A method for improving reliability of laser-assisted sintering of solar cells and a solar cell

Abstract

This invention relates to the field of solar cell technology, and discloses a method for improving the reliability of laser-assisted sintering of solar cells and a solar cell. The method includes: subjecting the solar cell, after metallization, to a forward bias process, forcing substances such as Ag... + Other cations (such as H+) surrounding the metal positive electrode + Keep away from the metal positive electrode and battery surface to prevent H + Other cation scavenging e ‑ Then, in conjunction with the LECO process, Ag is treated to... + Get more e ‑ The reduction and conversion to Ag electrodes can better leverage the advantages of the LECO process, improve its reliability, and significantly reduce the H2O degradation rate. + With e ‑ The combination of properties effectively avoids the occurrence of e ‑ +H + The conversion process of →H and H+H→H2 can also avoid problems such as the rupture of the passivation and antireflection film caused by hydrogen leakage, and the decrease in battery electrical performance (such as Voc, FF, Eta, etc.) caused by the reduction of elemental H.

Description

Technical Field

[0001] This invention relates to the field of solar cell technology, and specifically to a method for improving the reliability of laser-assisted sintering of solar cells and a solar cell. Background Technology

[0002] Solar cells typically require passivation and antireflection coatings, such as silicon nitride (SiNx), to reduce reflection, passivate, and protect the surface. During solar cell fabrication, after depositing the passivation and antireflection coating, a metallization process (such as screen printing and sintering) is required. This involves first screen printing a metal electrode paste (using silver paste as an example) onto a localized area of ​​the passivation and antireflection coating surface; then, sintering is used to allow the glass frit in the metal electrode paste to erode and locally open the passivation and antireflection coating on its surface, enabling the metal electrode to pass through the passivation and antireflection coating, contact the silicon substrate, and fully alloy.

[0003] During sintering, metallic materials (such as silver) need to undergo a melting and recrystallization process. Molten metallic materials usually exist in ionic form (such as Ag). + Upon cooling and recrystallization, the solidified elemental silver reacts with the silicon matrix to form a silver-silicon alloy. This process involves the conversion of silver ions into metallic silver (Ag...). + →Ag), requires a large number of negatively charged electrons (e - ) participates. However, in solar cells, taking N-type TOPCon cells (i.e., N-type tunneling oxide passivated contact cells) as an example, the front side of its silicon substrate is the positive electrode (i.e., p-type). + (Emitter), this side lacks e - This will lead to Ag + →The Ag process is not complete. Therefore, when using the traditional screen printing sintering process, there are often areas of insufficient alloying at the contact interface between the metal electrode (such as the silver electrode) and the silicon substrate of the solar cell, resulting in high contact resistance.

[0004] Based on this, LECO technology (laser-enhanced contact optimization technology, also known as laser-assisted sintering technology) was developed. LECO technology is a key process that has been widely used in high-efficiency crystalline silicon solar cells (especially TOPCon cells) in recent years. Its core principle is to utilize the precise localization of energy input by a laser. Through the synergistic effect of the laser and the application of a reverse bias voltage, on the one hand, the laser excites a large number of charge carriers on the cell surface to generate a large number of charge carriers. On the other hand, a 10V reverse bias voltage is applied to the cell, forcing the negatively charged electrons to... - Flowing towards the front, making Ag +→The Ag process is more complete; on the other hand, under laser irradiation, the current generated by the reverse bias can generate instantaneous high temperature when it passes through the interface contact point between the metal electrode and the semiconductor such as the silicon substrate, causing the silver paste and silicon to undergo co-fusion diffusion, promoting the local alloying reaction between silver and silicon surfaces, and forming a denser silver-silicon alloy; thus, the contact performance between the metal electrode and the silicon substrate can be optimized, the contact resistance can be reduced, and the photoelectric conversion efficiency of the solar cell (referred to as cell efficiency) can be improved.

