Back contact cells and methods of making the same

Abstract

The application discloses a back contact cell and a preparation method thereof. After a silicon wafer is subjected to laser patterning once, an alkaline etching solution containing a corrosion inhibitor is introduced to perform alkaline washing, wherein the corrosion inhibitor comprises ammonia compounds and organic amine compounds. The synergistic effect of the ammonia compounds and the organic amine compounds can form a dynamic adsorption layer on specific crystal surfaces or exposed areas of the silicon surface, so as to effectively inhibit the lateral diffusion of the etching solution to the pattern sidewall and the bottom. Therefore, the etching anisotropy is actively regulated from the etching chemical essence level without significantly sacrificing the etching efficiency or increasing the process complexity, the undercut phenomenon is fundamentally relieved, and the photoelectric conversion efficiency, batch consistency and industrial yield of the silicon-based devices such as the back contact solar cell are improved.

Description

Technical Field

[0001] This application relates to the field of solar energy, and more specifically, to a back contact battery and a method for its preparation. Background Technology

[0002] In the manufacturing process of silicon-based devices such as semiconductor chips and photovoltaic cells, patterning etching is a key process for constructing the core structure of the device. Its purpose is to selectively remove specific areas of silicon-based material using an etching solution to form predetermined lines, trenches, or channel structures. Alkaline etching solutions are widely used in this type of process due to their advantages such as good etching selectivity for silicon (especially polycrystalline silicon and monocrystalline silicon) and minimal damage to dielectric layers such as silicon dioxide and silicon nitride.

[0003] However, existing alkaline etching solutions (such as KOH and TMAH systems) commonly suffer from undercut during patterning etching. Undercut refers to the etchant spreading laterally and etching the silicon-based material beneath the pattern while vertically etching the target area, resulting in a "hollowing out" phenomenon at the bottom of the pattern and destroying the intended pattern morphology and dimensional accuracy. Undercut leads to a series of serious consequences: first, it causes deviations in critical device dimensions, affecting the device's electrical performance (such as a decrease in short-circuit current and fill factor in photovoltaic cells); second, it weakens the mechanical strength of the pattern structure, increasing the risk of pattern collapse and breakage in subsequent processes; and third, it reduces device consistency and yield, increasing the cost of industrial production.

[0004] To address the undercutting issue, current methods primarily involve adjusting process parameters and optimizing the process, such as changing laser power and wet alkaline washing time. Specific process optimizations are tailored to each step to suppress undercutting. However, these methods are essentially "passive suppression," often at the cost of sacrificing etching rate, increasing process complexity, or reducing overall production capacity, and they are unlikely to fundamentally reduce the undercutting phenomenon. Summary of the Invention

[0005] This application provides a back-contact solar cell and its fabrication method. Without significantly sacrificing etching efficiency or increasing process complexity, it actively controls etching anisotropy from the essential level of etching chemistry, fundamentally alleviating the undercut phenomenon, thereby improving the photoelectric conversion efficiency, batch consistency and industrial yield of silicon-based devices such as back-contact solar cells.

[0006] The embodiments of this application are implemented as follows: In a first aspect, the present invention provides a method for preparing a back contact battery, comprising: Texturing of silicon wafers; The back of the silicon wafer is etched and alkaline polished. A polycrystalline silicon thin film or an amorphous silicon thin film is formed on the back of the silicon wafer; Boron diffusion and annealing are performed on the silicon wafer; A single laser patterning is performed on the silicon wafer; The silicon wafer is subjected to alkaline etching using an alkaline etching solution containing an inhibitor, wherein the inhibitor includes ammonia compounds and organic amine compounds.

[0007] In an optional embodiment, the ammonia compound is ammonia water or methylamine.

[0008] In an optional embodiment, the organic amine compound is triethanolamine, diethanolamine, or ethanolamine.

[0009] In an optional embodiment, the ammonia compound is ammonia water, and the organic amine compound is triethanolamine.

[0010] In an optional embodiment, the ratio of the organic amine compound to the ammonia compound is 1:1 to 1:2.

[0011] In an optional embodiment, the ratio of the organic amine compound to the ammonia compound is 1:1.5.

[0012] In an optional embodiment, the corrosion inhibitor is added to the alkaline etching solution at a ratio of 0.02-0.5%.

[0013] In an optional embodiment, the corrosion inhibitor is added to the alkaline etching solution at a ratio of 0.1-0.3%.

[0014] In an optional embodiment, after the step of alkaline washing of the silicon wafer with an alkaline etching solution containing an inhibitor, the preparation method further includes: A tunneling oxide thin film is formed on the back side of the silicon wafer; A polycrystalline silicon thin film or an amorphous silicon thin film is formed on the back of the silicon wafer; Phosphorus diffusion and annealing are performed on silicon wafers; Laser secondary patterning of silicon wafers; Acid etching is performed on the silicon wafer; Texturing of silicon wafers; Apply antireflective coatings to both the front and back sides of the silicon wafer; Gate lines are printed on the back of the silicon wafer.

[0015] Secondly, the present invention provides a back contact battery, which is manufactured by the back contact battery preparation method described in any of the foregoing embodiments.

