A METHOD FOR FORMING A GRADIENT THIN FILM BY SPRAY PYROLYSIS

Abstract

The present invention proposes a method to form a gradient thin film using a spray pyrolysis technique. The method comprises providing a base substrate, preparing a spray aqueous solution by mixing at least two precursor compounds comprising at least two different elements and spraying the spray aqueous solution onto the base substrate. According to the present invention, the ratio of the concentration of the at least two different elements within the spray aqueous solution is varied while performing the method. In this way, a thin film having a gradient of elemental composition over its layer thickness may be formed.

Claims

1. A method for forming a front contact in a CdTe solar cell including a ZTO thin film with a concentration gradient and a transparent conducting layer, wherein the ZTO thin film is arranged between the transparent conducting layer and a CdS layer, the method comprising the steps: a) providing a base substrate, which is either the CdS layer or the transparent conducting layer, b) preparing a spray aqueous solution by mixing at least two precursor compounds comprising a Zn containing compound and a Sn containing compound, c) spraying the spray aqueous solution onto the base substrate thereby forming the ZTO thin film, and d) forming the transparent conducting layer or the CdS layer, respectively, on the ZTO thin film, characterized in that the ratio of the concentration of Zn and Sn within the spray aqueous solution is varied while performing the steps b) and c) such, that the zinc concentration in the resulting ZTO thin film decreases over the whole ZTO thin film from the interface of the ZTO thin film to the CdS layer in the direction to the interface of the ZTO thin film to the transparent conducting layer.

2. The method according to claim 1, characterized in that the ratio of the concentration of Zn and Sn within the spray aqueous solution is set to be in a range from 1 to 50.

3. The method according to claim 1, characterized in that one of the at least two precursor compounds is added continuously to the spray aqueous solution while performing the method.

4. The method according to claim 1, characterized in that step b) comprises at least two substeps b1) and b2), wherein a specific spray aqueous solution having a specific ratio of the concentration of Zn and Sn is prepared by mixing the at least two precursor compounds in each of the at least two substeps b1) and b2), and step c) comprises at least two substeps c1) and c2) each of them corresponding to one of the at least two substeps b1) and b2), wherein, in each of the at least two substeps c1) and c2), the specific spray aqueous solution prepared in the specific corresponding substep of step b) is sprayed onto the base substrate or onto a partial ZTO thin film formed in the previous substep of step c), respectively.

5. The method according to claim 4, characterized in that, in each of the at least two substeps c1) and c2), a specific partial ZTO thin film is formed with a thickness in a range of 10 nm to 50 nm, preferably in a range from 20 nm to 40 nm.

6. The method according to claim 1, characterized in that the base substrate is heated to a temperature in a range from 300° C. to 500° C., preferably in a range from 450° C. to 500° C., before spraying the spray aqueous solution onto it.

7. The method according to claim 1, characterized in that the ZTO thin film with a concentration gradient is formed with a thickness in a range from 20 nm to 1000 nm.

8. The method according to claim 1, characterized in that the ZTO thin film is formed with a thickness in a range from 20 nm to 150 nm, preferably in a range from 80 nm to 100 nm.

9. The method according to claim 1, characterized in that the Zn containing compound is Zn(CH3COO).sub.2.Math.H.sub.2O and in that the Sn containing compound is SnCl.sub.2H.sub.2O.

10. The method according to claim 1, characterized in that the transparent conducting oxide layer is a CTO layer with a concentration gradient or SnO.sub.2:F.

11. The method according to claim 10, characterized in that the transparent conducting layer is a CTO layer with a concentration gradient and providing the transparent conducting layer in step a) or forming the transparent conducting layer in step d), respectively, comprises 30 preparing a spray aqueous solution by mixing at least two precursor compounds comprise a Cd containing compound and a Sn containing compound and spraying the spray aqueous solution onto a substrate or the ZTO thin film, respectively, wherein the ratio of the concentration of Cd and Sn within the spray aqueous solution is varied.

12. The method according to claim 11, characterized in that the Cd containing compound is Cd(CH.sub.3COO).sub.2.Math.H.sub.2O and in that the Sn containing compound is SnCl.sub.2.Math.H.sub.2O.

