Method for forming a CdTe thin-film solar cell including a metal doping step and a system for carrying out the metal doping step

Abstract

Method for manufacturing a CdTe solar cell, comprising the steps: a) Providing a semi-finished CdTe solar cell (10) comprising at least a transparent substrate, a front contact layer, a CdS layer and a CdTe layer, wherein a surface of the CdTe layer opposite the transparent substrate forms a first surface of the semi-finished CdTe solar cell (10), b) Applying an aqueous solution (20) comprising metal ions or metal-containing ions to a back side (11) of the semi-finished CdTe solar cell (10), c) Removing the aqueous solution (20) from the back (11) of the semi-finished CdTe solar cell (10), d) Applying a metal layer to the first surface of the semi-finished CdTe solar cell (10), and e) additionally applying electrical energy to the semi-finished CdTe solar cell (10) by electrically connecting it to an electrical power supply (140), wherein the back side (11) of the semi-finished CdTe solar cell (10) is either the first surface of the semi-finished CdTe solar cell (10) or a first surface of the metal layer facing the transparent substrate, wherein step e) is carried out for a first period (t1) of the time during which the aqueous solution (20) is present on the back side (11) of the semi-finished CdTe solar cell (10).

Description

[0001] The present application relates to a method for manufacturing a CdTe solar cell including a metal doping step and a system for carrying out this metal doping step.

[0002] In the prior art, a CdTe solar cell has the following structure: A layer of transparent conducting oxide (TCO) is deposited as a front contact on a glass substrate. The TCO layer may include a high-resistance buffer layer, which helps to minimize the shunt effect in the solar cell. A layer of cadmium sulfide (CdS) is deposited on top of this, followed by a layer of cadmium telluride (CdTe). Finally, a metal layer, e.g., of molybdenum, nickel vanadium, tantalum, titanium, tungsten, gold, or any composition or compound containing any of these elements, is applied to collect the charge carriers. This process is called a superstrate configuration.

[0003] To achieve high solar cell efficiency, good ohmic contact between the CdTe layer and the metal layer should be established. This can be achieved by introducing copper into the CdTe layer at the interface with the metal layer. The copper can be applied to the CdTe layer as an elemental layer consisting solely of copper, as a dopant contained in another material, or as an ion or component of a chemical compound. For example, the copper can be deposited onto the CdTe layer from a gas, e.g., by sputtering, or from an aqueous solution, e.g., copper chloride or a copper salt. After the copper has been deposited onto the CdTe layer, a heat treatment can be performed. In the prior art, any process for introducing copper into the CdTe layer is referred to as the copper doping step.

[0004] German patent DE 10 2011 054 795 A1 describes such a conventional process for manufacturing a CdS / CdTe solar cell. In this process, a CdTe layer is produced on a CdS layer using electrodeposition. An aqueous solution containing Cu ions is then applied to the CdTe surface to dope the CdTe layer with copper, followed by a temperature step (annealing). Finally, a back-contact layer is produced on the CdTe layer by sputtering or evaporation.

[0005] US Patent 2016 / 126399 A1 discloses a method for manufacturing a heterojunction silicon solar cell and, in particular, a specific design of the collector electrode on the irradiated side of the solar cell and an insulating layer surrounding it. The collector electrode is produced locally in a three-step process: First, a seed layer is created in locally defined areas by printing an electrically conductive paste containing electrically conductive particles and an insulating material, or by sputtering. Next, an insulating layer is created that surrounds the locally defined areas, leaving the seed layer exposed. Finally, a second conductive layer of metal, e.g., copper, is produced locally on the seed layer by electroplating.

[0006] DE 28 44 712 A1 describes a method for forming a semiconducting layer on an electrically conductive substrate, wherein the semiconducting layer is used, for example, as an electrode layer in a photoelectrochemical cell. Such a cell comprises a semiconductor photoanode as a thin film on a metal electrode, a metal cathode, and an electrolyte (solution containing a redox couple) arranged between them. Test experiments to determine the efficiency of such a cell were conducted, necessarily involving illumination of the cell.

[0007] However, copper migrates very easily within the CdTe and can therefore degrade the properties of the CdTe solar cell over time. It is therefore crucial to precisely control the amount and position of copper introduced into the CdTe layer to ensure good ohmic contact while minimizing the risk of copper migration. This can be achieved, for example, by controlling the copper concentration in the aqueous solution, the duration for which an aqueous solution is applied to the CdTe layer, or the heat balance of the heat treatment step, defined by its temperature and duration. Unfortunately, some of these parameters cannot be controlled as precisely as necessary.

[0008] The object of the present invention is to provide a method for forming a CdTe thin-film cell, including a metal doping step, wherein this method offers improved control of the quantity and position of the metal ions introduced into the CdTe layer by the metal doping step. A further object is to provide a system suitable for carrying out this method.

