DE102010023407B4 - Glass ceramic object for the production of photovoltaic elements - Google Patents

Abstract

Glass-ceramic article for use in high-temperature process steps with process temperatures of at least 400 ° C for the production of photovoltaic elements, wherein the glass-ceramic
A lithium aluminum glass ceramic is,
- Has a water content of at least 0.04 moles per liter of glass ceramic volume, measured by transmission spectroscopy, and wherein
- The content of Na ₂ O and K ₂ O in the glass ceramic in total not more than one weight percent and not more than 0.2 at% Ag, 0.5 at.% Sb, 5 ppm V, 5 ppm Mg, 5 ppm Ti in the glass ceramic are included.

Description

The invention relates to glass ceramic articles and in particular glass ceramic articles which are suitable for the production of photovoltaic elements, in particular for the production of thin film solar cells.

For the production of photovoltaic elements such as solar cells, in particular thin-film solar cells, various methods can be used depending on the respective technology. In principle, one can distinguish between high-temperature and low-temperature paths during their production. For thin-film solar cells based on crystalline silicon, in particular the high-temperature CVD path is of interest in view of high deposition rates, since this makes it possible to produce very high-quality material. Here are temperatures in the range> 1000 ° C, sometimes up to 1300 ° C common. However, the requirements for the materials involved in the production of such a process are very high, since on the one hand they must withstand the manufacturing process as unscathed as possible and on the other hand should have the most passive possible role during the process. This applies not only to the materials to be processed to the product, but also to materials incorporated in the device for producing such solar cells, such as furnace liners, as well as auxiliary elements needed during manufacture, such as carrier plates for the manufacture of individual ones Components of the solar cells.

As an alternative to the high-temperature production path for semiconductors, there are low-temperature approaches which make it possible to use cheaper substrate material; in particular, solar cells based on amorphous or μ-crystalline silicon or combinations of these are to be mentioned here.

Very promising are thin-film solar cells based on compound semiconductors with regard to their potentially achievable efficiencies. Thus, efficiencies of more than 20% could be demonstrated in the laboratory. Different production technologies are also possible for these solar cells, but according to the current state of the art, high efficiencies are generally achieved only with the aid of at least one temperature step greater than 500 ° C. or between 450-700 ° C., and these processes additionally frequently occur a corrosive atmosphere such as, for example, an atmosphere with a high proportion of chalcogen (for example sulfur or selenium) or compounds thereof, eg. B. H ₂ S and H ₂ Se.

For the production of such solar cells, the materials used, device elements and auxiliary elements should both be sufficiently resistant to heat, and in particular to be able to withstand a corrosive atmosphere at the abovementioned temperatures.

The materials, device elements and auxiliary elements used for the production of the solar cells should deliver only small amounts of substances and / or particles, preferably no substances and / or particles to the atmosphere and / or the semiconductor to be produced. So-called semiconductor poisons, which are introduced into the active layers of the solar cell in the manufacturing process, can lead to drastic power losses of a solar cell and in extreme cases render it even unusable.

The use of transparent glass-ceramics as well as alkali-free aluminoborosilicate glasses was discussed with regard to the described requirements. However, it has been shown that many of the discussed glasses are not suitable for use temperatures greater than 500 ° C., in particular in conjunction with a corrosive atmosphere, since they do not achieve the required temperature resistance and exhibit degradation phenomena. Glass ceramics, in particular transparent, but also opaque, often show high damage under a corrosive atmosphere and are partially suitable only up to temperatures of 1000 ° C, which they especially as substrate materials, but also for coating or machining processes that require high temperatures, such as ., the healing of defects of crystalline silicon thin films, makes useless.

As a carrier material, such as backing plates, fiber-reinforced composites, in particular carbon fiber reinforced carbon (CFC) are usually used. Although these materials have good properties with respect to the aforementioned problem areas at high temperatures; However, the costs are very high, especially for the cost-driven solar industry, and delivery times are sometimes long.

It is furthermore known from the prior art to use glass ceramics as substrate carriers for coating installations (eg EP 1 855 324 A1 and DE 103 46 197 B4 ). The EP 1 855 324 A1 discloses glass-ceramic materials, such as NEOCERAM ^® N-11 and Schott Robax ^® that can be used as a substrate support or carrier plates. The DE 103 46 197 B4 teaches the composition of a MAS glass-ceramic (MgO) Al ₂ O ₃ -SiO ₂ glass-ceramic), which are used as a substrate for Si thin-film semiconductors, in particular thin-layer silicon.

The DE 199 39 787 A1 teaches an aluminosilicate glass-ceramic whose content of Na ₂ O and K ₂ O in total may be less than or equal to 1 wt .-%. The water content of the glass ceramic according to the DE 10 2004 024 583 A1 is between 0.01 and 0.06 mol / l. The proportion of Na ₂ O and K ₂ O is between 0.2 and 0.8 wt .-% Die DE 103 46 197 B4 shows alkali-free aluminosilicate glass-ceramics that can be used for the substrates of solar cells.

