Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
1999-03-23
2001-02-06
Chaudhuri, Olik (Department: 2814)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S061000, C438S074000, C438S080000, C136S251000, C136S244000, C136S259000
Reexamination Certificate
active
06184057
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is in the field of thin film photovoltaic cells. For example, amorphous silicon (i.e., a-Si:H) photovoltaic (PV) cells are known structures which comprise several layers, usually alternatingly of n-doped, intrinsic, and p-doped silicon, and which essentially have the ability of generating electric current from incident light. Since sunlight, for example, can be used to generate power, photovoltaic cells form an interesting alternative source of energy in principle: one much more environment-friendly than fossil fuels or nuclear power. However, for such PV cells to become a serious and economically attractive alternative, they need to be provided in a suitable form and made by relatively low-cost processes, using relatively inexpensive raw materials.
SUMMARY OF THE INVENTION
In order to satisfy this demand, the present invention is directed to a process by which photovoltaic cells can be made in the form of a foil. It is not only desirable to have photovoltaic cells in the form of a foil, since such may allow economic production on a large scale (in a “roll-to-roll” process), but also since flexible substrate based photovoltaic cells will be more versatile and easier to handle than the more conventional amorphous silicon PV cells made on glass substrates.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Thus, the invention pertains to: a method of manufacturing a photovoltaic foil supported by a carrier and comprising a plurality of layers which together have the ability of generating electric current from incident light (hereinafter referred to as “photovoltaic (PV) layers”), a back-electrode layer on one side adjacent and parallel to the photovoltaic layers, and a transparent conductor layer on the other side adjacent and parallel to the photovoltaic layers; the method comprising providing a substrate; and applying the transparent electrode layer and the photovoltaic layers (including any additional and/or adjuvant layers) onto the substrate. At some point, after the photovoltaic layers have been applied, the back-electrode layer is applied. This does not need to be a transparent electrode and, in fact, is preferably a reflector for visible light (both for reflectance and for conductivity, the back-electrode layer will generally be a metal layer). For the sake of clarity, in the context of the present invention, the term “back” pertains to the side of the PV foil that upon eventual use will be facing away from the side on which the light is to fall.
Such a method is known from, for example, Shinohara et al., First WCPEC, Dec. 5-9, 1994, Hawaii, pages 682 and following (copyright: IEEE), where the substrate used is poly(ethylene 2,6-naphthalene dicarboxylate) (PEN). The disclosed method has several serious drawbacks. For example, first the PV layers are built up, and then the transparent conductor. This is a logical consequence of the substrate not being sufficiently transparent, namely, it cannot eventually serve as a window for the transparent conductor layer (which is customary in amorphous silicon PV cells that are made on glass substrates). However, the necessary “reverse” order of first applying the PV layers and then the transparent conductor layer imposes serious limits on the transparent conductor materials used. For example, a very favorable transparent electrode layer is F-doped tin oxide. However, in order for this to have the desired properties and texture, it should preferably be applied at a temperature of at least 400° C. Such a high temperature may be devastating to the PV layers: among other things, as a result of crystallization, the diffusion of the dopants, and/or loss of hydrogen. The preferred temperature for the deposition of F-doped tin oxide also causes the PEN substrates to deteriorate and, therefore, this layer cannot be deposited prior to the PV layers. Thus, with the use of the desired application temperature of the transparent electrode any sequence of deposition on the PEN substrate would adversely affect the fundamental ability of the PV foil to generate power.
Hence, a process is required which allows the roll-to-roll manufacture of a (relatively tough) photovoltaic foil or device, while at the same time making it possible to use any desired transparent conductor material and deposition process, and without jeopardizing the current-generating action of the PV layers. These requirements, and other desirable objects, are met by the process of the invention. To this end, the invention relates to a method of the aforementioned known type, which method comprises the following subsequent steps:
providing a temporary substrate,
applying the transparent conductor layer,
applying the photovoltaic layers,
applying the back-electrode layer
applying the (permanent) carrier
removing the temporary substrate, and, preferably,
applying a top coat on the side of the transparent conductor layer.
In a preferred embodiment of the invention, the transparent conductor layer is applied at a temperature higher than the one to which the photovoltaic layers are resistant (e.g., for a-Si:H, the maximum temperature to which the PV-layers are resistant is about the same as the deposition temperature of the said layers. Higher temperatures will cause loss of hydrogen and diffusion of dopants and impurities, thus forming defects that reduce the efficiency of the PV-layers).
These steps and their sequence essentially make it possible for PV cells to be produced in the form of a foil, while still maintaining the desired order of manufacture that is customary in the case of PV cells produced on glass substrates (in which case one can start by applying the transparent conductor since the glass will act as a window for it). Thus, when following the process of the invention, the substrate can be selected so as to allow any further process steps (like the high-temperature application of a transparent conductor layer) without any concerns about its (i.e. the substrate's) transparency or other properties needed for the functioning of the eventual PV foil. The temporary substrate is removed after the last of the photovoltaic layers, the back-electrode layer, and also a permanent carrier back-substrate have been applied, this in order to have the thin PV foil supported during as many process steps as possible and to ensure that the foil exhibits sufficient strength and bending stiffness (preferably adapted to the intended end product). After removal of the temporary substrate, the transparent conductor (front-electrode) will generally be provided with a transparent protective layer, which preferably further adds to the mechanical properties of the foil and/or the end product.
Although the transparent conductor layer will generally be deposited directly onto the temporary substrate (sometimes preceded by one or more extremely thin layers serving as a process aid), it is also possible after providing the temporary substrate to first apply the eventual protective layer on the said temporary substrate, and then the transparent conductor layer, followed by the other layers making up the foil. In this case the protective layer should, preferably, be made of an inorganic material.
Both the temporary substrate itself and the method to remove it (suitably by means of dissolving or etching) can be selected by the man skilled in the art without great difficulty. For example, the temporary substrate may be a “positive” photoresist, namely, a photosensitive material that upon irradiation undergoes a change from solvent-resistant to solvent-extractable, e.g., cross-linked polyimides. In order to meet the object of using low cost materials, these are not the substrates of preferred choice. In this respect, it is more advantageous to use polymers that can be removed by means of plasma etching (for example, O
2
plasma or, for example, for polysiloxane polymers, SF
6
plasma). While basically any polymer will thus be suitable, in view of the above it is, of course, preferred to employ polymers which can withstand higher temperatures (250° C. and more preferably
Middelman Erik
Schropp Rudolf Emmanuel Isidore
Van Andel Eleonoor
Akzo Nobel nv
Chaudhuri Olik
Fennelly Richard P.
Pham Hoai
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