Method for manufacturing nanostructured thin film electrodes

TECHNICAL FIELD

The present invention relates to a method of manufacturing nanostructured thin film electrodes, and more particularly to a method of manufacturing a nanostructured thin layer of a semiconducting material on a substrate for use in an electrochemical or photoelectrochemical cell, such as a solar cell, a monitor, a battery etc.

PRIOR ART

The basic concept of nanostructured thin films is described by B. O'Regan and M. Grätzel in Nature, 353, 737 (1991), and by Grätzel et al in J. Am. Chem. Soc., 115, 6382 (1993).

The use of nanostructured thin films in monitors is described by A. Hagfeldt, L. Walder and M. Grätzel in Proc. Soc. Photo-Opt. Intrum. Engn., 2531, 60 (1995).

0 A method for manufacturing sintered porous multilayers is described by A. Kay and M. Grätzel in Sol Energy Mat. Solar. Cells, 44, 99 (1996).

Numerous patents relating to technology for nanostructured films, including manufacturing methods involving a firing step, have been issued to Grätzel et al, such as US 5,569,561, EP-A-0 333 641, US-A-5 350 644, US-A 5,525,440, US-A-5,441,827 and US-A-5,084,365. A screen printing technique is included in, for example, US 5,830,597 to Grätzel.

TECHNICAL BACKGROUND

In many applications where electrochemical or photoelectrochemical cells are used, such as in solar cells or in computer screens, there is a need to expose an electrolyte to a very large electrode surface. This could be expressed so that the electrode has a very large surface to volume ratio.

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A useful method for achieving a large area to volume ratio is to manufacture an electrode in the form of a nanostructured film, i.e. a network of interconnected nanometer-sized particles. The porosity of such a film is typically in the range of 50-60%. The particles are typically of a semiconducting material, such as a metal oxide, and the particle size is typically in the range of a few nanometers to several hundred nanometers. The thickness of a nanostructured film is typically on the order of 5-10 microns, but can be up to several hundred microns.

The electrode film is deposited on a substrate, such as a glass sheet. However, the nanostructured film must be electrically connected to peripherals. Since the substrate is typically an insulator, a conductive layer is provided on the substrate, and the nanostructured electrode is deposited on the conductive layer. A substrate (eg glass) coated with a conductive layer is called a conductive substrate (such as a conductive glass).

The function of the nanostructured film depends on the application. In a solar cell, for example, the function of the nanostructured film is to collect electrons from an excited state, created when light is absorbed into color molecules attached to the surface of the nanostructured film. The electrons are transported through the particle network of the film to the conductive substrate where the photocurrent is collected. In monitor applications, on the other hand, the nanostructured film is useful for delivering electrons to surface-colored dye molecules or to the nanostructured surface itself to effect intercalation of, for example, lithium ions. By changing the electrical potential of the conductive substrate, the apparent color of the nanostructured film can be controlled.

There are several prior art methods for making nanostructured films. Common to most of these is that the semiconducting material is applied to the conductive substrate in the form of very small particles, typically with a size of a few nanometers, which are present in a colloidal solution. These small particles are physically and electrically interconnected using a firing process. The firing process is carried out at a temperature of several hundred degrees and for a period of, typically, half an hour.

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In fact, in addition to the firing process described above, conventional methods of forming nanostructured films include several steps, each step often being rather time consuming and costly. For example, a step of preparing a colloidal solution includes measures to ensure a low degree of particle aggregation, such as the addition of organic additives. Thus, the firing step is needed not only to couple the particles, but also to remove aggregation-inhibiting organic additives in the colloidal solution by combustion. Furthermore, a film deposition step may involve the use of masks to create patterns or limit the extent of the film.

The firing step in conventional methods of forming nanostructured films sets limits on the choice of substrate material. The high firing temperature, especially in combination with a long residence time, excludes plastics as substrate material.

There is therefore a need for a rapid manufacturing method for nanostructured films which allows a wide variety of substrate materials.

0 DESCRIPTION OF THE INVENTION

The present invention aims to provide an improved method of coating a conductive substrate with a porous nanostructured film.

This object is achieved with the method according to claim 1.

