A laminated polycrystalline silicon tunnel junction structure, perovskite / crystalline silicon laminated cell and preparation method

Abstract

This invention discloses a stacked polycrystalline silicon tunneling junction structure, a perovskite / crystalline silicon tandem solar cell, and its fabrication method. The stacked polycrystalline silicon tunneling junction structure serves as an intermediate connecting layer in a perovskite / crystalline silicon tandem solar cell. It comprises, sequentially stacked from the backlight surface to the light-receiving surface: an ultrathin silicon oxide layer, a phosphorus-doped polycrystalline silicon layer, a first boron-doped polycrystalline silicon layer, and a second boron-doped polycrystalline silicon layer. The second boron-doped polycrystalline silicon layer is doped with a modifying element selected from one or more of carbon, nitrogen, and oxygen. This invention, by constructing a stacked structure consisting of an ultrathin silicon oxide layer, a phosphorus-doped polycrystalline silicon layer, and the first and second boron-doped polycrystalline silicon layers, and introducing modifying elements such as carbon, nitrogen, or oxygen into the surface layer, synergistically optimizes carrier tunneling transport and surface chemical properties. As an intermediate connecting layer in a perovskite / crystalline silicon tandem solar cell, this structure improves the crystallinity quality of the perovskite thin film and the overall photoelectric performance of the cell, providing crucial interface support for the efficient and stable operation of the tandem solar cell.

Description

Technical Field

[0001] This invention relates to the field of solar cell technology, and in particular to a tandem polycrystalline silicon tunnel junction structure, a perovskite / crystalline silicon tandem solar cell, and a method for its fabrication. Background Technology

[0002] Tandem solar cells effectively reduce heat dissipation losses from high-energy excitons and improve photoelectric efficiency by connecting materials with different bandgap ranges in series. In the current technological landscape, crystalline silicon, with its high conversion efficiency, mature manufacturing process, and significant economies of scale, has become the core bottom cell material in tandem solar cell systems. Perovskite materials, on the other hand, show great promise as top cell materials due to their tunable bandgap and rapidly improving conversion efficiency. Among various bottom cells, crystalline silicon cells based on tunneling silicon oxide passivated contact (TOPCon) structures have become the mainstream bottom cell choice for tandem solar cells due to their excellent surface passivation performance, high open-circuit voltage, and mature industrial foundation.

[0003] In tandem solar cell structures, the intermediate connecting layer is the core component for achieving photoelectric coupling between the top and bottom cells. It not only serves a mechanical connection function but also directly affects carrier transport, recombination behavior, and optical losses. Traditional technologies commonly use transparent conductive oxide (TCO) as the intermediate layer, but this has significant limitations: magnetron sputtering easily damages the passivation structure of the bottom layer; refractive index mismatch between the TCO and adjacent layers introduces interfacial optical reflection; furthermore, it exhibits parasitic absorption losses in the near-infrared region, limiting the current output of the tandem solar cell. In contrast, polycrystalline silicon-based tunneling composite layers exhibit unique advantages. This structure can form quasi-metallic contacts through heavy doping, achieving efficient carrier tunneling recombination. Simultaneously, it is naturally compatible with the polycrystalline silicon deposition stage in the TOPCon process, avoiding additional TCO deposition steps and sputtering damage, and possesses both high lateral resistance and low parasitic absorption characteristics.

[0004] However, applying polycrystalline silicon tunneling composite layers to practical stacked devices still faces key challenges—insufficient surface hydroxyl density hinders the uniform anchoring and high coverage of self-assembled molecular (SAM) layers. Particularly on the surface of bottom cells with pyramidal textures, uneven SAM distribution easily leads to intensified interfacial recombination and leakage, severely restricting device performance improvement. Existing methods to enhance SAM anchoring (such as ultraviolet ozone or nickel oxide intercalation) not only introduce additional process steps but also have limited applicability in polycrystalline silicon systems, potentially causing over-oxidation or passivation quality degradation. Furthermore, the low surface energy of polycrystalline silicon results in poor wettability of perovskite precursor solutions, easily leading to defects such as thin film pores.

[0005] Therefore, developing a novel polycrystalline silicon tunneling composite layer structure to effectively enhance its surface hydroxyl density, achieve uniform and dense anchoring of SAMs, and regulate surface hydrophilic and hydrophobic properties is of great significance for improving the interface quality and overall efficiency of perovskite / crystalline silicon tandem solar cells, and is also a key link in promoting the industrial application of this technology. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention aims to solve the following technical problems: in existing perovskite / crystalline silicon tandem solar cells, there are issues such as uneven self-assembly molecular anchoring, severe interfacial recombination, and poor wettability caused by the intermediate connecting layer.

[0007] To address the aforementioned problems, the first aspect of this invention provides a stacked polycrystalline silicon tunnel junction structure, which serves as an intermediate connection layer for a perovskite / crystalline silicon tandem solar cell. The stacked polycrystalline silicon tunnel junction structure comprises, in sequence from the backlight surface to the light-receiving surface, an ultrathin silicon oxide layer, a phosphorus-doped polycrystalline silicon layer, a first boron-doped polycrystalline silicon layer, and a second boron-doped polycrystalline silicon layer. The second boron-doped polycrystalline silicon layer is doped with a modifying element selected from one or more of carbon, nitrogen, and oxygen.

