043] In principle, the present invention has no special restrictions on other morphologies of the cadmium selenide film after annealing. Those skilled in the art can select and adjust according to actual conditions, raw material conditions and product requirements. The present invention is to improve the surface flatness of the cadmium selenide film after annealing, better eliminate cracks, reduce holes, and better improve the quality of the cadmium selenide film.

[0044] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An in-situ gas sealing annealing method for improving the quality of cadmium selenide film, characterized in that: The method is to treat a cadmium selenide film prepared by rapid thermal evaporation as a treatment object, and to perform in-situ annealing on the cadmium selenide film at a temperature of 450 to 600° C. in a protective gas atmosphere and a chamber pressure of 50 to 500 Torr. The cadmium selenide film obtained after annealing has fewer holes and smaller roughness than the cadmium selenide film before treatment; wherein: The cadmium selenide film to be treated is prepared by rapid thermal evaporation, specifically: in a deposition chamber, a conductive substrate is heated to 400-480° C. and maintained for not less than 15 minutes, cadmium selenide powder is used as an evaporation source, and the evaporation source is heated to 800-850° C. During the preparation process, the vacuum degree is not higher than 1 Pa, and the cadmium selenide film can be obtained; The in-situ annealing is specifically as follows: after the deposition is completed, a protective gas is immediately introduced into the deposition chamber, the chamber vacuum is adjusted to control the chamber pressure, and then the substrate temperature is adjusted to a target annealing temperature of 450-600°C, thereby performing an annealing treatment. In this way, by controlling the substrate temperature to 450-600°C and increasing the chamber pressure to 50-500Torr, the newly deposited non-steady-state film exchanges with the vapor in the chamber on the micro-surface, thereby prompting the filling of holes and making the surface morphology smoother.

2. The method according to claim 1, characterized in that: When the cadmium selenide film is prepared by rapid thermal evaporation, the evaporation time is 50-100 seconds, and the thickness of the corresponding cadmium selenide film is 1-3 microns.

3. The method according to claim 1, characterized in that: The heating rate used for heating the conductive substrate is 10-20°C / s; When adjusting the substrate temperature to a target annealing temperature of 450-600° C. is achieved by heating the substrate, the heating rate used is 10-20° C. / s.

4. The method according to claim 1, characterized in that: After the deposition is completed, a protective gas is introduced into the deposition chamber, and the specific gas introduction rate is 500-1000 sccm.

5. The method according to claim 1, characterized in that: The annealing time of the in-situ annealing is not less than 10 minutes.

6. The method according to claim 1, characterized in that: The protective gas is nitrogen or argon.

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