[0016] The advantages of this application compared to the prior art include: After a single laser patterning is completed on a silicon wafer, the present invention introduces an alkaline etching solution containing an etching inhibitor for alkaline washing. The etching inhibitor contains ammonia compounds and organic amine compounds. The two work synergistically to form a dynamic adsorption layer on a specific crystal plane or exposed area of ​​the silicon surface, effectively inhibiting the lateral diffusion of the etching solution to the sidewalls and bottom of the pattern. Specifically, because the amino groups (–NH2) in ammonia compounds can form weak hydrogen bonds with the Si–OH groups on the sidewall surface of polycrystalline silicon, they can be adsorbed during the initial etching stage. - The lateral diffusion path of ions creates a preliminary physical barrier, thereby achieving a slight inhibition of undercutting; at the same time, organic amine compounds, due to the presence of both hydroxyl (–OH) and amino (–NH2 / –NR2) in their molecular structure, can not only enhance their adsorption stability on the silicon surface through multi-point hydrogen bonding, but also coordinate with unsaturated bonds on the silicon surface through lone pair electrons, forming a more dense, continuous and chemically stable composite protective film. Therefore, when ammonia compounds and organic amine compounds are introduced into alkaline etching solution as composite etching inhibitors, they form a hierarchical synergistic adsorption structure on the sidewall of polycrystalline silicon. Ammonia compounds provide rapid initial coverage, while organic amines build a reinforced barrier on this basis, thereby significantly improving the sidewall passivation effect and greatly reducing the lateral etching rate. Therefore, the composite corrosion inhibition system exhibits a nonlinear enhancement in its ability to suppress undercutting (i.e., the "1+1>2" effect), resulting in a clear bottom outline and steep edges in the pattern. Furthermore, this composite film maintains moderate permeability in the vertical etching direction, allowing OH- - The ions react continuously in the vertical direction, thus effectively suppressing undercutting while maintaining a reasonable overall etching rate, and taking into account the high selectivity between polysilicon and masking dielectric layers (such as silicon oxide and silicon nitride).

[0017] Therefore, compared to traditional alkaline etching processes, this method can significantly reduce the undercut effect while maintaining a high vertical etching rate, thereby accurately preserving the graphic contours and key dimensions defined by laser patterning; Furthermore, since the undercut is effectively suppressed, the subsequent back contact structure has higher morphological fidelity and interface integrity, which not only improves the carrier collection efficiency and electrical contact performance, but also enhances the mechanical stability of the pattern structure and reduces the risk of pattern collapse or breakage in subsequent high-temperature or cleaning processes. Ultimately, this preparation method, without significantly sacrificing etching efficiency or increasing process complexity, actively controls etching anisotropy at the level of etching chemistry, fundamentally alleviating the undercut phenomenon, thereby improving the photoelectric conversion efficiency, batch consistency, and industrial yield of silicon-based devices such as back-contact solar cells. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, 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 this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 SEM image of the back contact battery prepared according to Example 1; Figure 2 SEM image of the back contact battery prepared according to Comparative Example 1; Figure 3 SEM image of the back contact battery prepared according to Comparative Example 2. Detailed Implementation

[0020] As used in this article: "Prepared from" is synonymous with "comprising". The terms "comprising", "including", "having", "containing", or any other variations thereof as used herein are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0021] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1–5” is disclosed, the described range should be interpreted as including ranges “1–4”, “1–3”, “1–2”, “1–2 and 4–5”, “1–3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0022] This application discloses a method for preparing a back contact battery, which includes the following steps: S1, texturing the silicon wafer; S2, etching and alkaline polishing of the back side of the silicon wafer; S3, forming a polycrystalline silicon thin film or an amorphous silicon thin film on the back of the silicon wafer; S4, Boron diffusion and annealing of silicon wafers; S5, laser patterning of the silicon wafer; S6, the silicon wafer is subjected to alkaline etching solution containing an inhibitor for alkaline washing, wherein the inhibitor includes ammonia compounds and organic amine compounds.

[0023] Thus, after the silicon wafer has been laser patterned once, the present invention introduces an alkaline etching solution containing an etching inhibitor for alkaline washing. The etching inhibitor contains ammonia compounds and organic amine compounds. The two work together to form a dynamic adsorption layer on a specific crystal plane or exposed area of ​​the silicon surface, effectively inhibiting the lateral diffusion of the etching solution to the sidewalls and bottom of the pattern. Specifically, because the amino groups (–NH2) in ammonia compounds can form weak hydrogen bonds with the Si–OH groups on the sidewall surface of polycrystalline silicon, they can be adsorbed during the initial etching stage. - The lateral diffusion path of ions creates a preliminary physical barrier, thereby achieving a slight inhibition of undercutting; at the same time, organic amine compounds, due to the presence of both hydroxyl (–OH) and amino (–NH2 / –NR2) in their molecular structure, can not only enhance their adsorption stability on the silicon surface through multi-point hydrogen bonding, but also coordinate with unsaturated bonds on the silicon surface through lone pair electrons, forming a more dense, continuous and chemically stable composite protective film. Therefore, when ammonia compounds and organic amine compounds are introduced into alkaline etching solution as composite etching inhibitors, they form a hierarchical synergistic adsorption structure on the sidewall of polycrystalline silicon. Ammonia compounds provide rapid initial coverage, while organic amines build a reinforced barrier on this basis, thereby significantly improving the sidewall passivation effect and greatly reducing the lateral etching rate. Therefore, the composite corrosion inhibition system exhibits a nonlinear enhancement in its ability to suppress undercutting (i.e., the "1+1>2" effect), resulting in a clear bottom outline and steep edges in the pattern. Furthermore, this composite film maintains moderate permeability in the vertical etching direction, allowing OH- - The ions react continuously in the vertical direction, thus effectively suppressing undercutting while maintaining a reasonable overall etching rate, and taking into account the high selectivity between polysilicon and masking dielectric layers (such as silicon oxide and silicon nitride).

