A method for fabricating photovoltaic cell metal grid lines based on melt forming technology

Abstract

This invention discloses a method for preparing photovoltaic cell metal grid lines based on fused deposition modeling (FDM) technology, comprising the following steps: S1 Metal slurry preparation: The slurry includes a conductive metal phase and a completely volatile organic carrier; S2 Grid line pattern forming and organic carrier removal: The metal slurry is screen-printed onto the surface of the photovoltaic cell passivation layer to form a preset grid line pattern, followed by low-temperature heat treatment under an inert atmosphere to volatilize and remove the organic carrier; S3 Metal grid line densification treatment: The grid lines are heated and melted, leveled and densified, and controlled solidified using conventional controlled atmosphere thermal melting or high-energy light pulse rapid melting processes to form dense metal grid lines. This invention provides a method for preparing photovoltaic cell metal grid lines based on FDM technology, improving the quality of grid line forming.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic cell manufacturing technology, and more specifically, to a method for preparing photovoltaic cell metal grid lines based on melt forming technology. Background Technology

[0002] The current fabrication of electrode grid lines in crystalline silicon solar cells (including mainstream technologies such as TOPCon, HJT, and BC) mainly employs a combination of "screen printing of metal paste + solid-state sintering". First, conductive paste (such as silver paste, aluminum paste, or silver-aluminum paste) is screen printed onto the surface of the cell according to a preset pattern to form wet grid lines. Then, heat treatment in a sintering furnace is performed to solidify the grid lines and form reliable ohmic contacts with the silicon substrate.

[0003] The microstructure of metal grids obtained by solid-state sintering is essentially a porous network formed by metal powder particles connecting through surface diffusion and neck growth. This results in incomplete densification between particles, with numerous micropores and voids. When current is conducted between particles, it must pass through a large number of narrow "neck" regions, which have high resistance. Non-metallic components such as the glass phase and organic residues in the paste can become trapped in the conductive network, further hindering electron transport. This structure makes the bulk resistivity of sintered metal pastes (such as photovoltaic silver paste) typically several times higher than that of pure metal / alloy bulk materials. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing photovoltaic cell metal grid lines based on melt forming technology, thereby improving the quality of grid line forming.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing photovoltaic cell metal grid lines based on melt forming technology, comprising the following steps: S1 Metal paste preparation: the paste includes a conductive metal phase and a completely volatile organic carrier; S2 Grid line pattern forming and organic carrier removal: the metal paste is screen-printed on the surface of the photovoltaic cell passivation layer to form a preset grid line pattern, and then subjected to low-temperature heat treatment in an inert atmosphere to volatilize and remove the organic carrier; S3 Metal grid line densification treatment: the grid lines are heated and melted, leveled and densified, and controlled solidified using a conventional controlled atmosphere hot melting or high-energy light pulse rapid melting process to form dense metal grid lines.

[0006] Furthermore, the metal paste also includes functional additives, with the following mass distribution ratios: conductive metal phase 60%-95%, organic carrier 5%-40%, functional additives 0%-20%, and photothermal absorption enhancer 0.1%-2%.

[0007] Furthermore, the conductive metallic phase is copper-aluminum alloy powder or aluminum-silicon alloy powder.

[0008] Furthermore, the organic carrier includes a high-boiling-point solvent and a small-molecule dispersant. The high-boiling-point solvent is selected from one or two of tetradecane and terpineol, and the small-molecule dispersant is selected from one or two of fatty acids and phosphate esters.

[0009] Furthermore, the functional additives include antioxidants and wetting accelerators, with the antioxidant being a benzotriazole derivative and the wetting accelerator being an organotitanate.

[0010] Furthermore, the conventional controlled atmosphere hot melting process in step S3 specifically involves: ① placing the battery substrate after removing the organic carrier in an inert gas tube furnace or a rapid hot annealing device; ② heating the metal grid lines to above the metal liquidus temperature but not exceeding 700°C; ③ holding the metal grid lines in a fully molten or semi-molten state; ④ cooling and solidifying.