[0009] The method according to the present application comprises the steps of providing a semi-finished CdTe solar cell, applying an aqueous solution comprising metal ions or metal-containing ions to a back surface of the semi-finished CdTe solar cell, removing the aqueous solution from the back surface of the semi-finished CdTe solar cell, and applying a metal layer to a first surface of the semi-finished CdTe solar cell to form a back contact. The semi-finished CdTe solar cell comprises at least a transparent substrate, a front contact layer, a CdS layer, and a CdTe layer, wherein a surface of the CdTe layer facing the transparent substrate forms the first surface of the semi-finished CdTe solar cell. The back surface onto which the aqueous solution is applied is either the first surface of the semi-finished CdTe solar cell or a first surface of the metal layer facing the transparent substrate.The semi-finished CdTe solar cell may include further layers between the aforementioned layers, and one or more layers, for example the front contact layer, may also be formed as a layer sequence, as known in the prior art. The front contact layer is typically transparent and is often implemented using a transparent conductive oxide (TCO). The CdS layer, the CdTe layer, and the front contact layer or layer sequence are formed by methods known in the prior art.

[0010] The aqueous solution can be applied to the back of the semi-finished CdTe solar cell using methods known in the prior art, such as, but not limited to: - Immersion of the semi-finished CdTe solar cell (or the back of the semi-finished CdTe solar cell) into the aqueous solution in a container, - Spray on, - Centrifugal coating, - Roller coating with a sponge roller, etc.

[0011] The aqueous solution can be a solution of a metal salt, for example, CuCl₂, CuSO₄, Cu(NO₃)₂, SbCl₃, AsCl₃, AgCl, or any other compound containing metal complexes. In the solution, the metal is present in the form of metal ions or bound in electrically charged complexes, i.e., in metal-containing ions. The aqueous solution contains the metal at a concentration between 0.05 mmol / L and 10 mmol / L. Consequently, ions of a metal, for example, copper (Cu), antimony (Sb), silver (Ag), or arsenic (As), are introduced into the CdTe layer; in other words, the CdTe layer is doped with metal ions.

[0012] The aqueous solution can be removed from the back of the semi-finished CdTe solar cell by removing the semi-finished CdTe solar cell from the aqueous solution contained in a container and / or by blowing, rinsing with a cleaning solution, drying or a combination thereof, or by other methods known in the prior art.

[0013] The step of applying a metal layer to the first surface of the semi-finished CdTe solar cell is known from the prior art and can include the deposition of a layer of molybdenum, nickel vanadium, gold, tantalum, tungsten, alloys of molybdenum, tantalum, titanium and tungsten, compounds comprising molybdenum or tungsten or other materials, or the deposition of combinations or layer sequences of various such materials using sputtering, evaporation / sublimation, chemical vapor deposition, or any other suitable deposition technique. Furthermore, the step of applying a metal layer can also include the formation of an additional layer of another material such as ZnTe, Cu₂O, Cu₂Te, CuTe, or other metal telluride compounds between the first surface of the semi-finished CdTe solar cell and the metal layer, as is also known from the prior art.

[0014] According to the present application, the semi-finished CdTe solar cell is additionally illuminated and / or electrical energy is applied to the semi-finished CdTe solar cell by electrically connecting the solar cell to an electrical power supply. As a result, an additional electric field is generated above the semi-finished CdTe solar cell. This step of additional illumination and / or application of electrical energy can be carried out for a first period while the aqueous solution is present on the back of the semi-finished CdTe solar cell, and / or for a second period after the step of removing the aqueous solution from the back of the semi-finished CdTe solar cell and before the step of applying a metal layer to the first surface.The first period can be the entire time between the start of the aqueous solution application step and the end of the aqueous solution removal step, i.e., the entire time during which the aqueous solution is at least partially present on the back side of the semi-finished CdTe solar cell, or only a portion of this time. The second period can be the entire time between the end of the aqueous solution removal step and the start of the metal layer application step, i.e., the entire time between these two steps, or only a portion of this time.

[0015] Therefore, there are four possible process sequences according to the present application, wherein the steps are carried out in the aforementioned sequence and wherein only the step of additional illumination and / or application of electrical energy can overlap with the steps of applying an aqueous solution and removing the aqueous solution in the first, the second and the first part of the fourth process sequence: 1. Providing the semi-finished CdTe solar cell, applying an aqueous solution, additional illumination and / or application of electrical energy, removing the aqueous solution and applying a metal layer; 2. Providing the semi-finished CdTe solar cell, applying a metal layer, applying an aqueous solution, additional illumination and / or application of electrical energy, and removing the aqueous solution; 3. Providing the semi-finished CdTe solar cell, applying an aqueous solution, removing the aqueous solution, additional illumination and / or application of electrical energy, and applying a metal layer; 4. Providing the semi-finished CdTe solar cell, applying an aqueous solution, additional illumination and / or application of electrical energy, removing the aqueous solution, additional illumination and / or application of electrical energy, and applying a metal layer.

