DE3908156A1 - SOLUTION OF SILICON IN A METAL, COATING PROCESS AND SILICON THIN FILM AVAILABLE BY THE PROCESS - Google Patents

Abstract

A method is described for coating a material surface with a silicon thin film, silicon being dissolved in a metallic solution and the dissolved silicon then being precipitated from the solution in a temperature range in which a silicon layer is formed on the metal surface. The solvent is a mixture of gold and a metal or metals which either have a melting point which is below the deposition temperature range or which, together with gold, form a eutectic which has a eutectic temperature which is below the deposition temperature range.

Description

The present invention relates to the production of thin film from silicon for solar cells and other electronic Areas of application, the manufacture of solar cells, the are coated with a silicon thin film and in particular a method for producing such solar cells.

The advantages in the production of silicon solar cells in thin film form on a carrier layer compared to the commercially available, thick, self-supporting crystalline or polycrystalline wafers are known.

In the consumer goods sector, e.g. B. in liquid crystal displays there is great interest in television screens Silicon deposits, the electronic quality an demands are sufficient. A separation technique exists too next in that you have silicon in a molten metal dissolves so that the melt is saturated with silicon. The melt is then cooled, reducing the amount of the silicon soluble in the melt decreases. The over the amount of the shot is made at a controlled speed precipitated on a substrate.

Tin (Sn) is usually used as the metal for opening dissolve silicon that is deposited in this way should. An advantage of this is that tin is electrical in silicon is quite inert, so the inevitable incorporation of  Tin into the deposited silicon layer the electro nical properties not affected. A disadvantage is that fairly high temperatures (greater than 900 ° C) are required to remove large amounts of silicon in tin dissolve. The high deposition temperature limits the Selection of the substrate material. Another limitation both for the choice of the substrate and for the choice the process conditions and purity requirements in the much greater likelihood of a contact Mination of the silicon layer due to further impurities at high temperatures.

Gold forms a eutectic with silicon and has under the only thing that metals have is the ability to handle large quantities Solve silicon at low temperatures. The eutectic mixture consists of about 18% silicon (on atomic basis) with a corresponding eutectic Temperature of about 363 ° C. This means that with a Temperature above that, a solution of silicon can be formed in gold in the form of a melt, wherein the silicon content corresponds at least to that of the eutectic speaks.

Gold adversely affects the electronic own of the silicon layer, even if it is only in very small amounts are built into this layer. But shows that because of the low deposition temperature practically no gold in the lattice structure of the silicon layer is installed. Similarly, only very small amounts of other contaminants at such low Temperatures built in. This allows less strict clean safety requirements for the substrate material, the equipment To provide devices and the solutions used. The low ones Deposition temperatures also reduce temperature problems between the deposited silicon layer and the substrate.  

A disadvantage of gold is its high solubility of silicon in the eutectic. The large amounts of Silicon used to form a melt at low Temperatures are required cause difficulties in controlling the rate of precipitation of the silicon the solution by cooling. This reduces the crystallo graphic quality of the deposited film and enables the formation of macroscopic gold inclusions in the deposited movie.

According to an embodiment of the present invention a solution is provided which is made of silicon stands in a solvent for the deposition of a Silicon thin film coating is solved, the Solvent alone or as a mixture or alloy comprises first metal, which at a low Temperature forms a eutectic with silicon, and one second metal or a mixture of second metals that either has a melting point (s), that is below the desired deposition temperature or that form a eutectic with the first metal, whose eutectic temperature is below the desired deposition temperature is.

The present invention also relates to a method for coating a material surface with a Silicon thin film comprising the following steps:

The dissolution of silicon in a metallic solution medium with low melting point and the subsequent Precipitation of the dissolved silicon from the solution inside half a deposition temperature range around a sili form cium layer on the material surface. Herein the solvent is a mixture or an alloy from a first metal, which is a low-temperature  Eutectic with silicon forms and a second metal or a mixture of second metals, which either one Melting point below the deposition temperature range sits (s) or which is a eutectic with the first metal form, which is a eutectic temperature below the mentioned temperature range. Preferably the first metal is gold and the solvent is a mixture Gold and a second metal.

