DE102011085078A1 - Method for manufacturing silicon layer on substrate in photovoltaic cell in laboratory, involves utilizing agent to form amorphous or part-crystalline silicon after heating layer at temperature below silicon melting temperature - Google Patents

Abstract

Method for producing a silicon layer on a substrate, in particular for use in a photovoltaic cell, wherein silicon microparticles (3) having an average particle size between 1-10 μm are used as layer formers, wherein the interstices between the silicon microparticles (3) are at least partially be filled with an agent that forms amorphous or semi-crystalline silicon (11) after a subsequent heating of the layer to a temperature below the silicon melting temperature.

Description

The invention relates to a method for producing a silicon layer on a substrate, in particular for use in a photovoltaic cell.

Substrates with an applied silicon layer are used in different areas. A well-known area is that of photovoltaics. There are for obtaining electrical energy from solar radiation u. a. photovoltaic systems based on inorganic solar cells used. These consist mostly of (partially) crystalline or amorphous silicon (c-Si and a-Si) or other semiconducting element compounds such as cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) or copper indium disulfide (CIS).

Here, cells made of crystalline silicon with the simplest structure of a so-called "single junction", ie only a light-absorbing layer sequence, with about 25% efficiency so far show the highest efficiency on a laboratory scale. However, they have a long energy payback time through the energy intensive process of the Czochralski process in wafer fabrication. Polycrystalline silicon solar cells achieve somewhat lower efficiency (about 20%), but can be made by manufacturing in cast silicon blocks with lower energy requirements, which reduces their relative energy payback time. The absolute values may vary depending on the system considered.

The competing thin-film technologies a-Si, CdTe, CIGS and CIS show so far only comparatively low efficiency at the module level (about 9-13%) or are of environmental concern in the case of the CdTe.

Therefore, it is expedient to provide an ecologically safe, fast and energy-efficient production of such silicon solar cells, in particular the realization of thin, pure silicon layers with good conductivity is of importance.

It is known to produce, for example, a silicon layer using nanoparticulate silicon US 2010/0047476 A1 , In this method, different sized nanoparticle fractions, which are in different proportions to one another, are used as layer formers, this nanoparticle mixture forming the precursor. The mixture may be added with a liquid silane to fill the pores. The deposited layer is then sintered to form a thin film. The nanoparticles used, which are present in the different defined size classes, have sizes of at most a few 10 nm, for example when using three different size classes of 39 nm, 16 nm and 8-9 nm.

However, the known method is extremely complicated. On the one hand, the nanoparticles are already complex to produce, either in solution or in the hot wall reactor. With regard to the use of defined different size classes, the particle production is ultimately even more complex, in particular with regard to the following required, precise classification, so that the individual size classes are as homogeneous as possible in order to process them accordingly. Finally, the nanoparticles used are also precisely weighed in terms of the required quantities for setting the appropriate proportions.

The invention is therefore based on the problem to provide a method that allows the production of a silicon layer on a substrate in a simpler manner.

To solve this problem, the invention provides a method for producing a silicon layer on a substrate, wherein silicon microparticles having an average particle size between 1-10 microns are used as a layer former, wherein the spaces between the silicon microparticles are at least partially filled with an agent which forms amorphous or semi-crystalline silicon after a subsequent heating of the layer to a temperature below the silicon melting temperature.

Unlike the known prior art, silicon microparticles according to the invention are used which have an average particle size of about 1-10 μm, preferably in the range between 1-5 μm, particularly preferably in the range between 1-3 μm. These microparticles can be prepared in a much simpler manner. It can ordinary silicon fragments or dusts, such as those incurred in the production of wafers or wafer cutting as waste products, are used and by a simple, energy-saving grinding process, eg. B. in a planetary ball mill, are shredded to the desired size. This milling process is followed, if necessary, only by screening to screen out the microparticles with the particle size in the target range. These silicon microparticles are the actual layer formers, that is to say that the silicon layer produced is based on these microparticles. The microparticles are, as will be discussed in more detail below to clean after grinding and optionally done screening, for example, abrasion or a Remove oxide layer. Subsequently, the particles should be applied as quickly as possible or handled in an inert gas atmosphere.

