Method for improving the ohmic contact between a front-side contact grid and a doped layer of a wafer solar cell
Patent Information
- Application Number
- EP2024713904
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-02-20
- Filing Date
- 2024-02-20
- Publication Date
- 2025-12-31
AI Technical Summary
High contact resistances between the front contact grid and the doped layer in wafer solar cells lead to reduced efficiency due to inhomogeneous voltage distribution and higher voltage losses, as existing methods apply reverse voltage on the sun-facing side, causing uneven current flow and resistance variations.
Applying a reverse voltage on the back side of the wafer solar cell, which has a higher degree of metallization and conductivity, to induce a current flow, thereby reducing electrical resistance and voltage losses, and improving the ohmic contact between the front contact grid and the doped layer.
This method results in a more homogeneous current distribution and lower voltage losses, enhancing the efficiency of the wafer solar cell by optimizing the contact resistance and metallization patterns on both sides.
Smart Images
Figure DE2024100135_29082024_PF_FP_ABST
Abstract
Description
[0001] Method for improving the ohmic contact between a front-side contact grid and a doped layer of a wafer solar cell
[0002] The invention relates to a method for improving the ohmic contact between a front-side contact grid and a doped layer of a wafer solar cell.
[0003] Depending on the process used to manufacture the wafer solar cell with the emitter layer in the form of a doped layer and the front-side contact grid, high contact resistances can occur in certain areas at the interface between the metal paste used to create the front-side contact grid and the doped layer. Such excessive contact resistances regularly lead to reduced efficiency of the wafer solar cell.
[0004] DE 10 2016 009 560 A1 and DE 10 2018 001057 A1 disclose methods for improving the ohmic contact between the front contact grid and the doped layer formed as an emitter layer. First, a silicon wafer solar cell is provided, comprising the doped layer intended to function as an emitter, a front contact grid, and a rear contact. The front contact grid is then electrically connected to one pole of a voltage source, and a contacting device electrically connected to the other pole of the voltage source is connected to the rear contact. A voltage directed counter to the forward direction of the silicon wafer solar cell, which voltage is lower in magnitude than the breakdown voltage of the silicon wafer solar cell, is applied to the voltage source. When the voltage is applied, a point light source is guided over the sun-facing side of the silicon wafer solar cell.In this process, a section of the sun-facing side is illuminated, thereby locally inducing a current flow in that section. This current flow, relative to the section, has a current density of 200 A / cm. 2 up to 20,000 A / cm 2 and acts on the partial area for 10 ns to 10 ms. However, as the current flows toward the solar cell contacts, a portion of the electrical voltage drops due to resistance. A contact point close to the contacting device thus experiences different effective process parameters than a contact point farther away from the contacting device. The applied reverse voltage, which has a significant influence on the quality of this process, is thus distributed inhomogeneously across the wafer solar cell. As a result of the process, this results in a wafer solar cell with reduced efficiency.
[0005] It is an object of the invention to provide a method for improving the ohmic contact between a front-side contact grid and a doped layer of a wafer solar cell, by means of which the wafer solar cell is provided with improved efficiency.
[0006] According to the invention, the object is achieved by a method having the features of patent claim 1. Advantageous further developments and modifications are specified in the subclaims.
[0007] The invention relates to a method for improving the ohmic contact between a front contact grid and a doped layer of a wafer solar cell, comprising the following steps:
[0008] - Providing the wafer solar cell with the doped layer, the front contact grid and a back contact grid,
[0009] - electrical contacting of the front contact grid with a contacting device electrically connected to a pole of a voltage source,
[0010] - electrical contacting of another contacting device electrically connected to the other pole of the voltage source with the rear contact grid,
[0011] - applying a voltage directed opposite to the forward direction of the wafer solar cell to the front contact grid and the rear contact grid by means of the voltage source, wherein the applied voltage is lower in magnitude than the breakdown voltage of the wafer solar cell,
[0012] - guiding a point light source over the rear side of the wafer solar cell facing away from the sun while the voltage is applied, whereby a section of a partial area of the rear side facing away from the sun is illuminated so that a current flow is induced in the partial area and acts on the partial area.