[0008] This invention constructs a stacked structure consisting of an ultrathin silicon oxide layer, a phosphorus-doped polycrystalline silicon layer, and first and second boron-doped polycrystalline silicon layers, and introduces modifying elements such as carbon, nitrogen, or oxygen into the surface layer to synergistically optimize carrier tunneling transport and surface chemical properties. This structure serves as the intermediate connecting layer in a perovskite / crystalline silicon tandem solar cell, ensuring efficient carrier transport while significantly enhancing surface hydroxyl density and self-assembling molecular anchoring capabilities. It effectively improves interfacial wettability and coverage uniformity, suppresses interfacial recombination, and thus enhances the crystallinity quality of the perovskite thin film and the overall photoelectric performance of the cell, providing crucial interfacial support for the efficient and stable operation of the tandem solar cell.

[0009] Furthermore, the thickness of the ultrathin silicon oxide layer is 1-3 nm, the thickness of the phosphorus-doped polycrystalline silicon layer is 10-250 nm, the thickness of the first boron-doped polycrystalline silicon layer is 10-250 nm, and the thickness of the second boron-doped polycrystalline silicon layer is 1-10 nm. By limiting the thickness of each functional layer, optical absorption loss is minimized while ensuring effective carrier tunneling.

[0010] Furthermore, the thickness of the ultrathin silicon oxide layer is 1.5-1.8 nm, the thickness of the phosphorus-doped polycrystalline silicon layer is 10-25 nm, the thickness of the first boron-doped polycrystalline silicon layer is 10-25 nm, and the thickness of the second boron-doped polycrystalline silicon layer is 2-5 nm. This further defines the preferred thickness range for each layer, achieving optimal performance within the limits of industrial feasibility. The 1.5-1.8 nm silicon oxide layer achieves the best balance between passivation effect and tunneling probability; the 10-25 nm polycrystalline silicon layer ensures sufficient electrical conductivity while minimizing optical loss; and the 2-5 nm surface layer thickness ensures effective doping of the modifying elements while avoiding the electrical performance degradation caused by excessive thickness.

[0011] Furthermore, the doping concentration of the modifying element in the second boron-doped polycrystalline silicon layer is 1×10⁻⁶. 17 -1×10 22 cm -3 The boron doping concentration is 1×10 17 -5×10 20 cm -3 By controlling the concentration of modifying elements and boron doping, the surface chemical properties were precisely regulated. Within a limited concentration range, the modifying elements could effectively increase the surface hydroxyl density, enhance the anchoring ability of self-assembled molecules, and maintain the electrical properties of the polycrystalline silicon layer, avoiding the decrease in conductivity caused by excessive doping.

[0012] Furthermore, the phosphorus doping concentration of the phosphorus-doped polycrystalline silicon layer is 1×10⁻⁶. 18 -2×10 21 cm -3 The boron doping concentration of the first boron-doped polycrystalline silicon layer is 1×10⁻⁶. 17 -5×10 20 cm -3 By limiting the doping concentration of each polycrystalline silicon layer, efficient carrier transport is ensured.

[0013] Furthermore, the water contact angle on the surface of the second boron-doped polycrystalline silicon layer is 0°-20°. A lower contact angle indicates a higher surface energy, which is conducive to the growth of self-assembled molecules in a two-dimensional spreading mode, forming a dense and uniform capping layer. This creates favorable conditions for the uniform deposition of perovskite films and effectively avoids the generation of defects such as pores.

[0014] Furthermore, the second boron-doped polycrystalline silicon layer has an amorphous / nanocrystalline mixed phase structure. The second boron-doped polycrystalline silicon layer significantly reduces the surface grain boundary density. Compared with fully crystalline polycrystalline silicon, the amorphous / nanocrystalline mixed phase has a more uniform surface energy distribution, which is beneficial to the orderly arrangement and uniform coverage of self-assembled molecules, thereby improving the interface potential uniformity and reducing interface recombination losses.

[0015] A second aspect of the present invention provides a method for preparing the above-mentioned multilayer polycrystalline silicon tunnel junction structure, comprising the following steps: S1. An ultrathin silicon oxide layer is formed on the surface of an n-type silicon substrate; S2. Deposit a phosphorus-doped amorphous silicon layer on the ultrathin silicon oxide layer; S3. Perform the first annealing treatment to form a phosphorus-doped polycrystalline silicon layer; S4. A first boron-doped amorphous silicon layer and a second boron-doped amorphous silicon layer are sequentially deposited on the phosphorus-doped polycrystalline silicon layer, wherein the second boron-doped amorphous silicon layer contains a modifying element selected from one or more of carbon, nitrogen, and oxygen; S5. Perform a second annealing process to form a stacked polycrystalline silicon tunnel junction structure containing a first boron-doped polycrystalline silicon layer and a second boron-doped polycrystalline silicon layer.

[0016] The preparation method of the present invention uses a two-step annealing process to effectively control the distribution and activation of dopant elements while ensuring the crystal quality of polycrystalline silicon, thus ensuring good passivation and contact performance of the tunnel junction structure.

[0017] Furthermore, the phosphorus-doped amorphous silicon layer in step S2 and the first boron-doped amorphous silicon layer and the second boron-doped amorphous silicon layer in step S4 are both deposited using plasma-enhanced chemical vapor deposition. The modifying element of the second boron-doped amorphous silicon layer is achieved by introducing methane, ammonia, or carbon dioxide in situ.

[0018] Furthermore, in step S3, the first annealing temperature is 800-900℃, and the annealing time is 10-200 minutes; in step S5, the second annealing temperature is 650-750℃, and the annealing time is 2-20 minutes. By optimizing the annealing process parameters, while ensuring sufficient crystallization of polycrystalline silicon and effective activation of dopant elements, dopant interdiffusion and excessive hydrogen overflow caused by high temperatures are avoided, thus ensuring excellent passivation and contact performance.

