A METHOD FOR THE PRODUCTION OF SOLAR CELLS

Abstract: An economical method of coating organic-polymeric carrier materials with at least one film and tempering the thus coated materials, wherein the carrier material has a thickness of at least 60 mu m and consists of a polymeric material with a glass transition temperature of at least 90 DEG C, the applied film has a thickness of a maximum of 30 mu m, the coating is performed at temperatures below the glass transition temperature and the tempering is performed at temperatures of at least 250 DEG C by means of a plasma.

Description: A Method for the Production of Solar Cells FIELD OF THE INVENTION The invention relates to an economically improved method for the production of thin- film solar cells, e.g. CdTe solar cells (CdTe = cadmium telluride). In the following CdTe serves merely as an example of all thin-film solar cells. CdTe- and CdTe/CdS solar cells can be produced in accordance with different methods (US 5 304499) commonly comprising a thermal treatment at a minimum of 575°C to ensure adequate efficiency. These temperatures only permit the use of expensive types of glass as carrier. Glass as a carrier has the disadvantage that the coating with CdTe can be performed only discontinuously on glass sheets, irrespectively of which coating method is selected. US 5 304499 describes a method wherein the coating temperatures amount only to 480 to 520°C and thus permit the use of cheap types of glass ("window glass"). For this, it is necessary for the glass firstly to be provided with a transparent, electrically conductive coating, consisting for example of doped tin oxide. A thin coating of cadmium sulphide (CdS) then follows, to which the light-sensitive CdTe film is then applied by sublimation at 480 to 520°C. The apparatus required to apply the CdTe film is complicated and costly: carrier material and CdTe source are accommodated by oppositely disposed graphite blocks heated to the required temperature, such that the CdTe source is arranged at a distance of only 2 to 3 mm from the carrier surface. The sublimation then takes place in a 0.1 mbar inert gas atmosphere, e.g. a nitrogen-, helium-, argon- or hydrogen atmosphere. Large surfaces of CdTe-coated material for the manufacture of solar cells cannot be produced cost-effectively in this way. This method and the other known methods do not permit the use of carrier sheets as polymeric organic materials. The object of the invention was to produce a material for the manufacture of solar cells with a support and a photovoltaically active layer on said support, e.g. a CdTe layer, in a cost-effective manner. A method has now been found which permits the use of flexible polymeric sheets for coating with CdTe and tempering without the polymeric carrier material undergoing damage due to the high temperatures. In this way a starting material for solar cells of high efficiency is obtained. The method is also suitable for coating with other substances and tempering, for example for the production of transparent, conductive films of indium-tin oxide (ITO). Therefore the invention relates to a method of coating organic-polymeric carrier materials with at least one substance, in particular at least one inorganic substance, preferably CdTe, and tempering of the film applied to the thus coated materials, characterised in that the carrier material has a thickness of at least 60 μm, in particular 90 to 120 μm, and consists of a polymeric material with a glass transition temperature of at least 90°C, the applied film has a thickness of a maximum of 30 μm, in particular 2 to 7 μm, the coating is performed at temperatures below the glass transition temperature and the tempering is performed at temperatures of at least 250°C, in particular 400 to 600°C, by means of a plasma. Plasma is to be understood as that aggregation state of the matter wherein a gas consist to equal parts of positive and negative charged particles and thereby is externally neutral. The production of a plasma at very high temperatures by separation of shell electrons from the atomic residue is known. The method according to the invention, known as plasma sintering, is performed by focusing a laser of sufficient strength that at the focal point the magnitude of the electric field strength becomes such that the electrons of the air molecules, preferably however a protective gas such as nitrogen or argon, break away from their atomic cores, whereby the plasma forms. This plasma is hot and expands. If the laser is pulsed, in focus with the pulse frequency of the laser one obtains a pulsating plasma with correspondingly frequent pressure surges to its environment. This pulsating plasma can be conducted over the film to be sintered, whereby three events then occur: the film absorbs the laser directly and is thereby heated, the film is additionally heated by the hot plasma sliding along its surface, the pulsating plasma produces pressure surges which mechanically compress the film. A suitable laser is, for example, a neodymium-YAG laser with 100 mJ pulse energy and 50 Hz pulse frequency. Preferably, the layer to be sintered is irradiated with a continuous laser which heats the layer but does not damage it at the given transportation speed of material. The pulsing laser then sinters the heated zone of the material. The coating is performed, for example, using an aqueous or solvent-containing CdTe suspension. Then the material is dried. Suitable coating methods consist for example of casting and blade coating. The tempering can be performed several times; cooling phases are preferably provided between two tempering steps. Suitable polymers are polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). Prior to the coating the polymeric carrier material can be provided with a substrate layer, consisting for example of indium-tin oxide, which improves the adhesion of the CdTe film. The substrate layer is to be transparent and electrically conductive. Organic-polymeric carrier materials are flexible and thus permit continuous coating in accordance with a suitable coating method, for example a continuous coating method using a casting device, for example a meniscus or curtain casting device as known in association with the coating of photographic film. It may be particularly advantageous for the CdTe particles to be especially fine- particled, and in particular to consist of so-called nano-particles, i.e. particles whose average diameter is in the nanometre range and amounts for example to 3 to 5 nm. In this case it is expedient, actually during the production of the nano-particles, to add an agent which prevents the agglomeration of the nano-particles, e.g. tributyl- phosphane. The invention further relates to a solar cell with at least one CdTe film having a thickness of a maximum of 30 μm on a carrier, characterised in that the carrier is a polymeric organic material having a thickness of at least 60 μm and a glass transition temperature of at least 90°C. Example 1 A 100 μm thick sheet of PEN with a width of 100 cm is continuously coated with a suspension containing a dispersant and, per litre, 31 g cadmium telluride. Then the coated sheet is dried and has a dry layer thickness of the applied film of 5 μm. The sheet is tempered as follows: A neodymium-YAG-laser with lOO mJ pulse energy and 50 Hz pulse frequency is focused such that a pulsating plasma forms closely above the sheet. The sheet is moved in grid-like manner under this plasma so that the entire surface to be sintered is heated successively, yet sufficiently briefly that the sheet is not damaged. After the tempering the sheet has a light-dependent electric resistance and thus is suitable for the production of a photovoltaic cell. Example 2 A sheet of PEN according to Example 1 is coated and dried according to Example 1. Then the sheet is continuously heated by a diode laser with a power of 8 kW/cm 2 and a duration of exposition of each square element of 100 ms, and thereafter a pulsating plasma with 100 mJ pulse energy and 50 Hz pulse frequency is produced closely above the surface of the layer to be sintered by a neodymium-YAG-laser. After tempering the sheet is suitable for the production of a photovoltaic cell.