[0013] In contrast to the methods described in DE 10 2016 009 560 and DE 10 2018 001057 A1, the point light source is guided not over the sun-facing front side of the wafer solar cell, but over the sun-facing back side of the wafer solar cell. It has been found that this results in a surprisingly effective improvement in the ohmic contact between the front contact grid and the doped layer, thereby providing a wafer solar cell with improved efficiency.
[0014] In a preferred embodiment, the front-side contact grid and the rear-side contact grid each cover the front and rear sides of the wafer solar cell with a degree of metallization, respectively, wherein the rear-side contact grid has a higher degree of metallization than the front-side contact grid. The degree of metallization refers to the area covered by the conductor surface in relation to the remaining free areas. This applies to the respective side, i.e. the front side facing the sun or the rear side facing away from the sun. The greater the degree of metallization of the conductor surface, the lower the current generated in a solar cell. The front side facing the sun, which normally generates approximately 90% of the cell output, usually contains less conductor surface in the form of contact fingers and busbars than the rear side facing away from the sun in order to minimize shading on the main current generation side of the wafer solar cell.Therefore, the conduction path of the reverse current to the light point of the point light source is usually longer and varies more than on the rear side facing away from the sun with a higher degree of metallization. The lower degree of metallization on the front side facing the sun leads to higher resistances and thus to higher voltage losses at the light point generated by the point light source when the reverse voltage is applied. The electrical resistance of a current generated by a light point close to the electrical contact is comparatively low and leads to a correspondingly low voltage loss. However, the resistance for a current generated by a light point further away from the electrical contact is significantly higher and leads to a significant voltage loss. Therefore, an inhomogeneous result results for this method when the front side facing the sun is illuminated with the lower degree of metallization.When the back side facing away from the sun is illuminated when voltage is applied, the higher degree of metallization leads to lower resistance and thus to lower voltage losses for the reasons described above. The electrical resistance of an electric current generated by a light point close to the electrical contact is very small and leads to a very low voltage loss. The electrical resistance of an electric current generated by a light point or a surface illumination farther away from the electrical contact is somewhat larger and accordingly leads to a somewhat greater voltage loss. As a result, the process leads to a more homogeneous result when the back side facing away from the sun is illuminated and to a higher efficiency for the wafer solar cell.
[0015] Preferably, the front contact grid and the rear contact grid each have an electrical conductivity, with the rear contact grid having a higher electrical conductivity than the front contact grid. The advantages correspond to those mentioned above regarding the degree of metallization.
[0016] In a preferred embodiment, the front contact grid and the rear contact grid each have a sheet resistance in the material on the front and rear sides, respectively, with the rear contact grid having a lower sheet resistance in the material on the rear side than the front contact grid on the front side. The advantages correspond to those mentioned above regarding the degree of metallization.
[0017] Preferably, the rear side contact grid has a higher degree of metallization, a higher conductivity than the front side contact grid and a lower sheet resistance in the material on the rear side than the front side contact grid on the front side.
[0018] Preferably, the front side contact grid has a plurality of front side contact fingers arranged parallel to one another and at least one front side busbar arranged transversely to the plurality of front side contact fingers.
[0019] Alternatively or additionally, the rear contact grid preferably has a plurality of rear contact fingers arranged parallel to one another and at least one rear busbar arranged transversely to the plurality of rear contact fingers. The front busbar is preferably arranged perpendicular to the plurality of front contact fingers. The rear busbar is preferably arranged perpendicular to the plurality of rear contact fingers.
[0020] In a preferred embodiment, the front-side contact fingers are arranged at a distance from one another that is greater than the distance between the rear-side contact fingers. This allows for a metallization level of the rear side facing away from the sun to be provided that is higher than the metallization level of the front side facing the sun.
[0021] Preferably, the number of rear-side contact fingers is greater than the number of front-side contact fingers. This is a further possibility of providing a degree of metallization on the rear side facing away from the sun that is higher than the degree of metallization on the front side facing the sun. In a preferred embodiment, the wafer solar cell is textured on the front side, and the rear side of the wafer solar cell is smoothed with an average surface roughness of < 2 μm. The term "textured" is understood to mean that the surface has an average surface roughness of > 2 μm. While pyramid-like structures are preferably formed on the front side surface, the rear side is preferably smoothed such that its surface only has truncated pyramids. The texturing reduces the penetration / illumination effectiveness of the light beam or light spot generated by the point light source.Therefore, it is more effective to illuminate the wafer solar cell from the back, away from the sun, when applying the voltage. While the incoming light is coupled in with less reflection by the texture, it is also highly scattered. This is advantageous when the wafer solar cell is in operation. However, in this process, the generated current flows over a larger area when the voltage is applied and the illumination is applied.
