EP2018644A2 - Photovoltaic cell - Google Patents

Abstract

The invention relates to a photovoltaic cell, particularly a color-sensitized solar cell, comprising a conductive support substrate (10) coated with a metal oxide semiconductor layer (16), a color layer (18) embodied so as to electronically interact with the metal oxide semiconductor layer, an electrolyte layer (32) that is applied to the color layer, and a counter-electrode (20) which is connected to the electrolyte layer. The support substrate and/or the counter-electrode is/are made from a flexible fabric composed of a plurality of interwoven fibers.

Description

 Photovoltaic cell

The present invention relates to a photovoltaic cell according to the preamble of the main claim, in particular a so-called. Dye-sensitized, nano-structured solar cell (DNSC = DYE-SENSITIVED NANO STRUCTURE SOLAR CELL), the invention nevertheless also for other solar cell technologies, such as organic solar cells, eig - net.

A generic device is well known in the art and is often referred to as a Grätzel cell, according to the inventor of US 4,927,721, which discloses essential structural features and photovoltaic or chemical details of the present technology, which is believed to be generic. The core of such a cell is provided on an electrode titanium dioxide layer on which a dye layer (= DYE layer) is formed on which in turn an electrolyte layer and a counter electrode is formed. Typically, the external electrodes are realized as thin, conductive glass substrates (to allow light to enter the cell), taking advantage of the effect that incident light excites an electron from the dye layer and enters the TiO 2 conduction band, resulting in a state of charge separation is reached. The charge in the conduction band is then conducted via a load to the counter electrode, where a redox electrolyte is reduced, which in turn leads to the reduction of the (oxidized) dye. The representation of FIG. 4 illustrates this basic process in a two-dimensional arrangement of sequence or geometry of the individual layers in the horizontal and the energy level in the vertical.

For many applications, however, such a rigid arrangement (due to the conductive glass plate electrodes) proves to be too rigid and correspondingly inflexible. so that efforts are also known to produce flexible DNSCs. Firstly, it has been necessary to provide low temperature processes (especially for the deposition of the metal oxide semiconductor) in order to use polymeric substrates instead of the glass plates (typically titanium dioxide is applied at high temperatures, which is incompatible with the use of plastics). Thus, there are efforts to use polymer-based substrates, such as in the form of conductive coated PET (such as ITO PET, so a conductive layer on PET, produced by indium-doped Snθ2). The conductive layers in polymer-based substrate films are limited to transparent materials, such as doped metal oxides, conductive polymers. Opaque coatings (such as metals) typically can not be used. Another problem of (conductive) polymers used here is their still unsuitable high sheet resistance.

Yet another disadvantage of such considerations (initially only existing in principle) for manufacturing flexible solar cells according to the DNSC principle is the mechanical problem that bending between the so-called active layer (ie the conductive substrate, the titanium dioxide layer formed thereon and the colored layer) on the one hand and the counter electrode on the other hand leads to unstable conditions due to the displacement or shearing at the contact surface.

Finally, there is a significant problem in designing flexible SECMs in making a stable, strong and yet flexible bond between the substrate and the metal oxide semiconductor material: the titanium dioxide (typically having a large effective surface area) with a surface roughness of between about 20 and 200 , defined as the ratio of an effective surface area with respect to the projected base area, eg by a nano-particulate structure), is in the inherent brittleness of the material, with the associated mechanical stability problem. In particular, such a metal oxide layer adheres only poorly on a (conductive) polymer as a carrier substrate.

The object of the present invention is therefore to provide an improved photoelectric cell, in particular a solar cell of the DNSC type, which combines improved mechanical flexibility of the end product with favorable production properties, advantageous photoelectric properties and good long-term stability. In addition, a cell is to create, which is potentially manufacturable for mass production with little effort and a high reproducibility of the photoelectric properties also outside the small series or laboratory environment allowed.

The object is achieved by the photovoltaic cell having the features of the main claim and the method for producing a photovoltaic cell according to independent claim 16; advantageous developments of the invention are described in the subclaims.

