How to improve contact in silicon solar cells

Abstract

The present invention relates to a method for improving contact in a silicon solar cell. The object of the present invention is to improve the contact of a plated metal contact grid in a silicon solar cell. The object of this invention is to first prepare a silicon solar cell having a contact grid and back contacts, then use a contacting device to bring the silicon solar cell into contact, the contact grid being electrically connected to one pole of a voltage source, the back contacts being electrically connected to the other pole of the voltage source, the voltage source being used to apply a voltage in the opposite direction to the forward direction of the silicon solar cell and having an absolute value smaller than the breakdown voltage of the silicon solar cell, a point light source being guided onto the solar active surface of the silicon solar cell while this voltage is applied, illuminating a portion of a partial area of ​​the solar active surface, with a voltage of 21,000 A / cm² applied to the portion. 2 From 200,000 A / cm² 2 This is achieved by inducing a current in a subregion that has a current density of 10 ns to 10 ms, and acts on that subregion for a period of 10 ns to 10 ms.

Description

[Technical Field]

[0001] The present invention relates to a method for improving contact in silicon solar cells. [Background technology]

[0002] Various known approaches exist for contacting crystalline silicon solar cells. One approach involves applying an anti-reflective layer (usually a silicon nitride layer) to the front surface of the silicon solar cell, followed by coating the front surface of the silicon solar cell with a metal paste (usually a silver conductive paste) in the form of a screen-printed contact grid (Kontaktgitter). After coating, the metal paste is fired into the anti-reflective layer at 800-900°C, thereby establishing electrical contact with the emitter layer. Such silicon solar cells are commonly referred to as PERC cells. In single-sided embodiments, the entire back surface of these silicon solar cells is usually covered with an additional metal paste (usually an aluminum conductive paste) and similarly fired. In contrast, in double-sided embodiments, the back surface of the silicon solar cell is also covered with a metal paste (usually a silver conductive paste) in the form of a screen-printed contact grid and then fired into a passivation layer on the back surface. During the firing of the metal paste, process control significantly affects contact formation. Improper process control results in high contact resistance at the transition zone between the metal paste and the emitter layer of the silicon solar cell, and / or, in the case of a bifacial embodiment, high contact resistance between the metal paste and the back-side electric field of the silicon solar cell. High contact resistance can lead to a decrease in the efficiency of the silicon solar cell.

[0003] Silicon solar cells are also known by an embodiment called TOPCon. In this case, a conductive path is applied to the front surface of the silicon solar cell using a metal paste (e.g., AlAg paste or pure Ag paste). A contact system consisting of Ag paste-polysilicon-tunnel oxide-Si is typically provided on the back surface of the silicon solar cell.

[0004] Another approach to silicon solar cell contact involves replacing the expensive screen-printed silver conductive paste used to form the contact grid with a more cost-effective plated metal layer (e.g., galvanically treated silver, copper, or nickel). Nevertheless, to achieve low-resistance transitions on the front and / or back surfaces of the silicon solar cell, the anti-reflective layer on the front or back surface beneath the future location of the contact grid is locally removed prior to the galvanic deposition of the plated metal layer (usually using a laser and subsequent chemical etching of the oxide layer). In this case, the quality of the anti-reflective layer removal and / or the etching of the formed oxide layer plays a crucial role in determining the subsequent contact quality. If the anti-reflective layer and / or the formed oxide layer are not adequately removed, it results in low contact quality, with high contact resistance between the contact grid and the emitter layer, or between the contact grid and the back surface electric field.

[0005] In both approaches, the contact grid typically consists of a system of contact grids, which are usually connected via busbars.

[0006] In the prior art, DE102018001057.1 discloses a method for improving the ohmic contact characteristics (ohmsches Kontaktverhalten) between the contact grid and the emitter layer of a silicon solar cell. In this case, the contact grid and the back surface of the silicon solar cell are electrically biased in the opposite direction to their forward direction. The electrically biased silicon solar cell is then scanned with a point light source. This applies 200 A / cm² to the irradiated portion (Ausschnitt). 2 From 20,000 A / cm² 2has a current density, and a current acting on the partial region is induced between 10 ns and 10 ms. This method compensates for inappropriate process control during firing of the metal paste and ensures that the solar cell still achieves an optimal series resistance for its design. This method has been demonstrated in solar cells having contact grids made of metal paste. However, in the case of solar cells having partially plated contacts, the improvement in contact is often smaller. Summary of the Invention Problems to be Solved by the Invention

