WO2010058012A2

WO2010058012A2 - Method for improving light trapping of series connected thin film solar cell devices

Info

Publication number: WO2010058012A2
Application number: PCT/EP2009/065665
Authority: WO
Inventors: Jens Guenster; Clau Maissen
Original assignee: Oerlikon Solar IP AG
Current assignee: TEL Solar Services AG
Priority date: 2008-11-24
Filing date: 2009-11-23
Publication date: 2010-05-27
Anticipated expiration: 2011-05-24
Also published as: WO2010058012A3

Abstract

The inventions relates to a method for improving the efficiency of converting light into electrical current of a solar module (1), said solar module (1) comprising photoelectrically inactive areas (9, 10) and photoelectrically active areas formed by a solar cell (11), characterized in that light approaching said inactive area (9, 10) is at least partially directed into said active area via at least one structural optical element (13, 14, 15).

Description

METHOD FOR IMPROVING LIGHT TRAPPING OF SERIES CONNECTED THIN

FILM SOLAR CELL DEVICES Technical Field

[0001] The present invention relates to a method for improving the efficiency of converting light into electrical current of a solar module and a respective solar module; in particular to a method for improving the light trapping of a solar module comprising an (areal) semiconductor thin film formed on a substrate which is divided or segmented into plural regions, in the following referred to as cells. Preferably, the thin film solar module comprises an amorphous and / or microcrystalline silicon film having a PIN (or NIP) junction structure arranged in parallel to the thin film surface. The PIN/NIP structures are sandwiched between transparent film electrodes, which are continuously extending in each of said plurality of regions on one main surface of a substrate, e. g. a light transmissive substrate, also referred to as superstrate.

[0002] Despite the known advantages of forming thin film solar modules comprising series connected cells, active area of the module is lost in the cells' contact and insulation zone or in the rim area of the solar modules where e.g. a framework supports the solar module. Said inactive zone cannot contribute to the conversion of the incoming light to an electrical current. The invention describes technologies for directing light, approaching said inactive zone, into the thin films modules active area.

Background Art

[0003] Various solar cell technologies are commercially available today. Among those, thin film solar cells that employ thin amorphous and/or microcrystalline silicon films are actively being developed. The possibility to process that cell at low temperatures and on large areas, e.g. > 1 m², places this technology as a good candidate to achieve the so called grid parity.

[0004] Such large area thin film photovoltaic modules with an area in a range up to a few square meters are typically divided into a plurality of series connected cells, e.g. by laser scribing.

[0005] In case of laser scribing for the manufacture of series connected thin film solar cells said inactive zone has a width of typically 300 - 600 μm.

[0006] Typically widths of individual cells are between 2 mm to 20 mm. The ratio between the active area and the inactive zone between adjacent cells or in the rim area lost for converting light into electrical current by dividing the module in a plurality of cells is given by the cells average width and the average width of said inactive zones. The ratio between active area and said zone is typically in the range from 4:1 to 50:1.

Disclosure of Invention

[0007] It is therefore the object of the invention to provide a method, solar module and system which is capable to at least reduce the loss of efficiency of the overall solar module caused by said inactive areas.

[0008] This object is achieved by the independent claims. Advantageous embodiments are detailed in the dependent claims.

[0009] Disclosed herein are apparatus, system, and methods for manufacturing a thin film solar cell. Such apparatus, system, and method can be used for a wide range of applications such as for manufacturing solar cells that convert solar into electrical energy. When such apparatus, system, and methods are used during a manufacturing process for thin film solar modules comprising series connected solar cells, they have the advantage of increasing the solar modules efficiency of converting light into electrical current. Solar cells manufactured by the disclosed apparatus, system, and methods according to the invention are thin film solar cells employing a thin amorphous and/or microcrystalline silicon film or a combination of those, as a photoelectric conversion layer.

[0010] Often, a solar module as described above is divided into a passive periphery close to the solar modules rim and an active inner region divided into a plurality of series connected cells. The separation of the active inner region into a plurality of series connected cells results in the generation of inactive area between adjacent cells. The invention provides ways for directing light approaching said inactive area via optical elements into the active area of the solar module. These optical elements can be formed via structuring the surface of the solar module or by adding material to the solar module at the side opposite to the photovoltaic thin films system, i.e. the side the light to be converted mainly approaches.