[0024] Therefore, compared to traditional alkaline etching processes, this method can significantly reduce the undercut effect while maintaining a high vertical etching rate, thereby accurately preserving the graphic contours and key dimensions defined by laser patterning; Furthermore, since the undercut is effectively suppressed, the subsequent back contact structure has higher morphological fidelity and interface integrity, which not only improves the carrier collection efficiency and electrical contact performance, but also enhances the mechanical stability of the pattern structure and reduces the risk of pattern collapse or breakage in subsequent high-temperature or cleaning processes. Ultimately, this preparation method, without significantly sacrificing etching efficiency or increasing process complexity, actively controls etching anisotropy at the level of etching chemistry, fundamentally alleviating the undercut phenomenon, thereby improving the photoelectric conversion efficiency, batch consistency, and industrial yield of silicon-based devices such as back-contact solar cells.

[0025] Optionally, the ammonia compound is ammonia water (NH3·H2O) or methylamine (CH3NH2), both of which contain active amino groups (–NH2) and can be effectively dissociated and adsorbed on the surface of the polycrystalline silicon sidewall in an alkaline etching environment.

[0026] Specifically, ammonia, as a weak base, not only provides –NH2 groups for forming hydrogen bonds with Si–OH, but also synergistically regulates the overall pH of the etching solution, preventing localized over-etching. Methylamine, as a small organic molecule ammonia compound, has a base level close to that of ammonia, but its amino group is more reactive, resulting in a more significant adsorption force on the silicon surface and a more stable initial adsorption layer formed on the silicon sidewalls. Its ionization produces CH3NH3. + It can form a dense adsorption film with TEA (or other organic amines) through electrostatic interaction, which strengthens the lateral etching barrier; its organic groups can reduce the volatility of the solution and improve the stability of the etching solution.

[0027] Therefore, regardless of whether ammonia or methylamine is chosen as the component of the ammonia compound, it can synergistically construct a highly efficient composite corrosion inhibition interface with organic amine compounds, further enhancing the inhibition effect on undercutting, improving the controllability and repeatability of the etching process, and meeting the process compatibility requirements of different production lines.

[0028] It should be noted that compared to ammonia, methylamine aqueous solution has a evaporation rate that is more than 40% lower, and the etching solution components have better stability, making it suitable for long-term continuous production scenarios; however, its cost is higher than that of ammonia, and the concentration used must be controlled to avoid excessive etching of the mask.

[0029] Optionally, the organic amine compound is triethanolamine (TEA), diethanolamine (DEA), or ethanolamine (MEA). These three compounds all contain one or more hydroxyl groups (–OH) and a secondary / primary amino group (–NH2 or –NHR), possessing both hydrophilicity and strong coordination ability, enabling them to effectively anchor onto the polycrystalline silicon surface in an alkaline etching environment.

[0030] Specifically: Ethanolamine has a simple molecular structure, containing one –OH and one –NH2 group. It is slightly more basic than TEA and DEA, diffuses rapidly, and can quickly regulate the interfacial tension of solutions. Ethanolamine can quickly adsorb onto the silicon sidewalls to form an initial protective film, which, combined with ammonia, enhances the inhibition of lateral corrosion. Its strong basicity can help improve the etching rate, balancing the slow etching issue when MEA is used alone. It is suitable for processes that require a balance between etching efficiency and basic undercut control. It can be compounded with TEA in a specific ratio to optimize cost and performance.

[0031] Diethanolamine contains two –OH groups and one –NH group, enhancing its ability to form multi-site hydrogen bonds, resulting in a more stable adsorption layer and improved corrosion inhibition durability. It combines complexing ability with surface activity, exhibiting a structure similar to TEA but with a shorter molecular chain, thus offering greater adsorption flexibility. Diethanolamine can form hydrogen bonds with Si-OH groups on silicon surfaces and simultaneously synergistically form a composite adsorption layer with ammonia to block OH groups. - Lateral penetration; its complexing ability can promote the detachment of etched products, improve etching uniformity, and is suitable for medium and low precision pattern etching. Its cost is lower than that of TEA, and it can replace part of TEA in processes with slightly less stringent requirements for undercut suppression.

