Fabrication method of high-porous layers

0001The invention relates to a method for producing porous layers on a substrate.

0002Highly porous layers are required, for example, in photoelectrochemical or dye-sensitized solar cells. These consist of an electrically conductive substrate with a semiconductor layer applied thereon. A dye is adsorbed on the semiconductor layer. After excitation of the dye by a photon, it can transfer an electron to the semiconductor with suitably chosen properties - generally a transition metal complex. The electrical field inside the semiconductor ensures that the electron is carried away. The positive charge remaining on the dye is transferred to a redox mediator, which is in the solution filling the cell, and from there passed on to the counterelectrode. With the last electron transfer, the redox mediator is reset to the reduced state and the electrical circuit is closed.

0003It is from the pamphlet <img file="EP0826432A2_D0001.tif" />B. O'Regan et al., Letters of nature, Vol. 353, pages 737-740, (1991), the semiconductor layer by knife application of an aqueous paste of titanium dioxide of certain particle size (Degussa, P 25) and structure (30% rutile , 70% anatase), ie by pushing the paste through a mask with a pusher onto a substrate using the screen printing process or the sol-gel process. The doctored paste is first exposed to air at room temperature in approx. 1 Dried for one minute and then sintered between 450 and 550 ° C for about 15 minutes.

0004The semiconductor layer of a dye-sensitized solar cell should have a large surface area in order to give as many photons as possible the chance to excite a dye molecule. It should have good adhesion to the conductive coating to ensure perfect electrical contact. It should have a high adsorption capacity and consist of a crystalline structure. Furthermore, it should be as translucent and porous as possible.

0005Drying the doctored paste in air at room temperature and ambient humidity before sintering results in a uniform, relatively smooth surface, the roughness of which can be determined by varying the size of the titanium dioxide particles in the paste. Titanium dioxide layers produced by this method disadvantageously have a relatively low porosity.

0006A method is known from US Pat. No. 4,874,414, according to which a paste made from a liquid and particles, namely from kaolin and water, is applied to a substrate, dried in air and then sintered.

0007The object of the invention is to create a production method for highly porous layers with improved porosity compared to the aforementioned prior art.

0008The object is achieved by a method having the features of the main claim. Advantageous embodiments result from the related claims.

0009Paste is understood to be a spreadable mass. This consists of particles and a liquid.

0010The paste particles consist of the material from which the porous layer to be produced is to be made. So z. B. used to produce a porous, consisting of titanium dioxide layer of titanium dioxide particles. Basically, all metal connections are suitable. However, preference is given to compounds which have a rutile structure, such as. B. the oxides ZrO₂, VO₂, NbO₂, CrO₂, MoO₂, WHERE₂, MnO₂, RuO₂, OsO₂, IrO₂, GeO₂, SnO₂, PbO₂, TeO₂ the fluorides MnF₂, FeF₂, CoF₂, NiF₂, PdF₂, ZnF₂ and MgF₂ or which can form an anatase structure. The diameter of the particles is preferably 0.1-1000 nm.

0011Water is preferably used as the liquid for the preparation of the paste. Toluene, benzene, methanol, ethanol, acids, bases, aldehydes, ketones, ethers, alkanes, alkenes, alkynes, amines, amides, alcohols, aromatics, (hetero-) cyclic compounds are also suitable.

0012The paste is applied to the substrate, for example, by knife coating.

0013Suitable substrates are preferably made of materials that do not absorb water and that have a smooth surface. Glass, SnO₂: F on glass or metal are therefore suitable.

0014Steam is a gas that is not too far from the state of saturation, i.e. from the liquefaction area. A steam in the sense of the invention is present when a drying process at a predetermined drying temperature takes 10 minutes and longer per 10 μm layer thickness due to the prevailing ambient steam.

0015The information regarding the layer thickness relates to the porous layer after sintering. Is the layer thickness after sintering z. B. 20 microns, the drying time controlled by the degree of saturation of the steam is at least 20 minutes. The porosity increases with the time required for the drying process. In an improved embodiment, the drying time is therefore at least 30 minutes - preferably 2 hours - per 10 μm layer thickness.

0016One made of 2 g TiO₂ and 2 g of Carbowax (polyethylene glycol, M = 20,000 g / mol) and 10 ml of water were applied to various substrates with a smooth surface, namely glass and metal, by knife coating. At high relative humidity close to the saturation limit, drying was carried out at temperatures between 10-59 ° C for up to 200 hours. It was then sintered in air at 500 ° C. for 15 minutes.

0017In contrast to drying in air at room temperature and ambient humidity, ie at a comparatively low relative humidity, a highly porous layer with a rough surface was obtained according to the process.

0018The roughness of the layer could be increased even further by reducing the size of the titanium dioxide particles in the paste. The same was true for pastes made from other particles and with other liquids. In all cases, drying in a vaporous atmosphere close to the saturation state was found to be advantageous. The greater the relative humidity, the more porous the layers produced according to the process.

0019Commercial TiO was also added to the paste₂ and processes water and uses them to produce highly porous layers. As an alternative to commercially available TiO₂ became TiO₂ Made from titanium isopropoxide and water and exposed to a pressure of 40 bar at 200 ° C. The TiO made from it₂-Paste led to a different structure compared to the aforementioned product.

0020The highly porous layers produced can act as a catalyst or as a substrate for catalysts. Furthermore, these can be used for (thin layer) chromatography.