Preparation method of vanadium-doped carbon composite back contact layer and application thereof in preparation of cadmium telluride solar cell
By preparing a vanadium-doped carbon composite back contact layer using vanadium-doped carbazole polymer, the problems of copper ion diffusion and interfacial recombination in traditional cadmium telluride solar cells were solved, thus improving cell efficiency and stability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- FLAT GLASS GROUP CO LTD
- Filing Date
- 2026-04-28
- Publication Date
- 2026-05-29
Smart Images
Figure CN122121318A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of cadmium telluride solar cell technology, and particularly to a method for preparing a vanadium-doped carbon composite back contact layer and its application in the preparation of cadmium telluride solar cells. Background Technology
[0002] In cadmium telluride (CdTe) solar cells, the performance of the back contact layer is a key factor restricting device efficiency and stability. Currently, the mainstream technology mainly adopts copper (Cu)-based inorganic material systems: one is the zinc telluride copper (ZnTe:Cu) composite back contact layer (such as CN102629630A), which reduces the Schottky barrier through copper (Cu) doping. However, copper ions easily diffuse along the grain boundaries to the cadmium telluride light absorption layer, forming deep-level defect recombination centers, resulting in an annual battery efficiency degradation rate of over 3%, and the device performance is extremely sensitive to doping concentration and annealing process parameters; the other is the copper-doped molybdenum oxide (MoO:Cu) back contact layer (such as CN201810813214A), which, although suppressing the bulk diffusion of copper to some extent, requires acid etching to form a textured structure to enhance adhesion, making process control complex, and the high copper content (MoO:copper=9:1) still poses a long-term stability risk. In addition, some copper-free alternatives, such as the molybdenum oxide (MoO) / molybdenum (Mo) / molybdenum nitride (MoN) multilayer structure (CN201611236907A), can effectively avoid copper diffusion, but due to the poor band matching between layers, the open circuit voltage (Voc) is difficult to exceed 820 mV, resulting in limited performance improvement.
[0003] In terms of doping processes, existing technologies also have significant limitations: while in-situ doping with copper chloride (CuCl2) solution (such as CN201210112099.5) can introduce copper, its diffusion depth is uncontrollable, and it tends to accumulate at the cadmium telluride (CdTe) / cadmium sulfide (CdS) heterojunction interface, leading to light-induced degradation. On the other hand, while non-in-situ doping with group V elements, such as bismuth trifluoride (BiF3), can avoid the copper diffusion problem, it requires high-temperature cadmium (Cd) vapor activation, resulting in a complex process and low bismuth (Bi) doping activation rate (<0.09%), severely restricting the improvement of the cell fill factor (FF). Therefore, developing a novel back contact layer technology that can balance efficient hole extraction, excellent interface stability, and process controllability has become an urgent need to promote the further development of cadmium telluride solar cells. Summary of the Invention
[0004] This invention addresses the problems of deep-level defects, ion diffusion, and high lattice stress in traditional copper-doped back contact layers by proposing a vanadium-doped alternative to copper doping. A vanadium-doped carbazole polymer is synthesized by coordinating a vanadium source compound with a carbazole monomer. The synthesized polymer is then dissolved and coated onto the surface of a cadmium telluride light-absorbing layer, followed by gradient annealing to transform it into a vanadium-doped carbon composite back contact layer. During gradient annealing, the coordination bonds in the polymer undergo selective breakage, releasing vanadium ions that preferentially diffuse along the cadmium telluride grain boundaries, forming localized P2 ions. + The doped region enhances hole extraction efficiency and avoids bulk lattice distortion; simultaneously, the carbonization of the polymer backbone forms a graphene-like dense conductive network, blocking diffusion from the metal back electrode and suppressing interfacial recombination (S<10). 3 (cm / s), while simultaneously transforming the intramolecular conjugation of the vanadium-doped carbazole polymer into a long-range delocalized π network, thereby enhancing carrier transport capability.
[0005] The technical solution provided by this invention is as follows: In a first aspect, the present invention provides a method for preparing a vanadium-doped carbon composite back contact layer, comprising the following steps: A vanadium-doped carbazole polymer (V-PCz) solution was deposited on the surface of a cadmium telluride light-absorbing layer to form a polymer film; Under a nitrogen atmosphere, the polymer film is subjected to gradient annealing to form a vanadium-doped carbon composite back contact layer.
[0006] In some specific implementations, the gradient annealing process includes: Pre-anneal at 180~220℃ for 25~35 minutes; Perform main annealing at 320~370℃ for 55~65 minutes; Perform short-term annealing at 380~400℃ for 5~15 minutes.