[0032] Triethanolamine, possessing three –OH groups and one tertiary amino group, can construct a more dense and sterically hindered three-dimensional adsorption network on the silicon surface, thus enhancing its ability to adsorb OH groups. - The lateral penetration pathways of ions form stronger physical and chemical barriers.

[0033] Therefore, any one of triethanolamine, diethanolamine, or ethanolamine can be selected as the organic amine component, which can synergistically work with ammonia compounds (such as ammonia water or methylamine) to construct a gradient, high-coverage composite protective film in situ on the sidewalls of polycrystalline silicon patterns. This film effectively suppresses undercutting while still allowing controllable etching in the vertical direction, thereby achieving excellent morphology fidelity and etching uniformity in the fabrication of precision back contact structures, and adapting to the differentiated requirements of different production lines for etching rate, selectivity, and pattern accuracy.

[0034] Optionally, the ratio of the organic amine compound to the ammonia compound is 1:1 to 1:2 (molar ratio or volume ratio, depending on the specific preparation method). The addition ratio of the corrosion inhibitor to the alkaline etching solution is 0.02-0.5% (volume fraction or mass fraction, preferably volume fraction).

[0035] Preferably, the ammonia compound is ammonia water, the organic amine compound is triethanolamine, the ratio of the organic amine compound (triethanolamine) to the ammonia compound (ammonia water) is 1:1.5, and the addition ratio of the corrosion inhibitor in the alkaline etching solution is 0.1-0.3%.

[0036] Ammonia and triethanolamine (TEA) are used as composite additives to synergistically etch polycrystalline silicon, combining the advantages of both to achieve a "1+1>2" effect. The core advantages are reflected in three aspects: enhanced undercut suppression, improved etching uniformity, and a balance between etching rate and selectivity, making it particularly suitable for precision pattern etching requirements. I. The undercut inhibition effect is significantly better than that of a single additive: The role of ammonia: Its amino group (-NH2) can form weak hydrogen bonds with the Si-OH on the sidewalls of polycrystalline silicon, thus initially blocking the OH groups in the etching solution. - Lateral osmosis of ions slightly inhibits undercutting.

[0037] TEA Enhancement: TEA molecules contain multiple hydroxyl (-OH) and amino groups, resulting in stronger bonding with the sidewalls of polycrystalline silicon (forming multi-point hydrogen bonds and coordination interactions). This allows for the formation of a denser composite protective film on top of the ammonia adsorption layer, further blocking lateral corrosion paths.

[0038] Interfacial adsorption synergy blocks lateral etching: ammonium ions (NH4+) generated by ammonia ionization + This composite adsorption film can combine with TEA molecules through electrostatic interactions, further enhancing the density and stability of the adsorption film. The adsorption capacity of this composite film is stronger on the lateral surfaces of silicon (i.e., the sides and bottom of the pattern), effectively blocking hydroxide ions (OH-) in the etching solution. - The adsorption film contacts the lateral silicon atoms, thereby suppressing lateral etching and reducing undercutting. On the longitudinal surface of silicon (the etching target area), the adsorption strength of the adsorption film is weak, which does not affect the normal progress of longitudinal etching, thus achieving an anisotropic etching effect of "fast longitudinal etching and slow lateral etching".

[0039] Coordinated control of etching kinetics optimizes reaction direction: ammonia slowly releases OH- through weak ionization. - It can buffer OH in the solution - A concentration gradient is used to prevent the etching solution from forming OH groups at the bottom of the pattern. - Aggregation leads to a surge in lateral etching rate; TEA can form stable complexes with etching products (such as Si(OH)4), promoting rapid detachment of products from the etched surface, reducing adsorption and accumulation of products at the bottom of the pattern, and further reducing the kinetics of lateral etching. The two work synergistically to regulate the kinetics of the etching reaction, ensuring that the etching reaction mainly proceeds along the vertical direction, thereby reducing undercutting at its source.

[0040] Synergistic optimization of wettability reduces etchant penetration: TEA has good surface activity, which can reduce the surface tension of the etchant solution and improve the wettability of the etchant solution on the photoresist mask surface, preventing the etchant solution from penetrating below the interface between the photoresist and the silicon substrate through capillary action; ammonia can adjust the interfacial tension between the photoresist and the etchant solution, further enhancing the barrier performance of the mask, reducing the penetration of the etchant solution to the bottom of the pattern, thereby inhibiting undercut etching at the interface.

[0041] II. Significantly improved etching uniformity and stability Suppressing concentration fluctuations: Ammonia is volatile, causing the concentration of a single system to change over time. TEA has high viscosity and strong chemical stability, which can slow down the volatilization of ammonia and make the etching solution components more stable.

[0042] Improved mass transfer uniformity: TEA can adjust the solution viscosity and reduce the "solution retention" phenomenon on the polysilicon surface (especially at pattern corners and the bottom of deep trenches). Combined with the weak alkaline diffusion characteristics of ammonia, it reduces the difference in etching rates between wafers and wafers.

