Method for fabricating a solar module of rear contact solar cells using linear ribbon-type connector strips and respective solar module

Abstract

A solar module and a method for fabricating a solar module comprising a plurality of rear contact solar cells are described. Rear contact solar cells (1) are provided with a large size of e.g. 156156 mm.sup.2. Soldering pad arrangements (13, 15) applied on emitter contacts (5) and base contacts (7) are provided with one or more soldering pads (9, 11) arranged linearly. The soldering pad arrangements (13, 15) are arranged asymmetrically with respect to a longitudinal axis (17). Each solar cell (1) is then separated into first and second cell portions (19, 21) along a line (23) perpendicular to the longitudinal axis (17). Due to such cell separation and the asymmetrical design of the soldering pad arrangements (13, 15), the first and second cell portions (19, 21) may then be arranged alternately along a line with each second cell portion (21) arranged in a 180-orientation with respect to the first cell portions (19) and such that emitter soldering pad arrangements (13) of a first cell portion (19) are aligned with base soldering pad arrangements (15) of neighboring second cell portions (21), and vice versa. Simple linear ribbon-type connector strips (25) may be used for interconnecting the cell portions (19, 21) by soldering onto the underlying aligned emitter and base soldering pad arrangements (13, 15). The interconnection approach enables using standard ribbon-type connector strips (25) while reducing any bow as well as reducing series resistance losses.

Claims

1. A solar module comprising a plurality of first and second cell portions of rear contact solar cells arranged along a longitudinal axis, wherein each of the first and second cell portions comprises soldering pad arrangements on top of each of emitter contacts and base contacts which soldering pad arrangements each comprise one or more soldering pads arranged linearly and the soldering pad arrangements being arranged on the rear surface of the semiconductor substrate asymmetrically with respect to a longitudinal axis of the semiconductor substrate; wherein the plurality of first and second cell portions of the rear contact solar cells are arranged alternately along a line such that the second cell portions are arranged in a 180 orientation with respect to the first cell portions and such that soldering pad arrangements of emitter contacts and of base contacts of the first cell portions are aligned with soldering pad arrangements of base contacts and of emitter contacts of the second cell portions, respectively; and wherein the plurality of first and second cell portions of the rear contact solar cells are connected in series by linear ribbon-type connectors strips each being arranged on top of a linear soldering pad arrangement of an emitter contact of each first cell portion and on top of an aligned linear soldering pad arrangement of a base contact of a second cell portion neighboring the respective first cell portion on one side and by linear ribbon-type connector strips each being arranged on top of a linear soldering pad arrangement of a base contact of the respective first cell portion and on top of an aligned linear soldering pad arrangement of an emitter contact of a second cell portion neighboring the respective first cell portion on an opposite side, wherein no insulation layer is interposed between each of the connector strips and the emitter and base contacts, wherein the soldering pad arrangement of an emitter contact continuously extends from a first end arranged close but spaced to a first edge of a cell portion to a second end arranged at an opposite second edge of the cell portion, wherein the first end is spaced apart from the first edge by between 4 and 96% of the distance between the first and second edges.

2. The solar module of claim 1, wherein the rear contact solar cell is a metal wrap-through solar cell.

3. The solar module of claim 1, wherein each of the first and second cell portions is rectangular and has a size of more than 50100 mm.sup.2.

4. The solar module of claim 1, wherein metal fingers extend from a soldering pad arrangement of a base contact arranged on one side of the continuous soldering pad arrangement of an emitter contact via a gap between the continuous soldering pad arrangement of the emitter contact and the first edge to an region at an opposite side of the continuous soldering pad arrangement of the emitter contact.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, features and advantages of embodiments of the present invention will be described with respect to the enclosed drawings. However, neither the drawings nor the description shall be interpreted as limiting the invention.

(2) FIGS. 1 and 2 show top views on a rear side surface of rear contact solar cells which may be used for fabrication methods and solar modules according to embodiments of the present invention.

(3) FIG. 3 show a top view onto a rear side surface of specifically oriented cell portions for a solar module to be fabricated according to an embodiment of the present invention.

(4) FIG. 4 shows a top view on a rear side surface of interconnected solar cells of a solar module according to an embodiment of the present invention.

(5) FIGS. 5 and 6 show top views on a rear side surface of cell portions to be used for a fabrication method and a solar module according to alternative embodiments of the present invention.

