DE102014202961A1 - Process for producing thin-film solar cells with a p-doped CdTe layer - Google Patents

Abstract

In the present invention, there is provided a process for producing thin film CdTe solar cells having a pin-pore free and uniformly doped CdTe layer of reduced film thickness. The inventive method effectively prevents short circuits in the solar cells, improves their reliability and long-term stability and provides the CdTe layer with a uniform doping. This is achieved by applying a sacrificial layer between a first coarse-grained CdTe layer and a second fine-grained CdTe layer, which together form the CdTe layer of the solar cells. It is also possible to dispense with the CdCl2 activation step if the sacrificial layer contains a halogen.

Description

The present invention is a process for producing CdTe solar cells with improved efficiency.

The spread of thin-film solar cells can be further accelerated by increasing the efficiency of light conversion. CdTe-based solar cells have proven particularly promising.

The CdTe solar cell according to the prior art has the following structure: On a glass substrate, a transparent (oxide-containing) conductor layer (TCO, transparent conductive oxides) is arranged as a front contact. The TCO layer may include a high-resistance buffer layer that helps to minimize the shunting effect (i.e., the occurrence of shunts) in the solar cells. A cadmium sulphide layer (CdS) is applied over this layer and a layer of cadmium telluride layer (CdTe) is applied thereon. Finally, a metal layer is applied to collect the charge carriers. This method is known as superstrate configuration.

The production of the solar cells is based on the substrate (preferably glass) on which the following layers are deposited successively.

In the manufacture of CdTe solar cells, the thickness of the CdS layer normally ranges from 4 to 5 μm. However, theoretical simulations of CdTe solar cells have shown that solar cells with a 1 μm thick CdTe layer could also have a fairly high efficiency. In principle, reducing the CdTe layer thickness from 4 μm to 2 μm could reduce the material consumption of CdTe by 30 to 40% during module production. Reducing the CdTe layer thickness would also reduce deposition time and thereby accelerate module fabrication.

Particularly high efficiency solar cells are usually made by depositing CdTe at substrate temperatures> 500 ° C. The CdTe layer has coarse grains at this temperature, which could lead to the formation of pinholes. Therefore, reducing the layer thickness in several ways may adversely affect the solar cell efficiency as well as its long-term stability. When the layer thickness is reduced to less than 3 .mu.m, shortcomings in the CdTe layer (pinholes or pinholes) form in the solar cells. This problem is more pronounced when the solar cell manufacturing involves etching, resulting in poor performance of the solar cells. In addition, the reduction of the short circuit resistance leads to a low fill factor and finally to reduced power. Therefore, it is necessary to minimize the formation of pinholes in the CdTe layer in order to obtain solar cells with high efficiency.

Moreover, increasing the p-type doping of the CdTe layer is also important in order to obtain solar cells with high efficiency. A further increase in performance of the CdTe solar cells can be achieved by doping the CdTe layer. According to the theoretical models, a massive p-doping of the CdTe is limited by the occurrence of a so-called self-compensation effect. Only a degree of p-type doping can be achieved by the use of a suitable doping element and a method that provides such a doping element after the deposition of the CdTe layer of that layer. In the production of the CdTe solar cell, the extrinsic p-type doping of the CdTe layer is usually carried out after the activation process and comprises a post-annealing treatment which causes the doping elements to diffuse. The well-known and problem-free dopant for the CdTe layer is Cu.

The object of the present invention is to achieve a solar cell which has a doped CdTe layer with reduced thickness and without pinholes. Moreover, it is an object of the present invention to simplify the manufacturing process of CdTe solar cells.

According to the invention, the manufacturing process of a CdTe solar cell comprises a step in which a first coarse-grained CdTe layer is formed on a base layer, a step of forming a sacrificial layer containing a dopant on the first CdTe layer, and a step Step for forming a second, fine-grained, CdTe layer on this sacrificial layer.

The preferred material of the sacrificial layer is selected from a group of materials comprising copper, phosphorus, antimony, bismuth, molybdenum or manganese as doping element. According to one Embodiment, the doping element is made available as an elementary layer. In another embodiment, the doping element is made available in a combination of various elements, for example, copper and antimony or antimony and bismuth, or in a composition, which is preferably a compound of any of said doping elements with a halogen, e.g. B. SbCl ₃ . The preferred halogen for the composition of the sacrificial layer is fluorine (F), most preferably chlorine (Cl). The preferred compounds used are chlorides.