It has been found, however, that glass-ceramic materials often have inadequate chemical resistance, especially in combination with a corrosive atmosphere at simultaneously prevailing temperatures greater than 500 ° C, which is directly reflected in a short life of these materials and thus leads to reduced reusability , In addition, there is the risk that potential semiconductor poisons, such as for compound semiconductor materials hydrogen, diffuse comparatively easily into the semiconductor from these glass-ceramic materials due to the chemical bonds of the respective microstructure at said temperatures and under the prevailing chemically aggressive conditions. This can lead to severe performance losses or even the uselessness of the produced solar cells.

The invention is therefore based on the object to provide glass ceramic articles which have a high chemical resistance to a corrosive atmosphere at high temperatures and thus show a long life or high reusability and at the same time a very good compatibility with the for Photovoltaic cells used semiconductors by having a low output of substances harmful to the semiconductor properties. This object is solved by the subject matter of the independent claims. Advantageous embodiments and further developments of the invention are specified in the respective dependent claims.

• – eine Lithium-Aluminium-Glaskeramik ist,
• – einen Wassergehalt von zumindest 0,04 Mol pro Liter Glaskeramikvolumen, gemessen mit Transmissionsspektroskopie aufweist, und wobei
• – der Gehalt an Na₂O und K₂O in der Glaskeramik in Summe höchstens ein Gewichtsprozent beträgt und maximal 0,2 Atom-% Ag, 0,5 Atom-% Sb, 5 ppm V, 5 ppm Mg, 5 ppm Ti in der Glaskeramik enthalten sind.

Accordingly, the invention provides a glass ceramic article for use in high-temperature process steps with process temperatures of at least 400 ° C for the production of photovoltaic elements, wherein the glass-ceramic

• A lithium aluminum glass ceramic is,
• - Has a water content of at least 0.04 moles per liter of glass ceramic volume, measured by transmission spectroscopy, and wherein
• - The content of Na ₂ O and K ₂ O in the glass ceramic in total not more than one weight percent and not more than 0.2 at% Ag, 0.5 at.% Sb, 5 ppm V, 5 ppm Mg, 5 ppm Ti in the glass ceramic are included.

In the determination of the water content via IR transmission spectroscopy IR spectroscopy preferably only the transmission of the first OH band - in total there are three OH bands - measured between 2.7 and 3.3 microns and then using the so-called practical molar Extinction coefficient of the total water content of the sample determined. The practical molar extinction coefficient is 110 l / (mol · cm) based on mol of H ₂ O.

The glass-ceramics, as used according to the invention for solar cell production, especially where high temperatures are used, are accordingly characterized by a very high OH content or water content derived therefrom. It is surprising here that the high OH content increases the corrosion resistance. In particular, the hydrogen which emerges from the water of the glass ceramic, in itself constitutes a semiconductor poison. However, it has surprisingly been found that the glass ceramic according to the invention is currently particularly compatible with the semiconductor layers, even if, for example, an oven used in the production is completely enclosed the glass ceramic is lined.

In particular, the combination of the high OH content with a low content of Na ₂ O and K ₂ O shows to be advantageous for achieving a high long-term stability and low emission rate of semiconducting poisons. The high long-term stability of glass-ceramic materials according to the invention, such as furnace linings and carrier plates, may possibly be explained as follows:
Typically, the interaction with a corrosive, in particular chalcogenide-containing, z. B. sulfur and / or selenium-containing atmosphere to a leaching of the glass-ceramic. This manifests itself in a release of the elements sodium, potassium and / or lithium and is associated with a salt formation (Na ₂ SO ₄ , etc.) on its surface. At the same time, this degrades the properties of the glass ceramic, which manifests itself, for example, in combustion chamber applications in a tendency to form cracks. Ultimately, cracks occur due to the gradient associated with leaching in the thermal expansion coefficient, the leads to stress formation within the glass ceramic. All of these processes will expire when the conditions for temperature, corrosive gas concentration and time are given.

Surprisingly, it has now been found that this corrosive attack can be avoided or at least brought below a critical level if, as stated above, the glass ceramic composition Na ₂ O and K ₂ O is at most 1 percent by weight and the water content is at least 0.04 mol / liter ,

Due to the high water content an H ⁺ release is possible. This is actually critical especially for processes for the production of thin-film solar cells based on compound semiconductors, since hydrogen is generally regarded as a very critical semiconductor poison. However, it is believed that high levels of hydrogen in the glass-ceramic have the advantage of providing hydrogen release, thereby forming the acids H ₂ SO ₄ and less the salts. Glass ceramics typically have high acid and alkali resistance due to their microstructure. Therefore, this process of acid formation is not critical and even leads to the advantage that the salt formation and thus the degradation of the glass-ceramic is suppressed. This then leads to an avoidance or attenuation of stress gradients, so that no cracking and this extremely in a non-existent particle delivery and a long life of glass ceramics in these processes. A low content of salt-forming alkali metals supports this mechanism.

If the ratio of water content to content of Na ₂ O and K ₂ O in the glass ceramic were too small, the mechanism of acid formation could no longer suppress salt formation. Although the mechanism of leaching would be slowed down even at high alkali content, but ultimately no longer stopped. According to a development of the invention, it is therefore provided that the ratio of the water content of the glass ceramic in mol / l to the total content of Na ₂ O and K ₂ O in percent by weight in the glass ceramic is at least 0.07 mol / (1 × wt. preferably more than 0.1 mol / (l.wt .-%, more preferably more than 0.2 mol / (l · wt .-%) is.