[0016] The fourth case is a combination of the first and third cases. In the first, third, and fourth cases, the back surface of the semi-finished CdTe solar cell, onto which the aqueous solution is applied, is formed by a surface of the CdTe layer, which is the first surface of the semi-finished CdTe solar cell. In the second case, the back surface of the semi-finished CdTe solar cell, onto which the aqueous solution is applied, is formed by the first surface of the metal layer.

[0017] The additional electric field across the semi-finished CdTe solar cell, generated by the additional illumination or the supplied electrical energy, superimposes upon the inherent electric field generated by the pn junction between the CdS and CdTe layers. "Superimposed" means that the additional electric field either reinforces or counteracts the inherent electric field. In the case of additional illumination or contacting the front contact layer with a negative terminal of the power supply, more metal ions or positively charged, metal-containing complexes migrate within the CdTe layer toward the CdS layer than would occur without the additional illumination or external electrical energy supply.When the front contact layer is brought into contact with a positive terminal of the power supply, the migration of metal ions or positively charged, metal-containing complexes within the CdTe layer towards the CdS layer is reduced compared to the case without external electrical energy input. Therefore, the quantity and location of metal ions introduced within the CdTe layer by a metal doping step can be controlled more precisely compared to the prior art copper treatment step, where only the copper concentration in the aqueous solution or the duration of the copper treatment step can be influenced.

[0018] Therefore, the metal doping step according to the present application comprises the following three substeps: applying an aqueous solution comprising metal ions or metal-containing ions to the back of the semi-finished CdTe solar cell, removing the aqueous solution from the back of the semi-finished CdTe solar cell, and additionally illuminating and / or electrically connecting the semi-finished CdTe solar cell to an electrical power supply for a first period or a second period, each substep being carried out at a defined location within the overall process sequence.

[0019] "Additional lighting" means lighting that is higher than the lighting provided by the light present during the copper treatment step according to the prior art. In the prior art, this step is not usually carried out in a darkroom, but under normal production conditions, including typical lighting conditions. The "additional lighting" is provided in addition to these lighting conditions by a special lighting unit and delivers additional light with an illuminance in the range of 5,000 to 200,000 ×.

[0020] The light with which the semi-finished CdTe solar cell is additionally illuminated has a wavelength in the absorption range of the CdTe solar cell and preferably in the range between 300 and 900 nm.

[0021] When the semi-finished CdTe solar cell is electrically connected to an electrical power supply, this power supply provides an additional electric field across the semi-finished CdTe solar cell. This additional electric field can be provided between the aqueous solution and the front contact layer of the semi-finished CdTe solar cell, or between ground and the front contact layer if the aqueous solution is not present on the back of the semi-finished CdTe solar cell. For example, the front contact layer of the semi-finished CdTe solar cell can be electrically connected to a first contact of the power supply, and the aqueous solution can be electrically connected to a second contact of the power supply. However, it is also possible to connect only the front contact layer and leave the aqueous solution unconnected.floating, or connecting them to the ground. The additional electric field can lead to an electric current flowing through the semi-finished CdTe solar cell, with an absolute value greater than zero and less than or equal to twice the short-circuit current of the CdTe solar cell. That is, the electric current can be positive or negative compared to the short-circuit current.

[0022] Only one of the measures, additional lighting and electrical power supply, can be implemented as a single measure, or both measures can be implemented separately, sequentially, in any order, or simultaneously. Implementing both measures can reduce exposure to light, i.e., the amount of energy from the additional lighting and / or electrical power, compared to implementing only one measure.

[0023] The first or second period, in which one or both measures are carried out, preferably lies in the range of 5 seconds to 30 minutes. The duration of the first or second period depends on the luminance and / or the electrical energy or the metal concentration within the aqueous solution and the desired distribution of the metal within the semi-finished CdTe solar cell.

[0024] Furthermore, the temperature of the semi-finished CdTe solar cell is controlled so that it remains between 25 °C and 80 °C during the first period, i.e., while one or both of the aforementioned measures are being carried out and the aqueous solution is present on the back. If the step of additional illumination and / or application of electrical energy is performed after the aqueous solution has already been removed, the temperature of the semi-finished CdTe solar cell is controlled so that it remains between 25 °C and 225 °C during the second period. The temperature of the semi-finished CdTe solar cell is another parameter used to control the distribution of the introduced metal within the semi-finished CdTe solar cell.To achieve a desired temperature, the semi-finished CdTe solar cell can be heated or cooled, or heated and cooled sequentially, in any order during the first period or the second period.

