Method for producing a semi-finished product for thin-film solar cells

Abstract

A method for producing a semi-finished product for a thin-film solar cell, comprising the steps: a. providing a transparent substrate with a front contact layer or front contact layer sequence, b. flame-treating the front contact layer or front contact layer sequence with one or more gas burners, wherein the surface temperature of the front contact layer after flame-treating is in the range of 80°C to 220°C, and wherein the flame-treating step of the front contact layer or front contact layer sequence is carried out for a duration between 2 s and 30 s, and wherein the increase in the surface temperature of the front contact layer or front contact layer sequence due to flame-treating is a maximum of 200 Kelvin.

Description

The present invention relates to an improved method for producing thin-film solar cells or a semi-finished product therefor, wherein the method provides for cleaning the front contact layer before the subsequent application of further layers.In the production of thin-film solar cells according to the prior art in the superlattice structure, a transparent front contact layer (preferably TCO-transparent conductive oxide) is applied to a substrate, usually glass. A window layer, preferably made of pure or modified CdS (cadmium sulfide), is deposited on this front contact layer, and the absorber layer, preferably a CdTe layer (Kadmiumtellurid), is subsequently deposited thereon. Finally, the back contact layer or layer sequence is applied.The CdS or CdTe layer is applied according to methods from the prior art. Frequently used in this case is the CSS (close spaced sublimation) method, in which the glass substrate with the prepared TCO front contact layer is moved over a crucible with CdS. This crucible is heated and the material to be evaporated (CdS) evaporates (sublimes) from the crucible and precipitates on the front contact layer of the substrate, which is maintained at a lower temperature than the crucible.The subsequent application of the CdTe layer is likewise preferably carried out using the CSS method.Finally, the back contact is applied, preferably as a layer sequence.In the method described and also in the following, annealing and sealing steps according to the prior art are assumed to be known and are not explained in more detail. Also the application of antireflection and protective layers (for example. Back side laminate or glass) is assumed.In industrial application, the application of the window and absorber layers is usually carried out by heating the substrates with the prepared front contact layer (which preferably points in the direction of the crucibles) and passing them over the crucibles at a constant speed, so that CdS and CdTe layers of uniform thickness form. According to the prior art, the process is carried out in heated vacuum chambers connected in series, by means of which the substrates are moved on a transport system comprising rollers or transport belts which support the substrates at their lateral edge, or are stored in carriers.In this process, it is desirable to make the window layer as thin as possible in order to limit the deterioration of the optical properties of the solar cell due to light absorption in this layer. At the same time, however, it must be ensured that the window layer (frequently CdS layer) does not have any defects (holes), which could lead to short circuits between the front contact and the subsequent absorber layer (frequently CdTe layer). According to the prior art, in order to meet these two requirements, a CdS layer thickness of 60 nm to 200 nm is preferred for a CdS window layer.It may be disadvantageous in the production of the window layer if the front contact layer (preferably TCO layer) has impurities. This can lead to defects in the front contact layer or the overlying layers, in particular the window layer. Such impurities are frequently organic substances, such as grease residues or plastic residues, such as, for example. Plasticizer of the packaging material of prefabricated glass substrates with TCO layer.Studies have shown that conventional washing or rinsing processes are not suitable for sufficiently removing organic contaminants on the TCO coating of the substrates.The problem of surface contamination of substrates is known and is addressed in the prior art by various methods.DE 10 2010 006 681 A1 describes a method in which, in the case of a tandem junction solar cell in the substrate structure, the TCO layer, which was deposited here before the application of a mirror layer and a substrate layer, is intensively cleaned before the subsequent deposition of a mirror layer. In this way, the adhesion of the mirror layer is to be improved and deposition is to be possible even without an adhesion-promoting layer. The purification is carried out using ion-assisted methods. Plasma cleaning methods using noble gas or an O 2 or N-containing gas are described. Ion sputtering or ion etching are also mentioned. However, this additional cleaning process is expensive in terms of apparatus.DE 195 16 446 A1 describes a procedure for cleaning the TCO-coated substrate for the production of solar cells based on amorphous silicon. A plasma method is also used. The plasma process is based on the use of CO 2 and provides specific energy management during the cleaning and subsequent deposition processes.WO 2010 / 120 902 A2 proposes a cleaning method for the TCO-coated surface of a glass substrate. The object is in particular to remove the metal ions on the TCO layer. For this purpose, it is provided to carry out an overetch of the TCO layer, including in two rinsing processes with deionized water. An organic acid is used as etchant.WO 2012 / 089 611 A2 provides for polishing off the surfaces of paper or cardboard in order to improve the adhesion capacity of the surface. In the process, the uppermost substrate layer