A cadmium telluride thin film solar cell with a cadmium oxide passivation layer and a method of manufacturing the same

Abstract

This invention discloses a cadmium telluride thin-film solar cell with a cadmium oxide passivation layer and its preparation method, including the following steps: (1) Pretreatment: The cadmium telluride thin film covered by the cadmium oxide layer formed by CdCl2 annealing is placed in a vacuum tube furnace, and the vacuum is drawn to a negative pressure state to remove residual O2; (2) Reduction treatment: The vacuum pump is turned off, an inert gas is introduced, the temperature is raised to the reduction reaction temperature and kept at the temperature, and then a mixture of reducing gas and inert gas is introduced under positive pressure to carry out the reduction reaction; the reduction reaction temperature is 200~400℃; the ratio of reducing gas to inert gas mixture is 1-20%; the reaction time is 5-30min; (3) Posttreatment: The heating is turned off, an inert gas is introduced to cool to room temperature to form a cadmium oxide layer covered by a metal Cd layer.

Description

Technical Field

[0001] This invention belongs to the field of cadmium telluride solar cell production, specifically relating to a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer and its preparation method. Background Technology

[0002] In the fabrication of CdTe solar cells, cadmium chloride (CdCl2) annealing is a crucial step in improving cell performance, as this process promotes grain growth, passivates grain boundaries, and optimizes carrier transport properties. However, during high-temperature annealing, the CdTe surface inevitably reacts with oxygen to form highly resistive cadmium oxide (CdO), creating a non-ideal interface layer. This insulating layer increases the device's series resistance, severely damaging cell performance. Therefore, removing or thinning the cadmium oxide layer helps improve cell performance.

[0003] Generally, industrial processes remove CdO layers using wet pickling (chemical etching) and plasma etching (dry processing). Wet pickling typically uses low-concentration nitric acid (HNO3), hydrochloric acid (HCl), or phosphoric acid (H3PO4) solutions to etch the CdTe surface and dissolve the CdO layer. This method is simple to operate but prone to over-etching, increasing the defect concentration on the CdTe back surface, thereby increasing non-radiative recombination on the back surface and reducing the device's open-circuit voltage and fill factor. Furthermore, this process generates a significant amount of Cd-containing chemical wastewater, and treating this wastewater greatly increases production costs. Dry etching primarily utilizes high-energy ion beams or plasma to bombard the CdTe surface, removing the CdO layer through physical sputtering. This approach avoids complex operating procedures and wastewater treatment, but requires vacuum equipment, resulting in relatively high costs. Simultaneously, plasma bombardment can potentially cause surface lattice damage, increasing the surface defect concentration and reducing the photoelectric conversion efficiency of the battery. Therefore, there is an urgent need to develop a more efficient and less damaging CdO layer treatment technology.

[0004] Among existing CdO layer processing technologies, wet acid washing is relatively complex, requiring additional treatment of acidic Cd-containing wastewater, increasing production costs and environmental burden. Furthermore, acid solutions may erode the CdTe bulk structure, causing lattice damage and increasing non-radiative recombination on the CdTe back surface. Plasma etching technology is limited by its high equipment cost, requiring high vacuum and plasma generation devices; simultaneously, high-energy ion bombardment may unavoidably damage the CdTe lattice, making uniform etching control relatively difficult, and uneven CdO removal is prone to occur during large-area etching, affecting battery performance.

[0005] Patent CN110021683A discloses a processing technology to improve the yield of cadmium telluride solar cells. This involves spraying a nitric acid solution with a pH of 0.3-5 onto the back surface of the cadmium telluride cell at 20-50°C for 5-60 seconds to etch it, resulting in a better Te-rich surface. Patent CN106252432A discloses a method for passivating defects in cadmium telluride cells and reducing defect density. This patent describes a method where, after annealing, the cell is etched with nitric acid and then placed in a PECVD vacuum chamber for plasma treatment to passivate defects.

[0006] The aforementioned patents disclose methods for improving wet or dry etching processes, reducing lattice damage to the CdTe surface, but in principle, they do not completely eliminate the possibility of damage caused by etching. On the other hand, the aforementioned patents do not disclose the use of redox methods to obtain CdO thin layers and use them as tunneling passivation layers. Nor do they disclose the use of redox methods to obtain metallic Cd thin layers and use them as part of a metal electrode. Summary of the Invention

[0007] This invention provides a method for fabricating a cadmium telluride thin-film solar cell with a cadmium oxide passivation layer and a method for reducing the cadmium oxide passivation layer, which is a highly efficient method for reducing the CdO layer in cadmium telluride solar cells. The method utilizes a reducing gas (such as H2 or CO) to perform in-situ chemical treatment on the CdO layer, reducing the relatively thick CdO insulating layer to a thin CdO layer covered by a thin metallic Cd layer. The thin CdO layer serves as a tunneling passivation layer with a thickness of 1.2-5.0 nm; the metallic Cd layer serves as part of the back electrode with a thickness of 5-12 nm.

[0008] To achieve the above objectives, in a first aspect, the present invention provides a method for reducing the cadmium oxide layer in a cadmium telluride battery, comprising the following steps: (1) Pretreatment: The cadmium telluride film with cadmium oxide layer formed on the surface after CdCl2 annealing is placed in a vacuum tube furnace and vacuumed to a negative pressure state to remove residual O2. (2) Reduction treatment: Turn off the vacuum pump, fill with inert gas, heat to the reduction reaction temperature and keep it at the temperature, and then introduce a mixture of reducing gas and inert gas under positive pressure to carry out the reduction reaction; the reduction reaction temperature is 200~400℃; the reducing gas accounts for 1-20% of the total volume of the mixture; the reaction time is 5-30min. (3) Post-treatment: turn off the heating and introduce inert gas to cool to room temperature to form a cadmium oxide layer covered with metallic cadmium.