[0025] Consequently, a skilled person now has a large number of parameters at their disposal that they can control to achieve a desired distribution of a metal within the semi-finished CdTe solar cell: the luminance of the additional illumination, the electrical energy supplied by the power supply, the duration of the first or second period, the temperature of the semi-finished CdTe solar cell during the first or second period, the concentration of the metal within the aqueous solution, and the duration of the aqueous solution's presence on the back surface. This allows for the reduction of over-doping of the semi-finished CdTe solar cell with the metal—that is, the introduction of a metal quantity higher than required to form a good ohmic contact—and the resulting degradation of the finished CdTe solar cell, while simultaneously widening the process window for the metal doping step.

[0026] The method according to the present application may include further steps, such as a temperature treatment step or a conditioning step that includes illuminating and / or providing electrical energy to the CdTe solar cell, or a combination of these steps. However, these steps, which are known in the prior art, are carried out at least after the step of applying a metal layer to the first surface of the semi-finished CdTe solar cell to form a backside contact and without the presence of the aqueous solution.

[0027] According to the present application, a system for performing a metal doping step on a semi-finished CdTe solar cell comprises a first unit for applying an aqueous solution containing metal ions or metal-containing ions to the back side of the semi-finished CdTe solar cell, a second unit for removing the aqueous solution from the back side of the semi-finished CdTe solar cell, and an illumination unit. The semi-finished CdTe solar cell comprises at least a transparent substrate, a front contact layer, a CdS layer, and a CdTe layer, wherein the back side of the semi-finished CdTe solar cell is a surface of the semi-finished CdTe solar cell facing the transparent layer.In particular, the semi-finished CdTe solar cell can comprise a transparent substrate, a front contact layer, a CdS layer, and a CdTe layer, with the back side formed by a surface of the CdTe layer; or the semi-finished CdTe solar cell can comprise a transparent substrate, a front contact layer, a CdS layer, a CdTe layer, and a metal layer, with the back side formed by a surface of the metal layer. The illumination unit is suitable for additionally illuminating the semi-finished CdTe solar cell for an initial period while the aqueous solution is present on the back side of the semi-finished CdTe solar cell.

[0028] The lighting unit can be combined with the first unit and / or the second unit so that all combined units perform their functions simultaneously for at least part of their respective process time. For example, if the first unit comprises a container holding the aqueous solution and a component for immersing the semi-finished CdTe solar cell into the aqueous solution, the lighting unit can be arranged so that the light it generates illuminates the semi-finished CdTe solar cell for at least part of the time the cell is immersed in the aqueous solution. However, it is also possible for the first unit and the lighting unit, or the second unit and the lighting unit, to perform their functions sequentially, even when combined.“Combination of the first unit and / or the second unit and the lighting unit” means that a process area of ​​the lighting unit overlaps at least partially with the first unit and / or with the second unit. “Process area” refers to the spatial area in which light generated by the lighting unit acts on the semi-finished CdTe solar cell. In one embodiment, the first unit and the lighting unit are arranged in a single unit.

[0029] However, it is also possible that the lighting unit is spatially separated from the first and / or second units, so that in a unit sequence, the lighting unit is positioned after the first unit and before the second unit. The unit sequence describes the order in which the units are used. The spatial arrangement of the units in a production hall may differ from the unit sequence. While the semi-finished CdTe solar cell is within the process area of ​​the lighting unit, the aqueous solution is at least partially present on the first surface of the semi-finished CdTe solar cell.For example, the first unit could include a container holding the aqueous solution and a component for immersing the semi-finished CdTe solar cell in the aqueous solution; or it could include a nozzle unit for spraying the aqueous solution onto the back of the semi-finished CdTe solar cell; or it could include a roller unit for rolling the aqueous solution onto the back of the semi-finished CdTe solar cell. In all cases, the process area of ​​the illumination unit is spatially separated from the first unit. Therefore, the first unit completes its work, i.e.,This is achieved by removing the semi-finished CdTe solar cell from the container holding the aqueous solution, or by stopping the spraying or roller coating of the aqueous solution onto the back of the semi-finished CdTe solar cell. The semi-finished CdTe solar cell is then transported from the first unit to the process area of ​​the illumination unit and illuminated there. In this case, the illumination unit is designed such that the aqueous solution remains on the back of the semi-finished CdTe solar cell during illumination. For example, the illumination unit has a holder that holds the semi-finished CdTe solar cell in such a way that the back is kept horizontal and that the lateral distribution of the aqueous solution across the lateral dimensions of the back remains unchanged compared to that resulting from the application of the aqueous solution by the first unit.The semi-finished CdTe solar cell is then transported from the lighting unit to the second unit, where the aqueous solution is removed from the back of the semi-finished CdTe solar cell.