In accordance with the invention, using the principle of the so-called Grätzel solar cell (for example according to US Pat. No. 4,927,721 or EP-B-0 525 070), a fabric is used as the basis for the conductive substrate according to the invention (complementary or alternatively also for realization) the counterelectrode), this flexible fabric offering numerous surprising advantages for solving the above-mentioned problem: Even if the fibrous material should be used which itself is not transparent, the use of a tissue, more preferably a tissue with predetermined Breakthroughs and / or fabric gaps, an adjustable or predetermined and advantageous transparency of the carrier substrate and thus potentially also of the overall arrangement. Even a fabric as such already offers a potentially large effective surface area (for example, by means of the individual jacket surfaces). the fiber-interwoven fibers), so that, with subsequent coating of the (in turn high surface area) metal oxide semiconductor material, an effective total area exists as a basis for the overlying (preferably monomolecular) dye layer, thus - to realize one so far Unmatched high efficiency - efficiency and stability can be optimized. (The inherently high effective surface area of the fabric makes it possible to apply the metal oxide semiconductor material only very thinly, preferably nano-particulate and / or nano-structured in a further development, with correspondingly positive effects on the efficiency - a lower dark current through a shorter distance to the conductive layer of the substrate for the electron and improved mechanical stability due to less brittleness of the thin coating). Also, this configuration allows the effective use of a much wider range of suitable dyes (especially those with lower extinction coefficients).

In this case, the tissue used according to the invention allows numerous possible configurations in order to achieve these advantageous effects. On the one hand, it is preferred to realize the fabric of fibers which are not or only slightly electrically conductive, which are then suitably coated in a conductive manner before or after interweaving, it being favorable for further development to use carbon or (conductive) polymer fibers. On the other hand, suitable copper, titanium or aluminum fibers can be used as examples of conductive fibers.

A further development according to the applied on the fabric for realizing the carrier substrate (especially in case of non / weakly conductive fibers) conductive layer may in turn be a (eg suitably doped) metal oxide, a metal or a conductive polymer. It is also particularly suitable to use the fabric itself in order to guide the lines required for feeding in or discharging the charges to corresponding terminal electrodes of the solar cell; According to a preferred embodiment of the invention, this is done by the fact that these leads in the form of metallic wires (which traditionally must be elaborately formed approximately on the conductive glass plates known solar cells) in the context of the development of the invention during the manufacture of the fabric with the other fibers are woven , In this way, in addition to favorable mechanical flexibility and connection properties, a favorable electrical contact is ensured (again with positive effects on the efficiency by reducing ohm ¹ shear contact resistance).

As already described, in the context of preferred embodiments of the invention, preferably nanostructured TiO ₂ or ZnO (by way of example) is used as metal oxide semiconductor material, since the above-described optimization between mechanical stability and elasticity with the desired effective surface area can be achieved here. In terms of process engineering within the scope of preferred developments of the invention, this material, dispersed in a suitable solvent, is also applied to the fabric by impregnation and pressed after drying (volatilization of the solvent). Other suitable methods which produce a favorable connection with the tissue without adversely affecting it are, for example, sintering, the so-called sol-gel method or sputtering.

In the context of the invention, a thin dye layer, monolayerwise, ie only with the layer thickness of a dye molecule, is then applied to the composite of (conductive) fabric-based fabric substrate with metal oxide semiconductor layer, again by a suitable solution. It is within the scope of the invention, both Ru-based metal complexes, as also organic dyes, wherein in the context of the selection of the dye layer according to the invention ensures that the energy levels of the dye, the semiconductor and the electrolyte are coordinated with each other to run the desired photochemical and -elektrischen processes optimized.