Examples of suitable metals with a low The melting points are Bi, Cd, Ga, Hg, In, Pb, Sn, Tl and Zn. Examples of metals that have a eutectic with gold lower eutectic temperature are Al, Bi, Cd, Ga, In, Pb, Sb, Sn and Tl as well as mixtures, such as. B. Low temperature eutectics containing the above metals or contain other metals that are complete with gold are miscible, e.g. B. Ag, Cu, Ni, Pd and Pt. Further Metals can melt, e.g. B. for doping silicon to be added. Examples include Al, As, Ga or Sb.

The advantage of a mixture of such metals with gold be is that the solubility of silicon in this Metals or mixtures is less than the solubility of silicon in gold at appropriate temperatures (with the proviso that such temperatures above the eutectic temperature of gold / silicon). Will these metals or starting mixtures mixed with gold, will the solubility of silicon in the final Mix at the respective temperature almost exclusively limited by its solubility in gold, while the lower limit due to its solubility in the output mixture is determined. In practice it is found that this greatly reduces the solubility in gold can be. So z. B. Solubility (in atomic percentages)  in gold-tin mixtures at 410 ° C of about 20% for pure gold to about 5.3%, 0.5% and 0.2% if one add 20, 25 and 30% by weight of tin to the gold.

The advantage of this reduced solubility is that that controls educational speeds more easily can be what better film quality and leads to a reduction in inclusions. Another Advantage is the possibility of the formation temperatures expand to temperatures lower than this with gold alone is possible. For example, one remains Mixture of 20 wt .-% tin with gold still at 278 ° C in a molten state. That is the temperature well below the eutectic temperature with gold of 363 ° C. These temperatures represent the lower limits of the Formation temperatures for the respective solutions.

A silicon film deposited in this way can under other than as an active layer in solar cells or as Substrate for transistor production for liquid crystal display (LCD) TV screens are used. Besides there are of course other possible uses.

The strong influence of tin on the solubility of Silicon in gold enables a new method for Generation of semiconductor films from molten solutions of metal alloys. In the present example, Add tin to a solution of silicon in gold Silicon solubility greatly reduced. If the solution originally was saturated with silicon, silicon would be added by this failed. Is a gold solution with both silicon also saturated with tin and this is in contact with a tin-rich compound, such as. B. pure tin or Au-Sn or tin-rich alloys further heating, more tin is dissolved in the solution.  

This reduces the solubility of silicon so that silicon fails. So you get in this special embodiment a film formation by heating the Solution and not, as conventional, by cooling it. This can be advantageous for. B. in the control of convection flow and the absorption of impurities and Inclusions in the film impact.

Experiments have shown that pure bismuth and lead Silicon basically cannot dissolve the addition of gold in a range of 5 to 50% by weight to each of these Metals but leads to an alloy that has good control silicon deposition at temperatures below 400 ° C possible. Due to the lower gold content, these can Alloys can be produced relatively inexpensively.

This method can also be carried out using other molten metal alloys as solvents and using other dissolved semiconductors. For example, Sb / Au alloys can be used as a molten alloy and Group III to V compounds and their alloys can be used as semiconductors. According to a particular embodiment, the methods listed above involve the introduction of Ge into the solution, so that the material deposited on the carrier layer is an alloy of Si and Ge. The inclusion of Ge in the solutions and its incorporation into the deposited film result in additional advantages. By adding Ge, the deposition temperature and the band gap of the deposited Si _x Ge ^{1 x} alloy are reduced. As described by Green (MA Green, IEEE Trans. Electron Devices, Vol. ED-31, pp. 681-689, 1984), with decreasing cell thickness, recombination on the cell surface becomes more important than recombination in bulk regions. By increasing the incorporation of Ge in the bulk regions or in part of the bulk region of thin-film solar cells (thin cells) than on the surface, a balance between surface and bulk recombination can be maintained. This leads to greater effectiveness due to the increased light absorption in the bulk regions with reduced bandwidth.

In some embodiments of the present invention, an optically transparent seeding layer is used between the carrier layer and the deposited silicon (or Si _x GE _{1- x} film). The purpose of this layer is to form an advantageous crystalline structure on which the silicon can be deposited in a comparatively favorable crystalline structure. The crystalline structure of the silicon is therefore independent of the choice of the underlying substrate layer. The seed layer has a relatively weak crystalline structure at the interface with the carrier layer, but it improves with increasing distance from the carrier layer. If the layer is made transparent, the distance required for good crystallinity is also not a critical variable. Preferred materials for the seed layer are ZnS, CaF ₂ , GaP, A / P and BP. aif an epitaxial growth of silicon is possible. These can be applied using a variety of known techniques, but deposition from a metallic solution is most convenient for the subsequent deposition of the silicon layer. However, this layer can also be used to passivate the surface of the immediately adjacent silicon film.