[0007] In some specific embodiments, the thickness of the polymer film is 20~150 nm.
[0008] In some specific embodiments, the vanadium-doped carbazole polymer is prepared by the following steps: A vanadium source compound and a carbazole monomer were dissolved in an organic solvent at a molar ratio of 1:(20~50). Under the action of a catalyst, a coordination reaction was carried out at 80~100°C for 20~30 hours to obtain a polymer with a vanadium content of 0.5 wt%~5 wt%. After the reaction was completed, a poor solvent was added to the reaction system to precipitate the polymer. After filtration and purification, the vanadium-doped carbazole polymer was obtained.
[0009] In some specific embodiments, the vanadium source compound is vanadium acetylacetonate, the carbazole monomer is 3,6-dibromo-9-n-octylcarbazole, the catalyst is tetra(triphenylphosphine)palladium, the organic solvent is a mixed solvent of toluene and water with added alkali, and the undesirable solvent is methanol.
[0010] In some specific embodiments, the vanadium-doped carbazole polymer is dissolved in an organic solvent to form a 5-15 mg / mL vanadium-doped carbazole polymer solution.
[0011] In some specific embodiments, the organic solvent is selected from at least one of chlorobenzene, N,N-dimethylformamide, or toluene.
[0012] Secondly, the present invention also provides a method for preparing a cadmium telluride solar cell, comprising the following steps: Provide a transparent conductive oxide substrate; A buffer layer, a window layer, and a cadmium telluride light-absorbing layer are sequentially deposited on the substrate; A vanadium-doped carbon composite back contact layer, prepared as described above, is fabricated on the pretreated cadmium telluride light-absorbing layer. A metal back electrode is fabricated on the vanadium-doped carbon composite back contact layer.
[0013] In some specific embodiments, the buffer layer is a tin oxide layer with a thickness of 20~50 nm; The window layer is a cadmium selenide layer with a thickness of 100~300 nm; The cadmium telluride light-absorbing layer was deposited using a near-space sublimation method, and its thickness was 3~5 μm. The metal back electrode is a Mo / Al / Cr composite electrode with a total thickness of 100~300 nm.
[0014] Thirdly, the present invention also provides a cadmium telluride solar cell, which is prepared by the method described above.
[0015] By adopting the above technical solution, the method for preparing a vanadium-doped carbon composite back contact layer provided by the present invention and its application in the preparation of cadmium telluride solar cells have the following beneficial effects: 1. Controllable doping and fewer defects: This invention forms a polycarbazole conjugated backbone through Suzuki coupling polymerization. Simultaneously, the vanadium source compound coordinates with nitrogen atoms in the polymer chain, achieving uniform and controllable introduction of vanadium into the polymer. This avoids the problem of metal ions (such as Cu) being introduced into the polymer in traditional processes. + The uncontrolled spread of V. 3+ Ionic radius and Cd 2+ Good matching, local P is formed after gradient annealing. +Doping effectively increases carrier concentration while greatly reducing the risk of introducing deep-level defects.
[0016] 2. Low interfacial recombination and superior performance: The formed vanadium-doped carbon composite back contact layer possesses high conductivity, suitable work function, and excellent interfacial passivation capability. It forms a good bandgap match with the CdTe light-absorbing layer, reducing the back contact barrier and interfacial recombination rate, thus expected to achieve higher open-circuit voltage (Voc) and fill factor (FF), thereby improving the cell conversion efficiency (PCE).
[0017] 3. High stability: The graphene-like network formed by carbonization not only provides a conductive path but also acts as an effective diffusion barrier layer, suppressing the migration of ions from the metal back electrode. Simultaneously, it avoids performance degradation caused by long-term copper ion migration, resulting in superior long-term device stability. Attached Figure Description
[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0019] Figure 1 The structural formula of the vanadium-doped carbazole polymer provided in the embodiments of the present invention; Figure 2 This is a schematic diagram of a cadmium telluride solar cell provided in an embodiment of the present invention.
[0020] The following is supplementary explanation of the attached figures: 1-Transparent conductive oxide substrate; 2-Buffer layer; 3-Window layer; 4-Cadmium telluride light absorption layer; 5-Vanadium-doped carbon composite back contact layer; 6-Metal back electrode. Detailed Implementation
[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.
[0022] The term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the invention. In the description of the invention, it should be understood that the terms "upper," "lower," "top," "bottom," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. 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 indicated technical features. Thus, a feature defined with "first" and "second" may explicitly or implicitly include one or more of that feature. Moreover, the terms "first," "second," etc., are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein.