In order to achieve the highest possible conductivity within the layer formed from the microparticles, it is further provided according to the invention to fill the intermediate spaces between the microparticles at least in part with an agent which forms amorphous or partially crystalline silicon as part of a temperature treatment. This means that the cavities between the silicon microparticles, that is to say the pores of the microparticle layer, are filled with additional material which is processed or converted into silicon by means of a low-temperature process below the melting temperature at which the microparticles, that is to say the bulk material, would be melted can. That is, to deposit the layer, the silicon microparticles are placed in a fluid to form a dispersion, which is either itself suitable to form silicon, or to which another agent is added in addition to form silicon in the pores. This dispersion can be applied to the substrate, for example, by a printing or spinning process and thermally treated.

As a result, a layer is obtained which, on the one hand, is very simple and inexpensive to produce, since only easy-to-use silicon microparticles are used, which can be obtained from waste material by a simple grinding process. This is possible, in particular, because the relevant layer thicknesses, for example when used in solar cells, are typically greater than a few micrometers, and consequently can be readily produced from silicon microparticles as layer formers. In addition, due to the at least partial filling of the cavities with silicon given between the microparticles, the layer obtained according to the invention also exhibits a very good layer conductivity, despite the use of the microparticles.

According to a first alternative of the invention, silicon nanoparticles may be used as means for silicon formation in the region of the cavities, which melt or melt at least superficially at the heating temperature and, after cooling, form amorphous or partially crystalline silicon which connects the silicon microparticles. This means that the dispersion for forming the silicon layer contains not only the silicon microparticles but also very small silicon nanoparticles, preferably with a size between 5-250 nm, preferably between 5-100 nm. The advantage of the additional use of the nanoparticles to the microparticles is that the Nanoparticles already at a significantly lower temperature than the actual silicon melting temperature, which is required for melting of bulk silicon, at least superficially melt and thus form the corresponding conduction bridges between the microparticles by attaching to the microparticles via the melt layer. The nanoparticles usually have a shell of organic material, which, however, "burns away" at elevated temperature. This means that areas of amorphous or partially crystalline silicon are formed in the former cavities by the molten or at least fused nanoparticles, while the larger, monocrystalline microparticles retain their state of aggregation, and therefore do not melt or melt. The heating temperature is below the melting temperature for silicon of about 1410 ° C, it is preferably in the range between 500-1350 ° C, and can be chosen the lower, the smaller the silicon nanoparticles are. Preferably, therefore, the smallest possible nanoparticles are used in order to be able to produce the layer composite at the lowest possible heating temperature and consequently to be able to make the layer formation as energy-efficient as possible. Since nanoparticles are more difficult to produce than the ground microparticles, their share of the total layer should be kept as low as possible. The majority of the layer is made from "cheap" because it simply consists of waste microparticles.

The silicon microparticles are expediently dispersed together with the silicon nanoparticles in a solvent, wherein the layer is formed from the dispersion, which is subsequently heated to the temperature for melting or fusing the silicon nanoparticles. For example, but not by way of limitation, ethanol or toluene or any other solvent may be used as the solvent, so that a total of a dispersion which can be processed by a printing or spin coating process is prepared. For the purposes of the invention are also solvent mixtures for adjusting the Trocknungseigenchaften.

An alternative to using void filling nanocrystalline silicon particles is to use as the means at least one liquid silane or a mixture of several silanes which, when heated, convert to amorphous silicon which joins the silicon microparticles. This silane or silane mixture can simultaneously form the carrier liquid for the formation of the microparticle suspension, that is, it has two functions, namely the one of the carrier liquid, on the other hand, the cavity filling and silicon formation. Of course, a solvent may be used as a carrier liquid in which the silane is mixed together with the microparticles. The heating temperature is between 150 ° C-400 ° C, it depends on the used silane or silane mixture. Again, consequently, in one simple low-temperature step an amorphous silicon for filling the cavities or pores between the microparticles or to form the silicon bridges between the microparticles can be used.

