WO1993024962A1

WO1993024962A1 - Process for the purification of electrolyte components

Info

Publication number: WO1993024962A1
Application number: PCT/GB1993/000946
Authority: WO
Inventors: Jeremy Barker; Rodney John Marshall; Mehran Sadeghi
Original assignee: BP Technology Ventures Ltd
Current assignee: BP Technology Ventures Ltd
Priority date: 1992-05-22
Filing date: 1993-05-07
Publication date: 1993-12-09
Anticipated expiration: 1994-11-22
Also published as: GB9210945D0; AU4077793A; MX9303008A; CN1082633A

Abstract

A method of preparing an aqueous solution of a Group IIB metal salt suitable for use in the electrodeposition of a IIB/VIB thin film semiconductor which comprises (a) bringing a solution of the Group IIB metal salt into contact with the corresponding IIB metal in the form of a powder with a purity greater than 99.99 %; (b) agitating the solution containing the powder for at least 10 minutes; and (c) removing the metal powder from the solution. A method of preparing an electrodeposited thin film IIB/VIB semiconductor by cathodic deposition from an electrolyte containing ions of Group II metal, is also claimed.

Description

PROCESS FOR THE PURIFICATION OF ELECTROLYTE COMPONENTS The present invention relates to the production of thin film IIB VIB semiconductor materials by electrodeposition.

Thin film IIB/VIB semiconductors are of value in the fabrication of photovoltaic cells. The production of thin film IIB/VIB semiconductors, e.g. CdTe, by electrodeposition is disclosed in numerous references. For example, Panicker et al, J. Electrochem Soc. 125. No. 4 pp 556-572, and US 4425 194. The use of electrodeposited thin film IIB/VIB semiconductors in the fabrication of photovoltaic cells is also disclosed in numerous references.

We have found that when preparing photovoltaic devices based on IIB/VIB thin film semiconductors formed by electrodeposition that the best performance is obtained when the impurity content is controlled at very low levels. The commercially available chemicals used to make the electrolyte solution in electrodeposition baths for depositing IIB/VIB semiconductors normally contain relatively high levels of certain metallic impurities. Certain electrochemically active impurities, i.e. those whose standard electrochemical reduction potential is less cathodic than cadmium, in the electrolyte will show significant concentration during the electrodeposition process. In other words, the concentration in the electrodeposited material is higher than that in the solution.

There is a need for a method of purifying electrolyte solutions used in the electrodeposition of IIB/VIB semiconductors. According to the present invention a method of preparing an aqueous solution of a Group IIB metal salt suitable for use in the electrodeposition of a IIB/VIB thin film semiconductor which comprises: (a) bringing a solution of the Group IIB metal salt into contact with the corresponding IIB metal in the form of a powder of high purity, preferably greater than 99.99%, (b) agitating the solution containing the metal powder for sufficient time to extract the impurities, and (c) removing the metal powder from solution.

According to a further aspect of the present invention a method of preparing an electrodeposited thin film IIB/VIB semiconductor by cathodic deposition from an electrolyte containing ions of the Group IIB metal characterised in that the electrolyte is prepared from a solution of a Group IIB salt which has been brought into contact with the corresponding Group IIB metal in powder form for a time sufficient to extract the impurities, conveniently at least 10 minutes followed by removal of the Group IIB metal powder from the solution. In this specification references to Group IIB and VIB are references to the Periodic Table of the Elements. There are various forms of the Periodic Table which differ in the way in which the letters A and B are used to to identify sub-Groups. References to the Periodic Table in this specification are references to the Periodic Table as given in "Advanced Inorganic Chemistry" , Cotton & Wilkinson, 4th Edition, in which Group IIB includes the elements Cd, Zn, and Hg, and Group VIB includes the main group elements of Group VI, e.g. 0, S, Se, and Te.

The Group IIB metal is preferably cadmium. The Group IIB salt used in the preparation of the electrolyte bath is preferably CdSθ4. The greater the size of the particles of metal powder the less will be the surface area per unit weight. Preferably the maximum size of the metal powder is 300 microns.

The minimum particle size will depend on the effectiveness of the filtration step. Preferably the metal powder is substantially free of particles with a size below 0.2 micrometres. More preferably the metal powder is substantially free of particles with a size belowlO microns.

The metal powder may conveniently be a metal powder, substantially all of which passes a 100 mesh British Standard sieve and is collected by a 200 mesh British Standard sieve. A mesh of 100 corresponds to a spacing between the mesh elements of 250 micrometres and a mesh size of 200 corresponds to 130 micrometers.

The concentration of the Group IIB salt in the aqueous solution may be in the range 0.01M to 3M, but is conveniently 2M. The temperature of the solution during the process of treatment may, for example, be in the range 0-80°C, but is conveniently around room temperature (18°-25°C) .

The purity of the Group IIB metal powder is at least 99.99%, more preferably at least 99.999%. Metals of this purity are available commercially in powder form

The duration of contact between the metal powder and the aqueous solution may, for example, be in the range 10 minutes to 2 hours. The Group IIB metal powder is then removed from the solution. This may be carried out by filtering the solution or allowing the metal powder to settle and then decant off the liquid. here it is desired to separate by filtration, this may be carried out using a fine mesh filter which contains no impurities which will effect the purity of the electrolyte. It is convenient to use a polymer filter such as polypropylene with a maximum pore size of 0.1 micrometres.