[0022] This results in a larger effective area, and the electrical contact properties of several contact fingers are sometimes influenced simultaneously. Therefore, the illumination process on the textured front side cannot be controlled as well when voltage is applied as on the smoothed back side.
[0023] Preferably, the front side of the wafer solar cell has a greater surface roughness than the back side. The difference in surface roughness is usually caused by the front side texturing. During illumination of the back side, with its lower surface roughness, a larger portion of the incident light is reflected compared to the front side due to the lower roughness, but it is less scattered within the wafer solar cell. The resulting current therefore flows over a smaller area. The effective range is thus smaller. Therefore, the process can be controlled more precisely.
[0024] Preferably, the method is carried out with the following parameters: Preferably, a voltage is applied by means of the voltage source to the front contact grid and the rear contact grid in the opposite direction to the forward direction, which voltage is in the range of 1 to 40 V. Preferably, the local illumination has a power density in the range of 200 to 500,000 W / cm 2 The process is preferably carried out so that a current of 0.1 to 10 A flows between the front and rear contacts.
[0025] The wafer solar cell provided in the process preferably has the following parameters:
[0026] Preferably, the wafer solar cell provided has a contact resistance of > 50 mOhmcm before carrying out the method according to the invention 2measured using the TLM method (transfer length method). In a preferred embodiment, the wafer solar cell provided has a contact area of less than 0.1% prior to carrying out the method according to the invention. This means that the metallized area of the metal-semiconductor contacts corresponds to less than 0.1% of the surface on which the metal-semiconductor contacts are located. The wafer solar cell provided preferably has one or more passivation and / or anti-reflection layers. The anti-reflection layer preferably has a thickness of more than 100 nm. The anti-reflection layer is made of, for example, SiN x (silicon nitride). The anti-reflection layer can also be SiN x (Silicon nitride) / SiO x N y (Silicon oxynitride) double layer or SiN x (Silicon nitride) / SiO x N y(Silicon oxynitride) / SiO2 (silicon dioxide) triple layer with thicknesses greater than 100 or greater than 110 nm. Preferably, the wafer solar cell provided has a higher sheet resistance of the doped layer on the front side than on the back side.
[0027] Preferably, the wafer solar cell is a bifacial solar cell.
[0028] A bifacial solar cell has the ability to utilize light incident on both the sun-facing front surface and the back surface, which faces away from the sun, to generate electricity. A monofacial solar cell, on the other hand, can only utilize light incident on the sun-facing front surface to generate electricity. Furthermore, the bifacial solar cell's incompletely metallized back surface allows for backlighting.
[0029] Alternatively, the wafer solar cell is designed as a subcell of a multi-junction solar cell. The multi-junction solar cell has at least two subcells. These subcells each have a specific p / n junction. The subcells are formed from different materials layered one above the other. The multi-junction solar cell therefore has an upper subcell facing the light, a lower subcell, and optionally one or more middle subcells arranged between the upper and lower subcells. Preferably, the wafer solar cell provided in the method according to the invention is the lower subcell of the multi-junction solar cell, which is subsequently provided with an upper subcell and optionally middle subcell(s) after carrying out the method according to the invention.A multi-junction solar cell can be a mechanically stacked multi-junction solar cell, in which the sub-cells are manufactured separately and then interconnected, or a monolithic multi-junction solar cell, in which all sub-cells are constructed on the same substrate with their layered structure, for example, by diffusion or layer deposition. The multi-junction solar cell is also provided with at least one contact each for its front and back sides.
[0030] The multi-junction solar cell can, for example, have a silicon solar cell as the lower sub-cell, which is subjected to the method according to the invention. The lower sub-cell is, for example, a p-type PERC (Passivated Emitter and Rear Cell) sub-cell, a heterojunction sub-cell, an n-type TOPCon (Tunnel Oxide Passivated Contact) sub-cell, or an IBC (Interdigitated Back Contact) sub-cell. The front and / or back contact grid can be optically transparent, preferably made of TCO (transparent conductive oxides) such as ITO (Indium Tin Oxide). The front and / or back contact grid can, however, also be made of metal, for example silver. The upper sub-cell can, for example, be made of a perovskite sub-cell.