[0023] When a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to an integer, it includes every integer between the minimum and maximum values of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included. For example, a specified range from “1 to 10” should be considered to include any and all subranges between the minimum value 1 and the maximum value 10. Exemplary subranges of the range 1 to 10 include, but are not limited to, 1 to 6.1, 3.5 to 7.8, 5.5 to 10, etc.
[0024] In traditional ZnTe:Cu or Cu-based back contact layers, the diffusion and migration of Cu ions easily form deep-level defects in cadmium telluride solar cells, leading to a decrease in long-term cell stability. Simultaneously, the work function mismatch between the back contact layer and the CdTe light-absorbing layer 4 causes severe recombination of interfacial carriers, limiting the improvement of open-circuit voltage (Voc) and fill factor (FF). To address these issues, this invention provides a method for preparing a vanadium-doped carbon composite back contact layer. This method is based on the design of a V-doped organic porous polymer, allowing the covalent bonds between V and polycarbazole to selectively break during annealing. The released V ions preferentially diffuse along the CdTe grain boundaries to form localized P ions. + Doping regions improve hole extraction efficiency while avoiding bulk lattice distortion. Specifically, this includes the following steps: First, a vanadium-doped carbazole polymer (V-PCz) solution is uniformly deposited on the surface of the cadmium telluride (CdTe) light-absorbing layer 4 using ultrasonic spraying or spin coating to form a polymer film with a thickness of 20-150 nm, preferably 100 nm. In a preferred embodiment, the cadmium telluride light-absorbing layer 4 is first activated with cadmium chloride and then etched on the surface before the V-PCz solution is deposited; wherein, the cadmium chloride activation treatment is preferably annealed at 400°C for 30 min, and the surface etching is performed using a nitric acid solution (nitric acid:water = 1:13) for 10 s to optimize the interface properties.
[0025] Subsequently, the polymer film was subjected to gradient annealing under a nitrogen atmosphere to transform it into a vanadium-doped carbon composite back contact layer. During the gradient annealing process, the VN / VO coordination bonds in the polymer underwent selective breakage, and the released vanadium ions preferentially diffused rapidly along the CdTe grain boundaries and entered the lattice, forming localized P + In the doped region, the polymer backbone carbonizes to form a dense, graphene-like conductive network. Its conjugated structure not only endows this layer with excellent conductivity but also serves as a highly efficient hole transport buffer layer. During this process, V, as a group V element dopant, occupies Cd. 2+ Site (acceptor state), V in CdTe lattice 3+ It exists in a low-spin state (d 2 Electronic configuration), its effective ionic radius (~0.78 Å) is similar to that of Cd. 2+ The difference (0.17 Å) between copper ions and Cd is less than that between copper ions and Cd ions. 2+ The difference (0.18 Å), and due to V 3+ The Jahn-Teller effect is suppressed in the octahedral field, resulting in lower lattice stress. Simultaneously, first-principles calculations show that V-doped vacancy has a higher defect formation energy than Cu, making it more difficult to form defect recombination centers. Therefore, its higher defect formation energy fundamentally avoids the deep-level defects and attenuation problems caused by Cu ion diffusion. Furthermore, the Te vacancies (V-doped vacancy) at grain boundaries... Te The polycarbazole carbonized layer effectively passivates and inhibits V. Te The formation of antisite defects, combined with the bandgap modulation and interface passivation of V-PCz, synergistically achieves effective control of the contact barrier between the CdTe light absorption layer 4 and the metal back electrode 6, thereby improving the hole injection and collection efficiency.
[0026] In some specific embodiments, the gradient annealing process includes the following three stages: First, a pre-annealing is performed at 180~220℃ for 25~35 min (preferably annealing at 200℃ for 30 min), which mainly promotes the full evaporation of the solvent and the initial densification of the film structure; subsequently, a main annealing is performed at 320~370℃ for 55~65 min (preferably annealing at 350℃ for 60 min), which can effectively trigger the selective breaking of coordination bonds in the polymer, promoting V 3+ Ions diffuse into the CdTe lattice to achieve p-type doping, while simultaneously inducing a carbonization transformation in the polymer backbone to form a graphene-like conductive network. Finally, a short-time annealing process is performed at 380–400 °C for 5–15 min (preferably 10 min at 400 °C) to further enhance the degree of carbonization and graphitization quality, thereby improving the conductivity of the carbon composite layer. Through this gradient annealing process, Vt can be precisely controlled. 3+ The diffusion behavior, the carbonization process of the polymer, and the microstructure of the carbon layer ultimately form a vanadium-doped carbon composite back contact layer 5 with excellent conductivity, good hole transport capability, and efficient interface regulation function.
