United States Patent [191 [11] Patent Number: 4,773,973 Griiniger et al. [45] Date of Patent: Sep. 27, 1988 [54] PROCESS FOR THE PRODUCTION OF POLYCRYSTALLINE SILICON COATINGS FOREIGN PATENT DOCUMENTS BY ELECTROLYTIC DEPOSITION OF 10415 l/l983 European Pat. Off. SILICON OTHER PUBLICATIONS [75] Inventors: Hans R. Griiniger, Olten; Rudolf Monnier, Chimia, 105-124 (1983). Kern, Maggia; Paul Rys, Zurich, all Elswell, J. Crystal Growth, 52, 7l4-752 (1981). of Sweden Dennis Elwell et al., Solar Energy Materials, 6, pp. 123-145, (1982). [73] Assignee: Ciba-Geigy AG, Basel, Switzerland D. Elimarskii et al., Chemical Abstracts, 96: 42968j (1982). [21] Appl. No.: 165,492 Primary Examiner-G. L. Kaplan . [22] ‘ Filed: Mar. 8, 1988 Attorney, Agent, or Firm—Wenderoth, Lind & Ponack [57] ABSTRACT Related US. Application Data A novel process for the electrolytic deposition of silicon [62] Division of Ser. No. 87,635, Aug. 18, 1987. from a melt containing covalent silicon compounds, in particular silicon tetrahalides, and furthermore alumin [30] Foreign Application Priority Data ium halides, alkali metal halides and halides of transition metals is carried out at relatively low temperatures of Aug. 19, 1986 [CH] Switzerland ....................... .. 3320/86 100° to 350° C. in an inert atmosphere. The silicon is deposited cathodically or anodically onto electrically [51] Int. Cl.4 .............................................. .. C25D 3/66 [52] US. Cl. 204/39 conductive material. [58] Field of Search .................................. .. 204/39, 60 The silicon coatings are homogeneous and adhere ?rmly to the substrate. The coated materials can be used [56] References Cited for the production of photoconductive or photovoltaic devices. U.S. PATENT DOCUMENTS 3,983,012 9/ 1976 Cohen ................................. .. 204/ 15 11 Claims, No Drawings 4,773,973 1 2 used for direct conversion of solar energy into electrical PROCESS FOR THE PRODUCTION OF energy. POLYCRYSTALLINE SILICON COATINGS BY This and the other subject matter to which the inven ELECTROLYTIC DEPOSITION OF SILICON tion relates are described below in more detail. The covalent silicon compound (a) supplies the sili This application is a division of Ser. No. 87,635 ?led con, which can be deposited, for example, cathodically. Aug. 18, 1987. Component (b) serves to prepare a homogeneous melt The present invention relates to the production of (good miscibility with the silicon compounds), compo thin coatings of elemental silicon on electrically con nent (c) is the conductive salt, which, for example, can ductive materials by electrolytic deposition of the sili be dissolved with components (b) (ALI3) to form a con from low-melting mixtures containing covalent complex, and component (d) is the so-called catalyst silicon compounds. which signi?cantly improves the deposition of silicon The materials thus coated can be used in the produc and the quality of the silicon coatings on, for example, tion of photoconductive or photovoltaic devices, for copper, chromium, molybdenum, nickel, iron and example solar cells. 15 chrome steel or inorganic glasses, for example of tin Processes for the electrolytic deposition of silicon are dioxide or tin dioxide/indium oxide mixtures, and already known from the literature [R. Monnier, Chimia makes the formation of a silicon coating on a silicon 37, 109 (1983); and D. Elswell, J. Crystal Growth, 52, carrier possible in the ?rst place under the conditions of 741 (1981)]. the process according to the invention. In fact, in the Thus, for example, silicon can be deposited by melt latter, it would not be possible to observe deposition of electrolysis at temperatures of about 700° to 1500" C. silicon without this catalyst. from melts containing silicon ?uorides and oxides and As an alternative to cathodic deposition of the silicon salts of aluminium, alkali metals and/or alkaline earth from the halide melt mentioned, it is also possible to metals. The high temperatures which cause consider deposit the silicon anodically. able material problems are a disadvantage in this pro 25 Since aluminium halides have a higher bonding en cess. Another process relates to the electrochemical depo ergy than the corresponding silicon halides, aluminium sition of silicon from solutions of suitable silanes, for should be capable of reducing silicon halides. If an alu example tetrahalogeno- or trihalogenosilanes, dissolved minium plate is connected as the anode in an electrolyte in polar organic solvents. The purity of the silicon coat containing a silicon halide as component (a), a compact ings deposited by this process and their continuity and silicon coating is formed on its surface. Exchange of adhesiveness to the electrically conductive substrate is aluminium for silicon takes place. This exchange can be not always completely satisfactory; the use of the mate carried out up to complete replacement of the alumin rials thus coated for the intended purpose is thus also ium by silicon. A pure silicon ?lm is obtained, but has