A lithium-doped crystalline silicon photovoltaic cell, silicon wafer, ingot, and doping method thereof

Abstract

This invention discloses a lithium-doped crystalline silicon photovoltaic cell, silicon wafer, ingot, and doping method thereof, belonging to the field of photovoltaic cells. By combining appropriate amounts of impurity lithium and impurity boron with lithium to form p-type first lithium silicon, lithium, with an electronegativity of only 0.98, saturates some of the boron atom holes when not exposed to high-energy space radiation. Upon exposure to high-energy space radiation, on the one hand, the outer electrons of lithium are easily attracted by electron traps, objectively passivating the electron traps and "releasing" the boron atom holes. As a result, the majority carrier holes in the p-type silicon material increase, and the conductivity increases instead of decreasing. On the other hand, due to the "accompaniment" of positive lithium ions to the electron traps, the probability of the electron traps capturing photogenerated minority carrier holes is greatly reduced, which is beneficial to improving the photoelectric conversion efficiency of the space cell.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic cells, specifically to a lithium-doped crystalline silicon photovoltaic cell, silicon wafer, ingot, and doping method thereof. Background Technology

[0002] The background technology of this invention is crystalline silicon photovoltaic cells, silicon wafers and ingots that exclude lithium as a harmful impurity.

[0003] Existing crystalline silicon photovoltaic cells have a bandgap of 1.12 eV and an absorption cutoff wavelength λc of 1107 nm. Therefore, they cannot convert the energy of longer wavelength light into electrical output. For example, on the ground, they cannot absorb the high-quantum region of the AM1.5 spectrum with a center wavelength around 1250 nm. To fully absorb the high-quantum region around 1250 nm, the bandgap of the photoactive material needs to be reduced to below 1 eV. However, reducing the bandgap means reducing the output voltage of the photovoltaic cell. Considering all factors, for ground-based photovoltaic cells, for single pn junction photovoltaic cells, a cell with a bandgap of 1.12 eV has a higher photoelectric conversion efficiency than a cell with a bandgap of 1 eV. However, in specific double-junction tandem cells, when the crystalline silicon photovoltaic cell is used as the bottom cell, appropriately reducing its bandgap can expand the cell's absorption bandwidth. Furthermore, the light absorption coefficient α of semiconductor silicon is α ∝ (hν - E). g ) 0.5 Reduced E g It also helps to improve the absorption coefficient α and reduce the thickness of the silicon wafer. The overall effect is to increase the output current of the bottom cell. In addition, the top cell of the stacked cell absorbs shorter wavelength light radiation and outputs a higher voltage, thereby improving the overall photoelectric conversion efficiency of the stacked cell.

[0004] For example, in perovskite / crystalline silicon tandem photovoltaic cells, reducing the bandgap width of the bottom crystalline silicon material is reasonable and beneficial.

[0005] This invention provides a crystalline silicon ingot, a crystalline silicon wafer, and a crystalline silicon photovoltaic cell doped with lithium impurities, as well as a method for lithium doping, to reduce the band gap width of the crystalline silicon material and expand the wavelength absorption limit of the crystalline silicon material to sunlight, thereby improving the photoelectric conversion efficiency of the tandem cell. Summary of the Invention

[0006] I. Nouns and Terminology:

[0007] The following terms and terms have the following meanings in this application:

[0008] Lithium silicon, lithium silicon ingots, lithium silicon wafers, lithium silicon photovoltaic cells:

[0009] Crystalline silicon doped with lithium impurities, crystalline silicon ingots, crystalline silicon wafers, and crystalline silicon photovoltaic cells.

[0010] Lithium region:

[0011] The lithium region in this application includes the lithium boron region, the lithium oxide region, the lithium boron oxide region, and the base region of the first lithium silicon photovoltaic cell.

[0012] Non-lithium region:

[0013] Areas outside the lithium region.

[0014] First lithium-silicon photovoltaic cell, first lithium-silicon wafer:

[0015] p-type combined doping with impurity lithium and impurity boron, wherein the doping concentration N of impurity lithium is... Li Satisfy: 1×10 12 / cm 3 <N Li ≤1×10 15 / cm 3 or 1×10 15 / cm 3 <N Li <1×10 16 / cm 3 The first lithium-silicon photovoltaic cell and the first lithium-silicon wafer were formed.

[0016] Second lithium-silicon photovoltaic cell, second lithium-silicon wafer, second lithium-silicon ingot:

[0017] In this application, apart from the p-type combined dopant containing impurity lithium and impurity boron, the doping concentration N of the impurity lithium is... Li Satisfy: 1×10 12 / cm 3 <N Li ≤1×10 15 / cm 3 or 1×10 15 / cm 3 <N Li <1×10 16 / cm 3 The first lithium silicon photovoltaic cell, other photovoltaic cells besides the first lithium silicon wafer, lithium silicon wafers or lithium silicon ingots are formed.

[0018] Lithium-oxygen region:

[0019] Regions rich in impurity lithium and impurity oxygen.

[0020] Lithium-boron region:

[0021] Regions rich in lithium and boron impurities.

[0022] Base region:

[0023] In the photoactive region of a crystalline silicon photovoltaic cell, there is a region on the side with a lower concentration of electroactive impurities and a thicker thickness in the pn junction.

[0024] Superbase region:

[0025] This application refers to the region near the ohmic contact between the base region and the electrode, where the concentration of active impurities is relatively high.

[0026] Launch area:

[0027] In the photoactive region of a crystalline silicon photovoltaic cell, there is a region on the side with a higher concentration of electroactive impurities in the pn junction and a thinner thickness.

[0028] Oxygen-deficient zone:

[0029] Silicon regions with impurity oxygen concentrations lower than the background concentration after gettering.

