Method for improving the ohmic contact behavior between the contact grid and the emitter layer of a silicon solar cell

Abstract

A method for improving ohmic contact behavior that further minimizes the impact on the material caused by sun-side radiation.SOLUTION: Initially the emitter layer, the contact grid and the rear contact are provided, wherein the contact grid is electrically connected to one pole of a power source, wherein a contact device connected to the other pole of the power source is connected to the rear contact, wherein by means of the power source a voltage below the breakdown voltage of the silicon solar cell is applied counter to the forward direction of the silicon solar cell, and wherein upon application of this voltage a point light source is guided onto the sunward side of the silicon solar cell, thereby irradiating the cross-section of a subsection of the sunward side, As a result, a current is induced in the subsection, and the current for the cross section has current densities of 200A / cm2 to 20, 000A / cm2, and acts on the subsection between 10ns and 10ms.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for improving the ohmic contact behavior between the contact grid and the emitter layer in a silicon solar cell. [Background technology]

[0002] When creating contacts to crystalline solar cells, a metal paste in the form of a contact grid is applied using screen printing techniques to the front surface of the cell, which is coated with a dielectric silicon nitride. After application, the metal paste is fired at 800-900°C to form the silicon nitride, forming electrical contact to the emitter layer. Process control during firing of the metal paste has a significant effect on the contacts that are formed; therefore, improper process control can result in high contact resistance at the transition between the metal paste and the emitter layer in the silicon solar cell. High contact resistance can reduce the efficiency of the silicon solar cell.

[0003] In the prior art, methods are known that allow for stabilizing the efficiency or improving the performance of solar cells. DE 10 2011 056 843 A1, for example, describes a method for "Stabilizing the efficiency of silicon solar cells." In this document, during the lamination process, a continuous flow of current is applied to the solar cell assembly, which essentially destroys the boron-oxygen complexes in the silicon material.

[0004] US 4,166,918 A proposes a method for improving the performance of solar cells, in which the solar cells are subjected to a voltage applied in the reverse direction of the forward direction, whereby a current is stimulated along a short circuit in the solar cell, causing the short circuit to be "burned out" and removed. These known methods have no known effect on the transition between the contact grid and the emitter layer of the silicon solar cell.

[0005] To form a low-resistance electrical contact to the emitter layer, a low film resistance (less than 100 Ω / sq) is required for the emitter layer. However, this results in insufficient conversion of short-wavelength light into electricity. Better conversion is achieved with a sheet resistance in the range of 110-150 Ω / sq. However, only a relatively high-impedance transition between the contact grid and the emitter layer can be created through a conventional bake-in process. To circumvent this, the selective emitter concept has been developed (e.g., EP 2 583 315 B1). Here, areas of the emitter layer that are subsequently printed with a metal paste are locally highly doped, thereby locally lowering the sheet resistance. However, this requires an expensive additional step in the process of manufacturing silicon solar cells.

[0006] In the field of electronic components, it is known from DD250247A3 that the ohmic contact behavior to semiconductor bodies contacted by a conductive adhesive can be improved by applying a voltage pulse. The operating mode of the contact improvement is not described in detail in this document. The conductive adhesive used for the contact essentially consists of conductive particles, usually silver balls or silver flakes surrounded by a polymer matrix.

[0007] German patent application DE102016009560.1, which has not yet been published, proposes a method for improving the ohmic contact behavior between the contact grid and the emitter layer in silicon solar cells. In this case, the contact grid of the silicon solar cell is in contact with a contact pin matrix, and a voltage pulse generates a current between the rear contact of the solar cell and the contact pin matrix. A pulse duration of 1 ms to 100 ms and an induced current 10 to 30 times the short-circuit current of the silicon solar cell can reduce the high contact resistance between the contact grid and the emitter layer, thereby correcting incorrect process control, for example, during the baking of a metal paste. Alternatively, a method is described in which an electrically biased silicon solar cell is scanned with a point light source, generating a current in the illuminated subsection with a short-circuit current density 10 to 30 times that of the silicon solar cell. Depending on the type and quality of the silicon solar cell, in some configurations, irradiating the sun-facing side of the silicon solar cell can cause undesirable effects on the material, even damaging it. Summary of the Invention

[0008] The aim of this invention is to develop a method for improving the ohmic contact behaviour between the contact grid and the emitter layer of a silicon solar cell so as to further minimize the impact on the material caused by irradiation of the sun-facing side, and furthermore, this method is applicable to silicon solar cells whose emitter layer has a high sheet resistance.