For layer formation, the silicon microparticles are dispersed together with the liquid silane or the silane mixture, it being possible either to use the silane or the silane mixture purely as the carrier fluid, or to disperse the microparticles together with the silane or the silane mixture in a solvent. Using the generated dispersion, the layer is formed, which is then heated to the temperature for conversion of the silane (s). In this case, too, silicon bridges can therefore be formed between the individual microparticles, that is, even in this procedure, the silicon microparticles are quasi soldered to one another.

It is also conceivable, of course, that both silicon nanoparticles and one or more liquid silanes are used as the means for forming the voided silicon, that is, the formation of the conductivity bridges both via the melting or melting nanoparticles and via the silane conversion produced silicon occurs. Of course, the heating temperature in this case is based on the type or size of the nanoparticles used.

Here too, a dispersion of the microparticles is formed together with the nanoparticles and the liquid silane or the silane mixture, wherein the silane or silane mixture itself can already form the carrier fluid. Of course, it is also conceivable to disperse the substances in a solvent. The resulting dispersion forms the layer which is deposited on the substrate and which is then heated to the appropriate temperature to melt the silicon nanoparticles and to convert the silane (s).

As described, the size of the nanoparticles used is between 5-250 nm, preferably in the range between 5-125 nm. The smallest possible nanoparticles are preferably used in order to keep the heating temperature as low as possible.

As silane, different silanes of the type Si _n H _{2n + 2} or Si _n H _2n can be used. Useful is, for example, cyclotrisilane, cyclobutasilane, cyclopentasilane, cyclohexasilane and cycloheptasilane, but the enumeration of the useful silane-based monomers is not exhaustive. Monosilane, disilane or trisilane can also be used. H (hydrogen) may also, in whole or in part, be substituted by saturated or unsaturated alkyl groups or aryl groups.

As solvents with which the dispersion can be prepared, various hydrocarbon-based solvents can be used, for example, n-hexane, n-heptane, n-octane, benzene, toluene, xylene, as well as other solvents in the form of various ethers, and the like.

The heating temperature when using silicon nanoparticles as the only voiding silicon forming agent or as an additional means used for silane is between 500-1350 ° C, whereby the aim is to be able to use the lowest possible heating temperature by using the smallest possible nanoparticles. If a silane or a silane mixture is used as the sole silicon-forming agent, the heating temperature is between 150-400 ° C, it depends on the silane used.

In order to avoid unwanted, poorly conductive SiO ₂ passivation on the surface of the silicon microparticles or to remove any surface contaminants, they may be applied to the substrate before application to the substrate, in particular before introduction into the solvent to remove a superficial oxide layer or, for example, any metal abrasion. which are adhered from the grinding process, in particular using hydrofluoric acid (HF), after which they are processed in the absence of oxygen to avoid reoxidation. This ensures that the deposited silicon microparticles form a more conductive network through better electrical contacts. After milling, the particles are thus cleaned to create a passivation or contamination-free surface. After cleaning with HF, the particles Si-H are terminated, which improves their compatibility with the silane. After cleaning, the particles should be applied as quickly as possible or handled in an inert gas atmosphere.

In addition, it is possible to surface-functionalize the purified silicon microparticles by applying a coating. Here, organic groups on the cleaned surface can be additively bonded, whereby the oxygen and water sensitivity can be reduced. As in the case of the silanes, these are preferably saturated or unsaturated alkyl or aryl radicals.

A particularly expedient development of the method provides for using doped silicon microparticles in order to be able to form distinctly doped layers. This is particularly preferred when two or more separate layers are applied one above the other, the layers containing silicon microparticles of different doping. That means that first a first layer is applied to the substrate and baked, so therefore completed, after which a further layer is applied and baked. This allows electrical components such. B. produce solar cells with p- and n-doped and intrinsic regions. In this case, the doping can already be introduced via internally doped microparticles, but it is also conceivable to use doping, for example by diffusion methods, after the respective layer has been deposited. In principle, however, the deposition of two or more layers allows the formation of corresponding pn or pin structures.

