Annealing method and equipment of cadmium telluride thin-film solar cell

Abstract

The invention discloses an annealing method and equipment for a cadmium telluride thin film solar cell, and the method comprises the steps: carrying out the irradiation of the surface of a cadmium telluride thin film through employing a laser beam which is shaped into a rectangular light spot or a linear light spot, and enabling an irradiation path to cover a to-be-annealed region of the cadmium telluride thin film. The linear light spot or the homogenized rectangular light spot is adopted for annealing, and the method has the following advantages that laser energy only acts on the thin film, overall heating is avoided, and energy consumption is reduced by 70% or above compared with traditional heat furnace annealing; the laser-induced rapid heating and cooling process is beneficial to formation of more uniform and larger cadmium telluride crystal grains, the crystal boundary is effectively passivated, and the service life of a carrier is prolonged; the glass substrate can be kept at a low temperature, and the risk of thin film damage caused by thermal mismatch is greatly reduced; the process speed is high, and the method can be integrated with deposition technologies such as magnetron sputtering and close-range sublimation to form a continuous production line.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic manufacturing technology, and more specifically, to an annealing method and apparatus for cadmium telluride thin-film solar cells. Background Technology

[0002] Cadmium telluride, as a direct bandgap semiconductor material, is one of the ideal materials for photovoltaic absorber layers. In the fabrication process of cadmium telluride thin-film solar cells, the deposited cadmium telluride film is usually polycrystalline with poor crystal quality and a large number of grain boundaries and defects. These defects can become recombination centers for photogenerated carriers, which seriously restricts the improvement of cell efficiency.

[0003] Currently, hot furnace annealing is commonly used in industry to improve the crystallinity of cadmium telluride thin films. This involves placing the entire glass substrate with the deposited film into a high-temperature furnace and holding it at 400-600°C for a period of time. This method has several drawbacks. First, it is energy-intensive and has low production efficiency: the entire substrate and even the entire furnace cavity need to be heated to a high temperature (typically 400-450°C), the heating and cooling processes are slow, and the holding time is 15-30 minutes, resulting in huge energy consumption and a slow production cycle.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide an annealing method and equipment for cadmium telluride thin-film solar cells, which helps to reduce energy consumption and improve production efficiency.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides an annealing method for a cadmium telluride thin-film solar cell, comprising: irradiating the surface of a cadmium telluride thin film with a laser beam shaped into a rectangular or linear spot, wherein the irradiation path covers the annealing area of ​​the cadmium telluride thin film.

[0007] In an alternative embodiment, the laser beam is generated by a continuous wave laser or a high repetition rate pulsed laser.

[0008] In an optional embodiment, the wavelength range of the laser beam is 808-915 nm.

[0009] In an optional embodiment, the power density of the laser beam is 10-1000 W / cm². 2 .

[0010] In an optional implementation, the scanning speed is 10-1000 mm / s.

[0011] In an optional embodiment, the overlap rate of the light spot generated by the laser beam irradiation on the cadmium telluride film surface is 50-90%.

[0012] In an optional embodiment, the length of the linear light spot is 100-900 mm and the width is 0.1-2 mm.

[0013] In an optional implementation, the annealing process is performed in an inert gas protective atmosphere or a vacuum environment; And / or, the atmosphere of the annealing process contains chlorine, or the cadmium telluride film surface has been pre-deposited with a chlorine-containing compound layer before laser annealing; And / or, a cadmium telluride thin film is deposited on a substrate comprising a glass substrate, a TCO layer and a Cds window layer disposed sequentially.

[0014] In an optional embodiment, the surface temperature of the cadmium telluride film during the annealing process is 400℃-600℃.

[0015] Secondly, the present invention provides an apparatus for the annealing method of cadmium telluride thin-film solar cells according to any one of the foregoing embodiments, comprising: A laser generating unit, used to emit a laser beam; A beam shaping unit is used to shape the laser beam into a linear or rectangular spot. A motion control platform is used to support the cadmium telluride thin film and realize the relative motion between the laser beam and the cadmium telluride thin film; An environmental chamber is used to provide an inert gas environment or a vacuum environment for the cadmium telluride thin film; The control unit is used to control the laser generating unit, motion control platform, and environmental chamber.

