WO2013015745A1 - Cu-zn-sn-s/se thin film and methods of forming the same - Google Patents

Cu-zn-sn-s/se thin film and methods of forming the same Download PDF

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WO2013015745A1
WO2013015745A1 PCT/SG2012/000270 SG2012000270W WO2013015745A1 WO 2013015745 A1 WO2013015745 A1 WO 2013015745A1 SG 2012000270 W SG2012000270 W SG 2012000270W WO 2013015745 A1 WO2013015745 A1 WO 2013015745A1
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thin film
water
nanoparticles
sulfide
mixture
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PCT/SG2012/000270
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French (fr)
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Zhenggang LI
Yeng Ming Lam
Subodh Mhaisalkar
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Nanyang Technological University
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0256—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by the material
    • H01L31/0264—Inorganic materials
    • H01L31/032—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312
    • H01L31/0326—Inorganic materials including, apart from doping materials or other impurities, only compounds not provided for in groups H01L31/0272 - H01L31/0312 comprising AIBIICIVDVI kesterite compounds, e.g. Cu2ZnSnSe4, Cu2ZnSnS4
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02367—Substrates
    • H01L21/0237—Materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02551—Group 12/16 materials
    • H01L21/02557—Sulfides
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02551—Group 12/16 materials
    • H01L21/0256—Selenides
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02568—Chalcogenide semiconducting materials not being oxides, e.g. ternary compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02587—Structure
    • H01L21/0259—Microstructure
    • H01L21/02601—Nanoparticles
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02612—Formation types
    • H01L21/02617—Deposition types
    • H01L21/02623—Liquid deposition
    • H01L21/02628—Liquid deposition using solutions

Definitions

  • the present invention relates to a method for forming a copper-zinc-tin-X (Cu-Zn- Sn-X) thin film, wherein X is sulfur (S), selenium (Se), or a mixture of sulfur and selenium (S, Se).
  • the invention also relates to a Cu-Zn-Sn-X thin film formed by the inventive method, wherein X is S, Se, or (S, Se).
  • the invention relates to a Cu-Zn-Sn-X thin film formed by a method according to the first aspect.
  • the invention relates to a device comprising a Cu-Zn-Sn-X thin film according to the second aspect.
  • the device is a solar cell.
  • FIG. 1 is a schematic diagram depicting the general scheme for a method for forming a Cu-Zn-Sn-X thin film, wherein X is S, Se, or a mixture of S and Se, according to the invention.
  • X is S, Se, or a mixture of S and Se, according to the invention.
  • Cu p X (1 ⁇ p ⁇ 2) nanoparticles (NPs) 101, ZnX NPs 103, and SnX q (1 ⁇ q ⁇ 2) NPs 105 are individually suspended in a dispersing agent.
  • a capping ligand 107 is present.
  • the Cu p X (1 ⁇ p ⁇ 2) NPs 101, ZnX NPs 103, and SnX q (1 ⁇ q ⁇ 2) NPs 105 are mixed together to form a dispersion 110.
  • the dispersion 110 is deposited on a substrate 109 to form a precursor thin film 120 as shown in Figure 1C.
  • the substrate 109 comprising the precursor thin film 120 is heated in an inert atmosphere at a temperature in the range of about 300 °C to about 600 °C to form the Cu-Zn- Sn-X thin film 130 as shown in Figure ID.
  • Figure 4 is a graph showing XRD pattern of as-synthesized ZnS nanoparticles, matches with ZnS sphalerite (JCPDS 00-005-0566).
  • Figure 5 is a graph showing XRD pattern of as-synthesized SnS 2 nanoparticles, matches with SnS 2 berndtite-2/ITT/RG (JCPDS 00-023-0677).
  • Figure 6 shows the TEM images of (a) CuS, (b) ZnS and (c) SnS 2 nanoparticles.
  • the scale bars in the images denote a length of 100 nm.
  • Figure 7 is a graph showing XRD pattern of the as-prepared CZTS thin film.
  • Figure 8 is (a) the topographical, and (b) cross-sectional scanning electron microscope (SEM) image of CZTS thin film. The scale bar denotes a length of 100 ran.
  • Figure 9 is scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) elemental mapping of elements Cu, Zn, Sn and S.
