Method for producing a CdTe thin-film solar cell

Abstract

The present invention describes a method for producing CdTe thin-film solar cells, in which special parameters of different processing steps and a special sequence of processing steps result in improved characteristics of the produced CdTe solar cells.

Claims

1. A method for producing a CdTe thin film solar cell, the method comprising the steps: providing a transparent substrate with a transparent conducting layer as a front contact on it, applying a CdS layer on the transparent conducting layer using closed space sublimation technique, wherein the CdS layer is applied with a thickness in the range between 20 nm to 40 nm, applying a CdTe layer on the CdS layer using closed space sublimation technique, wherein the CdTe layer is applied with a thickness in the range between 3 μm and 5 μm, applying a crystalline layer of CdCl.sub.2 with a thickness in the range of 50 nm to 100 nm on a first surface of the CdTe layer, performing a first temperature treatment step after applying the CdCl.sub.2 layer at a temperature in the range of 380° C. to 430° C. for a time in the range of 15 minutes to 45 minutes under atmospheric conditions, performing a first cleaning step after the first temperature treatment step, wherein the layer stack resulting from the previous processing steps is dipped into a solution of diammonium hydrogen citrate with a concentration in the range of 0.1% to 50% for a time in the range of 15 seconds to 5 minutes, applying a back contact layer on the first surface after the first cleaning step, wherein the back contact layer is applied with a thickness in the range of 200 nm to 400 nm and comprises molybdenum, providing copper ions to a first surface of the back contact layer by dipping a layer stack resulting from the previous process steps into a CuCl.sub.2 solution with a concentration in the range of 0.05 mmol/l to 1 mmol/l for a time in the range of 30 seconds to 2 minutes, performing a second temperature treatment step after removing the layer stack from the CuCl.sub.2 solution at a temperature in the range of 180° C. to 250° C. for a time period with a duration in the range of 10 minutes to 45 minutes under atmospheric conditions, performing an artificial aging step after performing the second cleaning step, wherein the artificial aging step includes an illumination of the layer stack resulting from the previous process steps for a time in the range of 1 minute to 48 hours with an illuminance in the range of 5000 lx to 200000 lx at a temperature in the range of 70° C. to 80° C. under atmospheric conditions, and performing a second cleaning step after performing the artificial aging step, wherein the layer stack resulting from the previous processing steps is cleaned by dipping the layer stack into a dimethylformamide solution with a concentration in the range of 50% to 100% for a time in the range of 1 minute to 10 minutes and subsequently rinsed with water and isopropanol.

2. The method according to claim 1, further comprising performing a further cleaning step after the second temperature treatment step and before the artificial aging step, wherein the layer stack resulting from the previous processing steps is cleaned by dipping the layer stack into a solution of dimethylformamide with a concentration in the range of 50% to 100% for a time in the range of 1 minute to 10 minutes.

3. The method according to claim 1, characterized in that the layer stack is dipped into a 100%-solution of dimethylformamide for 5 minutes in the second cleaning step or in the further cleaning step.

4. The method according to claim 1, characterized in that the CdS layer is applied with a thickness of 30 nm.

5. The method according to claim 1, characterized in that the CdTe layer is applied with a thickness of 4 μm.

6. The method according to claim 1, characterized in that the crystalline CdCl.sub.2 layer is applied with a thickness of 80 nm.

7. The method according to claim 1, wherein the CdCl.sub.2 is preferably applied as an aqueous salt solution by roller coating.

8. The method according to claim 1, wherein the first temperature treatment step is performed at a temperature of 410° C. for 25 minutes.

9. The method according to claim 1, wherein the first cleaning step is performed with a 1%-solution of diammonium hydrogen citrate for 1 minute.

10. The method according to claim 1, characterized in that the back contact layer is made of molybdenum and is applied with a thickness of 300 nm.

11. The method according to claim 1, wherein providing copper ions is performed by dipping the layer stack into a 0.1 mmol/l CuCl.sub.2 solution for 1 minute.

12. The method according to claim 1, characterized in that the second temperature treatment step is performed at a temperature of 200° C. for 15 minutes.