[0009] Preferably, in step (1), residual O2 is removed from the vacuum tube furnace to achieve a negative pressure state with a pressure of less than 0.1 Pa.

[0010] Preferably, in step (2), the mixture of reducing gas and inert gas is an H2 / Ar mixture, the proportion of H2 introduced is 3%~9% of the H2 / Ar mixture, the reduction reaction temperature is 280℃~360℃, and the reaction time is 5min~20min.

[0011] Preferably, in step (2), the mixture of reducing gas and inert gas is a CO / Ar mixture, the proportion of CO introduced is 10%~20% of the CO / Ar mixture, the reaction temperature of the CO / Ar mixture is 300℃~360℃, and the reaction time is 10min~30min.

[0012] Preferably, in step (2), the air pressure is 50~200Pa in the positive pressure state.

[0013] Secondly, the present invention provides a method for preparing a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer. The cadmium telluride thin-film solar cell includes a glass substrate, a transparent conductive layer, a window layer, an absorption layer, a passivation layer, and a back electrode layer. The CdO layer generated on the absorption layer after annealing is treated by the aforementioned method for reducing the cadmium oxide layer of a cadmium telluride cell to form a passivation layer containing a cadmium oxide layer covered by metallic cadmium. The method includes the following steps: S1. A transparent conductive oxide thin film is deposited on a glass substrate to form a transparent conductive layer; S2. Deposit a window layer on the transparent conductive layer; S3. A CdTe thin film is deposited on the window layer to obtain an absorption layer, and then a reduction treatment is performed; The reduction process includes: S31. Deposit CdCl2 on the surface of the absorber layer and perform annealing to generate a CdO layer on the surface; S32. The generated CdO layer is treated using the reduction method for the cadmium oxide layer of the cadmium telluride battery. S4. Deposit a back electrode on the surface of the absorption layer after reduction treatment; S5. Annealing is performed after the back electrode deposition is completed.

[0014] Preferably, in step S1, the material of the transparent conductive oxide film is selected from one of fluorine-doped tin oxide, tin-doped indium oxide, cadmium-doped tin oxide, and gallium-doped zinc oxide; The deposition method of the transparent conductive oxide thin film is selected from one of magnetron sputtering, chemical vapor deposition, pulsed laser deposition, and sol-gel method.

[0015] Preferably, in step S2, the material of the window layer is selected from one or more combinations of cadmium sulfide, cadmium selenide, aluminum oxide, zinc oxide, and magnesium-doped zinc oxide, and the thickness is 10~300nm. The deposition method of the window layer is selected from one of magnetron sputtering, near-space sublimation, atomic layer deposition and chemical vapor deposition, and the substrate temperature during deposition is 25℃~400℃.

[0016] Preferably, in step S3, the deposition method of the CdTe absorption layer is selected from one of near-space sublimation, vapor transport deposition, chemical bath deposition, and chemical vapor deposition. During deposition, the heating temperature of the cadmium telluride solid evaporation source is 600~1200℃, the substrate temperature is 400~600℃, and the thickness is 3~5μm. In step S31, the deposition method of CdCl2 is selected from one of spraying, spraying, solution deposition, and electrochemical deposition. The annealing temperature of the annealing treatment is 300~500℃, and the annealing time is 10~40min.

[0017] Preferably, in step S5, the annealing temperature is 200~300℃ and the annealing time is 10~40min.

[0018] Thirdly, the present invention provides a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer, prepared according to the aforementioned method for preparing a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer, comprising a glass substrate, a transparent conductive layer, a window layer, an absorber layer, a passivation layer, and a back electrode layer sequentially disposed therefrom, wherein the passivation layer is a cadmium oxide layer thinned by reduction; the cadmium oxide layer passivates the back surface of the solar cell, and the recombination rate of the back surface is less than 10. 3 cm / s.

[0019] Preferably, the thickness of the cadmium metal layer in the cadmium oxide passivation layer covered by cadmium metal is 1.2~5.0 nm, and the thickness of the cadmium oxide layer is 1~12 nm.

[0020] The beneficial effects of this invention are as follows: The reduction method in this invention is a highly efficient method for reducing the CdO layer in cadmium telluride (CdTe) batteries. In-situ chemical treatment of the CdO layer using a reducing gas (such as H2 or CO) thins the relatively thick CdO insulating layer, forming a cadmium oxide passivation layer covered by metallic cadmium. After reduction and thinning, this layer can act as a tunneling layer, allowing electrons or holes to pass through, improving carrier transport capacity. Simultaneously, this oxide layer can fix surface dangling bonds, reducing interface defect state density and interfacial contact resistance, and decreasing recombination centers. A thin metallic cadmium (Cd) layer is introduced between the CdTe layer and the metal electrode, forming a low-resistance ohmic contact. Therefore, it can effectively reduce production costs while improving battery open-circuit voltage, fill factor, and photoelectric conversion efficiency. The solution of this invention simplifies the equipment complexity and multi-step process of traditional methods, avoids surface damage caused by wet chemical and physical etching, ensures the integrity of the CdTe surface structure, and reduces the surface recombination rate; at the same time, it achieves effective control of the CdO layer thickness. Attached Figure Description

[0021] 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.

