Method for improving contact of silicon solar cells

Abstract

The invention relates to a method for improving the contact of a silicon solar cell. The object of the invention is to improve the contact of a plated metal contact grid of a silicon solar cell. This object is achieved in that the silicon solar cell having the contact grid and a rear contact is first prepared and the silicon solar cell is contacted using a contact device, the contact grid being electrically connected to one pole of a voltage source and the rear contact being electrically connected to the other pole of the voltage source; applying, using the voltage source, a voltage oriented opposite the forward direction of the silicon solar cell and lower in magnitude than the breakdown voltage of the silicon solar cell; and when this voltage is applied, directing a point light source over the solar active surface of the silicon solar cell, thereby illuminating a cross-section of a sub-region of the solar active surface and inducing a current in the sub-region, the current having from 21,000 A / cm2 to 200,000 A / cm2 with respect to the cross-section. A current density of 10,000 A / cm < 2 > and acting on the sub-region for 10 nanoseconds to 10 milliseconds.

Description

[0001] This invention relates to a method for improving the contact of silicon solar cells.

[0002] Various known methods exist for contacting crystalline silicon solar cells. One method involves applying a metal paste (typically silver conductive paste) in the form of a screen-printed contact grid to the front side of the silicon solar cell after an anti-reflective layer (typically silicon nitride) has already been formed thereon. After application, the metal paste is fired into the anti-reflective layer at 800-900°C, thereby establishing an electrical contact with the emitter layer. Such silicon solar cells are typically also referred to as PERC cells. In single-sided embodiments, the entire back surface of these silicon solar cells is typically coated with another metal paste (typically aluminum conductive paste) and similarly fired. In contrast, in bi-sided embodiments, the back surface of the silicon solar cell is also coated with a metal paste (typically silver conductive paste) in the form of a screen-printed contact grid, and then fired into a passivation layer on the back side. During the metal paste firing process, process control has a significant impact on contact formation. Inappropriate process control can lead to high contact resistance at the transition between the metal paste and the emitter layer of the silicon solar cell, and / or, in the case of bifacial embodiments, high contact resistance between the metal paste and the back surface field of the associated silicon solar cell. High contact resistance can then lead to a reduction in the efficiency of the silicon solar cell.

[0003] Silicon solar cells are also known in embodiments referred to as TOPCon. In this case, a conductor path is applied to the front side of the silicon solar cell using a metal paste (e.g., AlAg paste or pure Ag paste). Typically, a contact system comprising Ag paste-polycrystalline silicon-tunnel oxide-Si is provided on the back side of the silicon solar cell.

[0004] In another method for contacting silicon solar cells, the expensive screen-printed silver conductive paste used to create the contact grid is replaced by a more cost-effective electroplated metal layer (e.g., electroplated silver, copper, or nickel). However, to achieve a low-resistance transition on the front and / or back of the silicon solar cell, an anti-reflective layer located beneath the future location of the contact grid is locally removed on the front or back side before the electroplating deposition of the metal layer (typically using laser and subsequent chemical etching of the formed 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, accompanied by high contact resistance between the contact grid and the emitter layer or between the contact grid and the back surface field.

[0005] In both methods, the contact grid typically consists of a contact grid system, which is typically connected by busbars.

[0006] In the prior art, DE 10 2018 001 057.1 discloses a method for improving the ohmic contact behavior between the contact grid and the emitter layer of a silicon solar cell. In this method, the contact grid and back surface of the silicon solar cell are electrically biased relative to their forward orientation. The electrically biased silicon solar cell is then scanned with a point light source. A current is induced in this process, which has a current of 200 A / cm² relative to the illuminated cross-section. 2 Up to 20,000 A / cm 2 The current density is applied to the sub-region for 10 nanoseconds to 10 milliseconds. This method compensates for errors in process control during the metal paste firing process, thus ensuring that the solar cell still achieves the optimal series resistance for its design. This method has itself been demonstrated in solar cells with contact grids made of metal paste. However, in the case of solar cells with partially electroplated contacts, the improvement in contact is typically smaller.

[0007] The purpose of this invention is to improve the contact of the electroplated metal contact grid in silicon solar cells.

