EP0260223A1 - Process for the preparation of polycrystalline silicon layers by electrolytic deposition of silicon - Google Patents

Abstract

A new process for the electrolytic deposition of silicon from a melt, which contains covalent silicon compounds, in particular silicon tetrahlogenides, also aluminum halides, alkali metal halides and halides of transition metals, is carried out at relatively low temperatures of 100 to 350 ° C. in an inert atmosphere. Silicon is deposited cathodically or anodically on electrically conductive material. The silicon layers are homogeneous and adhere well to the surface. The coated materials can be used to manufacture photoconductive or photovoltaic devices.

Description

The present invention relates to the production of thin layers of elemental silicon on electrically conductive materials by electrolytic deposition of the silicon from low-melting mixtures which contain covalent silicon compounds.

The materials thus coated can be used in the manufacture of photoconductive or photovoltaic devices, e.g. of solar cells.

Methods for the electrolytic deposition of silicon are already known from the literature [R. Monnier, Chimia 37: 109 (1983); D. Elswell, J. Crystal Growth, 52, 741 (1981)].

So you can e.g. silicon deposit by melting electrolysis at temperatures of approximately 700 to 1500 ° C from melts containing silicon fluorides and oxides as well as aluminum, alkali metal and / or alkaline earth metal salts. The disadvantage of this process is the high temperatures, which cause considerable material problems.

Another method relates to the electrochemical silicon deposition from solutions of suitable silanes, for example tetrahalosilanes or trihalosilanes, which are dissolved in polar organic solvents. The The purity of the silicon layers deposited by this process, as well as their continuity and adherence to the electrically conductive base, cannot be fully satisfied; thus the use of the materials coated in this way is also impaired for the stated purposes. Attention is also drawn to the possible but undesirable chemical reaction of the halosilanes mentioned with the polar organic solvents.

It is an object of the present invention to provide a process for the electrochemical deposition of silicon which allows the production of high-purity silicon layers which may only have the necessary doping and which are continuously (coherently) formed on the corresponding substrates and adhere well to them .

The process according to the invention does not require high-temperature melt electrolysis or silicon deposition from an organic electrolysis bath, but rather uses a melt of a certain composition from which polycrystalline silicon can be electrochemically deposited in thin, continuous layers on suitable, electrically conductive material at relatively low temperatures.

• (a) ein Siliziumhalogenid,
• (b) einen Aluminiumhalogenid,
• (c) ein Alkalimetall- oder Ammoniumhalogenid und
• (d) ein Halogenid eines Uebergangsmetalles

enthält und die Elektrolyse bei Temperaturen von 100 bis 350°C in inerter Atmosphäre und gegebenenfalls unter Druck durchgeführt wird.The present invention therefore relates to a method for producing thin layers of elemental silicon on an electrically conductive material suitable as an electrode by electrolytic deposition of the silicon from a melt, which is characterized in that the melt

• (a) a silicon halide,
• (b) an aluminum halide,
• (c) an alkali metal or ammonium halide and
• (d) a halide of a transition metal

contains and the electrolysis is carried out at temperatures of 100 to 350 ° C in an inert atmosphere and optionally under pressure.

Further objects of the present invention are the electrically conductive material obtainable according to the invention provided with a thin layer of elemental silicon or the silicon layer itself, as well as its use for the production of photoconductive or photovoltaic devices, e.g. of solar cells that can be used for the direct conversion of solar energy into electrical energy.

These and other objects are explained in more detail below.

The covalent silicon compound (a) provides the silicon which e.g. can be deposited cathodically. Component (b) is used to produce a homogeneous melt (good miscibility with the silicon compounds), component (c) is the guiding principle that e.g. with component (b) (AlJ₃) can go into solution with complex formation, and component (d) represents the so-called catalyst, which reduces the silicon deposition and the quality of the silicon layers on e.g. Copper, chromium, molybdenum, nickel, iron and chromium steel or inorganic glasses e.g. from tin dioxide or tin dioxide / indium oxide mixtures, significantly improved and silicon layer formation on a silicon substrate under the conditions of the method according to the invention is made possible in the first place. Without this catalyst, no silicon deposition could be observed in the latter case.

As an alternative to the cathodic deposition of silicon from the halide melt mentioned, it is also possible to deposit silicon anodically.