[0027] In some specific embodiments, the thickness of the polymer film is 20-150 nm, preferably 100 nm. When the film thickness is less than 20 nm, it is difficult to form a continuous and dense capping layer, which easily leads to pinhole defects and intensifies interfacial recombination. When the film thickness exceeds 150 nm, the excessively thick organic layer increases the series resistance of the device and hinders carrier transport. This 20-150 nm thickness ensures that the film has complete coverage to effectively passivate CdTe surface defects while maintaining a low series resistance.
[0028] In some specific embodiments, the vanadium-doped carbazole polymer is prepared by the following steps: A vanadium source compound and a carbazole monomer are dissolved in an organic solvent at a molar ratio of 1:(20~50), and a coordination reaction is carried out at 80~100°C for 20~30 hours under the action of a catalyst to obtain a polymer with a vanadium content of 0.5 wt%~5 wt%. After the reaction is completed, a poor solvent is added to the reaction system to precipitate the polymer. After filtration and purification, a vanadium-doped carbazole polymer (V-PCz) with an extended π-conjugated fused-ring aromatic structure is obtained. For its specific structure, please refer to [reference needed]. Figure 1 This structure effectively enhances the stacking density and order between molecular chains, increasing the hole mobility of the material to >5×10⁻⁶. - ³ cm² / V·s.
[0029] Furthermore, the preferred vanadium source compound is vanadium acetylacetonate (VO(acac)2), which has good solubility and coordination stability, and can serve as an effective vanadium source precursor for polymerization; the preferred carbazole monomer is 3,6-dibromo-9-n-octylcarbazole, whose symmetrical bromine substitution sites are suitable for Suzuki coupling reactions and can form a conjugated polymer backbone; the preferred catalyst is tetrakis(triphenylphosphine)palladium (Pd(PPh3)4), which, as a classic homogeneous catalyst for the Suzuki reaction, can efficiently catalyze the formation of carbon-carbon bonds; the preferred organic solvent is a toluene-water mixture with added alkali (the volume ratio of toluene to water is usually 1:3 to 1:5), in which toluene dissolves the organic reactants and the aqueous phase dissolves the alkali to provide the necessary alkaline environment, wherein the alkali is preferably potassium carbonate; the preferred unsuitable solvent is methanol, which allows the polymer to precipitate effectively through the anti-solvent effect, facilitating separation and purification.
[0030] In some specific embodiments, the vanadium-doped carbazole polymer is dissolved in an organic solvent to form a vanadium-doped carbazole polymer solution with a concentration of 5-15 mg / mL, preferably 7 mg / mL. This concentration range ensures that the solution has a suitable viscosity for spin coating or spray coating processes: when the concentration is below 5 mg / mL, the solution viscosity is too low, resulting in insufficient cohesion of the liquid film, which easily leads to discontinuities in the polymer film and pinhole defects, failing to completely cover the surface of the CdTe light-absorbing layer 4; conversely, when the concentration exceeds 15 mg / mL, the solution viscosity is too high, which will seriously affect the leveling during the coating process, resulting in uneven film thickness and even coating streaks. Therefore, controlling the concentration within the range of 5-15 mg / mL can ensure the continuity and uniformity of film formation while providing a precursor film with a regular structure and appropriate thickness for subsequent gradient annealing.
[0031] Furthermore, chlorobenzene is preferred as the organic solvent. Chlorobenzene has good solubility for vanadium-doped carbazole polymer (V-PCz) and can form a stable homogeneous solution; its moderate boiling point (132℃) ensures that the solvent evaporates smoothly during the coating process, avoiding surface defects of the film due to excessive evaporation; at the same time, chlorobenzene is highly compatible with spin coating and spray coating processes, which can effectively ensure the film quality and thickness uniformity.
[0032] This invention also provides a method for preparing a cadmium telluride solar cell, comprising the following steps: A transparent conductive oxide substrate 1 is provided; a buffer layer 2, a window layer 3 and a cadmium telluride light-absorbing layer 4 are sequentially deposited on the substrate; a vanadium-doped carbon composite back contact layer 5 is prepared on the pretreated cadmium telluride light-absorbing layer 4 using the aforementioned method; and finally, a metal back electrode 6 is prepared on the vanadium-doped carbon composite back contact layer 5.
