A POWER GENERATING ELEMENT FOR CONVERSION OF LIGHT INTO ELECTRICITY AND PROCESS FOR MANUFACTURING THEREOF
FIELD OF THE INVENTION The invention relates to a power generating element for conversion of light into electricity and to a process for manufacturing thereof.
BACKGROUND OF THE INVENTION
A photovoltaic cell (PV) or solar cell converts light directly into electrical power. It exploits the photovoltaic effect in junctions between semiconductor materials. Photovoltaic cells are traditionally manufactured using silicon-based technology with Si substrates and semiconductor processes. Other technologies include CIGS (Copper, Indium, Gallium, Selenide) and dye sensitised titania or organic type of solar cells.
One of the purposes of a series connection structure in large-area solar cells is to obtain high output voltage from a large-area solar cell. In addition it is important to reduce the joule losses in electrodes. If one cell of the solar cell is formed on the entire surface of a substrate without forming a series connection structure, generated carriers may migrate over a long distance in the electrode and the metallic electrode on the rear side down to a lead take-out point disposed at the end of a solar cell. Because the metallic electrode is characterised by low resistance, joule loss caused by current flowing through the metallic electrode may be neglected. However, sheet resistance in the conductive thin-film can be significant; therefore, joule loss caused by current flowing over a long distance in the transparent electrode layer is also significant. For this reason, conventional technology usually disperses a large- area solar cell into several strip-formed cells, and constructs these cells with a width from 4 mm to 20 mm.
WO03079448 describes the state of the art in this technology, and relates to a self-adjusting serial circuit of thin layers and method for production thereof. The invention relating to WO03079448 is characterised in that electrically conducting conductor tracks are applied to a substrate whereupon several main deposit layers of conducting, semi-conducting or insulating materials are applied to the substrate. The application of the layers is carried out at various angles of incidence to the surface of the substrate.
One problem in this field is that, if a large-area solar cell is structured and manufactured using the aforementioned known techniques, costs incurred may be prohibitively high.
Still another important problem in this field is that the constructions described above generally require complex patterning steps each time each metallic electrode layer, thin-film semiconductor layer and transparent electrode layer is formed, which makes the manufacturing process complex. Moreover, unless ultra-fine patterning is carried out with sufficient positional accuracy, the ineffective area required for the series connection which does not contribute to power generation, increases, thereby reducing the efficiency.
Furthermore, in photovoltaic cell fabrication, to pattern the thin-film semiconductor layer and the metallic electrode layer, and the thin-film semiconductor layer and the electrode layer, it is necessary to cut the layers so that the layer beneath a particular layer is not damaged. Moreover, adopting normal wet processes such as photo-etching causes the manufacturing process to be complex, which makes it difficult to reduce manufacturing costs.
Additionally, adherence between various layers, either active layers or non- active, e.g. supporting or scaffolding/shaping layers, might be adversely affected during subsequent manufacturing steps and/or use, e.g. in high temperature conditions.
SUMMARY OF THE INVENTION An object of the invention is to provide for a series connection and process for manufacturing a series connection for photovoltaic cells, in particular a power generating element for conversion of light into electricity, which have simplified fabrication process steps so as to afford a cost effective and energy-efficient product.
Another object is to provide a power generating element and manufacturing process thereof, wherein one or more of the above mentioned drawbacks are reduced or eliminated.
In accordance with the present invention, there is provided a series connection for a series of photovoltaic (PV) cells comprising: a formed metal substrate having a series of spaced elongate protrusions extending upwardly on the substrate; at least one insulating layer over the substrate; active layers between the elongate protrusions; and at least one conducting layer over the substrate to provide conducting channels between the substrate and an overlying layer which is an active part of the PV cell.
The advantage of the series connection provided in accordance with this invention is that the current stays low, so that ohmic losses are minimised thereby providing an energy efficient system. An additional advantage is that the series connection conforms to a continuous production on a continuous production line. In particular, the invention provides a power generating element for conversion of light into electricity, the element comprising a plurality of photovoltaic cells, a support for individually supporting the photovoltaic cells, wherein the support comprises a metal substrate having integrally formed recesses for receiving photovoltaic cells.
According to this aspect of the invention the support for supporting the plurality of photovoltaic cells comprises a metal substrate. This metal substrate is provided with recesses. Each recess contains a photovoltaic cell. The recesses are integrally formed in the substrate. Such a support can be easily manufactured using standard
(de)forming techniques, advantageously in a continuous production line. Moreover, as the recesses, e.g. tray like recesses, are an integral part, there is no risk of detachment.