[0043] III. Etching rate and mask selectivity are easier to balance Rate controllability: TEA alone significantly reduces the etching rate, while ammonia alone results in an excessively fast and difficult-to-control etching rate; in the composite system, ammonia provides the main etching driving force (OH). - (Ions), TEA regulates the reaction rhythm, and the rate can be stabilized at 2-3 μm / h, balancing efficiency and precision.

[0044] Enhanced mask protection: Polysilicon etching commonly uses SiO2 or photoresist as a mask. Ammonia has a high etching rate for SiO2 (about 20 Å / min), while TEA can react with Si-OH on the SiO2 surface to form a protective film, reducing the SiO2 etching rate to 5-8 Å / min. The etching selectivity of polysilicon to SiO2 is improved from 15:1 to more than 30:1, reducing excessive mask consumption.

[0045] IV. Higher process tolerance for adapting to complex scenarios For grain boundaries and defect areas of polycrystalline silicon (where local over-etching is prone to occur), the strong adsorption properties of TEA can preferentially cover these highly active sites. Combined with the uniform corrosion properties of ammonia, it can reduce defects such as "pit erosion" and "local protrusion". It is especially suitable for etching heterogeneous materials such as doped polycrystalline silicon and polycrystalline thin films.

[0046] In summary, the synergistic system of ammonia and TEA, through "complementary strong and weak adsorption + balance of rate and stability", effectively improves the shortcomings of single additives in undercut control, uniformity or rate, and is the preferred solution for precision etching of polycrystalline silicon.

[0047] Optionally, after the step of alkaline washing of the silicon wafer with an alkaline etching solution containing a corrosion inhibitor, the preparation method further includes: S7, a tunneling oxide thin film is formed on the back side of the silicon wafer; S8, forming a polycrystalline silicon thin film or an amorphous silicon thin film on the back side of the silicon wafer; S9, phosphorus diffusion and annealing of silicon wafers; S10, laser secondary patterning of silicon wafers; S11, acid etching of silicon wafer; S12, texturing the silicon wafer; S13, applying anti-reflective coatings to the front and back of the silicon wafer; S14, grid line printing is performed on the back side of the silicon wafer.

[0048] Specifically, in step S1, the silicon wafer is texturized using a mixed solution of alkali and texturing additive. The alkali can be KOH or NaOH. After cleaning and texturing, the textured surface of the silicon wafer has a texture size of 0.5 μm to 4.0 μm.

[0049] In step S2, the KOH solution concentration is 1-5 wt%, the reaction temperature is 60-90℃, and the reaction time is 10-30 min.

[0050] In step S3, a polycrystalline silicon thin film or an amorphous silicon thin film can be formed on the back side of the silicon wafer using any applicable vacuum technique. For example, thermal oxidation, PECVD, low-pressure chemical vapor deposition (LPCVD), high-temperature diffusion, magnetron sputtering, and physical vapor deposition (PVD) techniques can be used.

[0051] The annealing in steps S4 and S9 involves crystallizing the silicon wafer using a high-temperature annealing furnace. The annealing method can be rapid thermal annealing or tubular high-temperature annealing. The annealing atmosphere is nitrogen or argon, the annealing temperature is 700-1080℃, and the annealing time is 10-90 minutes.

[0052] For example, N2 is introduced at a temperature of 700℃-900℃ and annealed for 400s-7000s; the flow rate of N2 can be 2000-80000 sccm.

[0053] Step S5 involves patterning. Patterning processes include screen printing, laser processing, and photolithography. For example, laser etching can be used for patterning, with laser power ranging from 200-1000W and laser spot size from 30μm x 30μm to 300μm x 300μm. In photolithography, a dry photoresist film can be used as a mask, and infrared light can be used as the exposure light source.

[0054] In step S6, KOH is used as the main etching solution, the alkaline washing solution is 1.0wt%-2.5wt%, the reaction time is 600s-1200s, and the reaction temperature is 65-85℃.

[0055] In step S7, oxygen and silane are used as process gases, with a silane to oxygen flow ratio of 1:10 to 1:20, an RF power density of 0.5-20 mW / cm², a pressure of 0.2-50 mbar, a temperature of 150℃-500℃, and a time of 1-10 min. The thickness of the formed tunneling oxide film is 1-2 nm.

[0056] In step S8, an n-amorphous silicon thin film is formed on the back side of the silicon wafer using LPCVD. The amorphous silicon thin film can be a single layer or a multilayer film, with a thickness of 10-200 nm.

[0057] In step S10, the patterning method is laser removal, with a laser wavelength of 556 nm, a frequency of 3000-54000 Hz, and an energy density of 20-100 mW / cm². 2 .

[0058] In step S11, the HF concentration is 3-6 wt%, the ratio of HNO3 to HF solution is 6-8:1, and acid washing is performed at a concentration of 1-2 wt% HCl.

[0059] In step S12, KOH is used as the main etching solution, the alkaline washing solution is 1.0wt%-2.0wt%, the alkaline washing additive is 0.1-0.2wt%, the reaction time is 600s-1000s, and the reaction temperature is 65-85℃.