[0030] The lighting unit is suitable for illuminating the semi-finished CdTe solar cell with light that has a wavelength in the absorption range of the CdTe solar cell, and preferably with light with a wavelength in the range between 300 and 900 nm.

[0031] In a particular embodiment, the system further comprises a third unit for controlling the temperature of the semi-finished CdTe solar cell so that it remains within the range of 25 °C to 80 °C during the initial period. The third unit can comprise a heating device, a cooling device, or a combination thereof, depending on the desired temperature of the semi-finished CdTe solar cell.

[0032] For example, the lighting unit and the third unit can be combined in a temperature-controlled tunnel containing one or multiple light-emitting lamps. This tunnel can be arranged in a sequence of units after the first unit, which applies the aqueous solution to the back of a semi-finished CdTe solar cell, for example by spraying or roller coating, and before the second unit.

[0033] In addition to the lighting unit, the system can include an electrical power supply and a contact device for connecting the front contact layer of the semi-finished CdTe solar cell to the electrical power supply during the initial phase. The electrical power supply is suitable for generating an additional electric field across the semi-finished CdTe solar cell when connected to it. Fig. Figures 1A to 1C schematically show exemplary process flows of the procedure according to the present application. Fig. Figure 2 schematically shows a first embodiment of the system according to the present application, wherein the first unit and the lighting unit are combined. Fig. Figure 3 schematically shows a second embodiment of the system according to the present application, wherein the first unit and the lighting unit are separate from each other.

[0034] The method and the system according to the invention are explained below in exemplary embodiments, the figures not being intended to imply any limitation of the embodiments shown.

[0035] Fig. Figure 1A shows a first exemplary process flow of the method according to the present application. Initially, in step S110, a semi-finished CdTe solar cell with a first surface as described above is provided. In a subsequent step S120, an aqueous solution comprising metal ions as described above is applied to the first surface, i.e., to a surface of the CdTe layer. The aqueous solution is at least partially present on the first surface of the semi-finished CdTe solar cell until it is completely removed from the first surface in step S140, i.e., during a period of aqueous presence defined by t A The time from the start of step S120 to the end of step S140 is the time of the metal doping step, denoted by t. D is designated as the period t in this example. A is. Over a first period t1 during time t AThe semi-finished CdTe solar cell is additionally illuminated (step S131) ​​and / or electrical energy is applied to the semi-finished CdTe solar cell (step S132). This means that one or both steps S131 and S132 can be performed. If both steps are performed, steps S131 and S132 can be carried out simultaneously, partially overlapping, or completely separated in time. This means that the time periods t 31 and t 32 The periods in which step S131 and step S132 are each performed can be the same or different, and they can overlap completely or partially, or be completely separated on the timescale. However, the total period, i.e., the sum of all periods in which at least one of step S131 and step S132 is performed, is what is defined in Fig. Figure 1A shows the first period t1. Steps S131 and / or S132 may overlap with step S120 and / or step S140. Following step S140, a metal layer is applied to the first surface of the semi-finished CdTe solar cell (step S150). The combination of all steps S120, S131, S132, and S140 is called the metal doping step according to the present application.

[0036] In Fig. Figure 1B schematically depicts a second exemplary process flow of the procedure according to the present application. Step S210 corresponds to step S110 of Fig. 1A. Then, in the next step S220, a metal layer is applied to the first surface of the semi-finished CdTe solar cell. As a result, the semi-finished CdTe solar cell has a back side formed by the metal layer. Subsequently, an aqueous solution containing metal ions, as described above, is applied to the back side, i.e., to a surface of the metal layer (step S230). The aqueous solution remains at least partially on the back side of the semi-finished CdTe solar cell until it is completely removed from the back side in step S250, i.e., during a period of aqueous presence defined by t A The time from the start of step S230 to the end of step S250 is the time of the metal doping step, denoted by t. D is designated as the period t A is. Over a first period t1 during time t AThe semi-finished CdTe solar cell is additionally illuminated (step S241) and / or electrical energy is applied to the semi-finished CdTe solar cell (step S242). This means that one or both steps S241 and S242 can be performed. If both steps are performed, steps S241 and S242 can be carried out simultaneously, partially overlapping, or completely separated in time. This means that the time periods t 41 and t 42 The periods in which step S241 and step S242 are each performed can be the same or they can be different, and they can overlap completely or partially, or be completely separated on the timescale. However, the total period, i.e., the sum of all periods in which at least one of step S241 and step S242 is performed, is what is defined in Fig. 1B shows the first period t1. As in relation to the first exemplary process flow in Fig. As described in Figure 1A, steps S241 and / or S242 can overlap with step S230 and / or with step S250. The combination of all steps S230, S241, S242 and S250 is called the metal doping step according to the present application.