A further, preferred embodiment of the present invention (best mode) provides that the electrolyte layer according to the invention (for example by using an acrylate resin or another, deformable and curable polymer) in a liquid or flow state, the deformation of the cells according to the invention in an almost arbitrary , allows the desired shape (in particular also for adaptation to an intended use environment, eg in the construction or building sector), whereupon this material is then hardenable and thus permanently fixes the shape in its design. For this purpose, the electrolyte layer suitably comprises a solvent, a redox couple and, if appropriate, additives which, in the manner of the construction with glass-fiber reinforced plastics, can enable mechanically very stable units, at the same time realizing the photochemical or photoelectric properties of a DNSC solar cell. In the context of suitable developments of the invention, it is also provided in particular that, in addition to a suitable hardenability of the electrolyte provided with corresponding properties (in addition to thermosetting resin, UV hardening resin is also suitable), such hardening or permanent deformation thereby to ensure that resins outside the electrolyte (which are not in connection with the electrolyte) receive such functionality, for example by an additional outer resin layer, which is then brought into its permanent form by appropriate formulation by curing in this continuing education and, independently, the electrolyte material may be selected. Within the scope of a preferred development of the invention, it is preferable in terms of process technology to apply one or more of the layers also by means of a screen printing process.

According to a further, preferred development of the invention, it is provided to stack a plurality of cells according to the invention on their flat sides, in order to create a very compact, efficient multi-cell structure, for example in the manner of a book with superimposed sides.

Particularly suitable for this implementation is then a lateral light entry (ie light input in the plane of the tissue), more preferably made possible by the use of photoconductive fibers as fibers for the fabric or films or thin glass layers through which then end- or Light can be introduced on the face side and, after appropriate modification of the fibers or light guide, can emerge on the shell side into the further, photoelectrically active layers of the cell arrangement (according to the present invention, a conventional direction of the light input would otherwise occur from the side of the conductive carrier substrate which, particularly suitable by the tissue used according to the invention, suitably transparent). As an advantage of such (corresponding to a book form) embodiment of the invention, it should be noted that the substrates used need not be transparent. In addition, the encapsulation can be optimized, since, in principle, the light-introducing layer can have an arbitrary thickness, and it is also a suitable adjustment or control of the light entry wavelength possible. As a result, a way is shown by the present invention in a surprisingly elegant and manufacturing technology favorable manner how flexible solar cells can be produced with favorable efficiency or efficiency properties and excellent mechanical stability, so that it is expected that numerous new applications for photovoltaic developed can be.

Further advantages, features and details of the invention will become apparent from the following description of preferred embodiments and from the drawings; these show in

1 shows a schematic, exploded sectional side view of the layer structure of the photovoltaic cell according to a first, preferred embodiment of the present invention;

Fig. 2 is a diagram of the molecular structure of the dye used for the dye layer in the embodiment of Fig. 1 (N719);

3 shows a current / voltage diagram to illustrate the electrical properties of the photovoltaic cell analogous to FIG. 1 and FIG

4 shows a schematic diagram with an energy level and slice representation to illustrate the basic mode of operation of a DNSC.

In the following, the structure and the production of the photovoltaic cell according to a first preferred embodiment of the present invention will be explained with reference to FIGS. 1 to 3. Since manufacturing aspects are important for achieving the desired properties, in the following description each effective layer according to FIG. 1 will be described and linked in connection with associated, particularly suitable production steps.

Thus, in the exemplary embodiment of the photovoltaic cell in FIG. 1, a conductive carrier substrate 10 has an underlying tissue layer 12 as PEEK tissue, which is coated with indium-doped SnO ₂ (= ITO) as the conductive layer 14 in an otherwise known manner. On these layers 12, 14, a metal oxide semiconductor layer of TiO _{2 is applied to} a thickness of 1 to 20 microns, for which purpose a 5 wt .-% Tiθ ₂ ~ solution in ethanol was sprayed onto the ITO-modified fabric and after the drying or evaporation of the solvent, the coating at a pressure of about 15000 min / cm ² over a period of 10 sec. To 10 min. was charged. Alternative ways of applying the semiconductor layer are (plasma) sputtering, corona + aerosol and screen printing.

The TiO ₂ layer 16 is then provided with a light-absorbing dye layer 18 as a monomolecular layer. In the present case, the dyestuff N719 (Solaronix, CH-Arbonne), a ruthinium metal complex of the structural formula according to FIG. 2, was used, the application to the substrate coated with metal oxide semiconductor material taking place in that the PEEK TiO ₂ substrate was placed in a 3 mmole dye solution for four hours.