[0011] Furthermore, the high-energy light pulse rapid melting process in step S3 also includes the following steps: S31. The photovoltaic cell substrate with the organic carrier removed is placed on a conveying mechanism protected by an inert atmosphere and continuously or stepwise conveyed to the light pulse region; S32. A pulsed xenon lamp system is used as the light source to apply a high-energy-density light pulse to the metal grid lines, causing the metal grid lines to rapidly heat up to above the melting point and melt; S33. After the light pulse stops, the metal grid lines cool down and solidify to form dense grid lines.

[0012] Furthermore, the metal paste also includes a photothermal absorption enhancer, which is a nanomaterial with strong absorption capacity for specific wavelengths.

[0013] In summary, the present invention has the following beneficial effects:

[0014] 1. Abandoning the porous network structure of traditional solid-state sintering, the metal particles are completely melted, the liquid phase flows and solidifies uniformly through a melting process. The formed grid line is a fully dense and continuous metal structure, which reduces micropores, gaps and high-resistivity interfaces at particle necks. This significantly reduces the resistivity of the grid line, making it close to the conductivity of pure copper, pure aluminum or copper-aluminum / aluminum-silicon alloy blocks. This meets the core requirements of high-efficiency batteries such as TOPCon, HJT, and BC for low-resistivity electrodes.

[0015] 2. The high-energy light pulse melting process adopts a three-stage progressive heating + real-time temperature closed-loop control, which can accurately determine the molten state of the metal, dynamically correct the pulse parameters, and avoid problems such as under-melting, over-melting, and substrate overheating. The process has high stability and is suitable for fully automated large-scale production lines. Attached Figure Description

[0016] Figure 1 The flowchart is for an example. Detailed Implementation

[0017] 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. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] like Figure 1 As shown, this embodiment discloses a method for fabricating photovoltaic cell metal grid lines based on melt-forming technology, including the following steps:

[0019] S1. Preparation of metal paste.

[0020] The slurry includes a conductive metal phase and a completely volatile organic carrier. Functional additives and photothermal absorption enhancers can be selectively added according to actual process requirements.

[0021] As a preferred embodiment, the composition ratio of the metal paste is as follows (by mass): conductive metal phase 60%-95%, organic carrier 5%-40%, functional additives 0%-20%, and photothermal absorption enhancer 0.1%-2%. This composition results in a paste viscosity suitable for screen printing, preventing pattern collapse and overflow after printing. The moderate organic carrier content allows for rapid and complete removal at low temperatures. Appropriate amounts of functional additives and photothermal absorption enhancers can respectively improve the metal's oxidation resistance, wettability, and photothermal conversion efficiency, accelerating the melting process.

[0022] The conductive metal phase is selected from copper-aluminum alloy powder or aluminum-silicon alloy powder, which significantly reduces the cost of raw materials. These metals have excellent conductivity, are suitable for melting processes, and can form dense grid lines with high conductivity. The preferred particle size of the metal powder is a combination of micron and nano-sized particles to increase the particle packing density and facilitate subsequent melting and leveling.

[0023] In another embodiment, the conductive metal phase is selected from copper powder or aluminum powder.

[0024] The organic carrier is a compound of a high-boiling-point solvent and a small-molecule dispersant. The high-boiling-point solvent has a boiling point greater than 100℃. The high-boiling-point solvent is selected from one or two of tetradecane and terpineol, possessing suitable boiling point and viscosity to ensure the printability of the paste and to completely evaporate at low temperatures. The small-molecule dispersant is selected from one or two of fatty acids and phosphate esters, effectively dispersing conductive metal particles, preventing particle agglomeration, and ensuring the uniformity of the paste and the uniformity of the printed lines.

[0025] Functional additives include antioxidants and wetting accelerators. The antioxidants are benzotriazole derivatives, which effectively inhibit the oxidation of metals such as copper and aluminum during the melting process and prevent the oxide layer from reducing the conductivity of the grid lines. The wetting accelerators are organic titanates, which improve the wettability of the molten metal and the battery passivation layer, enhance the adhesion between the grid lines and the substrate, and prevent the grid lines from falling off.

[0026] Photothermal absorption enhancers are nanomaterials with strong absorption capabilities for specific wavelengths, enabling rapid absorption of light energy. Specifically, they are surface-modified carbon nanomaterials and tungsten oxide with specific morphologies, which possess excellent photothermal conversion performance. In high-energy light pulse melting processes, they can rapidly absorb light energy and convert it into heat energy, accelerating the heating and melting of metal particles, shortening heat treatment time, and further reducing the risk of thermal damage.