The photovoltaic cells can be connected in parallel. Alternatively, the photovoltaic cells are serially connected.
In a preferred embodiment thereof the element comprises: a substrate having a series of spaced parallel protrusions extending upwardly on the substrate, the protrusions being integrally formed with the substrate made of metal, and neighbouring protrusions defining recesses between them; at least one insulating layer over the protrusions and recesses; spaced apart conductive parts leaving free a gap between neighbouring conductive parts, each conductive part (at least) partially covering each of a protrusion and adjacent recess; active layers of a photovoltaic cell in each recess on the respective conductive part; spaced apart front electrode elements on the uppermost active layer, a front electrode element being in electrical connection with the conductive part of one adjacent protrusion.
In this embodiment the protrusions or ridges are integral with the metal substrate. Such integral protrusions can be easily provided by mechanical forming operations, that can be applied to strip metal. Again because the protrusions are integral, these protrusions can not become detached or otherwise removed or repositioned during subsequent manufacturing processes or during use in high
temperature conditions. Partial detachment or repositioning may happen when the adherence between separately applied protrusions e.g. by (screen) printing and the substrate leaves something to be desired. Neighbouring protrusions define a recess. The substrate having the protrusions and recesses serves as a support for the series of photovoltaic cells, which are constructed in these recesses. Furthermore the protrusion itself is a useful tool for structuring a connection in serially connected photovoltaic cells as will be explained hereinbelow.
Preferably, the insulating layer is a coating made from vitreous enamel, glass or glass-ceramic, ceramic nitride or oxide, sol-gel or polymer. Advantageously, the insulating layer is applied using printing, sputter deposition, plasma deposition, chemical vapour deposition (CVD) or physical vapour deposition (PVD) processes, sol-gel, electrochemical (masking) deposition processes or lamination.
Preferably, the substrate is a metal panel or strip, advantageously made from carbon steel , low or ultra low carbon steel, stainless steel, aluminised steel, ECCS, aluminium or titanium.
Preferably, the conductive parts are made from a material selected from the group comprising transparent conductive oxide (TCO) coating, preferably indium- doped tin oxide (ITO), zinc oxide (ZnO), aluminium or molybdenum. Advantageously, the conductive parts are applied as a conducting layer. This layer can be applied using printing, sputter deposition, plasma deposition, chemical vapour deposition (CVD) or physical vapour deposition (PVD) processes, sol-gel, electrochemical (masking) deposition processes or lamination. At the appropriate positions the conducting layer is interrupted, e.g. by lasering, or removing material, thereby creating a gap between adjacent conductive parts. Preferably, this gap is filled with an insulating material in order to obtain a flush surface serving as a support and scaffold for building the subsequent active layers of a photovoltaic cell.
Preferably, a conductive part is a back contact.
Advantageously the active layer comprises porous titanium and electrolyte. Preferably, the active layer comprises CIGS or CdS.
Preferably, a protrusion or ridge has a bus bar as an electrical contact.
Preferably, the front electrode elements are made from a transparent material.
In accordance with the present invention there is further provided: a process for manufacturing a series connection for a series of (PV) cells comprising: - providing a formable metal substrate;
forming a series of spaced elongate protrusions extending upwardly on the substrate; applying at least one insulating layer over the substrate; applying active layers between the elongate protrusions; and - forming at least one conducting layer over the substrate to provide conducting channels between the substrate and an overlying layer which is an active part of the PV cell.
In particular a second aspect of the invention relates to a process for manufacturing a power generating element for conversion of light into electricity, the process comprising: providing a metal substrate; forming a number of recesses in the metal substrate; and manufacturing a photovoltaic cell in each recess.
This process provides advantages and benefits similar to those described above. Other preferred embodiments are defined in the dependent process claims.
In a preferred embodiment the invention provides a process for manufacturing a power generating element for conversion of light into electricity, the process comprising: providing a metal substrate; - forming a series of spaced parallel protrusions extending upwardly on the substrate, neighbouring protrusions defining recesses between them; applying at least one insulating layer over the protrusions and recesses; at least partially covering each of a protrusion and adjacent recess with electrically conductive parts, such that a gap is formed between neighbouring conductive parts; applying active layers of a photovoltaic cell in each recess on the respective conductive part; providing spaced apart front electrode elements on the uppermost active layer such that a front electrode element is in electrical connection with the conductive part on one adjacent protrusion.
Advantageously, the process permits the use of low cost substrates and the use of materials that are readily formable in accordance with the invention. Further, the process permits a continuous production in a continuous production line.