The method of the invention is useful for providing a conductive substrate with a thin porous nanostructured film in a considerably shorter process time than has been possible previously. The reason for this is that no non-volatile substances, which are generally used in previously known methods, need to be mixed with the particulate material of the electrode. Therefore, no prolonged high temperature firing is required to remove such non-volatile substances.

With the method according to the invention it is thus possible to use a wider selection of substrate materials, i.e. material with temperature properties

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BRIEF DRAWING DESCRIPTION

The invention will now be described in more detail with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view showing the preparation of a particle suspension according to a step according to the present invention;

FIG. 2 is a schematic view showing the application of the particle suspension to a substrate, according to another step of the present invention;

FIG. 3 is a schematic view showing the removal of the suspending agent from the suspension, according to another step of the present invention;

FIG. 4 is a schematic view showing the compression of the particles remaining on the substrate, according to another step of the present invention; and

FIG. 5 is a schematic view showing the porous nanostructured film after compression.

FIG. 6 is a schematic view showing the steps of the present invention utilized in a continuous production line.

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DETAILED DESCRIPTION OF EMBODIMENTS

According to the invention, a porous nanostructured film is manufactured by a method in which a suspension of electrode material particles in a suspending agent is prepared, the particle suspension is deposited on a conductive substrate, the suspending agent is removed and the particles are compressed to form an electrically conductive and mechanically stable porous nanostructured film on the conductive substrate.

In the step of preparing a suspension according to the invention, which is schematically illustrated in Fig. 1, a powder 11, consisting of particles 12 of a material selected to form the nanostructured film, is added to a suspending agent 13. Of course, particles from more than a suitable substance can be used, but in order to simplify the language of the description, in the following only the case of one material will be described. It is not critical to obtain a colloidal solution.

The electrode particle material 11 is selected from any suitable conductive or semiconducting materials capable of forming a film when compressed, such as metal oxides such as T1O2, ZnO, Nb2Os and SnC> 2. Of course, mixtures of different materials are possible, such as T1O2 mixed with carbon or Fe2O3 ·

Suitable particle sizes are within the nanometer range, i.e. up to 1000 nanometers. Preferably, the majority of the particles should have a size in the range of 10-100 nanometers. The particles are added to the suspending agent, typically to a concentration of young. 20% by weight. However, it has been shown that an addition of a small amount (up to young. 1 wt%) particles of larger size, typically in the range 1-10 μχη (approximately corresponding to the thickness of the applied particle layer) improve the smoothness of the resulting nanostructured film. Furthermore, the supply of larger particles also reduces the tendency of smaller particles to adhere to the tool that provides the pressure during the compression step (as described below).

Useful suspending agents 13 are found among arbitrary suspending agents with low surface tension and which are volatile in the room environment. Preferred

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examples of such suspending agents are ethanol, methanol and acetone. For environmental and health reasons, water can also be preferred as a suspending agent. This is possible with the method of the present invention.

In the step of the invention, which is schematically illustrated in Fig. 2, the suspension 21 is deposited on a conductive substrate 22, such as a glass or plastic sheet coated with F-doped SnCl 2, ITO (ie Sn-doped . The deposition is made by any conventional method, such as spraying or brushing 23. In order to obtain a uniform nanostructured film, an even coating of suspension should be sought on the substrate, on a micrometer scale.

The step according to the invention for removing the suspending agent, which is schematically illustrated in Fig. 3, is simply based on the fact that a volatile component is evaporated 31 under favorable pressure, temperature and ventilation conditions, to leave the particles of the suspension as a particle layer 32 on the conductive substrate 22. When using a highly volatile suspending agent, such as acetone, the suspending agent removal step occupies only a time of a few minutes or less, even at room temperature and atmospheric pressure provided good ventilation. Of course, good ventilation is also necessary in the event that a suspending agent that presents health and environmental risks is used, in which case the suspending agent is preferably disposed of in a suspending agent recycling plant. It is possible to shorten the process time for removing the suspending agent by raising the temperature, reducing the pressure and / or forced ventilation. This is especially preferred in case water is used as a suspending agent.

In an embodiment (not shown) of the present invention, the steps are combined with depositing the suspension and removing the suspending agent using a roller, on the surface of which the suspension is first distributed. In a subsequent operation, the roller is then rotated to deposit the suspension on the substrate, or the remaining particles in case the volatile suspension means has already evaporated before the particles have been deposited on the substrate.