• – Sputtern,
• – elektrochemische (galvanische) Abscheidung
• – Aufsprühen einer Lösung einer halogenhaltigen Verbindung, wobei die Verbindung in Wasser oder in einem anderen bekannten Lösemittel aufgelöst wird,
• – Spin-Coating (d. h. Aufschleudern einer Lösung auf die Wafer)
• – Eintauchen des Substrats (oder die Oberfläche der ersten CdTe-Schicht) in eine Lösung, die das Dotierelement oder eine Verbindung desselben enthält,
• – Sponge-Coating (mit Schwammgummi) usw.

The sacrificial layer can be applied by methods known in the art. Preference is given to using physical or dry chemical processes, or else wet chemical processes such as (but not limited to) the following:

• - sputtering,
• - Electrochemical (galvanic) deposition
• Spraying a solution of a halogen-containing compound, the compound being dissolved in water or in another known solvent,
• Spin coating (ie spinning a solution onto the wafers)
• Immersing the substrate (or the surface of the first CdTe layer) in a solution containing the doping element or a compound thereof,
• - Sponge coating (with sponge rubber) etc.

Halogen-containing compounds are preferably deposited by wet chemical methods; especially preferred by sponge coating with sponge rubber.

The thickness of the sacrificial layer depends on the size ratios of the CdTe layer resulting from the fusion of the first and second CdTe layers as well as the doping element used. With regard to the thickness of the CdTe layer, the thickness of the sacrificial layer is selected such that a predetermined doping level of the CdTe layer is achieved as soon as the sacrificial layer has completely degraded. If the sacrificial layer is elemental antimony, the thickness of the sacrificial layer is preferably approx.

one-thousandth of the thickness of the CdTe layer. Some examples are given in Table 1, where also the respective approximate strengths of the first and second CdTe layers are given. Thickness of the first CdTe layer (nm) Thickness of second CdTe layer (nm) Total thickness of the CdTe layer (nm) Thickness of the doping layer at Sb (nm) 4000 1000 5000 5 2400 600 3000 3 1600 400 2000 1-2 500 500 1000 0.5-1 Table 1

However, it is also possible to use other doping elements or compounds containing the doping element. In general, the thickness of the sacrificial layer preferably ranges from 2 nm to 15 nm. If copper is used as a doping element, the thickness of the sacrificial layer should be reduced with respect to the fact that, when other doping elements are used, a high copper content will over time lead to quality defects Solar cells would lead. Preferably, the thickness of the sacrificial layer comprising copper as a doping element should be 30% less than the thickness of a sacrificial layer comprising another doping element, such as antimony.

The sacrificial layer is preferably deposited on the first CdTe layer at a substrate temperature in the range of room temperature to 350 ° C. deposited. The substrate temperature should not exceed 350 ° C because higher substrate temperatures would make it difficult to apply a sacrificial layer of the mentioned low strength due to problematic re-evaporation. In the case that halogen-containing compounds are used for applying the sacrificial layer, the substrate temperature preferably moves in the range of room temperature to 100 ° C.

The first CdTe layer, which is a layer of coarse-grained structure, is deposited on a basecoat. The first CdTe layer has grains on the order of microns on, for example in the range of 2 microns to 5 microns. This is achieved by depositing the first CdTe layer at a substrate temperature in the range of 490 ° C to 540 ° C, the thickness of the first CdTe layer being between 0.5 μm and 6 μm, more preferably between 1 μm and 1.8 μm, lies. The base layer is a layer stack comprising a translucent substrate, a translucent front contact layer, and a CdS layer if the solar cell is manufactured in a superstrate configuration. The base layer is a layer stack comprising a substrate and a back contact layer if the solar cell is manufactured in a substrate configuration. Further details of these configurations are described below.

The thickness of the second CdTe layer is preferably between 1% and 100% of the total thickness of the first CdTe layer, depending on the required total thickness of the CdTe layer. Particularly preferably, the thickness of the second CdTe layer is between 20% and 30% of the total thickness of the CdTe layer. The total thickness of the CdTe layer can range from 0.5 μm to 8 μm. Only with a total of very thin total CdTe layers having a layer thickness of 0.5 μm to 1.5 μm, the thickness of the second CdTe layer can be about 40-50% of the total CdTe layer thickness to the pinholes and / or grain boundaries of the first To fill out the CdTe layer. The percent strength of the second layer is only an example. According to the invention, the method is suitable for layer thicknesses of the second layer in each of said thickness ranges.