[0030] Oxygen-rich zone:

[0031] Corresponding to the oxygen-deficient region is the silicon region with a higher concentration of impurity oxygen.

[0032] Boron-rich region:

[0033] Silicon regions with high concentrations of electroactive impurity boron.

[0034] Boron-deficient region:

[0035] Corresponding to the boron-rich region is the silicon region with a higher concentration of boron impurities.

[0036] Boron-deficient region:

[0037] In contrast to the boron-rich region, there is a silicon region with a lower concentration of electroactive boron impurities.

[0038] Quasi-i type:

[0039] In this application, the concentration of excess p-type or excess n-type impurities is less than 1 × 10⁻⁶. 12 / cm 3 Crystalline silicon materials are called quasi-i-type materials, or those with a concentration of excess p-type or excess n-type impurities below 1 × 10⁻⁶. 12 / cm 3 A certain region of crystalline silicon material is called the quasi-i-type region.

[0040] Tandem photovoltaic cells:

[0041] In this application, a photovoltaic cell refers to a photovoltaic cell consisting of two photovoltaic sub-cells with different spectral responses stacked together in series along an optical path. The top cell has higher absorption and conversion efficiency for short-wavelength light, while the bottom cell has higher absorption and conversion efficiency for long-wavelength light.

[0042] Space photovoltaic cells: Photovoltaic cells used in outer space. II. Summary of the Invention:

[0044] In a first aspect, the present invention provides a crystalline silicon photovoltaic cell, wherein the silicon material in the substrate region is doped with a p-type combination of lithium impurity and boron impurity to form a first lithium-silicon photovoltaic cell, wherein in the first lithium-silicon photovoltaic cell, the concentration N of the lithium impurity element in the substrate region is... Li Satisfy: 1×10 12 / cm 3 <N Li ≤1×10 15 / cm 3 or 1×10 15 / cm 3 <N Li <1×10 16 / cm 3 ;

[0045] And the concentration of impurity boron N B With the concentration of lithium impurity N Li Satisfy: 1×10 15 / cm 3 ≤(N B -N Li ≤5×10 17 / cm 3 (For example, it could be: (N) B -N Li ) equals (1×10 15 / cm 3 ~2×10 15 / cm 3 ); or (2×10 15 / cm 3 ~3×10 15 / cm 3 ), or (3×10 15 / cm 3 ~1×10 16 / cm 3 ), or (1×10 16 / cm 3 ~2×10 16 / cm 3 ), or (2×10 16 / cm 3 ~3×10 16 / cm 3 ), or (3×10 16 / cm 3 ~1×10 17 / cm 3 ), or (1×10 17 / cm 3 ~3×10 17 / cm3 This ensures that the resistivity requirements of the material are met while obtaining the beneficial effects of impurity lithium.

[0046] Preferably, the first lithium-silicon photovoltaic cell is a space photovoltaic cell or the base cell of a space tandem photovoltaic cell.

[0047] Alternatively, the first lithium-silicon photovoltaic cell is the base cell of a ground-mounted tandem photovoltaic cell.

[0048] The beneficial effects of this invention on space photovoltaic cells, or the bottom cell of space tandem photovoltaic cells, are as follows:

[0049] As is well known, silicon photovoltaic cells used in space are directly exposed to the space environment with high-energy particles during their on-orbit service. This high-energy radiation can excite silicon atoms in the silicon substrate into interstitial positions in the lattice, thereby generating vacancy defects and dangling bond defects, which are mainly manifested as electron traps.

[0050] For n-type silicon materials, on the one hand, the majority electrons (captured by traps) in n-type silicon are basically saturated with these electron traps, which reduces the conductivity of n-type silicon materials; on the other hand, these saturated, negatively charged electron traps are also likely to capture positively charged photogenerated minority carrier holes in n-type materials, becoming effective recombination centers for photogenerated minority carrier holes, thereby reducing the photoelectric conversion efficiency of photovoltaic cells.

[0051] However, for p-type silicon materials, on the one hand, the majority carriers in p-type silicon are holes, and the capture of minority carrier electrons does not reduce the conductivity of the material; on the other hand, because the number of minority carriers in p-type silicon is small, the probability of filling these electron traps is low, so the number of holes captured in p-type silicon after these electron traps capture electrons is also small, resulting in a low effective recombination rate of photogenerated minority carrier holes and a small impact on the photoelectric conversion efficiency of photovoltaic cells.

[0052] This invention combines appropriate amounts of impurity lithium and impurity boron to form a p-type first lithium silicon. Lithium has an electronegativity of only 0.98. When not exposed to high-energy space radiation, the doped lithium saturates some of the holes in the boron atoms. Upon exposure to high-energy space radiation, on the one hand, the outer electrons of lithium are easily attracted by the electron traps, objectively passivating the electron traps and "releasing" the holes in the boron atoms. As a result, the majority carrier holes in the p-type silicon material increase, and the conductivity increases instead of decreasing. On the other hand, due to the "accompaniment" of positive lithium ions to the electron traps, the probability of the electron traps capturing photogenerated minority carrier holes is significantly reduced, which is beneficial for improving the photoelectric conversion efficiency of the space battery.

[0053] The second beneficial effect of this invention is that the light absorption coefficient α of semiconductor silicon is α∝(hν-E). g )0.5 Reduced E g This is beneficial for increasing the absorption coefficient α, which can reduce the thickness of the silicon wafer, reduce the weight of the space photovoltaic cell, and lower the cost of rocket launch.