[0060] In step S13, silane, nitrogen, and ammonia are used as process gases. The volume ratio of silane to ammonia is 1:3 to 1:10, and the volume ratio of nitrogen to silane is 1:1 to 10:1. The temperature is 150℃-500℃, the time is 10-60 min, the pressure is 0.2-50 mbar, and the RF power density is 0.5-20 mW / cm³. 2 The thickness of the formed silicon nitride passivation film is 50-140 nm, and the refractive index of silicon nitride is 2.00-2.10.

[0061] In step S14, metal electrodes are formed by screen printing. The sintering temperature of the front side is 150-300℃, and the time is 20-30 min; the sintering temperature of the back side is 180-250℃, and the time is 20-30 min.

[0062] This application also discloses a back contact battery, which is made by the above-described method for preparing a back contact battery.

[0063] The embodiments of this application will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of this application. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0064] In these embodiments, unless otherwise specified, the parts and amounts are all by weight.

[0065] Example 1 This embodiment discloses a method for preparing a back contact battery, which includes the following steps: Silicon wafers were texturized using a mixed solution of KOH and texturing additives. After cleaning and texturing, the textured surface size of the silicon wafers ranged from 1.5 to 3.0 μm. Then, the silicon wafers were alkaline polished at 75°C with a 6 wt% KOH solution for 18 minutes.

[0066] Silane gas deposition was used at a temperature of 300°C for 20 minutes; the process pressure was 10 mbar; and the RF power density was 300 mW / cm². 2 A polycrystalline silicon thin film with a thickness of 20 nm is formed.

[0067] The silicon wafer is subjected to boron diffusion and annealing. N2 is introduced at 900°C with a flow rate of 20,000 sccm, and the annealing is carried out for 2200 seconds.

[0068] The silicon wafer is patterned using laser etching with a power of 500W and a spot size of 100μm x 100μm.

[0069] KOH was used as the main etching solution, with 1 wt% alkaline washing solution, 0.2% ammonia, and 0.3% TEA in a ratio of 1:1.5. The reaction time was 750 s and the reaction temperature was 75 °C.

[0070] A tunneling oxide film is formed on the back side of the silicon wafer using HDPCVD, employing oxygen and silane as process gases. The silane to oxygen flow ratio is 1:15, and the RF power density is 10 mW / cm². 2 The pressure was 15 mbar, the temperature was 300℃, and the time was 5 min; the thickness of the tunneling oxide film formed was 1.3 nm.

[0071] Silane gas deposition was used at a temperature of 100°C for 15 minutes; the process pressure was 10 mbar; and the RF power density was 250 mW / cm². 2 A 50nm thick amorphous silicon thin film is formed.

[0072] The silicon wafer underwent annealing and secondary patterning; the annealing atmosphere was nitrogen, the annealing temperature was 850℃, and the annealing time was 60 min; patterning was performed using laser stripping with a wavelength of 556 nm, a frequency of 30000 Hz, and an energy density of 40 mW / cm². 2 .

[0073] Silicon nitride passivation films were deposited on both the front and back sides of the silicon wafer using HDPCVD. Silane, nitrogen, and ammonia were used as process gases, with a silane to ammonia volume ratio of 1:6 and a nitrogen to silane volume ratio of 5:1. The temperature was 200℃, the time was 30 min, the pressure was 20 mbar, and the RF power density was 10 mW / cm². 2 The resulting silicon nitride film has a thickness of 80 nm and a refractive index of 2.05.

[0074] Electrodes are formed on the front and back sides of a silicon wafer, and metal electrodes are formed by screen printing. The front side is sintered at 150°C for 20 minutes, and the back side is sintered at 180°C for 30 minutes.

[0075] SEM image of the back contact battery fabricated according to this embodiment is shown below. Figure 1 As shown in the figure, the white border represents the undercut width. The undercut widths of the three examples are 1.3μm, 1.7μm and 2.2μm, respectively, and the average undercut width is approximately 1.8μm.

[0076] Example 2 Unlike Example 1, in this example, the total addition ratio of TEA and ammonia in the alkaline etching solution is 0.05%.

[0077] Example 3 Unlike Example 1, in this example, the total addition ratio of TEA and ammonia in the alkaline etching solution is 0.10%.

[0078] Example 4 Unlike Example 1, in this example, the total addition ratio of TEA and ammonia in the alkaline etching solution is 0.30%.

[0079] Example 5 Unlike Example 1, in this example, DEA is used to replace TEA, methylamine is used to replace ammonia, and the ratio of DEA to methylamine is 1:1.2. The total addition ratio of DEA and methylamine in the alkaline etching solution is 0.2%.

[0080] Example 6 Unlike Example 1, in this example, MEA is used instead of TEA, and the ratio of MEA to ammonia is 1:1.8. The total addition ratio of MEA and ammonia in the alkaline etching solution is 0.25%.

[0081] Example 7 Unlike Example 1, in this example, the ratio of TEA to ammonia is 1:1, and the total addition ratio of TEA and ammonia in the alkaline etching solution is 0.45%.

[0082] Example 8 Unlike Example 7, in this example, the ratio of TEA to ammonia is 1:1.2.