According to a preferred embodiment of the present Er is a self-supporting glass pane as a load-bearing Upper layer used for the deposition of the silicon film. The lower the deposition temperature, the wider  is the range of glass types that can be used. Example wise can at high temperatures quartz glass or "vicor" can be used. Borosilicate glasses are suitable for lower temperatures. At deposition temperatures Below 600 ° C, less expensive soda lime glass can be used will. In currently available solar cell modules on the The basis of self-supporting silicon wafers is generally mean a lime as a structural layer within the module natron flat glass with low iron content and a thickness of about 3 mm. The lowest cost for a load-bearing upper layer according to the invention.

Nucleation for centers of crystalline growth the glass surface can be structured by a Side of the upper class to which the Film or its vaccine layer is put down. These Structuring can be done using mechanical processes, such as a Rolling or by chemical or plasma etching. This structure then favors preferred orientations in the crystal film. It also changes the direction of the light entering the silicon film via the upper layer influenced. The formation of a structure on the upper area can also be used to weaken weakly capture sorbed light in the silicon layer. A Example of such a preferred geometric Structure in the upper class to favor such Small pyramids, which are formed effects Regarding the original surface of the top layer are inclined. A less precise structure as obtained by chemical etching of glass yourself as well. Other methods are also suitable  small, high density pores on the surface of the layer train on which the films are put down. A An example of this is plasma etching.

According to a further embodiment of the present invention, all cells of the entire module are produced simultaneously on the supporting substrate. Several new techniques have been developed to enable the simultaneous production of a large number of cells. One of them is the formation method shown in Figure 1 using partitioned solution growth method. In this case the molten solution, which contains the silicon in dissolved form, is stored in a suitable container which is compartmentalized by thin partitions. These partitions can be arranged with respect to the container fixed or slidable ver, so that better contact with the layer (the workpiece) on which the film should be knocked down is made possible. The purpose of such a separating device is that a wetting of the workpiece with the solution in the immediate area of the partition is avoided. The silicon is deposited by cooling the solution melt or by creating a temperature gradient between its upper regions, which are in contact with the silicon source (and possibly with Ge) and the workpiece. If the workpiece is held stationary, it is deposited on the sections of the workpiece that are below the individual chambers. In this case, the deposited layers reflect the geometry of the chambers. If the workpiece is moved slowly with respect to the solution, strips can be formed as shown in FIG. 1.

The separators described above provide one mechanical way to achieve deposition in certain  Prevent areas. The same purpose can also be used thermal or electrical path can be achieved. At for example, is that the separation devices at a higher temperature than the solution that keeps solution under the separator hotter than the surrounding solution, causing a supersaturation of this Areas with silicon can be avoided This avoids divorce in these areas.

If the workpiece moves with respect to the solution, a point source of heat, such as. For example, a heated needle can be used to give the layer formed a stripe structure similar to that shown for the end result of FIG. 1. The heated solution in the area of the needle can be used to dissolve a narrow area of a deposited film as it passes through the needle. If a transparent workpiece is present, a laser beam can be used for this purpose as a suitable local heat source, which irradiates the solution from below through the workpiece. If a strip laser is used, as in the case of the heated separation device, deposition along the strip irradiated by the laser can be avoided.

The thickness of the deposited layers can be about Temperature of the solution including temperature gradients or by varying the time period in which the solution in contact with the workpiece.

The strength of the doping in the deposited layer can be controlled by depositing the silicon from a metal solution in which the solvent is also a dopant for silicon, or by dissolving dopants in the solution or by using a combination of both variants. As will be shown later, successive coatings of different dopant content or type can be built up, which can be arranged in series or staggered to form a desired device. If the tool is arranged stationary, you are extremely flexible in the spatial distribution of these layers. On the other hand, if the workpiece is moving as shown in Fig. 1, additional techniques are required to allow changes in the properties of the deposited film with the workpiece moving. One technique for this is to maintain a temperature cycle in the metallic solutions. When the temperature in the area of the substrate is highest, the solution is no longer oversaturated with silicon and the deposition is ended. If the solution is coolest, the separation is strongest. With the help of localized heating or cooling of the substrate, the same effect can be achieved more easily due to the lower thermal measurement of the substrate compared to the metallic solution. As shown in Fig. 2, an arrangement pattern in the moving direction can be generated in both ways.