The second CdTe layer is deposited as a layer of fine-grained structure and serves to fill in the pinholes and / or grain boundaries of the first CdTe layer. The size of the grains of the second CdTe layer is in the range of nanometers, for example from 100 nm to 500 nm. Accordingly, both the occurrence of shorts between back and front contact of the solar cell and the propagation of impurities at the grain boundaries within the CdTe Layer can be reduced or completely avoided. The deposition of a fine grained layer is achieved by depositing the second CdTe layer at a substrate temperature in the range of 200 ° C to 350 ° C.

Deposition of the first and second CdTe layers may be by any known method including, but not limited to, close-space (d) sublimation (CSS), chemical bath (CBD) deposition, sputtering, electrochemical deposition, or any other physical or chemical processes.

According to one embodiment of the method for solar cell production, the method further comprises a step of heat treatment, which is carried out after the deposition of the second CdTe layer. That is, the step of heat treatment may be performed immediately after the step of depositing the second CdTe layer, or it may be carried out at a later step, for example, after the application of a cap layer, which may be a sacrificial cap layer.

The step of heat treatment includes heating the substrate to a temperature in the range of 300 ° C to 550 ° C. Most preferably, the substrate temperature during the heat treatment should not exceed 450 ° C if the second CdTe layer is exposed - d. H. if it is not covered by another layer - to prevent re-evaporation of CdTe.

Preferably, during the step of heat treatment, a halogen-containing substance is provided on the surface of the second CdTe layer. This process step corresponds to the so-called activation step, which is known from the prior art in the production of CdTe solar cells. Typically, CdCl _{2 is} used as the halogen-containing material in this heat-treatment step, where the CdCl ₂ is applied to the CdTe layer by wet-chemical or vacuum evaporation followed by annealing in air at a predetermined temperature (typically in the range of 380 ° C ₎ ° C to 440 ° C). Advantages of this activation step include reducing lattice mismatch between the CdS / CdTe layers and passivating grain boundaries in the CdTe layer. The CdCl ₂ -activation interdiffusion between the CdS and CdTe layers contributes to the smooth transition of the electron bands at the CdS and CdTe transition point. However, a disadvantage of this approach is that CdCl ₂ a is a potentially hazardous substance and therefore handling it is more difficult in the manufacturing process.

If the sacrificial layer contains a halogen, the use of CdCl _{2 can} be avoided since the halogen component contained in the sacrificial layer contributes to the passivation of the grain boundaries in the CdTe layer. Therefore, the heat treatment step in the present invention is preferably carried out without providing a halogen-containing substance on the surface of the second CdTe layer, because the invention process mimics the CdCl ₂ activation process under the conditions mentioned above.

The heat energy present during the heat treatment step leads to a dissolution of the sacrificial layer into its components and / or to the diffusion of its constituents, in particular of the doping element, into and / or through the CdTe layer. In this way, the sacrificial layer is degraded, which characterizes the doping layer as a sacrificial layer. As a result, the first and second CdTe layers now adjoin one another and form the CdTe layer of the solar cell.

However, the manufacturing process of solar cells may involve different process steps involving higher temperatures, such as the deposition of CdTe layers. Therefore, the dissolution of the sacrificial layer and the diffusion of its constituents may also occur, at least in part, during the deposition of the sacrificial layer, during the deposition of the second CdTe layer, and / or in other process steps that occur after the second CdTe layer has been deposited. such as in a process step for applying a contact layer. Therefore, the above-mentioned heat treatment step can be maintained if a halogen-containing sacrificial layer is used, and if the process steps following the sacrificial layer deposition step have sufficient heat balance / heat quantity to dissolve the sacrificial layer and diffuse the doping element bring.

Because dopant diffusion diffuses from the "inside" of the CdTe layer, the CdTe layer is more uniformly doped than if a dopant layer were deposited on top of a deposited, complete CdTe layer, as is currently known in the art. At least a nearly uniform doping of the CdTe layer is achieved by using a lower amount of heat for the solar cell in the manufacturing process compared to other prior art processes. "Almost uniform doping" means that no or only a small concentration gradient of the doping element can be measured within the CdTe layer.

In another embodiment of the invention, depending on the substance selected for the doping layer, the excess doping element, i. H. those atoms of the dopant that can not be incorporated into the CdTe crystals accumulate on the surface of the second CdTe layer. This is due to the preferential diffusion of impurities, supported by the grain boundaries, especially due to the second finer grain CdTe layer. The excess dopants may be rinsed off with suitable solvents, or removed in a subsequent nitric and phosphoric acid etching process.

The solar cell fabrication process of the present invention may be used to fabricate solar cells in a superstrate configuration or in a substrate configuration.

