Method for in-situ reduction of TCO film

Abstract

The invention discloses a method for in-situ reduction of a TCO film, and relates to the technical field of solar cells. The method comprises the steps that S1, a semi-finished heterojunction solar cell with a TCO thin film prepared on the front face is soaked in roughening liquid to be treated; s2, the treated sample serves as a cathode to be placed in an electrolytic tank, electrolytic reduction is carried out with preset current, and the current density of the preset current in the electrolytic reduction process is controlled to be gradually reduced; the coarsening liquid comprises hydrochloric acid and disodium ethylene diamine tetraacetate; an electrolyte solution in the electrolytic tank comprises sodium citrate, sodium sulfamate, sulfuric acid, disodium ethylene diamine tetraacetate and sodium potassium tartrate. The method has the beneficial effects that a seed-layer-free electroplating technology is provided, activating treatment is carried out on the surface of the TCO, and indium is electrolyzed and reduced in situ to serve as a binding layer for subsequent copper electroplating, so that the binding force of electroplated copper and the surface of the TCO is improved, the tedious step of preparing a metal seed layer through traditional PVD sputtering is avoided, the production cost is reduced, and the battery efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical fields of semiconductors and solar cells, and particularly to a method for in-situ reducing a TCO film. Background Art

[0002] Among many silicon-based solar cell technologies, silicon-based heterojunction solar cells have higher conversion efficiency because both the front and back sides are passivated by amorphous silicon thin films, and currently it is also the most likely to become the next-generation mass-produced photovoltaic technology. However, the biggest limitation in the large-scale mass production of heterojunction solar cells is the high cost of the low-temperature silver paste material used. A large amount of research work has been carried out on this problem - electroplating copper electrodes to replace screen-printed low-temperature silver paste electrodes to reduce material costs; but traditional electroplating copper technologies all require using a PVD device to sputter a metal seed layer on the TCO surface to improve the bonding force between the subsequent electroplated copper and the substrate. Without the metal seed layer, the direct bonding force between the electroplated copper and the TCO is too small, which easily leads to the shedding of the grid lines, thereby reducing the battery efficiency and stability; through research, it is found that the process of sputtering the metal seed layer by the PVD device will not only cause great sputtering damage to the TCO surface and result in an efficiency drop of more than 0.3%, but also increase a PVD device, thus increasing the equipment investment. Therefore, the above method is not a very suitable technical solution for mass production.

[0003] Therefore, there is an urgent need to propose a preparation method for heterojunction solar cells that is low-cost, simple and more suitable for large-scale mass production, which can achieve the process of no PVD sputtering of the metal seed layer and at the same time the bonding force between the subsequent electroplated copper and the TCO can meet the use requirements. Summary of the Invention

[0004] The purpose of the present invention is to solve at least one of the technical problems existing in the prior art, that is, to solve the problem of efficiency loss of heterojunction solar cells caused by sputtering damage generated by PVD to prepare the metal seed layer, and thus a seedless electroplating technology is proposed. The present invention uses in-situ electrolytic reduction of indium in the TCO film as the seed layer for subsequent electroplating copper, which can not only reduce the steps of sputtering the metal seed layer by PVD, but also reduce the equipment cost and improve the battery efficiency.

[0005] The technical solution of the present invention is as follows: The first aspect of the present invention provides a method for in-situ reducing a TCO film, including the following steps: S1. Immerse the semi-finished product with TCO films on the front and back in a roughening solution for roughening treatment; S2. Place the semi-finished product processed in S1 as the cathode in an electrolytic cell, with a current density of 0.1A / dm 2 -20A / dm 2Perform electrolytic reduction with a preset current, and control the current density of the preset current to gradually decrease during the electrolytic reduction process; Among them, the roughening solution in S1 includes hydrochloric acid and disodium ethylenediaminetetraacetate; The electrolyte solution in the electrolytic cell in S2 includes sodium citrate, sodium aminosulfonate, sulfuric acid, disodium ethylenediaminetetraacetate and potassium sodium tartrate, and the pH value of the electrolyte solution is 2-5.

[0006] Since the prior art uses a single sodium citrate system as the electrolyte for electrolytic reduction of indium in the TCO film, there are several disadvantages involved: (1) After sodium citrate hydrolysis, it shows weak alkalinity, and in the actual production process, the photosensitive ink on the battery surface is easily dissolved in the alkaline solution, causing damage to the line. (2) The wetting angle between the sodium citrate solution and the TCO surface is large, which is not conducive to the subsequent preparation of uniform indium particles. (3) The electrolysis process using a single current is adopted in the prior art, which is likely to cause the reduction rate to be too large or too small, resulting in agglomeration of indium particles and reducing the bonding force with the TCO.