[0033] The specific preparation process is as follows: First, a commercially available transparent conductive oxide substrate 1 (such as TCO, ITO, AZO, or FTO) is selected and ultrasonically cleaned sequentially with deionized water and ethanol to remove surface oil and impurities. Then, it is dried with high-purity nitrogen to ensure substrate cleanliness. Next, a 20-50 nm (preferably 30 nm) buffer layer 2 is deposited on the clean transparent conductive oxide substrate 1 using magnetron sputtering. The buffer layer can be made of materials such as SnO2 to improve interface matching and carrier transport. Then, a 100-300 nm (preferably 150 nm) window layer 3 is deposited on the buffer layer 2 using near-space sublimation (CSS). The window layer can be a cadmium selenide (CdSe) film, and its preparation method includes, but is not limited to, near-space sublimation. Subsequently, a cadmium telluride light-absorbing layer 4 is deposited using a near-space sublimation method. Preferably, a substrate temperature of 570°C and a source temperature of 700°C are used, with 99.999% high-purity CdTe powder as the source material, and a deposition thickness of 3-5 μm (preferably 3.5 μm) is achieved, serving as the core light-absorbing unit of the battery. To optimize interface performance and improve battery efficiency, the cadmium telluride light-absorbing layer 4 preferably undergoes activation treatment and surface etching. Specifically, a cadmium chloride (CdCl2) thin film is first deposited on its surface, then activated by heat treatment at 400°C for 30 min in an annealing furnace. After heat treatment, the film is rinsed with deionized water and dried with nitrogen. Then, the film is etched for 10 s using a nitric acid etching solution (preferably nitric acid:water = 1:13) to remove surface oxides. Etching methods include, but are not limited to, acid washing and plasma cleaning. After etching, the film is rinsed again with deionized water and dried with nitrogen. It should be noted that the activation treatment can be performed using conventional methods in the art, such as, but not limited to, cadmium chloride annealing; surface etching can be performed using acid etching, such as, but not limited to, nitric acid solution etching. Those skilled in the art will understand that, in certain circumstances, the cadmium telluride light-absorbing layer 4 without the above treatment can also be used to implement this invention.
[0034] Next, a vanadium-doped carbon composite back contact layer 5 is prepared on the cadmium telluride light-absorbing layer 4 treated as described above, using the aforementioned method. Finally, a metal back electrode 6 of 100-300 nm (preferably 200 nm) is deposited on the back contact layer 5 using magnetron sputtering. The material of the metal back electrode 6 includes, but is not limited to, Mo, Al, Cr, etc., and the deposition method includes, but is not limited to, magnetron sputtering, thereby completing the device fabrication. In some specific embodiments, the buffer layer 2 is a tin oxide (SnO2) layer with a thickness of 20-50 nm (preferably 30 nm). This appropriately thick SnO2 layer can effectively passivate surface defects of the TCO substrate and improve bandgap matching and interface quality with the window layer. The window layer 3 is a cadmium selenide (CdSe) layer with a thickness of 100-300 nm (preferably 150 nm). This thin layer helps reduce interfacial recombination losses, increase short-wavelength light transmission, and optimize the heterojunction band alignment with the CdTe light-absorbing layer 4, thereby improving carrier separation efficiency. The cadmium telluride light-absorbing layer 4 is deposited using a near-space sublimation method and has a thickness of 3-5 μm (preferably 3.5 μm). This thickness is sufficient to ensure sufficient absorption of the solar spectrum while maintaining effective diffusion and collection of photogenerated carriers. The metal back electrode 6 is a Mo / Al / Cr composite electrode with a total thickness of 100-300 nm (preferably 200 nm). In this composite structure, a molybdenum (Mo) layer serves as an adhesion layer and part of the hole collection layer, an aluminum (Al) layer serves as the main conductive layer, and a chromium (Cr) layer serves as an anti-oxidation protective layer. Together, these elements form a low-resistance, high-stability back electrode, which works in conjunction with the vanadium-doped carbon composite back contact layer 5 to construct a cadmium telluride solar cell with a rational structure and optimized performance.
[0035] This invention also provides a cadmium telluride solar cell, prepared by the above-described method for preparing cadmium telluride solar cells. For details of its structure, please refer to [link / reference needed]. Figure 2 .
[0036] The following detailed description of examples of the present invention is exemplary and is used only to explain the present invention, and should not be construed as limiting the present invention.
[0037] Example 1 I. Preparation method of vanadium-doped carbazole polymer (V-PCz): Vanadyl acetylacetonate vanadium oxyacetate (VO(acac)2, 0.053 g, 0.2 mmol) and carbazole monomer 3,6-dibromo-9-n-octylcarbazole (3.732 g, 6.0 mmol) were dissolved in a mixed solvent of toluene (30 mL) and water (10 mL) containing potassium carbonate (1.66 g, 12.0 mmol). Tetra(triphenylphosphine)palladium (Pd(PPh3)4, 0.139 g, 0.12 mmol) catalyst was added. The reaction was carried out at 90 °C for 24 h under nitrogen protection. After the reaction was completed, the reaction solution was cooled to room temperature, poured into 200 mL of methanol, and stirred vigorously to precipitate the polymer. The precipitate was collected by filtration and washed successively with methanol and deionized water. Finally, it was dried under vacuum at 60°C for 24 h to obtain a dark brown solid, namely vanadium-doped carbazole polymer (V-PCz).