Since aluminum halides have a higher binding energy than the corresponding silicon halides, aluminum should be able to reduce silicon halides. If an aluminum plate is connected as an anode in an electrolyte containing a silicon halide as component (a), a compact silicon layer forms on its surface. There is an exchange of aluminum for silicon instead of. This exchange can be carried out until the aluminum is completely replaced by silicon. A pure silicon film is obtained which, however, has insufficient stability due to the lack of a carrier. The effect of the anodic current is probably due to the removal of aluminum atoms from the electrode surface. This creates vacancies that can be occupied by silicon atoms after the binding partners have been released. The structure of a silicon layer can therefore be controlled by a predetermined current flow. Since the solubility of aluminum in silicon is very low at the temperatures used, very pure silicon layers can be produced. These layers are p-conductive because they contain traces of aluminum.

The molten salt (the electrolyte) for the anodic silicon deposition contains components (a) to (c). This electrolytic silicon deposition is carried out at temperatures of 100 to 350 ° C in an inert atmosphere and optionally under pressure, e.g. 1 to 5 bar. An anode made of aluminum is used, and suitable cathode materials are made of silicon or graphite.

The halides which are present as components (a) to (d) or (a) to (c) in the melt for carrying out the process according to the invention are, in particular, the chlorides, bromides and iodides, the latter being preferred.

Component (a) is silicon tetrahalide of the formula
(1) SiX₄,
wherein X is chlorine, bromine, preferably iodine or mixtures thereof, such as silicon tetrachloride, silicon tetrabromide or preferably silicon tetraiodide, furthermore for example SiClBr₃, SiClBr₂, SiCl₃Br, SiCl₂J₂, SiCl₂J, SiBr₃J, SiBr₂J₂ or SiBrJ₃; Halogen can also be used silanes of formulas
(2) H _n SiX _4-n and (3) Si _m Xʹ _{2m + 2} ,
wherein n and m are integers from 1 to 3 and 2 to 6, X is chlorine, bromine, iodine or a mixture thereof and Xʹ is chlorine, bromine or iodine. Examples include HSiCl₃, H₂SiCl₂, HSiBr₃, H₂SiBr₂, HSiJ₃ and H₂SiJ₂. The trihalosilanes are preferred.

Examples of di- and polysilanes of the formula (3) are Si₂Cl₆, Si₃Cl₈, Si₄Cl₁₀ and other homologues and the corresponding bromine and especially iodine compounds.

When selecting compounds of component (a) it may be necessary to ensure that the boiling point of these compounds is not too low and the volatility is therefore too high; the electrolysis in a normal pressure range - normal pressure or low overpressure could be made more difficult.

Component (b) is aluminum trihalides such as aluminum trichloride, aluminum tribromide or preferably aluminum triiodide; with component (c), the so-called conductive salt, around the chloride, bromide or preferably iodide of sodium, potassium or preferably lithium; ammonium halides, such as ammonium chloride, bromide or iodide, and also lower (C₁-C₄) tetraalkyl or alkanolammonium halides (tetraethylammonium, tetrabutylammonium halides); and in component (d), the so-called catalyst, chlorides, bromides or preferably iodides of transition metals.

In this context, transition metals are to be understood as the metals which are in the so-called subgroups (B groups, IB - VIIB and VIII) of the periodic system of the elements. Representatives of these groups include copper, zinc, scandium and the lanthanides, for example erbium or gadolinium; Titanium, vanadium, chromium, manganese, iron, cobalt and nickel (see NA Lange, Handbook of Chemistry, 10th Ed. 1961, Mc Graw Hill Book Co.).

Preferred halides of these transition metals which can be used as catalysts are chromium (II) iodide (CrJ₂), manganese (II) iodide (MnJ₂), iron (II) iodide (FeJ₂), nickel iodide (NiJ₂), copper (I) iodide ( CuJ), hafnium (IV) iodide (HfJ₄) or vanadium (II) iodide (VJ₂).

Mixtures of the halides mentioned as component (d) can also be used, e.g. those of VJ₂ and NiJ₂, NiJ₂ and FeJ₂, FeJ₂ and CrJ₂, VJ₂, NiJ₂ and FeJ₂ or NiJ₂, FeJ₂ and CrJ₂, the mixing ratios of which can have a wide range.