[0027] The proportions of each component are strictly controlled to ensure the printability, drying and removal properties of the paste, and the effect of subsequent melt molding. The organic carrier can be completely volatilized under low-temperature heat treatment, leaving no residual impurities and avoiding affecting the density and conductivity of the grid lines.

[0028] S2. Grid pattern forming and organic carrier removal.

[0029] ① The prepared metal paste is uniformly printed on the surface of the photovoltaic cell passivation layer using a high-precision screen printing process to form a preset grid pattern, ensuring pattern accuracy and line uniformity.

[0030] ② After printing, the battery cells are placed in an inert atmosphere protection device for low-temperature heat treatment. The inert atmosphere can effectively prevent the metal particles from oxidizing. During the low-temperature heat treatment, the organic carrier is completely volatilized and removed, leaving only a continuously distributed layer of conductive metal particles, which lays the foundation for subsequent melting and densification. This step avoids damage to the passivation layer caused by high-temperature pretreatment and preserves the passivation performance of the battery.

[0031] The low-temperature heat treatment is carried out at temperatures ranging from 100°C to 450°C. The purpose is to completely decompose and remove all organic carrier components, leaving a pure metal alloy powder preform pattern on the substrate.

[0032] S3. Densification treatment of metal grid lines.

[0033] The metal particle layer after the organic carrier has been removed is subjected to heating, melting, leveling, densification, and controlled solidification using either of two controllable melting processes: the traditional controlled atmosphere thermal melting process and the high-energy light pulse rapid melting process. Both processes can achieve full melting and leveling of the metal particles, eliminate internal pores and defects, and form a continuous metal grid line with a dense structure, smooth surface, and strong bonding, which greatly improves the conductivity of the grid line.

[0034] For the two melting processes in step S3, the following steps are included:

[0035] 1. Traditional controlled atmosphere thermal melting process:

[0036] ① Place the battery substrate after removing the organic carrier in a tube furnace or rapid thermal annealing equipment with an inert gas to prevent metal oxidation.

[0037] ② Heat at a uniform heating rate of 10-30℃ / min to avoid rapid heating that could cause grid line deformation. The grid line temperature should be raised above the liquidus temperature of the metal but not higher than 700℃. The temperature should be controlled within the tolerance range of the passivation layer to prevent thermal damage.

[0038] ③ Keep the grid metal in a fully molten or semi-molten state to ensure that the particles are fully fused and leveled.

[0039] ④ Cool slowly and evenly to complete solidification and molding, avoiding rapid cooling that could cause cracking of the grid lines, and finally forming dense metal grid lines.

[0040] 2. High-energy light pulse rapid melting process: includes the following steps:

[0041] S31. Place the photovoltaic cell substrate, from which the organic carrier has been removed, on a conveying mechanism protected by an inert atmosphere, and convey it continuously or in steps.

[0042] S32. A pulsed xenon lamp system is used as the light source. High-energy-density light pulses are applied, and the temperature is rapidly increased by photothermal conversion, so that the metal grid wires can instantly reach above the melting point and melt. The heating rate is much higher than that of traditional processes, and the heat treatment time is extremely short.

[0043] Specifically, the high-energy light pulse heating process is divided into three progressive stages: the preheating pulse stage, the main melting pulse stage, and the leveling and heat preservation pulse stage. The preheating pulse stage has a lower temperature to achieve preliminary preheating of the metal particles, remove residual trace moisture and impurities, and avoid splashing caused by rapid melting. The main melting pulse stage has the highest temperature to achieve complete melting and full leveling of the metal particles and eliminate internal pores. The leveling and heat preservation pulse stage has a lower temperature than the main melting stage to maintain the leveling state of the molten metal, ensure a smooth and uniform grid surface, and avoid defects caused by sudden temperature drops. The three-stage temperature control further improves the grid forming quality.

[0044] This step also includes the following sub-steps:

[0045] ①Preset full-process threshold: The melting point of the metal alloy (metal grid) is denoted as T. melt The highest tolerable temperature of the battery substrate is denoted as T. sub_limit Temperature deviation threshold ΔT (allowable deviation of detection temperature), maximum withstand temperature T of metal alloy (metal grid wire) max .