[0038] II. Preparation method of vanadium-doped carbon composite back contact layer: The vanadium-doped carbazole polymer prepared above was dissolved in chlorobenzene to prepare a coating solution with a concentration of 7 mg / mL. Using an ultrasonic spin-coating process, this coating solution was uniformly coated onto the surface of a pretreated cadmium telluride (CdTe) light-absorbing layer to form a polymer film with a thickness of 100 nm. The cadmium telluride light-absorbing layer was pre-treated with cadmium chloride activation and nitric acid solution etching: activation was performed by annealing in air at 400℃ for 30 min; etching was performed using a nitric acid to water solution with a volume ratio of 1:13 for 10 s. Subsequently, the resulting polymer film underwent gradient annealing: first pre-annealing at 200℃ for 30 min, then main annealing at 350℃ for 60 min, and finally short-time annealing at 400℃ for 10 min to obtain the vanadium-doped carbon composite back contact layer.
[0039] III. Preparation Method of Cadmium Telluride Solar Cells The vanadium-doped carbon composite back contact layer prepared by the above method is used to assemble a cadmium telluride solar cell. The specific steps are as follows: First, a 30 nm thick SnO2 buffer layer was deposited on an FTO transparent conductive oxide substrate using magnetron sputtering. Then, a 150 nm thick CdSe window layer and a 3.5 μm thick CdTe light-absorbing layer were sequentially deposited using near-space sublimation (CSS). Next, following the same method for preparing the vanadium-doped carbon composite back contact layer, a back contact layer was formed on the surface of the CdTe light-absorbing layer. Finally, a 200 nm thick Mo / Al / Cr composite metal back electrode was deposited on the back contact layer using magnetron sputtering, completing the fabrication of the cadmium telluride solar cell.
[0040] Example 2 The preparation method of the vanadium-doped carbazole polymer in Example 1 is the same, except that the molar ratio of the vanadium source compound to the carbazole monomer is 1:60.
[0041] Example 3 The preparation method of the vanadium-doped carbazole polymer in Example 1 is the same, except that the molar ratio of the vanadium source compound to the carbazole monomer is 1:10.
[0042] Example 4 The preparation method of the vanadium-doped carbazole polymer in Example 1 is the same, except that the Suzuki coupling polymerization reaction temperature is 70 °C and the reaction time is 24 h.
[0043] Example 5 The preparation method of the vanadium-doped carbazole polymer in Example 1 is similar, except that ethanol is used instead of methanol as a poor solvent.
[0044] Example 6 The preparation method of the vanadium-doped carbazole polymer in Example 1 is similar, except that the catalyst is not tetra(triphenylphosphine)palladium, but palladium on carbon (Pd / C).
[0045] Example 7 The preparation method of the vanadium-doped carbon composite back contact layer in Example 1 is the same, except that the pre-annealing temperature of the gradient annealing is 160 °C and the pre-annealing time is 30 min.
[0046] Example 8 The preparation method of the vanadium-doped carbon composite back contact layer in Example 1 is the same, except that the concentration of the coating solution is 3 mg / mL.
[0047] Example 9 The preparation method of the vanadium-doped carbon composite back contact layer in Example 1 is the same, except that the thickness of the polymer film is 180 nm.
[0048] Example 10 The preparation method of the vanadium-doped carbon composite back contact layer in Example 1 is the same, except that the organic solvent is N,N-dimethylformamide.
[0049] Comparative Example 1 The preparation method of the vanadium-doped carbon composite back contact layer in Example 1 is the same, except that gradient annealing is not performed, but only single-temperature annealing is performed at 200°C for 30 min.
[0050] Comparative Example 2 The preparation method of the vanadium-doped carbon composite back contact layer in Example 1 is similar, except that ZnTe:Cu is used as the precursor material for the back contact layer.
[0051] Test case The vanadium-doped carbazole polymer (V-PCz), the vanadium-doped carbon composite back contact layer derived therefrom, and the finally fabricated cadmium telluride solar cell prepared by the above method were tested.