[0011] The invention relates to a method for improving the efficiency of converting light into electrical current of a solar module formed by a solar cell, said solar module comprising photoelectrically inactive and photoelectrically active areas, which method is characterized in that light approaching said inactive area is at least partially is directed into said active area via at least one structural optical element.

[0012] In one embodiment, the light approaching the inactive area is directed into an active area by light diffraction, light scattering, and/or light reflection.

[0013] In another embodiment, the at least one structural optical element is arranged in a plane parallel to the region of the inactive area of the solar module.

[0014] In a further embodiment the structural optical element is formed by a groove, an optical element, and/or a light scattering layer. In another embodiment, the optical element comprises an elevated optical element.

[0015] Preferably, the inactive area in the meaning of the invention is a framework of the solar module and/or an area between two adjacent solar cells of the solar module.

[0016] In a further embodiment, the solar module comprises a substrate, the substrate is arranged such that light approaching said inactive area passes the substrate and the structural optical element is arranged on a surface of the substrate.

[0017] Furthermore, the invention relates to a solar module, said solar module comprising a photoelectrically inactive area and a photoelectrically active area, wherein the active area is formed by a solar cell, which solar module is characterized in that it comprises at least one structural optical element directing light, which approaches the inactive area, into said solar cell.

[0018] As state before, the structural optical element can be arranged in a plane parallel to the region of the inactive area and may be formed by a groove, an elevated optical element, and/or a light scattering layer. [0019] In one embodiment, when the structural optical element is formed by a groove, the structural optical element comprises a hemispherical, spherical, rounded, triangular, or polygonal shape.

[0020] In another embodiment, the solar module comprises a substrate, the substrate is arranged such that light approaching said inactive area passes the substrate and the structural optical element is arranged on a surface of the substrate.

[0021] Furthermore, the invention relates to a system for converting light into electrical current, said system comprising a solar module having at least one photoelectrically inactive area and at least one photoelectrically active area, wherein the photoelectrically active area is formed by a solar cell comprising a photoelectric conversion semi-conductor, characterized in that the system comprises at least one structural optical element by which light approaching the photoelectrically inactive area is at least partially directed into the solar cells, wherein the structural optical element with respect to the main direction of incidence of the incoming light is located above the inactive area in a plane parallel to the region of the inactive area, and wherein the size of the structural optical element is such that it overlaps or covers at least partially the path of incident light directed to said inactive area.

[0022] In another embodiment, the photoelectrically inactive area comprises grooves establishing a monolithic photovoltaic module composed of a number of solar cells electrically connected in series and/or the photoelectrically inactive area is caused by a framework supporting the solar module.

[0023] In a further embodiment, the structural optical element is a groove, an optical element, and/or a light scattering layer.

[0024] In an alternate embodiment, the solar module comprises a substrate, the substrate is arranged such that light approaching said inactive area passes the substrate and the structural optical element is arranged on a surface of the substrate.

[0025] According to the invention, a solar module is equipped with at least one structural optical element, arranged in a plane parallel to the region of the inactive area. Said plane may be adjacent to or distant from the inactive area. The size of said optical element is chosen such that it overlaps or covers at least partially the path of incident light directed to the inactive area. The functionality of the optical element allows a scattering, deflecting, or bending of light approaching the inactive area into the photoelectrically active areas of the solar module.

[0026] Such optical element may be realized as a groove being machined e. g. by mechanically structuring, laser ablating, etching or embossing a surface of solar module which is averted from the photoelectrically active layer, i.e. the surface side of the solar module the light mainly approaches. This groove may be realized e. g. as an elongated channel in parallel to at least one inactive area of the module. The number of sidewalls may be two or more, thereby forming a groove having a triangular or polygonal shape. Even a chamfer with at least partially spherical or rounded shape is possible.

[0027] Alternatively an elevated optical element may be foreseen to be placed in the same area as the grooves. Such elevated optical element may have, e.g. a hemispherical, triangular or polygonal shape. It can be realized as a strip to be glued or welded onto the respective surface of the solar module the light mainly approaches, or form an integral part of said surface. Also a transparent foil with embossed or elevated elements as described could be employed on the surface of solar module. If the foil already exhibits grooves or elevations at the periodic interval of the inactive zones on the cell, it can be applied "at once" instead of individually to each zone. If such a foil has been equipped with a hydrophobic and/or dirt-repellent surface, this will also be beneficial for the long term efficiency of the solar cell. Such a foil could also be used to be "wrapped" around the edges and thus contribute to the edge isolation.