The process for producing solar cells in a superstrate configuration also comprises providing a transparent substrate, preferably glass, applying a transparent front contact layer or a layer stack, for example made of TCO, and applying a CdS layer on the transparent front contact layer or the layer stack. After applying the CdS layer, the invented method described above is carried out, wherein the layer stack comprising the transparent substrate, the transparent front contact layer and the CdS layer serves as a base layer for the application of the first CdTe layer. That is, the first CdTe layer, the sacrificial layer, and the second CdTe layer are applied to the CdS layer in this order. Furthermore, the described heat treatment process, for example a CdCl ₂ activation process and nitric and phosphoric acid etching process, can be carried out before the back contact layer or layer stack is applied. The CdTe surface is treated with a suitable solution, such. As water or methanol, purified. According to the prior art, the back contact layer may comprise a metal, another suitable conductor material or a suitable semiconductor layer (such as, for example, Sb ₃ Te ₂ ).

In the method of producing solar cells in a substrate configuration, the further steps after the steps of applying the first coarse-grained CdTe layer, depositing the sacrificial layer, and applying the second fine-grained CdTe layer basically proceed in reverse order. The substrate may either be a flexible metal foil of, for example, molybdenum, which may serve as a back contact to collect the photo-induced electrical charges, or any other suitable substrate known in the art. In this way, firstly a substrate is provided on which the back contact layer or the layer stack is applied; this is followed by the application of the first CdTe layer, the sacrificial layer and the second CdTe layer. That is, the layer stack comprising the substrate and the back contact layer serves as a base layer for applying the first CdTe layer. Subsequently, the CdS layer and the transparent front contact layer, as TCO, for example, wherein a heat treatment process as described above, after depositing the second CdTe layer or even after application of the CdS layer and / or the transparent front contact layer can be performed. Optionally, and depending on the CdS and TCO deposition methods used, diffusion of the dopant may also take place during the CdS and / or TCO deposition process. If a CdCl ₂ activation process is included, the dopant may also diffuse during the activation process. Under such conditions, additional annealing to diffuse the dopant may not be required.

The process steps for applying a (translucent) substrate, application of a front contact layer, application of a CdS layer and application of a back contact layer are carried out according to common methods of the prior art and therefore not described in detail here. It should be noted that in the process of fabricating solar cells in substrate configuration, the step of depositing a CdS layer should be performed at relatively low substrate temperatures in the range of 200 ° C to 350 ° C to avoid re-evaporation of the CdTe layer. This can be achieved by using a common sputtering process for depositing the CdS layer.

pictures

schematically shows the layer structure of a solar cell according to the prior art. This solar cell comprises on the substrate ( 1 ) a layer sequence consisting of front contact ( 21 ), CdS layer ( 3 ), CdTe layer ( 4 ) as well as back contact ( 22 ).

to schematically show the layer sequences, as they are observed in the course of the process according to the invention.

embodiment

The method according to the invention is explained below in a first embodiment, which shows the production of a solar cell in superstrate configuration, without wishing to restrict it to this embodiment.

As in to see are the front contact ( 21 ) and the CdS layer ( 3 ) by means of prior art methods on the translucent substrate ( 1 ) has been applied. As a front contact ( 21 A 450 nm thick transparent bilayer [with fluorinated tin oxide (350 nm) as a conductor layer and tin oxide (100 nm) as a high-resistance buffer] (as TCO) was applied. The CdS layer ( 3 ) reaches a thickness of 90 nm and was deposited by CSS technique. In addition, the first CdTe layer according to the invention ( 41 ) having a thickness of 1.6 microns. The deposition was carried out in the CSS process at a substrate temperature of 530 ° C, resulting in a coarse-grained structure of the deposited layer.

schematically shows the applied sacrificial layer ( 5 ) above the first CdTe layer ( 41 ). The sacrificial layer ( 5 ) is made of elemental antimony (Sb) and was deposited to a thickness of 2 nm in a sputtering process at a substrate temperature of 280 ° C.

schematically shows the layer stack of the solar cell after deposition of the second CdTe layer ( 42 ) on the sacrificial layer ( 5 ). The second CdTe layer ( 42 ) was deposited at a thickness of 400 nm in the CSS method at a substrate temperature of 300 ° C. The second CdTe layer ( 42 ) has fine grains covering the grain boundaries of the first CdTe layer ( 41 ) cover. The sacrificial layer ( 5 ) covers the grain boundaries of the first CdTe layer ( 41 ) not completely, due to the very low layer thickness of the sacrificial layer ( 5 ). However, it is evenly distributed over the surface of the first CdTe layer. This ensures a uniform doping of the resulting CdTe layer.