  • Figure 10 is a graph showing UV-vis-NIR absorption spectrum of CZTS thin film on sodium lime glass. Inset shows the plot of (ohv) 2 vs hv.
  • the present invention refers to a method for forming a copper-zinc- tin-X (Cu-Zn-Sn-X) thin film, wherein X is sulfur (S), selenium (Se), or a mixture of sulfur and selenium (S, Se).
  • the method of the present invention is a simple and low-cost method for the production of Cu-Zn-Sn-S, Cu-Zn-Sn-Se, and Cu-Zn-Sn-(S,Se) thin films.
  • the use of readily available materials such as water and water-soluble metal salts including metal chlorides, sulfates, nitrates and acetates, coupled with low-cost solution deposition techniques such as spray coating, printing, drop-casting and dip-coating, translates into ease of fabrication of the films which may be easily scaled up for mass production.
  • the method of the invention may be carried out at room temperature under ambient conditions which renders the process energy efficient. As no toxic byproduct is formed from the process, the synthetic route is environmentally friendly.
  • the composition of the resulting film may be adjusted or controlled easily.
  • the Cu-Zn-Sn-X thin film formed by a method of the invention is also referred herein as a CZTX thin film or a CZTX film, where X is S, Se or SSe. Accordingly, the Cu- Zn-Sn-X thin film may be respectively termed as a CZTS, CZTSe, or CZTSSe film.
  • the Cu-Zn-Sn-X thin film contains copper, zinc, tin and X, where X denotes sulfur, selenium, or a mixture of sulfur and selenium.
  • the CZTX thin film consists essentially of copper, zinc, thin and X, where X denotes sulfur, selenium, or a mixture of sulfur and selenium. In some embodiments, the thin film consists of copper, zinc, tin and X, where X denotes sulfur, selenium, or a mixture of sulfur and selenium. In embodiments in which X denotes sulfur or selenium or a mixture of sulfur and selenium, the thin film may also be described as a multi-component CZTX film, wherein term "multi-component" refers to a material that is composed of four or more different elements. In various embodiments of the above, X is sulfur or selenium or a mixture of sulfur and selenium.
  • the method comprises forming a dispersion comprising Cu p X (1 ⁇ p ⁇ 2) nanoparticles, ZnX nanoparticles and SnX q (1 ⁇ q ⁇ 2) nanoparticles in a dispersing agent.
  • a dispersing agent Generally, each of the Cu p X nanoparticles, ZnX nanoparticles and SnX q nanoparticles are prepared individually, and subsequently mixed in together with a dispersing agent to form the dispersion.
  • the Cu p X (1 ⁇ p 2) nanoparticles, ZnX nanoparticles and SnX q (1 ⁇ q ⁇ 2) nanoparticles may each be prepared in an aqueous solution.
  • the Cu p X (1 ⁇ p ⁇ 2) nanoparticles may be prepared by reacting in an aqueous solution, a water-soluble salt of Cu and a water-soluble compound containing an anion of X.
  • p is 1, i.e. the Cu p X nanoparticles prepared are CuX nanoparticles.
  • water-soluble salts of Cu that may be used include, but are not limited to, copper chloride, copper sulfate, copper nitrate, copper acetate, and mixture thereof.
  • the ZnX nanoparticles may be prepared by reacting in an aqueous solution, a water-soluble salt of Zn and a water-soluble compound containing an anion of X.
  • water-soluble salts of Zn include, but are not limited to, zinc chloride, zinc sulfate, zinc nitrate, zinc acetate, and mixture thereof.
  • the SnX q (1 ⁇ q ⁇ 2) nanoparticles may also be prepared by reacting in an aqueous solution, a water-soluble salt of Sn and a water-soluble compound containing an anion of X.
  • q is 2, i.e. the SnX q nanoparticles prepared are SnX 2 nanoparticles.
  • water-soluble salts of Sn include, but are not limited to, tin chloride, tin sulfate, tin nitrate, tin acetate, and mixture thereof.
  • the reaction to form the nanoparticles may be carried out in the presence of a capping ligand.
  • the capping ligand comprises a water-soluble surfactant.
  • the water-soluble surfactant include, but are not limited to dodecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, octadecyltrimethylammonium bromide, octadecyltrimethylammonium chloride, cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, cetylbenzyldimethylammonium chloride, cetyltriethylammmonium bromide, and mixture thereof.