[0022] Figure 1 The diagram illustrates the composition and structure of a cadmium telluride battery and the method for reducing the cadmium oxide layer, as provided in an embodiment of the present invention.

[0023] Figure 2 The reduction process flow of cadmium oxide layer in cadmium telluride battery provided in the embodiments of the present invention.

[0024] Explanation of reference numerals in the attached figures: A07, Glass substrate; A06, Transparent conductive layer; A05, Window layer; A04, Absorption layer; A03, Cadmium oxide layer; A02, Metallic cadmium layer; A01, Back electrode layer. Detailed Implementation

[0025] In this invention, unless otherwise stated, directional terms such as "up," "down," "left," and "right" are generally understood in conjunction with the accompanying drawings and the directions shown in actual applications.

[0026] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0027] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0028] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. The terms "optional" and "discretionary" mean that they may or may not be included (or may or may not be present).

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. The described embodiments are some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. A method for preparing a cadmium oxide passivation layer for a cadmium telluride battery is provided, comprising the following steps: (1) Pretreatment: The CdTe solar cells that have been annealed with CdCl2 and have a cadmium oxide layer on the back surface are cleaned with deionized water and dried with dry nitrogen. They are placed in a vacuum tube furnace, evacuated to a negative pressure state, and the O2 in the furnace is removed. Inert gas is introduced to purge, and the vacuum is continuously evacuated and replaced 2 to 3 times to remove residual O2. (2) Reduction treatment: Turn off the vacuum pump, fill with inert gas as protective gas, gradually raise the furnace temperature to the reaction temperature and keep it at the temperature, and then introduce a mixture of H2 / Ar or CO / Ar under positive pressure to start the reduction reaction. (3) Post-processing: After the reaction is completed, the heating is turned off, and inert gas is continuously introduced until the sample cools down. After cooling to room temperature, the sample is taken out to obtain a thinned CdO passivation layer sample covered with a thin metal Cd layer. The thickness of the metal Cd layer is 1.2~5.0 nm, which can be one or any two of the following values: 1.2 nm, 1.5 nm, 2.0 nm, 2.5 nm, 3.0 nm, 3.5 nm, 4.0 nm, 4.5 nm, 5.0 nm; the thickness of CdO is 1~12 nm, which can be one or any two of the following values: 1 nm, 2 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm.

[0030] The reduction method in this invention introduces a thin cadmium (Cd) layer between the cadmium telluride layer and the metal electrode. The work function of cadmium is more closely matched to that of cadmium telluride, effectively reducing the contact barrier and creating an ideal ohmic contact, thus significantly reducing the resistance to current transmission. By forming a low-resistance ohmic contact, the series resistance (Rs) of the battery is significantly reduced. Reduced series resistance means reduced energy loss, directly improving the fill factor (FF) and final photoelectric conversion efficiency. The remaining cadmium oxide (CdO) passivation layer effectively passivates surface defects on the back side of the cadmium telluride absorber layer. These defects become recombination centers for charge carriers (electrons and holes), leading to current loss. The passivation layer reduces this recombination, improving carrier lifetime. Furthermore, both surface passivation and ohmic contact are achieved simultaneously in the same layer structure.

[0031] In this embodiment, the thickness of the CdO insulating layer is reduced (CdO+H2→Cd+H2O or CdO+CO→Cd+CO2), transforming the insulating layer into a tunneling layer capable of carrier transport. This reduces series resistance, passivates interface defects, and decreases recombination probability. Simultaneously, it avoids lattice damage to CdTe caused by wet acid washing and plasma etching, significantly improving the photoelectric conversion efficiency of the battery.

[0032] In one embodiment of the present invention, in step (1), the cadmium oxide layer (5cm×5cm) of the CdTe solar cell after CdCl2 annealing is cleaned with deionized water and dried with dry nitrogen gas, placed in a vacuum tube furnace, evacuated to a negative pressure state, O2 in the furnace is removed, inert gas is introduced for purging, and evacuation is continued for 2-3 times to remove residual O. 2。 CdO surfaces often adsorb OH groups. -Alternatively, a thin layer of Cd(OH)₂ may form. These hydroxyl groups can react with the reducing gas during the subsequent H₂ / CO reduction process, producing water or carbonates, thus reducing the reduction efficiency. Nitrogen purging can remove these adsorbed moisture and hydroxyl groups, resulting in a purer oxide surface. A dry, oxygen-free surface allows H₂ or CO to contact CdO more directly and uniformly, avoiding the formation of localized barrier layers due to the presence of water vapor or oxygen, thereby improving the reduction rate and product uniformity.

[0033] In one embodiment of the present invention, in step (2), the vacuum pump is turned off during the reduction process, an inert gas is introduced as a protective gas, the furnace temperature is gradually increased to the reaction temperature and held, and then a mixture of H2 / Ar or CO / Ar is introduced under positive pressure, and the reduction reaction begins. The inert gas replaces the residual oxygen in the furnace during the heating stage, significantly inhibiting the oxidation of metals or oxides. Subsequently, the reducing gas supplied under positive pressure can continue to work in the same oxygen-free environment, preventing the infiltration of outside air. The inert gas is used to slowly raise the temperature first, so that the temperature of the furnace body and the sample is evenly distributed, avoiding thermal stress or local overheating caused by rapid heating, thereby improving the uniformity of the subsequent reduction reaction and the product quality. Under positive pressure, the partial pressure of H2 or CO remains at a high level, driving the reduction potential to move in a favorable direction, accelerating the reduction rate of oxides, and improving the conversion rate. H2 in H2 / Ar has strong reducing properties and can quickly remove the surface oxide layer, which is suitable for metals with high redox potential requirements. CO in CO / Ar provides stronger carbon-oxygen reduction at high temperatures, which is suitable for deep reduction processes. The ratio of the two gases can be adjusted with reference to the temperature-redox balance diagram to achieve a precise reduction environment. Using an inert gas instead of a reducing gas during the preheating stage avoids unnecessary hydrogen or carbon monoxide consumption during heating, resulting in higher overall gas utilization. Positive pressure gas supply prevents external air from flowing back into the furnace, reducing the risk of accidental oxidation or explosion; simultaneously, precise control of flow rate and pressure allows for real-time adjustment of the reducing atmosphere, improving process repeatability.