Claims: Claims 1. A method of coating organic-polymeric carrier materials with at least one film and tempering the thus coated materials, characterised in that the carrier material has a thickness of at least 60 μm, in particular 90 to 120 μm, and consists of a polymeric material with a glass transition temperature of at least 90°C, the applied film has a thickness of a maximum of 30 μm, in particular 2 to 7 μm, the coating is performed at temperatures below the glass transition temperature and the tempering is performed at temperatures of at least 250°C, in particular 400 to 600°C, by means of a plasma. 2. A method according to Claim 1, characterised in that the applied film is a photovoltaically active film. 3. A method according to Claim 1 , characterised in that the applied film is a CdTe film. 4. A method according to Claim 1 , characterised in that the plasma is produced by means of a laser. 5. A method according to Claim 4, characterised in that the laser is operated in pulsed mode. 6. A method according to Claim 1, characterised in that the plasma is produced from protective gas. 7. A method according to Claim 1 , characterized in that the material prior to the treatment with the plasma is heated with a continuous laser. 8. A solar cell with at least one photovoltaically active film having a maximum thickness of 30 μm on a carrier, characterised in that the carrier is a polymeric organic material having a thickness of at least 60 μm and a glass transition temperature of at least 90°C. 9. A solar cell according to Claim 8, characterised in that the photovoltaically active film contains cadmium telluride. 10. A solar cell according to Claim 8, characterised in that the carrier consists of polyethylene terephthalate or polyethylene naphthalate.

Inventors: CRONE KLAUS PETER, CRONE, KLAUS, PETER
Applicants: AGFA GEVAERT NV, CRONE KLAUS PETER, AGFA-GEVAERT N.V., CRONE, KLAUS, PETER
Application year: 2000
Main classification: H01L31
IPC codes: H01L31