Method for producing thin-film solar cells

Abstract

A method to produce thin film solar cells in superstrate or substrate configuration is an efficient way to minimize the loss due to absorption in CdS layer and to eliminate the CdCl.sub.2 activation treatment step. This is achieved by applying a sacrificial metal-halide layer between the CdS-layer and the CdTe-layer of the solar cells.

Claims

1. A method for producing a solar cell in a superstrate configuration, comprising the steps: a. making available a transparent substrate, b. applying a transparent front contact layer, c. applying a CdS layer, d. applying a sacrificial layer of a metal-halide compound, e. applying a CdTe layer and break-up of the sacrificial layer, including diffusion of the major part of the metal ions into the CdS layer and the major part of the halide ions into the CdTe layer, wherein the CdTe layer from a first partial layer with a percentage of up to 25% of the total layer thickness is produced, in a temperature range of room temperature to 200 C., and the remaining second CdTe partial layer is produced in a temperature range of 350 C. to 550 C., and f. applying the back contact layer.

2. A method for producing a solar cell in substrate configuration, comprising the steps: a. making available a substrate, b. applying the back contact layer c. applying a CdTe layer d. applying a sacrificial layer of a metal-halide compound, e. applying a CdS layer and break-up of the sacrificial layer, including diffusion of the major part of the metal ions into the CdS layer and the major part of the halide ions into the CdTe layer, wherein the CdTe layer from a first partial layer with a percentage up to 75% of the total layer thickness is produced in a temperature range of 350 C. to 550 C. and the remaining second CdTe partial layer is produced in a temperature range of room temperature to 200 C., and f. applying a transparent front contact layer.

3. The method according to claim 1, wherein the step b. is performed with an additional suitable high resistive buffer layer.

4. A method for producing a solar cell in substrate configuration, comprising the steps: a. making available a substrate, b. applying the back contact layer c. applying a CdTe layer d. applying a sacrificial layer of a metal-halide compound, e. applying a CdS layer and break-up of the sacrificial layer, including diffusion of the major part of the metal ions into the CdS layer and the major part of the halide ions into the CdTe layer, the CdS layer from a first partial layer with a percentage up to 25% of the total layer thickness is produced in a temperature range of 100 C. to 200 C. and the remaining second CdS partial layer is produced in a temperature range of 350-550 C., and f. applying a transparent front contact layer.

5. The method according to claim 1, wherein step e. is performed at temperatures in the range of 100 C. to less than 550 C.

6. The method according to claim 1, wherein after step e. an additional CdCl.sub.2 activation step is performed.

7. The method according to claim 1, wherein after step e. a temperature treatment step at a temperature in the range of 300 C. to 450 C. is performed.

8. The method according to claim 1, wherein the sacrificial layer is made from ZnCl.sub.2 or ZnCl.sub.2 derivatives.

9. The method according to claim 8, wherein the sacrificial layer additionally includes other suitable metal chlorides appropriate to increase the band gap of the CdS layer.

10. The method according to claim 1, wherein the metal-halide-compound of the sacrificial layer is dissolved in a suitable solvent in step d.

11. The method according to claim 1, wherein the metal-halide-compound of the sacrificial layer consists of Zn as the metal and Fluorine or Chlorine as the halide.

12. The method according to claim 1, wherein the metal-halide-compound of the sacrificial layer contains additional Fluorine or Chlorine above the stoichiometric ratio of the metal-halide-compound.

13. The method according to claim 2, wherein step f. is performed with an additional suitable high resistive buffer layer.

14. The method according to claim 2, wherein step e. is performed at temperatures in the range of 100 C. to less than 550 C.

15. The method according to claim 1, wherein step e. is performed at temperatures in the range of 100 C. to less than 550 C.

16. The method according to claim 2, wherein step e. is performed at temperatures in the range of 100 C. to less than 550 C.

17. The method according to claim 4, wherein step e. is performed at temperatures in the range of 100 C. to less than 550 C.

Description

FIGURES

(1) FIG. 1 schematically shows the layer structure of a solar cell according to the state of the art. Said solar cell comprises on the substrate (1) a layer sequence consisting of front contact (21), CdS layer (3), CdTe layer (4) and back contact (22).

(2) FIGS. 2a to 2e schematically shows the layer sequences, as they may be observed during the course of the method according to the invention.

EXEMPLARY EMBODIMENT

(3) The method according to the invention is explained in the following in a first exemplary embodiment showing the making of a solar cell in superstrate configuration, without intending to imply a restriction to said embodiment.

(4) In FIG. 2a on the substrate (1) the front contact (21) and the CdS layer (3) have already been applied by means of methods according to the state of the art. As front contact (21), a 450 nm thick transparent bi layer [Fluorine doped tin oxide (350 nm) as conducting layer and tin oxide (100 nm) as high resistive buffer] was applied (as TCO). The CdS layer (3) reaches a thickness of 90 nm and was deposited using CSS technique. On this, the sacrificial layer (5) of ZnCl.sub.2 according to the invention is deposited. This was applied by spraying the ZnCl.sub.2 solution (dissolved in water) and by subsequently drying at 80 C. The thickness of the ZnCl.sub.2 layer is about 15 nm.

(5) FIGS. 2b and 2c schematically show how the sacrificial layer (5) above the CdTe layer is deposited. In a first step (FIG. 2b) a CdTe layer (4) with a thickness of 1500 nm is deposited by means of CSS at a temperature of 120 C. Afterwards (FIG. 2c) the substrate temperature is increased to 450 C. and about 3500 nm of CdTe is deposited. The total thickness of CdTe layer is about 5000 nm. The sacrificial layer starts to break up during the deposition of CdTe at 450 C., and the Zn ions preferably move into the CdS layer (3), while the CI ions diffuse preferably into the CdTe layer and thus helping for CdTe grain boundary passivation. In general, if required, an additional thermal step can be performed. This may help to completely break-up the sacrificial layer (5). In case if necessary, the regular CdCl.sub.2 activation treatment can be performed but with reduced CdCl.sub.2 amount and/or treatment time.

(6) FIG. 2d schematically shows that the sacrificial layer (5) is nearly completely broken down after the thermal treatment in the process of the CdTe deposition.

(7) FIG. 2e schematically shows that as a result of the method according to the invention, after completing the back contact procedure with metal layer. (22) (made of Mo), a solar cell has been created having a layer sequence which corresponds to that known from prior art. In detail, concentration gradients of the Zn ions in the CdS layer (3) and of the CI ions in the CdTe layer (4) arise through the diffusion process. These diffusion gradients point to the use of the method according to the invention.

REFERENCE NUMERALS

(8) 1 Substrate (glass) 21 Front contact (transparent, TCO) 22 back contact (metal) 3 CdS layer 4 CdTe layer 5 ZnCl.sub.2 layer