  • the water-soluble surfactant comprises or consists of the above-mentioned compounds.
  • the water-soluble surfactant comprises cetyltrimethylammonium bromide (CTAB).
  • CTAB cetyltrimethylammonium bromide
  • the water-soluble surfactant is CTAB.
  • the water-soluble compound containing an anion of S comprises or consists of a compound selected from the group consisting of lithium sulfide, sodium sulfide, potassium sulfide, rubidium sulfide, cesium sulfide, ammonium sulfide, lithium sulfide monohydrate, sodium sulfide nonahydrate, potassium sulfide nonahydrate, rubidium sulfide tetrahydrate, cesium sulfide tetrahydrate, and a mixture thereof.
  • the water- soluble compound containing an anion of S may comprise sodium sulfide nonahydrate or potassium sulfide.
  • the water-soluble compound containing an anion of S consists of sodium sulfide nonahydrate or potassium sulfide.
  • the water-soluble compound containing an anion of Se comprises or consists of a compound selected from the group consisting of lithium selenide, sodium selenide, potassium selenide, rubidium selenide, cesium selenide, ammonium selenide, and a mixture thereof.
  • the water-soluble compound containing an anion of Se may comprise sodium selenide or potassium selenide.
  • the water-soluble compound containing an anion of Se and S comprises or consists of a compound selected from the group consisting of lithium selenide sulfide, sodium selenide sulfide, potassium selenide sulfide, rubidium selenide sulfide, cesium selenide sulfide, ammonium selenide sulfide, and a mixture thereof.
  • the water-soluble compound containing an anion of Se and S may comprise sodium selenide sulfide or potassium selenide sulfide.
  • Preparation of the nanoparticles may be carried out at any suitable temperature, such as between about 20 °C to about 50 °C, between about 20 °C to about 40 °C or between about 25 °C to about 30 °C. Generally, preparation of the nanoparticles is carried out at about 25 °C. In various embodiments, no external heating is required for preparing the nanoparticles, i.e. the process may be carried out at room temperature.
  • a capping ligand such as CTAB may be added to deionized water to form the aqueous solution.
  • a water-soluble salt such as copper chloride and a water-soluble compound containing an anion of S such as sodium sulfide nonahydrate may be added to the aqueous solution.
  • the reaction between copper chloride and sodium sulfide nonahydrate may be carried out at room temperature and under ambient conditions. In various embodiments, the reaction is carried out under constant stirring. As a result, CuS nanoparticles are formed.
  • the size of the nanoparticle depends on factors such as concentration of the reagents used, time for reaction, temperature used and stirring speed.
  • the size of the nanoparticles may be characterized by their mean diameter.
  • the term "diameter” as used herein refers to the maximal length of a straight line segment passing through the center of a figure and terminating at the periphery.
  • the term “mean diameter” refers to an average diameter of the nanoparticles, and may be calculated by dividing the sum of the diameter of each nanoparticle by the total number of nanoparticles.
  • the term “diameter” is used normally to refer to the maximal length of a line segment passing through the centre and connecting two points on the periphery of a nanosphere, it is also used herein to refer to the maximal length of a line segment passing through the centre and connecting two points on the periphery of nanoparticles having other shapes, such as a nanocube or a nanotetrahedra, or an irregular shape.
  • the mean diameter of the nanoparticles may range from about 20 nm to about 50 nm. This is an ideal size range wherein the nanoparticles are small enough to allow for homogeneous mixing, and the reduction in surface energy provides the driving force for the solid state reaction between neighboring particles during annealing.
  • the nanoparticles are essentially monodisperse. Generally, smaller sized and/or monodisperse nanoparticles are desirable as they result in formation of a more homogeneous CZTX film.
  • the nanoparticles Prior to forming the dispersion comprising Cu p X (1 ⁇ p 2) nanoparticles, ZnX nanoparticles and SnX q (1 ⁇ q ⁇ 2) nanoparticles, the nanoparticles, which may be individually prepared, may be subjected to an optional washing step.
  • the Cu p X (1 p ⁇ 2) nanoparticles, ZnX nanoparticles and SnX q (1 ⁇ q ⁇ 2) nanoparticles may be washed in a washing reagent.
  • the washing reagent comprises an alcohol.
  • the washing reagent consists of an alcohol.
  • the method of the invention comprises dispersing the nanoparticles in a dispersing agent or dispersant.
