Kesterite material of CZTS, CZTSe or CZTSSe type

Abstract

A method of producing a kesterite material of CZTS, CZTSe or CZTSSe type, including the steps of: a) preparing an acidic solution by dissolving copper and zinc salts in water in desired molar ratio, b) preparing a basic solution by dissolving an alkali metal stannate together with an alkali metal carbonate or an alkali metal hydrogen carbonate or an alkali metal hydroxide or a combination thereof, and optionally with an alkali metal selenate or an alkali metal selenite or a mixture thereof, c) carrying out a precipitation reaction by mixing the acidic and the basic solution, d) drying the precipitate thereby providing a precursor for the kesterite material, and e) sulfurizing the precursor of step d to provide the kesterite material. Also, a precursor for a kesterite material of CZTS, CZTSe or CZTSSe type.

Claims

1. A method of producing a kesterite material of CZTS, CZTSe or CZTSSe type, comprising the steps of: a) preparing an acidic solution and dissolving copper salt and zinc salt in water in desired molar ratio, b) preparing a basic solution by dissolving an alkali metal stannate together with an alkali metal carbonate or an alkali metal hydrogen carbonate or an alkali metal hydroxide or a combination thereof, and optionally with an alkali metal selenate or an alkali metal selenite or a mixture thereof, c) carrying out a precipitation reaction by mixing said acidic and said basic solution, thereby obtaining a precipitate, d) washing and heat treating said precipitate thereby providing a precursor for the kesterite material, and e) sulfurizing the precursor of step d to provide the kesterite material.

2. The method according to claim 1, wherein the salts used for the acidic solution are nitrates, halides, sulfates, carboxylates or combinations thereof.

3. The method according to claim 1, wherein the copper salt is copper (II) nitrate and the zinc salt is zinc (II) nitrate.

4. The method according to claim 1, wherein said acidic solution further comprises nitric acid.

5. The method according to claim 1, wherein said alkali metal stannate comprises Na.sub.2SnO.sub.3 and/or K.sub.2SnO.sub.3 in hydrated or anhydrous form.

6. The method according to claim 1, wherein said basic solution further comprises selenium in the form of an alkali metal selenate and/or an alkali metal selenite.

7. The method according to claim 1, wherein the step of sulfurizing comprises subjecting the precursor to a sulfur comprising gas at a temperature above room temperature.

8. The method according to claim 7, wherein said sulfur comprising gas is one or more of the following: H.sub.2S, elemental sulfur in gaseous state (S.sub.2-S.sub.8), COS, CS.sub.2, organic sulfur compounds.

9. The method according to claim 8, wherein said organic sulfur compounds are CH.sub.3SH and/or CH.sub.3SSCH.sub.3.

10. The method according to claim 1, wherein said sulfur comprising gas further comprises selenium.

11. The method according to claim 1, further comprising the following steps between steps d) and e): grinding the precursor for the kesterite material obtained by step d), dispersing said grinded precursor in a liquid, thereby obtaining a slurry, depositing a thin layer of the slurry onto a substrate, and optionally, drying said thin layer.

12. The method according to claim 11, wherein the step of dispersing said grinded precursor in a liquid comprises wetting and dispersing said grinded precursor in a liquid using polymeric wetting and/or dispersion agents.

13. The method according to claim 1, further comprising the following steps subsequent to step e): grinding said kesterite material, dispersing said grinded kesterite material in a liquid, thereby obtaining a slurry, depositing a thin layer of the slurry onto a substrate, optionally, drying said thin layer, and optionally, heat treating such thin layer.

14. The method according to claim 13, wherein the step of dispersing said grinded kesterite material in a liquid comprises wetting and dispersing said grinded kesterite material in a liquid using polymeric wetting and/or dispersion agents.

15. A precursor material for a kesterite material of CZTS, CZTSe or CZTSSe type, where said precursor material is obtained by a process comprising the steps of: a) preparing an acidic solution and dissolving copper salt and zinc salt in water in a desired molar ratio, b) preparing a basic solution by dissolving an alkali metal stannate together with an alkali metal carbonate or an alkali metal hydrogen carbonate or an alkali metal hydroxide or a combination thereof, c) carrying out a precipitation reaction by mixing the acidic and the basic solutions, thereby obtaining a precipitate, and d) washing and heat treating the precipitate thereby providing a precursor for the kesterite material.

Description

EXAMPLES

Example 1a. Preparation of a CZTS Hydroxy Carbonate Precursor

(1) Two solutions were prepared: An acidic solution (solution A) and a basic solution (solution B).

(2) Solution A was prepared by mixing 189.1 g of a copper nitrate solution which was analyzed to contain 16.8% wt/wt Cu (0.50 mol Cu) with 66 g of 65% HNO.sub.3 (0.65 mol) and then adding 20.3 g of solid ZnO (0.25 mol) and diluting the resulting solution to 1.5 liter with deionized water.