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The step to compress them on substrate! The deposited particles to form a thinner but still porous film, as schematically illustrated in Fig. 4, have several important aspects. Thus, it is necessary to ensure a good electrical contact between adjacent particles in the film and between particles and the conductive layer of the conductive substrate to enable electron transport from any particle via the conductive layer to a current collecting device connected to the conductive substrate. By applying a pressure to the deposited particles, the particles are compressed and at the same time they are pressed against the conductive layer, in order to obtain sufficient contact surfaces to allow the resulting porous film to function as an electrical conductor. The compression also provides a mechanical stability to the film. Thereby, the film adheres to the conductive substrate and exhibits sufficient strength to handle subsequent handling.

Furthermore, in order to achieve a large surface to volume ratio of the porous film, it is necessary to break up the relatively large particle aggregates used in the method according to the invention, into smaller aggregates or particles. Using a suitable pressure, which is transferred to the particles via a pressing tool with sufficient hardness, the particles are broken down into such small parts, preferably with a size in the range a few nanometers up to several hundred nanometers.

It should be noted that it is not necessary to remove the suspending agent completely before the compaction step. A small amount of the agent remaining in the particle layer is not critical to the success of the compaction step.

In the case of relatively small substrates, such as substrates of 10 cm x 10 cm, the compression step shown in Fig. 4 is performed, preferably using a very simple method in which a steel pressure plate 41 with a selected compressive force F is lowered onto the particle layer 32 which is deposited on the substrate 22. After compression, a mechanically stable nanostructured film 51 covers the substrate 22 (Fig. 5).

In the case of larger dimension substrates, such as conductive plastic films provided from rollers, it is preferred to perform the compaction step on

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In the embodiment shown in Fig. 6, a roll of conductive substrate 61, i.e. a roll of flexible material such as a plastic film provided with an electrically conductive film on the side to be provided with the electrode, arranged to provide a continuous web of conductive substrate into the nip between two pressure rollers 62, 63. The rollers 62, 63 rotate towards each other to feed the conductive substrate web 64 past the rollers, and are mutually compressed with a force P calculated to provide a suitable pressure on the substrate to form a nanostructured film, which will be described. A container 65 houses the particle suspension 21. The suspension 21 is poured on the web 64 at a distance before the rollers, in relation to the feed direction of the web, and in such a way that it flows out evenly on the web. Consequently, the suspension follows the web towards the rollers, but on its way the volatile suspending agent evaporates and leaves the clean particles on the web. As they pass the nips of the rollers, the particles are compressed to form a porous nanostructured film 56 covering the substrate, as described above.

To avoid adhesion between the particles and the press tool, it is preferred to provide the press tool with a surface material which exhibits poor adhesion to the particles, such as stainless steel, gold or fluorinated plastics such as polytetrafluoroethylene (PTFE), PVDF or PVDC. Alternatively, a thin film of a non-adhesive material, such as a 50 gm aluminum foil, may be placed on the particle film before pressing, to separate the particles from the pressing tool. After pressing, the separating film is removed.

A step sequence will now be described as an example of the practice of an embodiment of the invention.

Experimental example

The conductive substrate was a 10 cm x 10 cm x 3 mm soda glass plate coated with a conductive layer of fluorine-doped tin oxide at 8 ohms / cm² resistance. A suspension was prepared by adding 20% ​​by weight of TiO2 particles (Degussa P25) to ethanol. The suspension was applied to a thickness of 50 gm thereon

514 600 conductive layer by brushing. The ethanol was allowed to evaporate into the air, and a 50 .mu.m thick separating film of aluminum foil was swept over the particle layer. The assembly consisting of substrate, particle layer and separation film was placed between two flat stainless steel plates. A pressure of 150 kg / cm²was applied via the steel plates to the assembly to obtain a nanostructured film with young. 55% porosity.

The above experiment was then repeated with new substrates, with variation of the pressure within 100-1000 kg / cm². As a result, the porosity achieved varied among young people. 50-60%, the higher the pressure the lower the porosity. The mechanical stability and film thickness also varied as a result of the applied pressure. Optimal properties for the nanostructured film were obtained at a pressure of young. 500 kg / cm² .