  • the dispersing agent may include, but not limited to, water, an alcohol or a mixture of alcohols.
  • Types of water that may be used have already been described herein and include, but are not limited to deionized water and distilled water.
  • Examples of alcohols that may be used have already been described herein and include, but are not limited to methanol, ethanol, 1 -propanol, iso-propanol, 1 -butanol, 2-butanol, tert-butanol, and glycol.
  • the washing reagent and the dispersing agent may comprise, or be formed from the same alcohol.
  • the washing reagent and the dispersing agent may both comprise ethanol.
  • the washing reagent and the dispersing agent may alternatively, comprise a different alcohol.
  • the washing reagent may comprise ethanol
  • the dispersing agent may comprise isopropyl alcohol.
  • the molar ratio of the respective elements Cu, Zn and Sn in the dispersion comprising Cu p X (1 p ⁇ 2), ZnX and SnX q (1 ⁇ q ⁇ 2) nanoparticles may differ slightly from that of the prepared Cu-Zn-Sn-X thin film. Accordingly, the amount of Cu p X (1 ⁇ p ⁇ 2), ZnX and SnX q (1 q ⁇ 2) nanoparticles in the dispersion used may be varied to obtain a desired molar ratio of the prepared Cu-Zn-Sn-X thin film.
  • the dispersion containing the nanoparticles may be deposited on a substrate to form a precursor thin film. Any suitable thin film forming method may be used.
  • the dispersion may be deposited on the substrate by spray coating, inkjet printing, drop- casting, dip-coating, painting, or spin coating. In one embodiment, the dispersion is deposited on the substrate by spray coating.
  • suitable substrates that may be used for forming the CZTX thin film include glass, silicon, metal foil or ceramic.
  • the substrate comprises glass or silicon.
  • the substrate consists of glass or silicon.
  • the precursor thin film that is formed by a method according to the invention may be of any suitable thickness.
  • the thickness of the precursor thin film may be from about 100 nm to about 10 ⁇ , such as about 100 nm to about 1 ⁇ , or about 1 ⁇ to about 10 ⁇ .
  • the precursor thin film undergoes annealing to form the Cu-Zn-Sn-X thin film.
  • the substrate comprising the precursor thin film may be annealed in a furnace, such as a quartz tube furnace operable in the range of about 25 °C to about 1000 °C.
  • the quartz tube is connected to an inert gas supply at one end and a rotary pump at the other end.
  • the quartz tube may first be purged with an inert gas, before heating of the precursor films at a specified reaction temperature in the range of between about 300 °C to about 600 °C.
  • the heating rate may be up to 20 °C/min. Subsequently, the samples may be allowed to react for about 30 minutes in this inert gas environment. At high temperatures, the Cu, Zn, Sn and X atoms are able to diffuse between different particles, and for the formation of Cu-Zn-Sn-X thin film from binary sulfides precursors, the process is driven by a reduction in surface energy of nanoparticles.
  • X is sulfur
  • the quaternary Cu-Zn-Sn-S thin film may be obtained by heating the substrate comprising the precursor thin film in an inert gas atmosphere at a temperature of about 400 °C to 600 °C.
  • the thickness of the resultant Cu-Zn-Sn-X thin film may depend on the thickness of the precursor thin film formed. In various embodiments, the thickness of the Cu-Zn-Sn-X thin film is about 100 nm to about 10 ⁇ .
  • the average grain size of the Cu-Zn-Sn-X thin film, without selenization may range from about 20 nm to about 200 nm. In one embodiment, the average grain size of CZTS thin film is about 100 nm.
  • the molar ratio of the respective elements Cu, Zn and Sn in the prepared Cu-Zn-Sn-X thin film may differ slightly from that contained in the dispersion comprising Cu p X (1 ⁇ p 2), ZnX and SnX q (1 ⁇ q 2) nanoparticles. This difference may come about as a result of the heating process. Accordingly, the molar ratio of the resultant
  • Cu-Zn-Sn-X thin film may be varied or controlled by varying the amount of Cu p X (1 ⁇ p ⁇ 2),
  • the Cu-Zn-Sn-X thin film formed by a method of the invention may comprise
  • the Cu-Zn-Sn-X thin film consists essentially of
  • the resultant CZTS film may have a graded profile or structure along the thickness of the film.