• (i) eine hohe chemische Beständigkeit, insbesondere gegenüber einer Auslaugung und korrosive Atmosphäre (S, Se-Atmosphäre unter mindestens 400°C, insbesondere 500°C und mehr),
• (ii) eine geringe Ausdiffusion von Elemente aus der Glaskeramik heraus,
• (iii) Eine hohe Langzeitbeständigkeit und damit eine gute Wiederverwendbarkeit, Weitere vorteilhafte Eigenschaften können sein:
• (iv) eine hohe Homogenität der Zusammensetzung,
• (v) eine genaue Oberflächenbeschaffenheit,
• (vi) eine hohe Transmission im IR-Bereich.

The glass-ceramic according to the invention exhibits the following basic properties:

• (i) a high chemical resistance, in particular to a leaching and corrosive atmosphere (S, Se atmosphere below at least 400 ° C, in particular 500 ° C and more),
• (ii) a slight outdiffusion of elements out of the glass-ceramic,
• (iii) High long-term stability and thus good reusability, Further advantageous properties can be:
• (iv) a high homogeneity of the composition,
• (v) a precise surface finish,
• (vi) high transmission in the IR range.

A preferred type of aluminosilicate glass ceramics are so-called lithium aluminosilicate glass ceramics (hereinafter also referred to as LAS glass ceramics). Among other things, these are typically characterized by a high content of crystalline phase. The invention is also applicable to other glass ceramics, such as magnesium aluminosilicate glass-ceramics.

LAS glass-ceramics with the following constituents have proven to be suitable: 60.0 to 73.0 Wt .-% SiO ₂ , 15.0 to 25.0 Wt .-% Al ₂ O ₃ , 2.2 to 5.0 Wt .-% Li ₂ O, 0.0-5.0 Wt .-% CaO + SrO + BaO, 0.0-5.0 Wt .-% ZrO ₂ , 0.0-4.0 Wt .-% ZnO, 0.0-3.0 Wt .-% SnO ₂ , 0.0-2.0 Wt .-% P ₂ O ₅ , 0.0 to 1.5 Wt .-% As ₂ O ₃ , 0.0-1.0 Wt .-% Na ₂ O + K ₂ O, where the respective Shares within the following specified ranges, 0.0-1.0 Wt .-% Na ₂ O, 0.0-0.5 Wt .-% K ₂ O.

Although LAS glass-ceramics due to the lithium contained an even higher alkali content than the proposed by the invention content of Na ₂ O and K ₂ O, but it is also to be assumed here that the water contained at least slows down the leaching of lithium. In addition, lithium is typically incorporated in the crystal phase, while the sodium and potassium ions are predominantly found in the residual glass phase. The mechanism of leaching of lithium ions therefore differs from the leaching of Na ⁺ and K ⁺ ions.

The water in the glass-ceramic can be easily introduced via crystal water present in the crystalline raw materials for producing the green glass.

The spectroscopic determination of the OH or of the corresponding water content can furthermore be carried out in a simple manner spectroscopically via the infrared absorption and the characteristic OH band.

An advantage for the durability of the glass-ceramics is furthermore an at least 30 nm, preferably at least 100 nm and especially preferably at least 500 nm thicker, largely amorphous edge region, which preferably envelops a crystalline interior without any gaps. The largely amorphous edge region forms an additional protective layer against an acid-like attack.

As already described above, an essential object of the invention is the provision of a glass-ceramic article, the temperature of at least at temperatures greater than 400 ° C, the smallest possible amount of substances, in particular so-called semiconductor poisons, and thus the otherwise mainly the Alkaline earth leaching atmosphere passivated in their effect exactly this mechanism.

The term "substances" in the context of this invention comprises both particles that are exposed to high temperatures, ie. H. at temperatures greater than 400 ° C, in particular at least 500 ° C, preferably greater than 700 ° C, and under the influence of a corrosive atmosphere from the glass ceramic object flake off, crumble or rubbed or otherwise removed, as well as chemical compounds, elements or ions , which can be delivered under the conditions mentioned by the glass ceramic article. In particular, "substances" means elements, ions and compounds which diffuse out of the glass-ceramic article and can diffuse directly or via the detour of the furnace atmosphere into functional layers of the later photovoltaic device, such as, for example, photoactive layers. Due to the diffusion of these substances into, for example, a photoactive layer, their function can be impaired, as a result of which the performance of the subsequent solar cells can be significantly reduced. In particular, the term "substances" so-called semiconductor poisons meant that in the production z. B. of solar cells can lead to electrically active defects. Depending on the solar cell technology, a whole series of elements are known that can act as a semiconductor poison, such. For example, for compound semiconductor-based solar cells halogens, boron, iron, silver, antimony, vanadium, magnesium or titanium. In general, however, especially the hydrogen present in large quantities in glass ceramics according to the invention represents a harmful substance for the semiconductor layers.