impaired. Reference should also be made to the possible only an inadequate stability because of the lack of a but undesirable chemical reaction of the halogenosi carrier. The action of the anodic current is probably lanes mentioned with the polar organic solvents. based on the aluminium atoms being dissolved out of the The object of the present invention is thus to provide electrode surface. Empty sites are thereby formed and a process for the electrochemical deposition of silicon can be occupied by silicon atoms after release of the bonding partner. The build-up of a silicon coating can which allows the production of highly pure silicon 40 coatings, if appropriate provided only with the neces therefore be controlled by a given flow of current. sary doping agents, which are formed continuously Since the solubility of aluminium in silicon is very low (coherently) on the corresponding substrates and ad at the temperatures used, very pure silicon coatings can here ?rmly to these. be produced. These coatings are p-conducting, since they contain traces of aluminium. For this, the process according to the invention re 45 quires neither the high temperature melt electrolysis The salt melt (the electrolyte) for the anodic deposi nor the silicon deposition from an organic electrolysis tion of silicon contains components (a) to (c). This elec bath, which uses a melt of certain composition, from trolytic deposition of silicon is carried out at tempera which polycrystalline silicon can be deposited electro tures of 100° to 350° C. in an inert atmosphere and chemically in thin continuous coatings onto suitable optionally under pressure, for example under 1 to 5 bar. electrically conductive material at relatively low tem The anode used is an anode of aluminium, and suitable peratures. cathode materials are those of silicon or graphite. The present invention therefore relates to a process Halides which are present as components (a) to (d) or for the production of thin coatings of elemental silicon (a) to (c) in the melt for carrying out the process accord on an electrically conductive material which is suitable ing to the invention are above all the chlorides, bro as the electrode by electrolytic deposition of the silicon mides and iodides, the latter being preferred. from a melt, which comprises a procedure wherein the Component (a) is thus a silicon tetrahalide of the melt contains (a) a silicon halide, (b) an aluminium hal formula ide, (c) an alkali metal halide or ammonium halide and (d) a halide of a transition metal and the electrolysis is 5iX4 (1) carried out at temperatures from 100' to 350° C. in an inert atmosphere, optionally under pressure. in which X is chlorine, bromine or, preferably, iodine or mixtures thereof‘, such as silicon tetrachloride, silicon The present invention also relates to the electrically tetrabromide or preferably silicon tetraiodide, or fur conductive material which is obtainable according to thermore, for example, SiClBr3, SlClzBl‘g, SiCl3Br, the invention and is provided with a thin coating of 65 SiCl2I2, SiCl3I, SiBr3I, SiBrgIz or SlB1‘I3; halogenosi elemental silicon and to the silicon coating itself, and to lanes of the formulae its use for the production of photoconductive or photo voltaic devices, for example solar cells, which can be HnSiX4-n and (3) $imX'2m+z (2) 4,773,973 3 4 such highly pure compounds are known from the litera in which n and m are integers from 1 to 3 or 2 to 6, X is ture (compare, for example, R. C. Ellis, J. Elektrochem. chlorine, bromine, iodine or a mixture thereof and X’ is Soc. 107, 222 (l960)--Herstellung von Siliziumtetrajo chlorine, bromine or iodine, can also be used. Examples did und Verwendung zur Herstellung von Silizium which may be mentioned are HSiCl3, H2SiCl2, HSiBr3, [Preparation of silicon tetraiodide and use for the prepa HzsiBrz, HSiI3 and HZSiIZ. The trihalogenosilanes are ration of silicon]). preferred. The preparation of pure aluminium iodide and lith Examples of di- and polysilanes of the formula (3) are ium iodide is described below. Si1Cl6, Si3Clg, Si4Cl10 and further homologues and the The process according to the invention can be carried corresponding bromine and, in particular, iodine com= out in an electrolysis vessel of the customary construc pounds. tion. The vessel can be made, for example, of glass, in In selecting compounds of component (a), optionally particular quartz glass, or of a non-corroding metal, and it should be ensured that the boiling point of this com if appropriate can contain a porous sinter plate of pound is not too low and the volatility is not therefore quartz, a metal or a ceramic material as the partition too high; electrolysis in a normal pressure range—nor 15 between the anode and cathode chamber. Such a parti mal pressure or slightly increased pressure-could tion can prevent, for example, the anodically (use of an thereby be impeded. inert anode) formed halogen (for example C12 or 1;) Component (b) is an aluminium trihalide, such as being converted back on the cathode.