[0008] This objective is achieved by first preparing a silicon solar cell with a contact grid and a back contact, and then 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. The voltage source is then applied with a voltage oriented opposite to the forward direction of the silicon solar cell and a value lower than the breakdown voltage of the silicon solar cell. When this voltage is applied, a point light source is then directed above the solar active surface of the silicon solar cell, thereby illuminating a cross-section of a sub-region of the solar active surface and thereby inducing a current in the sub-region, the current having a cross-section of 21,000 A / cm. 2 Up to 200,000 A / cm 2 The current density is applied to the sub-region for 10 nanoseconds to 10 milliseconds.

[0009] In an advantageous embodiment, the current has a relative current of 40,000 A / cm². 2 Up to 200,000 A / cm 2 The current density.

[0010] In another advantageous embodiment, the current has 80,000 A / cm² relative to the cross-section. 2 Up to 200,000 A / cm 2 The current density.

[0011] In another advantageous embodiment, the current has a relative current of 120,000 A / cm². 2 Up to 200,000 A / cm 2 The current density.

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

[0013] In one embodiment, the point light source has a strength of 500 W / cm² on the cross-section. 2 Up to 1,000,000 W / cm 2 The power density.

[0014] One embodiment envisions the point light source emitting radiation in the range of 400 nm to 1500 nm.

[0015] In another embodiment, the area of ​​the cross-section is 1.10 3 μm 2 Up to 1.10 8 μm 2 Within the range, preferably within 1.10 3 μm 2 Up to 1.10 7 μm 2 Within the range and more preferably within 1.10 3 μm 2 Up to 1.10 6 μm 2 Within the range.

[0016] It is suggested that the voltage oriented opposite to the forward direction of the silicon solar cell is in the range of 1V to 30V.

[0017] In one embodiment, the silicon solar cell is single-sided, and the point light source is directed above the solar-active front side.

[0018] In another embodiment, the silicon solar cell is bifacial, and the point light source is directed above the solar-active front side and / or solar-active back side of the silicon solar cell.

[0019] One embodiment envisions the back contact of the bifacial silicon solar cell taking the form of a contact grid.

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

[0021] One embodiment envisions the silicon solar cell taking the form of a PERC cell or a TOPCon cell.

[0022] One embodiment envisions the contact grid of the silicon solar cell being composed of an electroplated metal layer. The method of the present invention improves the contact behavior of the electroplated contact grid. Specifically, when the silicon solar cell has a single-sided design, the contact resistance between the front contact grid and the emitter is improved (contact resistance is reduced). In the case of a double-sided design, the contact resistance between the back contact element, which takes the form of a contact grid on the back side, and the back surface field of the silicon solar cell can also be reduced.

[0023] Surprisingly, it has been found that the method according to the invention improves contact quality without damaging the silicon solar cell, even in the case of contact grids made from sintered metal paste. Specifically, the method according to the invention compensates for errors in process control during the sintering of the metal paste, thereby ensuring that the contact grid still achieves low contact resistance. Again, when the silicon solar cell has a single-sided design, the contact resistance between the front contact grid and the emitter is improved (contact resistance is reduced). In the case of a double-sided design, the contact resistance between the back contact, which takes the form of a contact grid on the back side, and the back surface field of the silicon solar cell can also be reduced.

[0024] The illuminated section of the sub-region may have a circular outer contour. However, the invention is not explicitly limited to this. In principle, the illuminated section of the sub-region may have any contour. For example, the outer contour may also form a rectangle or another polygonal shape. The corners of these shapes may also be rounded. Furthermore, linear sections within the sub-region are also possible.

[0025] The following explains embodiments of the present invention.

[0026] First, a crystalline single-sided silicon solar cell is prepared. This crystalline single-sided silicon solar cell has a silicon nitride anti-reflective layer on its solar active front side. The emitter layer of the silicon solar cell is arranged below the anti-reflective layer. A contact grid made of electroplated metal layers is applied to the solar active front side, which can be enhanced by printing and curing metal paste to increase conductivity. On the side facing away from the sun, the silicon solar cell is equipped with a back contact. This back contact consists of a planar metal layer.

[0027] The silicon solar cell is then contacted using a contact device, such that the contact grid on the front side is connected to one pole of a voltage source and the back contact is connected to the other pole. The voltage source is then applied with a voltage oriented opposite to the forward direction of the silicon solar cell and a magnitude lower than the breakdown voltage of the silicon solar cell. When this voltage is applied, a point light source is directed above the solar active surface of the silicon solar cell. This point light source illuminates a cross-section of a sub-region of the solar active front side of the silicon solar cell, thereby inducing a current in that sub-region. The current has a cross-section of 21,000 A / cm. 2 Up to 200,000 A / cm 2 The current density is applied to the sub-region for 10 nanoseconds to 10 milliseconds.

[0028] The high current density required to improve the ohmic contact behavior between the contact grid and the emitter layer can be specifically achieved by shifting the operating point of the illuminated cell region without causing radiation-induced material damage. The necessary current density can be achieved through the interaction between the radiation density of the radiation source on the cross section, the exposure time, and the applied voltage, without causing any radiation that could damage the material.

[0029] In another example embodiment, the method is applied to a bifacial silicon solar cell. First, a bifacial silicon solar cell is prepared. This bifacial silicon solar cell has an anti-reflective layer of silicon nitride on its solar-active front side. The emitter layer of the silicon solar cell is arranged below the anti-reflective layer. A contact grid made of an electroplated metal layer is applied to the solar-active front side. An additional contact grid containing an electroplated metal layer is applied to the solar-active back side. The contact grid applied to the solar-active back side forms the back contact of this silicon solar cell. Optionally, the electroplated metal layer can also be reinforced with printed and cured metal paste to increase conductivity.

[0030] The bifacial silicon solar cell is then contacted using a contact device, such that the contact grid on the solar-active front side is connected to one pole of a voltage source, and the contact grid formed on the solar-active back side (in this case, the back contact) is connected to the other pole of the voltage source. The voltage source then applies a voltage oriented opposite to the forward direction of the silicon solar cell and a value lower 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. When this voltage is applied, a point light source is directed above the solar-active front side and / or the solar-active back side of the silicon solar cell. This point light source illuminates a cross-section of a sub-region of the light-receiving surface of the silicon solar cell, thereby inducing a current in that sub-region. The current has a cross-section of 21,000 A / cm. 2 Up to 200,000 A / cm 2The current density is applied to the sub-region for 10 nanoseconds to 10 milliseconds.

[0031] In another embodiment of the method for a silicon solar cell with 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 manufacturer's instructions for the metal paste. After this type of silicon solar cell has been prepared, the subsequent steps of the method are performed as described above.

[0032] In another embodiment of the method for a bifacial silicon solar cell, the contact grids on the solar-active front side and / or on the solar-active back side are formed from a metal paste, which is applied, for example, by screen printing and then cured according to the manufacturer's instructions for the metal paste. After this type of silicon solar cell has been prepared, the subsequent steps of the method are performed as described above.

[0033] In all the described embodiments, the point light source is a laser, a light-emitting diode, or a (focused) white light source, but the invention is not limited thereto. The point light source has a power of 500 W / cm² relative to the illuminated cross-section. 2 Up to 1,000,000 W / cm 2 The power density is [not specified]. However, the present invention is not limited to these power densities used for illumination. Ideally, the point light source should emit radiation with wavelengths in the range of 400 nm to 1500 nm. The area of ​​the illuminated cross-section should ideally be [not specified]. 3 μm 2 Up to 1.10 8 μm 2 Within the range, preferably within 1.10 3 μm 2 Up to 1.10 7 μm 2 Within the range and more preferably within 1.10 3 μm 2 Up to 1.10 6 μm 2 Within the range. However, the method of the present invention also allows for deviations from these measurement areas. Ideally, the voltage oriented opposite to the forward direction of the silicon solar cell should be in the range of 1V to 30V (but not limited to this range).

[0034] In one embodiment, the illuminated cross-section of the sub-region has a circular outer contour. However, the invention is not explicitly limited to this. In other embodiments, the outer contour of the illuminated cross-section of the sub-region is rectangular or forms another polygonal shape. The corners of these shapes may also be rounded. Well-known beam optics with beam-shaping elements are typically used to form the outer contour of the illuminated cross-section of the sub-region. Similarly, the laser may also have multiple laser diodes, which are arranged, for example, in a rectangular shape or along a straight line. Another embodiment envisions the illuminated cross-section of the sub-region as linear.

[0035] For example, in the case of an illuminated cross-section with a surface diameter of approximately 150 μm, typically an applied voltage of 10 V and 20,000 W / cm² are used. 2 Up to 100,000 W / cm 2 Under certain illuminance, it generates a current in the range of 50 mA to 5,000 mA, resulting in approximately 21,000 A / cm² relative to the area of ​​the illuminated cross-section. 2 Up to 30,000 A / cm 2 The current density is low. The absolute current remains low, especially due to the relatively small area of ​​the illuminated cross section.

[0036] In another embodiment of the method according to the invention described above, the current has a relative current of between 40,000 A / cm² to the illuminated cross section. 2 With 200,000 A / cm 2 The current density between.

[0037] In another embodiment of the method according to the invention described above, the current has a relative current of 80,000 A / cm² to the illuminated cross section. 2 With 200,000 A / cm 2 The current density between.

[0038] In another embodiment of the above-described scheme of the method according to the invention, the current has a relative current of 120,000 A / cm² to the illuminated cross section. 2 With 200,000 A / cm 2 The current density between.

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

Claims

1. A method for improving the contact behavior of a contact grid in a silicon solar cell, characterized in that: First, prepare the silicon solar cell having the contact grid and the back contact, and then contact the silicon solar cell using a contact device, wherein the contact grid is electrically connected to one pole of a 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 that is oriented opposite to the positive direction of the silicon solar cell and is lower in magnitude than the breakdown voltage of the silicon solar cell; And when this voltage is applied, a point light source is directed above the solar active surface of the silicon solar cell, thereby illuminating a cross-section of a sub-region of the solar active surface and inducing a current in the sub-region, the current having a relative cross-section of 21,000 A / cm. 2 Up to 200,000 A / cm 2 The current density is applied to the sub-region for 10 nanoseconds to 10 milliseconds.

2. The method according to claim 1, characterized in that, The current has 40,000 A / cm relative to the illuminated cross section. 2 Up to 200,000 A / cm 2 The current density.

3. The method according to claim 1, characterized in that, The current has 80,000 A / cm relative to the illuminated cross section. 2 Up to 200,000 A / cm 2 The current density.

4. The method according to claim 1, characterized in that, The current has 120,000 A / cm² relative to the illuminated cross section. 2 Up to 200,000 A / cm 2 The current density.

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

6. The method according to any one of the preceding claims, characterized in that, The point light source exhibits a power density of 500 W / cm² on the cross-section. 2 Up to 1,000,000 W / cm 2 .

7. The method according to any one of the preceding claims, characterized in that, The point light source emits radiation with wavelengths ranging from 400 nm to 1500 nm.

8. The method according to any one of the preceding claims, characterized in that, The area of ​​the cross section is 1.10 3 μm 2 Up to 1.10 8 μm 2 Within the range, preferably within 1.10 3 μm 2 Up to 1.10 7 μm 2 Within the range and more preferably within 1.10 3 μm 2 Up to 1.10 6 μm 2 Within the range.

9. The method according to any one of the preceding claims, characterized in that, The cross-section of the sub-region has a circular or polygonal outer contour.

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

11. The method according to any one of claims 1 to 8, characterized in that, The cross section of the sub-region is linear.

12. The method according to any one of the preceding claims, characterized in that, The voltage oriented 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 preceding claims, characterized in that, The silicon solar cell is single-sided, and the point light source is directed above the solar active front side.

14. The method according to any one of claims 1 to 12, characterized in that, The silicon solar cell is bifacial, and the point light source is directed above the solar active front side and / or solar active back side of the silicon solar cell.

15. The method according to claim 14, characterized in that, The back contact of the bifacial silicon solar cell is in the form of a contact grid.

16. The method according to any one of the preceding claims, characterized in that, The one or more contact grids are made of sintered metal paste or electroplated metal layers.

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