[0019] A third aspect of the present invention provides a perovskite / crystalline silicon tandem solar cell, comprising, in sequence from the backlight surface to the light-receiving surface: a back electrode, a back passivation layer, an n-type silicon substrate, an intermediate connection layer, a hole transport layer, a perovskite light-absorbing layer, an electron transport layer, a buffer layer, a transparent conductive layer, and a top electrode, wherein the intermediate connection layer is composed of the above-described tandem polycrystalline silicon tunnel junction structure.

[0020] By applying the tandem polycrystalline silicon tunnel junction structure of this invention as an intermediate connecting layer in perovskite / crystalline silicon tandem solar cells, highly efficient optical and electrical coupling between the top and bottom cells is achieved. This structure not only provides excellent carrier recombination channels but also improves the coverage of SAMs and the crystallinity quality of the subsequent perovskite layer through surface modification, ultimately significantly enhancing the open-circuit voltage, fill factor, and conversion efficiency of the tandem solar cell.

[0021] In summary, the present invention has the following beneficial effects: (1) Optimize the photoelectric properties of the interface and improve the carrier transport efficiency: This invention introduces doping elements such as carbon, nitrogen or oxygen into the surface layer of the polycrystalline silicon tunnel junction, which effectively increases the surface hydroxyl density, greatly improves the anchoring uniformity and coverage of the self-assembled molecular layer, and forms a dense and ordered hole transport channel. This interface modification significantly enhances the hole extraction capability, reduces the interface recombination rate, and promotes efficient tunneling transport of carriers through band matching optimization, thereby significantly improving the open circuit voltage and fill factor of the tandem battery.

[0022] (2) Improve film growth quality and enhance optical utilization efficiency: Thanks to the hydrophilicity regulation of the second boron-doped polycrystalline silicon layer, the wettability of the perovskite precursor solution is significantly improved, effectively avoiding film pore defects and promoting uniform growth and increased crystallinity of perovskite grains; at the same time, the refractive index matching between the polycrystalline silicon layer and the silicon substrate reduces interface reflection loss, and the high transmittance of the second boron-doped polycrystalline silicon layer in the near-infrared region significantly reduces parasitic absorption, allowing more long-wavelength photons to be absorbed by the bottom cell, thus jointly improving the short-circuit current density of the cell.

[0023] (3) Achieving synergistic optimization of surface properties and electrical performance: A two-layer structure design is adopted, consisting of polycrystalline silicon with surface doping modified elements and conventional polycrystalline silicon in the inner layer. The thin surface layer optimizes the surface chemical properties through an amorphous / nanocrystalline mixed phase structure, while the thick inner layer maintains a high doping concentration to ensure low contact resistivity. This gradient design not only improves the surface hydroxyl density and the anchoring effect of the self-assembled molecular layer, but also ensures the efficient longitudinal transport of charge carriers, thus solving the technical contradiction that it is difficult to balance surface modification and electrical performance in traditional structures.

[0024] (4) Enhanced process adaptability: The surface modification strategy of this invention adopts PECVD in-situ doping, which shows good adaptability to different morphological substrates (polished surface, pyramidal textured surface, micro-nano texture).

[0025] (5) Break through the bottleneck of industrialization technology and realize low-cost large-scale manufacturing: The entire doping process can be achieved by introducing cheap gas sources such as methane, ammonia or carbon dioxide through conventional PECVD equipment without additional process steps or complex equipment modifications. This simple preparation method is highly compatible with traditional TOPCon production lines, avoids the vacuum magnetron sputtering process of traditional TCO intermediate layers, greatly reduces production costs, and provides a feasible technical path for the industrialization and promotion of perovskite / crystalline silicon tandem solar cells. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the stacked polycrystalline silicon tunnel junction structure in a specific embodiment of the present invention.

[0027] Figure 2This is a schematic diagram of the structure of a perovskite / crystalline silicon tandem solar cell in a specific embodiment of the present invention.

[0028] Figure 3 These are water contact angle images of the tunnel junctions in Embodiment 1 and Comparative Example 1 of the present invention.

[0029] Figure 4 The images are AFM-IR images of the tunnel junctions of Embodiment 2 and Comparative Example 2 after being coated with Me-4PACz.

[0030] Figure 5 The images show the contact angles of the perovskite precursors after coating the tunnel junctions with Me-4PACz in Examples 4 and 5 of this invention.

[0031] Figure 6 These are top and cross-sectional SEM images of the tunnel junction coated with perovskite thin films in Examples 5 and 6 of the present invention.

[0032] Explanation of reference numerals in the attached figures: 1-Perovskite top cell, 11-Transparent conductive layer, 12-Buffer layer, 13-Electron transport layer, 14-Perovskite light-absorbing layer, 15-Hole transport layer, 16-Top electrode, 2-Intermediate connection layer, 21-Ultra-thin silicon oxide layer, 22-Phosphorus-doped polycrystalline silicon layer, 23-First boron-doped polycrystalline silicon layer, 24-Second boron-doped polycrystalline silicon layer, 3-Crystal silicon bottom cell, 31-n-type silicon substrate, 311-Phosphorus diffusion region, 312-Boron emitter, 32-Back passivation layer, 33-Back electrode. Detailed Implementation

[0033] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0034] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0035] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art.