[0031] In a preferred embodiment, the point light source is guided directly next to the backside contact fingers of the backside contact grid across the backside of the wafer solar cell facing away from the sun. This further allows the production of a wafer solar cell with improved efficiency. The term "direct" refers to a distance of less than two, preferably less than one, millimeters.
[0032] The point light source can be, for example, a laser, a light-emitting diode, or the focused radiation of a flash lamp. The point light source preferably emits radiation with wavelengths in the range of 400 nm to 1500 nm. The point light source is preferably a laser. The beam from a laser or laser diode can be well shaped, focused, and directed.
[0033] Further advantages and properties of the method are explained using the preferred embodiments described below. The figures are not drawn to scale, but are to be understood purely schematically and as examples.
[0034] They show schematically and not to scale:
[0035] Fig. 1 is a plan view of a front side of a wafer solar cell undergoing a prior art method;
[0036] Fig. 2 is a plan view of a back side of the wafer solar cell shown in Fig. 1, which is subjected to a method according to the invention;
[0037] Fig. 3 is a cross-sectional view of the wafer solar cell shown in Fig. 1;
[0038] Fig. 4 is a cross-sectional view of the wafer solar cell shown in Fig. 2;
[0039] Fig. 5 is a partial cross-sectional view of the wafer shown in Fig. 3 and 4 respectively.
[0040] Solar cell. Fig. 1 shows a plan view of a front side of a wafer solar cell undergoing a prior art method. The wafer solar cell 1 has a front side contact grid 2 comprising a plurality of front side contact fingers 21 arranged parallel to one another and at least one front side busbar 22 arranged transversely, in particular perpendicularly, to the plurality of front side contact fingers 21. The front side contact fingers 21 are arranged at a distance d1 from one another.
[0041] The wafer solar cell 1 shown in Fig. 1 is subjected to a process for improving the ohmic contact between a front-side contact grid 2 and a doped layer (not shown) of the wafer solar cell 1. The process comprises the following steps
[0042] - Providing the wafer solar cell 1 with the doped layer, the front contact grid 2 and a rear contact grid (not shown in Fig. 1),
[0043] - electrical contacting of the front contact grid 2 with a contacting device 4 electrically connected to a pole of a voltage source 9,
[0044] - electrically contacting a further contacting device (not shown in Fig. 1) electrically connected to the other pole of the voltage source 9 with the rear contact grid,
[0045] - applying a voltage directed opposite to the forward direction of the wafer solar cell 1 to the front contact grid 2 and the rear contact grid by means of the voltage source 9, the applied voltage being lower in magnitude than the breakdown voltage of the wafer solar cell 1,
[0046] - Guiding a point light source (not shown in Fig. 1) over the sun-facing front side of the wafer solar cell 1 while the voltage is applied, wherein a section of a partial area of the sun-facing front side is illuminated such that a current flow is induced in the partial area and acts on the partial area. Two light points 8a, 8b generated by the point light source are shown purely as an example. An electrical resistance 5a for the current flow generated by light point 8a near the contacting device 4 is relatively low and thus leads to a relatively low voltage loss. However, the electrical resistance 5b for the current generated by light point 8b further away from the contacting device 4 is significantly higher, as indicated by the thickness of the line, and inevitably leads to a high voltage loss.
[0047] Fig. 2 shows a plan view of a back side of the wafer solar cell shown in Fig. 1, which is subjected to a method according to the invention. The plan view shown in Fig. 2 corresponds to the plan view shown in Fig. 1 with the difference that it has a back side contact grid 3 consisting of a multiplicity of back side contact fingers 31 arranged parallel to one another and at least one back side busbar 32 arranged transversely, in particular perpendicularly, to the multiplicity of back side contact fingers 31. The back side contact fingers 31 are arranged at a distance d2 from one another which is significantly smaller than the distance d1 shown in Fig. 1. In addition, the number of back side contact fingers 31 is greater than the number of front side contact fingers shown in Fig. 1. The degree of metallization of the back side contact grid 3 is therefore greater than the degree of metallization of the front side contact grid shown in Fig. 1.