[0009] This objective is achieved by first providing a silicon solar cell having an emitter layer, a contact grid, and a rear contact, and electrically connecting the contact grid to one pole of a power source. The other pole of the power source is electrically connected to a contact device disposed on the rear contact. The power source then applies a voltage in the forward and reverse directions of the silicon solar cell that is lower than the breakdown voltage of the silicon solar cell. When this voltage is applied, a point light source is then guided to the solar face side of the silicon solar cell, illuminating a cross section of a subsection of the solar face side in the process. Thus, a current of 200 A / cm2 is applied to that section. 2 ~20,000A / cm 2 A current is induced in the subsection having a current density of 0.1 to 1.0 ns and acts on the subsection for 10 ms.

[0010] The method according to the present invention compensates for process control errors during firing of the metal paste, so that the solar cell achieves optimal series resistance. Furthermore, the method according to the present invention achieves very good ohmic contact between the contact grid and the emitter layer, even when using an emitter layer with a high film resistance, eliminating the process steps required to form selective emitters. Furthermore, by using the method according to the present invention, the firing process can be performed at a lower temperature, saving energy in the process of manufacturing silicon solar cells.

[0011] It is proposed that the point light source is a laser, a light emitting diode, or a flash lamp. In one embodiment, the point light source has a cross-sectional area of ​​500 W / cm. 2 ~200,000W / cm 2 One version contemplates that the point source emits radiation at wavelengths in the range of 400 nm to 1500 nm. In a further embodiment, the cross section is 10 3 μm 2 〜10 4 μm 2The silicon solar cell has an area in the range of 1 V to 20 V. It is proposed that the forward and reverse voltages of the silicon solar cell are in the range of 1 V to 20 V. It is further proposed that the point light source is guided immediately adjacent to the contact fingers of the contact grid on the sun-facing side of the silicon solar cell. In one version, the silicon solar cell has a single-sided or double-sided form. In another version, the silicon solar cell has an n-doped or p-doped silicon substrate. One embodiment contemplates that the emitter layer has a sheet resistance greater than 100 ohms / sq. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the invention is described below. First, a crystalline silicon solar cell is prepared. It has an anti-reflection layer of silicon nitride on the solar-facing side. The emitter layer of the silicon solar cell is placed under this anti-reflection layer. On the solar-facing side, a front metallization is printed in the form of a contact grid consisting of collection contacts (busbars) and contact fingers made from a commercially available metal paste (e.g., silver paste), which is cured according to the manufacturer's specifications and baked onto the silicon nitride layer. On the side of the silicon solar cell opposite the solar-facing side, a rear contact is provided. This rear contact consists of a metal layer that can be designed with or without passivation (PERC concept).

[0013] The contact grid is electrically connected to one pole of a power supply. The other pole of the power supply is connected to a contact device connected to the rear contact. The power supply then applies a voltage in the forward direction of the silicon solar cell and in the reverse direction, which is lower than the breakdown voltage of the silicon solar cell. When this voltage is applied, a point light source is guided to the solar-face side of the silicon solar cell. The point light source can be, for example, a laser, a light-emitting diode, or a focused beam of a flash lamp. However, the invention is not limited to these radiation sources. The point light source emits radiation having a wavelength in the range of 400 nm to 1500 nm. A cross section of a subsection of the solar-face side of the silicon solar cell is illuminated by this point light source, thereby inducing a current in the cross section. A current of 200 A / cm2 associated with the cross section is 2 ~20,000A / cm 2 and acts on the subsection for a period of 10 ns to 10 ms.