[0034] In this embodiment, regarding the rate and temperature control of the reduction reaction: excessively high temperatures will exacerbate the volatilization of cadmium, so it is necessary to precisely control the reaction temperature and rate. Therefore, this scheme selects a mild reduction scheme with a lower temperature, and uses inert gas protection in a closed system to reduce the volatilization of cadmium, and performs the reduction of cadmium oxide on the cadmium telluride surface.

[0035] Alternatively, in one embodiment, the reducing agents for cadmium oxide mainly include hydrogen, carbon monoxide, carbon, or metallothermic reduction, all of which require a high-temperature, closed system to cope with the volatilization of cadmium. Each of these reducing agents has its advantages and disadvantages, and the reasons why weakly acidic aqueous solutions cannot be used to reduce cadmium oxide are as follows: First, thermodynamic limitations (electrode potential): in weakly acidic solutions, the reduction of hydrogen ions is preferential to that of cadmium ions (the standard reduction potential of hydrogen ions is greater than that of cadmium ions), which will result in only hydrogen gas being released from the solution, not metallic cadmium. Second, there is a lack of a spontaneous reduction pathway kinetically. Cadmium oxide is insoluble in water, but dissolves in acid to generate cadmium ions, requiring an external reducing agent. Metallic cadmium can only be obtained through electrolysis, strong reducing agents, or displacement reactions; spontaneous reduction is not possible.

[0036] The necessity of gas reduction in this invention is as follows: First, hydrogen reduction of cadmium oxide occurs at a relatively low reaction temperature (200-400℃), which reduces cadmium volatilization, produces a high-purity product, and is more conducive to the formation of a high-purity cadmium layer. Carbon monoxide reduction of cadmium oxide has high reduction efficiency. Reduction with carbonaceous reducing agents (carbon, coke) is low-cost, but the reaction temperature is high, the reaction is vigorous, and it exacerbates cadmium volatilization, requiring strict temperature control and inert gas protection. Metallothermic reduction (aluminum, magnesium) is highly exothermic and requires careful control to avoid the risk of explosion. Therefore, in summary, reduction with hydrogen and carbon monoxide is more suitable for this scheme, as it is simple to operate, does not introduce new impurities, facilitates the formation of metallic cadmium, is easy to control, and carries lower risk.

[0037] In one embodiment of the present invention, in step (3), after post-processing, heating is turned off after the reaction is completed, and inert gas is continuously introduced until the sample cools down. After cooling to room temperature, the cadmium oxide layer of the CdTe solar cell is removed. By cooling under inert gas protection, after thinning the oxide layer, a thin cadmium oxide layer (1-12 nm) and a metallic cadmium layer (1.2-5.0 nm) will be formed on the surface. After thinning the oxide layer, tunneling passivation is achieved. Cooling and removing the oxide layer in an inert gas environment avoids the re-oxidation of the CdTe surface by moisture and oxygen in the air, ensuring the controllability of subsequent processes and improving the long-term stability of the device.

[0038] In this embodiment, the cadmium oxide layer thickness is generally controlled to be 1~12 nm. The main technical effect within this range is a significant improvement in the electrical performance of the cadmium telluride battery prepared by this method, with obvious optimizations in open-circuit voltage, fill factor, conversion efficiency, and series resistance. CdO is not simply removed, but thinned through a conversion reaction, which helps to form a carrier tunneling layer. By controlling the degree of reduction, the CdO layer thickness can be further precisely controlled, reducing damage to the CdTe lattice and other interface problems. This avoids the discharge of Cd-containing waste liquid and reduces the burden of environmental protection costs.

[0039] Optionally, in one embodiment, in step (1), the pressure at which oxygen is removed from the tubular furnace to achieve a negative pressure state is less than 0.1 Pa. It can be a value within the range of one or any two of 0.02 Pa, 0.04 Pa, 0.06 Pa, and 0.08 Pa. The ultra-low pressure vacuum reduces the residual oxygen concentration in the furnace to ppm or even lower, almost eliminating the re-oxidation of the formed CdO layer by oxygen, and maintaining the chemical composition and thickness of the layer unchanged.

[0040] Optionally, in one embodiment, in step (2), an H2 / Ar mixed gas is introduced, wherein the proportion of hydrogen to the total volume of hydrogen and argon is 3% to 9%, which can be any one or any two of 3%, 4%, 5%, 6%, 7%, 8%, and 9%. The H2 gas can reduce and thin CdO in a reducing atmosphere. The reaction temperature of the H2 / Ar mixed gas is 280℃ to 320℃, which can be any one or any two of 280℃, 290℃, 300℃, 310℃, and 320℃. The reaction time is 5min to 20min, which can be any one or any two of 5min, 10min, 15min, and 20min.