[0054] When this invention is used as the bottom cell of a ground-mounted tandem photovoltaic cell, its beneficial effects are as follows:

[0055] In the first lithium-silicon photovoltaic cell, lithium impurities exist in the crystalline silicon as interstitial elements. Lithium ions existing in the interstitial state will expand the interatomic spacing of the crystalline silicon, thereby reducing the band gap width of the crystalline silicon and expanding the absorption wavelength range of light energy by the crystalline silicon.

[0056] Because lithium atoms have the lowest electronegativity and are easily ionized, their small ion radius results in a high diffusion coefficient within the crystalline silicon lattice at room temperature, affecting the long-term operational stability of photovoltaic cells. Therefore, this invention incorporates boron impurities into the silicon material of the substrate region. The bond energy between boron and lithium elements delays lithium-ion migration, stabilizes lithium distribution, and improves the long-term operational stability of the first lithium-silicon photovoltaic cell. Different silicon lattice expansion designs require different lithium concentrations, and different lithium concentrations necessitate different boron impurities to delay lithium-ion migration and improve the long-term operational stability of the photovoltaic cell.

[0057] Furthermore, the concentration of boron in the emitter region and superbase region outside the base region is lower than or equal to the concentration of boron in the base region. The beneficial effect is that, by using the binding effect of impurity boron on impurity lithium, the migration of impurity lithium in the base region is restricted, which is conducive to ensuring the stability of photovoltaic cell performance.

[0058] Furthermore, the first lithium silicon photovoltaic cell is a bottom cell in the form of a tandem photovoltaic cell with a back electrode structure, which is beneficial for maintaining the stability of lithium impurity elements in the base region.

[0059] Secondly, the present invention also provides a crystalline silicon photovoltaic cell, wherein the substrate silicon material is doped with lithium impurities to form a second lithium-silicon photovoltaic cell, the second lithium-silicon photovoltaic cell having one of the following characteristics:

[0060] 1) In the second lithium-silicon photovoltaic cell, there exists a lithium-oxygen region co-doped with impurity oxygen and impurity lithium. In this lithium-oxygen region, the concentration N of the impurity lithium element is... Li Satisfy: 1×10 12 / cm 3 <N Li ≤1×10 15 / cm 3 or 1×10 15 / cm 3 <N Li <1×1016 / cm 3 or 1×10 16 / cm 3 ≤N Li <2×10 19 / cm 3 The concentration of impurity oxygen element N O Satisfy: N O ≥0.5N Li (For example, it can be N) O =(a±0.05)N Li (b±0.5)N Li (c±5)N Li (d±50)N Li (e±500)N Li (f±5000)N Li (g±50000)N Li Where a = 0.55, 0.65, 0.75, 0.85, 0.95; b = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5; c = 10b, d = 10c; e = 10d, f = 10e, g = 10f, etc.

[0061] 2) In the second lithium-silicon photovoltaic cell, there exists a lithium-boron region where impurity boron and impurity lithium are co-doped. In this lithium-boron region, the concentration N of the impurity lithium element is... Li Satisfy: 1×10 16 / cm 3 ≤N Li <2×10 19 / cm 3 The concentration of impurity boron N B Satisfy: N B ≥N Li (For example, it can be N) B =(a±0.05)N Li (b±0.5)N Li Where a = 1.05, 1.15, 1.25, 1.35, 1.45, 1.55, 1.65, 1.75, 1.85, 1.95, and b = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5, etc.

[0062] 3) In the second lithium-silicon photovoltaic cell, there exists a lithium boron-oxygen region co-doped with impurity oxygen, impurity boron, and impurity lithium. In this lithium boron-oxygen region, the concentration N of the impurity lithium element is... Li Satisfy: 1×10 12 / cm 3 <N Li ≤1×10 15 / cm3 or 1×10 15 / cm 3 <N Li <1×10 16 / cm 3 or 1×10 16 / cm 3 ≤N Li <2×10 19 / cm 3 The concentration of impurity boron N B With the concentration of impurity oxygen element N O Satisfy: N B +2N O ≥N Li (For example, it can be N) B +2N O =(a±0.05)N Li (b±0.5)N Li (c±5)N Li (d±50)N Li (e±500)N Li (f±5000)N Li (g±50000)N Li Where a = 1.05, 1.15, 1.25, 1.35, 1.45, 1.55, 1.65, 1.75, 1.85, 1.95; b = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5; c = 10b, d = 10c; e = 10d, f = 10e, g = 10f, etc.

[0063] Its beneficial effect is that, in the second lithium-silicon photovoltaic cell, the impurity lithium exists in the crystal silicon in the form of interstitial elements. The lithium ions existing in the interstitial state will expand the interatomic spacing of the crystal silicon, thereby reducing the band gap width of the crystal silicon and expanding the absorption wavelength range of light energy of the crystal silicon.

[0064] Because lithium atoms have the lowest electronegativity and are easily ionized, their small ion radius results in a high diffusion coefficient within the crystalline silicon lattice at room temperature, affecting the long-term operational stability of photovoltaic cells. Therefore, this invention dops the lithium region with at least one impurity element, such as boron or oxygen. The bond energy between boron or oxygen and lithium hinders lithium-ion migration, stabilizing the lithium distribution and improving the long-term operational stability of the second lithium-silicon photovoltaic cell. Different silicon lattice expansion designs require different lithium concentrations, and different lithium concentrations necessitate different oxygen and / or boron impurity concentrations to achieve localization, thereby improving the long-term operational stability of the photovoltaic cell.

[0065] Furthermore, the second lithium-silicon photovoltaic cell serves as the base cell in the tandem photovoltaic system. This second lithium-silicon photovoltaic cell, with its better response to near-infrared waves in sunlight, can absorb more solar energy, thereby improving the photoelectric conversion efficiency of the tandem photovoltaic system.