In addition to the method, the invention further relates to a component comprising a substrate having at least one silicon layer, produced according to the method described above.

Further advantages, features and details of the invention will become apparent from the embodiments described below and with reference to the drawings. Showing:

1 a schematic diagram for explaining the method according to the invention for producing a silicon layer according to a first embodiment,

2 an enlarged schematic representation of the silicon layer formed,

3 A second embodiment of the method according to the invention,

4 a third example of the method according to the invention,

5 an example of a device according to the invention, with two layers, which are produced by the method according to the invention, and

6 a device with three layers produced according to the invention.

1 shows in the form of a flow chart a method for producing a silicon layer on a substrate according to a first embodiment variant. Starting material is a silicon fragment in the example shown 1 , as is the case for example in silicon production or wafer production. Additionally or alternatively, silicon saw dust or abrasive dust may also be used. This silicon fragment 1 is in the step 2 ground so that silicon microparticles 3 arise. The milling can be done for example in a planetary ball mill. The silicon microparticles 3 have an average particle size of 1-10 microns, preferably in the range of 1-5 microns and more preferably in the range 1-3 microns. Optionally, these microparticles are filtered out of the ground material by sieving, whereby a very sharp distribution of the screenings obtained can be achieved by the sieving. This means that the screenings contain at least 85%, preferably at least 90% of silicon microparticles in the required size range.

The silicon microparticles are then - optionally after prior purification in hydrofluoric acid to remove a SiO ₂ layer and any metallic impurities - to form a dispersion in a solvent 4 given, for example, ethanol or toluene or another of the initially mentioned (but not limiting) solvents. In addition, become the solvent 4 silicon nanoparticles 5 given, giving a total dispersion 6 arises containing the silicon microparticles 3 , the silicon nanoparticles 5 as well as the solvent 4 ,

This dispersion 6 will now be in one step 7 for example, by spinning, printing, knife coating or slot coating on a substrate 8th to form a layer 9 applied. In a following low-temperature step 10 becomes the substrate 8th together with shift 9 baked. The heating temperature is ≤ 1350 ° C, it preferably moves in a temperature range between 500 ° C-1350 ° C. The concrete heating temperature is selected so that, in conjunction with the selected holding time, at least superficial melting of the silicon nanoparticles 5 within the shift 9 takes place. During heating, the solvent volatilizes on the one hand 4 , which causes the silicon microparticles 3 anneal. The silicon nanoparticles 5 store due to size in the area of the cavities or pores between the silicon microparticles 3 at. By heating to the heating temperature, optionally with intermediate holding steps, it is now ensured that the silicon nanoparticles 5 at least superficially melt, possibly even completely melt because of their extremely small size, since the size of the on or melting temperature of the nanoparticles is significantly lower than that of large bulk silicon, such as the silicon microparticles 3 represent.

Due to this at least partial liquefaction of the silicon nanoparticles consequently forms in the region between the silicon microparticles 3 a molten silicon structure 11 which consequently at least partially fills the cavities, respectively the silicon microparticles 3 connects with each other, so quasi soldered. This improves the conductivity or the charge carrier mobility of the silicon layer formed 12 ,

A schematic diagram of the silicon layer formed 12 shows 2 , Shown are the monocrystalline silicon microparticles 3 , as well as the amorphous or partially crystalline silicon bridges 13 that ultimately the silicon structure 11 form.

3 shows in the form of a schematic representation of another embodiment of the method according to the invention, again, similar to 1 shown from a suitable silicon fragment 1 and / or silicon dust while performing a grinding step 2 the silicon microparticles 3 which are then made into a solvent 4 are given. This solvent 4 are used to form the dispersion 6 as a silicon-forming agent, liquid silane 14 or a mixture of different liquid silanes attached, as in 3 is indicated by the dashed line. Silanes are known to contain silicon, which can be deposited when heated in the form of compact silicon layers.

The resulting dispersion 6 is then in a processing step 7 by printing or spinning again on the substrate 8th to form the layer 9 Applied to the silicon microparticles here 3 , the solvent 4 as well as the silane 14 or the silane mixture.