The thus constructed, photoelectrically (and chemically) active layer 14, 16, 18 opposite, a counter electrode 20 is provided, which in turn has a conductive PEEK / ITO substrate 22, 24 (about 100 nm), which on the conductor side with a platinum layer of conventional thickness is coated. Concretely, the platinization was carried out by introducing the counter electrode 20 into a 0.5 nM solution of H ₂ PtCl 2 in 2-propanol for a few seconds. After removing the counter electrode from the solution, it was dried and heated at a temperature of 200 ^° C. for 10 minutes.

Both the counter electrode 20 and the photoelectrically active substrate 10 each have an electrical input and output in the form of an electrical contact electrode 28 and 30, which is realized in the illustrated embodiment by silver paint, but also in other suitable ways, in particular by Weaving appropriate conductive fibers into the fabric 12, 22, may be realized. These Feed lines 28, 30 are then used for external contacting of the solar cell in FIG. 1.

In order to realize an electrolyte layer 32 to be provided between the respective coated electrodes (FIG. 1 shows an exploded exploded view in schematic form), an electrolyte of the type PEG20000 (Aldrich) in combination with LiI (0.1 M) and I ₂ ( 0.01 M) was used. Since PEG20000 is solid at room temperature, melting was required to mix with the active redox components.

For bonding the coated counter electrode 20 to the photoelectrically coated substrate electrode 10, the electrolyte was applied in liquid form to the active layer of the electrode 10 (i.e., the ink surface 18), and the counter electrode was set while the electrolyte was still liquid. After cooling and curing of the electrolyte, an adhesive bond of the entire layer arrangement was created, whereby care was taken that the contact electrodes 28, 30 did not come into contact with the electrolyte and that no short circuit occurs between the two electrodes.

The current / voltage diagram in FIG. 3 shows, between the no-load voltage and the short-circuit current, the electrical behavior of the photovoltaic cell thus produced under ambient light and room temperature.

The present invention is not limited to the illustrated embodiment or the described process steps. Thus, for example, the substrate may also consist of conductive material (AI fibers or optionally self-coated carbon fibers), with sufficient electrical conductor properties then the conductor layer 14 may be omitted. This in turn may itself have a doped metal oxide to achieve the desired conductivity properties, as described in the exemplary embodiment, alternatively a metal (eg Ti or Al) or a conductive polymer (eg PDOT). Another variant for realizing the (main) electrode is the use of the so-called Carbotex, a fabric offered by the company Sefar, CH-Thal, which has carbon-coated polyamide fibers and makes the ITO coating unnecessary by virtue of its conductivity properties ,

Also, the application of the metal oxide semiconductor (ZnO is also used instead of TiO ₂ ) by sintering a corresponding powder, by the so-called. SoI gel process or by sputtering.

While a Ru-based metal complex was used as the dye layer, metal-free dyes are also possible, in the form of so-called organic dyes, such as AZO dyes, oligoenes, merocyanines or others.

While the counterelectrode 20 in FIG. 1 had a platinum coating 26, it is also possible, alternatively with comparatively good catalytic properties, for a SnO 2 .nano.-powder or the like. be applied.

It should also be taken into consideration that the above description is to be understood merely as an example and that other suitable process steps and / or materials are also possible for realizing the respective functionality of the individual layers.

In particular, a preferred development of the invention provides for providing the electrolyte layer 32 with acrylate resin, polyethylene oxide or polyethylene glycol, so that, in the manner of a procedure in the processing of glass fiber reinforced plastics, the solar cell arrangement of the described, inventive manner integral part from various object and / or construction projects, whereby flexibility in processing with thermal stability and rigidity can be advantageously combined with given transparency. Furthermore, an alternative procedure for realizing the photovoltaic cell according to a second embodiment of the present invention is described as a sequence of the steps required or preferred according to the invention:

a) A tissue for electrode (eg PEEK coated with ITO) as well as for the counter electrode (eg Carbotex) is cut to produce the solar cell pattern in each case to a format 1.5 x 4 cm and contacted with strip-shaped present aluminum foil and tissue adhesive tape, wherein Place a strip of commercially available aluminum foil 0.8 x 6 cm in the middle of the adhesive surface of a strip Apply tissue tape (1 x 4 cm format) so that the aluminum foil protrudes 2 cm on one long side. The electrode fabric is placed on the adhesive surface or the aluminum foil, then folded over the supernatant section of the aluminum foil and pressed. Furthermore, a fabric strip (PET 1000) is cut as intermediate fabric in dimensions of 0.8 x 4 cm.