[0036] A specific embodiment of the present invention provides a perovskite / crystalline silicon tandem solar cell, comprising a crystalline silicon bottom cell 3, an intermediate connecting layer 2, and a perovskite top cell 1 sequentially stacked from the backlight surface to the light-receiving surface. The intermediate connecting layer 2 is composed of a tandem polycrystalline silicon tunnel junction structure, combined with… Figure 1 As shown, the structure includes, from bottom to top, an ultrathin silicon oxide layer 21, a phosphorus-doped polycrystalline silicon layer 22, a first boron-doped polycrystalline silicon layer 23, and a second boron-doped polycrystalline silicon layer 24. The second boron-doped polycrystalline silicon layer 24 is doped with a modifying element, which is selected from one or more of carbon, nitrogen, and oxygen.

[0037] This tandem solar cell uses a tandem polycrystalline silicon tunnel junction structure with specific doping characteristics as an intermediate connecting layer 2 inserted between the perovskite top cell 1 and the crystalline silicon bottom cell 3. In a specific implementation, the thickness of the ultrathin silicon oxide layer is 1-3 nm, the thickness of the phosphorus-doped polycrystalline silicon layer is 10-250 nm, the thickness of the first boron-doped polycrystalline silicon layer is 10-250 nm, and the thickness of the second boron-doped polycrystalline silicon layer is 1-10 nm. The doping concentration of the modified element in the second boron-doped polycrystalline silicon layer is 1×10⁻⁶. 17 -1×10 22 cm -3 This innovative structure effectively overcomes the technical bottlenecks of traditional polycrystalline silicon tunneling layers, such as uneven anchoring of self-assembled molecular (SAMs) layers, severe interfacial recombination, and poor wettability caused by insufficient surface hydroxyl density, thereby achieving a significantly improved photoelectric conversion efficiency.

[0038] Specifically, the second boron-doped polycrystalline silicon layer 24, containing modified elements, exhibits tunable surface properties. Unlike the nonpolar Si-Si bonds on conventional polycrystalline silicon surfaces, the polar C-Si, N-Si, and O-Si bonds formed after doping with carbon, nitrogen, or oxygen significantly enhance surface energy, allowing the contact angle to be precisely controlled within the range of 0°-20°. This enables perfect adaptation to self-assembled molecular layers and perovskite precursor solutions with different wetting properties. This improved wetting property not only promotes the two-dimensional spreading growth mode of SAMs but also significantly enhances their coverage and distribution uniformity on the tunnel junction surface. More importantly, the second boron-doped polycrystalline silicon layer 24 provides excellent anchoring sites for phosphate groups in SAMs by increasing the surface hydroxyl density, achieving dense adsorption of the molecular layer. The surface modified with SAMs exhibits excellent wettability to perovskite precursor solutions, with the contact angle reduced to 0-10°. This uniform molecular coverage effectively improves the spreading performance of perovskite precursor solutions, significantly reduces structural defects in perovskite films, and thus greatly enhances the crystallinity of the films.

[0039] Furthermore, the second boron-doped polycrystalline silicon layer 24 possesses a unique amorphous / nanocrystalline mixed-phase microstructure, with significantly superior structural uniformity compared to traditional polycrystalline silicon materials. This uniform microstructure provides an ideal spreading platform for self-assembled molecules, facilitating uniform molecular distribution and thereby improving the potential distribution uniformity of the SAMs layer, enhancing the hole carrier extraction efficiency.

[0040] In the preferred embodiment, the thickness parameters of each functional layer are further optimized as follows: 1.5-1.8 nm for the ultrathin silicon oxide layer, 10-25 nm for the phosphorus-doped polycrystalline silicon layer, 10-25 nm for the first boron-doped polycrystalline silicon layer, and 2-5 nm for the second boron-doped polycrystalline silicon layer. This optimized thickness configuration plays a crucial role in device performance optimization: the 1.5-1.8 nm silicon oxide layer achieves the best balance between surface passivation effect and carrier tunneling probability; the 10-25 nm polycrystalline silicon layer ensures sufficient electrical conductivity while minimizing optical loss; and the 2-5 nm surface layer thickness ensures effective doping of the modified elements while avoiding the electrical performance degradation that may result from excessively thick layers.

[0041] In some specific embodiments, the doping concentration of each functional layer is carefully optimized to achieve optimal carrier transport and interface characteristics. The phosphorus doping concentration of the phosphorus-doped polycrystalline silicon layer 22 is controlled at 1×10⁻⁶. 18 -2×10 21 cm -3 Within this range, this high concentration of doping forms n + The heavily doped region effectively promotes electron tunneling transport while ensuring sufficient conductivity. The boron doping concentration of the first boron-doped polycrystalline silicon layer 23 is set to 1×10⁻⁶. 17 -5×10 20 cm -3 This ensures efficient hole transport. The boron doping concentration of the second boron-doped polysilicon layer 24 is also maintained at 1×10⁻⁶. 17 -5×10 20 cm -3 Simultaneously, this layer also incorporates modifying elements such as carbon, nitrogen, or oxygen, with doping concentrations ranging from 1 × 10⁻⁶. 17 -5×10 22 cm -3 This dual-doping strategy significantly improves surface chemical properties while maintaining electrical performance. The synergistic effect of this doping concentration system enables the tandem structure to achieve an optimal match between carrier transport, interface passivation, and optical performance, providing a key guarantee for achieving high conversion efficiency in perovskite / crystalline silicon tandem solar cells.