[0007] The problem of the present invention is to improve the contact of the plated metal contact grid of a silicon solar cell. Means for Solving the Problems

[0008] This problem is achieved by first preparing a silicon solar cell provided with a contact grid and a back contact, and electrically connecting the contact grid to one pole of a voltage source using a contact device. The other pole of the voltage source is electrically connected to the back contact via the contact device. Next, the voltage source applies a voltage in the direction opposite to the forward direction of the silicon solar cell, and the absolute value of the voltage is smaller than the breakdown voltage of the silicon solar cell. When this voltage is applied, a point light source is guided (gefuhrt) onto the solar active surface of the silicon solar cell, irradiating a part of the partial region of the solar active surface, whereby, for said part, 21,000 A / cm 2 to 200,000 A / cm 2 has a current density, and a current acting on the partial region is induced in the partial region between 10 ns and 10 ms.

[0009] In one advantageous embodiment, the current has a current density of 40,000 A / cm 2 to 200,000 A / cm 2 for said part.

[0010] In a further advantageous embodiment, the current has a current density of 80,000 A / cm 2 to 200,000 A / cm 2 for the part.

[0011] In a further advantageous embodiment, the current has a current density of 120,000 A / cm 2 to 200,000 A / cm 2 for the part.

[0012] It is proposed that the point light source consists of a laser, a light-emitting diode or a concentrated white light source.

[0013] <00,000100>In one embodiment, the point light source has an irradiation intensity of 500 W / cm 2 to 1,000,000 W / cm 2 in the part.

[0014] In one embodiment, it is assumed that the point light source emits electromagnetic radiation having a wavelength in the range from 400 nm to 1500 nm.

[0015] In a further embodiment, the part has an area in the range of 1·10 3 μm 2 to 1·10 8 μm 2 , preferably in the range of 1·10 3 μm 2 to 1·10 7 μm 2 , more preferably in the range of 1·10 3 μm 2 to 1·10 6 μm<000002,]]4]]In a further embodiment, it is proposed that the voltage in the opposite direction to the forward direction of the silicon solar cell is in the range from 1 V to 30 V.

[0016] In one embodiment, the silicon solar cell is of the single-sided type and the point light source is guided onto the solar-active front surface.

[0017]

[0018] In a further embodiment, the silicon solar cell is double-sided, and a point light source is guided onto the solar-active front and / or solar-active back surface of the silicon solar cell.

[0019] In one embodiment, it is assumed that the back surface contacts of a double-sided silicon solar cell take the form of a contact grid.

[0020] It is further proposed that a point light source be guided directly next to the contact fingers of the contact grid on the solar active surface of a silicon solar cell.

[0021] In one embodiment, the silicon solar cell is assumed to take the form of a PERC cell or a TOPCon cell.

[0022] In one embodiment, it is assumed that the contact grid of a silicon solar cell is composed of a plated metal layer. The method according to the present invention improves the contact characteristics of the plated contact grid. In particular, when the silicon solar cell has a single-sided design, the contact resistance between the front contact grid and the emitter is improved (reduction of contact resistance). In the case of a double-sided design, the contact resistance between the back contacts, which take the form of a contact grid on the back surface, and the back field of the silicon solar cell can also be reduced.

[0023] Surprisingly, the method according to the present invention has been found to improve contact quality without damaging the silicon solar cell, even in the case of contact grids made from fired metal paste. In particular, the method according to the present invention compensates for improper process control during the firing of the metal paste, ensuring that the contact grid nevertheless achieves low contact resistance. Here again, if the silicon solar cell has a single-sided design, the contact resistance between the front contact grid and the emitter is improved (reduction of contact resistance). In the case of a double-sided design, the contact resistance between the back contacts, which take the form of a contact grid on the back surface, and the back field of the silicon solar cell can also be reduced.

[0024] A portion of a partial region that has been illuminated may have a circular outline. However, the present invention is not expressly limited thereto. In principle, a portion of a partial region that has been illuminated may have any outline. For example, the outline may form a rectangular or other polygonal shape. The corners of these shapes may be rounded. Furthermore, it may also be a linear portion within the partial region. [Modes for carrying out the invention]

[0025] Embodiments of the present invention are described below.

[0026] First, a crystalline, single-sided silicon solar cell is prepared. It has an anti-reflective layer of silicon nitride on its solar active front surface. Below this anti-reflective layer lies the emitter layer of the silicon solar cell. A contact grid, fabricated from a plated metal layer, is applied to the solar active front surface and can be reinforced with printed and cured metal paste to enhance conductivity. On the side opposite to the sun-facing side, the silicon solar cell has a back contact. This back contact consists of a planar metal layer.

[0027] Next, the silicon solar cell is brought into contact using a contact device such that the contact grid on the front is connected to one pole of the voltage source and the contacts on the back are connected to the other pole. Then, the voltage source applies a voltage opposite to the forward direction of the silicon solar cell and has an absolute value less than the breakdown voltage of the silicon solar cell. While this voltage is applied, a point light source is guided onto the solar active surface of the silicon solar cell. This point light source illuminates a portion of a partial area of ​​the solar active front surface of the silicon solar cell, thereby inducing a current in that partial area. The current to the aforementioned portion is 21,000 A / cm². 2 From 200,000 A / cm² 2 It has a current density and acts on a subregion for a period of 10 ns to 10 ms.

[0028] The high current density required to improve the ohmic contact characteristics between the contact grid and the emitter layer can be achieved, in particular, by shifting the operating point of the irradiated cell region without causing radiation-induced material damage. The required current density is achieved without causing material-damaging irradiation through the interaction of the radiation density of the radiation source, exposure time, and applied voltage in part.

[0029] In a further embodiment, the method is applied to a bifacial silicon solar cell. First, a bifacial silicon solar cell is prepared. It has an anti-reflective layer of silicon nitride on its solaractive front surface. The emitter layer of the silicon solar cell is located below this anti-reflective layer. A contact grid made of a plated metal layer is applied to the solaractive front surface. A further contact grid made of a plated metal layer is applied to the solaractive back surface. The contact grid applied to the solaractive back surface forms the back contacts of this silicon solar cell. Optionally, the plated metal layer may be reinforced with a printed and cured metal paste to enhance conductivity.

[0030] The bifacial silicon solar cell is then contacted using a contact device such that the contact grid on the front of the solar active is connected to one pole of the voltage source, and the contact grid formed on the back of the solar active (in this case, the back contact) is connected to the other pole of the voltage source. Next, the voltage source applies a voltage opposite to the forward direction of the silicon solar cell and less in absolute value than the breakdown voltage of the silicon solar cell. For example, the silicon solar cell is biased with a voltage opposite to the forward direction of the silicon solar cell. While this voltage is applied, a point light source is guided over the front and / or back of the solar active of the silicon solar cell. This point light source illuminates a portion of the sun-facing side of the silicon solar cell, thereby inducing a current in that portion. The current is 21,000 A / cm² to the portion. 2 From 200,000 A / cm² 2It has a current density and acts on a subregion for a period of 10 ns to 10 ms.

[0031] In a further embodiment of the method, if the silicon solar cell has a single-sided design, the contact grid is formed from a metal paste, which is applied, for example, by screen printing and then cured according to the instructions of the metal paste manufacturer. After such a silicon solar cell is prepared, the subsequent steps of the method are carried out as described above.

[0032] In a further embodiment of the method, when the silicon solar cell has a double-sided design, the contact grid on the front of the solar active and / or the contact grid on the back of the solar active are formed from a metal paste, which is applied, for example, by screen printing and then cured according to the instructions of the metal paste manufacturer. After such a silicon solar cell is prepared, the subsequent steps of the method are carried out as described above.

[0033] In all described embodiments, the point light source is a laser, a light-emitting diode, or a (focused) white light source, but the present invention is not limited thereto. The point light source provides 500 W / cm² of light to the irradiated portion. 2 From 1,000,000 W / cm² 2 The irradiation intensity is as follows. However, the present invention is not limited to these irradiation intensities. Ideally, the point source should emit electromagnetic radiation having wavelengths in the range of 400 nm to 1500 nm. The irradiated portion is, ideally, 1.10 3 μm 2 From 1.10 8 μm 2 A range of 1 to 10 3 μm 2 From 1.10 7 μm 2 Range, comfortable 1-10 3 μm 2 From 1.10 6 μm 2The area should be within the range of these dimensions. However, the method according to the present invention also allows for areas that deviate from these dimensions. The voltage in the opposite direction to the forward direction of the silicon solar cell should ideally be in the range of 1V to 30V (but not limited to this range).

[0034] In one embodiment, the illuminated portion of a partial region has a circular outline. However, the present invention is not expressly limited thereto. In other embodiments, the outline of the illuminated portion of a partial region is rectangular or forms another polygonal shape. The corners of these shapes may be rounded. A beam optical system with a commonly known beam shaping element is used to form the outline of the illuminated portion of a partial region. Similarly, the laser may have multiple laser diodes arranged, for example, in a rectangular shape or along a line. In another embodiment, it is assumed that the illuminated portion of a partial region is linear.

[0035] For example, if the irradiated area has a surface diameter of approximately 150 μm, then an applied voltage of 10 V and a power load of 20,000 W / cm² would be used. 2 From 100,000 W / cm² 2 At this illuminance, a current of 50mA to 5,000mA is typically generated, resulting in approximately 21,000A / cm² of irradiated area. 2 From 30,000 A / cm² 2 This results in a current density. The absolute current is kept low, especially by the relatively small area that is irradiated.

[0036] In a further embodiment of the embodiment of the method according to the present invention described above, the current is 40,000 A / cm² to the irradiated portion. 2 From 200,000 A / cm² 2 It has a current density of .

[0037] In a further embodiment of the embodiment of the method according to the present invention described above, the current is 80,000 A / cm² to the irradiated portion. 2 From 200,000 A / cm² 2 It has a current density of .

[0038] In a further embodiment of the embodiment of the method according to the present invention described above, the current is 120,000 A / cm² to the irradiated portion. 2 From 200,000 A / cm² 2 It has a current density of .

[0039] The described method can be applied to silicon solar cells with PERC design and silicon solar cells with TOPCon design, but the present invention is not limited to these cell types.

Claims

1. A method for improving the contact characteristics of a contact grid of a silicon solar cell, First, the silicon solar cell having a contact grid and back contacts is prepared, and the silicon solar cell is brought into contact using a contact device. Here, the contact grid is electrically connected to one pole of the voltage source, and the back contact is electrically connected to the other pole of the voltage source. The voltage source is used to apply a voltage in the opposite direction to the forward direction of the silicon solar cell, and whose absolute value is smaller than the breakdown voltage of the silicon solar cell. When the aforementioned voltage is applied, a point light source is guided onto the solar active surface of the silicon solar cell, thereby illuminating a portion of the solar active surface with 21,000 A / cm² of light. 2 From 200,000 A / cm 2 Having a current density such that a current acts on the subregion for a period of 10 ns to 10 ms, the current is induced in the subregion. A method characterized by the following.

2. The current is 40,000 A / cm² to the irradiated portion. 2 From 200,000 A / cm 2 The method according to claim 1, characterized in that it has a current density of the following.

3. The current is 80,000 A / cm² to the irradiated portion. 2 From 200,000 A / cm 2 The method according to claim 1, characterized in that it has a current density of the following.

4. The current is 120,000 A / cm² to the irradiated portion. 2 From 200,000 A / cm 2 The method according to claim 1, characterized in that it has a current density of the following.

5. The method according to any one of the above claims, characterized in that the point light source is a laser, a light-emitting diode, or a focused white light source.

6. The point light source has an irradiation intensity of 500 W / cm 2 to 1,000,000 W / cm 2 in the part, and the method according to any one of the preceding claims is characterized in that.

7. The method according to any one of the above claims, characterized in that the point light source emits electromagnetic radiation having a wavelength in the range of 400 nm to 1500 nm.

8. The aforementioned part is 1.10 3 μm 2 From 1.10 8 μm 2 A range of 1.10 3 μm 2 From 1.10 7 μm 2 The range, more preferably 1.10 3 μm 2 From 1.10 6 μm 2 The method according to any one of the above claims, characterized in that it has an area within the range of [the specified range].

9. The method according to any one of the above claims, characterized in that the part of the aforementioned partial region has a circular or polygonal outline.

10. The method according to claim 9, characterized in that the corners of the outer contour of the polygon are rounded.

11. The method according to any one of claims 1 to 8, characterized in that the part of the aforementioned partial region is linear.

12. The method according to any one of the above claims, characterized in that the voltage in the direction opposite to the forward direction of the silicon solar cell is in the range of 1V to 30V.

13. The method according to any one of the above claims, characterized in that the silicon solar cell is single-sided and the point light source is guided onto the front surface of the solar active.

14. The method according to any one of claims 1 to 12, characterized in that the silicon solar cell is double-sided and the point light source is guided on the solar active front and / or solar active back surface of the silicon solar cell.

15. The method according to claim 14, characterized in that the back surface contacts of the double-sided silicon solar cell take the form of a contact grid.

16. The method according to any one of the above claims, characterized in that the contact grid or a plurality of contact grids are made from a fired metal paste or a plated metal layer.

17. The method according to any one of the above claims, characterized in that the silicon solar cell is configured as a PERC cell or a TOPCon cell.

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