(6) The figures are only schematically and not to scale. Same or similar features are designated with same reference signs throughout the figures.

DESCRIPTION OF PREFERRED EMBODIMENTS

(7) Embodiments of a fabrication method and a solar module according to the present invention shall be described in the following with regard to an exemplary embodiment of metal wrap-through (MWT) solar cells. However, the proposed method and solar module may also be applied to other rear contact solar cells such as e.g. emitter wrap-through (EWT) solar cells, interdigitated back contact (IBC) solar cells, etc.

(8) Various techniques and approaches for fabricating MWT solar cells and interconnecting a plurality of such MWT solar cells for fabricating a solar module have been developed. An overview may be found for example in Florian Clement: Die Metal Wrap Through SolarzelleEntwicklung undCharakterisierung (electronically published on http://www.freidok.uni-freiburg.de/volltexte/6832/).

(9) One of the main problems of state-of-the-art MWT solar cell technology is the complexity and cost of manufacturing a module. Frequently, additional foils carrying complex patterns of printed metal structures thereon are used for interconnecting neighboring solar cells within a solar module. For conventional ribbon-type interconnection, either additional process steps such as application of insulating layers have to be applied or specific non-linear ribbons have to be used or an amount of ribbons for interconnecting emitter and base contacts is uneven. While applying additional insulating layers or using complex shaped ribbon-type interconnectors may add to processing complexity and costs, the provision of an uneven number of ribbons interconnecting emitter and base contacts may result in non-homogeneous distribution of electrical current flow within the solar cell, finally resulting in reduced solar cell efficiency.

(10) Furthermore, while using ribbon-type connector strips for interconnecting solar cells in a module in principle may include many advantages like using well-established technology for soldering such connector strips to soldering pads of the solar cell, cheap availability of simple connector strips, etc., using ribbon-type connector strips on rear contact solar cells of a large size such as the currently common standard size of 156 mm156 mm may result in tremendous bow induced by the ribbons. The metal material of the ribbons and the semiconductor material of the solar cell significantly differ in their thermal expansion coefficient. During a soldering process, temperatures of around 200 C. may be reached, resulting in thermally induced stress when cooling down. Due to such mechanical stress, the semiconductor substrate may significantly bend to a concave form. The induced bowing is proportional, inter alia, to the length of the ribbon-type connector strips, their cross-section and to a contact area between the strips and the solar cell. The induced bowing may be mainly responsible for mechanical yield losses during solar module fabrication. For example, in standard size solar cells of 156156 mm.sup.2, using ribbon-type connector strips of dimensions of 2 mm0.1 mm may result in an excessive bow of more than 4 mm. For connector strips having even larger cross-section of 3.5 mm0.3 mm, as they can be beneficially used for reducing serial resistances in the connector strips, excessive bowing of even more than 9 mm may be observed. However, in standard solar module fabrication, a bow of 2-3 mm is regarded to be the maximum allowable in mass production to avoid yield loss due to breakage during ribbon stringing and lamination.

(11) Accordingly, prior to the present invention, is was assumed that using ribbon-type connector strips for interconnecting rear contact solar cells was no option for solar module fabrication from large sized solar cells. This is particularly true as MWT solar cells typically provide 2-3% higher electrical currents compared to standard solar cells with busbars on the front side such that any reduction of the cross-section of the ribbon-type connector strips would result in even severe series resistance problems.

(12) With the fabrication method as well as the solar module proposed herein, the above-mentioned problems may be solved or at least significantly relaxed. The proposed approach allows using simple linear ribbon-type connector strips for interconnecting rear contact solar cells. While the entire solar cell may be produced with a large size thereby enabling using established high-through-put industrial solar cell processing, it is proposed to applying a specific asymmetrical pattern of soldering pad arrangements on the rear side surface of a semiconductor substrate and separating each rear contact solar cell into at least two cell portions before arranging the cell portions in an alternating manner and in an alternating orientation and finally soldering linear ribbon-type connector strips onto the aligned soldering pad arrangements of neighboring cell portions. Thereby, both the number of connector strips per cell portion as well as the length of connector strips may be reduced thereby reducing any bowing of the semiconductor substrate upon cooling-off after soldering the connector strips to the soldering pad arrangements.

(13) Furthermore, as the size of the cell portions is significantly smaller, preferably half the size of the non-separated rear contact solar cells, the electrical current produced by each cell portion is smaller than in entire rear contact solar cells. Accordingly, power losses due to serial resistance within the connector strips may be reduced by a factor of 4.