[0042] Combination Figure 2As shown, in some embodiments, the perovskite top solar cell 1 includes a hole transport layer 15, a perovskite light-absorbing layer 14, an electron transport layer 13, a buffer layer 12, and a transparent conductive layer 11 sequentially stacked on an intermediate connecting layer 2, with a metal grid top electrode 16 on its front side. The hole transport layer 15 is directly deposited on the second boron-doped polycrystalline silicon layer 24, and its hole extraction performance is optimized through self-assembled molecular layers. The crystalline silicon bottom solar cell 3 is a crystalline silicon cell based on a TOPCon structure, which includes an n-type silicon substrate 31, a back passivation layer 32, and a back electrode 33 sequentially from the light-receiving surface to the back side. The n-type silicon substrate 31 has a pyramidal textured or planar structure, a phosphorus diffusion region 311 on the front side, and a boron emitter 312 on the back side.

[0043] The above-mentioned method for fabricating perovskite / crystalline silicon tandem solar cells includes the following steps: (1) Provide an n-type silicon substrate 31, with boron diffusion on the back side to form a boron emitter 312.

[0044] (2) An ultrathin silicon oxide layer 21 is formed on the front side of the n-type silicon substrate 31 by thermal oxidation or plasma oxidation.

[0045] (3) A phosphorus-doped amorphous silicon layer is deposited on the ultrathin silicon oxide layer 21 using a tubular PECVD equipment.

[0046] (4) Perform the first annealing treatment, annealing at 800-900℃ in a nitrogen atmosphere for 10-200 minutes to crystallize amorphous silicon to form a phosphorus-doped polycrystalline silicon layer 22, and form a phosphorus diffusion region 311 on the front side of the n-type silicon substrate 31.

[0047] (5) A tubular PECVD equipment is used to sequentially deposit a first boron-doped amorphous silicon layer and a second boron-doped amorphous silicon layer on the phosphorus-doped polycrystalline silicon layer 22. The second boron-doped amorphous silicon layer is doped with modified elements by introducing methane, ammonia or carbon dioxide.

[0048] (6) Perform a second annealing treatment, and rapidly anneal at 650-750℃ in a nitrogen atmosphere for 2-20 minutes to form a complete stacked structure including the first boron-doped polysilicon layer 23 and the second boron-doped polysilicon layer 24.

[0049] (7) Hole transport layer 15, perovskite light-absorbing layer 14, electron transport layer 13, buffer layer 12 and transparent conductive layer 11 are sequentially prepared on the second boron-doped polycrystalline silicon layer 24.

[0050] (8) Prepare the back passivation layer 32, and prepare the top electrode 16 and back electrode 33 by double-sided metallization to complete the device fabrication.

[0051] This invention provides a reliable technical solution for high-efficiency, stable, and low-cost perovskite / crystalline silicon tandem solar cells by combining an innovative tandem polycrystalline silicon tunnel junction structure design with PECVD technology.

[0052] The technical solution and effects of the present invention will be illustrated below with specific embodiments.

[0053] Example 1 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was prepared on both sides. RCA standard cleaning was then performed. The treated silicon wafer was immersed in a concentrated nitric acid solution at 110°C for 10 minutes to grow an ultrathin silicon oxide layer with a thickness of approximately 1.5 nm. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD device. A first annealing treatment was performed at 860°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 25 nm boron-doped amorphous silicon layer and a 5 nm carbon-containing boron-doped amorphous silicon layer were then sequentially deposited in the PECVD reaction chamber. A second annealing treatment was performed at 700°C for 10 minutes to complete the fabrication of the stacked polycrystalline silicon tunnel junction structure.

[0054] Comparative Example 1 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was prepared on both sides, followed by RCA standard cleaning. The treated silicon wafer was then immersed in a concentrated nitric acid solution at 110°C for 10 minutes to grow an ultrathin silicon oxide layer with a thickness of approximately 1.5 nm. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD device. A first annealing treatment was performed at 860°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 30 nm boron-doped amorphous silicon layer was then deposited in the PECVD reaction chamber. A second annealing treatment was performed at 700°C for 10 minutes to complete the fabrication of the tunnel junction structure.

[0055] The surface water contact angles of the tunneling junction structures prepared in Example 1 and Comparative Example 1 were tested, and the results are as follows: Figure 3 As shown, the surface water contact angle of Example 1 is 0°, while that of Comparative Example 1 is 76°. A lower contact angle means that the polycrystalline silicon surface has higher surface energy, which is beneficial for SAMs to grow in 2D mode and improve the coverage and uniformity of SAMs on the tunnel junction.

[0056] Example 2 An n-type single-crystal silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was fabricated on both sides. RCA standard cleaning was then performed. The treated silicon wafer was immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer with a thickness of approximately 1.5 nm. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD apparatus. A first annealing treatment was performed at 880°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 25 nm boron-doped amorphous silicon layer and a 5 nm carbon-containing boron-doped amorphous silicon layer were then sequentially deposited in the PECVD chamber. A second annealing treatment was performed at 700°C for 10 minutes to obtain the tunnel junction. Me-4PACz was coated on the tunnel junction, and ultraviolet photoelectron spectroscopy (UPS) was performed, with measurements taken at 1230 cm⁻¹. -1 Atomic force microscopy-infrared spectroscopy (AFM-IR) was performed at (representing the carbazole group), and the results are as follows: Figure 4 As shown.

[0057] Comparative Example 2 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was fabricated on both sides. RCA standard cleaning was then performed. The treated silicon wafer was immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer with a thickness of approximately 1.5 nm. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD system. A first annealing treatment was performed at 880°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 30 nm boron-doped amorphous silicon layer was then deposited in the PECVD chamber. A second annealing treatment was performed at 700°C for 10 minutes to obtain the tunnel junction. Me-4PACz was coated on the tunnel junction, and ultraviolet photoelectron spectroscopy (UPS) was performed, with measurements taken at 1230 cm⁻¹. -1 Atomic force microscopy-infrared spectroscopy (AFM-IR) was performed at (representing the carbazole group), and the results are as follows: Figure 4 As shown.

