ADDITIONAL TEMPERATURE TREATMENT STEP FOR THIN-FILM SOLAR CELLS

Abstract

The present invention refers to a method for producing CdTe thin-film solar cells, respectively a semi-finished CdTe thin-film solar cell, where in an additional temperature step is carried out after applying the CdTe layer on to a substrate. In particular, the temperature step is performed after activating the CdTe layer using a suitable activation agent and removing the residual activation agent from the CdTe layer. The temperature treatment is performed under vacuum or in a heating chamber filled with either air or inert gas, during which treatment the substrate is exposed to a temperature between 180 C. and 380 C. for a time between 5 minutes and 60 minutes. Due to the inventive additional temperature step, the number and extension of crystal defects in the CdTe layer is reduced and the electric efficiency of the solar cell is further improved.

Claims

1. A method for producing a thin-film solar cell, comprising the steps: a. Applying a CdTe layer onto a substrate, b. Activating the CdTe layer using a suitable activation agent, c. Removing the residual activation agent from the CdTe layer after activation, and d. Performing a temperature treatment under vacuum or in a heating chamber filled with either air, or inert gas, after removing the residual activation agent, during which treatment the substrate is exposed to a temperature between 180 C. and 380 C. for a time between 5 minutes and 60 minutes.

2. The method according to claim 1, wherein in step c) the residual activation agent is evaporated off, and afterwards the additional temperature treatment takes place without any interim cooling.

3. The method according to claim 1, wherein, after activation of the CdTe layer, the substrate containing the layer sequence already present is cooled down to room temperature, and the residual activation agent is rinsed off with a suitable solvent in step c), after which the additional temperature treatment takes place.

4. The method according to claim 3, wherein the solvent is either deionized water, an alcohol, a weak acid, a lye, or a mixture of various substances.

5. The method according to claim 1, wherein the activation agent is CdC12.

6. The method according to claim 1, further comprising the steps: e. Providing a transparent substrate, f. Applying a front contact layer onto the substrate, g. Applying a window layer onto the front contact layer, h. Applying a back contact layer, wherein the steps a) to d) are performed after step g), and step d) is performed either before or after step h).

7. The method according to claim 6, wherein step d) is performed before step h), wherein the resulting temperature of the substrate during step d) is less than 350 C.

8. The method according to claim 7, wherein the method further comprises an etching step for the surface treatment of the CdTe layer, wherein said etching step takes place after step d).

9. The method according to claim 6, wherein step d) takes place after step h), wherein during step d) the substrate is exposed to a temperature of more than 200 C.

10. The method according to claim 1, further comprising the steps: i. Providing a substrate, j. Applying a back contact layer onto the substrate, k. Applying a window layer, l. Applying a front contact layer onto the window layer, wherein the steps a) to c) are performed between step j) and step k), and step d) is performed after step c) and before step k), or between the steps k) and l), or after step l).

11. The method according to claim 10, wherein step d) is performed before step k), wherein the resulting temperature of the substrate during step d) is less than 350 C.

12. The method according to claim 10, wherein step d) is performed after step l), wherein during step d) the substrate is exposed to a temperature of more than 200 C.

13. The method according to claim 6, wherein the window layer consists of pure or modified CdS.

14. The method according to claim 2, wherein the activation agent is CdC12.

15. The method according to claim 3, wherein the activation agent is CdC12.

16. The method according to claim 4, wherein the activation agent is CdC12.

17. The method according to claim 2, further comprising the steps: e. Providing a transparent substrate, f. Applying a front contact layer onto the substrate, g. Applying a window layer onto the front contact layer, h. Applying a back contact layer, wherein the steps a) to d) are performed after step g), and step d) is performed either before or after step h).

18. The method according to claim 3, further comprising the steps: e. Providing a transparent substrate, f. Applying a front contact layer onto the substrate, g. Applying a window layer onto the front contact layer, h. Applying a back contact layer, wherein the steps a) to d) are performed after step g), and step d) is performed either before or after step h).