[0005] The core functions of LECO technology are: a) reducing contact resistance (e.g., a 30-50% reduction) and increasing the fill factor (FF), such as a 0.5-1.5% increase in FF. b) reducing metal recombination, suppressing interfacial carrier recombination, and increasing open-circuit voltage (Voc), such as a 3-5mV increase in Voc. c) repairing sintering defects, fixing contact points that have not fully penetrated the passivation antireflective film in traditional screen printing sintering processes, and improving yield. d) increasing process tolerance, allowing for a more flexible screen printing sintering process window and reducing the requirements for screen printing precision.

[0006] However, existing LECO technology also has the following drawbacks: laser-assisted sintering technology forces negatively charged electrons to undergo laser-assisted sintering. - The flow is directed towards the front of the battery, causing Ag to... + The conversion to Ag is more complete. However, the cations on the front side of the solar cell are not only Ag. + There are also other cations, such as H introduced during the preparation of various film structures like passivation antireflection films. + Na, a major metal contaminant in battery manufacturing + etc., among which H is actively introduced to reduce dangling bonds in order to achieve hydrogen passivation. + They make up the majority.

[0007] In laser-assisted sintering, to reduce contact resistance, it is necessary to make more negatively charged electrons... - +Ag + →Ag. However, in Ag + During the reduction process, other cations (such as H+) + (etc.) also has some reducing properties and will steal negative charges (e). - ; and this will further affect Ag + The reduction of [the material] has a negative impact. Therefore, directly performing laser-assisted sintering on screen-printed sintered solar cells can lead to the reduction of other cations (such as H+). + (etc.) the e that takes away the negative charge - As for Ag + The reduction has a negative impact, which greatly reduces the role or advantage of laser-assisted sintering technology, hinders the further reduction of contact resistance, and thus affects the further improvement of battery efficiency.

[0008] Moreover, a large number of hydrogen ions (H) + ) and electrons (e - ) combine to form elemental H, and elemental H readily combines with other substances to form hydrogen gas (e.g., e.g.) - +H + →H, and H+H→H2), the overflow of hydrogen gas can cause the passivation and antireflection film to break, thus affecting its antireflection, passivation and protection functions, and the reduction of elemental H will lead to problems such as the decline in the electrical performance of the battery. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method and a solar cell for improving the reliability of laser-assisted sintering of solar cells.

[0010] Based on this, the present invention discloses a method for improving the reliability of laser-assisted sintering of solar cells, comprising the following process steps:

[0011] Step 1: Prepare the solar cells after metallization process in advance;

[0012] The solar cell includes a silicon substrate, a passivation film, and a metal electrode. The passivation film is disposed on the surface of the silicon substrate, and the metal electrode includes a positive electrode, one end of which passes through the passivation film and contacts the silicon substrate.

[0013] Step 2: Perform a forward voltage process on the solar cell to allow the H+ cations in the solar cell to undergo a forward voltage process. + They migrate and leave the metal electrodes and battery surface;

[0014] Step 3: Perform laser-assisted sintering on the solar cells processed in Step 2 to optimize the contact performance between the metal electrodes and the silicon substrate, thus obtaining the optimized solar cells.

[0015] Preferably, in step 2, the forward voltage process includes: placing the solar cell in a fixed external electric field, with the electric field strength controlled at 10000-15000V / m and the processing time controlled at 10-30s, to complete the forward voltage process of the solar cell.

[0016] Alternatively, the forward voltage process includes: using an electroluminescent device to energize the solar cell for a duration of 5-10 seconds to complete the forward voltage process of the solar cell.

[0017] More preferably, in step 2, when using an external electric field for forward voltage processing, the electric field strength is 10000V / m and the processing time is 20s;

[0018] Alternatively, when using an electroluminescent device for forward voltage processing, the energizing time is 5 seconds.

[0019] Preferably, in step 3, the reverse bias voltage of the laser-assisted sintering process is controlled at 20-25V, the laser scanning speed is controlled at 8000-10000mm / s, the laser frequency is controlled at 50-60KHz, and the processing time is 1-2s.