The effect of these semiconductors is based on different mechanisms, which have in common, however, that they can lead to a reduction in performance of the subsequent solar cell. For example, silver, chromium, vanadium and magnesium can form metal segregations along the grain boundaries of compound semiconductor layers, which in turn can lead to increased series resistance of the solar cells and thus to corresponding power losses. Hydrogen ions in such a material can occupy lattice sites or interstitial sites and thus restrict the p-conduction of intrinsically conductive semiconductors. Other elements can act as semiconductor poisons and occupy lattice sites in functional layers of the solar cell, whereby the growth of these layers can be hindered or electrically active defect arise, which can act as so-called. Recombination centers.

According to the invention, a glass-ceramic article comprises in each case not more than 0.2 atom% Ag, 0.5 atom% Sb, 5 ppm V, 5 ppm Mg, 5 ppm Ti. These elements can likewise act as semiconductor etchers and are therefore advantageously not more than contain said small amounts in the glass-ceramic article.

There are three particularly preferred fields of application for the use of glass ceramics in the production of solar cells.

According to a first embodiment, the glass ceramic is used directly as a substrate for thin-film solar cells, that is, the layers of the solar cells are deposited directly on the glass ceramic. This is especially intended for the production of c-Si thin-film cells. The glass ceramic should preferably remain stable during the production at temperatures above 1000 ° C. and should not corrode.

According to a second application, the glass-ceramic is used as a support plate for substrates of photovoltaic cells. In this case, the carrier plate can in principle be used for all coating methods, in particular all deposition methods for photovoltaic thin films and all coatings for wafer systems. As fulfilled by the glass ceramic requirements are: a high dimensional stability due to the low temperature expansion, no or negligible few semiconductor poisons, high temperature stability at temperatures of more than 700 ° C, no corrosion, especially in chalcogen-containing atmosphere, such as H ₂ S, H ₂ Se or S, Se) and associated long-term stability. A third application is the use of glass-ceramic as a lining or construction of ovens and process chambers. In this case, essentially the same requirements apply as when used as a carrier plate. The temperature load is generally even higher here because the glass-ceramic is permanently heated during the operating time of the furnace.

Particularly advantageously, a glass-ceramic article can also be produced in very small thicknesses, so that, for example, as mentioned above, it can be used as a substrate for thin-film solar cells. Thus, glass ceramic articles, which have only a thickness of 0.5 to 2 mm, yet largely dimensionally stable, d. H. without bending, ripples or corner elevations, are produced and this dimensional stability even in use by measures such as temperature homogeneity and turning, for example. Of support plates are obtained.

Furthermore, a glass-ceramic article preferably has a temperature difference resistance of at least 400 ° C, preferably greater than 600 ° C, more preferably greater than 700 ° C or even up to 800 ° C.

Its surface advantageously has a low roughness rms of less than 100 nm, which is necessary for a number of advanced glass-ceramic applications. For example, in the case of using the glass-ceramic article as a support plate having surface roughnesses of less than 100 nm, the caking or adhesion of the substrate to be coated thereon to the glass-ceramic article can be avoided. Glass-ceramic articles may be pre-ceramized, i. H. still as a green compact, extensively machined. For example, their surface may be ground or polished with high precision, they may be cut to size, etc., without this processing affecting the quality of the glass-ceramic article. Machining prior to the ceramization of the glass-ceramic article is particularly advantageous because a processing subsequent to the ceramization may be disadvantageous if, for example, the amorphous edge region would be damaged or even partially eliminated by the machining. In this case, a glass-ceramic article of the invention would lack an important barrier to the possible diffusion of large ions.

Preferably, a glass-ceramic article has a very good chemical resistance. According to preferred developments of the invention, a glass ceramic article has an acid resistance of class 1 according to DIN 12 116 and / or a caustic resistance of class A1 according to ISO 695 and / or a hydrolytic resistance of class HGB 1 according to DIN / ISO 719.

In addition, a glass-ceramic article may advantageously have a low coefficient of thermal expansion α _(20,700) of less than 1.5 × 10 ^-6 / K. Thus, it is excellently suitable as a carrier material for substrates to be coated, since the glass-ceramic article then exerts no influence, in the sense of compression or elongation effects, on the substrate and the layers deposited thereon. In particular, for coating processes that take place in a vacuum or under a defined atmosphere, ie, require lock systems, a high flexural strength of the glass-ceramic article is advantageous. If, for example, the glass-ceramic article bent up at the corners, this can lead to great problems, in particular during insertion and removal processes, such as glass breakage (in the coating system), which is equivalent to an additional downtime of the production plant. In addition, this would be extremely detrimental to the reusability of the article. A glass-ceramic article may therefore advantageously have a flexural strength of at least 100 MPa, measured according to EN 1288, part 5, R45.

According to another embodiment of the invention, it is possible to provide the glass-ceramic article with a coating. This coating can be applied by a chemical or physical vapor deposition method or by a liquid phase deposition method. The coating can then function as an additional diffusion barrier and keep semiconductor poisons, for example from a solar cell substrate, away from the photovoltaically active layers. Such a coating can comprise, for example, an SiO ₂ layer applied by means of medium-frequency magnetron sputtering.

Basically, all materials are possible for this coating, which result in a dense, low-pored coating and can not pass or even release semiconductors. Particularly suitable for the coating are SiO _x , Si _x N _y , Cr ₂ O ₃ , TiO ₂ , Al ₂ O ₃ , as well as combinations of these and other layers or layer systems familiar to the person skilled in the art.