[0066] Furthermore, in the region outside the lithium-boron region, the concentration of boron is lower than or equal to the concentration of boron in the lithium-boron region, to prevent the high concentration of boron from attracting lithium ions and disrupting the predetermined distribution of lithium impurities in crystalline silicon.

[0067] Furthermore, in the region outside the lithium-oxygen region, the concentration of oxygen is lower than or equal to the concentration of oxygen in the lithium-oxygen region, to prevent the high concentration of oxygen from attracting lithium ions and disrupting the predetermined distribution of lithium impurities in crystalline silicon.

[0068] Furthermore, in the region outside the lithium boron-oxygen region, the concentration of oxygen is lower than or equal to the concentration of oxygen in the lithium boron-oxygen region, and the concentration of boron is lower than or equal to the concentration of boron in the lithium boron-oxygen region, to prevent the attraction of high concentrations of oxygen and boron to lithium ions and disrupt the predetermined distribution of lithium impurities in crystalline silicon.

[0069] Furthermore, the second lithium-silicon photovoltaic cell is an HJT cell, or the second lithium-silicon photovoltaic cell is the bottom cell of a tandem photovoltaic cell.

[0070] The bottom cell, which absorbs infrared-limited absorption, is a type of lithium-silicon photovoltaic cell that helps improve the overall photoelectric conversion efficiency of the stacked cell; the HJT cell is a region where the electrode contact area is prepared by deposition, making it easy to control the migration of lithium elements.

[0071] Thirdly, the present invention also provides a crystalline silicon wafer in which a p-type combined dopant of impurity lithium and impurity boron is doped to form a first lithium silicon wafer, wherein in the first lithium silicon wafer, in the base region doped with the combined impurity boron and impurity lithium, the concentration N of the impurity lithium element is... Li Satisfy 1×10 12 / cm 3 <N Li ≤1×10 15 / cm 3 or 1×10 15 / cm 3 <N Li <1×10 16 / cm 3 And the concentration of impurity boron N B With the concentration of lithium impurity N Li The following condition must be met: 1×10 15 / cm 3 ≤(N B -N Li≤5×10 17 / cm 3 (For example, it could be: (N) B -N Li ) equals (1×10 15 / cm 3 ~2×10 15 / cm 3 ); or (2×10 15 / cm 3 ~3×10 15 / cm 3 ), or (3×10 15 / cm 3 ~1×10 16 / cm 3 ), or (1×10 16 / cm 3 ~2×10 16 / cm 3 ), or (2×10 16 / cm 3 ~3×10 16 / cm 3 ), or (3×10 16 / cm 3 ~1×10 17 / cm 3 ), or (1×10 17 / cm 3 ~3×10 17 / cm 3 This ensures that the resistivity requirements of the crystalline silicon wafer are met while still obtaining the beneficial effects of the impurity lithium element.

[0072] The first lithium silicon wafer can be used to prepare a first lithium silicon photovoltaic cell, preferably a space photovoltaic cell, with the same beneficial effects as before, which will not be repeated here.

[0073] Fourthly, the present invention also provides a crystalline silicon wafer, wherein the substrate silicon material is doped with lithium impurities to form a second lithium silicon wafer, which has one of the following characteristics:

[0074] 1) In the second lithium-silicon wafer, there exists a lithium-oxygen region co-doped with impurity oxygen and impurity lithium, wherein the concentration N of impurity lithium element in the lithium-oxygen region is... Li Satisfy 1×10 12 / cm 3 <N Li ≤1×10 15 / cm 3 or 1×10 15 / cm 3 <N Li <1×10 16 / cm3 or 1×10 16 / cm 3 ≤N Li <2×10 19 / cm 3 The concentration of impurity oxygen element N O Satisfying N O ≥0.5N Li (For example, it can be N) O =(a±0.05)N Li (b±0.5)N Li (c±5)N Li (d±50)N Li (e±500)N Li (f±5000)N Li (g±50000)N Li Where a = 0.55, 0.65, 0.75, 0.85, 0.95; b = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5; c = 10b, d = 10c; e = 10d, f = 10e, g = 10f, etc.

[0075] 2) In the second lithium-silicon wafer, there exists a lithium-boron region where impurity boron and impurity lithium are co-doped. In this lithium-boron region, the concentration N of the impurity lithium element is... Li Satisfy 1×10 16 / cm 3 ≤N Li <2×10 19 / cm 3 The concentration of impurity boron N B Satisfying N B ≥N Li (For example, it can be N) B =(a±0.05)N Li (b±0.5)N Li Where a = 1.05, 1.15, 1.25, 1.35, 1.45, 1.55, 1.65, 1.75, 1.85, 1.95, and b = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5, etc.

[0076] 3) In the second lithium-silicon wafer, there exists a lithium boron-oxygen region co-doped with impurity oxygen, impurity boron, and impurity lithium. In this lithium boron-oxygen region, the concentration N of the impurity lithium element is... Li Satisfy 1×10 12 / cm 3 <N Li ≤1×10 15 / cm 3 or 1×10 15 / cm 3 <N Li <1×10 16 / cm 3 or 1×10 16 / cm 3 ≤N Li <2×10 19 / cm 3 The concentration of impurity boron N B The concentration of impurity oxygen element N O Satisfying N B +2N O ≥N Li (For example, it can be N) B +2N O =(a±0.05)N Li (b±0.5)N Li (c±5)N Li (d±50)N Li (e±500)N Li (f±5000)N Li (g±50000)N Li Where a = 1.05, 1.15, 1.25, 1.35, 1.45, 1.55, 1.65, 1.75, 1.85, 1.95; b = 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5, 8.5, 9.5; c = 10b, d = 10c; e = 10d, f = 10e, g = 10f, etc.