13. The method according to claim 11, wherein the ratio of the concentration of Cd and Sn within the spray aqueous solution is varied during performing steps a) or d), respectively such that the cadmium concentration in the resulting CTO layer decreases over the whole CTO layer in a direction away from the interface of the CTO layer to the ZTO thin film.

14. The method according to anyone of claims 11, characterized in that the CTO layer is formed with a thickness in a range from 100 nm to 1000 nm, preferably in a range from 300 nm to 500 nm.

15. The method according to claim 1, characterized in that hydrochloric acid is added to the spray aqueous solution in a volume percentage in a range from 0.5% to 0% of the whole volume of the prepared spray aqueous solution.

16. The method according to claim 2, characterized in that one of the at least two precursor compounds is added continuously to the spray aqueous solution while performing the method.

17. The method according to claim 2, characterized in that step b) comprises at least two substeps b1) and b2), wherein a specific spray aqueous solution having a specific ratio of the concentration of Zn and Sn is prepared by mixing the at least two precursor compounds in each of the at least two substeps b1) and b2), and step c) comprises at least two substeps c1) and c2) each of them corresponding to one of the at least two substeps b1) and b2), wherein, in each of the at least two substeps c1) and c2), the specific spray aqueous solution prepared in the specific corresponding substep of step b) is sprayed onto the base substrate or onto a partial ZTO thin film formed in the previous substep of step c), respectively.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 schematically shows the layer structure of a solar cell according to the state of the art. Said solar cell comprises on a transparent substrate (1) a layer sequence consisting of front contact (2), CdS layer (3), CdTe layer (4) and back contact (5). The front contact (2), also called transparent conducting oxide layer (TCO), comprises a transparent conducting layer (21), made for instance of CTO, and a high resistive buffer layer (22), made for instance of ZTO.

[0039] FIG. 2A and 2B schematically show the process steps of the inventive method for forming a gradient thin film, wherein FIG. 2A shows the process steps for a continuous change of the elemental concentration and FIG. 2B shows the process steps for a gradual change of the elemental concentration.

[0040] FIG. 3A to 3D schematically shows the layer sequences, as they may be observed during the course of the method as described with respect to FIG. 2B, wherein the ZTO layer (22) is formed with a gradient of the zinc concentration and comprises a plurality of partial ZTO layers (221 to 223).

[0041] FIG. 4 schematically shows an apparatus for performing the spray pyrolysis according to the inventive method.

DETAILED DESCRIPTION

[0042] The method according to the invention is explained with respect to the following figures by way of example. The shown and described details are only for the purpose of better understanding and are in no way limiting with respect to the scope of the claims.

[0043] An embodiment of the inventive method comprises steps S21 to S23 as shown in FIG. 2A. In step S21, a base substrate is provided. This base substrate is for instance the transparent conducting layer (21) arranged on the transparent substrate (1). In this exemplary embodiment the base substrate is commercial SnO.sub.2:F. However, it can be also considered that instead of FTO, CTO can also be deposited by spray pyrolysis with variation in elemental concentration during the coating based on the similar procedure as described for the ZTO. The base substrate is heated to a temperature of 480° C. to enable a pyrolysis of the precursor compounds. In step S22, a spray aqueous solution is prepared by mixing at least two precursor compounds, for instance Zn(CH.sub.3COO).sub.2.Math.H.sub.2O and SnCl.sub.2.Math.H.sub.2O, comprising at least two different elements, being Zn and Sn in the described example. Furthermore, 1 ml of hydrochloric acid is added to 100 ml of the spray aqueos solution. In step S23, the spray aqueous solution is sprayed onto the base substrate, where the pyrolysis of the Zn(CH.sub.3COO).sub.2 and a reaction take place resulting in the formation of a ZTO layer. Step S23 actually consists of a sequence of spraying the spray aqueous solution on the base substrate for around 2 s and of waiting for 30 s for performing pyrolysis and forming a stable ZTO film, whereas this sequence is repeated several times. The time duration of the whole method is calculated based on the growth rate and the desired film thickness. As an example, an about 100 nm ZTO layer can be made in about 30 min of spray coating, based on the spraying conditions mentioned above.