[0016] The present invention has the following beneficial effects: The use of linear or uniform rectangular laser spots for annealing in this application has the following advantages: the laser energy acts only on the thin film, avoiding overall heating, and reducing energy consumption by more than 70% compared to traditional hot furnace annealing; the laser-induced rapid heating and cooling process helps to form more uniform and larger cadmium telluride grains, effectively passivating grain boundaries and improving carrier lifetime; it can keep the glass substrate at a low temperature, greatly reducing the risk of thin film damage due to thermal mismatch; the process speed is fast and can be integrated with deposition technologies such as magnetron sputtering and close-range sublimation to form a continuous production line. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 The fabrication process of CdTe cells; Figure 2 This is a schematic diagram of the annealing equipment for the cadmium telluride thin-film solar cell in Example 1; Figure 3 SEM images of cadmium telluride films after annealing in Example 1 (right) and Comparative Example 1 (left); Figure 4 SEM images of cadmium telluride films after annealing in Example 1 (right) and Comparative Example 2 (left); Figure 5 This is a comparison of the midline spot and the Gaussian spot in this application.

[0019] Diagram: 1-Semiconductor laser; 2-Transmission fiber; 3-Beam shaping unit; 4-Substrate; 5-Motion control platform; 6-Environmental chamber; 7-Infrared thermal imager; 8-Control unit. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0021] The industrial practice of using hot furnace annealing to improve the crystallinity of cadmium telluride films has the following drawbacks: 1. High energy consumption and low production efficiency: The entire substrate and even the entire furnace cavity need to be heated to a high temperature (usually 400-450°C). The heating and cooling process is slow and the temperature is held for 15-30 minutes, resulting in huge energy consumption and slow production cycle.

[0022] 2. Thermal stress and deformation issues: Heating flexible substrates such as glass or large-area substrates as a whole can easily lead to warping, deformation, or even cracking of the substrate due to mismatch in thermal expansion coefficients, which limits the selection of substrate materials and the development of lightweight and flexible batteries.

[0023] 3. Inaccurate process control: Overall heating makes it difficult to achieve precise local control of annealing temperature, which may lead to uneven performance in different areas of the film, affecting the overall efficiency and yield of the battery.

[0024] 4. Unfavorable for online continuous production: Batch-type furnace annealing is difficult to seamlessly integrate with deposition processes such as magnetron sputtering and close-range sublimation, hindering the construction of fully automated production lines.

[0025] Laser annealing, as an advanced material processing technology, offers advantages such as concentrated energy, rapid heating and cooling, non-contact processing, and selective area processing. However, its application to the annealing of heat-sensitive cadmium telluride / glass structures, and the resolution of technical challenges such as laser-material interaction, thermal management, and narrow process windows, remain obstacles for researchers in this field. Therefore, this application provides an annealing method and apparatus for cadmium telluride thin-film solar cells.

[0026] This invention provides an annealing method for cadmium telluride thin-film solar cells, comprising: irradiating the surface of a cadmium telluride thin film with a laser beam shaped into a rectangular or linear spot, wherein the irradiation path covers the annealing area of ​​the cadmium telluride thin film.

[0027] The annealing process employs a single-scan laser line beam scan of the CdTe thin film surface, using laser energy to selectively and locally heat the CdTe film. This application uses a linear or uniform rectangular laser spot for annealing, offering the following advantages: laser energy acts only on the thin film, avoiding overall heating and reducing energy consumption by more than 70% compared to traditional furnace annealing; the laser-induced rapid heating and cooling process helps form more uniform, larger-grained cadmium telluride grains, effectively passivating grain boundaries and improving carrier lifetime; it allows the glass substrate to remain at a low temperature, greatly reducing the risk of film damage due to thermal mismatch; and the process is fast, allowing integration with deposition technologies such as magnetron sputtering and close-range sublimation to form a continuous production line. A flowchart of the cadmium telluride battery fabrication process using this annealing method is shown below. Figure 1 As shown.

[0028] In an alternative embodiment, the laser beam is generated by a continuous wave laser or a high repetition rate pulsed laser.

[0029] In an optional embodiment, the wavelength range of the laser beam is 808-915 nm.