[0020] More preferably, in step 3, the reverse bias voltage of the laser-assisted sintering process is 25V, the laser scanning speed is 10000mm / s, the laser frequency is 50KHz, the laser pulse width is 10us, and the processing time is 1s.

[0021] Preferably, in step 1, the solar cell is a TOPCon cell, a PERC cell, or an HJT cell; the thickness of the solar cell is 100-130 μm.

[0022] More preferably, in step 1, the solar cell is an N-type TOPCon cell, and the silicon substrate includes a silicon wafer and a p-type solar cell disposed on the front side of the silicon wafer. + Emitter and passivated contact structure located on the back of the silicon wafer;

[0023] Both the front and back sides of the silicon substrate are provided with passivation films; one end of the positive electrode passes through the front passivation film and then interacts with the p-type electrode. + The emitter is in contact with the metal electrode; the metal electrode also includes a negative electrode, one end of which passes through the back passivation film and comes into contact with the passivation contact structure.

[0024] Preferably, in step 1, the metal electrode is made of at least one of silver, aluminum, copper, and nickel.

[0025] More preferably, in step 1, the metal electrode is made of silver.

[0026] The present invention also discloses a solar cell, which is prepared by the method described above for improving the reliability of laser-assisted sintering of solar cells.

[0027] Compared with the prior art, the present invention has at least the following beneficial effects:

[0028] The method of this invention first performs a forward bias process on the solar cell after metallization (such as screen printing and sintering) (i.e., applies a voltage from the positive electrode to the negative electrode), forcing the positive electrode (such as Ag) to... + Other cations around ) such as H + (etc.) away from the positive electrode and the surface of the solar cell (such as H) + Due to their smaller size, cations like Ag migrate away from the battery surface (e.g., the front) under a positive voltage; while cations like Ag... + Because of their larger size, the positive electrode cations are blocked at the surface of the battery by the silicon lattice, preventing them from penetrating too deeply into the battery and causing damage. This is to avoid situations like H...+ Other cations steal negatively charged electrons. - Furthermore, by combining this with LECO (laser-assisted sintering) processing, Ag can be... + Get more e - More reduction and conversion to Ag can better leverage the process advantages of the LECO process, improve its reliability, and enable H... + With e - The likelihood of combination is greatly reduced, effectively avoiding the occurrence of e. - +H + The conversion processes of →H and H+H→H2 can also avoid problems such as the rupture of passivation and antireflection films caused by hydrogen leakage, and the decrease in battery electrical performance (such as Voc, FF, Eta, etc.) caused by the reduction of elemental H.

[0029] Therefore, compared to existing technologies (existing methods that directly process solar cells after screen printing and sintering using the LECO process), the method of this invention (a method that performs forward bias processing + LECO processing on solar cells after screen printing and sintering) enables more solar cells to achieve higher efficiency after metallization processes such as screen printing and sintering, through the synergistic cooperation of the forward bias process and the LECO process. - Acting on Ag + In terms of reduction and conversion, the advantages of the LECO process are further highlighted, improving its reliability; and enabling H + The reduction in the proportion of hydrogen reduced to hydrogen enhances the battery's hydrogen passivation capability; thus, the battery's electrical properties, such as open-circuit voltage and fill factor, are further improved, thereby further enhancing battery efficiency. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the forward voltage process performed on the N-type TOPCon battery semi-finished product after screen printing and sintering in step 2 of Example 1.

[0031] Figure 2 The graph shows the photoelectric conversion efficiency data of the optimized N-type TOPCon cells in Example 1 and Comparative Example 1.

[0032] Reference numerals: Silicon wafer 1; p + Emitter 2; Alumina film 3; Front silicon nitride film 4; Positive electrode 5; Tunneling silicon oxide 6; n + 7. Doped polycrystalline silicon; 8. Backside silicon nitride film; 9. Negative electrode; 10. External electric field. Detailed Implementation

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

[0034] The present invention provides a method for improving the reliability of laser-assisted sintering of solar cells, comprising the following process steps:

[0035] Step 1: Prepare the solar cells after they have undergone the metallization process.