The molten salts used in the process according to the invention preferably contain silicon tetrabromide or silicon tetraiodide as component (a), aluminum triiodide as component (b), lithium iodide as component (c) and vanadium (II) iodide or in particular the aforementioned salt mixtures as component (d).

Components (a) to (c) can also be mixtures of the specified halides.

Components (a) to (d) are used in the melt in approximately the following amounts: 20 to 90, preferably 20 to 75% by weight of component (a), 5 to 95, preferably 20 to 60% by weight of component (b), 1 to 20% by weight of component (c) and 0.1 to 10% by weight of component (d).

In particular, the melt contains the components in the following amounts: 40 to 75% by weight of component (a), 20 to 50% by weight of component (b), 1 to 12% by weight of component (c) and 0 , 1 to 5 wt .-% of component (d).

Components (a) to (d) must have a very high chemical purity. Corresponding processes for producing such high-purity compounds are known from the literature (cf., for example, RC Ellis, J. Elektrochem. Soc. 107, 222 (1960) - production of silicon tetraiodide and use for the production of silicon).

The production of pure aluminum iodide and lithium iodide is discussed below.

The method according to the invention can be carried out in an electrolysis vessel of the usual type. The vessel can e.g. be made of glass, in particular quartz glass or of a non-corroding metal and optionally contain a porous sintered plate made of quartz, a metal or ceramic material as a partition between the anode and cathode spaces. Such a partition can e.g. prevent the conversion of the halogen (e.g. Cl₂ or J₂) formed anodically (using an inert anode) on the cathode.

However, since the halogens formed quickly escape from the gold electrode space at the electrochemical silicon deposition temperatures required according to the invention, a partition between the anode and cathode spaces is generally not necessary.

The escaping gaseous halogens can be collected and separated in a fractionation column connected to the electrolysis vessel.

If the silicon deposition is carried out at a predetermined cathodic potential, a reference electrode is generally used, which is separated from the cathode space by a diaphragm (porous sintered plate). A suitable reference electrode is e.g. Made of high-purity aluminum (99.999%), which is in an aluminum halide / alkali metal halide melt (e.g. AlJ₃ / LiJ) (reference element). The reference electrode is used as the third currentless electrode. With it you can e.g. check the electrical conditions (e.g. potential changes) during the electrolysis process.

Suitable electrode materials for cathodic silicon deposition are: Under the conditions of electrochemical deposition, corrosion-resistant metals / alloys or semimetals or non-metals, such as copper, chromium, molybdenum, nickel, iron, platinum or stainless steels, such as chromium steel, and preferably aluminum, silicon or graphite as cathode material and platinum, silicon or graphite as (inert) anodes material. Particularly corrosion-resistant materials are molybdenum, platinum, graphite and silicon. For the anodic silicon deposition, the anode material is made of aluminum, as indicated, while the cathodes are preferably made of graphite or silicon.
The silicon anodes can be etched with a mixture of 5 parts nitric acid, 3 parts concentrated hydrofluoric acid, 3 parts acetic acid and 0.1 part bromine before use. Their surfaces are then designed in such a way that they are hardly attacked anodically and can thus serve as inert anodes.

The working temperature for carrying out the method according to the invention from 100 to 350 ° C is by indirect heating of the electrolysis vessel, e.g. achieved with a suitable, electrically heated heating bath. For melts which contain the individual components as iodides, temperature ranges from preferably 200 to 350 ° C and in particular from 260 to 320 ° C can be specified.

The electrochemical deposition of the silicon is carried out at a current density of approximately 0.5 to 20, preferably 1 to 20, in particular 1 to 10 or 1 to 5 mA / cm 2. The electrochemical silicon deposition can be carried out galvanostatically or potentiostatically using a conventional energy source.

The power yield (power consumption) is in the range of about 50 to 100%, usually 100%, i.e. corresponds to the theoretical value and indicates that there are practically no side reactions, e.g. Dimer or polymer formation takes place, which could reduce the current efficiency.

The duration of the electrochemical deposition depends on the thickness of the desired silicon layer and thus fluctuates within wide limits. A period of about 1 to 24, preferably 1 to 10 hours can be mentioned as an example. The thickness of the silicon layer on the electrically conductive bodies used as electrodes can be specified as 0.01 to 300, preferably 0.01 to 100 μm.