Advantageously, the spaced parallel protrusions are formed substantially upright relative to the substrate.
Preferably, the protrusions are formed by a forming operation including metal- embossing, coining, engraving at one or both sides, profiling, laser marking, pressing, machining or mechanical grinding/sharpening.
Advantageously, the PV cells are encapsulated by an encapsulating layer, e.g. by applying a laminate on top of the structure. Such an encapsulation is useful, if a non-solid active PV layer is used. The bottom of a recess, the upstanding walls of the protrusions including their insulating layer and conductive parts. define a container containing the active PV layers, while the encapsulating layer seals the container like a lid or cap.
Embodiments of the present invention will now be described, by way of example only with reference to the figures as described hereunder, wherein:
Figure 1 refers to the formation by forming operations of an upright ridge or protrusion in a metal substrate; Figure 2 shows that the metal substrate is electrically insulated from the subsequently applied layers which form the photovoltaic cells;
Figure 3 shows the application of the scribing process to form scribed regions;
Figure 4 shows an electrically conductive layer (not shown) is deposited so as subsequently to form bus bars. Figure 5 shows an embodiment of the series connection indicating the flow of current in the PV cell series connection.
Figure 6 shows shows an embodiment of the series connection with TCO and electrolyte materials.
Referring to figure 1 , substrate 10 is a metal panel upon which the layers of the PV cell are applied in accordance with the invention. In a first step of the process, there is provided a metal substrate 10 which can be shaped or deformed using processes such as forming operations, metal-embossing, coining, engraving at one or both sides, profiling, laser marking, pressing, machining or mechanical grinding/sharpening. Hydro-forming is an alternative process which may be used to deform the substrate in accordance with the invention. This forming or shaping step provides the substrate 10 with ridges or protrusions 12 which are formed substantially vertical to the substrate surface 13 typically having a height within the range 5-200 microns with a variability of no more than 10%. Further, the formation of the spaced apart parallel protrusions 12 along the substrate 10 may span the entire panel or may be truncated towards the edge of the panel so as to leave sufficient space at the edges for bonding subsequent overlying encapsulating layers.
The protrusions 12 function as a support for electrically conductive tracks. Inter- protrusion spacing is of the order 10-100 mm, the width dimension of the ridge or protrusion being of the order 0.5-5 mm, while the upright walls 15 of the protrusions with respect to the substrate surface 13 are substantially vertical and preferable at ninety degrees to the surface 13. This angle could also deviate variably from ninety degrees. The walls 15 facing each other, of neighbouring protrusions 12 define recesses 14. The flat surface 13 is the bottom 21 of the recess 14.
A suitable metal for the substrate 10 in accordance with the invention is carbon steel, preferably steel strip or sheet material, (ultra) low carbon steel or aluminium. Other materials such as glass or ceramics tend to be expensive and difficult to form.
In a second step, one or more coatings are applied to the metal substrate 10 before or after the metal substrate is formed into the linear protrusion pattern. An insulating coating 16 can be formed from enamel such as vitreous enamel, glass or glass-ceramic, ceramic nitride or oxide such as chromium oxide, sol-gel or polymer and can be applied by printing, sputter deposition, plasma deposition, chemical vapour deposition (CVD) or physical vapour deposition (PVD) processes, in addition to sol-gel, electrochemical (for masking) deposition processes or lamination. For example, in addition to the above metals, an additional suitable candidate for the substrate includes electro-coated chromated steel (ECCS). Further, a thin layer of amorphous SiO2 using CVD may be used as the insulating layer or stabilised zirconium oxide. Alternative materials include high silicon steel or high aluminium steel.
Figure 2 shows that the metal substrate 10 is electrically insulated by insulating layer 16 from the subsequently applied layers which form the photovoltaic cells. The function of this insulating layer 16 is to provide a corrosion protective layer or diffusion barrier which is preferentially resistant to high temperatures, for example up to 5500C, and resistant to corrosive atmospheres. Insulating layer 16 can also be used as an effective substrate for deposition of a conducting coating layer 18 thereon to function as the back-contact. Advantageously, the insulating layer 16 should be resistant to the conditions of subsequent mechanical, chemical or thermal removal processes such as scribing of the conducting layer 18 applied on top of it, in order to provide interruptions or gaps 20 in the conducting layer 18 so that spaced apart conductive parts 19 are obtained.
In addition to the electrically conductive layer 118 being resistant to high temperatures and corrosive environments, preferably, the layer 18 should not decompose when an electrical potential is created by the operation of the photovoltaic
cells. Furthermore, the conductive coating 18 should not be sensitive to the electrolyte when using a dye sensitized titania (Graetzel) cell.
