Patent Application
20120298198
The invention relates to a layered system for producing a solar cell on a metal substrate, method of producing said layered system.
Owing to demands imposed on future energy requirements coupled with desire for environmentally friendly energy, photovoltaic power generation systems have gained momentum in recent decades and are now being commercially exploited and are growing in prevalence. While the traditional solar cell or photovoltaic device market has advocated cells based on p-n junction silicon wafer-based solar cells which are characterised by high manufacturing costs, recent research efforts are focussed on providing low cost continuous manufacturing processes to provide inexpensive solar cells for photovoltaic power generation systems.
It is an object of the invention to provide a layered system for continuous production of inexpensive solar cells. A further object of the invention is to provide a method for producing such a layered system.
In a first aspect of the invention, a layered system for producing a solar cell is provided comprising:
The metal substrate is impermeable for oxygen and water vapour thus the active material is less exposed to oxygen and water vapour. Metal substrate is a formable substrate for solar cells, giving additional strength as well. Preferably, the metal substrate is a metal which has inherent corrosion protection such as stainless steels, or a metal provided with additional protection against corrosion such as an electroplated mild steel. An additional advantage of such additional protection is that the adhesion between the substrate and the polymer coating layer may be improved. The polymer coating layer electrically isolates the active solar cells from the steel substrate. In this way individual solar cells on the polymer coating layer can be connected in series. This increases the voltage of the assembly. In addition the polymer coating layer provides an additional corrosion protection to the steel substrate. The polymer coating layer needs to be embossed for instance to provide a hologram or a periodic grating. This hologram or grating provides a layer with a rainbow effect. The holograms found on credit cards are examples of rainbow holograms. The effect of these holograms or gratings in the invention is that it increases scattering of light which impinges onto the surface. The scattered light increases the angle of the light towards the surface and increases the light path through the photovoltaic active layers of the solar cell. This improves light absorption into the photovoltaic active layers and increases the performance of the solar cells. The solar cell comprises an active solar cell back electrode following the shape of the embossed polymer coating layer and having a high light reflectivity. The layer with high reflectivity is needed to enhance the reflection of the incoming light. It is necessary that the back electrode follows the shape of the embossed polymer coating to benefit from the embossing to scatter the light. The phrase ‘follows the shape’ intends to mean that the surface texture resulting from the embossing survives after the application of the back-electrode. If the back-electrode would fill the embossed pattern partly or completely, the scattering enhancing function of the embossing is (partly) lost. The photovoltaic active layer or layers, or series of layers, absorb the light falling onto the system and generate electrons and holes essential for producing electricity. The longer the light travels through these photovoltaic active layer(s), the more light is absorbed by the photovoltaic active layers. As a result more electrons and holes will be generated and as a consequence the efficiency of the solar cell improves. The embossed polymer layer in combination with the solar cell back electrode with a high light reflectivity ensures that the path the light takes through the photovoltaic active layer(s) is prolonged. It is noted that any surface texture of the polymer layer will have a similar effect to a certain extent as surface roughness also scatters the light, particularly when provided with the high reflectivity active solar cell back electrode but uncontrolled surface roughness can create imperfect layers as the layers can not follow the roughness. This reduces the performance of the solar cells and even can cause complete malfunction if imperfect layers cause electrical short cuts. The advantage of an embossed polymer layer is that the scattering of light is controlled and optimised for this purpose. Finally a transparent top electrode is provided to enable the transport of the electrons. Additional layers may be present such as a protective top layer, adhesion layers between polymer layer and substrate or between polymer layer and active back electrode. Also metal fingers and/or busbars may be provided on top of the transparent top electrode to improve electron transport.
In a preferred embodiment of the invention the embossed polymer coating layer is provided with an adhesion layer, such as a chromium oxide layer or an aluminium layer. The purpose of this layer is to improve the adhesion of the solar cell layers onto the polymer coating layer.
In another preferred embodiment of the invention the back electrode preferably has a thickness of between 0.1 and 0.6 μm, preferably wherein the back electrode has a high reflectivity. The back electrode may be a thin metal layer, such as a silver or aluminium layer. Metals, and particularly silver and aluminium have a high reflection coefficient and consequently most of the incoming light is reflected by it. However, for instance for PV cells based on thin photovoltaic active layers of small organic molecules or polymer or hybrid based active donor-acceptor layers system combined with hole conducting and exciton blocking layers non-metallic electrode layers may be used. The thickness of the layer is governed by its function as a back electrode, resulting in a minimum thickness of 0.1 μm. The maximum thickness is governed by the ability to follow the shape of the embossed polymer layer. When the thickness exceeds 0.6 μm, the light scattering effect of the embossed polymer decreases.