Since the high-purity compounds (e.g. components (a) and (b)) used in the process according to the invention are sensitive to atmospheric moisture, the electrochemical silicon deposition is carried out in an inert atmosphere at normal pressure, optionally also at about 1 to 5 bar gauge pressure. Before starting the process, the electrolytic cell is filled with an inert gas, e.g. Flushed with nitrogen or argon, creating an inert gas atmosphere that remains throughout the process. Components (a) to (d) are also usually filled into the electrolytic cell under inert conditions (dry blox).

The process according to the invention can be used to produce relatively large-area, uniform polycrystalline silicon layers which are firmly bonded to the electrically conductive base. The coated materials thus obtained show very good electrical and thermal dissipation, so that they e.g. can be used for the production of or in photoconductive or photovoltaic devices. Photovoltaic devices are e.g. (Silicon) solar cells that are capable of converting light energy into electrical energy (Photo-Volta effect).

Doping to p- or n- (p = positive, n = negative charge carriers) conductive material can be achieved by appropriate addition of suitable compounds during the electrochemical deposition of the silicon. Suitable are e.g. BJ₃, GaJ₃ or JnJ₃ for the production of p-type material and PJ₃, AsJ₃ or SbJ₃ for the production of n-type material.

For the process according to the invention for the production of silicon layers i.a. required aluminum iodide and lithium iodide, new processes for the preparation of these compounds in very pure form are given below. The following is carried out in detail:

Aluminum iodide is usually made from the elements (aluminum and iodine) at higher temperatures and in an inert atmosphere. As by-products it contains iodine and certain impurities from the starting components. The cleaning of the aluminum iodide thus obtained is very cumbersome. If very pure starting materials (aluminum and iodine) are used, the conversion takes place only very slowly and incompletely.

It has now been found - and this is another object of the present invention - that very pure aluminum iodide can be produced from aluminum and hydrogen iodide, the hydrogen iodide being expediently formed in situ from iodine and hydrogen. The hydrogen iodide can be produced from iodine and hydrogen in the presence of a platinum catalyst at around 500 ° C.

It is also possible to work without a platinum catalyst and thus avoid contamination of the hydrogen iodide and platinum if the reaction is carried out in the presence of traces of water at temperatures of about 600 to 800 ° C.

• (1) reines, gegebenenfalls mit Chlorwasserstoffsäure angeätztes Aluminium bei Temperaturen von 300 bis 500°C, vorzugsweise 350 bis 450°C, und in Gegenwart katalytischer Mengen von Wasser mit
• (2) Jodwasserstoff umsetzt.

The inventive method for producing aluminum iodide (AlJ₃) from aluminum and hydrogen iodide is characterized in that

• (1) pure, optionally etched with hydrochloric acid aluminum at temperatures of 300 to 500 ° C, preferably 350 to 450 ° C, and in the presence of catalytic amounts of water with
• (2) implement hydrogen iodide.

The hydrogen iodide is expediently prepared in-situ from iodine and hydrogen at temperatures from 600 to 800 ° C. and in the presence of catalytic amounts of water and used directly for the further reaction with aluminum.

The catalytic amounts of water are introduced into the process, for example, by passing the hydrogen through a wash bottle of water before the reaction with the iodine.

The preferred component (c) - lithium iodide - can be present as mono-, di- or trihydrate and is usually purified by recrystallizing the trihydrate (LiJ · 3H₂0) from water.

It has been found - and this is also another object of the present invention - that a very pure product can be obtained by zone melting the trihydrate mentioned at 60 to 100 ° C., preferably 60 to 80 ° C., which can be dewatered by subsequent drying, working at temperatures up to 250 ° C and under vacuum.

The mono- or dihydrate mentioned can also be used for the zone melting process, temperatures of 50 to 140 ° C. being possible.

The invention is illustrated further in the examples below, but is not restricted thereto.

Unless otherwise stated, parts and percentages are by weight.

example 1

(a) A rectangular silicon wafer (dimensions 40/8/2 mm), which has been sawed off from a silicon single crystal, is treated in a 20% strength alkaline aqueous solution of a commercial surfactant for 1 hour at 90 ° C., washed with bidistilled water and then at 150 ° C air dried.

The silicon single crystal is drawn from a silicon melt by known methods; by appropriate doping, it is made p- or n-type and has a resistance of 0.04 ohm cm.