is oxidized.US 2010 / 0 162 761 A1 describes a method for polishing off the surfaces of glass bodies, in particular flat glass. The flat glass substrates for LCD or plasma panel production must meet very high requirements with regard to the surface roughness. In order to avoid the complicated and highly lossy process of polishing the surfaces, it is proposed to deflamp the surface. This is done to an extent at which the top glass layer is melted and the roughnesses are leveled.US 2007 / 0 031 593 A1 discloses a method for producing a semi-finished product coated with a transparent conductive oxide (TCO), which contains a glass substrate. The semi-finished product is tempered in a furnace at high temperatures of greater than 580° C. for several minutes in order to minimize stresses in the glass. During this tempering, the excess oxygen near the TCO surface is burned by means of flames from one or more gas burners in order to prevent or reduce oxidation of the TCO during the tempering. The flames are used only when the glass and the TCO have reached or exceeded a temperature of greater than approximately 500° C.CN 1 01 236 872 A describes a method for producing field emission cathodes from carbon nanotubes, which are used, for example, for displays. In this case, a metal layer as buffer layer, an activation layer and a catalyst layer are first deposited on a substrate. This layer structure is treated with a flame of a carbon-containing gas, wherein the metal melts due to the low melting point (30° C. to 660° C.) and forms islands of a metal-catalyst mixture with the catalyst, on which islands the carbon nanotubes can in turn grow.US 2011 0311732 A1 discloses a method for heat treatment of at least one thin film on a glass substrate in order to increase the degree of crystallization and / or the crystallite size in the thin film. The heat treatment is carried out by means of at least one gas burner, so that the thin film is heated to a temperature of at least 300° C. for a duration of at most 2 seconds. After the heat treatment, the substrate is cooled, for example by air cooling at rates of 50 to 150 m / s.The processes described are either very complicated in terms of apparatus or energy or they can be integrated poorly in the continuous continuous process of solar cell production according to the prior art.The object is to propose a cleaning method (also as a supplement to conventional washing processes) for the TCO-coated surface of a glass substrate, which cleaning method can be easily integrated into known production processes and ensures stable cleaning success.According to the invention, the object is achieved by a method according to claim 1. Advantageous procedures are set out in the dependent claims which refer back.The method according to the invention provides for the TCO front contact layer or TCO front contact layer sequence of the thin-film solar cell to be polished.The main object of the process according to the invention is to remove organic impurities from the surface. This is done by treating the surface with a gas flame. The aim here is to heat the surface not up to the softening or even melting temperature of the TCO or of the substrate arranged beneath it (preferably float glass). This is ensured by the treatment being carried out only for a short period of time. Studies have shown that the surface temperature of the TCO surface of the substrate after the flame-retardant treatment is in the range from about 80° C. to 220° C. (starting temperature: 20° C.), particularly preferably in the range from 80° C. to 150° C., and very particularly preferably in the range from 90° C. and 120° C. Experience has shown that increasing the temperature of the surface of the front contact layer (TCO layer) by a maximum of 200 K (Kelvin) due to the flame-removal is sufficient to remove the adhering contaminants on the front contact layer. If necessary, however, it is possible to choose the increase in the temperature of the front contact layer to be higher, as long as the temperature of the front contact layer or of the substrate remains below the softening temperature. The temperature increase is preferably in the range from 60° C. to 130° C., particularly preferably in the range from 70° C. to 100° C. In each case, the temperature remains below the softening temperature of the TCO layer or of the substrate.According to the invention, the TCO-coated surface of the substrate is blown off with one or more gas flames. The duration of the flame action on the surface is between 2 s and 30 s and preferably between 3 s and 10 s. Excellent results were obtained with a flame-retardant treatment over about 5 s.In a particularly preferred embodiment, the flame-removal process follows directly the application of the front contact layer (for example by sputtering) or a conventional wet-chemical cleaning process. In a further preferred embodiment, the CdS layer is applied to the blown-off front contact layer immediately after the flame-removal. It is further preferred to deposit an adhesion-promoting layer for the CdS layer on the blown-off front contact layer immediately after the blowing-off.The gas flames are preferably produced with the combustion of commercially available combustible gases, such as propane or butane or preferably a propane / butane mixture (80% / 20%). The combustion guide is designed to stably maintain a blue flame. This is done using methods according to the prior art, in particular nozzle design and air ratio regulation. The blue flame (premix flame) prevents soot particles or unburned hydrocarbons from precipitating on the surface which has been blown off. The temperature in the region of the flame tip is preferably about 900° C. to 1000° C.As a result of the inventive polishing, it was observed that the TCO surface of the substrate is free from pre-existing impurities. In particular, it was found that the surface was wetted much better by water