The role of temperature gradients between the top and bottom of the molten solution has already been mentioned. Lateral temperature gradients within the solution can also be used to create profiled structures as shown in FIG. 2.

Using the techniques described, Struct doors for solar cells made of superimposed silicon strips are made that differ in concentration of the dopant, in the thickness and the Ge content below divorce. An advantage of the present invention is that overlapping and separated layers without additional Masking, photolithographic measures or additional processing steps can be generated.  

Fig. 3 shows such an embodiment. The structure in the direction of the sheet remains unchanged, so that the various layers shown therein are in the form of long strips. A schematic representation of the method by means of which the layers can be deposited is shown in FIG. 4. Fig. 3 shows several solar cells, each having an n⁺-pp⁺ transition structure, which is known from the prior art. The cells are serially connected to each other in the areas in which the contact between the n⁺ and p⁺ areas is made. Contacts between such heavily doped areas do not act as rectifying transitions, but as ohmic contacts with low resistance. The rectification ability between the n⁺ and p⁺ areas, on the other hand, can be destroyed by a deliberate disturbance in the crystallographic quality of these areas. These cells also contain an "isolation area", whereby the contact area is isolated from the base body of the cell by the high lateral resistance of the p-like area. The n⁺ and p⁺ regions must be doped sufficiently high and have sufficient strength to have a low lateral resistance to the current flow. This is particularly important for the n⁺ areas, the strength of which is limited for other reasons. On the other hand, this area must be sufficiently thin to enable those carriers (charge carriers) to make a transition that were generated by light in the area of the contact point with the top layer or inoculation layer. An attempt to reduce the resulting losses is to apply a lateral temperature gradient within the molten metal in order to achieve a variation in the lateral strength or the lateral doping level or a combination of both variations. In addition, the geometry of the weld pool can be used to control the layer thickness, as described below. The p⁺ range can be similar, although in this case the benefit is less. By varying the lateral layer thickness, however, there are further advantages, e.g. B. easier control of the rear geometry of the cell, which enables an optimization of the light trapping of the cell. Another or complementary approach concerns the use of "semiconductor fingers". A method for their formation and the structures resulting therefrom are shown in FIG. 6. By periodically changing the temperature of the first bath, highly doped n ⁺⁺ finger areas can be deposited periodically and largely in accordance with the dimensions of the bath in which the deposition takes place. As a result, the lateral resistance of the n⁺ layer of the cell is reduced and the unfavorable effects of excessive doping and layer thickness are restricted to a partial area of the cell. The contact between successive cells can be limited to the n ⁺⁺ areas. In some embodiments, this can reduce the area required for the insulating region. Although the n ⁺⁺ regions in FIG. 6 are suitable as sharply delimited units, the temperature change in the corresponding bath or substrate can be controlled in such a way that the boundaries of these regions run. For example, temperature control in a single bath can produce transitions from n ⁺⁺ to n⁺ properties, for which the second bath shown in FIG. 6 is not necessary. The non-rectangular shape of the first bath favors a change in the layer thickness of the final film. It should be mentioned that the structures described do not necessarily require metallic contacts. This has many advantages. Examples of this are greater durability by eliminating possible metal-silicon interactions, eliminating shielding losses and simplifying further processing. However, such metal contacts can be produced using standard methods by precipitation on the carrier layer or on the deposited film. The formation of metal silicide contacts with epitaxial bonding to silicon in solution is another preferred embodiment.

Several cells can be arranged one above the other and, as shown in FIG. 7, connected in series. In this case, the germanium content in the less doped areas in each subsequent cell can increase in order to take advantage of the increasing red shift of the light as it passes through the cells. An advantage of such a tandem arrangement of cells is that it reduces the lateral current flow in each of the heavily doped regions of a cell. Another improvement, as shown in Fig. 8, is the provision of a bypass diode over each cell. If the cell protected by the bypass diode emits less current than predetermined due to either shielding of the module during use or due to production errors, the bypass diode provides a current path that bypasses the cell, not only that Reliability of the module but also improved the effectiveness of module production. By including a few additional cells in the module, modules in which not all cells function fully still meet all requirements.