[0028] Further alternatively, a light scattering layer or layer stack can be foreseen. To do so, at least one thin layer of optically active material, e. g. realized as a vacuum coating on selected areas, determined according to rules as described above, will allow deflecting of light away from inactive areas and into photoelectrically active areas. Alternatively an overall coating can be applied on solar modules surface and subsequently be patterned and etched, e.g. as it is known for structuring large area substrates in LCD panel manufacturing.

[0029] Further alternatively, the thin film module can be initially deposited on a very thin glass substrate. This substrate is then bonded to a thicker glass sheet supporting the thin glass substrate. Optical active elements in the thicker glass substrate can be used for reflecting light away from the inactive areas and into the photoelectrical^ active areas of the thin film module.

[0030] Such optical element may be realized as a groove being machined e. g. by mechanically structuring, laser ablating, etching or embossing a surface thicker glass substrate. The groove may be coated with a light reflecting and/or scattering material.

[0031] Further alternatively, also a combination of different optical elements to direct the light approaching the inactive area into the solar modules active area is possible, like e.g. using a groove to direct the light approaching an inactive area in the rim area of a solar module into the active area of a solar cell a an elevated optical element to direct the light approaching an inactive area in the central area of the solar module into the active area of a solar cell.

[0032] Structural optical elements as described above may be applied before, during or after application of the photoelectric layer stack. Proper alignment of said structural optical elements with said inactive areas is important for the effectiveness of the invention, therefore the laser scribing of grooves will align with said structural optical elements, if the latter have been applied before. Alternatively the application of the structural optical elements can align with an already manufactured layer stack of the solar module or parts thereof.

[0033] An approach for generating a structural element is via laser ablation / evaporation. In one particular embodiment a Trumpf RF CO₂ Laser, available from Trumpf Laser und Systemtechnik, Ditzingen, Germany, was used as the light source. The laser beam generated by said laser source can be focused onto the glass substrate surface opposite to the one supporting a thin film solar module, divided in a plurality of series connected cells. Between the cells an inactive zone, not able to convert light into electrical current, exists. The projection of said inactive zone to the side of the glass substrate not supporting the thin film solar module can be treated by a focused CO₂ laser beam. CO₂ lasers are emitting light with a wavelength of about 10 μm. Almost all glasses strongly absorb laser energy in the 10 μm wavelength region. In most cases, the absorption is strong enough that light is being absorbed completely within the uppermost 10-20 μm thickness. If sufficient energy is provided, material from the glass substrate surface can be evaporated locally by a focussed CO₂ laser beam.

[0034] The CO₂ laser is operated at continuous wave mode at a power output of up to e.g. 3000 Watts. The focused laser beam is scanned over the glass substrate surface via a galvanoscanner from Scanlab AG Puchheim, Germany. By evaporating material from the glass surface, a groove is formed on the substrate surface opposite to the one supporting the thin film module, in an area projection of said inactive zone to the side of the glass substrate not supporting the thin film solar module.

Brief Description of Drawings

[0035] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. [0036] In the drawings: [0037] Fig. 1 shows an arrangement of layers and laser-scribed grooves according to Prior Art in cross-section,

[0038] Fig. 2 shows the cell structure of Fig. 1 on a module level in a top view and [0039] Fig. 3 shows several embodiments according to the invention.

Detailed Description of the Drawings

[0040] Fig. 1 is a schematic cross section of a portion of a conventional thin film solar module 1 according to the state of the art. On a transparent insulator substrate 2 a transparent (front) electrode layer 3 is being arranged; a photoelectric conversion semiconductor 4 is formed on said transparent (front) electrode layer 3 and a further transparent (back) electrode layer 5 on said photoelectric conversion semiconductor 4. The photoelectric conversion semiconductor 4 comprises a thin amorphous and / or a microcrystalline silicon film stack.

[0041] Fig. 1 further shows grooves 6, 7 and 8. The purpose of this structuring is to establish a monolithic photovoltaic module composed of a number of solar cells 11 electrically connected in series. The transparent electrode layer 3 is divided by a first isolation groove 6, which determines the cell width. The photoelectric conversion semiconductor layer 4 is filling these grooves when the overall layer stack is being build up in the order: Layer 3 - groove 6 - layer 4 - groove 7 - layer 5 - groove 8 during the manufacturing process.