The silicon wafer cleaned as stated is installed as a cathode in an electrolysis cell. The anode is made of graphite or silicon. Anode and cathode compartments can be separated from one another by a porous sintered plate in order to prevent a possible conversion of the halogen at the cathode. As a rule, however, the halogen (iodine) escapes from the electrolysis cell so quickly that it is not necessary to separate the anode and cathode compartments. The escaping halogen can be recovered, for example, by condensation.

In the electrolytic cell, a compound mixture consisting of 73 wt .-% SiJ₄, 22 wt .-% AlJ₃, 3.5 wt .-% LiJ and 1.5 wt .-% VJ₂ is added, which is then at 310 ° C and a current density of 2 mA / cm² is electrolyzed for 4 hours. The voltage depends on the distance between the electrodes. It is in the range of 300 to 500 mV. The electrolysis is carried out under inert conditions in a closed system. For this purpose, the electrolysis cell is flushed with nitrogen or argon before the electrolysis; the inert gas atmosphere is maintained during the electrolysis.

The measured current yield is slightly higher than 100%, probably due to a certain thermal decomposition of SiJ₄ during the electrolysis.

After the electrolysis had ended, a continuous, well-adhering silicon layer of about 10 μm thickness had formed on the silicon wafer. The material coated in this way shows very good electrical and thermal dissipation.

(b) A likewise continuous and well-adhering silicon layer is obtained if a molten salt of 43% by weight SiJ₄, 43% by weight AlJ₃, 12% by weight LiJ and 2% by weight VJ₂ as described in (a ) specified, but electrolyzed at a current density of 5 mA / cm². The current yield is 100%.

• (c₁) 73 Gew.-% SiJ₄, 22 Gew.-% AlJ₂, 3,5 Gew.-% LiJ, 0,75 Gew.-% VJ₂ und 0,75 Gew.-% NiJ₂;
• (c₂) 73,5 Gew.-% SiJ₄, 22 Gew.-% AlJ₃, 3,7 Gew.-% LiJ, 0,65 Gew.-% NiJ₂ und 0,15 Gew.-% FeJ₂;
• (c₃) 73 Gew.-% SiJ₄, 21,9 Gew.-% AlJ₃, 3,5 Gew.-% LiJ, 1,5-Gew.-% CrJ₂ und 0,1 Gew.-% FeJ₂;
• (c₄) 73 Gew.-% SiJ₄, 22 Gew.-% AlJ₃, 3,4 Gew.-% LiJ, 0,75 Gew.-% VJ₂, 0,75 Gew.-% NiJ₂ und 0,1 Gew.-% FeJ₂;
  und
• (c₅) 73,5 Gew.-% SiJ₄, 22 Gew.-% AlJ₃, 3,74 Gew.-% LiJ, 0,37 Gew.-% NiJ₂, 0,37 Gew.-% CrJ₂ und 0,02 Gew.-% FeJ₂

bei jeweils 320°C und einer Stromdichte von jeweils 2,5 mA/cm² während 4 Stunden elektrolysiert. Die Spannung ist vom Elektrodenabstand ab­hängig. Sie liegt etwa im Bereich von 300 bis 500 mV. Die Elektrolysen werden unter inerten Bedingungen in einem geschlossenen System durch­geführt. Die Elektrolysezelle wird dazu jeweils vor der Elektrolyse mit Stickstoff oder Argon gespült; die Inertgasatmosphäre bleibt während der Elektrolyse aufrechterhalten.(c) in an electrolysis cell according to Example (1a), molten salts of the following compositions:

• (c₁) 73 wt% SiJ₄, 22 wt% AlJ₂, 3.5 wt% LiJ, 0.75 wt% VJ₂ and 0.75 wt% NiJ₂;
• (c₂) 73.5 wt% SiJ₄, 22 wt% AlJ₃, 3.7 wt% LiJ, 0.65 wt% NiJ₂ and 0.15 wt% FeJ₂;
• (c₃) 73 wt% SiJ₄, 21.9 wt% AlJ₃, 3.5 wt% LiJ, 1.5 wt% CrJ₂ and 0.1 wt% FeJ₂;
• (c₄) 73% by weight SiJ₄, 22% by weight AlJ₃, 3.4% by weight LiJ, 0.75% by weight VJ₂, 0.75% by weight NiJ₂ and 0.1% by weight % FeJ₂;
  and
• (c₅) 73.5 wt% SiJ₄, 22 wt% AlJ₃, 3.74 wt% LiJ, 0.37 wt% NiJ₂, 0.37 wt% CrJ₂ and 0.02 wt .-% FeJ₂