[0030] The choice of wavelength is crucial to the final process effect of the thermal reaction between the laser and the thin film. 808nm is considered optimal. When cadmium telluride (CdTe) thin films are subjected to electromagnetic radiation, they exhibit emission (R), absorption (A), and transmission (T), with the relationship: R + A + T = 1. When a CdTe film absorbs heat, it emits electromagnetic radiation, represented by emissivity (E). When the heat reaches equilibrium, E = A, or E = 1 - RT. At 808nm, the transmission T ≈ 0, meaning E = 1 - R. CdTe is a direct bandgap semiconductor, and its absorption characteristics are closely related to photon energy (wavelength). The photon energy at 808nm is slightly higher than the bandgap energy of CdTe. The absorption coefficient is moderate (approximately 10). 2 - 10 4 cm -1 The advantages are as follows: Individual absorption: Laser energy can penetrate and be absorbed relatively uniformly throughout the entire CdTe film thickness (typically 2-4µm), achieving bulk heating of the entire absorption layer, which is beneficial for overall crystallization and grain growth.

[0031] Highly efficient thermal conversion: Almost all laser energy is absorbed by the CdTe layer and converted into heat energy, resulting in the highest efficiency.

[0032] Substrate-friendly: Very little energy penetrates to the substrate, ensuring that the substrate remains at a low temperature and avoiding thermal damage and deformation.

[0033] Wide process window: The moderate absorption coefficient makes temperature control more stable and less prone to surface ablation.

[0034] Therefore, excessively short wavelengths, such as 532nm, can easily cause damage to the thin film surface, while excessively long wavelengths, such as 1064nm, have low absorption efficiency. The optimal wavelength is between 808-915nm.

[0035] In an optional embodiment, the power density of the laser beam is 10-1000 W / cm². 2 .

[0036] This power density range ensures sufficient laser energy to rapidly heat the cadmium telluride film to the required annealing temperature (400–600°C), enabling grain recrystallization and defect repair, while avoiding excessive energy leading to surface ablation or insufficient energy for effective annealing. Moderate energy input allows for precise control of the thermal depth, heating only the CdTe layer without damaging the glass substrate, achieving efficient and controllable localized bulk heating, and improving crystallization quality and process stability.

[0037] In an optional implementation, the scanning speed is 10-1000 mm / s.

[0038] Adjusting the scanning speed allows for precise control of laser irradiation time, thereby synergistically regulating the energy absorbed by the thin film and the temperature rise process. Lower speeds are suitable for thick films or applications requiring deep heating, while higher speeds meet the needs of continuous online production. This wide range allows for flexible process matching to different production line cycles, making it suitable for the industrial application of large-area power-generating glass.

[0039] In an optional embodiment, the overlap rate of the light spot generated by the laser beam irradiation on the cadmium telluride film surface is 50-90%.

[0040] Appropriate beam overlap ensures uniform heating within the scanning area, preventing uneven annealing or performance fluctuations caused by gaps, while also reducing the risk of localized overheating due to excessive repeated irradiation. This overlap rate strikes a balance between ensuring process consistency and thermal management, contributing to the formation of a uniform, large-grain structure and improving the consistency of thin film electrical properties.

[0041] In an optional embodiment, the length of the linear light spot is 100-900 mm and the width is 0.1-2 mm.

[0042] In an optional implementation, the annealing process is carried out in an inert gas protective atmosphere (such as N2, Ar) or a vacuum environment; And / or, the atmosphere of the annealing process contains chlorine (such as chlorine gas, carbon tetrachloride vapor, etc.), or the cadmium telluride film surface has been pre-deposited with a chlorine-containing compound layer before laser annealing; And / or, a cadmium telluride thin film is deposited on a substrate comprising a glass substrate, a TCO layer and a Cds window layer disposed sequentially.

[0043] In an optional embodiment, the surface temperature of the cadmium telluride film during the annealing process is 400℃-600℃.

[0044] This temperature range represents the optimal thermodynamic window for CdTe materials to achieve sufficient recrystallization and grain growth. Temperatures that are too low hinder grain boundary migration, offering limited improvement; temperatures that are too high may lead to elemental volatilization or interfacial reactions. Within this range, combined with rapid heating / cooling characteristics, large and uniform grains can be obtained, and the chlorine doping effect can be effectively activated, significantly improving carrier lifetime and battery conversion efficiency.

[0045] This invention also provides an apparatus for the annealing method of cadmium telluride thin-film solar cells according to any one of the foregoing embodiments, comprising: A laser generating unit, used to emit a laser beam; A beam shaping unit is used to shape the laser beam into a linear or rectangular spot. A motion control platform is used to support the cadmium telluride thin film and realize the relative motion between the laser beam and the cadmium telluride thin film; An environmental chamber is used to provide an inert gas environment or a vacuum environment for the cadmium telluride thin film; The control unit is used to control the laser generating unit, motion control platform, and environmental chamber.