[0052] I. Characterization of Vanadium-Doped Carbazole Polymer (V-PCz) The V-PCz prepared in Examples 1-6 were characterized in terms of structure and composition, and tested in terms of basic physical properties and photoelectric performance. The vanadium content of V-PCz was determined by elemental analysis. V~N coordination can be observed by the redshift of the V=O stretching vibration peak in FTIR and the newly appearing VN bond characteristic peak (~450 cm⁻¹). -1 The integrity of the molecular structure was confirmed by nuclear magnetic resonance (NMR) signals of aromatic and alkyl chain protons. The thermal decomposition temperature (Td) was obtained by thermogravimetric analysis. Hole mobility was obtained by space charge confinement current (SCLC) method. The characterization results are shown in Table 1.
[0053] II. Characterization of Vanadium-Doped Carbon Composite Back Contact Layer The vanadium-doped carbon composite back contact layers prepared in Examples 1-10 and Comparative Examples 1-2 were characterized: the degree of graphitization of the carbon network (Id / Ig ratio) was analyzed by Raman spectroscopy to evaluate its conductive network structure; and the distribution and valence state of V in the interface region (e.g., Vd) were confirmed by X-ray photoelectron spectroscopy (XPS) in depth analysis. 3+ The diffusion and doping effects of V ions were verified. The resistivity was tested by the four-probe method, and the work function was measured by Kelvin probe force microscopy (KPFM) to verify its conductivity and bandgap matching. Finally, the recombination rate of the back surface was determined by time-resolved photoluminescence (TRPL) to systematically evaluate its electrical transport performance and interface passivation effect. The characterization results are shown in Table 2.
[0054] III. Characterization of the overall performance of cadmium telluride solar cells Comprehensive electrical, optical, and stability tests were conducted on the cadmium telluride solar cells prepared in Examples 1-10 and Comparative Examples 1-2. Current-voltage (J-V) measurements under standard AM 1.5G illumination were performed to obtain and compare open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE), verifying the overall performance improvement. External quantum efficiency (EQE) spectroscopy was used to analyze the improvement in carrier collection efficiency in the long-wavelength region (near-infrared) by the back contact layer. The carrier concentration of the four-body CdTe light-absorbing layer was extracted using capacitance-voltage (C-V) measurements. Further long-term aging experiments (e.g., 85°C / 85%RH humid heat environment) were conducted to evaluate the device's operational stability. The characterization results are shown in Table 3.
[0055] Table 1. Performance characterization results of vanadium-doped carbazole polymer (V-PCz)
[0056] Analysis of the data in Table 1 shows that the vanadium-doped carbazole polymer (V-PCz) in Example 1 exhibits the best overall performance: its vanadium content is moderate (1.5 wt%), and its VN coordination characteristic peak is clear (-450 cm⁻¹). -1 ),and 1 H / 13 C10 NMR confirmed the molecular structure was intact, exhibiting high thermal stability (Td>350℃) and the highest hole mobility (>5×10⁻⁶). -3 cm 2The ratio of vanadium to vanadium (V·s) indicates that the synthesis conditions can effectively introduce vanadium active sites while maintaining the regular conjugated structure and charge transport capability of the polymer to the greatest extent. In contrast, excessive doping (Example 3) may destroy conjugation and lead to a decrease in mobility. On the other hand, too low a ratio (Example 2, 1:60) or insufficient reaction temperature (Example 4, 70°C) will result in insufficient vanadium doping and low degree of polymerization. Changing the solvent (Example 5) or catalyst (Example 6) has little effect on the core structure of the polymer, but the mobility is slightly reduced.
[0057] Table 2 Performance characterization results of vanadium-doped carbon composite back contact layer
[0058] Analysis of the data in Table 2 shows that the vanadium-doped carbon composite back contact layer of Example 1 exhibits the best overall interface performance: it has the highest degree of graphitization (Id / Ig ~0.85) and the lowest resistivity (<0.1 Ω·cm), confirming that gradient annealing (200°C→350°C→400°C) effectively promotes the ordering of the carbon network; XPS shows suitable V 3+ Interfacial concentrations (0.5–2.0 at%) confirmed the controllable diffusion and doping of V ions into the CdTe layer; simultaneously, its work function (~5.2 eV) matched well with CdTe, and the recombination rate measured by TRPL was the lowest (<1×10⁻⁶). 3 The figure of cm / s indicates that the back contact layer achieves excellent interface passivation while improving conductivity. In contrast, Comparative Example 1, which did not use gradient annealing, had incomplete carbonization (Id / Ig ~1.2) and uneven V diffusion, resulting in high resistivity and a surge in recombination rate; while Example 3, which was over-doped with V, had high recombination due to V agglomeration (>3.0 at%); in addition, using different organic solvents (Example 8, DMF) or reducing the pre-annealing temperature (Example 7) slightly degraded the degree of graphitization and increased recombination, while defects in the synthesis of polymer precursors (Examples 2, 4) directly led to insufficient active V concentration in the back contact layer, decreased graphitization degree and deterioration of electrical performance, and Comparative Example 2 had Cu deep-level defects leading to high recombination.