[0083] Example 9 Unlike Example 7, in this example, the ratio of TEA to ammonia is 1:1.5.

[0084] Example 10 Unlike Example 7, in this example, the ratio of TEA to ammonia is 1:1.8.

[0085] Example 11 Unlike Example 7, in this example, the ratio of TEA to ammonia is 1:2.

[0086] Example 12 Unlike Example 7, in this example, the ratio of TEA to ammonia is 1:0.9.

[0087] Example 13 Unlike Example 7, in this example, the ratio of TEA to ammonia is 1:2.2.

[0088] Example 14 Unlike Example 9, in this example, the total addition ratio of TEA and ammonia in the alkaline etching solution is 0.01%.

[0089] Example 15 Unlike Example 9, in this example, the total addition ratio of TEA and ammonia in the alkaline etching solution is 0.8%.

[0090] Comparative Example 1 Unlike Example 1, only 0.3% TEA was added instead of ammonia.

[0091] The SEM image of the back contact battery made according to this comparative example is shown below. Figure 2 As shown in the figure, the white border represents the undercut width. The undercut widths of the three examples are 1.7μm, 2.1μm and 2.6μm, respectively, and the average undercut width is approximately 2.2μm.

[0092] Comparative Example 2 Unlike Example 1, only 0.2% ammonia was added instead of TEA.

[0093] The SEM image of the back contact battery made according to this comparative example is shown below. Figure 3 As shown in the figure, the white border represents the undercut width. The undercut widths of the three examples are 2.1μm, 2.9μm and 3.4μm, respectively, and the average undercut width is approximately 2.7μm.

[0094] Comparative Example 3 Unlike Example 9, DEA was used instead of ammonia.

[0095] Comparative Example 4 Unlike Example 9, methylamine was used instead of TEA.

[0096] The performance of the back contact batteries prepared according to the above embodiments and comparative examples was tested, and the results are shown in Table 1 below: Table 1

[0097] It can be seen from the above table: I. The combined use of corrosion inhibitors is significantly superior to that of single components: The undercut widths of Comparative Example 1 (TEA 0.3% only) and Comparative Example 2 (ammonia 0.2% only) were 2.2 μm and 2.7 μm, respectively, which were significantly higher than those of Example 1 (ammonia 0.2% + TEA 0.3%, undercut 1.8 μm).

[0098] This indicates that the synergistic effect of ammonia and TEA can effectively inhibit undercutting, while single additives are difficult to form an efficient sidewall protective layer due to insufficient adsorption capacity or coverage integrity.

[0099] II. There is an optimal window for the total amount of corrosion inhibitor added (0.1–0.3%): In Example 2 (0.05%), the undercut still reached 1.8 μm, indicating limited inhibition effect; In Example 3 (0.10%), the undercut thickness was reduced to 1.3 μm, and in Example 4 (0.30%), it was further reduced to 1.0 μm. However, the undercut thickness in Example 14 (0.01%) deteriorated to 2.7 μm, and the undercut thickness in Example 15 (0.8%) rebounded to 2.3 μm, with a decrease in efficiency.

[0100] Note: If the concentration is too low, a continuous film cannot be formed; if it is too high, it may cause an increase in solution viscosity, bubble adhesion, or excessive passivation, which will weaken the etching uniformity.

[0101] When the total proportion is in the range of 0.05%-0.3%, the undercut width gradually decreases as the proportion increases, and the battery electrical performance (Voc, FF, efficiency) continues to be optimized. When the proportion exceeds 0.3%, the rate of decrease in undercut width slows down, while the etching rate decreases slightly (e.g., Jsc decreases slightly in Supplementary Example 4 compared to Supplementary Example 3). Therefore, the optimal proportion range for minimizing undercut and ensuring process stability is 0.1%-0.3%.

[0102] III. The optimal performance is achieved when the ratio of ammonia water to TEA is 1:1.5. The total addition amount was fixed at 0.45% (Examples 7–13): Example 9 (1:1.5) achieved an undercut of 0.8 μm and an efficiency of 24.23%, which was the best among all examples. Deviations from this ratio (e.g., Example 81: 1.2 → 1.2 μm; Example 101: 1.8 → 1.4 μm; Example 111: 2 → 2.0 μm; Example 121: 0.9 → 1.7 μm) all resulted in increased undercut. Example 13 (1:2.2) showed an undercut of up to 2.2 μm, indicating that an excess of organic amines could disrupt the adsorption equilibrium.

[0103] Therefore, when the ratio of ammonia to TEA is 1:1.5, the overall performance is optimal: the undercut width is the smallest (1.0μm) and the battery efficiency is high. This is because the "preliminary adsorption" of ammonia and the "enhanced protective film" of TEA form the best synergy at this ratio, completely blocking the transverse corrosion path and forming a composite corrosion-inhibiting interface with high coverage and high stability.

[0104] When the ratio of ammonia to TEA deviates from 1:1.5, the performance gradually decreases: when the ratio is lower than 1:1.5 (such as 1:1 or 1:1.2), the TEA content is insufficient, the composite adsorption film is not dense enough, and the undercut suppression effect is weakened; when the ratio is higher than 1:1.5 (such as 1:1.8 or 1:2), the excess TEA leads to an increase in the viscosity of the etching solution, a slight decrease in the longitudinal etching rate, and a slight decrease in Isc and FF.