[0037] A third exemplary process flow of the procedure according to the present application relates to Fig. 1C is shown. Step S310 corresponds to step S110 of Fig. 1A and step S210 of Fig. 1B. Then, an aqueous solution containing metal ions as described above is applied to the first surface, i.e., to a surface of the CdTe layer (step S320). The aqueous solution remains at least partially present on the first surface of the semi-finished CdTe solar cell until it is completely removed from the first surface in step S330, i.e., during a period of aqueous presence lasting t AThis is referred to as step S330. Following step S330, the semi-finished CdTe solar cell is processed over a second period t2 during an intermediate period t. l Between step S330 and step S350, in which a metal layer is applied to the first surface, additional illumination is applied (step S341) and / or electrical energy is applied to the semi-finished CdTe solar cell (step S342). This means that one or both steps S341 and S342 can be performed. If both steps are performed, steps S341 and S342 can be carried out simultaneously, partially overlapping, or completely separated in time. This means that the time intervals t 41 and t 42The periods in which step S341 and step S342 are each performed can be the same or they can be different, and they can overlap completely or partially, or be completely separated on the timescale. However, the total period, i.e., the sum of all periods in which at least one of step S341 and step S342 is performed, is what is defined in Fig. The second period t2 shown in 1C. The combination of all steps S320, S330, S341 and S342 is called the metal doping step according to the present application.

[0038] In Fig. Figure 2 schematically illustrates a first embodiment (100) of the system according to the present invention. The system (100) comprises a first unit (110), a second unit (120), a lighting unit (130), and an electrical power supply (140). The first unit (110) is suitable for applying an aqueous solution (20) as described above to a back surface (11) of a semi-finished CdTe solar cell (10). The back surface can be a surface of the CdTe layer or it can be a surface of a metal layer serving as a back contact layer. For this purpose, the first unit (110) comprises a container (111) in which the aqueous solution (20) is contained and a component (112) for immersing the semi-finished CdTe solar cell (10) in the aqueous solution (20). The component (112) can, for example, be a holder with a clamp that is attached to the semi-finished CdTe solar cell (10).The aqueous solution (30) can be heated by a third unit (150) so that the aqueous solution (20) has a temperature between 25 °C and below 100 °C (below the boiling point of the aqueous solution). As a result, the semi-finished CdTe solar cell (10) preferably has a temperature between 25 °C and 80 °C when immersed in the aqueous solution (20). Furthermore, an electrode (113) is provided, which is also immersed in the aqueous solution (20). When the electrical power supply (140) is electrically connected at one end to a front contact layer of the semi-finished CdTe solar cell (10) and at the other end to the electrode (113) by means of a contact device comprising, for example, electrical conductors (141), an electric field is formed between the first electrode (113) and the aqueous solution (20) on one side and the front contact layer of the semi-finished CdTe solar cell (10) on the other side.This allows the movement of metal ions or ions containing charged metal to be controlled within the aqueous solution (20) and / or within the semi-finished CdTe solar cell (10). Furthermore, a lighting unit (130), for example, a halogen lamp with an illuminance of 100,000 ix, is combined with the first unit (110) so that the lighting unit (130) illuminates the semi-finished CdTe solar cell (10) while it is immersed in the aqueous solution (20). After the semi-finished CdTe solar cell (10) is removed from the aqueous solution (20), it is transported to the second unit (120) (as indicated by the arrow). The second unit (120), which is known from the prior art, comprises, for example, a rinsing device (121) and a drying device (122) which remove the aqueous solution (20) or residues thereof from the back (11) of the semi-finished CdTe solar cell (10).

[0039] Fig. Figure 3 schematically shows a second embodiment (200) of the system according to the present invention. The system (200) comprises a first unit (210), a second unit (220), and a lighting unit (230). The first unit (210) comprises a nozzle assembly or nozzle device (211) connected by a fluid line (213) to a container (212) containing an aqueous solution (20), as described above. The aqueous solution (20) is sprayed by the nozzle device (211) onto the back surface (11) of a semi-finished CdTe solar cell (10) (as indicated by the dashed arrows). The back surface can be a surface of the CdTe layer or it can be a surface of a metal layer serving as the back contact layer. The semi-finished CdTe solar cell (10) is held on a holder (214), which may be rotatable.After the aqueous solution (20) is applied to the back (11) of the semi-finished CdTe solar cell (10), the semi-finished CdTe solar cell (10) with the aqueous solution present on its back (11) is transported to a temperature-controlled tunnel (240) in which the lighting unit (230) is mounted. The lighting unit (230) is a lamp arrangement that provides light with a wavelength of 300 to 800 nm and an illuminance of 30,000 to 200,000 ×. While the semi-finished CdTe solar cell (10) is held on a temperature-controlled holder (241) within the temperature-controlled tunnel (240), the lighting unit (230) illuminates the semi-finished CdTe solar cell (10) for an initial period. Therefore, in the second embodiment (200) of the system shown, the first unit (210) and the lighting unit (230) are separate from each other.To maintain the semi-finished CdTe solar cell (10) at a desired temperature during illumination, a third unit (250), for example a cooling device, is mounted inside the temperature-controlled tunnel (240). After illumination, the semi-finished CdTe solar cell (10) is transported from the temperature-controlled tunnel (250) to the second unit (220), which may include the same or different devices as described above. Fig. 2 described.