[0059] Table 3 Performance test results of cadmium telluride solar cells
[0060] Analysis of the data in Table 3 shows that the cadmium telluride solar cell prepared in Example 1 achieved the best overall performance: its open-circuit voltage (Voc>0.865 V), short-circuit current density (Jsc>28.5 mA / cm²), and fill factor (FF>78%) were all at the highest levels, jointly contributing to its significantly higher photoelectric conversion efficiency (PCE>19.0%); simultaneously, the carrier concentration in the CdTe light-absorbing layer of this cell was >5×10⁴ ppm. 14 cm -3 A higher level indicates that V 3+ The effective doping increases the hole concentration, and the fact that it can still maintain an efficiency of over 95% after 1000 hours of aging proves that the vanadium-doped carbon composite back contact layer not only achieves efficient ohmic contact and excellent interface passivation, but also has excellent long-term stability, fundamentally overcoming the efficiency decay problem caused by copper ion migration and deep energy level defects in traditional Cu-based back contacts (Comparative Example 2).
[0061] In summary, this invention successfully solves the technical problems of deep-level defects, uncontrollable ion diffusion, and poor stability in the back contact layer of traditional cadmium telluride solar cells, providing an innovative solution for the preparation of high-efficiency and stable cadmium telluride solar cells.
Claims
1. A method for preparing a vanadium-doped carbon composite back contact layer, characterized in that, Includes the following steps: A vanadium-doped carbazole polymer (V-PCz) solution was deposited on the surface of a cadmium telluride light-absorbing layer to form a polymer film; Under a nitrogen atmosphere, the polymer film is subjected to gradient annealing to form a vanadium-doped carbon composite back contact layer.
2. The preparation method according to claim 1, characterized in that, The gradient annealing process includes: Pre-anneal at 180~220℃ for 25~35 minutes; Perform main annealing at 320~370℃ for 55~65 minutes; Perform short-term annealing at 380~400℃ for 5~15 minutes.
3. The preparation method according to claim 1, characterized in that, The thickness of the polymer film is 20~150nm.
4. The preparation method according to claim 1, characterized in that, The vanadium-doped carbazole polymer is prepared by the following steps: A vanadium source compound and a carbazole monomer were dissolved in an organic solvent at a molar ratio of 1:(20~50). Under the action of a catalyst, a coordination reaction was carried out at 80~100°C for 20~30 hours to obtain a polymer with a vanadium content of 0.5 wt%~5 wt%. After the reaction was completed, a poor solvent was added to the reaction system to precipitate the polymer. After filtration and purification, the vanadium-doped carbazole polymer was obtained.
5. The preparation method according to claim 4, characterized in that, The vanadium source compound is vanadium acetylacetonate, the carbazole monomer is 3,6-dibromo-9-n-octylcarbazole, the catalyst is tetra(triphenylphosphine)palladium, the organic solvent is a mixed solvent of toluene and water with added alkali, and the undesirable solvent is methanol.
6. The preparation method according to claim 1 or 4, characterized in that, The vanadium-doped carbazole polymer was dissolved in an organic solvent to form a vanadium-doped carbazole polymer solution of 5-15 mg / mL.
7. The preparation method according to claim 6, characterized in that, The organic solvent is selected from at least one of chlorobenzene, N,N-dimethylformamide, or toluene.
8. A method for preparing a cadmium telluride solar cell, characterized in that, Includes the following steps: Provide a transparent conductive oxide substrate; A buffer layer, a window layer, and a cadmium telluride light-absorbing layer are sequentially deposited on the substrate; A vanadium-doped carbon composite back contact layer prepared by any one of claims 1 to 7 is prepared on the pretreated cadmium telluride light-absorbing layer. A metal back electrode is fabricated on the vanadium-doped carbon composite back contact layer.
9. The method for preparing a cadmium telluride solar cell according to claim 8, characterized in that, The buffer layer is a tin oxide layer with a thickness of 20~50 nm; The window layer is a cadmium selenide layer with a thickness of 100~300 nm; The cadmium telluride light-absorbing layer was deposited using a near-space sublimation method, and its thickness was 3~5 μm. The metal back electrode is a Mo / Al / Cr composite electrode with a total thickness of 100~300 nm.
10. A cadmium telluride solar cell, characterized in that, Prepared by the method described in claim 8 or 9.