It may be desirable to repeat the steps of bringing the solution into contact with the Group IIB metal powder and the separation step. Preferably, the metal powder used is fresh metal powder.

The electrodeposition of IIB/VIB thin film semiconductors is well known and there is no need to describe it in detail. Thus electrodeposition processes are disclosed in GB 1 532 616. In order to reduce the impurity level to the minimum it may be desirable to separate the anode from the electrolyte bath from which the semiconductor is deposited by a cation/exchange polymer membrane as disclosed in our co-pending European application EP-A-0538041.

The production of photovoltaic cells by depositing a IIB/VIB thin film semiconductor on a cathode covered with a thin film of CdS is well known and does not need to be described in detail here. It will generally be necessary to heat the deposited film of IIB/VIB semiconductor to convert it to a p-type semiconductor. Such a process is disclosed in US 4 388 483. A contact may then be deposited on the IIB/VIB thin film by various well known methods. The invention will now be described with reference to the following examples in which comparative tests, not according to the invention, are identified by letters, and examples of the invention are identified by numbers. Comparative Test A A photovoltaic cell was prepared by conventional methods from glass coated with tin oxide on which a layer of CdS had been deposited. The method is disclosed in N R Pavaskar, C A Menzes, ABP Sinha, J Electrochemical Soc. 124 (1967) page 743. The glass sheet with the layer of CdS was used as the cathode in an electrodeposition process for depositing CdTe.

The aqueous electrodeposition bath contained 0.9M Cd introduced as CdS04, 600 ppm Cl^", 50 ppm of Te introduced as Teθ2- The electrodeposition step was carried out at 70°C.

The CdS0 was not subjected to any pre-treatment but was used as supplied.

Photovoltaic cells were formed from the resulting glass/tin oxide/CdS/CdTe structure after heating to convert the CdTe to p-type material as disclosed in USP 4388483, 2 mm^ gold dot contacts were then applied by thermal evaporation following an etch as described in USP 4456630.

The resulting photovoltaic cell was tested under standard test conditions and the characteristics at three different regions of the sheet identified as strips A, B and C each covering a third of the plate were determined. The results are given in Table 1. TABLE 1

Example 1

A photovoltaic cell was prepared as in Comparative Test A, except that the electrolyte bath was prepared from a CdS0 aqueous solution which had been treated as set out below. Cd powder was added to the CdSO_ή. solution with mechanical stirring at the rate of 1 g of Cd powder to 1 litre of CdS0 solution. The Cd powder had a purity of 99.999%. The powder passed a 100-mesh sieve but was collected by a 200-mesh sieve (British Standard sieve) . The solution and powder are agitated together at room temperature for 20 minutes. The solution was then filtered through a 0.1 micrometre polypropylene filter to remove the Cd powder. The resulting filtered solution was treated a second time as above with Cd powder and filtered. The results for the photovoltaic cell prepared as in Example 1 are given in Table 2.

TABLE 2

Claims

Claims:

1. A method of preparing an aqueous solution of a Group IIB metal salt suitable for use in the electrodeposition of a IIB/VIB thin film semiconductor which comprises:

(a) bringing a solution of the Group IIB metal salt into contact with the corresponding IIB metal in the form of a high purity powder, agitating the solution containing the metal powder for sufficient time to extract the impurities, and (c) removing the metal powder from solution.

2. A method of preparing an electrodeposited thin film IIB/VIB semiconductor by cathodic deposition from an electrolyte containing ions of the Group IIB metal characterised in that the electrolyte is prepared from a solution of a Group IIB salt which has been brought into contact with the corresponding Group IIB metal in powder form for a time sufficient to extract the impurities followed by removal of the Group IIB metal powder from the solution.

3. A method according to either of the preceding claims wherein the Group IIB metal is cadmium.

4. A method according to Claim 2 wherein the Group IIB metal salt is cadmium sulphate.

5. A method according to any one of the preceding claims wherein metal powder is substantially free of particles with a size below 0.2 micrometres.

6. A method according to Claim 5 wherein the metal powder is substantially free of particles with a particle size below 10 micrometres.

7. A method according to any one of the preceding claims wherein the metal powder is substantially free of particles with a particle size above 300 micrometres.

8. A method according to any one of the preceding claims wherein the concentration of the Group IIB salt is in the range 0.01M to 3M.

9. A method according to any one of the preceding claims wherein the temperature is in the range 0-80* C.

10. A method according to claim 9 wherein the temperature is in the range 18° to 25".

11. A method according to any one of the preceding claims wherein the metal powder is at least 99.999% pure.

12. A method according to any one of the preceding claims wherein the metal powder is removed from the solution by filtration.

13. A method according to claim 12 wherein the metal powder is removed by filtration through a polymer filter with a maximum pore size of 0.1 micrometres.

14. A method according to any one of the preceding claims which comprises bringing the Group IIB salt solution after removal of the metal powder into contact with fresh metal powder, agitating, and separating the metal powder as claimed in any one of Claims 1 to 12.