[0105] IV. Different combinations of ammonia compounds / organic amines are substitutable, but their performance is slightly inferior to ammonia water + TEA: Example 5 (methylamine + DEA, 0.2%) undercut 1.5 μm, efficiency 23.92%; Example 6 (ammonia + MEA, 0.25%): Undercut 1.4 μm, efficiency 23.99%; Although superior to the comparative example, it is still inferior to Example 9 (0.8 μm, 24.23%).

[0106] Note: TEA, due to its denser three-dimensional network formed by its trihydroxy structure, exhibits superior corrosion inhibition performance compared to DEA or MEA. Ammonia, on the other hand, demonstrates better overall performance than methylamine due to its low cost, moderate volatility, and good pH buffering capacity. While the combination system of alternative organic amines (DEA, MEA) and ammonia (methylamine) is slightly inferior to the TEA-ammonia system, it can still control the undercut width within 1.5 μm, meeting the requirements.

[0107] VI. Confirmation of the Optimal Solution: Based on all the data, Example 9 (ammonia:TEA = 1:1.5, total addition 0.3%) shows that: Minimal undercut (0.8μm), highest conversion efficiency (24.23%), excellent Voc (734.1mV) and FF (79.82%).

[0108] Therefore, the alkaline etching solution system with ammonia and triethanolamine in a 1:1.5 ratio and a total added concentration of 0.3% is the optimal technical solution in the back contact battery preparation method of this invention, which fully verifies the effectiveness and industrial feasibility of the "composite corrosion inhibitor synergistic suppression of undercut" strategy.

[0109] In summary, this series of examples, through systematic control of the type, ratio, and concentration of corrosion inhibitors, confirms that: 1. Significant undercut suppression effect: Through the synergistic effect of ammonia and TEA, the undercut during the etching process can be reduced by more than 60%. Experimental verification shows that when using the etching solution of this invention to etch polycrystalline silicon patterns, the minimum undercut width can be controlled at 0.8 μm, which is far superior to existing single-additive etching solutions, significantly improving the pattern dimensional accuracy and structural integrity.

[0110] 2. Balanced etching performance: While significantly reducing undercut, the etching solution of the present invention still maintains a high etching rate (50-180nm / min) and the etching uniformity error is ≤2.5%, avoiding the problem of "sacrificing etching efficiency to suppress undercut".

[0111] 3. Good solution stability: The synergistic effect of ammonia and TEA can inhibit the decomposition and volatilization of the main etching agent, extending the service life of the etching solution (more than 30% longer than existing solutions); and the solution has mild corrosiveness, high operational safety, and low cost and difficulty in waste liquid treatment.

[0112] The above description is merely a specific embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method of fabricating a back contact cell, characterized by, include: Texturing of silicon wafers; The back of the silicon wafer is etched and alkaline polished. A polycrystalline silicon thin film or an amorphous silicon thin film is formed on the back of the silicon wafer; Boron diffusion and annealing are performed on the silicon wafer; A single laser patterning is performed on the silicon wafer; The silicon wafer is subjected to alkaline etching using an alkaline etching solution containing an inhibitor, wherein the inhibitor includes ammonia compounds and organic amine compounds.

2. The method of claim 1, wherein, The ammonia compound is ammonia water or methylamine.

3. The method of claim 1, wherein the back contact cell is prepared by a process comprising: The organic amine compound is triethanolamine, diethanolamine, or ethanolamine.

4. The method of producing a back contact cell according to claim 2 or 3, characterized in that, The ammonia compound is ammonia water, and the organic amine compound is triethanolamine.

5. The method of claim 1, wherein, The ratio of the organic amine compound to the ammonia compound is 1:1 to 1:

2.

6. The method of claim 5, wherein the back contact cell is prepared by, The ratio of the organic amine compound to the ammonia compound is 1:1.

5.

7. The method of claim 1, wherein the back contact cell is prepared by a method comprising: The corrosion inhibitor is added to the alkaline etching solution at a ratio of 0.02-0.5%.

8. The method of claim 7, wherein the back contact cell is prepared by, The corrosion inhibitor is added to the alkaline etching solution at a ratio of 0.1-0.3%.

9. The method of claim 1, wherein, After the step of alkaline washing of the silicon wafer with an alkaline etching solution containing a corrosion inhibitor, the preparation method further includes: A tunneling oxide thin film is formed on the back side of the silicon wafer; A polycrystalline silicon thin film or an amorphous silicon thin film is formed on the back of the silicon wafer; Phosphorus diffusion and annealing are performed on silicon wafers; Laser secondary patterning of silicon wafers; Acid etching is performed on the silicon wafer; Texturing of silicon wafers; Apply antireflective coatings to both the front and back sides of the silicon wafer; Gate lines are printed on the back of the silicon wafer.

10. A back contact cell characterized in that, It is made by the method of preparing a back contact battery according to any one of claims 1-9.

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