[0041] Optionally, in one embodiment, in step (2), a CO / Ar mixed gas is introduced, and the proportion of carbon monoxide to the total volume of carbon monoxide and argon is 10%~20%, which can be one or any two of the following: 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%. The reaction temperature of the CO / Ar mixed gas is 300℃~360℃, which can be one or any two of the following: 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃. The reaction time is 10min~30min, which can be one or any two of the following: 10min, 15min, 20min, 25min, 30min.

[0042] Optionally, in one embodiment, in step (2), the gas pressure in the positive pressure state is 50-200 Pa, which can be one or any two of the following values: 50 Pa, 60 Pa, 70 Pa, 80 Pa, 90 Pa, 100 Pa, 110 Pa, 120 Pa, 130 Pa, 140 Pa, 150 Pa, 160 Pa, 170 Pa, 180 Pa, 190 Pa, and 200 Pa. The positive pressure inert gas forms a slight positive driving force, making it difficult for external air to penetrate into the furnace, thereby preventing the reduced and thinned cadmium oxide surface from being oxidized again at high temperatures. Within this positive pressure range, it is possible to ensure that the inert gas forms a uniform flow layer on the sample surface, promoting the uniform distribution of reducing gas and avoiding defects caused by insufficient or excessive reduction in certain areas. Cd has a certain vapor pressure at high temperatures, and vacuum conditions will accelerate the sublimation loss of Cd. Positive pressure can suppress the evaporation of Cd to a certain extent, keeping the Cd content of the material at the design value, and improving the activity and lifespan of the final battery. Using a positive pressure inert gas of 50-200 Pa in the CdO reduction process can not only effectively prevent re-oxidation and reduce Cd volatilization, but also improve the reduction uniformity of the reducing gas and improve the structure of the film.

[0043] This invention also provides a method for fabricating a thin-film solar cell containing a cadmium oxide passivation layer. The CdTe thin-film solar cell includes a glass substrate, a transparent conductive layer, a window layer, an absorber layer, a passivation layer, and a back electrode layer. The CdO layer generated on the absorber layer after annealing is treated with a method for reducing the cadmium oxide layer in cadmium telluride solar cells, including the following steps: S1. A transparent conductive oxide thin film is deposited on a glass substrate to form a transparent conductive layer; In one embodiment of the present invention, a transparent conductive oxide layer with a thickness of 300-600 nm is formed on glass by magnetron sputtering or chemical vapor deposition (CVD).

[0044] S2. Deposit a window layer on the transparent conductive layer; In one embodiment of the present invention, one or more combinations of CdS, CdSe, ZnO, ZnO:Mg, SnO2, SnO2:Mg, or In2O3:Ga are deposited on a transparent conductive layer by magnetron sputtering or evaporation to reduce front-interface recombination. S3. A CdTe thin film is deposited on the window layer to obtain an absorption layer, and then a reduction treatment is performed; In one embodiment of the invention, p-type CdTe is deposited on the window layer by near-space sublimation (CSS) or magnetron sputtering.

[0045] The reduction process includes: S31, depositing CdCl2 on the surface of the absorber layer and annealing it to generate a CdO layer on the back surface of CdTe; In one embodiment of the present invention, during annealing in a high-temperature oxygen-containing atmosphere, the CdTe surface is partially oxidized to form a thick CdO layer. The thick oxide layer has a high resistance, which hinders the collection of current from the back side.

[0046] S32. The generated CdO layer is thinned by reduction method; In one embodiment of the present invention, the insulating CdO is reduced and thinned by reducing gas, thereby achieving carrier tunneling and enhancing carrier collection; at the same time, interface defects are passivated, nonradiative recombination on the back surface is reduced, and the open-circuit voltage, fill factor and conversion efficiency of the battery are improved to a certain extent.

[0047] S4. Deposit a back electrode on the surface of the absorption layer after reduction treatment. After the absorption layer surface is modified by reduction treatment, part of the layer structure changes. After reduction treatment, a thinned passivation layer and a covered metal cadmium layer are formed on the surface of the absorption layer after reduction treatment. A back electrode layer is deposited on the surface of the metal cadmium layer. The metal cadmium layer and the back electrode layer can be regarded as forming a composite multi-metal mixed electrode layer. One or more combinations of metals such as Au, Ag, Ni, Mo, and Al are deposited using vacuum evaporation or sputtering methods, with a thickness of 100-200 nm.

[0048] S5. Annealing is performed after the back electrode deposition is completed.

[0049] In one embodiment of the present invention, the preparation process of cadmium telluride battery mainly includes transparent conductive oxide, window layer, absorption layer, high-temperature annealing with Cl treatment, acid etching (or plasma cleaning or cadmium oxide reduction treatment mentioned in the present invention), and back electrode deposition.

[0050] Optionally, in one embodiment, the reduction of cadmium oxide in this invention is due to the fact that after high-temperature annealing with Cl, the surface of cadmium telluride reacts with oxygen to form cadmium oxide. This layer is non-conductive, which increases series resistance and severely damages battery performance. Therefore, reducing and thinning CdO to produce a tunneling passivation effect will facilitate carrier transport and reduce recombination.

[0051] Optionally, in one embodiment, in step S1, the material of the transparent conductive oxide film is selected from one of fluorine-doped tin oxide, tin-doped indium oxide, cadmium-doped tin oxide, and gallium-doped zinc oxide.