19. The method according to claim 4, further comprising the steps: e. Providing a transparent substrate, f. Applying a front contact layer onto the substrate, g. Applying a window layer onto the front contact layer, h. Applying a back contact layer, wherein the steps a) to d) are performed after step g), and step d) is performed either before or after step h).

20. The method according to claim 5, further comprising the steps: e. Providing a transparent substrate, f. Applying a front contact layer onto the substrate, g. Applying a window layer onto the front contact layer, h. Applying a back contact layer, wherein the steps a) to d) are performed after step g), and step d) is performed either before or after step h).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 shows the heating ramp for heating a substrate under vacuum.

[0041] FIG. 2 shows the heating ramp for heating a substrate in an oven in a standard atmosphere.

DETAILED DESCRIPTION

[0042] The substrates used in the following exemplary embodiments serve, for the major part, for research purposes. The substrates are made from float glass, having dimensions of 10 cm10 cm and a thickness of 3.2 mm. Onto these substrates, in the exemplary embodiments 1 and 2, the TCO layers, the CdS layers and the CdTe layers have already been deposited. If substrates are used on an industrial scale, these dimensions may significantly exceed an edge length (i.e. feed size) of 1 metre. For these substrates, the heating ramp requires adjustment to reduce the danger of breaking caused by thermal stress. The corresponding permissible heating rates are known from prior art. The most essential factor for the method according to the invention is however, that the substrate is kept for the time indicated at the temperatures indicated.

1. Vacuum Heating Chamber

[0043] The heating ramp is shown in FIG. 1.

[0044] A substrate according to the state of the art is introduced into a vacuum heating chamber. For heating, a resistance heating according to the state of the art is used. The initial temperature of the substrate is at room temperature (20 C.). The end temperature of the substrate is 250 C. The oven temperature during feed-in of samples is 310 C. The substrate is heated up at a rate of about 11.5 C./minute over a time of about 20 minutes. The substrate reaches its end temperature of 250 C. Subsequently the oven temperature is reduced to 270 C. and the substrate is heated for 20 minutes at the same temperature. After the temperature step, the substrate is discharged from the heating chamber and left to cool under ambient conditions.

2. Heating Chamber with Air

[0045] Die heating ramp is shown in FIG. 2.

[0046] The substrate having ambient temperature (20 C.) is introduced into the heating chamber, which has been heated up to 250 C. and has a standard atmospheric pressure, and left there for a duration of 40 minutes. The temperature rise in the substrate is about 300 C./minutes.

3. Method for Producing a Solar Cell Using a Temperature Step According to the Invention

[0047] Cadmium sulfide and cadmium telluride are applied using CSS deposition onto a glass substrate coated with TCO. The layer thickness is c. 70 nm for the CdS and 4 m for the CdTe. Deposition takes place at about 500 C. Subsequently, a CdC12-containing aqueous solution is applied, followed by a layer activation at 400 C. for a duration of 25 minutes exposed to air. Afterwards, the substrate is cooled down, the excess CdC12 is rinsed off using water, and the substrate is dried exposed to air.

[0048] Method 3a: The substrate (containing the layer sequence already present) is fed into a pre-heated vacuum chamber and the sample is annealed at 250 C. for a total duration of 40 minutes (the temperature profile corresponds to that shown in FIG. 1). Subsequently the substrate is discharged and cooled down (6-9 C./minutes) to room temperature, being exposed to air.

[0049] Method 3b: The substrate (containing the layer sequence already present) is fed into an atmospheric oven pre-heated to 250 C. (placing the substrate onto a hot graphite sheet) and subsequently annealed for 20 minutes. Thereafter cooling of the substrate to room temperature (6-9 C./minutes) takes place, exposed to air.

[0050] Only after this step (in method 3a or 3b), nitric-phosphoric acid etching takes place at ambient temperature (20 C.) for a duration of 30 seconds (NP-etching: HNO3:H3PO4:H2O). As a result of the etching a Te-rich layer of about 150 nm thickness is formed.

[0051] Finally, the substrate is fed into a vacuum chamber and the metallic back contact is deposited.