[0046] Initial pulse parameters are set in stages: the initial parameter for the preheating stage is E. p0 t p0 The initial parameters of the main melting section are E m0 t m0 The initial parameters for the leveling section are E. i0 t i0 .

[0047] ② Multi-pulse time-sequence segmented execution: Following a fixed timing sequence of "preheating pulse stage → main melting pulse stage → leveling and heat preservation pulse stage," the pulse light source is driven by a real-time controller to complete irradiation stage by stage and pulse by pulse. The sub-pulse sequence number within each stage is defined as n (n is a positive integer, initially n=1 for each stage), and the parameter E of the nth pulse in the current stage is used. n t n The metal grid lines are locally heated, paused after a single pulse ends, and then the data acquisition phase begins.

[0048] ③ Real-time data acquisition after pulse: Within 10ms after the end of a single subpulse irradiation, the peak temperature T of the corresponding grid region is acquired in real time using a high-speed infrared thermometer. peak Temperature T in the non-grid region of the battery substrate sub Simultaneously calculate the cumulative heat Q under the action of the pulse.

[0049] ④ Real-time identification of melting state in stages: Combining the process objectives of each time stage, the collected T peak Compared with the corresponding stage temperature threshold, T sub With T sub_limit In comparison, three melting states are distinguished: when T peak If the temperature is less than the target temperature - ΔT, it is considered an undermelting state, where ΔT is 20-40℃; if the target temperature - ΔT ≤ T, it is considered an undermelting state. peak ≤T max And T sub <T sub_limit It is judged to be in a qualified state; when T peak >T max or T sub ≥T sub_limit It was determined to be in an overmelted / substrate overheating state.

[0050] A special criterion for exceeding the standard is added during the preheating stage: if T peak >500℃ or T sub A temperature ≥280℃ is directly considered an excessive preheating temperature, triggering emergency protection. The target temperature for the preheating stage is 400-500℃, the target temperature for the main melting stage is the alloy melting point (Tmelt), and the target temperature for the leveling stage is slightly lower than the melting point (T). sub_limit ≤300℃.

[0051] ⑤ Pulse parameter correction:

[0052] E n Given the current pulse energy density (J / cm²), t n E represents the current pulse width (ms). max t represents the upper limit of energy for a given stage. max T represents the upper limit of the stage pulse width. target For the target temperature of the stage, T peak This is the measured peak temperature.

[0053] undermelting state (T) peak <Target temperature T for each stage target - Under the condition of ΔT (ΔT=20-40℃), directly increase the energy with a fixed step size of ΔE and the pulse width with a fixed step size of Δt, where the value range of ΔE is 2-10 J / cm², the value range of Δt is 50-200 ms, ΔE is preferably 3 J / cm², and Δt is preferably 75 ms. After increasing the pulse multiple times, when the target temperature T is reached... target The number of pulse increments is denoted as n (n=0,1,2,3....).

[0054] E based on the number of pulse increases n The adjustments are made to determine the first pulse energy and pulse width values ​​for the next metal gate region:

[0055] Energy Correction: E n = min(E n + n△E, E max ).

[0056] Pulse width correction: t n = min(t n + n△t, t max ).

[0057] Formula Explanation and Numerical Constraints: A fixed step size is used for incremental increases, rather than unconstrained amplification based on temperature difference. Furthermore, the corrected parameters must not exceed the preset upper limit for each stage (preheating stage E). max =30J / cm², t max =1s; Main melting section E max =60J / cm², t max =2s; Leveling section E max =35J / cm², t max =1.5s), ensuring that each amplification is gentle and controllable, and that no instantaneous overheating or metal splashing is caused by excessive amplification in a single instance.

[0058] To reduce overheating, the initial parameters for each stage are all lower than the aforementioned upper limit values ​​(i.e., preheating stage E). max =30J / cm², t max =1s; Main melting section E max=60J / cm², t max =2s; Leveling section E max =35J / cm², t max The setting of 1.5s makes it easy for the device to experience under-melting when it is first started. This can be adjusted using the method described above.

[0059] When T peak >Target temperature T for the stage target - 2△T / 3, triggering over-temperature warning, E n , t n Both are automatically downgraded by one ΔE and Δt level, i.e., E n =E n -△E,t n = t n - △t.