[0042] The groove 7, filled with material from transparent back electrode layer 5 permits the electrical contact between the adjacent cells. In fact the back electrode of one cell contacts the front electrode of the adjacent cell. The back surface electrode layer 5 and the photoelectric conversion semiconductor 4 are finally divided by a third isolation groove 8. This structuring process is achieved preferably by employing a laser light or the like. The zone 10 comprising the area between groove 6 and groove 8 plus the area of groove 6 and 8 is lost for the conversion of light into electrical current. In case of laser scribing for the manufacture of series connected thin film solar cells said zone has a width of typically 300 - 600 μm.

[0043] The thin film solar module 1 can be fabricated for example as follows: Initially over a transparent insulator substrate 2, a transparent electrode layer 3 is deposited e. g. by means of LPCVD (low pressure CVD). This transparent electrode layer 3, also called transparent conductive oxide TCO (e. g. ZnO, SnO2, Indiumtinoxide ITO) is thereafter laser-scribed to remove a portion of said transparent electrode layer 3 to form a first isolation groove 6 dividing the transparent electrode layer 3 into a plurality of thus isolated, adjacent layers. Subsequently, over this patterned transparent electrode layer 3, plasma CVD is employed to deposit a photoelectric conversion layer stack 4. [0044] Said layer stack comprises at least one p doped layer, an intrinsic i-layer and an n-doped layer of e. g. thin amorphous silicon. This operation can be repeated in order to form a multi junction amorphous silicon thin film solar cell. Thus additional p-i-n junctions can be formed from microcrystalline materials or a mix from amorphous and microcrystalline materials in order to establish said photoelectric conversion semiconductor layer 4. The photoelectric conversion semiconductor layer 4 is then laser-scribed in order to remove a portion of the photoelectric conversion semiconductor layer 4 to form a groove 7, in the following referred to as contact line, dividing the photoelectric conversion semiconductor layer 4 into a plurality of such isolated layers. Subsequently, the back surface electrode layer 5 is deposited to fill the groove 7 and thereby resulting into a contact line and also to cover photoelectric conversion semiconductor layer 4. This back surface electrode layer 5 can again be a transparent conductive oxide TCO (e. g. ZnO, SnO₂, Indiumtinoxide ITO).

[0045] Finally, the photoelectric conversion semiconductor layer 4 and back-surface electrode layer 5 are laser-scribed forming the second isolation groove 8 that divides the photoelectric conversion semiconductor layer 4 into a plurality of photoactive layers electrically connected in series. The thin film solar cell 1 shown in Fig. 1 is thus fabricated. Typically further process steps follow, such as for providing an electrical contact to the thin film solar cell, building up a back reflector layer and protecting the thin film solar cell by laminating protective layers and substrates. Such solar modules may comprise a framework 9 in the rim area of the module which supports the structure of the module and/or is used for mounting the solar module.

[0046] Fig. 2 is a top view on a module as described in Fig. 1. Large area thin film photovoltaic modules with an area in a range up to a few square meters are typically divided into a plurality of series connected cells as illustrated in Fig. 2.

[0047] Fig. 3 shows different embodiments (a - c) according to the invention.

[0048] As shown as embodiment a, the optical element 13 directing light approaching the photoelectrically inactive area 10 into the photoelectrically active solar cell 11 is formed by a groove having 2 sidewalls. In a lateral cut this groove has a triangular shape. The groove is located above the inactive area 10 and is covering at least partially the path of incident light directed to the inactive area 10. Light approaching the inactive area 10 is diffracted by the sidewalls of the groove in a manner, that it is directed to the photoelectrically active solar cell 11.

[0049] The groove may be machined e.g. by mechanically structuring, laser ablating, etching or embossing a surface of the transparent insulating substrate 2. The depth of the groove and the sloop of the sidewalls depends on the size of the inactive area 10, the index of refraction of the material used for the optical element 13, what in the case of a groove is identical to the material used as transparent insulating substrate 2, and the distance between the top surface of the transparent insulating substrate 2 and the transparent electrode layer 3.

[0050] In embodiment b, a light scattering layer 14 is used to direct the light approaching the photoelectrically inactive area 10 into the photoelectrically active solar cell 11. The scattering level of the layer 14 has to be chosen dependent on the distance between the top surface of the transparent insulating substrate 2 and the transparent electrode layer 3 and the size of the photoelectrically inactive area 10.