[0046] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0047] Example 1 This embodiment provides an annealing method for cadmium telluride thin-film solar cells, specifically including the following steps: 1. Take a substrate on which a 400 nm thick TCO layer, a 1 μm thick CdS window layer and a 3 μm thick CdTe absorber layer have been deposited on a soda-lime glass substrate, and the substrate has been pre-deposited with CdCl2 with a thickness of 300 nm.

[0048] 2. Place the substrate into the environmental chamber, evacuate it, and then fill it with nitrogen to a slightly positive pressure.

[0049] 3. Set the laser annealing parameters: linear spot size 350mm×0.4mm, laser power 9KW (power density approximately 130W / cm²), and platform scanning speed adjusted between 100-500mm / s. The purpose is to ensure that the absorption temperature of the cadmium telluride material on the substrate film surface is within the range of 550±50℃, in accordance with the PID feedback.

[0050] 4. Start the equipment. The motion control platform drives the substrate through the laser irradiation area at a constant speed. The laser beam performs a one-time scanning annealing on the CdTe thin film.

[0051] 5. After annealing, remove the substrate.

[0052] The equipment used in this embodiment is as follows: Figure 2 As shown, the system includes: an optical fiber-coupled semiconductor laser 1 (wavelength 808nm), whose emitted laser light reaches a beam shaping unit 3 via a transmission optical fiber 2. This unit homogenizes the circular laser beam into a uniform linear spot with a length of 350mm and a width of 0.4mm. Taking cadmium telluride photovoltaic glass with short-side feeding as an example, a 1200mm length can be covered by a combination of four laser shaping heads, and the substrate speed on the platform is adjustable from 1-10m / min. The cadmium telluride photovoltaic glass substrate 4 to be processed is placed on a precision motion control platform 5. The entire processing is carried out in a sealed environmental chamber 6, which is filled with high-purity nitrogen as a protective gas. An infrared thermal imager 7 monitors the temperature of the thin film surface in real time and feeds the signal back to the central control unit 8. The control unit 8 dynamically adjusts the power of the laser 1 according to a preset temperature curve (e.g., 550℃±10℃) and synchronously controls the movement speed of the motion control platform 5.

[0053] Note: In other embodiments, the number of homogenizing laser heads can be freely adjusted according to the substrate advance width, and can be freely matched between homogenization length from 100-900mm and homogenization spot width from 0.1-2mm.

[0054] Comparative Example 1: Take the substrate from step 1 of Example 1, place it in a hot furnace, and anneal it at 580°C for 30 minutes to complete the annealing.

[0055] Back electrodes were prepared on the substrates that had undergone annealing in the above embodiments and comparative examples to obtain CdTe cells, and the prepared CdTe cells were tested.

[0056] According to the test results, the short-circuit current density and fill factor prepared in Example 1 were significantly higher than those in Comparative Example 1, increasing by 10% and 1% respectively, and the average photoelectric conversion efficiency was relatively improved by about 8%. SEM images of the CdTe films after laser annealing in Example 1 and Comparative Example 1 are shown below. Figure 3 As shown, the CdTe thin film annealed in Example 1 has a more uniform grain size and fewer, clearer grain boundaries.

[0057] Experimental Example 1: This embodiment provides an annealing method for cadmium telluride thin-film solar cells, specifically including the following steps: 1. Take a substrate on which a 400 nm thick TCO layer, a 1 μm thick CdS window layer and a 3 μm thick CdTe absorber layer have been deposited on a soda-lime glass substrate, and the substrate has been pre-deposited with CdCl2 with a thickness of 300 nm.

[0058] 2. Place the substrate into the environmental chamber, evacuate it, and then fill it with nitrogen to a slightly positive pressure.

[0059] 3. Setting laser scribing parameters: Laser scribing of the substrate was performed using a circular Gaussian spot with a diameter of 10 μm and a square flat-top spot with a side length of 10 μm, respectively, with a power density of approximately 1.9 J / cm². 2 The platform scanning speed was adjusted between 10000-15000 mm / s to ensure that the absorption temperature of the cadmium telluride material on the substrate film surface was within the range of 550±50℃, in accordance with the PID feedback. The SEM image of the substrate after laser scribing is shown below. Figure 4 As shown, the left side shows the Gaussian spot scribing situation, and the right side shows the flat-top spot scribing situation after shaping in a similar embodiment.