[0007] Therefore, on the one hand, the present invention uses a multi-component system mixture as the electrolyte solution, with sodium citrate as the main reducing agent and crystal form control agent, sodium aminosulfonate as the buffer agent and auxiliary reducing agent, sulfuric acid as the surface activator, and a composite complexing agent system composed of disodium EDTA and 0.001-0.01 mol / L potassium sodium tartrate, which can not only reduce the damage to the line, but also be conducive to the subsequent preparation of uniform indium particles. On the other hand, control the current density of the preset current to gradually decrease during the electrolytic reduction process, avoid agglomeration of indium particles, and improve the bonding force between indium and TCO.

[0008] In a preferred embodiment of the present invention, in S1, the roughening solution includes 0.01 mol / L - 0.1 mol / L hydrochloric acid and 0.001 - 0.01 mol / L disodium ethylenediaminetetraacetate.

[0009] In a preferred embodiment of the present invention, in S1, the roughening treatment time is 10 s - 120 s.

[0010] In a preferred embodiment of the present invention, in S1, the roughening treatment temperature is 15°C - 45°C.

[0011] In a preferred embodiment of the present invention, in S1, the surface of the TCO film after roughening treatment has micro-etch pits with a size of 50 nm - 100 nm.

[0012] In a preferred embodiment of the present invention, in S2, the electrolyte solution includes 0.01 mol / L - 0.5 mol / L sodium citrate, 0.01 mol / L - 0.2 mol / L sodium aminosulfonate, 0.01 mol / L - 0.1 mol / L sulfuric acid, 0.005 mol / L - 0.02 mol / L disodium ethylenediaminetetraacetate, and 0.001 mol / L - 0.01 mol / L potassium sodium tartrate.

[0013] In a preferred embodiment of the present invention, in S2, the temperature of the electrolytic cell is 5°C - 35°C.

[0014] In a preferred embodiment of the present invention, in S2, during the electrolytic reduction process, the first preset current, the second preset current, and the third preset current are sequentially used for electrolytic reduction, wherein the current density of the first preset current is greater than that of the second preset current, and the current density of the second preset current is greater than that of the third preset current.

[0015] In a preferred embodiment of the present invention, the specific process of the electrolytic reduction in S2 includes: First, electrolyze with a first preset current having a current density of 5 A / dm 2 - 20 A / dm 2 for 20 s - 120 s; Then, electrolyze with a second preset current having a current density of 1 A / dm 2 - 5 A / dm 2 for 40 s - 200 s; Finally, electrolyze with a third preset current having a current density of 0.1 A / dm 2 - 1 A / dm 2 for 5 s - 20 s.

[0016] The second aspect of the present invention provides a TCO thin film prepared by the above method.

[0017] The third aspect of the present invention provides a heterojunction solar cell prepared by the above method or including the above TCO thin film.

[0018] The present invention has at least one of the following beneficial effects: In order to solve the problem of efficiency loss of heterojunction solar cells caused by sputtering damage generated during the preparation of metal seed layers by PVD, the present invention proposes a seedless electroplating technique. By performing an activation treatment on the surface of the TCO and in-situ electrolytic reduction of indium as a bonding layer for subsequent copper electroplating, that is, first treating with a roughening solution, and then using a mixture of a multi-component system composed of sodium citrate, sodium aminosulfonate, sulfuric acid, disodium EDTA, and sodium potassium tartrate as the electrolyte solution for in-situ electrolytic reduction of indium, and controlling the current density of the preset current to gradually decrease during the electrolytic reduction process, thereby improving the bonding force between the electroplated copper and the TCO surface. This not only avoids the cumbersome steps of traditional PVD sputtering for preparing metal seed layers, reduces production costs, but also improves the cell efficiency. Brief Description of the Drawings

[0019] Figure 1 It is a surface microscopic test diagram of the TCO thin film obtained by the method in Example 1; Figure 2 It is a surface microscopic test diagram of the untreated TCO thin film; Figure 3 It is a surface microscopic test diagram of the TCO thin film obtained by the method in Comparative Example 1; Figure 4 It is a surface microscopic test diagram of the TCO thin film obtained by the method in Comparative Example 2. Detailed Embodiments

[0020] In order to make the technical problems, technical solutions, and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0021] An embodiment of the present invention provides a method for in-situ reducing a TCO thin film, which includes process parameters and solution formulations. The main steps and formulations are as follows: Step 1: Immerse the semi-finished heterojunction solar cell with TCO thin films prepared on the front and back into the roughening solution. The function of the roughening solution is to perform micro-etching on the TCO surface with a certain concentration of acid to form micro-etch pits with uniformity and a size of 50 - 100 nm, which is beneficial for increasing the contact area with the TCO during subsequent indium reduction to improve the bonding force; The formulation of the roughening solution is: a. 0.01 mol / L - 0.1 mol / L hydrochloric acid (that is, in the prepared roughening solution, the concentration of hydrochloric acid is 0.01 mol / L - 0.1 mol / L), and its function is to perform micro-etching on the TCO surface.