On the other hand, a glass-ceramic article according to the invention emits only very little or no semiconductor poisons at the temperatures and chemically aggressive conditions prevailing in the production of the compound semiconductor. It is therefore advantageously possible to deposit such compound semiconductors on a glass-ceramic article according to the invention or to use this as a carrier plate material.

In addition, glass-ceramic articles according to the invention can also serve as intermediate substrates for layers or layer components which are combined during production into a composite layer, in particular to a semiconductor and particularly preferably to an absorber of a solar cell. In the patent US 2005/0186805 A1 discloses the production of CIS compound semiconductors (CIS = copper indium selenides), in which first constituents of the compound semiconductor are deposited on a first substrate and second constituents are deposited on a second substrate, a so-called intermediate substrate. An "intermediate substrate" is understood here to mean a carrier for holding the second constituents or precursors of the compound semiconductor, wherein the carrier or the intermediate substrate is removed again after the combination with the first constituents on the first substrate by means of a detachable connection. The intermediate substrate is therefore not part of the finished product. Advantageously, such an intermediate substrate can be reused because it is chemically and mechanically resistant and therefore reusable.

The glass-ceramic article according to the invention can in principle be manufactured and used in different geometries and also in very different sizes. Thus, not only are rectangular geometries possible, but of course also, for example, round geometries, which are particularly suitable for accommodating wafers.

In addition, glass ceramic articles described above can also be used as a lining of a furnace or a process chamber for the production of photovoltaic cells, in particular as a furnace lining for the interior of a furnace. This is particularly advantageous if the purest possible production conditions are required under a highly corrosive atmosphere. Advantageously, such a furnace lining is durable and gives little or no undesirable substances to the furnace atmosphere.

The glass-ceramic according to the invention can generally be used wherever coating technologies, in particular for photovoltaic applications, require high chemical resistance of the substrates and / or substrate carrier plates and / or intermediate substrate carrier plates and / or the furnace lining and / or in processes which are larger at temperatures 400 ° C, in particular at least 500 ° C and corrosive atmosphere are connected.

In the following the invention will be explained in more detail by means of embodiments and with reference to the drawings, wherein the same and similar elements are provided with the same reference numerals and the features of various embodiments can be combined.

Show it:

1 a cross section through a thin film solar cell;

2 a cross section through an intermediate substrate;

3 : the use as a support plate;

4 and 5 : Transmission spectra for determining the water content of glass ceramic samples;

6 and 7 SIMS spectra on glass-ceramic samples before and after exposure to a corrosive atmosphere;

8th Parts of a device for the production of thin-film solar cells.

For the following on the basis of 1 to 3 Applications described can have a LAS glass-ceramic with the above-mentioned components in weight percent: 60.0 to 73.0 Wt .-% SiO ₂ , 15.0 to 25.0 Wt .-% Al ₂ O ₃ , 2.2 to 5.0 Wt .-% Li ₂ O, and optionally other ingredients, such as 0.0-5.0 Wt .-% CaO + SrO + BaO, 0.0-5.0 Wt .-% ZrO ₂ , 0.0-4.0 Wt .-% ZnO, 0.0-3.0 Wt .-% SnO ₂ , 0.0-2.0 Wt .-% P ₂ O ₅ , 0.0 to 1.5 Wt .-% As ₂ O ₃ .

The content of Na ₂ O + K ₂ O is not more than 1.0% by weight in total. Furthermore, via water of crystallization of the raw materials, a water content of at least 0.04 moles per liter, preferably at least 0.08 moles per liter of glass-ceramic volume is contained.

At most 0.2 atomic% Ag, 0.5 atomic% Sb, 5 ppm V, 5 ppm Mg, 5 ppm Ti are contained in the green glass starting materials and correspondingly also in the glass ceramic.

A glass-ceramic article according to the invention can be produced, for example, by means of the following method. First, a green glass precursor article is prepared from the selected green glass raw materials. This green glass precursor article can then be extensively processed. It can be polished, for example, to achieve a low surface roughness. It can be tailored so that even unusual geometries can be produced.

This green glass precursor article is then first converted to a glass ceramic precursor article having a high quartz solid solution phase. This can, for example, by heating to about 660 ° C with a heating rate of about 58 K / min. happen. Subsequently, the heating rate is slowly lowered to zero, so that the volume crystallization of the high quartz solid solution phase can take place at about 790 ° C. For this, the temperature for 20 to 40 min. kept approximately constant at 790 ° C.

After completion of volume crystallization of the high quartz solid solution phase, the glass-ceramic precursor article may be cooled to room temperature. The conversion of the high quartz mixed crystal phase into a keatite mixed crystal phase can thus take place temporally and spatially separately from the preparation of the glass-ceramic precursor article with the high-quartz mixed-crystal phase.

However, it is also possible to directly convert the high quartz mixed crystal phase to the keatite mixed crystal phase, i. H. without intermediate cooling, to connect to the volume crystallization of the high quartz solid solution phase. Although the spatial and / or temporal separation of the two ceramization steps is preferred because it allows a higher dimensional stability in the conversion, but not necessarily. It may also be advantageous, due to process-economic considerations, to carry out the ceramization in one step.