[0052] Optionally, in one embodiment, in step S1, the deposition method of the above-mentioned transparent conductive oxide thin film is selected from one of magnetron sputtering, chemical vapor deposition, pulsed laser deposition, and sol-gel method.

[0053] Optionally, in one embodiment, in step S2, the material of the window layer is selected from one or two of cadmium sulfide, cadmium selenide, aluminum oxide, zinc oxide, and magnesium-doped zinc oxide, and the thickness is 10~300nm, which can be a range of one or any two of 10nm, 50nm, 100nm, 150nm, 200nm, 250nm, and 300nm.

[0054] Optionally, in one embodiment, in step S2, the deposition method of the window layer is selected from magnetron sputtering, near-space sublimation, atomic layer deposition, and chemical vapor deposition. The substrate temperature during deposition is 25°C to 400°C, which can be any combination of 25°C, 50°C, 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, and 400°C. Within the temperature window of 25°C to 400°C, the CdTe film can maintain good adhesion and mechanical integrity, while also improving crystal quality, carrier concentration, and optical bandgap through moderate heating, ultimately achieving higher photoelectric conversion efficiency, while simultaneously considering cost, energy consumption, and substrate material limitations.

[0055] Optionally, in one embodiment, in step S3, the CdTe absorber layer deposition method is selected from near-space sublimation, vapor transport deposition, chemical bath deposition, and chemical vapor deposition. In actual production, these deposition technologies are often selected or combined based on cost, production capacity, device structure, and equipment conditions to achieve the goal of high efficiency and low cost for CdTe thin-film solar cells. The source temperature during deposition is 600~1200℃, which can be any one or any two of 600℃, 700℃, 800℃, 900℃, 1000℃, 1100℃, and 1200℃. A high source temperature provides sufficient CdTe vapor pressure, which increases the deposition rate and is beneficial for forming a dense and uniform thin film. The substrate temperature is 400~600℃, which can be any one or any two of 400℃, 450℃, 500℃, 550℃, and 600℃. A higher substrate temperature is beneficial for reducing grain boundary recombination and improving carrier lifetime. The thickness is 3~5μm, which can be one of 3μm, 4μm, 5μm or any two of them; within this thickness range, it is beneficial to reduce grain boundary recombination and improve the fill factor and open circuit voltage.

[0056] Optionally, in one embodiment, in step S31, the deposition method for depositing CdCl2 on the surface of the absorber layer is selected from one of spraying, atomizing, solution deposition, and electrochemical deposition. The annealing temperature of CdCl2 is 300~500℃, which can be any one or any two of 300℃, 350℃, 400℃, 450℃, and 500℃. Within this temperature range, CdCl2 annealing is beneficial for passivating grain boundary (GB) defects, reducing grain boundary resistance, and decreasing the probability of carrier recombination. The annealing time is 10~50 min, which can be any one or any two of 10 min, 20 min, 30 min, 40 min, and 50 min. In the flexible CdTe thin-film battery structure, CdCl2 annealing combined with the deposition of the metal back electrode can improve the ohmic contact between the metal and CdTe, promote -p- carrier transport, and further reduce series resistance.

[0057] Optionally, in one embodiment, in step S4, the back electrode material is selected from at least one of molybdenum, nickel, aluminum and chromium.

[0058] Optionally, in one embodiment, in step S5, the annealing temperature is 200~300℃, which can be a range of one or any two of 200℃, 220℃, 240℃, 260℃, 280℃, and 300℃, and the annealing time is 10~40min, which can be a range of one or any two of 10min, 20min, 30min, and 40min.

[0059] This invention also provides a thin-film solar cell with a cadmium oxide passivation layer, comprising a glass substrate, a transparent conductive oxide thin film layer, a window layer, an absorber layer, a passivation layer, and a back electrode layer sequentially disposed therefrom, wherein the passivation layer is a cadmium oxide layer; the cadmium oxide layer passivates the back surface of the solar cell, thereby reducing the recombination rate of the back surface to below 10. 3 cm / s. The aforementioned back electrode layer comprises a thin Cd metal layer.

[0060] Optionally, in one embodiment, the thickness of the cadmium oxide layer in the cadmium oxide passivation layer covered by the metal cadmium is 1.2~5.0 nm, and the thickness of the metal cadmium layer is 5~12 nm.

[0061] The present invention will be described in detail below through embodiments.

[0062] Example 1 This embodiment provides a method for preparing a CdTe thin-film battery. After annealing, the surface CdO layer of the CdTe absorber layer undergoes a reduction treatment. The specific process is as follows: Figure 1 and Figure 2As shown, the structure of the CdTe thin-film battery includes a glass substrate A07, a transparent conductive layer A06, a window layer A05, an absorber layer A04, a cadmium oxide layer A03, a cadmium metal layer A02, and a back electrode layer A01. The back electrode layer A01 is another metal layer, and the cadmium metal layer A02 can be considered as forming a composite electrode with other metal layers; the absorber layer A04 has a cadmium oxide layer A03 on its surface, and the CdO layer is covered with a cadmium metal layer A02 obtained after reduction treatment. In the embodiment, the glass substrate A07 is soda-lime glass; the transparent conductive layer A06 is fluorine-doped tin oxide (FTO); the window layer A05 is cadmium selenide (CdSe); the absorber layer A04 is CdTe; and the back electrode layer A01 is an aluminum (Al) layer.