(14) FIGS. 1 and 2 are top views onto rear surfaces of rear contact solar cells with a 2-busbar and 3-busbar design as they may be used for fabricating a solar module according to an embodiment of the present invention.

(15) A square semiconductor substrate 3 of an MWT rear contact solar cell 1 has a size of 156 mm156 mm. Such MWT solar cell 1 comprises emitter contacts not only on a front surface but emitter contacts are also lead through through-holes 5 to the rear surface of the semiconductor substrate 3. In small areas adjacent to these through-holes 5, soldering pads 9 are arranged on the rear surface of the semiconductor substrate 3. Both the front side emitter contacts as well as the soldering pads 9 contacting the emitter contacts 5 lead through the through-holes towards the rear side of the substrate 3 may be applied using e.g. screen-printing technologies and using e.g. silver-containing screen-printing pastes.

(16) The remainder of the rear surface of the semiconductor substrate 3 apart from the areas of the soldering pads 9 contacting the emitter contacts 5 is covered with a base contact 7 and/or a back surface field layer (BSF). Both the base contacts 7 as well as the back surface field layer may be applied e.g. by screen-printing an aluminium-containing paste onto the entire rear surface of the semiconductor substrate 3 except for the regions of the emitter soldering pads 9. As an aluminium layer may not be soldered, soldering pads 11 comprising a solderable material such as a silver-aluminum compound are arranged on the base contacts 7 locally.

(17) Both the single soldering pad 9 contacting the emitter contacts 5 as well as the multiple soldering pads 11 contacting the base contact 7 form soldering pad arrangements 13, 15 having a linear geometry, i.e. extending along a straight line. Furthermore, as shown in the figures, the linear soldering pad arrangements 13, 15 may extend parallel to a longitudinal axis 17 running through the center of the semiconductor substrate 3.

(18) The soldering pad arrangements 13, 15 are arranged asymmetrical with respect to the longitudinal axis 17. In other words, when mirroring one of the soldering pad arrangements 13, 15 at the longitudinal axis 17, there is no corresponding soldering pad arrangement 13, 15 at the mirrored position but, to the contrary, there is a soldering pad arrangement 15, 13 of the other type at this position.

(19) When fabricating a solar module from a plurality of rear contact solar cells 1 as shown in FIG. 1 or 2, after processing the entire solar cell 1 in its original large size and applying soldering pads 9, 11 at the desired locations, each solar cell 1 is separated into two cell portions 19, 21 along a line 23 perpendicular to the longitudinal axis 17 of the semiconductor substrate 3. Preferably, the separating line 23 is positioned at the middle of the semiconductor substrate 3 such that the two cell portions 19, 21 are halves of the original solar cell 1 and have same sizes.

(20) The solar cell 1 may be separated by first generating a linear trench along the separating line 23 using e.g. a laser. Such separating trench may not go through the entire thickness of the semiconductor substrate 3 but may have a depth of e.g. between 10 and 100 m. Subsequently, the solar cell 1 may be broken along this trench wherein the trench serves as a predetermined breaking line.

(21) While such separation process using a laser-scribed trench and subsequently mechanically breaking the substrate 3 along this trench appears to provide advantages when incorporated into an industrial scale fabrication procedure, other techniques for separating the solar cell 1 such as sawing, etching, etc. may be applied.

(22) After having separated the solar cell 1 into first and second cell portions 19, 21, these first and second cell portions are alternately arranged along a line as schematically shown in FIGS. 3 and 4. Therein, every first cell portion 19 is arranged in a first orientation and the second cell portions 21 arranged at opposite sides of the first cell portion 19 are arranged in an opposite orientation, i.e. rotated by 180.

(23) Due to such alternating arrangement and orientation of first and second cell portions 19, 21 and due to the specific asymmetrical design of the soldering pad arrangements 13, 15 arranged on the emitter contacts 5 and the base contacts 7, respectively, the cell portions 19, 21 may be arranged such that a soldering pad arrangement 13 of emitter contacts on a first cell portion 19 may be linearly aligned with a soldering pad arrangement 15 of base contacts on a neighboring second cell portion 21, and vice versa, as shown in FIG. 3.