To produce the keatite mixed crystal phase, the glass-ceramic precursor article can then be re-heated at a heating rate of about 30 K / min. to a maximum temperature of 900 to 1300 ° C, preferably 1100 to 1150 ° C and particularly preferably heated to 1120 ° C. The heating rate should preferably at least in a range between 14 and 36 K / min. lie. After a very short, ie a few minutes, holding time, preferably even without observing a holding time, the ceramizing glass-ceramic precursor article is then at temperatures below 400 ° C, for example with an average cooling rate between 15 to 350 K / min. and then cooled to room temperature.

The deliberate selection of the batch components described above and the ceramization in a dry oven atmosphere ensures that such a glass-ceramic substrate 10 has only unavoidable levels of semiconductor poisons. In combination with the amorphous edge region forming during the ceramization, the high OH content ensures that such a glass-ceramic substrate 10 only very little semiconductor poisons in the production of the absorber and contacting layers of a solar cell gives off and corrosion is slowed down even at high process temperatures at least.

In order to minimize the outdiffusion of semiconductor poisons from a glass-ceramic article according to the invention, it is furthermore advantageous if the amorphous edge region has as few pores or cracks as possible or generally defects. Any defect in the amorphous edge area represents a potential pathway for semiconductor poisons.

Optionally, such a glass ceramic article may still have a coating applied, for example, by a vacuum deposition method or a liquid deposition method. Advantageously, for example, a SiO ₂ coating can be applied. Such a coating provides an additional diffusion barrier for semiconductor poisons, or can prevent or at least slow down leaching.

1 shows the use of a glass-ceramic article according to the invention as a substrate 10 for a solar cell 1 , The glass-ceramic substrate 10 is between 0.5 and 21 mm thick. In the solar cell shown here 1 it is a thin-film solar cell that acts as a photoactive layer 11 a CIS (copper-indium-sulfur and / or selenium) and / or CIGS (copper-indium-gallium-sulfur and / or selenium) layer 11 having. For the exemplary embodiment, this type of solar cell was chosen because the substrate 10 must meet extreme demands for its production. Of course, solar cells of a different type, in which less extreme conditions are produced, may likewise be provided with a glass-ceramic article according to the invention as a substrate.

The illustrated thin-film solar cell 1 points next to the substrate 10 and the photoactive layer 11 typically a molybdenum back contact 12 and a zinc oxide front contact 13 on top of that with a nickel / aluminum contact grid 15 is provided for contacting. Between the photoactive layer 11 and the front contact 13 is a cadmium sulfide buffer layer 14 intended.

Such solar cells based on compound semiconductors can be produced, for example, via a so-called sequential process. Other technology options such as 2- or 3-step processes are known to those skilled in the art. In the sequential process, metallic or metal chalcogen-containing precursor layers are applied first, preferably via a sputtering process, but also electroplating, liquid coating, CVD, other PVD, etc. These precursor layers are then heated under a chalcogen-containing atmosphere and converted into compound semiconductor layers. Typical temperature profiles are with a ramp of 3 K / min, preferably> 5 K / min to 550 ° C and hold for a few minutes, preferably 60 seconds. It is also possible to vary the holding temperature, for example up to 650 ° C, the holding time also 30 s-5 min. Both elemental selenium or sulfur, their corresponding process gases (eg H2Se or H2S) or their corresponding acids can be corrosive.

Another advantage of using a glass-ceramic article according to the invention as a substrate 10 for a solar cell 1 is that such objects can be easily produced with very smooth and relatively "low-interference" surfaces. On the one hand, the glass-ceramic articles basically have an amorphous edge region, which preferably envelops the entire glass-ceramic article without gaps. Therefore, such substrates have 10 even by their nature a low roughness, which comes close to that of glass substrates. A low roughness of the substrate 10 , are then applied to the contacting and absorber layers, since unevenness of the substrate can affect the coating quality of the absorber. The unevenness of the substrate can break through the molybdenum contact and generate lattice defects in the application of the absorber.

In addition, the green compacts that make up the glass-ceramic substrates 10 be ceramized, be machined before ceramification, so that the achievable roughness can be further reduced. A glass-ceramic article may accordingly have a roughness of less than 100 rms. The processing before the ceramization does not restrict the good properties of the glass-ceramic article in any way, in particular the amorphous edge region is not damaged, since this only arises during the ceramization.

Another embodiment of a use of a glass-ceramic article according to the invention is shown in FIG 2 shown. The glass-ceramic article may here as well be the final substrate 10 for a composite layer 24 serve as well as a so-called intermediate substrate 20 , An intermediate substrate 20 serves as a temporary carrier for applying a portion of the components, for example, for a precursor layer 21 . 22 ,

The use of composite layers 24 is particularly advantageous if individual precursor layers 21 . 22 need different manufacturing conditions in their production. For example, temperature-sensitive precursors can be prepared separately from precursors, which must be produced at higher temperatures.

Such precursors 21 . 22 can be beneficial to reusable intermediate substrates 20 be applied, and with these over detachable connections 23 , for example, as in the US 2005/0186805 A1 disclosed to be connected by means of calcium or strontium fluoride layers.