[0044] During step S22, one of the at least two precursor compounds is continuously added to the spray aqueous solution while performing the method. That is, the spray aqueous solution is permanently mixed in a reservoir, from which the spray aqueous solution is permanently taken and sprayed onto the base substrate in step S23. In the beginning, the spray aqueous solution has a first ratio for the Zn/Sn concentration, e.g. of 1:1. By adding Zn(CH.sub.3COO).sub.2.Math.H.sub.2O continuously to the spray aqueous solution, the ratio continuously increases, for instance to a value of 6:1 at the end of the method. By this method, the formed ZTO film has a gradient of the concentration of Zn over its thickness, wherein the concentration changes continuously.

[0045] FIG. 2B shows another embodiment of the inventive method. The step S21 is the same as in the embodiment described in FIG. 2A. However, steps S22 and S23 are each divided into at least two substeps, for instance into three substeps, i.e. 5221, 5222 and 5223 and 5231, 5232 and 5233. The numbers of substeps of step S22 and of step S23 equal each other. In each substep 5221, S222 and S223, a spray aqueous solution having a constant ratio of the concentration of the two elements, for instance Zn and Sn, is prepared by mixing the at least two precursor compounds, for instance Zn(CH.sub.3COO).sub.2.Math.H.sub.2O and SnCl.sub.2.Math.H.sub.2O, wherein the concentration ratio of the spray aqueous solution prepared in substep S221 is smaller than that of the spray aqueous solution prepared in substep S222, which is smaller than that of the spray aqueous solution prepared in substep S223. For instance, the first ratio is 1:1, the second ratio is 2:1 and the third ratio is 4:1. In each substep S231, S232 and S233, the respective spray aqueous solution prepared in the corresponding substep S221, S222 or S223, respectively, is sprayed onto the base substrate or onto the partial thin film formed in the substep S231 or S232, respectively. Each substep S231, S232 and S233 is a sequence of spraying and waiting as described with respect to step S23 in FIG. 2A. By this method, the formed ZTO film comprises three partial ZTO films each having a different Zn concentration. That is, the ZTO film as a whole has a gradient of the concentration of Zn over its thickness, wherein the concentration changes gradually. The number of substeps of steps S22 and S23 and thereby the number of partial layers is not limited to three, as described with respect to FIG. 2B, but may be two or more than three.

[0046] At least substep S221 may be performed in any order with respect to step S21. Furthermore, some of or all of the substeps S221, S222 and S223 may be performed in any order with respect to each other, for instance in parallel to each other, in case that different reservoirs are used for mixing the different spray aqueous solutions. However, it is also possible to use only one reservoir. In this case, substep S222 has to be performed after completing the substep S231, and substep S223 has to be performed after completing the substep S232.

[0047] As described above, the process may be further continued in order to have multiple layers with different layer concentration. Furthermore, both methods according to the embodiments of FIG. 2A and of FIG. 2B may be followed by similar process steps using different kinds of precursor materials thereby forming different kinds of gradient films above each other. For instance a gradient CTO film may be formed followed by a gradient ZTO film which is followed by a gradient CdS film. For each method for forming a gradient film, the appropriate method according to the embodiment of FIG. 2A or of FIG. 2B may be used.

[0048] With respect to FIGS. 3A to 3D, the making of a ZTO buffer layer for a solar cell in superstrate configuration according to the method of FIG. 2B is described, without intending to imply a restriction to said embodiment. In particular, the method of forming a ZTO film is not limited to the use in processes for manufacturing a solar cell, but can also be used in processes for manufacturing any other device, e.g. a light emitting device or a sensor.

[0049] As shown in FIG. 3A, the transparent conducting layer (21) has already been applied on the transparent substrate (1) by means of methods according to the state of the art. The transparent conducting layer (21) is, for instance, a Fluorine doped tin oxide having a thickness d.sub.21 of 350 nm.