[0022] b. A solution of disodium EDTA (ethylenediaminetetraacetic acid disodium) with a concentration of 0.001 - 0.01 mol / L (i.e., in the prepared roughening solution, the concentration of disodium EDTA is 0.001 - 0.01 mol / L). Its function is to act as a complexing agent to prevent the indium ions (In 3+ ), and tin ions (Sn 4+ ) from agglomerating to form precipitates, which would affect the service life of the liquid medicine. Disodium EDTA will capture indium ions and tin ions to form water-soluble complexes, thus preventing the formation of precipitates; The roughening process conditions are as follows: a. The roughening time is between 10 s and 120 s. If the time is too short, the roughening effect will not be achieved; if the time is too long, it is easy to cause excessive corrosion of the TCO, which will affect the performance of the silicon wafer substrate and the subsequent effect of electrolytic reduction of indium.

[0023] b. The roughening temperature is 15°C - 45°C. If the temperature is too low, the corrosion rate of hydrochloric acid is too slow to achieve the roughening effect; if the temperature is too high, the corrosion rate is too fast, causing excessive damage.

[0024] Step 2: Place the sample as the cathode in the electrolytic cell for electrolytic reduction: The electrolyte solution is a multi-component system mixture, specifically including the following components with the following concentrations: a) 0.01 - 0.5 mol / L sodium citrate (i.e., in the prepared electrolyte solution, the concentration of sodium citrate is 0.01 - 0.5 mol / L, the same below) as the main reducing agent and crystal form control agent; b) 0.01 - 0.2 mol / L sodium sulfamate as a buffer and auxiliary reducing agent; c) 0.01 - 0.1 mol / L sulfuric acid as a surface activator; d) A composite complexing agent system composed of 0.005 - 0.02 mol / L disodium EDTA and 0.001 - 0.01 mol / L potassium sodium tartrate; e) Sulfuric acid adjusts the pH value of the electrolyte solution to between 2 and 5.

[0025] Controlling the concentrations of each substance in the electrolyte is very important. When the concentration of sodium citrate > 0.5 mol / L, dendrites are generated, which is not conducive to subsequent uniformity. When the concentration of sodium sulfamate < 0.05 mol / L, indium atoms are prone to agglomeration.

[0026] The temperature of the electrolytic cell is 5°C - 35°C. If the temperature is too high, the electrolytic reduction rate is too fast, and the reduced indium atoms are prone to agglomeration to form large indium grains, resulting in poor surface coverage and uniformity; while if the temperature is too low, the reduction rate is too slow to form good indium particles. The current process during electrolysis is also a key factor. The present invention adopts a segmented current, using a current process with a gradually decreasing gradient: The first stage: applying a current density of 5 - 20 A / dm 2 for 20 s - 120 s. The purpose of the large current is to quickly nucleate and reduce uniform and fine indium particles. If the current in the first stage is too small, indium particles cannot be formed. In addition, a large amount of hydrogen bubbles will be generated, breaking the photosensitive resin on the surface and causing changes in the grid morphology.

[0027] The second stage: reducing the current density to 1 - 5 A / dm 2 for 40 s - 200 s. The purpose of reducing the current in this stage is to reduce the reduction rate. If a large current is still used, the indium atoms reduced will agglomerate to form large - sized indium particles, which will seriously reduce the surface adhesion and surface uniformity.

[0028] The third stage: continuously reducing the current density to 0.1 - 1 A / dm 2 and electrolyzing for 5 s - 20 s. This process is mainly for surface modification to increase the adhesion between the subsequent electroplated copper and indium.

[0029] In summary, the present invention proposes a method for electrolytically reducing indium in TCO to prepare an electroplated copper seed layer. This method can completely replace the traditional PVD sputtering process for preparing the copper seed layer, reduce the sputtering damage to the solar cell, and improve the conversion efficiency. At the same time, it also reduces the use of one PVD device and lowers the equipment investment cost. The present invention also proposes an effective solution formula and electrolysis process. Applying this process can prepare an indium seed layer with controllable size and high uniformity, and the adhesion after electroplating copper is very high, fully meeting the usage requirements of solar cells.