The joining of the individual layer components or precursors 21 . 22 can z. B. according to the US 2005/0186805 A1 done under pressure and at high temperatures. Are the substrates used 10 or intermediate substrates 20 not sufficiently temperature resistant, it can twist the substrates 10 . 20 come. In the manufacture of solar cells, such distortions of the substrate 10 . 20 by partial detachment of the applied layers lead to strong performance losses.

In addition, chemically aggressive gases can arise in the assembly of the precursors, the lifetime of a reusable intermediate substrate 20 can shorten considerably. In the US 2005/0186805 A1 For example, selenium fumes are called which can cause corrosion of the furnace material or materials used.

It is therefore extremely advantageous if intermediate substrate 20 and substrate 10 both have a corresponding temperature resistance, and are resistant to chemical attack as long as possible.

In addition, it is also required when it comes to the composite layer 24 is an absorber layer of a solar cell, that in the preparation of the substrates 10 and 20 no semiconducting toxins are released that can affect the functionality of the solar cell.

The use of the glass ceramics according to the invention in the production plant, or in general the production process, in particular where the glass ceramic is exposed to high temperatures of more than 400.degree. C., is particularly preferred, without being limited to the illustrated embodiments.

3 shows the use of a glass-ceramic article according to the invention as a carrier 30 or substrate carrier, in particular as a carrier plate. Such carriers 30 for substrates 31 find application in a wide variety of manufacturing processes and serve predominantly for receiving or holding and transporting substrates to be processed 31 , The 3a and 3b show examples of different carrier geometries. Conceivable are rectangular, but also round geometries, which may be useful for wafer production. It can be a carrier for a single substrate or a plurality of substrates of photovoltaic cells. In addition to a smooth support plate, in principle also shaped surfaces are possible, for example with recesses or recesses for receiving the substrates.

The in the 3a and 3b featured glass ceramic carrier 30 are due to their temperature resistance and chemical resistance particularly suitable for manufacturing processes in which temperatures greater than 400 ° C, especially greater than 500 ° C in combination with a chemically aggressive atmosphere occur.

In addition, such carriers can be 30 advantageous to use as a substrate support for the production of solar cells, as they deliver in contrast to substrate carriers from the prior art under high temperature load only extremely small amounts of semiconductor poisons.

For the same reasons, it is also advantageous to provide a furnace lining for a furnace interior, in particular for furnaces in which semiconductors are produced, of glass-ceramic articles according to the invention, in this case glass-ceramic plates.

An exemplary embodiment of a method with which the water content of the glass ceramic can be determined is described below. For this purpose, the infrared absorption is measured by the OH groups, preferably the absorption by the stretching vibration of the OH groups. In particular, the transmission minimum is determined according to the OH absorption band in the wavelength range from 2.7 to 3.3 micrometers. The water content is calculated therefrom using a conversion factor, in particular the practical extinction coefficient of water based on 1 mol of H ₂ O. This practical extinction coefficient takes into account the different bond types of the OH groups, which result in frequency-shifted absorption bands that do not or only partially contribute to the transmission minimum in the wavelength range of 2.7 to 3.3 microns.

A sample of the glass ceramic polished on both sides is used with a sample thickness in the range of 1-10 mm. In general, samples with an edge length of about 20-30 mm are sufficiently large. A Nicolet FTIR spectrometer is used for the measurement. First, the sample thickness is determined exactly, for example with a digital dial gauge. The spectrometer measures a transmission curve with a sufficiently large wavelength range. For example, a transmission curve in the wavelength range of 2.5 microns to 6.5 microns recorded.

Now, the transmission minimum in the wavelength range 2.7-3.3 microns is determined. In this wavelength range, there is an absorption band of the hydroxyl group.

The absorption coefficient a in the unit cm ^-1 can now be determined in the found minimum according to the following relationship:

Here, d denote the glass thickness, and T _i the pure transmission taking into account the reflection factor P. For the pure transmission applies accordingly T _i = T / P, (2) where T denotes the transmission.

The water content c is then calculated according to the following relationship: c = a / ε, (3) where ε denotes the practical extinction coefficient of water in l mol ^-1 cm ^-1 . The value of ε for water in the stated wavelength range can be set to a good approximation with a value of 110 l.mol ^-1 .cm ^-1 . This value for the respective wavelength can be found in a suitable table. An example is the work of H. Franz and H. Scholze from the Glastechnischen Berichte, 36th Jahrg. Issue 9 page 350.

An example of a transmission spectrum shows 4 , The sample on which this spectrum was measured had a thickness of 1.01 millimeters. The P-factor is 0.91.

The transmission minimum is clear in the spectrum 45 detect in the wavelength range between 2500 nm and 3000 nm wavelength. From the reduction in transmission in the minimum 45 a water content of 0.454 moles per liter was calculated. This water content is still relatively low. Even higher values are preferred in order to improve the long-term stability of the glass-ceramic.

5 shows, as a further example, the transmission spectrum of a glass ceramic in which the spectrum was used to determine a water content of 0.0829 mol per liter. The sample thickness of 1.04 millimeters was almost identical to the sample of the spectrum in 4 , But it is clear in 5 the even clearer minimum 35 to recognize the transmission.