[0058] The work function of Example 2 was measured to be 4.6 eV, while that of Comparative Example 2 was 4.3 eV. The higher work function is beneficial to promoting hole enhancement. The higher AFM-IR intensity means that Example 2 has more and denser carbazole groups, which proves that SAMs significantly improve the coverage and uniformity of the modified tunnel junction.

[0059] Example 3 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was fabricated on both sides. RCA standard cleaning was then performed. The treated silicon wafer was immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer with a thickness of approximately 1.5 nm. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD apparatus. A first annealing treatment was performed at 820°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 25 nm boron-doped amorphous silicon layer and a 5 nm carbon-containing boron-doped amorphous silicon layer were then sequentially deposited in the PECVD chamber. A second annealing treatment was performed at 700°C for 10 minutes to obtain a tunnel junction. A metal electrode was deposited on the tunnel junction using thermal evaporation, and the longitudinal contact resistivity was measured to be 3 mΩ·cm using the CS method. 2 .

[0060] Comparative Example 3 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was fabricated on both sides. This was followed by RCA standard cleaning. The treated silicon wafer was then immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer with a thickness of approximately 1.5 nm. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the wafer using a tubular PECVD system. A first annealing treatment was performed at 880°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 30 nm boron-doped amorphous silicon layer was then sequentially deposited in the PECVD chamber. A second annealing treatment was performed at 700°C for 10 minutes to obtain a tunnel junction. A metal electrode was deposited on the tunnel junction using thermal evaporation, and the longitudinal contact resistivity was measured to be 4 mΩ·cm using the CS method. 2 .

[0061] Comparative Example 4 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was fabricated on both sides. This was followed by RCA standard cleaning. The treated silicon wafer was then immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer approximately 1.5 nm thick. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the wafer using a tubular PECVD system. A first annealing treatment was performed at 880°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 30 nm carbon-doped boron-doped amorphous silicon layer was then sequentially deposited in the PECVD chamber. A second annealing treatment was performed at 700°C for 10 minutes to obtain a tunnel junction. A metal electrode was deposited on the tunnel junction using thermal evaporation, and the longitudinal contact resistivity was measured to be 90 mΩ·cm using the CS method. 2 .

[0062] The test results above indicate that the bilayer structure composed of carbon-doped boron polycrystalline silicon and boron-doped polycrystalline silicon has a thin surface layer of carbon-doped boron polycrystalline silicon used to improve wettability and SAMs adsorption, while the thick inner layer of boron-doped polycrystalline silicon can ensure sufficient film conductivity and avoid the decrease in polycrystalline silicon conductivity due to excessive doping, thus ensuring the low contact resistivity of the tunnel junction structure.

[0063] Example 4 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was fabricated on both sides. This was followed by RCA standard cleaning. The treated silicon wafer was then immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer approximately 1.5 nm thick. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the wafer using a tubular PECVD system. A first annealing treatment was performed at 880°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 20 nm boron-doped amorphous silicon layer and a 5 nm carbon-containing boron-doped amorphous silicon layer were then sequentially deposited in the PECVD chamber. A second annealing treatment was performed at 720°C for 5 minutes to obtain a tunnel junction. Me-4PACz was then coated onto the tunnel junction.

[0064] Comparative Example 5 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was fabricated on both sides. This was followed by RCA standard cleaning. The treated silicon wafer was then immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer approximately 1.5 nm thick. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the wafer using a tubular PECVD system. A first annealing treatment was performed at 880°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 25 nm boron-doped amorphous silicon layer was then deposited in the PECVD chamber. A second annealing treatment was performed at 720°C for 5 minutes to obtain a tunnel junction. Me-4PACz was then coated onto the tunnel junction.

[0065] The wettability of the perovskite precursors in Example 4 and Comparative Example 5 was tested, and the results are as follows: Figure 5 As shown, Example 4 has a lower perovskite precursor contact angle, which means that the Me-4PACz arrangement on this structure is more uniform, which is more conducive to the uniform growth of perovskite and improves the performance of perovskite solar cells.

[0066] Example 5 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was fabricated on both sides. This was followed by RCA standard cleaning. The treated silicon wafer was then immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer with a thickness of approximately 1.5 nm. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD apparatus. A first annealing treatment was performed at 850°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 25 nm boron-doped amorphous silicon layer and a 5 nm carbon-containing boron-doped amorphous silicon layer were then sequentially deposited in the PECVD chamber. A second annealing treatment was performed at 700°C for 10 minutes to obtain a tunnel junction. A Me-4PACz layer and a perovskite thin film were then coated on top of the tunnel junction.

[0067] Comparative Example 6 An n-type monocrystalline silicon wafer was selected as the substrate. After a standard cleaning process, a submicron-scale pyramidal textured structure was fabricated on both sides. This was followed by RCA standard cleaning. The treated silicon wafer was then immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer with a thickness of approximately 1.5 nm. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD apparatus. A first annealing treatment was performed at 850°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer. A 30 nm boron-doped amorphous silicon layer was then deposited in the PECVD chamber. A second annealing treatment was performed at 700°C for 10 minutes to obtain a tunnel junction. A Me-4PACz layer and a perovskite thin film were then coated on top of the tunnel junction.