In a subsequent low-temperature step 10 This structure is also baked out. The heating temperature is significantly lower than in the exemplary embodiment 1 , because it is only necessary to convert the silanes, a melting is not necessary here. The heating temperature is in the range of 150-400 ° C, it depends of course on the silane or silanes used 14 ,

As a result, a silicon layer is also formed here 12 on the one hand consisting of the juxtaposed silicon microparticles 3 , which after evaporation or evaporation of the solvent 4 anneal. These microparticles 3 are about the formed silicon structure 11 (or the silicon bridges formed), which fills the cavities, connected to each other, wherein this silicon formed may be either crystalline or amorphous. That is, here too, by creating bridging silicon in the cavities, a well-conductive layer 12 can be formed, which has only a low porosity.

4 shows a further process variant of the method according to the invention. This variant of the method represents a combination of in the 1 and 3 The method shown in turn obtained by suitable milling silicon microparticles 3 turn into a solvent 4 given together with silicon nanoparticles 5 and a silane 14 or a silane mixture. That is, here for the formation of the cavity silicon both silicon nanoparticles 5 as well as one or more silanes 14 be used.

The resulting dispersion 6 is again in a coating process step 7 as a layer 9 applied to the substrate and then in the step 10 heated. The heating temperature is based on the temperature used to melt or melt the silicon nanoparticle 5 is required.

In turn, a silicon layer is formed 12 consisting of the silicon microparticles not melted and thus still present as bulk material 3 and the silicon filling the cavities or the silicon structure 11 , which may be amorphous or partially crystalline, depending on what it is made from, be it the silicon nanoparticles 5 be it silane

5 shows an inventive embodiment of a device 15 , for example in the form of a pn diode. The substrate 8th has an electrical contact 16 on, on or with which contacted a first layer 12 by carrying out the method according to the invention for the first time in a variant of the method according to FIGS 1 . 3 or 4 was applied. By performing again the method according to the invention in one of the process variants according to the 1 . 3 or 4 becomes a second layer 12 ' on the first layer 12 generated. In this case, the first layer is not attacked by the second temperature step, since it already has the melting temperature of bulk silicon. The difference between the two layers is that at the layer 12 as silicon microparticles 3 those that are inherently p-doped while as silicon microparticles 3 ' to form the layer 12 ' Silicon microparticles are used, which are n-doped by nature. Alternatively to the use of already doped silicon microparticles 3 . 3 ' Of course, it is also conceivable to separately dope the individual layers after generation. There is also a second contact 17 intended.

Regardless, each of the layers 12 . 12 ' very easy to prepare as well as very conductive, after every microparticle layer 3 . 3 ' via corresponding silicon structures 11 . 11 ' are conductively connected.

6 finally shows another component 15 , wherein like reference numerals are used for the same components. Again, a substrate is provided 8th with an electrical contact 16 on which a first layer 12 , Again, a p-doped layer is applied. She is followed by a shift 12 '' , in this embodiment, an intrinsic, undoped layer of silicon on which an n-doped layer 12 ' is applied. Each of the layers 12 . 12 '' . 12 ' is separately processed according to one of the process variants as described above. The doping of the layers 12 and 12 ' can be achieved either by using already doped silicon microparticles 3 . 3 ' but it can also after application of the layer 12 first, this layer is p-doped, after which the layer 12 ' with the microparticles 3 ' is applied as undoped, pure silicon layer, then on the layer 12 ' is applied, which is then n-doped. Each of the layers 12 . 12 ' . 12 '' has a silicon structure 11 . 11 ' . 11 '' on. Here, therefore, forms a pin diode, on the other side also via a second electrical contact 17 features.