b) Under an argon atmosphere, an IM TTIP / ethanol solution is then prepared and the electrode is removed by means of an EBFE

Airbrush gun (model Double Action CI) sprayed (0.4 bar pre-pressure (argon), spray distance to the fabric about 10 cm, twice five spray cycles on one side). Between the spray cycles, the solvent is evaporated on the fabric in the argon stream of the gun.

c) treating the assembly in a miniclave, 10 ml of bidistilled water; Heat treatment 100 ⁰ C for 12 hours, then cooling to room temperature. The elec- trode is washed with ethanol on both sides and the fabric is left for approx. 1 min. dried in a stream of hot air. d) A 3mmol dye solution (N719 / ethanol 100% absolute) is prepared; the entire dye is to be dissolved (eg by means of ultrasonic bath).

The electrode assembly is light and moisture protected inserted for about 3 hours in this dye solution, then removed, washed with ethanol and about 1 min. dried in a stream of hot air.

e) The electrolyte is prepared as follows: 20 ml acetonitrile (ACN, pure grade) with 0.0160 g TiO ₂ (about 10% to polyethylene oxide, Degussa P25), 1.3384 g LiI (0.5 M, purity 99.9%, Aldrich), 0.2538 g 12 (0.05 M, polyurethane> = 99.5%, Fluka). 0.16 g of polyethylene oxide (Mw = 2,000,000, Fluka) are over a period of 1 min. fed. This electrolyte mixture is stirred at room temperature for 12 hours.

f) To assemble the solar cell, the electrolyte is concentrated on a hotplate at 100 ^° C. until it is barely flowable. The counter electrode - when using a counter electrode from Carbotex (Fa. Sefar) may possibly be dispensed with a further coating - is placed on a glass base, the intermediate fabric (step a) in the heated electrolyte by immersion coated on both sides and placed on the counter electrode. The electrode is immersed 1 mm deep in the electrolyte and then placed on top. By resting the assembly (about 10 minutes), the curing of the electrolyte takes place, followed by a treatment in a stream of hot air (about 1 min.), So that residual solvent can evaporate. To seal both sides clear coat or a transparent resin can be applied, which in turn can be adjusted mechanical properties, eg protection against weathering and / or permanent shape fixation. Comment: Carbotex can be used untreated as a counter electrode. Otherwise you can use the method 1 analog method.

As a result, numerous advantages can be realized with the present, tissue-based technology. As a result of the structure, the arrangement is at least partially translucent even after coating with non-transparent material; moreover, the realized flexibility allows virtually any fixing and curing on different shaped surfaces.

Compared to films, a substantially larger effective surface is realized, and not least because of the larger surfaces, the metal oxide semiconductor layer (such as TiO ₂ ) can be correspondingly thinner (and thus more flexible), with the further advantages of reduced delamination and less material consumption. Thus, the electrical contacting or derivation is easily possible by means of (woven) or sewn-in threads, and the book structure which can be realized further opens up additional fields of application and application.