  • the invention relates to a device comprising the Cu-Zn-Sn-X thin film, wherein X is S, Se or a mixture of S and Se, according to the second aspect.
  • the device is a solar cell.
  • the CZTS, CZTSe, or CZTSSe films formed may be used as a photovoltaic absorber materials in solar cells, in view that it is a direct band gap material with a band gap of about 1.5 eV and a very high absorption coefficient in the order of l0 4 cm Tl .
  • Example 1 Synthesis of Cu-, Zn- and Sn- Binary Sulfide Nanoparticles
  • CuS, ZnS and SnS 2 nanoparticles were synthesized through precipitate reactions in water between S 2" and Cu 2+ , Zn 2+ , Sn 4+ respectively, (n-hexadycyl) Cetyl trimethylammonium bromide (CTAB) was used as the capping ligand for all three types of nanoparticles.
  • CTAB cetyl trimethylammonium bromide
  • the synthesis experiments were performed at room temperature (25 °C) in ambient condition.
  • the as-synthesized CuS, ZnS and SnS 2 nanoparticles were washed thoroughly and undergo the same centrifugation process using water and ethanol. After the final wash, the collected CuS, ZnS and SnS 2 nanoparticles are re-suspended separately into ethanol to form respective CuS, ZnS and SnS 2 stock solutions.
  • CZTS precursor solution Prior to film deposition, CuS, ZnS and SnS 2 stock solutions are mixed together according to stoichiometry of CZTS under constant stirring, as described in Figure 2.
  • CZTS precursor solution is sprayed onto sodium lime glass (SLG) or Si substrate to form precursor film. After that, the CZTS precursor film is annealed to above 400 °C under Ar gas protection. There is obvious color change from dark green to black.
  • Example 3 CuS, ZnS and SnS 2 nanoparticles
  • Example 4 CZTS thin film
  • Figure 8 shows the (a) topographical and (b) cross-sectional scanning electron microscopy (SEM) images of as-prepared CZTS film.
  • CZTS shares very similar unit cell structure with ZnS (JCPDS 5-0566) and Cu 2 SnS 3 (JCPDS 1-089-4714).
  • ZnS JCPDS 5-0566)
  • Cu 2 SnS 3 JCPDS 1-089-4714
  • the three compounds have very similar XRD patterns.
  • STEM-EDS scanning transmission electron microscopy - energy dispersive X-ray spectroscopy
  • the TEM sample was prepared by re- dispersing CZTS nanocrystals film into ethanol and drop-casting the solution onto holey carbon-coated Ni-grid. It was found that all the four elements Cu, Zn, Sn and S were distributed in the inspected nanocrystals, which exclude the existence of ZnS and Cu 2 SnS 3 .
  • the average composition of various areas of CZTS thin film determined by EDS is Cu 1 .9 8 Zn 1 .o 2 Sni.o 4 S4.i8, close to stoichiometric ratio of 2:1 :1 :4.
  • Figure 10 displays the UV-vis- NIR absorption spectroscopy and the band gap is estimated to be 1.48 eV.
  • the band gap of CZTS can be controlled by varying the ratio of Cu 2 Sn and Zn cations, making it possible to tune the optical properties.
  • FIG. 11 shows the photoelectrochemical response of as prepared CZTS film coated on a fluorine-doped tin oxide (FTO) substrate. The measurement was performed with an AM 1.5 light source. The results reveal a good photoresponse of as prepared CZTS film with high stability.
  • FTO fluorine-doped tin oxide
  • Example 5 Advantages of Aqueous Solution Synthesized Binary Sulfides Nanoparticles Route
  • CZTS quaternary Cu 2 ZnSnS 4
  • the method according to the invention offers a low-cost fabrication method using for example, water as solvent for synthesis of binary metal sulfides nanoparticles, commonly available, inexpensive starting chemicals, and low cost solution deposition techniques.
  • water as solvent for synthesis of binary metal sulfides nanoparticles
  • common water-soluble metal compounds such as metal chlorides, sulfates, nitrates and acetates may be used.
  • the synthesis of binary metal sulfides nanoparticles is typically carried out at room temperature under ambient conditions, which is very user friendly.
  • Low-cost solution deposition techniques such as spray coating, printing, drop- casting and dip-coating may be used.