[0060] The principle of annealing the substrate using a circular Gaussian spot and a square flat-top spot in Experiment Example 1 is as follows: Figure 5As shown in Example 1, when annealing is performed using a Gaussian-distributed pulsed laser spot, the point-by-point scanning method results in uneven energy distribution, with high intensity at the center and significant attenuation at the edges. This easily leads to localized overheating and expansion of the heat-affected zone, resulting in uneven heating of the film. This method requires multiple scans to cover the entire surface, resulting in overlapping areas. It is not only inefficient but may also cause lattice damage due to repeated heating. Furthermore, it is sensitive to process parameters and has poor stability, making it suitable for experimental research or small-batch production, but unsuitable for the continuous manufacturing requirements of large-size photovoltaic glass. In contrast, Example 1 uses a square flat-top laser spot with highly uniform energy distribution, enabling one-time full-surface scanning annealing without point-by-point splicing. A single exposure completes the full-width annealing, with no areas of repeated irradiation, avoiding overheating and defect accumulation problems. This technology reduces annealing time from minutes to seconds, significantly improving production efficiency. The heat-affected zone is controllable, and the process window is wide and highly stable. A comparison shows that the flat-top spot obtained by using a shaper such as a DOE laser shaper is more suitable for the online continuous production of large-size cadmium telluride photovoltaic glass such as 1.2m×1.6m, and has good prospects for industrial application.

[0061] As can be seen from the comparison between Example 1 and Comparative Example 2, Example 1 can achieve full-area annealing with a single exposure, without scanning motion; there are no overlapping areas, avoiding lattice defects caused by repeated heating; it is suitable for large-size power-generating glass (such as 1.2m × 1.6m), and the annealing time is reduced from minutes to seconds; it can achieve selective annealing, strengthen inefficient areas, and improve yield.

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An annealing method for cadmium telluride thin-film solar cells, characterized in that, include: A laser beam shaped into a rectangular or linear spot is used to irradiate the surface of a cadmium telluride thin film, and the irradiation path covers the area of ​​the cadmium telluride thin film to be annealed.

2. The annealing method for cadmium telluride thin-film solar cells according to claim 1, characterized in that, The laser beam is generated by a continuous wave laser or a high repetition rate pulsed laser.

3. The annealing method for cadmium telluride thin-film solar cells according to claim 1, characterized in that, The wavelength range of the laser beam is 808-915nm.

4. The annealing method for cadmium telluride thin-film solar cells according to claim 1, characterized in that, The power density of the laser beam is 10-1000 W / cm². 2 .

5. The annealing method for cadmium telluride thin-film solar cells according to claim 1, characterized in that, The scanning speed is 10-1000 mm / s.

6. The annealing method for cadmium telluride thin-film solar cells according to claim 1, characterized in that, The overlap rate of the light spot generated by the laser beam irradiation on the cadmium telluride film surface is 50-90%.

7. The annealing method for cadmium telluride thin-film solar cells according to claim 1, characterized in that, The linear light spot has a length of 100-900 mm and a width of 0.1-2 mm.

8. The annealing method for cadmium telluride thin-film solar cells according to claim 1, characterized in that, The annealing process is carried out in an inert gas protective atmosphere or a vacuum environment; And / or, the atmosphere of the annealing process contains chlorine, or the cadmium telluride film surface has been pre-deposited with a chlorine-containing compound layer before laser annealing; And / or, a cadmium telluride thin film is deposited on a substrate comprising a glass substrate, a TCO layer and a Cds window layer disposed sequentially.

9. The annealing method for cadmium telluride thin-film solar cells according to claim 1, characterized in that, During the annealing process, the surface temperature of the cadmium telluride film is 400℃-600℃.

10. An apparatus used in the annealing method for a cadmium telluride thin-film solar cell according to any one of claims 1-9, characterized in that, include: A laser generating unit, used to emit a laser beam; A beam shaping unit is used to shape the laser beam into a linear or rectangular spot. A motion control platform is used to support the cadmium telluride thin film and realize the relative motion between the laser beam and the cadmium telluride thin film; An environmental chamber is used to provide an inert gas environment or a vacuum environment for the cadmium telluride thin film; The control unit is used to control the laser generating unit, motion control platform, and environmental chamber.

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