[0040] In the Fig. 2 and Fig.In the three examples shown, the illumination unit (130, 230) is arranged such that the light emitted by the illumination unit (130, 230) falls on the back side (11) of the semi-finished CdTe solar cell (10). However, this is only an exemplary arrangement of the illumination unit that could be used if the back side of the semi-finished CdTe solar cell is the first surface of the semi-finished CdTe solar cell, i.e., a surface of the CdTe layer. If a metal layer has already been applied to the CdTe layer, so that the back side of the semi-finished CdTe solar cell is a surface of the metal layer, the illumination unit should be arranged within the system for carrying out a metal doping step such that the light emitted by the illumination unit falls on the sun-facing side of the semi-finished CdTe solar cell, i.e., on the transparent substrate.Nevertheless, the semi-finished CdTe solar cell can in any case be illuminated on the sunny side. Reference numbers 10 semi-finished CdTe solar cells 11 Back side of the semi-finished CdTe solar cell 20 aqueous solution 100, 200 System for performing a metal doping step 110, 210 first unit 111, 212 containers 112 components for immersion 113 Electrode 120, 220 second unit 121 Flushing device 122 Drying device 130, 230 lighting unit 140 electrical power supply 141 electrical conductor 211 Nozzle device 213 Fluid line 214 holders 240 temperature-controlled tunnel 241 temperature-controlled holder 150, 250 third unit t1 first period t2 second period t Aperiod for the aqueous present t D Time period for the metal doping step t l Interim period t 31 , t 41 Period for additional lighting t 32 , t 42 Period for applying electrical energy