  • this fabrication method allows the replacement of costly vacuum deposition techniques including sputtering, and evaporation for the fabrication of CZTS, CZTSe, or CZTSSe.
  • This synthetic route is environmentally friendly as no toxic byproduct is produced.
  • composition of the film may be adjusted and controlled.
  • composition of the film may be adjusted and controlled easily. Furthermore, this synthetic route can be easily upscale for mass production.

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Abstract

A method for forming a copper-zinc-tin-X (Cu-Zn-Sn-X) thin film, wherein X is sulfur (S), selenium (Se), or a mixture of sulfur and selenium (S, Se), is provided. The method comprises a) forming a dispersion comprising CupX(1 ≤ p ≤ 2) nanoparticles, ZnX nanoparticles and SnXq(1 ≤ q ≤ 2) nanoparticles in a dispersing agent; b) depositing the dispersion on a substrate to form a precursor thin film; and c) heating the substrate comprising the precursor thin film in an inert atmosphere at a temperature in the range of about 300 °C to about 600 °C to form the Cu-Zn-Sn-X thin film. A Cu-Zn-Sn-X thin film formed by the method, and a device comprising the Cu-Zn-Sn-X thin film is also provided. Photoresponse data of a Cu-Zn-Sn-X thin film formed according to an embodiment is also provided.

Description

CU-ZN-SN-S/SE THIN FILM AND METHODS OF FORMING THE SAME
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application makes reference to and claims the benefit of priority of an application for "Aqueous Solution Synthetic Route to Cu2ZnSnS4 Thin Films" filed on July 25, 2011 , with the United States Patent and Trademark Office, and there duly assigned serial number 61/511 ,229. The content of said application filed on July 25, 2011, is incorporated herein by reference in its entirety for all purposes. TECHNICAL FIELD
[0002] The present invention relates to a method for forming a copper-zinc-tin-X (Cu-Zn- Sn-X) thin film, wherein X is sulfur (S), selenium (Se), or a mixture of sulfur and selenium (S, Se). The invention also relates to a Cu-Zn-Sn-X thin film formed by the inventive method, wherein X is S, Se, or (S, Se).
BACKGROUND
[0003] Presently, the photovoltaic market is dominated by crystalline silicon-based (Si- based) devices. The success of Si-based solar cells can be attributed to (i) mature technology adopted from microelectronics industry, (ii) an abundant supply of Si raw material, and (iii) highly viable conversion efficiency of more than 15 % for module production. Unfortunately, as an indirect band gap material, Si requires a thick absorber layer (in the order of 102 μηι), which results in raw material wastage, and limits the possibility of flexible solar cells.
[0004] In contrast thereto, thin film solar cells are formed from direct band gap materials having a thickness of a few micrometers or less. Such thin film solar cells may be fabricated using low-cost fabrication techniques, such as roll-to-roll processes and printing. Current state of the art thin film solar cells are based on cadmium telluride (CdTe) and copper indium gallium selenide (Cu(In,Ga)(S,Se)2) (CIGSSe) technologies. However, both technologies suffer from the following limitations: (i) for CdTe, the usage of toxic cadmium (Cd) results in environmental issue; (ii) for CIGSSe, the use of rare metals indium (In) and gallium (Ga) greatly limits the production capacity of such cells. Furthermore, the raw material costs are high. [0005] Cu2ZnSn(S,Se)4 (CZTSSe) is a promising alternative absorber material owing to its attractive optoelectronic properties, as well as abundance of its constituent elements Cu, Zn, Sn, S and Se. Methods used for preparing CZTS films include atom beam sputtering, radio frequency magnetron sputtering, hybrid sputtering, thermal evaporation, and sulfurization of electron beam-evaporated precursors. However, these methods have problems such as expensive precursors and complicated equipments. In addition, some of the methods are performed at high temperatures, which can result in inter-diffusion of the component elements, thereby degrading the quality of devices incorporating the films. Furthermore, these methods cannot be scaled up easily for mass production. In some cases, toxic byproducts are formed, which means that the processes are not environmentally friendly.
[0006] In view of the above, there is a need for an improved method to form Cu-Zn-Sn-S, Cu-Zn-Sn-Se, or Cu-Zn-Sn-(S,Se) thin films that alleviates at least some of the above- mentioned problems.