[0063] The CdTe thin-film battery preparation method in this embodiment includes the following steps: S1. FTO with a thickness of 400 nm is deposited on glass substrate A07 by chemical vapor deposition to obtain transparent conductive layer A06. S2. CdSe is deposited on the transparent conductive layer A06 by magnetron sputtering. The process conditions can be: deposition gas pressure of 1.0-1.5 Pa and power density of 1 W / cm³. 2 Other conditions may also be used, and no specific limitations are imposed; the substrate temperature is 300℃ and the thickness is 120nm to obtain window layer A05. S3. CdTe is deposited on the window layer A05 by near-space sublimation method, with a source temperature of 650℃, a substrate temperature of 550℃, and a thickness of 4μm, to obtain the absorption layer A04. S4. Spray CdCl2 onto the surface of the absorbent layer A04 and perform high-temperature annealing at 400℃ for 20 minutes to produce an oxide layer. S5. The oxide layer is reduced and thinned by using reducing gas in a vacuum tube furnace, which mainly includes the following steps; (1) Clean the CdTe solar cell (5cm×5cm) that has completed CdCl2 annealing treatment with deionized water and blow dry the surface moisture. Place it in a vacuum tube furnace, evacuate to a negative pressure state, remove O2, and purge with argon gas 2-3 times. (2) Introduce a small amount of argon as a protective gas, raise the temperature to 300℃ and keep it at that temperature for 10 min, then introduce 3% H2 and react for 15 min; (3) After the reaction is complete, turn off the heating and continue to purge with argon gas until the sample cools down. After cooling to room temperature, remove the battery. The cadmium oxide layer A03 and the metallic cadmium layer A02 are thinned by the reduction reaction.

[0064] S6. Continue to deposit metal Al on the metal cadmium layer A02 by magnetron sputtering, with a total thickness of 180-200 nm, to obtain the back electrode layer A01.

[0065] S7. Perform metallization annealing on the battery at a temperature of 210℃ for 20 minutes.

[0066] Example 2 This embodiment provides a method for preparing a CdTe thin-film battery, which is basically the same as that in Example 1. The difference is that the H2 content used in the reduction process is 6%.

[0067] Example 3 This embodiment provides a method for preparing a CdTe thin-film battery, which is basically the same as that in Example 1. The difference is that the H2 content used in the reduction process is 9%.

[0068] Example 4 This embodiment provides a method for preparing a CdTe thin-film battery, which is basically the same as that in Example 1. The difference is that the CO content used in the reduction process is 15%, and the reaction time is 20 minutes.

[0069] Example 5 This embodiment provides a method for preparing a CdTe thin-film battery, which is basically the same as that in Example 4. The difference is that the temperature used in the reduction process is 330°C.

[0070] Example 6 This embodiment provides a method for preparing a CdTe thin-film battery, which is basically the same as that in Example 4. The difference is that the temperature used in the reduction process is 360°C.

[0071] Comparative Example 1 This embodiment provides a method for preparing a CdTe thin-film battery, which is basically the same as that in Example 1. The difference is: S5. The oxide layer is completely removed using an acid etching process, which mainly includes the following steps: (1) Clean the CdTe solar cell (5cm×5cm) that has undergone CdCl2 annealing with deionized water and blow dry the surface moisture; (2) Wash in 5% HNO3 solution for 10-30 seconds, then blow dry the surface moisture; S6. Deposit ZnTe on the acid-washed CdTe surface by magnetron sputtering with a thickness of 30-35 nm to obtain the back contact layer. S7. A metal layer Al with a thickness of 180-200 nm is deposited on the back contact layer by magnetron sputtering to obtain the back electrode layer.

[0072] S8. Perform metallization annealing on the battery at a temperature of 210℃ for 20 minutes.

[0073] The preparation method of CdTe thin-film solar cells and the CdO layer reduction process of this invention, as shown in Table 1, result in the following improvements in cell performance compared to the acid washing process: the fill factor is increased by approximately 6%, the open-circuit voltage is increased by approximately 50mV, and the conversion efficiency is improved by approximately 1.6%-2.6%. Regarding process advantages: no acidic Cd-containing waste liquid is generated, resulting in better environmental performance; and regarding cell uniformity: the efficiency fluctuation between batches is <0.3% (compared to >0.8% for traditional processes).

[0074] Table 1: Comparison of battery performance under different CdO layer treatments

[0075] The microstructure of the above embodiments was observed after reduction, and the battery was tested using IV. The data are shown in Tables 2 and 3 below: Table 2: CdTe surface micromorphology of each embodiment

[0076] The reduced CdO has a thickness of approximately 1-10 nm, which allows for efficient reduction of the CdO layer to a certain extent. Compared with acid etching, the reduction method does not have the problem of preferred etching, so the surface roughness of the reduced CdTe does not increase significantly (as shown in Table 2), and a continuous and stable structure can be formed, which is better than the acid etching process. At the same time, the uniformity of the thinned cadmium oxide can reach more than 95%.

[0077] In addition, the H2 and CO reduction method of this invention forms a high-quality interface, which can repair CdTe surface defects to a certain extent and reduce interfacial recombination. Secondly, the reaction conditions are controllable, which can achieve precise control of the cadmium oxide layer thickness and form a dense and uniform film.

[0078] Table 3: Comparison of battery performance between various embodiments and comparative examples

[0079] IV tests were performed on the sample. The results showed that the H2 reduction reaction was optimal with an H2 / Ar mixed gas ratio of 6% and a reaction time of 15 min at 300℃, while the CO reduction reaction was optimal with a CO / Ar mixed gas ratio of 15% and a reaction time of 20 min at 330℃. These data indicate that this approach can significantly improve battery performance (as shown in Tables 1 and 3).