13. The method according to claim 1, characterized in that, during the artificial aging step, the layer stack is held at a temperature of 75° C., while it is illuminated with an illuminance of about 35000 lx for 48 hours.

14. The method according to claim 2, characterized in that the layer stack is dipped into a 100%-solution of dimethylformamide for 5 minutes in the second cleaning step or in the further cleaning step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate an embodiment of the present invention and together with the description serve to explain the principles. Other embodiments of the invention are possible and lie within the scope of the invention. The elements of the drawings are not necessarily to scale relative to each other. Like reference numbers designate corresponding similar parts.

(2) FIG. 1 schematically shows the sequence of process steps according to an embodiment of the method of the application.

(3) FIGS. 2A to 2D show the normalized performance parameters for CdTe solar cells produced by the method according to the application versus CdTe solar cells produced by a method according to the prior art.

DETAIL DESCRIPTION OF THE INVENTION

(4) The exemplary embodiment of the method according to the application shown in FIG. 1 starts with providing a transparent substrate made of glass with a transparent conducting layer made of ITO on it (S10), wherein the transparent conducting layer serves as a front contact of the CdTe solar cell which will be produced.

(5) On the transparent conducting layer, a 30 nm thick CdS layer is applied using closed space sublimation (CSS) technique (S20). The low thickness of the CdS layer is important for achieving good characteristics of the produced CdTe solar cell. However, the thickness cannot be reduced in any order due to other process steps, in particular to the activation step and temperature treatment steps, and due to the necessity to prevent microshunts or pinholes within the CdS layer. Therefore, 30 nm has been found by the inventors to be the best choice.

(6) On the CdS layer, a 4 μm thick CdTe layer is applied using CSS technique (S 30).

(7) Thereafter, a 80 nm thick crystalline layer of CdCl.sub.2 is applied on a first surface of the CdTe layer by roller coating using an aqueous CdCl.sub.2 solution of 25%, wherein the semi-finished CdTe solar cell comprising the previous mentioned layers on the transparent substrate is held at a temperature of 60° C. (S40). The thickness of the CdCl.sub.2 layer should be controlled very precisely, since the amount of chloride ions should be sufficient to activate the recrystallization of the CdTe layer and yet small enough to limit a degradation of the CdS layer, e.g. a delamination of the CdS layer. The first surface of the CdTe layer is that surface which lies free, i.e. it is that surface which does not adjoin to the CdS layer. The layer of CdCl.sub.2 is applied at room temperature, which is a temperature in the range between 15° C. and 30° C., usually around 20° C.

(8) After applying the CdCl.sub.2 layer, a first temperature treatment step is performed (S50). The layer stack resulting from the previous steps, i.e. with the CdCl.sub.2 layer on top of it, is held at a temperature of 410° C. for a time of 25 minutes under atmospheric conditions. This temperature and time in connection with the amount of CdCl.sub.2 applied in step S40 is sufficient for achieve a good recrystallization of the CdTe layer without largely dissolving or degrading the CdS layer.

(9) Subsequently, a first cleaning step is performed, wherein the layer stack resulting from the previous processing steps is dipped into a 1%-solution of diammonium hydrogen citrate for a time of 1 minute (S60). This step serves for removing residuals of the CdCl.sub.2 layer from the first surface of the CdTe layer, wherein the inventors found that diammonium hydrogen citrate and the used parameters are most suitable and give the best results.

(10) Following first cleaning step, a 300 nm thick back contact layer made of molybdenum is applied on the first surface of the CdTe layer using sputtering according to the prior art (S70).

(11) After terminating the step of applying the back contact layer, the layer stack resulting from the previous process steps is dipped into an aqueous 0.1 mmol/l CuCl.sub.2 solution for a time of 1 minute (S80). The layer stack and the solution are held at room temperature. During this step, copper ions being present in the CuCl.sub.2 solution adhere to the surface of the back contact layer.