In addition, the sacrificial layer ( 5 ) during the deposition of the second CdTe layer ( 42 ), with the antimony in the first CdTe layer ( 41 ) and in the first partially applied second CdTe layer ( 42 ) immigrates. However, since the antimony in this process step is not extensively incorporated into the first CdTe layer ( 41 ) into the second CdTe layer ( 42 ) diffuses, the already diffused Antimonatome and the reduced layer thickness of the sacrificial layer ( 5 ) in Not shown.

Subsequently, the known CdCl ₂ activation step is carried out at a temperature of 385 ° C for 20 minutes.

schematically shows a solar cell after completion of the application of the back contact. A back contact ( 22 ), which contains a metal, here molybdenum (Mo), has been formed with a layer sequence which corresponds to that known from the prior art. As shown, the sacrificial layer ( 5 ) is completely degraded and is in the CdTe layer ( 40 ) resulting from the combination of the first CdTe layer ( 41 ) with the second CdTe layer ( 42 ), wherein the newly formed CdTe layer ( 40 ) is doped with antimony [represented by the dots in the CdTe layer ( 40 )]. Diffusing the doping element into the first and second CdTe layers ( 41 . 42 ) as well as the complete degradation of the sacrificial layer ( 5 ) can at any time during the CdCl ₂ activation step and / or the formation of the back contact ( 22 ), resulting in the illustrated layer arrangement.

The CdTe layer ( 40 ) is almost uniformly doped; this means that no or only a slight concentration gradient of the antimony in the CdTe layer ( 40 ) is observable.

LIST OF REFERENCE NUMBERS

1

Substrate (glass)

21

Front contact (translucent, TCO)

22

Back contact (metal)

3

CdS layer

4

CdTe layer (prior art)

40

CdTe layer

41

first CdTe layer

42

second CdTe layer

5

sacrificial layer

Claims (14)

Process for producing a solar cell, comprising the steps: a) application of a first CdTe layer of coarse-grained structure on a base layer, b) applying a sacrificial layer having a doping element on the first CdTe layer, and c) applying a second CdTe layer of fine-grained structure on the sacrificial layer. A method according to claim 1, characterized in that the sacrificial layer comprises an element selected from the group consisting of copper, phosphorus, antimony, bismuth, molybdenum, or manganese as doping element. A method according to claim 1 or 2, characterized in that the doping element is provided as an elementary layer. A method according to claim 1 or 2, characterized in that the doping element is provided in a combination of different doping elements or in a compound. A method according to claim 4, characterized in that the compound comprises a halogen. Method according to one of the preceding claims, characterized in that the sacrificial layer is applied by sputtering or by a method in which a liquid solution is used which contains the doping element. Method according to one of the preceding claims, characterized in that the sacrificial layer is applied with a thickness in the range of 2 nm to 15 nm. Method according to one of the preceding claims, characterized in that the sacrificial layer is applied at a substrate temperature ranging from room temperature to 350 ° C. Method according to one of the preceding claims, characterized in that the first CdTe layer at a substrate temperature in the range of 490 ° C to 540 ° C and with a thickness in the range of 0.5 microns to 6 microns, preferably in the range of 1 micron to 1.8 microns, is deposited. Method according to one of the preceding claims, characterized in that the second CdTe layer at a substrate temperature in the range of 200 ° C to 350 ° C and with a thickness of 20% to 40% of the total layer thickness of a CdTe layer consisting of the first CdTe layer and the second CdTe layer is deposited. Method according to one of the preceding claims, characterized in that the method further comprises a step of heat treatment after step c) at a temperature in the range of 300 ° C to 550 ° C, preferably in the range of 300 ° C to 450 ° C. , is carried out. A method according to claim 11, characterized in that during the step of heat treatment, a halogen-containing material is provided on the surface of the second CdTe layer. Method according to one of the preceding claims, characterized in that the method further comprises the following steps: d) providing a translucent substrate, e) applying a transparent front contact layer, f) applying a CdS layer, and g) applying a back contact layer, wherein the steps d), e) and f) are carried out in this order and take place before steps a), b) and c), and step g) is carried out after steps a), b) and c), and wherein the layer stack comprising the translucent substrate, the transparent front contact layer, and the CdS layer forming the base layer. Method according to one of the preceding claims, characterized in that the method further comprises the following steps: h) providing a substrate, i) applying a back contact layer j) applying a CdS layer, and k) applying a transparent front contact layer, wherein steps h) and i) are carried out in this order and take place before the steps a), b) and c), and the steps j) and k) are carried out after the steps a), b) and c), and wherein the layer stack, which comprising the substrate and the back contact forming the base layer.

Images