The method according to the invention is characterized primarily by the fact that silicon microparticles are used whose production can be based on waste already produced in the production of silicon (sawdust, fragments of bulk material such as for example wafer fragments or wafers with poor crystallinity). This waste material can be ground energy-saving. The resulting from the layer formation disadvantage of higher porosity and thus lower conductivity is compensated by the addition of a small amount of silicon nanoparticles and / or liquid silane. However, the main component of the deposited silicon and thus also of the finished layer still remains that produced from the energy-saving silicon microparticles, only the interstices are filled by the admixed silicon nanoparticles and / or the liquid silane. The annealing step required to produce the amorphous silicon regions formed therefrom between the microparticles also requires only a low heating temperature, so that the entire energy expenditure for the deposition of a very good conductive silicon layer can be kept low. The resulting layer contains no other fillers of other materials that may hinder optical or electrical processes in the material.

Although the invention has been further illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples, and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• US 2010/0047476 A1 [0006]

Claims (15)

Method for producing a silicon layer on a substrate, in particular for use in a photovoltaic cell, silicon microparticles ( 3 ), which have an average particle size of between 1 and 10 μm, can be used as a layer former, whereby the between the silicon microparticles ( 3 ) spaces are at least partially filled with an agent which after a subsequent heating of the layer to a temperature below the silicon melting temperature amorphous or partially crystalline silicon ( 11 ). Process according to claim 1, characterized in that silicon nanoparticles ( 5 ) which melt at the heating temperature and, after cooling, amorphous or semi-crystalline silicon ( 11 ) containing the silicon microparticles ( 3 ). Method according to claim 2, characterized in that the silicon microparticles ( 3 ) together with the silicon nanoparticles ( 5 ) in a solvent ( 4 ) are dispersed, whereby from the dispersion ( 6 ) the layer ( 9 ), which is then heated to the temperature for melting the silicon nanoparticles ( 5 ) is heated. Process according to claim 1, characterized in that as agent at least one liquid silane ( 14 ) or a mixture of a plurality of liquid silanes, which on heating in amorphous silicon ( 11 ) containing the silicon microparticles ( 3 ) converts. Method according to claim 4, characterized in that the silicon microparticles ( 3 ) together with the liquid silane ( 14 ) or the silane mixture, optionally in a solvent ( 4 ) are dispersed, whereby from the dispersion ( 6 ) the layer ( 9 ), which is then heated to the temperature for conversion of the silane (s) ( 14 ) is heated. Method according to one of claims 2 or 3 and one of claims 4 or 5, characterized in that as means both silicon nanoparticles ( 5 ) as well as one or more liquid silanes ( 14 ) be used. Method according to claim 6, characterized in that the silicon microparticles ( 3 ) together with the silicon nanoparticles ( 5 ) and the liquid silane ( 14 ) or the silane mixture, optionally in a solvent ( 4 ) are dispersed, whereby from the dispersion ( 6 ) the layer ( 9 ), which is then heated to the temperature for melting the silicon nanoparticles ( 5 ) and the conversion of the silane (s) ( 14 ) is heated. Method according to one of claims 2, 3, 6 or 7, characterized in that silicon nanoparticles ( 5 ) with a size between 5-250 nm. Method according to one of claims 4 to 8, characterized in that silane ( 14 ) of the type Si _n H _{2n + 2} or Si _n H _2n is used. Method according to one of claims 2 to 9, characterized in that the heating temperature when using silicon nanoparticles ( 5 ) between 500 ° -1350 ° C and when using a silane ( 14 ) is between 150 ° -400 ° C. Method according to one of the preceding claims, characterized in that the silicon microparticles ( 3 ) before application to the substrate ( 8th ), in particular before introduction into the solvent ( 4 ) are cleaned to remove a surface contaminant, in particular an oxide layer, in particular using hydrofluoric acid. Process according to claim 11, characterized in that the purified silicon microparticles ( 3 ) are functionalized on the surface by applying a coating. Method according to one of the preceding claims, characterized in that doped silicon microparticles ( 3 ) be used. Method according to one of the preceding claims, characterized in that two or more separate layers ( 12 . 12 ' . 12 '' ) are applied one above the other, the layers ( 12 . 12 ' . 12 '' ) Silicon microparticles ( 3 . 3 ' . 3 '' ) contain different doping. Component comprising a substrate with a silicon layer produced by the method according to one of the preceding claims.

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