electrolyzed at 320 ° C and a current density of 2.5 mA / cm² for 4 hours. The voltage depends on the distance between the electrodes. It is in the range of 300 to 500 mV. The electrolysis is carried out under inert conditions in a closed system. For this purpose, the electrolysis cell is flushed with nitrogen or argon before the electrolysis; the inert gas atmosphere is maintained during the electrolysis.

After the electrolysis had ended, a continuous, well adhering silicon layer of about 5 to 10 μm thickness was formed on the silicon wafers. The materials coated in this way show very good electrical and thermal dissipation.

Depending on the composition of the catalyst, the nature of the surface of the electrode and the presence of traces of oxygen or water, which can react with the electrolyte to form surface-active species, the silicon deposits more or less easily. The current yields therefore fluctuate between 50 and 100% and more or less large amounts of the catalyst can also be separated.

Example 2

A disc of high-purity aluminum (99.99%) is treated in a 20% alkaline aqueous solution of a commercially available surfactant for 1 hour at room temperature and then dried in air at 150 ° C.

The aluminum disk is then anodically polarized at a current density of 2 mA / cm 2 at 260 to 270 ° C. for 20 minutes and then used as a cathode in an electrolysis process according to Example 1 (a).

The anodic polarization of the aluminum cathode accelerates the silicon deposition and improves the quality of the silicon layer. There is probably a (partial) exchange of aluminum for silicon on the aluminum surface before the actual deposition of the silicon. The cathodically deposited silicon adheres better to this surface than to aluminum itself.

After an electrolysis time of 4 hours, a continuous and well-adhering silicon layer of about 10 µm thick has formed on the aluminum disc. The current yield is 100%.

This coated material also exhibits the properties given in Example 1 (a).

Example 3

(a) a rectangular rolled aluminum disc (purity: 99.99%, dimensions 40/8/2 mm) is cleaned with methylene chloride in an ultrasonic bath and then rinsed with methylene chloride. Then it is sanded dry with emery paper and finally polished with a slurry of aluminum oxide in isopropanol. The polished disc is cleaned with isopropanol in an ultrasonic bath, rinsed with acetone and dried at room temperature.

The aluminum disk cleaned in this way is installed as an anode in an electrolysis cell according to Example 1 (a). The cathode is made of graphite or silicon. A compound mixture consisting of 74.2% by weight SiJ₄, 22.1% by weight AlJ₃ and 3.7% by weight LiJ is placed in the electrolysis cell under inert conditions in a suitably closed housing (dry box). The electrolytic cell is then removed from this housing and heated to about 320 ° C. until the mixture boils. Inert conditions are maintained in the electrolysis cell by introducing nitrogen (slight excess pressure). After thorough mixing of the electrolyte, the mixture is cooled to 260 to 270 ° C. Then electrolysis is carried out for 20 minutes with a current density of 10 mA / cm² and then for 5 hours with 1 mA / cm². The electrodes are then cooled by a stream of nitrogen and cleaned with propionitrile and alcohol. A continuous, well adhering silicon layer of about 50 µm thick has formed on the aluminum disc. The material coated in this way shows very good electrical and thermal dissipation.

(b) The procedure is as described under (3a), but instead of direct current, the pulse current is used: 2 seconds - 10 mA / cm², 20 seconds - 0.5 mA / cm², 2 seconds - 10 mA / cm² etc. Duration: 5 hours. Or: 5 seconds - 10 mA / cm², 20 seconds - 0.5 mA / cm², 5 seconds - 10 mA / cm² etc. Duration: 3 hours. After the end of the electrolysis, a continuous, well adhering silicon layer of about 75 µm thick has formed on the aluminum disc. The material coated in this way shows very good electrical and thermal dissipation.

(c) An aluminum plate (99.99%) is ground to a thickness of 100 µm and prepared as described in (3a). Then electrolysis is carried out as indicated for 4 hours with a current density of 5 mA / cm². The part of the aluminum plate immersed in the electrolyte is completely replaced by silicon. A silicon foil was created.