[0060] If overheating occurs, immediately cut off the current pulse, force heat dissipation for 50-100ms, and then re-execute the preheating pulse. It is strictly forbidden to jump directly to the next stage. All parameters will be restored to the initial settings. After that, the product will gradually increase the temperature by increasing the pulse from the underheating state.

[0061] ⑥ If the current stage has passed two consecutive tests and the heat accumulation meets the standard, it will automatically jump to the next time sequence stage; after all three time sequences are completed and meet the standards, the light pulse irradiation will be terminated, the grid line melting and densification will be completed, and it will wait for cooling.

[0062] S33. After the light pulse stops, the battery substrate cools down rapidly, and the metal grid lines solidify quickly, forming a dense, defect-free continuous grid line.

[0063] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for the production of metal grid lines for photovoltaic cells based on melt forming techniques, characterized by, Includes the following steps: S1 Metal Paste Preparation: The metal paste includes a conductive metal phase and a completely volatile organic carrier; S2 Grid Line Pattern Forming and Organic Carrier Removal: The metal paste is screen-printed onto the surface of the photovoltaic cell passivation layer to form a preset grid line pattern, followed by low-temperature heat treatment in an inert atmosphere to volatilize and remove the organic carrier; S3 metal grid densification process: The grid lines are heated and melted using a traditional controlled atmosphere thermal melting or high-energy light pulse rapid melting process to make the molten grid lines flow and densify, and then solidified to form dense metal grid lines.

2. A method for fabricating metal grid lines for photovoltaic cells based on melt- forming techniques according to claim 1, characterized in that, The metal paste also includes functional additives, with the following mass distribution ratios: 60%-95% of the conductive metal phase, 5%-40% of the organic carrier, 0%-20% of the functional additives, and 0.1%-2% of the photothermal absorption enhancer.

3. A method for fabricating metal grid lines for photovoltaic cells based on melt- forming techniques according to claim 1, characterized in that, The conductive metal phase is copper-aluminum alloy powder or aluminum-silicon alloy powder.

4. The method of claim 1, wherein the method is characterized by: The organic carrier comprises a high-boiling-point solvent and a small-molecule dispersant, wherein the high-boiling-point solvent has a boiling point greater than 100°C.

5. A method of fabricating metal grid lines for photovoltaic cells based on melt- forming techniques according to claim 4, characterized in that, The high-boiling-point solvent is selected from one or two of tetradecane and terpineol, and the small molecule dispersant is selected from one or two of fatty acids and phosphate esters.

6. The method of claim 2, wherein the metal grid lines are formed by a melt- forming technique. The functional additives include antioxidants and wetting promoters.

7. The method for preparing photovoltaic cell metal grid lines based on melt forming technology according to claim 6, characterized in that, The antioxidant is a benzotriazole derivative, and the wetting promoter is an organotitanate.

8. The method for preparing photovoltaic cell metal grid lines based on melt forming technology according to claim 1, characterized in that, The conventional controlled atmosphere thermal melting process described in step S3 is specifically as follows: ① Place the battery substrate after removing the organic carrier in an inert gas tube furnace or a rapid thermal annealing device; ②The metal grid wire is heated to a temperature above the liquidus temperature of the metal, but not higher than 700°C; ③ The metal grid wires are kept at a constant temperature while they are in a fully molten or semi-molten state; ④ Cool and solidify into shape.

9. The method for preparing photovoltaic cell metal grid lines based on melt forming technology according to claim 1, characterized in that, The high-energy light pulse rapid melting process in step S3 also includes the following steps: S31. Place the photovoltaic cell substrate, from which the organic carrier has been removed, on a conveying mechanism protected by an inert atmosphere, and continuously or stepwise transport it to the light pulse region; S32. A pulsed xenon lamp system is used as a light source to apply a high-energy-density light pulse to the metal grid line, causing the metal grid line to heat up rapidly to above its melting point and melt. S33. After the light pulse stops, the metal grid lines cool down and solidify to form dense grid lines.

10. A method for preparing photovoltaic cell metal grid lines based on melt forming technology according to claim 2, characterized in that, The metal paste also includes a photothermal absorption enhancer, which is a nanomaterial with strong absorption capacity for a specific wavelength.

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