A METHOD OF MANUFACTURING THIN-FILM SOLAR CELLS

Abstract: A method of manufacturing thin-film solar cells, wherein a layer of copper indium selenide (CuInSe2) (3) is applied in one manufacturing step on a structure which includes a metal layer (2) forming an electrical back contact in the solar cell, this back contact being applied to a substrate (1). The invention is characterized in that a layer (6) which contains an alkali metal is formed on the structure prior to applying the CuInSe2 layer (3).

Description: . . METHOD OF MANUFACTURING THIN-FILM SOLAR CELLS The present invention relates to a method of manufacturing thin-film solar cells. CuInSe 2 (copper indium selenide) - thin-film solar cells are normally produced by depositing a molybdenum (Mo) back contact onto a substrate, for instance a glass substrate. Copper, indium and selenide are then deposited onto the back contact to form a CuInSe 2 layer, the CuInSe 2 being formed by chemical reaction in a selenium-containing atmosphere. In this stage of manufacture, the structure is subjected to a temperature in the range of 350-600 * C. The CuInSe 2 layer is a p-type semiconductor. An n-type semiconductor is deposited on the CuInSe 2 layer to form a pn- junction, which forms the actual solar cell. The n-type semiconductor is most often comprised of a thin layer of CdS and a much thicker layer of ZnO, which provides the best structure. The ZnO layer forms a front side contact at the same time. It is desired to increase the efficiency of solar cells of this kind. The present invention provides a method of manufacturing solar cells which greatly increases the efficiency of the solar cell produced. Accordingly, the present invention relates to a method of manufacturing thin-film solar cells wherein a structure which includes a metal layer that forms an electrical back contact in the solar cell is coated with a layer of copper indium selenide (CuInSe 2 ) in a manufacturing stage, said back contact being applied to a substrate, wherein the method is characterized by applying a layer of alkali metal to the structure prior to applying the CuInSe 2 layer. According to one particularly preferred embodiment of the invention, the alkali metal that is applied to the structure prior to applying the CuInSe 2 layer is sodium (Na) or potassium (K) . By copper indium selenide layer as used in the present context and in the Claims, is meant a copper indium selenide layer of different compositions and containing different alloying substances, primarily gallium and sulphur. The term copper indium selenide as used here concerns primarily the compounds CuInSe 2 , CuInxGai-j-Sβ a and CuInxGa 1 _ 1 -SySe 2 _ y . The invention will be described below with reference to a layer of copper indium selenide that has the formula CuInSe 2 . The present invention will now be described in more detail partly with reference to an exemplifying embodiment of the invention shown in the accompanying drawing, the single Figure of which illustrates a solar cell structure. The present invention is based on the understanding that when sodium or elementary potassium in the form of elementary sodium or potassium, or a compound in which these basic substances are present, is applied to the surface on which the CuInSe 2 layer is to be built-up, for instance to the molybdenum layer, the grains in the polycrystalline CuInSe 2 film will be orientated more in a columnar structure. The grains will also be larger and the structure more dense. The resistivity in the CuInSe 2 layer is also reduced, meaning that a more effective p-doping is obtained, therewith resulting in a higher cell voltage. Alkali metals other than sodium and potassium will provide the same effect. This applies at least to lithium, which has a low atomic weight. Figure 1 is a schematic sectional view of a thin-film solar cell. The substrate on which thin-film solar cells are built up may be large in size, for instance have a size of 1 x 0.4 metre. The substrate surface will therewith carry a very large number of cells that connected together electrically. Figure 1 shows only one part of one such cell. The technique of producing a large number of separate but electrically connected cells on a substrate surface is well known to the art and will not be described in detail here. A solar cell to which the invention can be applied may be constructed in the following way. The substrate 1 will normally have the form of a glass sheet of suitable thick¬ ness, for instance a thickness of 2.0 mm. A molybdenum (Mo) layer 2 is first sputtered onto the glass surface. The molybdenum layer forms the back contact and the positive terminal of the finished cell. The molybdenum layer may have a thickness of 1,000 nanometers. There is then applied a CuInSe 2 layer 3, which may be given a thickness of 2,500 nanometers for instance. There is first applied to the layer 3 a cadmium sulphide (CdS) layer 4, which may be given a thickness of 50 nanometers for instance, and thereafter an electrical contact layer in the form of a transparent doped zinc-oxide layer (ZnO) 5, which may be given a thickness of 500 nanometers, for instance. When sunlight falls on the solar cell, an electric voltage is produced between the electrical contact 5 (ZnO), which is the negative terminal, and the electrical back contact 2. The invention is described below with reference to the use of sodium by way of example. It will be understood, however, that other alkali metals may be used with equal effect. It will be understood that although the layer 6 of the illus- trated solar cell is referred to as sodium, the layer 6 will in fact consist of the alkali metal that is used when this alkali metal is other than sodium. According to the invention, a layer 6 containing an alkali metal, in the illustrated case sodium (Na), is formed on top of the metal layer 2, i.e. on top of the molybdenum layer of the illustrated example, prior to applying the CuInSe 2 layer 3. According to one preferred embodiment of the invention, sodium is applied on the electrical back contact, i.e. the metal layer by vaporizing sodium selenide (Na 2 Se). According to another preferred embodiment of the invention, potassium is applied on the back contact, i.e. the metal layer by vaporizing potassium selenide (K 2 Se). According to another preferred embodiment, the alkali metal, in the illustrated case sodium, is applied simultaneously with the electrical back contact when said back contact is comprised of molybdenum. According to another preferred embodiment of the invention, the layer which includes the alkali metal, in the illustrated case sodium, is applied to a thickness of from 50 to 500 nanometers. According to another embodiment, the layer also contains oxygen (O) . It has been found that when the CuInSe 2 layer is applied on top of the layer that contains sodium or potassium, the sodium or potassium will essentially disappear from the surface of the metal contact. When the CuInSe 2 layer is applied, sodium or potassium is again found at the grain boundaries in the CuInSe 2 layer and on the surface of the layer. When the CdS layer is then applied in a wet process, the sodium or the potassium compound will disappear from the surface, provided that the sodium or the potassium compound is soluble in the liquid used in the wet process. This method results in a very surprising increase in the efficiency of the solar cell described and shown by way of example, namely an increase of about 25%. The typical efficiency of such a solar cell of this nature to which the invention has not been applied is 12%. The efficiency increases to 15% when the invention is applied. Although the present invention has been described with reference to a given solar cell structure, it will be understood that the present invention can be applied to other structures in which the electrical back contact is not comprised of molybdenum but of some other suitable metal, such as tungsten, nickel, titanium or chromium. Furthermore, substrates other than glass that contains sodium may also be used. For instance, glass which does not contain sodium may be used, as may also glass which contains sodium and which includes a diffusion barrier against sodium on the surface on which the electrical back contact is to be built up. The invention shall not therefore be considered to be limited to the aforedescribed exemplifying embodiments thereof, since variations can be made within the scope of the following Claims.