[0048] Using the wafer solar cell 1 shown in Fig. 2, a method is performed to improve the ohmic contact between a front-side contact grid 2 and a doped layer (not shown in Fig. 2) of the wafer solar cell 1. The method comprises the following steps:
[0049] - Providing the wafer solar cell 1 with the doped layer, the front contact grid (not shown in Fig. 2) and a rear contact grid 3,
[0050] - electrically contacting the front contact grid with a contacting device (not shown in Fig. 2) electrically connected to one pole of a voltage source 9, - electrically contacting a further contacting device 4 electrically connected to the other pole of the voltage source 9 with the rear contact grid 3,
[0051] - applying a voltage directed counter to the forward direction of the wafer solar cell 1 to the front contact grid and the rear contact grid 3 by means of the voltage source 9, the applied voltage being lower in magnitude than the breakdown voltage of the wafer solar cell 1,
[0052] - guiding a point light source (not shown) over the rear side of the wafer solar cell 1 facing away from the sun while the voltage is applied, wherein a section of a partial area of the rear side facing away from the sun is illuminated, so that a current flow is induced in the partial area and acts on the partial area.
[0053] Fig. 2 also shows, purely by way of example, two light points 8a, 8b generated by the point light source. When the rear side facing away from the sun is illuminated when the voltage is applied, the higher degree of metallization leads to lower electrical resistances 5a, 5b of the induced current and thus to lower voltage losses. The resistance 5a for a light point 8a near the contact is very small and accordingly leads to a very low voltage loss. The resistance 5b for the light point 8b, which is slightly further from the contact, is low and leads to a correspondingly low voltage loss. The difference between the electrical resistances is smaller compared to the front side, and thus the process leads to a more homogeneous result when the rear side facing away from the sun is illuminated.
[0054] Fig. 3 shows a cross-sectional view of the wafer solar cell shown in Fig. 1. As explained in Figs. 1 and 2, the wafer solar cell 1 has the front contact grid 2 with the front contact fingers 21 and the front busbar 22, and the rear contact grid 3 with the rear contact fingers 31 and the rear busbar 32. The contacting device 4 electrically contacts the front contact grid 2, while the further contacting device 4 electrically contacts the rear contact grid 3, wherein in Fig. 3 the latter electrically contacts the rear side over a large area. The point light source (not shown) is guided across the sun-facing front side of the wafer solar cell 1 while the voltage is applied. The resistance 5b for the current induced by the light point 8b is comparatively high, as indicated by the thickness of the line.
[0055] Fig. 4 shows a cross-sectional view of the wafer solar cell shown in Fig. 2. The wafer solar cell shown in Fig. 4 corresponds to the cross-sectional view shown in Fig. 3 with the difference that the contacting device 4 electrically contacts the rear contact grid 3, while the further contacting device 4 electrically contacts the front contact grid 2, whereby in Fig. 4 the latter electrically contacts the front side over a large area. The point light source (not shown) is guided over the rear side of the wafer solar cell 1 facing away from the sun while the voltage is applied. The resistance 5b for the current generated by the light point 8b is significantly lower compared to the front-side illumination, as is illustrated by the thickness of the horizontal line.
[0056] Fig. 5 shows a partial cross-sectional view of the wafer solar cell shown in Fig. 3 and 4 respectively. The rear contact grid 3 has been omitted for the sake of clarity, but the front contact fingers 21 of the front contact grid 2 are shown. The front surface is comparatively rough due to texturing, while the rear surface is smoothed and thus less rough in comparison. Fig. 5 shows that a point light source 6 illuminates the front of the wafer solar cell 1 with a light beam 7 and another point light source 6 illuminates the rear of the wafer solar cell 1 with another light beam 7, the resulting effects being illustrated. Although the incident light beam 7 is coupled into the semiconductor material by the texture with fewer reflection losses, it is also spatially scattered more strongly there.However, when the front of the wafer solar cell 1 is illuminated with the point light source 6 and the voltage is applied, the induced current flows through a larger solid angle. As a result, its effective area is more distributed and, in some cases, several contact fingers 21 are processed simultaneously; however, the induced current is also distributed across several contact fingers 21. This reduces the current strength per contact finger 21. Therefore, the process cannot be controlled as well when the textured front is illuminated when the voltage is applied. When the back of the wafer solar cell 1 is illuminated and the voltage is applied, a larger portion of the light is reflected at the interface due to the lower roughness of the back. However, it is subsequently scattered less strongly inside the semiconductor material of the wafer solar cell. The induced current flows through a smaller solid angle. The effective area is therefore more focused.Therefore, this process can be better controlled and leads to a more homogeneous result.