[0050] FIG. 3B schematically shows an applied first partial ZTO layer (221) above the transparent conducting layer (21). The first partial ZTO layer (221) has a ratio of 1:1 of the Zn/Sn concentration and was deposited with a thickness d.sub.221 of 40 nm during substep S231 of the method described in FIG. 2B using a first spray aqueous solution with a corresponding first ratio of the Zn/Sn concentration.

[0051] FIG. 3C schematically shows the layer stack after depositing a second partial ZTO layer (222) on the applied first partial ZTO layer (221). The second partial ZTO layer (222) has a ratio of 2:1 of the Zn/Sn concentration and was deposited with a thickness d.sub.222 of 20 nm during substep S232 of the method described in FIG. 2B using a second spray aqueous solution with a corresponding second ratio of the Zn/Sn concentration.

[0052] FIG. 3D schematically shows the layer stack after depositing a third partial ZTO layer (223) on the applied second partial ZTO layer (222). The third partial ZTO layer (223) has a ratio of 4:1 of the Zn/Sn concentration and was deposited with a thickness d.sub.223 of 20 nm during substep S233 of the method described in FIG. 2B using a third spray aqueous solution with a corresponding third ratio of the Zn/Sn concentration.

[0053] The first, second and third partial ZTO layers (221, 222, 223) together form the ZTO layer (22) similar to that shown in FIG. 1. However, the ZTO layer (22) has a gradient of the zinc concentration in contrast to the state of the art ZTO layer. The zinc concentration gradually increases from the first partial ZTO layer (221) to the third partial ZTO layer (223). The total thikness of the ZTO layer is about 100 nm.

[0054] As mentioned above, the number of partial layers is not limited to that of the described example. Furthermore, also the thicknesses and/or the ratios of the Zn/Sn concentration of the specific partial ZTO layers (221, 222, 223) may be changed and adapted to the desired conditions. For instance, a further process step for depositing a fourth partial ZTO layer on the applied third partial ZTO layer may be performed, wherein the fourth partial ZTO layer has a ratio of 6:1 of the Zn/Sn concentration and a thickness of 20 nm.

[0055] Subsequently, known process steps for forming the CdS layer (3), the CdTe layer (4) and the back contact layer (5) as shown in FIG. 1 are performed. During and/or after these process steps, zinc atoms or zinc ions diffuse from the third partial ZTO layer (223) into the CdS layer (3).

[0056] Furthermore, zinc atoms or zinc ions diffuse within and between the first, second and third partial ZTO layers (221, 222, 223), thereby balancing the zinc concentration within the ZTO layer (22). Thus, the originally generated gradient in zinc concentration essentially vanishes over the time.

[0057] FIG. 4 shows schematically an exemplary apparatus for performing the inventive method. A substrate 10 is arranged on a substrate holder 20. The substrate 10 comprises the base substrate described with respect to FIGS. 2A and 2B, which is arranged on that side of the substrate 10 not being adjacent to the substrate holder 20. The substrate holder 20 comprises a heating element 20a for bringing the substrate 10 to the process temperature mentioned with respect to FIG. 2. In other embodiments, the apparatus for performing the inventive method may comprise other means for heating the substrate 10, wherein these means may not be comprised by the substrate holder. Further, the apparatus for performing the inventive method includes a spray nozzle arrangement 30 comprising a plurality of spray nozzles 30a. In other embodiments, the spray nozzle arrangement 30 may comprise only one nozzle 30a. Through the spray nozzles 30a, the spray aqueous solution, which is provided to the spray nozzle arrangement 30 through the solution inlet 40, is sprayed onto the base substrate on top of the substrate 10 (indicated by arrows in FIG. 4) using a carrier gas, e.g. air, which is provided to the spray nozzle arrangement 30 through the air inlet 50.

REFERENCE NUMERALS

[0058] 1 transparent substrate (glass)
2 front contact
21 transparent conducting layer
22 high resistive buffer layer (ZTO layer)
221-223 partial ZTO layer
3 CdS layer
4 CdTe layer
5 back contact (metal)
10 substrate with base substrate as the top layer
20 substrate holder
20a heating element
30 spray nozzle arrangement
30a spray nozzle
40 solution inlet
50 air inlet