[0077] The second lithium silicon wafer can be used to prepare a second lithium silicon photovoltaic cell, preferably as the bottom cell of a tandem photovoltaic cell, and its beneficial effects are the same as before, so they will not be repeated.

[0078] Fifthly, the present invention provides a method for lithium impurity doping of lithium silicon, comprising one of the following methods:

[0079] Method 1), Solid source method: Solid lithium silicate and crystalline silicon raw material are placed together in a quartz crucible, the crystalline silicon raw material is melted, and the impurity lithium is incorporated into the molten silicon and enters the crystalline silicon ingot product;

[0080] Method 2), Sol-source method: Using sol-state lithium silicate, an amorphous lithium silicate film is prepared on the surface of crystalline silicon raw material, crystalline silicon ingot, crystalline silicon wafer, or photovoltaic cell semi-finished product. After drying, the crystalline silicon raw material with the lithium silicate film is melted to prepare lithium silicon crystalline ingot; or the crystalline silicon ingot with the lithium silicate film, crystalline silicon wafer, or photovoltaic cell semi-finished product is placed in a high temperature environment of 100℃~600℃ or 600℃~1100℃ to diffuse and redistribute impurity lithium to obtain lithium silicon crystalline silicon ingot, lithium silicon crystalline silicon wafer, or lithium silicon photovoltaic cell.

[0081] Method 3), Sol-based source re-purification method: Using sol-based lithium silicate, an amorphous lithium silicate film is prepared on the surface of crystalline silicon raw materials, crystalline silicon products, or crystalline silicon wafers. After drying, the diffusion and distribution of lithium impurities in the silicon material are completed within the following temperature range:

[0082] a) 100℃≤T<400℃;

[0083] b) 400℃≤T<500℃;

[0084] c) 500℃≤T<600℃;

[0085] After the above diffusion and impurity distribution are completed, the crystalline silicon raw material is used to prepare lithium silicon ingots after corrosion cleaning; the crystalline silicon ingots or the crystalline silicon wafers are used to prepare lithium silicon photovoltaic cells.

[0086] Furthermore, the solid lithium silicate used in method 1) includes lithium orthosilicate (Li4SiO4) and lithium metasilicate (Li2SiO3); the modulus of the sol-state lithium silicate used in methods 2) and 3) is 2 to 8.5.

[0087] Furthermore, the dry film thickness of the sol-sol lithium silicate in methods 2) and 3) is 1 μm to 1 mm.

[0088] The beneficial effects of this preparation method are as follows:

[0089] Solid source method:

[0090] 1) Lithium silicate source can achieve high purity without other interfering impurities;

[0091] 2) Simple, convenient, and easy to measure.

[0092] Sol-based source method:

[0093] 1) Lithium silicate is chemically stable and safe to use;

[0094] 2) The lithium silicate dry film coated on the crystalline silicon raw material does not come into direct contact with the quartz crucible, thus avoiding corrosion of the quartz crucible at high temperatures.

[0095] Sol-based source re-purification method:

[0096] The purity requirements for the dopant source are reduced because, at low temperatures, the diffusion coefficients of most metallic or non-metallic impurities in silicon crystals are much lower than those of lithium impurities. Once the lithium impurity diffusion meets the requirements, the remaining lithium source and the impurity contamination layer on the shallow surface of the silicon wafer can be etched away to obtain a satisfactory doping effect. Low-temperature diffusion processes cause much less damage to the silicon crystal itself than high-temperature diffusion processes, which helps maintain the higher quality of the silicon wafer.

[0097] Sixthly, the present invention provides a method for preparing a second lithium silicon ingot, comprising the following steps:

[0098] 1) For quasi-i-type or p-type second lithium silicon ingots: Based on the preset required amount of impurity lithium doping in the second lithium silicon wafer, add the corresponding amount of impurity lithium to the crystalline silicon raw material, and calculate the concentration N of impurity oxygen entering the silicon single crystal during the neutralization process in the crystalline silicon raw material. O Adjusting the concentration of impurity boron N B Satisfying N B +2N O ≥N Li ; and by adjusting the concentration of impurity boron N B The conductivity type of the second lithium silicon is made to be quasi-i-type or p-type. The crystalline silicon raw material and the impurity source are melted to prepare a quasi-i-type or p-type second lithium silicon ingot.

[0099] 2) For n-type second lithium silicon ingot: Based on the preparation of quasi-i-type second lithium silicon ingot, n-type impurities, including at least one of phosphorus, arsenic or antimony, are added as needed, and the same subsequent steps as in step 1) are followed to prepare an n-conductivity type second lithium silicon ingot.

[0100] The beneficial effect of this preparation method is that the excessive impurity oxygen in CZ silicon single crystal has a basically negative impact on the performance of photovoltaic cells, and at the same time, impurity oxygen is difficult to avoid. This invention not only uses impurity lithium to passivate impurity oxygen and reduce the adverse effect of impurity oxygen on the performance of photovoltaic cells, but also uses the high bond energy between impurity oxygen and impurity lithium to cooperate with impurity boron to suppress the migration of impurity lithium, thereby achieving the purpose of extending the wavelength reception limit of photovoltaic cells and improving the stability of the second lithium silicon cell.

[0101] In a seventh aspect, the present invention also provides a method for preparing a crystalline silicon wafer, which is obtained by slicing the aforementioned crystalline silicon ingot.