Claims

Pa divides claims 1. Photovoltaic cell, in particular dye-sensitized solar cell, with a conductive carrier substrate (10) coated with a metal oxide semiconductor layer (16), a dye layer (18) designed for electronic interaction with the metal oxide semiconductor layer An electrolyte layer (32) resting on the dye layer and a counterelectrode (20) connected to the electrolyte layer, characterized in that the carrier substrate and / or the counterelectrode are realized by means of a flexible fabric woven from a plurality of fibers. 2. Cell according to claim 1, characterized in that the fibers consist of an electrically conductive material or have an electrically conductive coating on a non-electrically or weakly electrically conductive core, in particular of a polymer, glass, ceramic or composite material. 3. Cell according to claim 1 or 2, characterized in that the fibers are selected from the group consisting of carbon fibers, fibers of conductive polymers, metal fibers, in particular AI fibers or combinations of these. 4. Cell according to one of claims 1 to 3, characterized in that the fabric (12, 22) with a conductive Layer (14, 24) for realizing the conductive carrier substrate is coated. 5. Cell according to claim 4, characterized in that the conductive layer comprises a doped metal oxide, a metal and / or an electrically conductive polymer. 6. Cell according to one of claims 1 to 5, characterized in that the fabric has at least one woven, electrically conductive fiber for supply and discharge of the current generated by the photovoltaic cell to a connection electrode. 7. A cell according to any one of claims 1 to 6, characterized in that the tissue has a plurality of predetermined fabric gaps and / or openings for realizing a partial transparency of the carrier substrate has, wherein preferably a width of a fabric gap between 50 microns and 500 microns , 8. Cell according to one of claims 1 to 7, characterized in that the metal oxide semiconductor layer has a preferably nanostructured trained and / or applied TiO ₂ and / or ZnO and / or BaTiθ ₃ . 9. Cell according to one of claims 1 to 8, characterized in that the metal oxide semiconductor layer by injecting and / or pressing a solution of the metal-semiconductor material into the tissue, by sintering, by a sol-gel process, by corona Aerosol, by screen printing or by plasma sputtering is applied. 10. Cell according to one of claims 1 to 9, characterized in that the dye layer has a thickness in the molecular range, in particular mono-particulate and / or nano-structured, is applied. 11. A cell according to any one of claims 1 to 10, characterized in that a dye of the dye layer is a preferably Ru-based metal complex and / or an organic dye, in particular a dye selected from the group consisting of azo dyes, oligoenes, merocyanines or mixtures of these. 12. Cell according to one of claims 1 to 11, characterized in that the electrolyte layer (32) comprises a deformable and solidifiable material, in particular an acrylate resin, wherein the material is formed so that this deformable for shaping into a predetermined shape and subsequently is solidifiable in this shaping by curing, and / or the deformable and solidifiable material is provided on or in the photovoltaic cell outside the electrolyte layer. 13. A cell according to any one of claims 1 to 12, which is designed such that it can be produced by stacking a plurality of cells as a multi-layer, wherein a preferred direction of light incidence is provided on the cell perpendicular to the direction of the multi-layering. 14. A cell according to claim 13, characterized in that the multi-layer arrangement in their respective dye layer different dyes, preferably with different absorption spectra, having. 15. A cell according to any one of claims 1 to 14, characterized in that the tissue has light-conducting fibers as fibers, which are formed so that light can be introduced into the fibers on the front side. 16. A method for producing a photovoltaic cell, in particular method for producing the cell according to one of claims 1 to 15, characterized by the steps Coating a conductive fabric with a metal oxide semiconductor layer; Applying a dyeing layer formed for electronic interaction with the metal oxide semiconductor layer; - Applying an electrolyte layer on the dye layer and - Applying a counter electrode to the electrolyte layer. 17. The method according to claim 16, characterized in that the metal oxide semiconductor layer is emulsified in Form, by plasma sputtering, by a sol-gel method and / or by pressing and / or pressing on or in the Tissue is brought. 18. The method according to claim 16 or 17, characterized in that a dye of the dye layer in dissolved form and with a coating thickness in the molecular range, in particular monomolecular, is applied to the metal oxide semiconductor layer. 19. The method according to any one of claims 16 to 18, characterized in that the application of the electrolyte layer in the flowable state to the counter electrode and the composite of tissue, metal oxide semiconductor layer and dye layer is carried out and subsequently solidified. 20. The method according to claim 19, characterized in that the solidification by solidification of the electrolyte lytschicht and / or an additionally used polymer, preferably outside of the electrolyte layer, takes place after a deformation of the overall arrangement in a predetermined shape.