Claims

[1] Method for manufacturing a CdTe solar cell, comprising the steps: a) Providing a semi-finished CdTe solar cell (10) comprising at least a transparent substrate, a front contact layer, a CdS layer and a CdTe layer, wherein a surface of the CdTe layer opposite the transparent substrate forms a first surface of the semi-finished CdTe solar cell (10), b) Applying an aqueous solution (20) comprising metal ions or metal-containing ions to a back side (11) of the semi-finished CdTe solar cell (10), c) Removing the aqueous solution (20) from the back (11) of the semi-finished CdTe solar cell (10), d) Applying a metal layer to the first surface of the semi-finished CdTe solar cell (10), and e) additionally applying electrical energy to the semi-finished CdTe solar cell (10) by electrically connecting it to an electrical power supply (140), wherein the back side (11) of the semi-finished CdTe solar cell (10) is either the first surface of the semi-finished CdTe solar cell (10) or a first surface of the metal layer facing the transparent substrate, wherein step e) is carried out for a first period (t1) of the time during which the aqueous solution (20) is present on the back side (11) of the semi-finished CdTe solar cell (10). [2] Method according to claim 1, characterized by , that steps b) and c) are carried out before step d) and the back (11) is the first surface of the semi-finished CdTe solar cell (10). [3] Method according to claim 1, characterized by , that steps b) and c) are carried out after step d) and the back side (11) is the first surface of the metal layer [4] Method according to any one of claims 1 to 3, characterized by , that the temperature of the semi-finished CdTe solar cell (10) is controlled to remain in the range between 25 °C and 80 °C during the first period (t1). [5] Method for manufacturing a CdTe solar cell, comprising the steps: a) Providing a semi-finished CdTe solar cell (10) comprising at least a transparent substrate, a front contact layer, a CdS layer and a CdTe layer, wherein a surface of the CdTe layer opposite the transparent substrate forms a first surface of the semi-finished CdTe solar cell (10), b) Applying an aqueous solution (20) comprising metal ions or metal-containing ions to a back side (11) of the semi-finished CdTe solar cell (10), c) Removing the aqueous solution (20) from the back (11) of the semi-finished CdTe solar cell (10), d) Applying a metal layer to the first surface of the semi-finished CdTe solar cell (10), and e) additionally applying electrical energy to the semi-finished CdTe solar cell (10) by electrically connecting it to an electrical power supply (140), wherein the back side (11) of the semi-finished CdTe solar cell (10) is the first surface of the semi-finished CdTe solar cell (10), wherein step e) is carried out for a second period (t2) after step c) and before step d). [6] Method according to claim 5, characterized by , that the temperature of the semi-finished CdTe solar cell (10) is controlled to remain in the range between 25 °C and 225 °C during the second period (t2). [7] Method according to any one of claims 1 to 6, characterized by, that the electrical power supply (140) provides an electric current which flows through the semi-finished CdTe solar cell (10) and has an absolute value greater than zero and less than or equal to twice the short-circuit current of the CdTe solar cell. [8] Method for manufacturing a CdTe solar cell, comprising the steps: a) Providing a semi-finished CdTe solar cell (10) comprising at least a transparent substrate, a front contact layer, a CdS layer and a CdTe layer, wherein a surface of the CdTe layer opposite the transparent substrate forms a first surface of the semi-finished CdTe solar cell (10), b) Applying an aqueous solution (20) comprising metal ions or metal-containing ions to a back side (11) of the semi-finished CdTe solar cell (10), c) Removing the aqueous solution (20) from the back (11) of the semi-finished CdTe solar cell (10), d) Applying a metal layer to the first surface of the semi-finished CdTe solar cell (10), and e) additionally illuminating the semi-finished CdTe solar cell (10), wherein the back (11) of the semi-finished CdTe solar cell (10) is either the first surface of the semi-finished CdTe solar cell (10) or a first surface of the metal layer facing the transparent substrate, wherein step e) is carried out for a first period (t1) of the time during which the aqueous solution (20) is present on the back (11) of the semi-finished CdTe solar cell (10) and / or for a second period (t2) after step c) and before step d). [9] Method according to claim 8, characterized by, that steps b) and c) are carried out before step d), the back side (11) is the first surface of the semi-finished CdTe solar cell (10) and step e) is carried out for the first period (t1) in which the aqueous solution (20) is present on the back side (11) of the semi-finished CdTe solar cell (10). [10] Method according to claim 8, characterized by , that steps b) and c) are carried out after step d), the back side (11) is the first surface of the metal layer and step e) is carried out for the first period (t1) in which the aqueous solution (20) is present on the back side (11) of the semi-finished CdTe solar cell (10). [11] Method according to claim 8, characterized by , that steps b) and c) are carried out before step d), the back side (11) is the first surface of the semi-finished CdTe solar cell (10) and step e) is carried out for the second period (t2) after step c) and before step d). [12] Method according to any one of claims 8 to 11, characterized by , that the light with which the semi-finished CdTe solar cell (10) is additionally illuminated has a wavelength in the absorption range of the CdTe solar cell. [13] Method according to claim 12, characterized by that the wavelength of the light lies in the range of 300 to 900 nm. [14] Method according to claims 9 or 10, characterized by , that the temperature of the semi-finished CdTe solar cell (10) is controlled to remain in the range between 25 °C and 80 °C during the first period (t1). [15] Method according to claim 11, characterized by , that the temperature of the semi-finished CdTe solar cell (10) is controlled to remain in the range between 25 °C and 225 °C during the second period (t2). [16] Method according to any one of claims 1 to 15, characterized by that the first period (t1) or the second period (t2) lies in a range between 5 s and 30 min. [17] System (100, 200) for carrying out a metal doping step of a semi-finished CdTe solar cell (10), wherein the system (100, 200) comprises: - a first unit (110, 210) for applying an aqueous solution (20) comprising metal ions or metal-containing ions to a back side (11) of a semi-finished CdTe solar cell (10) comprising at least a transparent substrate, a front contact layer, a CdS layer and a CdTe layer, wherein the back side (11) of the semi-finished CdTe solar cell (10) is a surface of the semi-finished CdTe solar cell (10) facing the transparent substrate, - a second unit (120, 220) for removing the aqueous solution (20) from the back (11) of the semi-finished CdTe solar cell (10), and - a lighting unit (130, 230) suitable for the additional illumination of the semi-finished CdTe solar cell (10) for a first period (t1), while the aqueous solution (20) applied to the semi-finished CdTe solar cell (10) by the first unit (110, 210) is present on the back (11) of the semi-finished CdTe solar cell (10). [18] System according to claim 17, characterized by , that the lighting unit (130, 230) is suitable to illuminate the semi-finished CdTe solar cell (10) with light with a wavelength in the absorption range of the CdTe solar cell. [19] System according to claim 18, characterized by that the wavelength of the light lies in the range of 300 to 900 nm. [20] System according to claim 17, further comprising a third unit (150, 250) for controlling a temperature of the semi-finished CdTe solar cell (10) so that it is in the range between 25 °C and 80 °C during the first period (t1). [21] System according to claim 17, further comprising an electrical power supply (140) and a contact device for connecting the semi-finished CdTe solar cell (10) to the electrical power supply (140), wherein the electrical power supply (140) is suitable to form an additional electric field over the semi-finished CdTe solar cell (10) during the first period (t1) when it is connected to the semi-finished CdTe solar cell (10).

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