[0102] Eighthly, the present invention also provides a method for preparing the lithium region of a photovoltaic cell doped with lithium impurities, comprising the following steps:

[0103] 1) Oxygen-containing crystalline silicon wafers are obtained by slicing crystalline silicon ingots containing oxygen impurities, wherein the concentration N of the oxygen impurities is... O The concentration of boron impurities in the preset lithium region (N) B The concentration N of impurity lithium in the preset lithium region Li Satisfying N B +2N O ≥N Li ;

[0104] 2) An oxygen-deficient region is prepared in the pre-defined non-lithium region of the oxygen-containing crystalline silicon wafer using a gettering technique;

[0105] 3) A lithium-rich region is prepared in a certain area of ​​the oxygen-containing crystalline silicon wafer. Under a temperature higher than room temperature and / or under the action of an external electric field, lithium ions are redistributed and migrated, and enriched in the oxygen-rich preset lithium region to form a lithium-rich region. The lithium-rich region is one of the following: lithium-oxygen region, lithium-boron region, and lithium-boron-oxygen region, as well as the base region of the first lithium-silicon photovoltaic cell.

[0106] Its beneficial effect is that, compared with the oxygen-rich and boron-rich region, the oxygen-poor and boron-poor region acts as a barrier to lithium-ion migration, which helps to limit lithium-ion migration and ensure the stability of photovoltaic devices.

[0107] In a ninth aspect, the present invention also provides a method for preparing the lithium region of a photovoltaic cell doped with lithium impurities, comprising the following steps:

[0108] Oxygen-deficient or oxygen- and boron-deficient regions are prepared in the pre-defined emitter region and superbase region of the oxygen-containing crystalline silicon wafer using gettering techniques. Attached Figure Description

[0109] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0110] Figure 1 This is a schematic diagram of Embodiment 5 of the present invention, a space photovoltaic cell and its lithium region preparation method;

[0111] Figure 2 This is a schematic diagram of Embodiment 6 of the present invention, a stacked battery, and a method for preparing the lithium region thereon;

[0112] Figure 3 This is a schematic diagram of a method for impurity lithium doping of a Czochralski-grown lithium silicon single crystal ingot in this embodiment 1;

[0113] Figure 4 This is a schematic diagram of an impurity lithium doping method for a zone-melting p-type lithium silicon single crystal ingot in Embodiment 2.

[0114] Figure 5 This is a schematic diagram of an impurity lithium doping method for a Czochralski-grown p-type lithium silicon single crystal ingot in this embodiment 3;

[0115] Figure 6 This is a schematic diagram of a method for impurity lithium doping of a Czochralski n-type lithium silicon single crystal ingot in Embodiment 4.

[0116] Explanation of reference numerals in the attached figures:

[0117] 10-Crystal silicon raw material; 11-Lithium-ion film crystalline silicon raw material; 12-Lithium-containing crystalline silicon raw material; 21-Solid-state lithium silicate impurity source; 22-Lithium silicate film layer; 3-Boron impurity source; 4-Phosphorus impurity source; 5-Silicon single crystal rod; 50-Lithium-free silicon single crystal ingot; 51-Czochralski-grown p-type lithium silicon single crystal ingot; 52-Czochralski-grown n-type lithium silicon single crystal ingot; 53-Zone-melted p-type lithium silicon single crystal ingot; 63-Zone-melted p-type lithium silicon wafer; 7-Stacked battery; 71-Bottom cell; 711-First electrode of tandem cell; 712-Second electrode of tandem cell; 713-Third electrode of tandem cell; 72-Top cell; 8-Czochralski silicon single crystal wafer; 81-Preset lithium region; 82-Preset oxygen- and lithium-deficient region; 83-Segregated oxide layer; 84-Lithium region; 85-Oxygen- and lithium-deficient region; 86-Amorphous lithium silicate film; 91-Quartz crucible; 92-Induction heater; 99-Impurity oxygen. Detailed Implementation

[0118] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The apparatus of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0119] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0120] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0121] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the product of this invention is usually placed when in use. They are only for the convenience of describing this invention and 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. Therefore, they should not be construed as limiting this invention.

[0122] Furthermore, the terms "first" and "second" are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0123] It should be noted that, where there is no conflict, the features in the embodiments of the present invention can be combined with each other.

[0124] Example 1:

[0125] See Figure 3 This embodiment describes a method for lithium impurity doping in a Czochralski-grown lithium-silicon single crystal ingot, employing a solid-state source method: a solid lithium silicate impurity source 21, crystalline silicon raw material 10, and a boron impurity source 3 are placed together in a quartz crucible 91. The crystalline silicon raw material 10 is melted, allowing the lithium and boron impurities to combine and integrate into the molten silicon. This mixture is then introduced into the silicon single crystal rod 5 and processed to obtain a Czochralski-grown p-type lithium-silicon single crystal ingot 51. The concentration N of the lithium impurity element in the Czochralski-grown p-type lithium-silicon single crystal ingot 51 in this embodiment is... Li ≈1×10 18 / cm 3 The concentration of impurity oxygen N o ≈1×10 18 The concentration of impurity boron N B ≈1×10 16 Satisfying N O ≥0.5N Li The relationship between these properties can be used to prepare the bottom cell of a tandem solar cell with an extended long-wavelength absorption limit.

[0126] The beneficial effects of this embodiment are as follows:

[0127] 1) Lithium silicate is chemically stable and safe to use;

[0128] 2) The process is simple, the measurement is accurate, the operation is convenient, and a high doping concentration can be achieved;

[0129] 3) The combined incorporation of impurity boron and impurity lithium can precisely control the resistivity of silicon single crystal ingots.