The aluminum iodide formed can be separated off and split back into aluminum and iodine by electrolysis.

Example 4 Production of aluminum iodide (AlJ₃)

Apparatus for carrying out the method: A heatable piston for receiving the iodine and provided with a gas inlet tube is connected to a vertically arranged reaction tube made of quartz glass, which is surrounded by a heating jacket. The hydrogen iodide is synthesized in this tube. A fractionation column filled with glass bodies is connected to the reaction tube. This fractionation column is kept at about 120 ° C. during the reaction. The reflux from this fractionation column flows back through a heatable feed line (likewise heated to about 120 ° C.) into the heatable flask, in which the iodine boils at the reflux temperature (185 ° C.). At the top of the fractionation column is a cooler which is kept at room temperature to condense residual iodine. From time to time, the cooler is heated to temperatures above the melting point of the iodine in order to melt the condensed iodine, which then flows back into the heatable flask via the fractionation column. Behind the cooler is a cold trap (-20 ° C), in which the last traces of iodine and impurities are separated.

Since the reaction of iodine with hydrogen does not proceed completely at the specified temperatures, iodine can be separated from hydrogen iodide and hydrogen with the aid of this device and flow back into the heatable storage flask. In this way, continuous hydrogen iodide synthesis is possible. The iodine in the flask constantly boils under reflux (185 ° C) and a continuous stream of hydrogen is introduced through the gas inlet pipe.

The cold trap is followed by a second reaction tube which contains the aluminum and in which the reaction with the hydrogen iodide to aluminum iodide (AlJ₃) takes place.

The aluminum iodide formed flows with the gas stream (hydrogen) to the end of the reaction tube and condenses there in a flask. The hydrogen recovered from the reaction and the unreacted hydrogen are discharged at the end of the apparatus through wash bottles (sulfuric acid or paraffin oil) or returned to the flask containing the iodine.

Process: Turned aluminum chips (purity 99.999%) are washed in methylene chloride, etched with semi-concentrated (18%) hydrochloric acid, then rinsed with bidistilled water and finally dried in air at 120 ° C.

The dried aluminum chips and the iodine are placed in the heated flask in the second reaction tube. The apparatus is then purged with argon for one hour to remove the air.

The first reaction tube is then heated to 750 ° C and the second to 400 ° C and the heatable flask containing the iodine to the reflux temperature (185 ° C) of the iodine. Weak iodine reflux is maintained in this flask.

The inert argon atmosphere inside the reaction apparatus is then displaced by a stream of hydrogen.

To start the reaction, hydrogen is first passed through a wash bottle containing water and from there into the heated flask containing the iodine. The hydrogen flow is regulated so that it flows through the first reaction tube in about 30 seconds. From time to time, the hydrogen flow is passed through the wash bottle mentioned in order to reactivate the formation of hydrogen iodide and aluminum iodide.

The colorless AlJ₃ thus obtained is spectroscopically pure. No disturbing impurities could be detected.

Example 5 Manufacture of lithium iodide (LiJ)

Milled lithium iodide trihydrate is filled into a quartz tube under inert gas. The salt is melted into a coherent block using the industrial blow dryer. The tube is closed and fastened horizontally in a zone melting apparatus. The quartz tube is only about half full, so it cannot break during the melting process.

Zone melting is carried out by slowly passing the heating ring (about 1 - 2 cm / h) over the quartz tube. The contaminants in the lithium iodide migrate with the melting zone during zone melting (70 to 80 ° C) and collect at the ends of the quartz tube. After about 20 melting cycles, the process is ended, the tube is broken into several pieces after cooling, the salt is melted out in an inert atmosphere and the solidified melt is finally ground.

The lithium iodide trihydrate thus obtained is spectroscopically pure and contains no troublesome impurities.

1,3·10⁻³ mbar) wie folgt getrocknet: 24 Stunden bei Raumtemperatur, je 12 Stunden bei 50, 100, 150 und 200°C und schliesslich 48 Stunden bei 250°C.The trihydrate is then in a vacuum (10⁻³ Torr

1.3 · 10⁻³ mbar) dried as follows: 24 hours at room temperature, 12 hours at 50, 100, 150 and 200 ° C and finally 48 hours at 250 ° C.