(24) Accordingly, as shown in FIG. 4, linear ribbon-type connector strips 25 may be arranged on top of the aligned soldering pad arrangements 13, 15 of neighboring first and second cell portions 19, 21 and electrically connected thereto for example by a soldering procedure. The connector strip 25 may be a simple linear ribbon as used conventionally for interconnecting solar cells having emitter contacts on a front side and base contacts on a rear side. The ribbons may have a highly conductive copper core enclosed by a solderable material such as silver.

(25) FIGS. 5 and 6 visualize electrical current flow densities within the base on the rear side of a cell portion 17, 19.

(26) As may be seen in FIGS. 1 and 2, the soldering pad arrangements 13 of emitter contacts continuously extend from a first end arranged close to a first edge 27 of the semiconductor substrate 3 to a second end close to a second edge 31 of the semiconductor substrate 3. However, the soldering pad arrangement 13 does not reach directly to the first and second edges 27, 31 but the first and second ends of the soldering pad arrangement 13 is spaced apart from these edges 27, 31 by a certain distance thereby forming a gap 33 between the end of the soldering pad arrangement 13 and the associated edge 27, 31. However, the soldering pad arrangement 13 extends from the first end continuously via a center region to the second end such that it crosses the separation line 23.

(27) Accordingly, as shown in FIGS. 5 and 6, the soldering pad arrangement 13 of the emitter contacts has one end thereof spaced apart from an upper first edge 23 by a gap 33, but reaches a lower opposite edge 29 of the cell portion 17, 19 directly, i.e. without any gap.

(28) Such design of the emitter soldering pad arrangement 13 may have two effects. First, a connector strip 25 arranged on top of the emitter soldering pad arrangement 13 as shown in FIG. 4 has no risk of getting into electrical contact with portions of the base or base contact of the respective cell portion. Thus, there is no risk of short-circuiting. Second, electrical current generated within an area adjacent to the emitter soldering pad arrangement 13 where there are no base soldering pads 11 may flow through the gap 33 towards the base soldering pad arrangement 15 as shown with the arrows 37 in FIG. 5. Accordingly, there is no need of providing base soldering pad arrangements 15 on both sides of an emitter soldering pad arrangement 13.

(29) In order to further improve current collection within the base of the solar cell, additional metal fingers 39 extending from a soldering pad arrangement 15 of a base contact into an area 35 may be provided. Accordingly, these metal fingers 39 extend from the base soldering pad arrangement 15 arranged on the right side of the continuous emitter soldering pad arrangement 13 via the gap 33 into the region 35 at the opposite left side of the continuous emitter soldering pad arrangement 13, thereby shortening any current paths as visualized with the arrows 41. The metal fingers 39 may have a significantly higher electrical conductivity than the base contact or the back surface field provided at the rear surface of the semiconductor substrate 3.

(30) Summarized, the proposed fabrication method and solar module enables cheap and simple cell interconnection using standard linear ribbon-type connector strips while minimizing any bowing of the semiconductor substrate 3 as well as minimizing series resistance losses. A key feature of embodiments of the present invention may be seen in separating a large size rear contact solar cell 1 into portions 17, 19, e.g. by cutting into halves, and providing a specific asymmetrical design for soldering pad arrangements thereby enabling that resulting first and second cell portions 17, 19 may be arranged alternately and aligned with each other such that linear ribbon-type connector strips 25 may be soldered onto associated soldering pad arrangements 13, 15.

(31) It shall be noted that embodiments of the present invention are described herein only with respect to the substantial features and processing steps. One skilled in the art realizes that, in a fabrication method, further processing steps may be added or some of the described processing steps may be replaced by equivalent processing steps for fabricating the solar cell. Similarly, one skilled in the art realizes that the proposed solar cell module may comprise further features and components additional to the features described herein or as equivalent replacements.

(32) Finally, it should be noted that the term comprising does not exclude other elements or steps and the a or an does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

LIST OF REFERENCE SIGNS

(33) 1 rear contact solar cell 3 semiconductor substrate 5 emitter contacts 7 base contacts 8 emitter soldering pad 11 base soldering pads 13 emitter soldering pad arrangement 15 base soldering pad arrangement 17 longitudinal axis 19 first cell portion 21 second cell portion 23 separation line 25 linear ribbon-type connector strip 27 first edge 29 second edge of cell portion 31 second edge of solar cell 33 gap 35 region without base soldering pad arrangement 37 current flow arrows 39 metal fingers 41 current flow arrows