[0130] Example 2:

[0131] See Figure 4 This embodiment describes a method for impurity lithium doping of a zone-melted p-type lithium-silicon single crystal ingot, employing a sol-gel method: an amorphous lithium silicate film 22 is prepared on the surface of a crystalline silicon raw material 10 using sol-state lithium silicate. After drying, the lithium-film crystalline silicon raw material 11 with the lithium silicate film 22 is melted together with a boron impurity source 3 through an induction heater 92 to prepare a zone-melted p-type lithium-silicon single crystal ingot 53. The concentration N of the impurity lithium element in the zone-melted p-type lithium-silicon single crystal ingot 53 of this embodiment is... Li ≈1×10 16 / cm 3 The concentration of impurity boron N B ≈2×10 16 , (N B -N Li ) = 1 × 10 16 / cm 3It can be used to prepare the base cell for space photovoltaic cells or space tandem photovoltaic cells.

[0132] The beneficial effects of this embodiment are as follows:

[0133] 1) Lithium silicate is chemically stable, safe to use, and the method of lithium doping is simple and reliable;

[0134] 2) The combined incorporation of impurity boron and impurity lithium can precisely control the resistivity of silicon single crystal ingots.

[0135] Example 3:

[0136] See Figure 5 This embodiment is a method for impurity lithium doping of Czochralski p-type lithium silicon single crystal ingots, namely, sol purification method: using sol-state lithium silicate, an amorphous lithium silicate film layer 22 is prepared on the surface of crystalline silicon raw material 10. After drying, the diffusion of impurity lithium from the surface of crystalline silicon raw material 10 into the bulk is completed at a temperature of 500°C. By controlling the modulus, diffusion temperature and time of the amorphous lithium silicate film layer 22, the total amount of lithium impurities can be controlled.

[0137] After the above diffusion and impurity distribution are completed, the lithium-silicon crystalline raw material 11 with lithium silicate film layer 22 is etched and cleaned to remove the residual lithium silicate film layer 22 along with the excess impurities therein, resulting in lithium-containing crystalline silicon raw material 12. The crystalline silicon raw material 10, the lithium-containing crystalline silicon raw material 12 and the boron impurity source 3 are placed in a quartz crucible 91, and a silicon single crystal rod 5 containing impurity lithium, impurity boron and impurity oxygen 99 precipitated from the quartz crucible 91 is pulled out. After processing, a Czochralski p-type lithium silicon single crystal ingot 51 is obtained.

[0138] The beneficial effects of this embodiment are:

[0139] The purity requirements for the dopant source are reduced because, at a low temperature of 500℃, the diffusion rate of most metallic or non-metallic impurities in the silicon crystal is much lower than that of lithium impurities. Once the lithium impurity diffusion meets the requirements, the remaining lithium source and the impurity contamination layer on the shallow surface of the silicon wafer can be etched away to obtain a satisfactory doping effect. Low-temperature diffusion processes cause much less damage to the silicon crystal itself than high-temperature diffusion processes, which is beneficial for maintaining the high quality of the silicon wafer.

[0140] Example 4:

[0141] See Figure 6This embodiment describes a method for lithium doping of a Czochralski-grown n-type lithium-silicon single crystal ingot. The sol-source method involves preparing an amorphous lithium silicate film 22 on the surface of a crystalline silicon raw material 10 using sol-state lithium silicate. After drying, the lithium-film crystalline silicon raw material 11 and a phosphorus impurity source 4 are placed together in a quartz crucible 91, and a silicon single crystal rod 5 is drawn from the melt. By controlling the modulus and coating amount of the amorphous lithium silicate film 22, the total amount of lithium impurities is controlled, as are the concentration and distribution of impurity oxygen 99. This results in a controlled concentration of lithium impurity N in the Czochralski-grown n-type lithium-silicon single crystal ingot 52. Li ≈5×10 17 / cm 3 The concentration of impurity oxygen N o ≈5×10 17 The concentration of impurity phosphorus N B ≈3×10 15 Satisfying N O ≥0.5N Li The relationship between these properties can be used to prepare the bottom cell of a tandem solar cell with an extended long-wavelength absorption limit.

[0142] Example 5:

[0143] See Figure 1 This embodiment is a space photovoltaic cell. It is prepared by slicing a zone-melted p-type lithium silicon monocrystalline ingot 53 to obtain a zone-melted p-type lithium silicon wafer 63. Amorphous silicon heavily doped with gallium is fabricated on the back side of the zone-melted p-type lithium silicon wafer 63 to form the first electrode region of the base cell 71, which is also the first electrode 711 of the tandem cell. Amorphous silicon heavily doped with phosphorus is fabricated on the back side of the zone-melted p-type lithium silicon wafer 63 to form the second electrode region of the base cell 71, which is also the second electrode 712 of the tandem cell. The heavily doped gallium tandem cell first electrode 711 and the heavily doped phosphorus tandem cell second electrode 712 inhibit lithium-ion diffusion, which helps maintain the stability of the space photovoltaic cell base cell 71.

[0144] The substrate silicon material of the bottom cell 71 is doped with a p-type combined dopant of lithium impurity and boron impurity, wherein the concentration of lithium impurity N Li =1×10 15 / cm 3 The concentration of impurity boron N B =3×10 17 / cm 3 The bottom cell 71 and the top cell 72 are stacked together to form a stacked cell 7.

[0145] Compared with the prior art, the beneficial effects of this embodiment are as follows:

[0146] 1) This invention combines appropriate amounts of impurity lithium and impurity boron to form a p-type first lithium silicon. When not exposed to high-energy space irradiation, the impurity lithium is partially saturated with boron atoms and is in a "non-ionized" state, contributing no to the conductivity of the material. When exposed to high-energy space irradiation, on the one hand, the outer electrons of the impurity lithium are easily attracted by the irradiation traps, resulting in passivation of the electron traps and "release" of boron atoms. As a result, the conductivity of the p-type silicon material increases, which is beneficial to reducing the conduction resistance of the battery. On the other hand, due to the "accompaniment" of positive lithium ions, the probability of the irradiation traps that have captured electrons capturing photogenerated minority carrier holes again is greatly reduced, which is beneficial to improving the photoelectric conversion efficiency of the space battery.