(12) A second temperature treatment step is performed after removing the layer stack from the CuCl.sub.2 solution (S90). The layer stack is held at a temperature of 200° C. for a time period of 15 minutes under atmospheric conditions. This step results in migration of copper ions from the surface of the back contact layer to the interface of the back contact layer with the CdTe layer and slightly into the CdTe layer. Due to providing copper ions after applying the back contact layer instead of providing copper directly onto the surface of the CdTe layer and due to the low thermal budget applied to the layer stack during the second temperature treatment step, copper migration and the amount of copper incorporated into the CdTe layer can be controlled better than in the prior art.

(13) After the second temperature treatment step, a further cleaning step is performed (S100). In this step, the layer stack resulting from the previous processing steps is cleaned by dipping the layer stack into a 100% solution of dimethylformamide for a time of 5 minutes. The dimethylformamide solution rinses off copper ions which did not diffuse into the back contact layer during the second temperature treatment step and simultaneously dissolves any residuals, like compounds of copper and molybdenum, chlorine and molybdenum or other compounds generated during the previous process steps on the surface of the back contact layer.

(14) An artificial aging step, also called Open-Circuit Light Soak, OCLS, is performed after the second cleaning step (S110). This step includes an illumination of the layer stack resulting from the previous process steps for 48 hours with an illuminance of about 35000 lx at a temperature of 75° C. under atmospheric conditions. Due to the furthercleaning step (S100), only copper ions already migrated into the back contact layer or the CdTe layer may now further migrate due to the electrical field inside the CdTe solar cell caused by the illumination. The parameters of this step, i.e. low temperature and low luminance, result in good control of copper migration within the CdTe solar cell and reduce risk of degradation of any layer within the CdTe solar cell.

(15) Subsequent to the artificial aging step, a second cleaning step is performed (S120). Again, a 100% dimethylformamide solution is used to clean the layer stack resulting from the previous processing steps, wherein the layer stack is dipped into the dimethylformamide solution for a time of 5 minutes and subsequently rinsed first with water and second with isopropanol.

(16) All cleaning steps are performed at room temperature if not otherwise mentioned.

(17) FIGS. 2A to 2D show the normalized performance parameters for CdTe solar cells produced by the method according to the application as described above with respect to FIG. 1 versus CdTe solar cells produced by a method according to the prior art. That is, the reference onto which the performance parameters are normalized are performance parameters of CdTe solar cell produced by a method according to the prior art comprising the following steps in the mentioned order: providing a transparent substrate with a transparent conductive layer on top of it, applying a 80 nm thick CdS layer on the transparent conductive layer, applying a 4 μm thick CdTe layer on the CdS layer, applying a crystalline CdCl.sub.2 layer on the surface of the CdTe layer, performing a first temperature treatment step at 400° C. for 25 minutes, providing copper to a surface of the CdTe layer by dipping the layer stack resulting from the previous steps into an aqueous 1 mmol/l CuCl.sub.2 solution for a time of 1 minute, applying a back contact layer of molybdenum on the surface of the CdTe layer, and performing a second temperature treatment step at 200° C. for 25 minutes. After this treatment, the produced CdTe solar cells were electrically characterized, wherein the obtained values of the parameters are the reference values for all other measurements.

(18) FIG. 2A shows the normalized efficiency (η), FIG. 2B the normalized open circuit voltage (V.sub.oc), FIG. 2C the normalized fill factor (FF) and FIG. 2D the normalized short-circuit current (J.sub.sc) of CdTe solar cells processed with the method according to the application (solid line with rectangular dots) and of CdTe solar cells processed with the above mentioned prior art method (dashed line with diamond dots). For each parameter, three different values are given: First the (normalized) value measured after the last temperature treatment step of the respective process flow, second the (normalized) value measured directly after performing an artificial aging step (OCLS) for 48 hours with an illuminance of about 35000 lx at 75° C. under open-circuit conditions, and third the (normalized) value measured after dark degradation. Dark degradation describes a storage of the CdTe solar cells for 7 days in darkness, i.e. without any illumination, and under open-circuit conditions. For the CdTe solar cells processed with the method according to the application, the third cleaning step according to the application was performed before this last measurement. As can be seen by comparing the third values for each parameter, the combination of all steps and changes with respect to the prior art method as given in the method according to the application results in a great improvement of the characteristics of the CdTe solar cells.