Another embodiment of cell structures according to the invention is the double connection shown in FIG. 9. In this case, the p-layer is more heavily doped compared to the structure shown in FIG. 3. This region forms a transition with the n⁺ regions immediately above and below, the transitions being connected in parallel. Is the life of the carrier in the p-area so large that the diffusion length is greater than the layer thickness of this area, the current generated is distributed between these two transitions in a way that takes into account possible resistance losses along the top n⁺ layer . If, on the other hand, the diffusion length is smaller, a thin, highly doped p p region in the p region can be used to reduce the lateral resistance in this region. The rear n⁺ region is deposited in two stages in the structure shown in Fig. 9 to allow an accurate adjustment with respect to the p⁺ region over which the successive cells are connected.

As shown in Fig. 10, multiple cells can be arranged one above the other. In the arrangement shown, the cells in the stack are connected in parallel. The advantage of this arrangement enables diffusion lengths that are greater than the layer thickness of the p-report. This also means that each layer has less current to conduct, reducing the loss due to lateral resistance. A bypass diode can be included in the rear area of the cell similar to that described above.

The flexibility of the present invention in the manufacture of doped or undoped semiconductor layers in any desired arrangement relative to one another facilitates the inclusion of additional circuits as an integral part of the plate (of the module). Configurations of diodes, transistors, and resistors, such as shown in FIG. 11 ge, can be incorporated into the structures using the techniques and methods described herein, thereby allowing the output of each module to be controlled so that this is completely independent of temperature in the temperature range expected for the use of the module. The combination with the bypass diodes makes it possible to provide a module with an output (IV curve) that fluctuates independently of temperature, regardless of the failure of a small number of cells and / or regardless of shading of individual areas of the module is.

The insertion of additional circuits as an integral part Part of the module can not only be used to regulate the output of a module, also for generating a variety of other effects can be used.

In all of the structures described above, all of the Be realms of the n-type are replaced by areas of the p-type, if you simultaneously pass p-type areas by areas of n type replaced. Within the scope of the present invention other semiconductors and alloys than those described here level silicon layers and alloys with Ge used will. Although the breeding method described herein device of doped semiconductor layers on a precipitate a solution melt are a variety of those herein discussed techniques, features and structures in the same Other methods for film growth or Film formation applicable.

The embodiments described in the text and shown in FIGS . 1 to 11 can also be carried out using solutions in which silicon is dissolved in metal or alloy melts other than gold-based. For example, solutions based on Al, As, Bi, Cd, Cu, Ga, Hg, In, Ni, Pb, Pd, Pt, Sb, Sn, Tl and Zn or their alloys are also suitable.

The embodiments described in the text and shown in FIGS . 1 to 11 can also be modified in a conventional manner to be used in other areas of application, such as, for. B. can be used in the manufacture of large displays. Since the costs involved are less critical in the method according to the invention, certain known techniques, such as photolithography in conjunction with low-temperature deposition, can be used from the described gold solutions to produce the desired structures.

FIG. 11a shows how, for example, by utilizing the present invention, a regulating circuit as an integral part of a solar module can be produced by deposition of individual layers.

Fig. 11b shows a schematic representation of a simple voltage regulator for a solar module with (x + y) serially connected cells. x and y are freely selectable. This circuit symbolizes (approximately) the layer structure from FIG. 11a with additional contacts on the transistor and on the Zener diode.

Reference symbol list

1 melted solution
2 partition
3 generated film
4 workpiece
5 glass top layer
6 vaccination shift
7 contact area
8 Isolation area
9 n⁺ layer
10 p layer
11 p⁺ layer
12 h ⁺⁺ shift
13 bypass diode
14 zener diode
15 thin p-type layer
16 npn transistor
17 first cell
18 resistance
19 transistor or darlington element
20 resistance

Claims (24)