(End of experiment)

The method of the invention for producing a porous nanostructured electrode has several advantages over known methods.

For example, the step of preparing a suspension is simple and fast and preferably uses only a cheap and commercially available suspending agent. The suspending agent can be selected based on environmental and health considerations. Since it is not critical to ensure the absence of particulate aggregates, there is no need to add components to prevent the formation of particulate aggregates. The particles are supplied in a powdered state which is commercially available at low cost.

The deposition step is easily done with simple methods, and is well suited for automation.

The step of removing the suspending agent is very easy when a volatile suspending agent is used. By recycling the evaporated suspending agent, the cost of the suspending agent can be kept very low, while reducing environmental and health risks.

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The step of compressing the deposited film to obtain a thin yet porous film is also performed with simple techniques. A particularly important aspect is the ability to achieve a mechanically stable and electrically conductive porous nanostructured film at room temperature. It is therefore possible to select the substrate from a wider selection of materials than is possible with conventional firing techniques. This opens up the possibility of using plastic materials that offer cheaper substrates, the possibility of manufacturing large electrodes, the possibility of manufacturing several electrodes on a large substrate that can be cut out at a later stage, and even the possibility of easily manufacturing non-planar electrodes.

However, although no firing step is necessary to form the nanostructured layer, in applications where a finishing with a functional dye or other molecule is to be performed, a short firing step may still be performed to remove impurities from the surface of the particle layer. Such a firing step, which is typically performed by blowing hot air for a few minutes (young. 400 ° C) over the electrode material, is done after the suspending agent removal step and preferably after the compression step. Such a firing step also removes any remaining traces of the suspending agent.

An important advantage of the method of the present invention is the possibility of continuous production of the porous nanostructured film. This is most pronounced in the embodiment which comprises flexible substrates, such as plastic film substrates, compressed in a rolling mill. A structure, on a microscale, on the roll surface (such as a corrugated profile cut along the longitudinal direction of the cylindrical surface of the roll) could be provided, and via the compression step this structure would then be transferred to the nanostructured layer.

It is even possible to provide a printing roll with a relief of a pattern to be reproduced on the nanostructured film. That is, a pattern to be transferred to the nanostructured film on the substrate is "printed" directly with the pressure from the roll, without the need for masks, while the loose particles remaining in the areas between the relief areas of the roll are washed away.

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The pattern can be, for example, segments, numbers or letters for use with a monitor or solar cell. Thus, in this embodiment, the nanostructured film is formed on selected surfaces of the conductive substrate using a technique similar to embossing.

Furthermore, a major advantage of the method is that all steps in the method are very fast, thereby allowing a very high productivity, especially when used in an automated process.

In fact, the steps of depositing the suspension, removing the suspending agent and pressing the particle film could be done in a cooperating operation by pouring the suspension, the suspending agent being highly volatile, onto the substrate feed side of a roller in a substrate / particle aggregate compactor. . To enhance the evaporation of the suspending agent, the ventilation and temperature surrounding the rolling mill are selected to remove the suspending agent from the particles mainly in the short time necessary for the particles to reach the nip between the upper and lower rollers of the rolling mill. Heating the rollers facilitates the removal of the suspending agent. Thus, very high productivity is possible.

Furthermore, the method of the present invention requires less energy, compared to a conventional method which requires high temperature firing and removal of organic additives.

It is obvious that the present invention can be varied in several ways in relation to the above detailed description. Such variations are not to be construed as departing from the spirit and scope of the invention, and all such modifications as may be apparent to those skilled in the art are intended to be included within the scope of the following claims.

For example, the method known per se for modifying the surface of the particles of the nanostructured film by depositing inorganic material, such as TiCl 4 in aqueous solution, can be performed before, as well as after, the compression step.

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As described above, it is further possible to treat the particles in the manufactured nanostructured film with organic dyes according to any method known in the art.

It is also possible to manufacture a nanostructured electrode consisting of several layers of nanostructured films. This is accomplished by performing the suspension coating several times, either using a suspension of the same composition each time or varying the composition of one or more of the layers to obtain a film aggregate having non-homogeneous properties. The multiple layers can be compressed between each suspension coating step, or compressed in a single work step after all separate layers have been applied.