[0147] 2) Lithium doping reduces the bandgap width of semiconductor silicon, which helps to improve the light absorption coefficient, reduce the thickness of silicon wafers, reduce the weight of space photovoltaic cells, and reduce the cost of rocket launch;

[0148] 3) By taking advantage of the high mobility of lithium ions, uniform lithium-doped silicon single crystal ingots can be obtained;

[0149] 4) The simple structure of the HJT battery allows for the localization of lithium impurities, resulting in device stability;

[0150] 5) By leveraging the extension of the wavelength absorption limit by the bottom cell, the photoelectric conversion efficiency of the tandem cell is improved.

[0151] Example 6:

[0152] See Figure 2 This embodiment is a tandem battery, consisting of an oxygen impurity concentration of N... O =5×10 17 / cm 3 A Czochralski-grown lithium-free silicon single crystal ingot 50 is sliced ​​to obtain a Czochralski-grown silicon single crystal wafer 8. The Czochralski-grown silicon single crystal wafer 8 includes a predetermined lithium region 81 and a predetermined oxygen- and lithium-deficient region 82. A segregated oxide layer 83 is prepared on the surface of the predetermined oxygen- and lithium-deficient region 82 by high-temperature wet oxygen oxidation. The segregated oxide layer 83 is removed to obtain an oxygen- and lithium-deficient region 85. An amorphous lithium silicate film 86 is prepared on the back side of the Czochralski-grown silicon single crystal wafer 8. Impurity lithium is diffused into the lithium region 84 at a high temperature of 450°C, causing lithium ions in the amorphous lithium silicate film 86 to redistribute and migrate. The amorphous lithium silicate film 86 is removed, and the top cell 72 of the tandem cell 7 is prepared on the front side of the Czochralski-grown silicon single crystal wafer 8. The first electrode 711 and the second electrode 712 of the tandem cell 7 are prepared on the back side of the tandem cell 7, and the third electrode 713 of the tandem cell 7 is prepared on the front side of the tandem cell 7 to form the tandem cell 7.

[0153] In this embodiment, the concentration of lithium impurity element N in lithium region 84 is... Li =8×10 17 / cm 3 The concentration of impurity oxygen N B=5×10 17 / cm 3 The concentration of impurity boron N B =3×10 17 / cm 3 .

[0154] The beneficial effects of this embodiment are:

[0155] 1) Reduce the purity requirements of the dopant source. At a low temperature of 500℃, the diffusion rate of most metallic or non-metallic impurities in silicon crystals is much lower than that of lithium impurities. Once the lithium impurity diffusion meets the requirements, the remaining lithium source and the impurity contamination layer on the shallow surface of the silicon wafer can be etched away to obtain a satisfactory doping effect. Low-temperature diffusion processes cause much less damage to the silicon crystal itself than high-temperature diffusion processes, which is beneficial for maintaining the high quality of the silicon wafer.

[0156] 2) By taking advantage of the high mobility of lithium ions and the binding of impurity lithium by impurity oxygen in the lithium region, a localized and uniform lithium region can be obtained.

[0157] 3) By leveraging the extension of the wavelength absorption limit by the bottom cell, the photoelectric conversion efficiency of the tandem cell is improved.

[0158] In summary, the technical solution provided by this invention has significant technical, environmental, and cost advantages, and is of great significance to the development of the industry.

[0159] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A lithium-doped crystalline silicon photovoltaic cell, characterized in that, The substrate silicon material is p-type doped with a combination of lithium and boron impurities to form a first lithium-silicon photovoltaic cell. In the first lithium-silicon photovoltaic cell, the concentration N of the lithium impurity element in the substrate region is... Li Satisfy: 1×10 12 / cm 3 <N Li ≤1×10 15 / cm 3 or 1×10 15 / cm 3 <N Li <1×10 16 / cm 3 ; And the concentration of impurity boron N B With the concentration of lithium impurity N Li Satisfy: 1×10 15 / cm 3 ≤(N B -N Li ≤5×10 17 / cm 3 .

2. The crystalline silicon photovoltaic cell according to claim 1, characterized in that, The first lithium-silicon photovoltaic cell is a space photovoltaic cell, or the base cell of a space tandem photovoltaic cell.

3. The crystalline silicon photovoltaic cell according to claim 1, characterized in that, The first lithium-silicon photovoltaic cell is the base cell of a ground-mounted tandem photovoltaic cell.

4. The crystalline silicon photovoltaic cell according to claim 1, characterized in that, The concentration of boron in the emitter region and superbase region outside the base region is lower than or equal to the concentration of boron in the base region.

5. The crystalline silicon photovoltaic cell according to claim 1, characterized in that, The first lithium-silicon photovoltaic cell is a bottom cell in the form of a tandem photovoltaic cell (HJT).

6. A crystalline silicon wafer, characterized in that, A first lithium-silicon wafer is formed by doping a p-type combined dopant of lithium impurity and boron impurity into the crystalline silicon wafer. The concentration N of the lithium impurity element in the first lithium-silicon wafer is... Li Satisfy: 1×10 12 / cm 3 <N Li ≤1×10 15 / cm 3 or 1×10 15 / cm 3 <N Li <1×10 16 / cm 3 And the concentration of impurity boron N B With the concentration of lithium impurity N Li Satisfy: 1×10 15 / cm 3 ≤(N B -N Li ≤5×10 17 / cm 3 .

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