Claims (17)

1. A method for producing thin layers of elemental silicon on an electrically conductive material suitable as an electrode by electrolytic deposition of the silicon from a molten salt, characterized in that the molten salt
(a) a silicon halide,
(b) an aluminum halide,
(c) an alkali metal or ammonium halide and
(d) a halide of a transition metal
contains and the electrolysis is carried out at temperatures of 100 to 350 ° C in an inert atmosphere and optionally under pressure. 2. The method according to claim 1, characterized in that the salt melt
(a) Silicon halides of the formula
(1) SiX₄,
(2) H _n SiX _4-n or
(3) Si _m Xʹ _{m + 2}
in which X is chlorine, bromine, iodine or mixtures thereof, Xʹ chlorine, bromine or iodine, n is an integer from 1 to 3 and m is an integer from 2 to 6,
(b) aluminum trichloride, aluminum tribromide or preferably aluminum triiodide,
(c) a chloride, bomide or preferably an iodide of sodium, potassium or preferably lithium and
(d) a chloride, bromide or preferably an iodide of a transition metal
contains, wherein components (a) to (d) can also be present as mixtures of the compounds mentioned. 3. The method according to claim 2, characterized in that component (a) is silicon tetrachloride, silicon tetrabromide or preferably silicon tetraiodide. 4. The method according to claim 2, characterized in that the melt as component (d) chromium (II) iodide (CrJ₂), manganese (II) iodide (MnJ₂), iron (II) iodide (FeJ₂), copper (I) iodide (CuJ), hafnium (IV) iodide (HfJ₄), nickel iodide (NiJ₂), vanadium (II) iodide (VJ₂) or mixtures of these iodides. 5. The method according to any one of claims 1 to 4, characterized in that the molten salt
20 to 90% by weight of component (a),
5 to 95% by weight of component (b),
1 to 20% by weight of component (c) and
0.1 to 10% by weight of component (d)
contains. 6. The method according to claim 5, characterized in that the melt
20 to 75% by weight of component (a),
20 to 50% by weight of component (b),
1 to 20% by weight of component (c) and
0.1 to 10% by weight of component (d)
contains. 7. The method according to any one of claims 1 to 6, characterized in that one carries out the electrolytic deposition of the silicon at temperatures of 260 to 320 ° C under normal pressure, optionally also at an excess pressure of 1 to 5 bar, with a melt which as Components (a) to (d) contains silicon tetraiodide, aluminum iodide, lithium iodide and a transition metal iodide or a mixture of transition metal iodides. 8. The method according to claims 1 to 7, characterized in that one carries out the electrolysis at a current density of 1 to 20 mA / cm². 9. The method according to any one of claims 1 to 8, characterized in that the electrolytic deposition of silicon is carried out cathodically, the cathode material being copper, chromium, molybdenum, nickel, platinum, iron or stainless steels, preferably aluminum, silicon or graphite and used as an anode material made of platinum, silicon or graphite. 10. The method according to claim 1, characterized in that the electrolytic deposition of the silicon is carried out anodically from a melt which contains the components (a) to (c), the aluminum anode material and the silicon or graphite cathode material . 11. The method according to claim 10, characterized in that the electrolytic deposition of the silicon takes place from a melt which contains silicon tetraiodide, aluminum triiodide and lithium iodide. 12. The method according to any one of claims 2 and 11, characterized in that one uses an aluminum triiodide, which by reaction of
(1) pure aluminum optionally etched with hydrochloric acid at temperatures of 300 to 500 ° C and in the presence of catalytic amounts of water
(2) hydrogen iodide
has been obtained. 13. The method according to claim 12, characterized in that the hydrogen iodide is prepared in situ from iodine and hydrogen at temperatures of 600 to 800 ° C and in the presence of catalytic amounts of water and used directly for the further reaction with aluminum. 14. The method according to any one of claims 2 and 11, characterized in that a lithium iodide is used which has been obtained by zone melting of lithium iodide trihydrate at 60 to 100 ° C and subsequent dewatering in a vacuum at temperatures of up to 250 ° C. 15. The electrically conductive material obtainable by the method according to one of claims 1 to 14 provided with a thin layer of elemental silicon. 16. Use of the material according to claim 15 for the production of or in photoconductive or photovoltaic devices. 17. Use according to claim 16, characterized in that one produces solar cells.