[0030] Another embodiment of the present invention provides a method for manufacturing a heterojunction solar cell, that is, the method for manufacturing a heterojunction solar cell includes a method for in - situ electrolytic reduction of indium in a TCO thin film and methods for other processes of manufacturing a heterojunction solar cell. The methods for other processes of manufacturing a heterojunction solar cell are all prior arts and are well - known to those skilled in the art, so they will not be elaborated in this embodiment.

[0031] Another embodiment of the present invention also provides a heterojunction solar cell obtained by the above - mentioned manufacturing method.

[0032] The following uses specific embodiments to further elaborate on the present invention in detail, but the present invention is not limited to the following specific embodiments.

[0033] Embodiment 1 A method for in - situ reduction of a TCO thin film is provided, including process parameters and solution formula. The main steps and formula are as follows: Step 1: Immerse the semi-finished heterojunction solar cell with TCO films on both the front and back sides into the roughening solution. The function of the roughening solution is to perform micro-etching on the TCO surface using acid at a certain concentration to form micro-etch pits with uniformity and a size of 50 - 100 nm. This is beneficial for increasing the contact area with TCO during subsequent indium reduction and improving the bonding strength; The formula of the roughening solution is: a. 0.1 mol / L hydrochloric acid, whose function is to perform micro-etching on the TCO surface.

[0034] b. 0.01 mol / L disodium EDTA (ethylenediaminetetraacetic acid disodium) solution, whose function is to act as a complexing agent to prevent the indium ions (In 3+ ) and tin ions (Sn 4+ ) produced by hydrochloric acid etching of TCO from agglomerating to form precipitates, which affects the service life of the liquid medicine. Disodium EDTA will capture indium ions and tin ions to form water-soluble complexes, thus avoiding the formation of precipitates; The roughening process conditions are: a. The roughening time is 80 s. If the time is too short, the roughening effect cannot be achieved; if the time is too long, it is easy to cause excessive corrosion of TCO, thereby affecting the performance of the silicon wafer substrate and the effect of subsequent electrolytic reduction of indium.

[0035] b. The roughening temperature is 30 °C. If the temperature is too low, the corrosion rate of hydrochloric acid is too slow to achieve the roughening effect; if the temperature is too high, the corrosion rate is too fast, causing excessive damage.

[0036] Step 2: Place the sample as the cathode in the electrolytic cell for electrolytic reduction: The electrolyte solution is a multi-component system mixture: a) 0.5 mol / L sodium citrate as the main reducing agent and crystal form control agent; b) 0.2 mol / L sodium sulfamate as a buffer and auxiliary reducing agent; c) 0.1 mol / L sulfuric acid as a surface activator; d) A composite complexing agent system composed of 0.02 mol / L disodium EDTA and 0.01 mol / L potassium sodium tartrate; e) Sulfuric acid adjusts the pH value of the electrolyte solution to between 2 - 5.

[0037] The temperature of the electrolytic cell is 20 °C. If the temperature is too high, the electrolytic reduction rate is too fast, and the reduced indium atoms are prone to agglomerating to form large indium grains, resulting in poor surface coverage and uniformity; while if the temperature is too low, the reduction rate is too slow to form good indium particles; The current process during electrolysis is also a key factor. In this embodiment, a segmented current is adopted, using a current process with a gradually decreasing gradient: The first stage: Apply a current density of 15 A / dm 2 for 100 s. The purpose of using a large current is to achieve rapid nucleation and reduce uniform and fine indium particles. If the current in the first stage is too small, indium particles cannot be formed. In addition, a large number of hydrogen bubbles will be generated, causing the photosensitive resin on the surface to be broken through, resulting in changes in the grid morphology.

[0038] The second stage: Reduce the current density to 4 A / dm 2 for 150 s. The purpose of reducing the current in this stage is to reduce the reduction rate. If a large current is still used, the indium atoms reduced will agglomerate to form large-sized indium particles, which will seriously reduce the surface adhesion and surface uniformity.

[0039] The third stage: Further reduce the current density to 0.5 A / dm 2 , and electrolyze for 10 s. This process is mainly for surface modification to increase the adhesion between the subsequent electroplated copper and indium.

[0040] Figure 1 is the surface microscopic test diagram of the TCO film obtained by the method of Example 1. Figure 2 is the surface test diagram of the untreated TCO film. From the obvious comparison of the two microscopic diagrams, it can be seen that a new substance has been significantly formed on the surface of the TCO after being treated by the method of the present invention. After elemental analysis, we know that this layer of substance is metallic indium; while no substance is formed on the untreated surface. This shows that the method is feasible, can reduce the metallic indium in the TCO film, and exhibits good adhesion.