[0051] In embodiment c, the structural optical element 13, 14, 15 is formed by a transparent insulator substrate 2, e.g. a glass substrate, which substrate 2 comprises at least one groove 15 to direct light approaching the photoelectrically inactive area 10 into the photoelectrically active solar cell 11. The transparent insulator substrate 2 can be realized as a layer to be glued or welded onto of the transparent insulating layer 12, e.g. a very thin glass substrate, with the surface of substrate 2 comprising the groove oriented to the transparent insulating layer 12.

[0052] The groove 15 may be machined e.g. by mechanically structuring, laser ablating, etching or embossing a surface of the transparent insulating substrate 2. The depth of the groove 15 and the sloop of its sidewalls depend on the index of refraction of the material used as transparent insulator substrate 2 as well as the size of the photoelectrically inactive area 10. While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A method for improving the efficiency of converting light into electrical current of a solar module (1), said solar module (1) comprising photoelectrical^ inactive areas (9, 10) and photoelectrical^ active areas formed by a solar cell (11), characterized in that light approaching said inactive area (9, 10) is at least partially directed into said active area via at least one structural optical element (13, 14, 15).

2. The method according to claim 1 , wherein the light approaching the inactive area (9, 10) is directed into active area by light diffraction, light scattering, and/or light reflection.

3. The method according to claim 1 , wherein the at least one structural optical element (13, 14, 15) is arranged in a plane parallel to the region of the inactive area (9, 10) of the solar module (1).

4. The method according to claim 3, wherein the structural optical element (13, 14, 15) is a groove (15), an optical element (13), and/or a light scattering layer (14).

5. The method according to claim 1 , wherein the inactive area (9, 10) is a framework (9) supporting the solar module (1) and/or an area (10) between two adjacent solar cells (11).

6. The method according to claim 1 , wherein the solar module (1) comprises a substrate (2), the substrate (2) is arranged such that light approaching said inactive area (9, 10) passes the substrate (2) and the structural optical element (13, 14, 15) is arranged on a surface of the substrate (2).

7. A solar module (1), said solar module (1) comprising a photoelectrically inactive area (10) and an photoelectrically active area, wherein the active area is formed by a solar cell (11), characterized in that said solar module (1) comprises at least one structural optical element (13, 14, 15) directing light, which approaches the inactive area (9, 10), into said solar cell (11).

8. The solar module (1) according to claim 7, wherein the at least one structural optical element (13, 14, 15) is arranged in a plane parallel to the region of the inactive area (9, 10).

9. The solar module (1) according to claim 7, wherein the structural optical element (13, 14, 15) is a groove (15), an optical element (13), and/or a light scattering layer (14).

10. The solar module (1) according to claim 9, wherein the optical element (13, 14, 15) has a hemispherical, spherical, rounded, triangular, or polygonal shape.

11. The solar module (1) according to claim 9, wherein the solar module (1) comprises a substrate (2), the substrate (2) is arranged such that light approaching said inactive area (9, 10) passes the substrate (2) and the structural optical element (13, 14, 15) is arranged on a surface of the substrate (2).

12. A system for converting light into electrical current, said system comprising a solar module (1) having at least one photoelectrically inactive area (9, 10) and at least one photoelectrically active area, wherein the photoelectrically active area is formed by a solar cell (11) comprising a photoelectric conversion semi-conductor (4), characterized in that the system comprises at least one structural optical element (13, 14, 15) by which light approaching the photoelectrically inactive area (9, 10) is at least partially directed into the solar cells (11), wherein the structural optical element (13, 14, 15) with respect to the main direction of incidence of the incoming light is located above the inactive area (9, 10) in a plane parallel to the region of the inactive area (9, 10), and wherein the size of the structural optical element (13, 14, 15) is such that it overlaps or covers at least partially the path of incident light directed to said inactive area (9, 10).

13. The system according to claim 12, wherein the photoelectrically inactive area (9, 10) comprises grooves (6, 7, 8) establishing a monolithic photovoltaic module composed of a number of solar cells (11) electrically connected in series and/or the photoelectrically inactive area (9,10) is caused by a framework (9) supporting the solar module (1).

14. The system according to claim 12, wherein the structural optical element (13, 14, 15) is a groove (13), an optical element (15), and/or a light scattering layer (14).

15. The system according to claim 12, wherein the solar module (1) comprises a substrate (2), the substrate (2) is arranged such that light approaching said inactive area (9, 10) passes the substrate (2) and the structural optical element (13, 14, 15) is arranged on a surface of the substrate (2).