In another preferred embodiment of the invention the photovoltaic active layers comprise thin films of photovoltaic active silicon, such as in n(-i-)p-cells, or thin photovoltaic active layers based on small organic molecules or polymer or hybrid based active donor-acceptor layers system combined with hole conducting and exciton blocking layers. The photovoltaic active layer may also comprise tandem cells in which two or more cells are stacked on top of each other, for instance wherein each cell absorbs complementary parts of the solar spectrum. The function of the photovoltaic active layer, which is made of a semiconducting material, is to absorb the photons. These photons bring electrons in the conduction band. The electrons flow through the material to produce electricity. Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. Complementary positive charges, called holes, are also created and flow in the opposite direction to the electrons. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity.
In another preferred embodiment of the invention an additional scattering layer is provided on the back electrode, and which has a refractive index between that of the back electrode and the photovoltaic active layer, and wherein the scattering layer preferably is a transparent conductive oxide (TCO) such as aluminium doped zinc oxide (ZnO:Al), fluorine-doped tin oxide (SnO2:F or FTO) or indium tin oxide (ITO), and wherein the scattering layer is optionally provided with a silicon-oxide nanolayer. This additional scattering layer further increases the path of the incident photons through the photovoltaic active layer(s) thus increasing the chances of it generating an electron-hole pair.
In another preferred embodiment of the invention the polymer coating layer comprises a crystallisable thermoplastic polymer, such as polyesters like polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphtalate (PEN) or polyolefins such as polypropylene or biaxially oriented polypropylene (BOPP), polyethylene (PE) or polyvinylchloride (PVC). PET has a particular preference because it adheres to a metal substrate very well, and it has a high melting temperature. The current process of producing a solar cell typically involves process steps at temperatures of between 120 and 200° C. The embossing and the polymer layer need to be able to withstand these temperatures and PET has proved to perform well. The PET may be modified with CHDM (1,4-cyclohexanedimethanol) or IPA (isophtalic acid). For similar reasons, polymers like polyamides or polyimide and the like may be suitable as long as they have a sufficiently high melting temperature. Mixtures (blends) or copolymers of the polymers mentioned hereinabove may also be used. The ability to crystallisation of the polymer coating layer ensures that an embossing remains stable during further processing. Preferably the melting temperature of the crystallisable thermoplastic polymer is higher than 200° C., more preferably at least 220° C. If so desired, the polymer layer may be provided with additional fillers, additives, colorants or corrosion inhibitors, such as titania. Preferably the polymer material is well suited for use under vacuum conditions, e.g. during application of the solar cell layers, to prevent vacuum evaporation. The minimum thickness of the polymer coating layer is determined by the breakthrough voltage, whereas the maximum thickness is a cost issue. A suitable minimum thickness of the polymer coating layer is 5 μm, and preferably 7 μm. For polymer-based solar cell technology the processing is initially done at low temperatures, i.e. where crystallisation of the polymer coating does not occur. However, in most cases an after treatment at 120-150° C. is needed to optimally function. In this case embossing and crystallisation is needed to increase the light absorption and reduce crazing due to crystallisation of the polymer after the application of the solar cell layers. A polyester-based coating wherein the crystallisable polymer comprises PET, or a copolymers or blends comprising PET and/or an IPA- or CHDM-modified PET has proven to work excellently.
In another preferred embodiment of the invention the metal substrate is a steel strip or foil, preferably a mild steel provided with a corrosion protection coating or a stainless steel, aluminium or titanium strip. Mild steel substrates with electroplated metal coating are relatively inexpensive and can be provided with a polymer coating layer on an industrial scale reliably and cost effectively. Stainless steel substrates are more expensive than mild steel substrates, but these steels do not require additional protection against corrosion, whereas the mild steel substrates are preferably provided with a protective coating layer such as a nickel or chromium or cobalt or molybdenum and their alloys based plating layer, or a combination thereof, or a chromium-oxide layer such as in the ECCS-steel (also known as tin-free steel). It is also possible to use galvanised steel as a substrate. However, it is not critical for the purpose of this invention which type of coating is used to provide the additional corrosion protection as long as it improves or at least does not impair the adhesion between the polymer coating layer and the substrate. Additional adhesion layers may be provided between the polymer coating layer and the substrate as known from conventional production of polymer coated metal substrates for the packaging industry. The thickness of the metal substrate is mainly determined by the mechanical properties required. Steel substrates preferably have a thickness of between 0.05 and 1 mm. Weight restrictions may limit the maximum thickness, although there is no maximum thickness above which the invention no longer technically functions. The thinner the substrate is, the bigger the risk of damage, creases and wrinkles, during installation and use of the solar cells, thereby potentially damaging the solar cells.
In another preferred embodiment of the invention the embossed polymer coating layer provides a holographic effect. The primary function of the holographic effect is the scattering function to increase the path of the incident photons through the photovoltaic active layer(s). However, the holograms may be useful for counterfeit prevention, brand promotion and product authentication because the hologram may be produced in a form comprising a company logo or a product trade name or trademark.