[0068] The perovskite films obtained in Example 5 and Comparative Example 6 were characterized by scanning electron microscopy (SEM), and the results are as follows: Figure 6 As shown, the perovskite film of Comparative Example 6 has small grains, surface defects such as pores, and a large number of voids between the silicon substrate and the perovskite film; while the perovskite film of Example 5 has larger grains, excellent crystallization performance, and the perovskite film completely covers the surface of the silicon substrate.

[0069] Example 7 n-type single-crystal silicon wafers were selected as substrates. Submicron-scale pyramids were fabricated on the front side, and texturing and BCl3 boron diffusion were performed on the back side. RCA standard cleaning was then performed. The treated silicon wafers were immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer of approximately 1.5 nm thickness. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD system. A first annealing treatment was performed at 840°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer, forming a TOPCon structure. The sample was then placed in a tubular PECVD system to sequentially deposit a 25 nm boron-doped amorphous silicon layer and a 5 nm carbon-containing boron-doped amorphous silicon layer. A second annealing treatment was performed at 700°C for 10 minutes to form a tunnel junction structure. A Me-4PACz layer and a perovskite film were coated on the tunnel junction. The Me-4PACz layer was coated on the tunnel junction, and then a perovskite film was prepared using spin coating. A C-4PACz film was prepared using thermal evaporation. 60 An electron transport layer was prepared; a buffer layer SnO2 was prepared by ALD; a transparent electrode IZO was prepared by magnetron sputtering; and then silver electrodes and passivation layers on both sides were prepared by thermal evaporation to obtain a perovskite / crystalline silicon tandem solar cell.

[0070] Battery performance testing: V oc =1.98 V, J sc =20.2 mA / cm 2 , FF =81.8%, η =32.7%.

[0071] Example 8 n-type single-crystal silicon wafers were selected as substrates. After standard cleaning procedures, submicron-scale pyramidal textured structures were fabricated on both sides. RCA standard cleaning was then performed. The treated silicon wafers were immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer of approximately 1.5 nm thickness. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD system. A first annealing treatment was performed at 840°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer, forming a TOPCon structure. The sample was then placed in a tubular PECVD system to sequentially deposit a 25 nm boron-doped amorphous silicon layer and a 5 nm nitrogen-containing boron-doped amorphous silicon layer. A second annealing treatment was performed at 700°C for 10 minutes to form a tunnel junction structure. A Me-4PACz layer and a perovskite film were coated on the tunnel junction. The Me-4PACz layer was coated on the tunnel junction, and then a perovskite film was prepared using spin coating. C2 was prepared using thermal evaporation. 60 An electron transport layer was prepared; a buffer layer SnO2 was prepared by ALD; a transparent electrode IZO was prepared by magnetron sputtering; and then silver electrodes and passivation layers on both sides were prepared by thermal evaporation to obtain a perovskite / crystalline silicon tandem solar cell.

[0072] Battery performance testing: V oc =1.97 V, J sc =20.19 mA / cm 2 , FF =81.5%, η =32.4%.

[0073] Example 9 n-type single-crystal silicon wafers were selected as substrates. After standard cleaning procedures, submicron-scale pyramidal textured structures were fabricated on both sides. RCA standard cleaning was then performed. The treated silicon wafers were immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer of approximately 1.5 nm thickness. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD system. A first annealing treatment was performed at 840°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer, forming a TOPCon structure. The sample was then placed in a tubular PECVD system to sequentially deposit a 25 nm boron-doped amorphous silicon layer and a 5 nm oxygen-containing boron-doped amorphous silicon layer. A second annealing treatment was performed at 700°C for 10 minutes to form a tunnel junction structure. A Me-4PACz layer and a perovskite film were coated on the tunnel junction. The Me-4PACz layer was coated on the tunnel junction, and then a perovskite film was prepared using spin coating. C2 was prepared using thermal evaporation. 60 An electron transport layer was prepared; a buffer layer SnO2 was prepared by ALD; a transparent electrode IZO was prepared by magnetron sputtering; and then silver electrodes and passivation layers on both sides were prepared by thermal evaporation to obtain a perovskite / crystalline silicon tandem solar cell.

[0074] Battery performance testing: V oc =1.97 V, J sc =20.2 mA / cm 2 , FF =81.6%, η =32.5%.

[0075] Comparative Example 8 n-type single-crystal silicon wafers were selected as substrates. After standard cleaning procedures, submicron-scale pyramidal textured structures were fabricated on both sides. RCA standard cleaning was then performed. The treated silicon wafers were immersed in a concentrated nitric acid solution at 110°C for 10 minutes to oxidize and grow an ultrathin silicon oxide layer with a thickness of approximately 1.5 nm. A 30 nm phosphorus-doped amorphous silicon layer was deposited on the front side of the silicon wafer using a tubular PECVD system. A first annealing treatment was performed at 840°C in a nitrogen atmosphere for 30 minutes to crystallize the amorphous silicon into a phosphorus-doped polycrystalline silicon layer, forming a TOPCon structure. A 30 nm boron-doped amorphous silicon layer was deposited in a tubular PECVD system. A second annealing treatment was performed at 700°C for 10 minutes to form a tunnel junction structure. A Me-4PACz layer and a perovskite film were coated on the tunnel junction. A Me-4PACz layer was coated on the tunnel junction, followed by spin coating to prepare a perovskite film. C0 was prepared by thermal evaporation. 60 An electron transport layer was prepared; a buffer layer SnO2 was prepared by ALD; a transparent electrode IZO was prepared by magnetron sputtering; and then silver electrodes and passivation layers on both sides were prepared by thermal evaporation to obtain a perovskite / crystalline silicon tandem solar cell.