than was the case before the treatment. In addition, the adhesion of layers applied to the treated TCO surface of the substrate is significantly improved. The TCO surface is apparently activated.The inventive flame-retardant treatment as an additional cleaning is particularly suitable when the TCO-coated glasses (substrates) have been produced and supplied on a large industrial scale and are stored for a greater or lesser length of time before the first washing and coating. During storage, impurities form on the TCO glasses, the inventive method advantageously contributing to their elimination. The polishing advantageously supplements the conventional wet chemical cleaning (glass washing) of the TCO-coated substrates before the coating with CdS.The method according to the invention can be integrated very well into the industrial production of thin-film solar cells. After the TCO layer has been applied to the flat glass substrate (or the prefabricated TCO / substrate semi-finished product has been provided), the latter is produced by means of the transport systems (for example, those present in the installations. The roller transport systems) are moved into and continuously through a combustion chamber. If, for example, the TCO layer is directed downward, one or more rows of gas burners are arranged perpendicular to the direction of movement, which extend over the entire width of the substrate and flame it off from below. The substrates are subsequently moved further by the transport system and fed to the process step for depositing the window layer, e.g. introduced into the vacuum chambers for CSS deposition of the CdS layer and subsequently the CdTe layer. This bottom-polishing is advantageous if, as in the case of the CSS method, for example, the deposition of window layer and absorber layer also takes place from below. It is furthermore advantageous that a rotation of the substrates after the slurry, before the introduction into the vacuum chamber for CSS deposition, is thus not necessary. However, it is also possible to flame-treat the TCO layer from above. This has the additional advantage that any combustion residues are entrained by the upwardly rising heat and are conducted away from the TCO surface of the substrate. Of course, the method can also be carried out with the substrate at rest and the burners moved or switched on only briefly.After the TCO layer of the substrate has been blown off, further processing can be carried out using methods according to the prior art to give the finished solar cell. Thus, for example, the CdTe layer and the back contact layer sequence can be applied by known methods. Variations and additional layers above the layer of pure or modified CdS are also possible and are not influenced by the use of the method according to the invention. Instead, studies have shown that the adhesion of the CdS layer to the front contact layer is significantly improved.Exemplary EmbodimentA substrate having dimensions 1600 mm x 1200 mm x 3.2 mm is coated with a layer of indium tin oxide (ITO) having a thickness of 250 nm as a transparent front contact layer.The substrate is then transported on a roller transport device with the downwardly directed front contact layer into a combustion chamber. The substrate travels through the combustion chamber at a continuous velocity of 1.5 m / min and passes a series of gas flames at the center of the combustion chamber, which flame extends over the full width of the substrate and perpendicular to the direction of travel. A mixture of propane and butane in the ratio 80:20 is burned in the gas flames. The combustion management ensures that the flame color remains blue and no soot particles or the like are formed. The substrates enter the combustion chamber at a surface temperature of 21° C. They are exposed to the gas flames in the interior of the combustion chamber for about 5 s. The surface temperature after the slurrying is 105° C. After the slurrying, the substrates are transported into the subsequent treatment chambers. There, the CdS layer is now applied in the CSS method. The layer thickness of the CdS achieved is 60 nm. The CdTe layer is then applied with a thickness of 5000 nm in the CSS method. Thereafter, the back contact layer or layers are applied using methods according to the prior art. The back contact layer here consists of a layer sequence of matching layer and actual contact layer. Here, a matching layer of Te (50 nm) is formed by NP etching the CdTe layer, after which the Mo layer (250 nm) is deposited as the actual contact layer.Finally, the further processing steps according to the prior art are carried out.

Claims

Method for producing a semi-finished product for a thin-film solar cell, comprising the steps: a. providing a transparent substrate having a front contact layer or front contact layer sequence, b. flame-coating the front contact layer or front contact layer sequence with one or more gas burners, wherein a surface temperature of the front contact layer after the flame-coating is in the range from 80°C to 220°C, and wherein the step of flame-coating the front contact layer or front contact layer sequence is carried out for the duration between 2 s and 30 s, and wherein an increase in the surface temperature of the front contact layer or front contact layer sequence by the flame-coating is at most 200 Kelvin.Method according to claim 1, wherein the flame-removal is effected immediately after application of the front contact layer or layer sequence.Method according to claim 1 or 2, wherein the flame-removal takes place immediately before the application of the subsequent window layer.Method according to one of the preceding claims, wherein the flame-removal is carried out using one or more gas burners which are operated with a blue premix flame.The method of any preceding claim, wherein the substrate is continuously conveyed through a combustor in which the slurrying occurs.