1. Solution consisting of silicon, dissolved in a solvent for the deposition of a thin film coating of silicon, characterized in that the solvent comprises a mixture of gold and a metal or metals, which either have a melting point below the temperature required for deposition own or which form a eutectic with gold that has a eutectic temperature below the temperature required for the deposition. 2. Solution according to claim 1, characterized in that you choose the metal or metals under Al, Bi, Cd, Ga, Hg, In, Pb, Sb, Sn, Tl, Zn, Ag, Cu, Ni, Pd and Pt. 3. Solution according to claim 1 or 2, characterized in that the solvent is selected from an alloy Au-Sn, Au-Sb, Au-Bi and Au-Pb. 4. Solution according to one of the preceding claims, characterized in that the silicon saturation content at a temperature of less than 450 ° C is at least 1% (atomic). 5. Solution according to one of the preceding claims, characterized in that the melting point of the Solvent lower than the melting point of is pure gold.   6. Solution according to one of the preceding claims, characterized in that the solvent is a Tin-gold alloy is whose tin content is between 20 and Is 30% by weight. 7. Process for coating a material surface with a silicon thin film, characterized in that that he Dissolves silicon in a metallic solvent and then the dissolved silicon from the solution within a deposition temperature range falls to a silicon layer on the material top deposit area, wherein the solvent alone or in a mixture or as an alloy comprises a first metal, the one Low temperature eutectic with silicon forms and one second metal or a mixture of second metals comprises either a melting point below the Deposition temperature range or a Eutectic with the first metal that forms a eutectic temperature below the deposition temperature range. 8. Process for coating a material surface with a silicon thin film, characterized in that that he Dissolves silicon in a metallic solvent and then the dissolved silicon from the solution inside fails in a deposition temperature range, a layer of silicon on the surface of the material to separate, wherein the solvent is a mixture of gold and a Metal or metals is either an enamel  point below the deposition temperature range own or form a eutectic with gold that a eutectic temperature below the Ab separating temperature range. 9. The method according to claim 7 or 8, characterized records that one for the precipitation of the solved Silicon from the solution from a saturated silicon solution with an initial temperature goes out and the solvent with the Material surface in contact while holding the Temperature of the solvent reduced in a controlled manner, so that the deposition of silicon from the material surface at the desired speed. 10. The method according to claim 8 or 9, characterized records that the metal or metals under Al, Bi, Cd, Ga, Hg, In, Pb., Sb, Sn, Tl, Zn, Ag, Cu, Ni, Pd and Pt selects. 11. The method according to any one of claims 7 to 9, characterized characterized in that the solvent is an alloy is made of tin and gold and has a tin content of 20 to Has 30 wt .-% and that to precipitate the dissolved silicon from the solution at a temperature of about 400 ° C and with a silicon content of about 1 to 5% (atomic) begins. 12. The method according to claim 8, characterized in that the metal is tin and that you can precipitate of the dissolved silicon at an initial temperature begins at the silicon and tin in a saturated Solution in gold and to precipitate the ge silicon dissolved the solution in with a tin source Contact is kept and the solution from the output  temperature is heated at a controlled rate so that the silicon with a certain speed fails. 13. The method according to claim 8, characterized in that the solvent is a mixture of gold and Is bismuth or lead and has a gold content of 5 to 50% by weight. 14. The method according to any one of claims 8 to 13, characterized characterized in that germanium to the solution in one certain ratio is added to a thin Film coating made of a silicon germanium alloy to manufacture. 15. The method according to any one of claims 8 to 14, characterized characterized in that the thin film on the material surface is deposited after the metal surface surface was subjected to texturing. 16. The method according to any one of claims 8 to 15, characterized characterized in that the solution in molten form is kept in a container that is thin Partitions opened in a variety of compartments is divided, the partitions with the material surface in contact so that in this area avoid wetting the material surface with the solution will, or that they are heated to prevent failure of the silicon from the solution in the area of this separation to prevent walls. 17. The method according to any one of claims 8 to 15, characterized characterized in that during the precipitation of the dissolved silicon from the solution the solution immediately kept in contact with the material surface at an early stage  and a controlled, localized Er heating certain areas of the solution and / or the Material surface is made to deposition areas to form for silicon. 18. The method according to claim 17, characterized in that that for local heating a LASER ver is applied. 19. The method according to claim 18, characterized in that the laser is a stripe LASER. 20. The method according to any one of claims 8 to 19, characterized in that characterized in that when completing the solved Silicon from the solution a temperature gradient in the solution is discontinued and maintained Direction runs parallel to the material surface, so that the thickness of the silicon thin films changes this direction changes. 21. The method according to any one of claims 17 to 20, there characterized in that to precipitate the dissolved Silicon from the solution simultaneously the material surface relative to the solution with a controlled Speed is moved. 22. Process for coating a material surface with a silicon thin film, characterized in that that the precipitation using a solution according to one of claims 1 to 6. 23. Silicon thin film, obtainable by a process according to one of claims 7 to 23. 24. Material conforming to a thin silicon film Claim 23 is coated.