(3) Solution B was prepared by adding 74.7 g K.sub.2SnO.sub.3*3H.sub.2O (0.25 mol) to 241 g of a 33% wt/wt solution of K.sub.2CO.sub.3 (0.575 mol) and diluting the suspension to 1.5 liter with deionized water.

(4) When almost all solids had dissolved (solution B does not become completely transparent), solution A and solution B were mixed in a large beaker which was mechanically stirred. The two solutions were simultaneously pumped into the beaker at constant and almost equal flow rates. pH was measured throughout this step and it was found to be fairly constant at approximately 6.5. After the two solutions had been mixed, the resultant blue precipitate was ripened by heating to 70° C. and keeping the temperature at 70° C. for one half hour. The product was filtered off, washed several times with hot, demineralized water and dried in an oven at 100° C. for 3 days.

(5) XRD analysis (Rietveld refinement) of this material showed a Zn[Sn(OH).sub.6] phase with average crystal size D=280 Å and lattice constant a=7.78 Å together with an amorphous phase. No copper-containing phase was detected.

Example 1b. Preparation of CZTS Material

(6) The dried precursor obtained in Example 1a was sulfided in the following way. A sieved fraction of the precursor (0.15-0.30 mm) was used. A total of 0.5 g was loaded into a tubular reactor and heated to 380° C. in a stream of N.sub.2 containing 100 ppm H.sub.2S. The sulfur uptake was determined by gas chromatograph measurements. When the sample did not take up any more sulfur, the treatment was disrupted and the sample was cooled to ambient temperature.

(7) The resultant black material was analyzed by XRD and Raman spectroscopy. XRD analysis (Rietveld refinement) showed phase pure Cu.sub.2ZnSnS.sub.4 with average crystal size D=446 Å and lattice constants a=5.438 Å and c=10.839 Å. Raman spectroscopy showed a single Raman shift at 327 cm.sup.−1.

Example 2. Preparation of CZTS Film Based on Sulfur Free CZTS Precursor

(8) A CZTS precursor was prepared as described in Example la with the exception that Cu:Zn:Sn ratio is 1.76:1.20:1.00. This precursor was heat treated in stagnant air at 350° C. for 4 h in order to transform the precursor into a Cu—Zn—Sn oxide precursor. A slurry for coating the Cu—Zn—Sn oxide precursor was prepared in the following way. 5 g ethanol (EtOH), 2 g of a VOC-free structured acrylic copolymer dispersion agent and 1 g of a polyvinylbutyral-based binder was added to a 50 ml PE bottle together with 30 g of ZrO.sub.2 pearls, ø 1.5 mm and shaken in a paint shaker for 2 min. 10 g of the Cu—Zn—Sn oxide precursor was added and the slurry was further shaken for 600 min in the paint shaker. Diffuse reflectance spectroscopy was used to determine that 90% of the volume weighted particle size distribution was below 1 μm after this. Some of the slurry was applied as a thin layer onto a piece of soda-lime glass and dried in ambient air. By scanning electron spectroscopy, it was verified that the coated layer was approximately 1-2 μm thick, homogeneous and essentially free of large cracks. The coated soda-lime glass was then treated in a stream of nitrogen containing 10 vol % H.sub.2S at a temperature of 600° C., in order to transform the precursor film into a CZTS film. The optical quality of the film was checked by photoluminescence. The photoluminescence from the film had maximum intensity at a photon energy of 1.37 eV and the peak had a FWHM of 0.26 eV which indicates that a good quality CZTS with relative few defects that would be detrimental to the performance of a solar cell based on the material.

Example 3. Preparation of CZTS Film Based on CZTS Powder

(9) A CZTS powder was prepared as described in Example 1b with the exception that it was sulfided in a stream of nitrogen containing 10 vol % H.sub.2S at 620° C. for 2 h. A slurry for coating this CZTS powder was prepared in the following way. 1.5 g EtOH, 0.6 g a VOC-free structured acrylic copolymer dispersion agent and 0.3 g of a polyvinylbutyral-based binder was added to a 10 ml PE bottle together with 9 g of ZrO.sub.2 pearls, ø 1.5 mm and shaken in a paint shaker for 2 min. 3 g of CZTS powder was added and the slurry was further shaken for 180 min in the paint shaker. Diffuse reflectance spectroscopy was used to determine that 90% of the volume weighted particle size distribution was below 1 μm after this. Some of the slurry was applied as a thin layer onto a piece of soda-lime glass and dried in ambient air. By scanning electron spectroscopy, it was verified that the coated layer was approximately 1-2 μm thick, homogeneous and essentially free of large cracks. The optical quality of the film was checked by photoluminescence. The photo luminescence from the film had maximum intensity at a photon energy of 1.37 eV and the peak had a FWHM of 0.28 eV which indicates that a good quality CZTS with relative few defects that would be detrimental to the performance of a solar cell based on the material.

(10) While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept.