Methods for improving the contacting of a silicon solar cell

Abstract

The invention relates to a method for improving the contacting of a silicon solar cell. The object of the invention is to improve the contacting of electroplated, metallic contact grids of a silicon solar cell.The problem is solved by first providing the silicon solar cell with the contact grid and a back contact, and by contacting the silicon solar cell with a contacting 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, and by applying a voltage directed against the forward direction of the silicon solar cell, which is less in magnitude than the breakdown voltage of the silicon solar cell, and by passing a point light source over a solar-active side of the silicon solar cell while this voltage is applied, thereby illuminating a section of a partial area of ​​the solar-active side, and thereby inducing a current flow in the partial area, and this current flow, relative to the section, has a current density of 21,000 A / cm. 2 up to 130,000 A / cm 2has and acts on the sub-area for 10 ns to 10 ms.

Description

[0001] The invention relates to a method for improving the contacting of a silicon solar cell.

[0002] Various approaches are known for contacting crystalline silicon solar cells. One approach involves applying a metal paste (usually a silver conductive paste) to the front of the silicon solar cell, which is provided with an anti-reflective layer (usually a silicon nitride layer), in the form of a contact grid using screen printing. After application, the metal paste is baked into the anti-reflective layer at 800–900°C, thus forming an electrical contact with the emitter layer. Such silicon solar cells are often also referred to as PERC cells. In a monofacial design, the back of these silicon solar cells is usually fully coated with another metal paste (usually an aluminum conductive paste) and also baked.In contrast, in a bifacial design, the back of the silicon solar cell is also coated with a metal paste (usually a silver conductive paste) in the form of a contact grid using screen printing and baked into the passivation layer on the back. The process control during baking of the metal pastes has a decisive influence on contact formation. Faulty process control leads to high contact resistances at the interface between the metal paste and the emitter layer of the silicon solar cell and / or, in the bifacial design, to high contact resistances between the metal paste and the back surface field of the silicon solar cell in question. High contact resistances can then result in reduced efficiency of the silicon solar cell.

[0003] Silicon solar cells are also known in a so-called TOPCon design. Here, conductor tracks are applied to the front of the silicon solar cells using metal paste (e.g., AlAg paste or pure Ag paste). The back of the silicon solar cells usually features a contact system consisting of Ag paste, polysilicon, tunnel oxide, and Si base.

[0004] In another approach to contacting silicon solar cells, the expensive screen-printed silver conductive pastes used to construct the contact grid are replaced with more cost-effective, electroplated metal layers (e.g., electroplated silver, copper, or nickel). To still achieve a low-resistance transition on the front and / or back of the silicon solar cell, the anti-reflective layer on the front or back is removed locally below the future contact grid before the metal layers are electroplated (usually using a laser followed by chemical etching of the oxide layers that form). The subsequent contact quality is crucially determined by the quality of the removal of the anti-reflective layer and / or the etching of the oxide layers that form.If the anti-reflective layer and / or formed oxide layers are not adequately removed, the contact quality is poor, resulting in high contact resistance between the contact grid and the emitter layer or between the contact grid and the back surface field.

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

[0006] In the prior art, DE 10 2018 001 057.1 proposes a method for improving the ohmic contact behavior between a contact grid and an emitter layer of a silicon solar cell. Here, the contact grid and the back of the silicon solar cell are electrically biased against their forward direction. The electrically biased silicon solar cell is then scanned with a point light source. This induces a current flow, with this current flow having a current density of 200 A / cm2 relative to the illuminated section. 2 up to 20,000 A / cm 2and acts on the partial area for 10 ns to 10 ms. This process compensates for faulty process control during the firing of the metal paste, so that the solar cells still achieve the optimal series resistance for their construction. This process has proven successful for solar cells with contact grids constructed from metal paste. In contrast, smaller improvements in contacting are often observed for solar cells with partially galvanically deposited contacts.

[0007] The object of the invention is to improve the contacting of galvanically deposited, metallic contact grids of a silicon solar cell.

[0008] This task is solved by first providing a silicon solar cell with the contact grid and a back contact, and electrically connecting the contact grid to one pole of a voltage source using a contacting device. The other pole of the voltage source is electrically connected to the back contact using the contacting device. A voltage is then applied to the voltage source, directed counter to the forward direction of the silicon solar cell and having a magnitude lower than the breakdown voltage of the silicon solar cell. When this voltage is applied, a point light source is then passed over a solar-active side of the silicon solar cell, illuminating a section of a partial area of ​​the solar-active side, thereby inducing a current flow in the partial area, and this current flow has a current density of 21,000 A / cm² relative to the section. 2 up to 200,000 A / cm 2and acts on the sub-area for 10 ns to 10 ms.

[0009] In an advantageous embodiment, the current flow has a current density of 40,000 A / cm 2 up to 200,000 A / cm 2 .

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

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

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

[0013] In one embodiment, the point light source on the cutout has a power density of 500 W / cm 2 up to 1,000,000 W / cm 2 on.

[0014] One embodiment provides that the point light source emits radiation with a wavelength in the range of 400 nm to 1500 nm.

[0015] In a further embodiment, the cutout has an area in the range of 1·10 3 µm 2 up to 1·10 6 µm 2 .

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

[0017] In one version, the silicon solar cell is monofacial and the point light source is guided over the solar-active front side.

[0018] In a further embodiment, the silicon solar cell is bifacial and the point light source is guided via the solar-active front side and / or solar-active back side of the silicon solar cell.

[0019] One embodiment provides that the back contact of the bifacial silicon solar cell is designed as a contact grid.

[0020] It is further proposed that the point light source is guided directly next to contact fingers of the contact grid over the solar-active side of the silicon solar cell.

[0021] One embodiment provides that the silicon solar cell is designed as a PERC cell or as a TOPCon cell.

[0022] One embodiment provides for the contact grid of the silicon solar cell to be constructed from galvanically deposited metal layers. The method according to the invention improves the contact behavior of the galvanically deposited contact grid. In particular, in a monofacial design of the silicon solar cell, the contact resistance between the contact grid on the front side and the emitter is improved (reduction in contact resistance). In a bifacial design, the contact resistance between the back contact, formed as a contact grid, on the rear side and the back surface field of the silicon solar cell can also be reduced.

[0023] Surprisingly, it has been shown that the method according to the invention leads to an improvement in contact quality even with contact grids made of fired metal pastes, without damaging the silicon solar cells. In particular, the method according to the invention compensates for faulty process controls during the firing of the metal paste, so that the contact grid still achieves a low contact resistance. Here, too, in the monofacial design of the silicon solar cell, the contact resistance between the contact grid on the front side and the emitter is improved (reduction in contact resistance). In the bifacial design, the contact resistances between the back contact, formed as a contact grid, on the rear side and the back surface field of the silicon solar cell can also be reduced.

[0024] Embodiments of the invention are explained below.

[0025] First, a crystalline, monofacial silicon solar cell is prepared. This has an anti-reflective layer made of silicon nitride on its solar-active front side. An emitter layer of the silicon solar cell is arranged beneath this anti-reflective layer. A contact grid composed of electroplated metal layers is applied to the solar-active front side, which can be reinforced with printed and cured 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 flat, metallic layer.

[0026] The silicon solar cell is then contacted using a contacting device, so 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 of the voltage source. A voltage is then applied to the voltage source, directed counter to the forward direction of the silicon solar cell and lower in magnitude than the breakdown voltage of the silicon solar cell. When this voltage is applied, a point light source is passed over the solar-active side of the silicon solar cell. This point light source illuminates a section of a portion of the solar-active front side of the silicon solar cell, inducing a current flow in that portion. The current flow has a current density of 21,000 A / cm² relative to the section. 2 up to 200,000 A / cm 2 and acts on the sub-area for 10 ns to 10 ms.

[0027] The high current densities required to improve the ohmic contact behavior between the contact grid and the emitter layer can be achieved, in particular, by shifting the operating point of the illuminated cell area without causing radiation-induced material damage. The interaction between the radiance of the radiation source on the cutout, the exposure time, and the applied voltage allows the necessary current densities to be achieved without causing material-damaging irradiation.

[0028] In a further embodiment, the method is applied to a bifacial silicon solar cell. First, the bifacial silicon solar cell is prepared. This has an anti-reflective layer made of silicon nitride on its solar-active front side. An emitter layer of the silicon solar cell is arranged beneath this anti-reflective layer. A contact grid composed of electroplated metal layers is applied to the solar-active front side. Another contact grid composed of electroplated metal layers is applied to the solar-active back side. This contact grid applied to the solar-active back side forms the back contact of this silicon solar cell. Optionally, the electroplated metal layers can also be reinforced with printed and cured metal paste to increase conductivity.

[0029] This bifacial silicon solar cell is also subsequently contacted using a contacting device, so 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 (here the back contact) is connected to the other pole of the voltage source. The voltage source then applies a voltage directed counter to the forward direction of the silicon solar cell, which is lower in magnitude than the breakdown voltage of the silicon solar cell. For example, the silicon solar cell is biased with a voltage directed counter to the forward direction of the silicon solar cell. When this voltage is applied, a point light source is guided over the solar-active front and / or solar-active back side of the silicon solar cell.This point light source illuminates a section of the sun-facing side of the silicon solar cell, inducing a current flow in that section. The current flow has a current density of 21,000 A / cm2 relative to the section. 2 up to 200,000 A / cm 2 and acts on the sub-area for 10 ns to 10 ms.

[0030] In a further embodiment of the method, the contact grid in a monofacial silicon solar cell is formed from a metal paste, which was applied, for example, by screen printing and then cured according to the manufacturer's instructions for the metal paste. After such a silicon solar cell has been prepared, the further process steps are carried out as described above.

[0031] In a further embodiment of the method, in a bifacial design of the silicon solar cell, the contact grid on the solar-active front side and / or the contact grid on the solar-active back side are formed from a metal paste, which was applied, for example, by screen printing and subsequently cured according to the manufacturer's instructions for the metal paste. After such a silicon solar cell has been prepared, the further process steps are carried out as described above.

[0032] In all described embodiments, the point light source is a laser, a light-emitting diode, or a (focused) white light source, without the invention being limited thereto. The point light source has a power density of 500 W / cm² relative to the illuminated section. 2 up to 1,000,000 W / cm 2However, the invention is not limited to these illumination power densities. Preferably, the point light source emits radiation with a wavelength in the range of 400 nm to 1500 nm. The illuminated section preferably has an area in the range of 1 10 3 µm 2 up to 1·10 6 µm 2 However, surfaces other than these are also possible with the method according to the invention. The voltage directed counter to the forward direction of the silicon solar cell is preferably (but not limited to) in the range of 1 V to 30 V.

[0033] For example, for an irradiated section with a diameter of approximately 150 µm and an applied voltage of 10 V and an illuminance of 20,000 W / cm 2 up to 100,000 W / cm 2Typically, currents with a strength of 50 mA to 5,000 mA are generated, so that, based on the area of ​​the irradiated section, a current density of the order of 21,000 A / cm 2 up to 30,000 A / cm 2 The absolute currents flowing are kept low, particularly due to the relatively small area of ​​the irradiated section.

[0034] In further embodiments of the above-described embodiments of the method according to the invention, the current flow has a current density of 40,000 A / cm 2 up to 200,000 A / cm 2 .

[0035] In further embodiments of the above-described embodiments of the method according to the invention, the current flow has a current density of 80,000 A / cm 2 up to 200,000 A / cm 2 .

[0036] In further embodiments of the above-described embodiments of the method according to the invention, the current flow has a current density of 120,000 A / cm 2 up to 200,000 A / cm 2 .

[0037] The described methods are applicable to silicon solar cells in PERC design and TOPCon design, although the invention is not limited to these cell types. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2018 001 057.1

[0006]

Claims

[1] Method for improving the contact behavior of a contact grid of a silicon solar cell, characterized bythat firstly the silicon solar cell is provided with the contact grid and a back contact and that the silicon solar cell is contacted with a contacting 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 and that a voltage directed counter to the forward direction of the silicon solar cell, which voltage is lower in magnitude than the breakdown voltage of the silicon solar cell, is applied with the voltage source and that when this voltage is applied a point light source is guided over a solar-active side of the silicon solar cell and in the process a section of a partial area of ​​the solar-active side is illuminated and that a current flow is thereby induced in the partial area and that this current flow has a current density of 21,000 A / cm 2 up to 200,000 A / cm 2and acts on the sub-area for 10 ns to 10 ms. [2] Method according to claim 1, characterized by that the current flow relative to the illuminated section has a current density of 40,000 A / cm 2 up to 200,000 A / cm 2 has. [3] Method according to claim 1, characterized by that the current flow in relation to the illuminated section has a current density of 80,000 A / cm 2 up to 200,000 A / cm 2 has. [4] Method according to claim 1, characterized by that the current flow relative to the illuminated section has a current density of 120,000 A / cm 2 up to 200,000 A / cm 2 has. [5] Method according to one of the preceding claims, characterized by that the point light source is a laser, a light-emitting diode or a focused white light source. [6] Method according to one of the preceding claims, characterized by that the point light source on the cutout has a power density of 500 W / cm2 up to 1,000,000 W / cm 2 has. [7] Method according to one of the preceding claims, characterized by that the point light source emits radiation with a wavelength in the range of 400 nm to 1500 nm. [8] Method according to one of the preceding claims, characterized by that the section has an area in the range of 1·10 3 µm 2 up to 1·10 3 µm 2 has. [9] Method according to one of the preceding claims, characterized by that the voltage opposite to the forward direction of the silicon solar cell is in the range of 1 V to 30 V. [10] Method according to one of the preceding claims, characterized by that the silicon solar cell is monofacial and the point light source is guided over the solar-active front side. [11] Method according to one of claims 1 to 9, characterized bythat the silicon solar cell is bifacial and the point light source is guided over the solar-active front side and / or solar-active back side of the silicon solar cell. [12] Method according to claim 11, characterized by that the back contact of the bifacial silicon solar cell is designed as a contact grid. [13] Method according to one of the preceding claims, characterized by that the contact grid or grids are constructed from fired metal pastes or from galvanically deposited metal layers. [14] Method according to one of the preceding claims, characterized by that the silicon solar cell is designed as a PERC cell or as a TOPCon cell.