[0080] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for reducing the cadmium oxide layer in a cadmium telluride battery, characterized in that, Includes the following steps: (1) Pretreatment: The cadmium telluride film with cadmium oxide layer formed on the surface after CdCl2 annealing is placed in a vacuum tube furnace and vacuumed to a negative pressure state to remove residual O2. (2) Reduction treatment: Turn off the vacuum pump, fill with inert gas, heat to the reduction reaction temperature and keep it at the temperature, and then introduce a mixture of reducing gas and inert gas under positive pressure to carry out the reduction reaction; the reduction reaction temperature is 200~400℃; the reducing gas accounts for 1-20% of the total volume of the mixture; the reaction time is 5-30min. (3) Post-treatment: turn off the heating and introduce inert gas to cool to room temperature to form a cadmium oxide layer covered with metallic cadmium.

2. The method for reducing the cadmium oxide layer in a cadmium telluride battery according to claim 1, characterized in that, In step (1), residual O2 is removed from the vacuum tube furnace to achieve a negative pressure state with a gas pressure of less than 0.1 Pa.

3. The method for reducing the cadmium oxide layer in a cadmium telluride battery according to claim 1, characterized in that, In step (2), the mixture of reducing gas and inert gas is H2 / Ar mixture, the proportion of H2 introduced is 3%~9% of H2 / Ar mixture, the reduction reaction temperature is 280℃~360℃, and the reaction time is 5min~20min.

4. The method for reducing the cadmium oxide layer in a cadmium telluride battery according to claim 1, characterized in that, In step (2), the mixture of reducing gas and inert gas is a CO / Ar mixture. The proportion of CO introduced into the CO / Ar mixture is 10%~20%. The reaction temperature of the CO / Ar mixture is 300℃~360℃ and the reaction time is 10min~30min.

5. The method for reducing the cadmium oxide layer in a cadmium telluride battery according to claim 1, characterized in that, In step (2), the air pressure is 50~200Pa in the positive pressure state.

6. A method for preparing a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer, characterized in that, A cadmium telluride thin-film solar cell includes a glass substrate, a transparent conductive layer, a window layer, an absorber layer, a passivation layer, and a back electrode layer. The CdO layer formed on the absorber layer after annealing is treated by the reduction method for the cadmium oxide layer of a cadmium telluride solar cell according to any one of claims 1-5 to form a passivation layer containing a cadmium oxide layer covered by metallic cadmium, comprising the following steps: S1. A transparent conductive oxide thin film is deposited on a glass substrate to form a transparent conductive layer; S2. Deposit a window layer on the transparent conductive layer; S3. A CdTe thin film is deposited on the window layer to obtain an absorption layer, and then a reduction treatment is performed; The reduction process includes: S31. Deposit CdCl2 on the surface of the absorber layer and perform annealing to generate a CdO layer on the surface; S32. The generated CdO layer is treated using the reduction method for the cadmium oxide layer of the cadmium telluride battery. S4. Deposit a back electrode on the surface of the absorption layer after reduction treatment; S5. Annealing is performed after the back electrode deposition is completed.

7. The method for preparing a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer according to claim 6, characterized in that, In step S1, the material of the transparent conductive oxide film is selected from one of fluorine-doped tin oxide, tin-doped indium oxide, cadmium-doped tin oxide, and gallium-doped zinc oxide; The deposition method of the transparent conductive oxide thin film is selected from one of magnetron sputtering, chemical vapor deposition, pulsed laser deposition, and sol-gel method.

8. The method for preparing a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer according to claim 6, characterized in that, In step S2, the material of the window layer is selected from one or more combinations of cadmium sulfide, cadmium selenide, aluminum oxide, zinc oxide, and magnesium-doped zinc oxide, and the thickness is 10~300nm. The deposition method of the window layer is selected from one of magnetron sputtering, near-space sublimation, atomic layer deposition and chemical vapor deposition, and the substrate temperature during deposition is 25℃~400℃.

9. The method for preparing a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer according to claim 6, characterized in that, In step S3, the CdTe absorption layer deposition method is selected from one of near-space sublimation, vapor transport deposition, chemical bath deposition, and chemical vapor deposition. During deposition, the heating temperature of the cadmium telluride solid evaporation source is 600~1200℃, the substrate temperature is 400~600℃, and the thickness is 3~5μm. In step S31, the CdCl2 deposition method is selected from one of spraying, spraying, solution deposition, and electrochemical deposition. The annealing temperature of the annealing treatment is 300~500℃, and the annealing time is 10~40min.

10. The method for preparing a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer according to claim 6, characterized in that, In step S5, the annealing temperature is 200~300℃ and the annealing time is 10~40min.

11. A cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer, prepared by the method of preparing a cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer according to any one of claims 6 to 10, characterized in that, The solar cell comprises, in sequence, a glass substrate, a transparent conductive layer, a window layer, an absorbing layer, a passivation layer and a back electrode layer, wherein the passivation layer is a reduced and thinned cadmium oxide layer; the cadmium oxide layer passivates the back surface of the solar cell, and the recombination rate of the back surface is less than 10 3 cm / s.

12. The cadmium telluride thin-film solar cell containing a cadmium oxide passivation layer according to claim 11, characterized in that, The thickness of the cadmium metal layer in the cadmium oxide passivation layer covered by cadmium metal is 1.2~5.0 nm, and the thickness of the cadmium oxide layer is 1~12 nm.

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