[0076] Battery performance testing: V oc =1.89 V, J sc =19.7 mA / cm 2 , FF =80.2%, η =29.9%.

[0077] Table 1. Performance comparison of perovskite / crystalline silicon tandem solar cells between the examples and comparative examples.

[0078] The battery performance test results above show that the photoelectric parameters of the tandem battery in the embodiment are significantly better than those in the comparative example. This result proves that by doping the second boron-doped polycrystalline silicon layer with carbon, nitrogen or oxygen modification elements, the surface chemical properties can be significantly optimized. The tandem polycrystalline silicon tunnel junction structure of the present invention can effectively break through the performance bottleneck of the traditional intermediate connection layer, and provides key technical support for the efficient and stable photoelectric conversion of perovskite / crystalline silicon tandem batteries.

[0079] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of this disclosure, and all such changes and modifications will fall within the scope of protection of this invention.

Claims

1. A stacked polysilicon tunnel junction structure, comprising: It is used as an intermediate connection layer for perovskite / crystalline silicon tandem solar cells. The tandem polycrystalline silicon tunnel junction structure includes, in sequence from the backlight surface to the light-receiving surface: an ultrathin silicon oxide layer, a phosphorus-doped polycrystalline silicon layer, a first boron-doped polycrystalline silicon layer, and a second boron-doped polycrystalline silicon layer. The second boron-doped polycrystalline silicon layer is doped with a modifying element, which is selected from one or more of carbon, nitrogen, and oxygen.

2. The multilayered polycrystalline silicon tunnel junction structure according to claim 1, characterized in that, The thickness of the ultrathin silicon oxide layer is 1-3 nm, the thickness of the phosphorus-doped polycrystalline silicon layer is 10-250 nm, the thickness of the first boron-doped polycrystalline silicon layer is 10-250 nm, and the thickness of the second boron-doped polycrystalline silicon layer is 1-10 nm.

3. The multilayered polycrystalline silicon tunnel junction structure according to claim 2, characterized in that, The thickness of the ultrathin silicon oxide layer is 1.5-1.8 nm, the thickness of the phosphorus-doped polycrystalline silicon layer is 10-25 nm, the thickness of the first boron-doped polycrystalline silicon layer is 10-25 nm, and the thickness of the second boron-doped polycrystalline silicon layer is 2-5 nm.

4. The multilayer polycrystalline silicon tunnel junction structure according to claim 1, characterized in that, The doping concentration of the modified element in the second boron-doped polycrystalline silicon layer is 1×10⁻⁶. 17 -1×10 22 cm -3 The boron doping concentration is 1×10 17 -5×10 20 cm -3 .

5. The multilayer polycrystalline silicon tunnel junction structure according to claim 4, characterized in that, The phosphorus doping concentration of the phosphorus-doped polysilicon layer is 1 x 10 18 -2 x 10 21 cm -3 The boron doping concentration of the first boron-doped polysilicon layer is 1 x 10 17 -5 x 10 20 cm -3 .

6. The multilayer polycrystalline silicon tunnel junction structure according to claim 1, characterized in that, The water contact angle on the surface of the second boron-doped polycrystalline silicon layer is 0°-20°.

7. The multilayered polycrystalline silicon tunnel junction structure according to claim 1, characterized in that, The second boron-doped polycrystalline silicon layer has an amorphous / nanocrystalline mixed phase structure.

8. A method for preparing a multilayered polycrystalline silicon tunnel junction structure as described in claims 1-7, characterized in that, Includes the following steps: S1. An ultrathin silicon oxide layer is formed on the surface of an n-type silicon substrate; S2. Deposit a phosphorus-doped amorphous silicon layer on the ultrathin silicon oxide layer; S3. Perform the first annealing treatment to form a phosphorus-doped polycrystalline silicon layer; S4. A first boron-doped amorphous silicon layer and a second boron-doped amorphous silicon layer are sequentially deposited on the phosphorus-doped polycrystalline silicon layer, wherein the second boron-doped amorphous silicon layer contains a modifying element selected from one or more of carbon, nitrogen, and oxygen; S5. Perform a second annealing process to form a stacked polycrystalline silicon tunnel junction structure containing a first boron-doped polycrystalline silicon layer and a second boron-doped polycrystalline silicon layer.

9. The preparation method according to claim 8, characterized in that, The phosphorus-doped amorphous silicon layer in step S2 and the first boron-doped amorphous silicon layer and the second boron-doped amorphous silicon layer in step S4 are both deposited using plasma-enhanced chemical vapor deposition. The modifying elements of the second boron-doped amorphous silicon layer are in-situ doped by introducing methane, ammonia, or carbon dioxide.

10. The preparation method according to claim 8, characterized in that, In step S3, the first annealing temperature is 800-900℃ and the annealing time is 10-200 minutes; in step S5, the second annealing temperature is 650-750℃ and the annealing time is 2-20 minutes.

11. A perovskite / crystalline silicon tandem solar cell, characterized in that, The structure comprises, in sequence from the backlight surface to the light-receiving surface, a back electrode, a back passivation layer, an n-type silicon substrate, an intermediate interconnect layer, a hole transport layer, a perovskite light-absorbing layer, an electron transport layer, a buffer layer, a transparent conductive layer, and a top electrode, wherein the intermediate interconnect layer is composed of a multilayer polycrystalline silicon tunnel junction structure as described in any one of claims 1-7.

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