[0057] List of reference symbols: d1 distance d2 further distance
[0058] 1 wafer solar cell
[0059] 2 front contact grids
[0060] 21 front contact fingers
[0061] 22 front busbar
[0062] 3 Rear contact grid
[0063] 31 Back - Contact fingers
[0064] 32 rear busbars
[0065] 4 Contacting device
[0066] 5a, 5b resistance
[0067] 6 Point light source
[0068] 7 light beam
[0069] 8a, 8b light point
[0070] 9 Voltage source
Claims
Patent claims: 1 . Method for improving the ohmic contact between a front contact grid (2) and a doped layer of a wafer solar cell (1), comprising the following steps - Providing the wafer solar cell (1) with the doped layer, the front contact grid (2) and a rear contact grid (3), - electrical contacting of the front contact grid (2) with a contacting device (4) electrically connected to a pole of a voltage source (9), - electrical contacting of a further contacting device electrically connected to the other pole of the voltage source (9) (4) with the rear contact grid (3), - applying a voltage directed counter to the forward direction of the wafer solar cell (1) to the front contact grid (2) and the rear contact grid (3) by means of the voltage source (9), the applied voltage being lower in magnitude than the breakdown voltage of the wafer solar cell (1), - guiding a point light source (6) over the rear side of the wafer solar cell (1) facing away from the sun while the voltage is applied, wherein a section of a partial area of the rear side facing away from the sun is illuminated so that a current flow is induced in the partial area and acts on the partial area.
2. Method according to claim 1, characterized in that the front side contact grid (2) and the rear side contact grid (3) each cover the front and rear sides of the wafer solar cell (1) with a degree of metallization, have an electrical conductivity and have an electrical sheet resistance in the material on the front and rear sides, wherein the rear side contact grid (3) has a higher degree of metallization than the front side Contact grid (2) has a higher electrical conductivity than the front contact grid and / or has a lower electrical sheet resistance in the material on the back than the front contact grid (2) on the front.
3. Method according to claim 1 or 2, characterized in that the front side contact grid (2) has a plurality of front side contact fingers (21) arranged parallel to one another and at least one front side busbar (22) arranged transversely to the plurality of front side contact fingers (21) and / or the rear side contact grid (3) has a plurality of rear side contact fingers (31) arranged parallel to one another and at least one rear side busbar (32) arranged transversely to the plurality of rear side contact fingers (31).
4. Method according to claim 3, characterized in that the front-side contact fingers (21) are arranged at a distance (d1) from one another which is greater than a further distance (d2) at which the rear-side contact fingers (31) are arranged from one another.
5. Method according to claim 3 or 4, characterized in that a number of the rear contact fingers (31) is greater than a number of front contact fingers (21).
6. Method according to one of the preceding claims, characterized in that the front side of the wafer solar cell (1) has a greater surface roughness than the back side of the wafer solar cell (1).
7. Method according to one of the preceding claims, characterized in that a voltage is applied by means of the voltage source to the front contact grid and the rear contact grid against the forward direction, which voltage is in the range of 1 to 40 V, that the local illumination has a power density which in the Range from 200 to 500,000W / cm 2 and / or that a current of 0.1 to 10A flows between the front and rear contacts while the voltage and the lighting are applied 8. Method according to one of the preceding claims, characterized in that the wafer solar cell (1) is a bifacial solar cell or is formed as a sub-cell of a multiple solar cell.
9. Method according to one of the preceding claims, characterized in that the point light source (6) is guided directly next to rear contact fingers (31) of the rear contact grid (3) over the rear side of the wafer solar cell (1) facing away from the sun.
10. Method according to one of the preceding claims, characterized in that the point light source (6) is a laser.