Claims: CLAIMS 1. A method of manufacturing thin-film solar cells, wherein a layer of copper indium selenide (CuInSe 2 ) (3) is applied in one manufacturing step to a structure which includes a metal layer (2) that forms an electrical back contact in the solar cell, said back contact being applied to a substrate (1), characterized by forming on the structure a layer (6) which contains an alkali metal, prior to applying the CuInSe 2 layer (3). 2. A method according to Claim 1, characterized by forming on the structure a layer (6) which contains the alkali metal sodium (Na) or the alkali metal potassium (K) prior to applying the CuInSe 2 layer (3). 3. A method according to Claim 1 or Claim 2, characterized in that sodium is applied on the back contact by vaporizing sodium selenide (Na 2 Se). 4. A method according to Claim l or Claim 2, characterized in that sodium is applied on the back contact by vaporizing potassium selenide (K 2 Se) onto the back contact. 5. A method according to Claim 1 or Claim 2, characterized in that the alkali metal is applied simultaneously with the back contact by sputtering when said back contact is com¬ prised of molybdenum. 6. A method according to Claim 1, 2, 3, 4 or 5, character¬ ized in that the layer containing the alkali metal is applied to a thickness of 50 to 500 nanometers. 7. A method according to Claim 1, 2, 3, 4, 5 or 6, charac- terized in that the layer which includes the alkali metal also includes oxygen (O) . 8. A method according to Claim 1, 2, 3, 4, 5, 6 or 7, characterized in that the back contact is comprised of molybdenum (Mo). 9. A method according to any one of the preceding Claims, characterized by applying cadmium sulphide (CdS) on top of the copper indium selenide layer, and by applying an electric contact layer on top of the layer of cadmium sulphide, said electrical contact layer preferably being a layer of doped zinc oxide (ZnO).

Inventors: BODEGAARD MARIKA, HEDSTROEM JONAS, STOLT LARS, BODEGAARD, MARIKA, HEDSTROEM, JONAS, STOLT, LARS
Applicants: BODEGAARD MARIKA, HEDSTROEM JONAS, NORDIC SOLAR ENERGY AB, STOLT LARS, BODEGAARD, MARIKA, HEDSTROEM, JONAS, STOLT, LARS
Application year: 1995
Main classification: H01L21
IPC codes: H01L21, H01L31