In a second aspect of the invention, a solar cell is provided comprising a layered system as described hereinabove. The photovoltaic active solar cell layers consist of a stack of thin films of photovoltaic active silicon, such as in n(i-)-p cells. Alternatively, these can be photovoltaic active thin layers based on small organic molecules or polymer or hybrid based active donor-acceptor layers system combined with hole conducting and exciton blocking layers to obtain optimal performance.
In a third aspect of the invention a method of producing a layered system for use in a solar cell is provided comprising the steps of:
The optional pretreatment may be an oxygen plasma treatment. Oxygen plasma surface treatment not only removes organic residues but also chemically reacts with the surface to form strong covalent carbon-oxygen bonds, which are much more polar and more reactive than the initial carbon-hydrogen bonds. The increased polarity of the surface accounts for the substantial increases in wettability and adds a degree of covalent bonding to the surface-adhesive interface. The polymer layer on the metal substrate is preferably an amorphous or partially crystallised polymer layer. The polymer layer may be provided upon the substrate by laminating a polymer film, which was produced earlier, onto the substrate by known means, or by direct extrusion of a polymer layer onto the substrate by known means, such as by extruding a film through a T-die directly upon the substrate, or onto a casting roll which film is then laminated onto the substrate. These methods for providing a polymer layer onto a substrate are known, for instance from the production of the Protact® polymer coated steel substrates produced for the packaging industry. Protact® is a registered trade name of Tata Steel IJmuiden BV.
In another preferred embodiment of the invention the embossing of the polymer coating layer is brought about by thermal embossing a surface texture such as a hologram or a periodic grating thereby at least partly crystallising the polymer coating layer. The embossing is preferably applied at an embossing temperature comparable to that of the processing temperatures in producing a solar cell which typically involves processing at temperatures of between 120 and 200° C. When PET is used as the polymer layer the PET coating crystallises during the embossing step while at the surface an imprint is pressed. This way a controlled embossed surface is obtained which increases the solar panel performance. The effect of the crystallisation also alleviates any internal stresses that may have been present in the polymer layer as a result of the lamination or extrusion onto the metal substrate. This alleviation or removal of stresses reduces or eliminates the risk of crazing of the solar cell layers during application of these layers or post-processing, because these are often processed at higher temperatures (typically up to 200° C.) or in a plasma environment. If the polymer layer is then still amorphous or partially crystallised, the high temperature will cause it to recrystallise then, causing crazing and complete failure of the solar cell. The embossing temperature is maximised by the temperature at which the polymer starts to stick to the embossing tools (also known as tacking). A minimum temperature is required to achieve crystallisation and to minimise the forces required to achieve the embossing. A suitable minimum temperature for polyesters such as PET was found to be about 120° C. and the maximum about 200° C. These minimum and maximum temperatures may vary slightly for other polymers but in general the temperature range of 120 to 200° C. is useful for all polymer layers suitable for use in a solar cell, i.e. capable of withstanding the typical processing temperatures during manufacture of a solar cell. The thermal embossing cannot be applied after solar cell deposition, because then the solar cell layers craze as a result of crystallisation of the polymer layer. As an alternative, the polymer layer may be embossed prior to lamination onto the metal substrate. In that case the embossing of the polymer coating layer is brought about by laminating a polymer coating layer already provided with a holographic film onto a metal substrate. As a further alternative, the embossing step may be performed during extrusion coating of a polymer film onto a metal substrate followed by embossing the extruded polymer film by means of an embossed casting roll, or an embossing roll mounted in the extrusion-lamination device. As a further alternative, the embossing step may be performed during or after lamination of a prefabricated polymer film by using an embossed lamination roll, or an additional embossing roll mounted in the extrusion-lamination device. As a further alternative, the embossing may be provided by applying a thixotropic coating composition or a sol-gel based composition onto the polymer layer, embossing the surface relief into the coating composition with an embossing device, and curing the embossed coating composition. The thixotropic coating may be provided with conductive properties, e.g. by incorporating conductive particles in it, so that the embossed layer may act as a conductive layer upon which the back electrode may be provided. Again the curing temperature should be chosen such that the polymer coating layer at least partly crystallises to alleviate or remove the internal stresses which might otherwise cause crazing in the solar cell layers. In another embodiment a substrate is provided with a polymer coating which is crystallised thermally after coating the substrate, followed by the coating of the coated substrate with a second coating, which second coating is partially cured, embossed to obtain a surface texture for light scattering and then finish cured. Said second coating may optionally be provided with conductive properties for reasons given above.
In another preferred embodiment of the invention the method comprises one or more of the following steps:
The invention will now be described by means of the following, non limitative and schematic drawings in which
In
In
| Number | Date | Country | Kind |
|---|---|---|---|
| 10001343.2 | Feb 2010 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP11/52002 | 2/10/2011 | WO | 00 | 8/6/2012 |