[0041] Comparative Example 1 The difference from Example 1 is that the roughening treatment time is 140 s, and the others are the same as in Example 1.

[0042] Figure 3 is the surface microscopic test diagram of the TCO film obtained by the method of Comparative Example 1. From Figure 3 it can be clearly seen that a very large number of micropores have appeared on the surface of the TCO. After further analysis, we found that the reason is that the roughening time is too long, resulting in excessive corrosion of the TCO and damage to the bottom amorphous silicon, affecting the performance of the final device; it shows that the roughening time should not be too long.

[0043] Comparative Example 2 The difference from Example 1 is that the temperature of the electrolytic cell is 45 °C, and the others are the same as in Example 1.

[0044] Figure 4 is the surface microscopic test diagram of the TCO film obtained by the method of Comparative Example 2. From Figure 4It is obvious that when the electrolysis temperature is 45 °C, the reduced indium agglomerates, with too large a size and poor uniformity; through in-depth testing of its binding force with the surface, we also found that it is relatively poor. This is because a high electrolysis temperature leads to too fast a reduction rate, and the newly produced small grains will agglomerate into large grains, resulting in poor uniformity and binding force.

[0045] The cell efficiency and comprehensive metallization cost of the heterojunction solar cells prepared by the traditional PVD sputtering metal seed layer method, Example 1, and Comparative Examples 1-2 were measured, and the results are shown in Table 1: Table 1 As can be seen from Table 1, compared with the method of preparing heterojunction solar cells by the traditional PVD sputtering metal seed layer method, the method of Example 1 of the present invention not only has a much higher cell efficiency but also a lower comprehensive metallization cost. Therefore, the method of the present invention can reduce the production cost and improve the cell efficiency.

[0046] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A method for in-situ reducing a TCO thin film, characterized in that, It includes the following steps: S1. Immerse the semi-finished product with TCO films on the front and back in a roughening solution for roughening treatment; S2. Place the semi-finished product after S1 treatment as the cathode in the electrolytic cell, and perform electrolytic reduction with a preset current of 0.1 A / dm 2 - 20 A / dm 2 , and control the current density of the preset current to gradually decrease during the electrolytic reduction process; Among them, the roughening solution in S1 includes hydrochloric acid and disodium ethylenediaminetetraacetate; In S2, the electrolyte solution in the electrolytic cell includes sodium citrate, sodium aminosulfonate, sulfuric acid, disodium ethylenediaminetetraacetate and potassium sodium tartrate, and the pH value of the electrolyte solution is 2-5.

2. The method according to claim 1, wherein In S1, the roughening solution includes 0.01mol / L - 0.1mol / L hydrochloric acid and 0.001 - 0.01mol / L disodium ethylenediaminetetraacetate.

3. The method according to claim 1, characterized in that In S1, the time of roughening treatment is 10s - 120s, and the temperature of roughening treatment is 15°C - 45°C.

4. The method according to claim 1, characterized in that, In S1, the surface of the TCO film after roughening treatment has micro-etching pits with a size of 50nm - 100nm.

5. The method according to claim 1, characterized in that, In S2, the electrolyte solution includes 0.01mol / L - 0.5mol / L sodium citrate, 0.01mol / L - 0.2mol / L sodium aminosulfonate, 0.01mol / L - 0.1mol / L sulfuric acid, 0.005mol / L - 0.02mol / L disodium ethylenediaminetetraacetate and 0.001mol / L - 0.01mol / L potassium sodium tartrate.

6. The method according to claim 1, characterized in that, In S2, the temperature of the electrolytic cell is 5°C - 35°C.

7. The method according to claim 1, characterized in that In S2, during the electrolytic reduction process, the first preset current, the second preset current and the third preset current are sequentially used for electrolytic reduction. Among them, the current density of the first preset current is greater than that of the second preset current, and the current density of the second preset current is greater than that of the third preset current.

8. The method according to claim 7, characterized in that The specific process of electrolytic reduction in S2 includes: First, electrolyze for 20 s - 120 s with a first preset current density of 5 A / dm 2 - 20 A / dm 2 . Then, electrolysis is carried out at a second preset current density of 1 A / dm 2 - 5 A / dm 2 for 40 s - 200 s; Finally, electrolysis is carried out for 5 s - 20 s with a third preset current density of 0.1 A / dm 2 - 1 A / dm 2 .

9. A TCO thin film, characterized in that, Prepared by the method described in any one of claims 1 to 8.

10. A heterojunction solar cell, characterized in that, Prepared by the method described in any one of claims 1 to 8, or includes the TCO film described in claim 9.

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