[0014] The high current densities required to improve the ohmic contact behavior between the contact grid and the emitter layer can be achieved by shifting the operating point of the irradiated cell area without radiation-induced material damage. In an interaction between the radiation density of the cross-sectional radiation source, the contact time, and the applied voltage, the required current density is achieved without the need for material-damaging radiation. For an emitting cross-section with a surface diameter of about 60 μm, currents with magnitudes of 50 mA to 600 mA are typically generated with an applied voltage of 10 V, resulting in a current of about 200 A / cm2 based on the area of ​​the emitting cross-section. 2 ~20,000A / cm 2 The full current is kept low, especially due to the relatively small area receiving the light beam.

[0015] Essentially, when scanning the sun-facing side of a silicon solar cell, it is sufficient for the point source to move directly to the left and right of the contact fingers. Thus, the processing time for processing a 6" cell with the method according to the present invention is approximately 1 second.

[0016] In a further embodiment, the method according to the invention is applied to silicon solar cells in which the contact grid has been baked at a lower temperature than that recommended by the metal paste manufacturer. Typically, baking is performed at approximately 800°C. If the metal paste is baked at a temperature of, for example, only 700°C, the silicon solar cell has a high contact resistance at the transition between the contact grid and the emitter layer. Even with this type of silicon solar cell, the method according to the invention has been shown to improve the ohmic contact behavior between the contact grid and the emitter layer. When the method according to the invention is combined with a baking process performed at a lower temperature, the same contact resistance at the transition between the contact grid and the emitter layer is achieved, while simultaneously saving energy.

[0017] The method according to the invention is applicable to both single-sided and double-sided silicon solar cells, in the latter case where treatment of only one side is sufficient to optimize the contacts on both sides.

[0018] In another embodiment, this process is applied to silicon solar cells that do not have a selectively formed emitter layer and therefore have a high sheet resistance (greater than 100 Ω / sq) across their entire surface. As described above, these silicon solar cells are printed with a metal paste and then subjected to a baking process, which can be performed according to the manufacturer's instructions or at a lower temperature. After the baking process, the silicon solar cells only have a relatively high contact resistance at the transition between the contact grid and the emitter layer. By applying the method according to the present invention, the contact resistance is also reduced in these silicon solar cells due to the interaction of the radiation density of the cross-sectional radiation source, the contact time, and the applied voltage, reducing the value required for optimal operation of the silicon solar cells. Therefore, selective emitters are not required, and the costly steps required for their manufacture can be omitted.

Claims

1. In a process for improving ohmic contact behavior between a contact grid and an emitter layer in a silicon solar cell, the silicon solar cell is first provided with an emitter layer, a contact grid, and a rear contact, the contact grid is electrically connected to one pole of a power supply, a contact device connected to the other pole of the power supply is connected to the rear contact, a voltage less than a breakdown voltage of the silicon solar cell is applied to the silicon solar cell in a forward direction and a reverse direction, and when this voltage is applied, a point light source is guided to the solar face side of the silicon solar cell, thereby irradiating a cross section of a subsection on the solar face side, thereby inducing a current in the subsection, the current in the cross section being 200 A / cm ２ ~20,000A / cm ２ and acting on said subsection for 10 ns to 10 ms.

2. The method of claim 1 , wherein the point light source is a laser, a light emitting diode, or a flash lamp.

3. The point light source has a cross-sectional area of ​​500 W / cm ２ ~20,000W / cm ２ 10. The method of claim 1, wherein the method demonstrates a power density of

4. The method of claim 1 , wherein the point light source emits radiation having a wavelength in the range of 400 nm to 1500 nm.

5. The cross section is 10 ３ μm ２ to 10 ４ μm ２ The method of claim 1 , having an area in the range of

6. 2. The method of claim 1, wherein the forward and reverse voltages of the silicon solar cell are in the range of 1V to 20V.

7. 2. The method of claim 1, wherein the point light source is guided on the sun-facing side of the silicon solar cell, immediately adjacent to the contact fingers of the contact grid.

8. The method of claim 1 , wherein the silicon solar cell is single-sided or double-sided.

9. The method of claim 1 , wherein the silicon solar cell is constructed from an n-doped or p-doped silicon substrate.

10. The method of claim 1 , wherein the emitter layer has a sheet resistance greater than 100 ohms / sq.