[0036] In step 1, the metallization process includes, but is not limited to, existing screen printing sintering processes. The solar cell includes a silicon substrate, a passivation film, and a metal electrode. The passivation film is disposed on the surface of the silicon substrate, and the metal electrode includes a positive electrode 5 (e.g., ...). Figure 1 As shown in the figure, one end of the positive electrode 5 passes through the passivation film and comes into contact with the silicon substrate.

[0037] In step 1, the metal electrode is made of at least one of silver, aluminum, copper, and nickel.

[0038] In practice, solar cells are TOPCon, PERC, or HJT cells; the thickness of the solar cell is 100-130μm (e.g., 130μm).

[0039] Taking an N-type TOPCon cell as an example, the silicon substrate includes a silicon wafer 1 and a p-type solar cell disposed on the front side of the silicon wafer 1. + Emitter 2 and passivated contact structure (i.e., tunneling silicon oxide 6 and n) on the back side of silicon wafer 1 + Doped polycrystalline silicon 7); passivation films are provided on both the front and back sides of the silicon substrate; one end of the positive electrode 5 passes through the front passivation film (such as the front silicon nitride film 4 and the aluminum oxide film 3) and then connects with p + The emitter 2 is in phase contact; the metal electrode also includes a negative electrode 9, one end of which passes through the back passivation film (such as the back silicon nitride film 8) and then contacts the passivation contact structure (such as...). Figure 1 (As shown).

[0040] Step 2: Perform a forward voltage process on the solar cell to allow the H+ cations in the solar cell to undergo a forward voltage process. + It migrates and leaves the positive electrode 5 and the battery surface.

[0041] In step 2, the forward voltage process is carried out using the following two methods:

[0042] Option 1 involves placing the solar cell in a fixed external electric field 10, with the electric field strength controlled at 10000-15000V / m and the processing time controlled at 10-30s, to complete the forward voltage process of the solar cell.

[0043] Alternatively, in Option 2, an EL (electroluminescent) device is used to power the solar cell. The voltage of the EL device is 24V. Taking a solar cell with a thickness of 130μm as an example, the electric field strength applied to the solar cell is E=U / d=24V / 130μm≈184615V. The power-on time is controlled within 5-10s to complete the forward voltage process of the solar cell.

[0044] Step 3: Perform laser-assisted sintering on the solar cell processed in Step 2 to optimize the contact performance between the positive electrode 5 and the silicon substrate, thus obtaining the optimized solar cell.

[0045] In step 3, the reverse bias voltage of the laser-assisted sintering process is controlled at 20-25V, the laser scanning speed is controlled at 8000-10000mm / s, the laser frequency is controlled at 50-60KHz, and the processing time is 1-2s.

[0046] The following provides a specific embodiment of a method for improving the reliability of laser-assisted sintering of solar cells according to the present invention.

[0047] Example 1

[0048] This embodiment provides a method for improving the reliability of laser-assisted sintering of solar cells. See [link to previous document]. Figure 1 The process includes the following steps:

[0049] Step 1: Prepare the solar cells after screen printing and sintering.

[0050] In step 1, the solar cell is an N-type TOPCon cell semi-finished product, which is prepared by existing processes such as texturing, diffusion, coating and screen printing sintering (metallization process) using a silicon wafer 1 with a size of 183.75mm×182.2mm. The preparation process of the N-type TOPCon cell semi-finished product after screen printing and sintering refers to the existing technology, so it will not be described in detail here.

[0051] The semi-finished N-type TOPCon battery after screen printing and sintering in step 1, such as Figure 1 As shown, it includes: a silicon wafer 1; the front side of the silicon wafer 1 is sequentially provided with p + Emitter 2, aluminum oxide film 3, front silicon nitride film 4, and positive electrode 5 (wherein, positive electrode 5 is a silver electrode; aluminum oxide film 3 and front silicon nitride film 4 serve as front passivation films); and the lower end of positive electrode 5 passes through front silicon nitride film 4 and aluminum oxide film 3 in sequence, and then connects with p + Emitter phase 2 contact; tunneling silicon oxide 6 and n are sequentially formed on the back side of silicon wafer 1. + 7. Doped polycrystalline silicon, 8. Backside silicon nitride film, and 9. Negative electrode (wherein, tunneling silicon oxide 6 and n +Doped polycrystalline silicon 7 serves as its back-side passivation contact structure, while the back-side silicon nitride film 8 serves as the back-side passivation film; the negative electrode 9 is a silver electrode; and the upper end of the negative electrode 9 passes through the back-side silicon nitride film 8 and connects with n + Doped polycrystalline silicon 7-phase contact.

[0052] The thickness of the N-type TOPCon battery semi-finished product after screen printing and sintering in step 1 is 130μm.

[0053] Step 2: Perform forward voltage processing on the solar cell from Step 1: Place the solar cell in a fixed external electric field 10, electrically connect the positive electrode of the external electric field 10 to the positive electrode 5, and electrically connect the negative electrode of the external electric field 10 to the negative electrode 9 (e.g., Figure 1 As shown), the electric field strength is 10000 V / m, and the processing time is 20 s; thus, the cations H in the solar cell are... + It migrates and leaves the metal positive electrode 5 and the battery surface.

[0054] Step 3: Perform laser-assisted sintering on the solar cell processed in Step 2. The laser-assisted sintering process conditions are as follows: the reverse bias voltage applied to the solar cell is 25V, and the solar cell is laser-scanned with a laser scanning speed of 10000mm / s, a laser frequency of 50KHz, a laser pulse width of 10us, and a processing time of 1s. In this way, the contact performance between the metal positive electrode 5 and the silicon substrate is optimized, thus obtaining the optimized solar cell (N-type TOPCon cell) of this embodiment.

[0055] Comparative Example 1

[0056] This comparative example describes a laser-assisted sintering method for solar cells. The process steps are the same as in Example 1, but the difference lies in:

[0057] This comparative example omits the forward voltage process in step 2 and directly proceeds to the laser-assisted sintering process in step 3, thus obtaining the optimized solar cell (N-type TOPCon cell) of this comparative example.

[0058] Performance testing

[0059] The solar cells (N-type TOPCon cells) obtained by the methods of Example 1 and Comparative Example 1 were subjected to performance tests, and the test results are shown in Table 1 below (in Table 1, the count represents the number of cells tested, and the electrical performance data in Table 1 is the average of the test data of each cell; Eta is the cell efficiency, Voc is the open-circuit voltage, Isc is the short-circuit current, and FF is the fill factor):

[0060] Table 1

[0061] Comparative Example 1 30 26.44 739.7 14.48 82.62 Example 1 28 26.48 740.0 14.48 82.75

[0062] Combining Table 1 and Figure 2 Therefore, we can conclude that:

[0063] The method in Embodiment 1 of this invention first applies a forward bias process to the solar cell after the screen printing and sintering process, which can force other cations (such as H+) around the positive electrode to undergo a forward bias process. + (etc.) Keep away from the positive metal electrode to prevent other cations from competing for the negatively charged electrons. - Furthermore, combined with the LECO process, the Ag can be processed during the LECO process. + Get more e - More reduction is converted into Ag, and the occurrence of e can be avoided. - +H + The conversion processes of →H and H+H→H2 can also avoid problems such as the rupture of passivation and antireflection films caused by hydrogen leakage, and the decrease in battery electrical performance (such as Voc, FF, Eta, etc.) caused by the reduction of elemental H.

[0064] Therefore, compared to Comparative Example 1 (the existing method of directly processing solar cells after screen printing and sintering with LECO process), the method of Embodiment 1 of this invention (the method of forward biasing and then LECO processing solar cells after screen printing and sintering) performs forward biasing first and then LECO processing after screen printing and sintering. Through the synergistic cooperation of the forward biasing and LECO processes, the advantages of the LECO process are more prominent, the reliability of the LECO process is improved, and more solar cells can be processed using this method. - Acting on Ag + On the reduction and transformation, and make H + The decrease in the reduction to H improves the battery's hydrogen passivation capability, which in turn increases the battery's open-circuit voltage Voc by 0.3mV, the fill factor by 0.13%, and the battery efficiency (Eta) by 0.04%.

[0065] Although preferred embodiments of the present invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present invention.

[0066] The technical solution provided by the present invention has been described in detail above. Specific examples have been used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for improving the reliability of laser-assisted sintering of solar cells, characterized in that, The process includes the following steps: Step 1: Prepare the solar cells after metallization process in advance; The solar cell includes a silicon substrate, a passivation film, and a metal electrode. The passivation film is disposed on the surface of the silicon substrate, and the metal electrode includes a positive electrode, one end of which passes through the passivation film and contacts the silicon substrate. Step 2: Perform a forward bias process on the solar cell to allow the H+ cations in the solar cell to undergo a forward bias process. + They migrate and leave the metal electrodes and battery surface; Step 3: Perform laser-assisted sintering on the solar cells processed in Step 2 to optimize the contact performance between the metal electrodes and the silicon substrate, thus obtaining the optimized solar cells.

2. The method for improving the reliability of laser-assisted sintering of solar cells according to claim 1, characterized in that, In step 2, the forward biasing process includes: placing the solar cell in a fixed external electric field, with the electric field strength controlled at 10000-15000V / m and the processing time controlled at 10-30s, to complete the forward biasing process of the solar cell. Alternatively, the forward bias process includes: using an electroluminescent device to energize the solar cell for a duration of 5-10 seconds to complete the forward bias process of the solar cell.

3. The method for improving the reliability of laser-assisted sintering of solar cells according to claim 2, characterized in that, In step 2, when using an external electric field for forward biasing, the electric field strength is 10000V / m and the processing time is 20s. Alternatively, when using an electroluminescent device for forward bias processing, the energizing time is 5 seconds.

4. The method for improving the reliability of laser-assisted sintering of solar cells according to claim 1, characterized in that, In step 3, the reverse bias voltage of the laser-assisted sintering process is controlled at 20-25V, the laser scanning speed is controlled at 8000-10000mm / s, the laser frequency is controlled at 50-60KHz, and the processing time is 1-2s.

5. A method for improving the reliability of laser-assisted sintering of solar cells according to claim 4, characterized in that, In step 3, the reverse bias voltage of the laser-assisted sintering process is 25V, the laser scanning speed is 10000mm / s, the laser frequency is 50KHz, the laser pulse width is 10us, and the processing time is 1s.

6. The method for improving the reliability of laser-assisted sintering of solar cells according to claim 1, characterized in that, In step 1, the solar cell is a TOPCon cell, a PERC cell, or an HJT cell; the thickness of the solar cell is 100-130µm.

7. A method for improving the reliability of laser-assisted sintering of solar cells according to claim 6, characterized in that, In step 1, the solar cell is an N-type TOPCon cell, and the silicon substrate includes a silicon wafer and a p-type solar cell disposed on the front side of the silicon wafer. + Emitter and passivated contact structure located on the back of the silicon wafer; The silicon substrate has passivation films on both its front and back sides; one end of the positive electrode passes through the front passivation film and then interacts with the p-type electrode. + The emitter is in contact with the metal electrode; the metal electrode also includes a negative electrode, one end of which passes through the back passivation film and comes into contact with the passivation contact structure.

8. The method for improving the reliability of laser-assisted sintering of solar cells according to claim 1, characterized in that, In step 1, the metal electrode is made of at least one of silver, aluminum, copper, and nickel.

9. A method for improving the reliability of laser-assisted sintering of solar cells according to claim 8, characterized in that, In step 1, the metal electrode is made of silver.

10. A solar cell, characterized in that, It is prepared by a method for improving the reliability of laser-assisted sintering of solar cells as described in any one of claims 1-9.

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