The electrically conductive parts 19, serving as the back contact, may include a transparent conductive oxide (TCO) coating such as indium-doped tin oxide (ITO), zinc oxide (ZnO), aluminium, titanium or molybdenum. CVD is a suitable process for depositing TCO layers as it is conducive to obtaining the optimal electrical conductivity.
Moreover, this electrically conductive deposit is typically about 1 micrometer in thickness. Further, the electrically conductive Iayer18 may be a metallic coating, a metal foil, an electrically conductive polymer, or a polymer that can be reinforced with the addition of components so as to induce electrical conductive properties thereto, to the desired extent.
In a next step, figure 3 shows the application of a scribing process to form gaps
20 in the conducting layer 18 thereby obtaining conductive parts 19 as described above, wherein it is important that there is no contact between the metal substrate 10 and the electrically conductive parts 19, and that there is no contact between the conductive coating layer parts 19 that lie at either side of the gap 20. Such contact would cause short circuiting and malfunctioning of the PV cell.
Referring to figure 3, if the conductive coating 18 is a metallic material with a lower thermal expansion coefficient than the substrate metal 10 (for example if the back electrode is titanium foil and the substrate is steel), then gaps 20 or single blade cuts of the scribing process mentioned above can be performed at a lower temperature, e.g. using a laser. As the metallic material warms up to room temperature, the substrate 10 will expand more than the metallic material and the gaps
20 will tend to expand into voids that are large enough to prevent electrical contact between the conductive parts 19, obtained from the conductive layer 18. Clearly, in this configuration, the metal substrate 10 and the foil should be joined together at low temperatures.
In a further step, the scribing/cutting step forming the gaps 20 in the conductive coating 18 is followed by the application of an insulating element 30 in the gaps 20 and deposition of additional components generally indicated by reference numeral 22 of the solar cell as shown in figure 4.
In a further step (see figure 4), along the top of the upright protrusions, an electrically conductive layer (not shown) is deposited so as subsequently to form bus bars40, and in the inter- protrusion recesses 14 between the upright ridges the additional components of the solar cell are deposited with materials such as porous titania and electrolytes. The bus bars 40 can be applied e.g. by screen printing of a
precursor or pre-sinter paste, which then reacts further with the desired active layer under the influence of temperature and atmosphere.
Another step in this multi-step process is the deposition of the active layer, for example CIS, CdS or dye-sensitized titania and electrolyte or a polymer-based PV system. CIS is Chalcopyrite which has the general formula Cu(In, Ga)(Se, S)2 and is used in thin film technology for manufacturing solar cells. In such solar cells the chalcopyrite semiconductors acts as absorber layers for polycrystalline thin film solar cells.
In the case of CIS, a TCO layer is sputtered over the profile shown in figure 4, and cuts or scribes are made at regular intervals into the TCO layer for electrical interruptions. Herein the back contact and front contacts TCO could also be applied using shadowing processes in the deposition of these layers so as to produce the electrical interruptions, provided that the layers are deposited obliquely relative to the substrate. Moreover in the case of a Graetzel cell, the TCO is applied to a laminate, which is then connected to the solar cell via the top of the upright protrusions 12 having electrically conductive bus bars 40 thereon.
Figure 5 shows an embodiment of the series connection indicating the flow of current in the PV cell series connection. In essence, the arrows show the resulting current flow. The current is conducted into the conductive parts 19 acting as back contact and via the active layer 22 into the front electrode elements 24. From there, it flows once again through a bus bar or conductor track 40 into the next conductive part (back contact) and from there, again via the front electrode element, into the next conductor track.
Referring to figure 6 (identical components are identified by the same numbers + 100), in a further embodiment of the invention there is provided a substrate 1 10 having an insulating layer 116. A conductive part 118 (back contact) deposited on the insulating layer 116 is connected to front electrode element 124 at the top of the protrusion 112 while the inter-protrusion region or recess 1 14 comprises porous titania 122a and electrolyte 122b. The arrangement is further encapsulated or packaged by covering the front surface of the element with, for example, a transparent material such as transparent foil 150. The encapsulating materials are applied over the entire PV panel shown in figure 6, and the bonding of the foil to the PV cell arrangement takes place at the edges of the panel 100.
The main advantage of this series connection is that the current stays low, so that the ohmic losses are minimised. Herein, it is desirable to manufacture these series
connections continuously by using the formability properties of sheet metal materials.