The effect of the high water content of the samples according to the invention is illustrated below on the basis of TOF-SIMS depth profiles. 6 shows SIMS depth profiles of the potassium content of a glass-ceramic according to the invention (sample 1a) and a comparative sample with a lower water content (sample 2a), each in the initial state after their preparation. As can be seen from the depth profiles, the potassium content of the sample 2a is higher than the potassium content of the sample 1a of inventive glass-ceramic.

The two samples were now exposed in 10 cycles to conditions that occur in the manufacture of thin-film solar cells. Per run, the samples were stored at a temperature between 500 ° C and 600 ° C in a corrosive, selenium-containing atmosphere for one hour each.

Subsequently, SIMS depth profiles were again obtained from the samples. 7 shows the course of the potassium content of the treated sample according to the invention (sample 1b) and the treated sample with a lower water content (sample 2b). After the above-described multiple treatment of the two samples in a corrosive atmosphere at process temperatures of 500-600 ° C shows a changed picture with respect to the elemental profile of potassium. Both treated samples now have an approximate potassium content, suggesting that sample 2 with lower water content leached potassium during the course of the process.

The invention further relates to a device for producing thin-film solar cells, comprising one or more glass-ceramic articles according to the invention as a lining of a furnace or a process chamber, or as a support for the substrates of the thin-film solar cells. A schematic embodiment of parts of such a device 3 shows 8th ,

The apparatus comprises an oven in this embodiment 4 whose inner wall with plates 5 are lined from inventive glass ceramic. It is also conceivable, the oven 4 from such plates 5 build so that a lining can be omitted. One or more additional plates 6 serve as substrates for substrates 7 on which the thin semiconductor layers are deposited, which then form the solar cell.

At the stove 4 are one or more containers 8th connected with precursor substances that are used for layer deposition in the oven 4 be admitted. The substrate 7 is heated so that the precursor substances on the surface of the substrate 7 Separate and convert to a semiconductive layer. Due to the process, both the carrier and the furnace lining are heated to temperatures of at least 400 ° C. In addition, there is the often highly corroding gas atmosphere acting in many cases. Many materials have only grown for a short time under these conditions. The invention here allows not only to increase the service life of components such as a furnace or process chamber liner or support for the substrates to be coated, but also to reduce the delivery of semiconductor poisons.

Claims (13)

Glass-ceramic article for use in high-temperature process steps with process temperatures of at least 400 ° C for the production of photovoltaic elements, wherein the glass ceramic - a lithium-aluminum glass-ceramic, - a water content of at least 0.04 moles per liter of glass ceramic volume, measured with transmission spectroscopy, and wherein - the content of Na ₂ O and K ₂ O in the glass ceramic in total is at most one percent by weight and not more than 0.2 at% Ag, 0.5 at% Sb, 5 ppm V, 5 ppm Mg, 5 ppm of Ti are contained in the glass-ceramic. Glass-ceramic article according to the preceding claim, characterized in that the ratio of the water content of the glass-ceramic in mol / l to the total content of Na ₂ O and K ₂ O in percent by weight in the glass-ceramic is at least 0.07 mol / (l · wt. ), preferably more than 0.1 mol / (l.wt .-%), more preferably more than 0.2 mol / (l · wt .-%). Glass-ceramic article according to one of the preceding claims, characterized in that the glass-ceramic article has a thickness of 0.5 to 21 mm. Glass-ceramic article according to one of the preceding claims, characterized in that the glass-ceramic article has a temperature difference resistance of at least 400 ° C, preferably greater than 600 ° C, more preferably greater than 700 ° C or even up to 800 ° C. Glass-ceramic article according to one of the preceding claims, characterized in that a surface of the glass-ceramic article has a roughness rms of less than 100 nm. Glass-ceramic article according to one of the preceding claims, characterized in that the glass-ceramic article has an acid resistance of class 1 according to DIN 12 116, a alkali resistance of class A1 according to ISO 695 and a hydrolytic resistance of class HGB 1 according to DIN / ISO 719. Glass-ceramic article according to one of the preceding claims, characterized in that the glass-ceramic article has a thermal expansion coefficient α _(20-700) of less than 1.5 · 10 ^-6 / K. Glass-ceramic article according to one of the preceding claims, characterized in that the glass-ceramic article has a bending strength of at least 100 MPa, measured according to EN 1288, part 5, R45. Glass-ceramic article according to one of the preceding claims, characterized in that a coating by means of a chemical or physical vapor deposition method or a liquid phase deposition method is applied. Use of a glass-ceramic article according to one of the preceding claims as a substrate for solar cells, in particular for thin-film solar cells, or as an intermediate substrate for layers or layer components, which are combined during production into a composite layer, in particular a semiconductor and particularly preferably an absorber of a solar cell. Use of a glass-ceramic article according to one of the preceding claims as a carrier in different geometries, in particular as a carrier plate for a single substrate or even a plurality of photovoltaic cell substrates. Use of a glass-ceramic article according to one of the preceding claims as a lining of a furnace or of a process chamber for the production of photovoltaic cells. An apparatus for manufacturing thin-film solar cells, comprising one or more glass-ceramic articles according to one of claims 1 to 9 as a lining of a furnace or a process chamber, or as a support for the substrates of the thin-film solar cells.

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