SUMMARY OF THE INVENTION
[0007] In a first aspect, the invention relates to a method for forming a copper- zinc-tin- X (Cu-Zn-Sn-X) thin film, wherein X is sulfur (S), selenium (Se), or a mixture of sulfur and selenium (S, Se). The method comprises
a) forming a dispersion comprising CupX (1 < p < 2) nanoparticles, ZnX nanoparticles, and SnXq (1 <q <2) nanoparticles in a dispersing agent; b) depositing the dispersion on a substrate to form a precursor thin film; and c) heating the substrate comprising the precursor thin film in an inert atmosphere at a temperature in the range of about 300 °C to about 600 °C to form the Cu-Zn-Sn-X thin film.
[0008] In a second aspect, the invention relates to a Cu-Zn-Sn-X thin film formed by a method according to the first aspect.
[0009] In a third aspect, the invention relates to a device comprising a Cu-Zn-Sn-X thin film according to the second aspect. In various embodiments, the device is a solar cell.
BRIEF DESCRIPTION OF THE DRAWINGS [0010] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:
[001 1] Figure 1 is a schematic diagram depicting the general scheme for a method for forming a Cu-Zn-Sn-X thin film, wherein X is S, Se, or a mixture of S and Se, according to the invention. In Figure 1A, CupX (1 <p <2) nanoparticles (NPs) 101, ZnX NPs 103, and SnXq (1 < q < 2) NPs 105 are individually suspended in a dispersing agent. In various embodiments, a capping ligand 107 is present. In Figure IB, the CupX (1 <p≤2) NPs 101, ZnX NPs 103, and SnXq (1 <q≤2) NPs 105 are mixed together to form a dispersion 110. The dispersion 110 is deposited on a substrate 109 to form a precursor thin film 120 as shown in Figure 1C. The substrate 109 comprising the precursor thin film 120 is heated in an inert atmosphere at a temperature in the range of about 300 °C to about 600 °C to form the Cu-Zn- Sn-X thin film 130 as shown in Figure ID.
[0012] Figure 2 is a schematic diagram depicting experiment flow of the preparation of a Cu-Zn-Sn-S (CZTS) film through CuS, ZnS and SnS2 nanoparticles according to an embodiment of the invention. In Figure 2A, CuS nanoparticles (NPs), ZnS NPs, and SnS2 NPs are individually suspended in a dispersing agent. In various embodiments, a capping ligand is present along with the respective NPs in solution. In Figure 2B, the CuS NPs, ZnS NPs, and SnS2 NPs are mixed together to form a dispersion. The dispersion is deposited on a substrate to form a precursor thin film as shown in Figure 2C. In Figure 2D, the substrate comprising the precursor thin film is heated in an inert atmosphere at a temperature of about 400 °C to 600 °C to form the Cu-Zn-Sn-S thin film.
[0013] Figure 3 is a graph showing X-ray diffraction (XRD) pattern of as-synthesized CuS nanoparticles, matches with CuS covellite (JCPDS 00-006-0464).
[0014] Figure 4 is a graph showing XRD pattern of as-synthesized ZnS nanoparticles, matches with ZnS sphalerite (JCPDS 00-005-0566).
[0015] Figure 5 is a graph showing XRD pattern of as-synthesized SnS2 nanoparticles, matches with SnS2 berndtite-2/ITT/RG (JCPDS 00-023-0677).
[0016] Figure 6 shows the TEM images of (a) CuS, (b) ZnS and (c) SnS2 nanoparticles. The scale bars in the images denote a length of 100 nm.
[0017] Figure 7 is a graph showing XRD pattern of the as-prepared CZTS thin film. [0018] Figure 8 is (a) the topographical, and (b) cross-sectional scanning electron microscope (SEM) image of CZTS thin film. The scale bar denotes a length of 100 ran.
[0019] Figure 9 is scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) elemental mapping of elements Cu, Zn, Sn and S.
[0020] Figure 10 is a graph showing UV-vis-NIR absorption spectrum of CZTS thin film on sodium lime glass. Inset shows the plot of (ohv)2 vs hv.
[0021] Figure 11 is the photoelectrochemical response of as prepared CZTS film. The measurement is performed at -300 mV vs Ag|AgCl in a 0.1 M europium nitrate (Eu(N03)3) aqueous electrolyte.
DETAILED DESCRIPTION OF THE INVENTION
[0022] In a first aspect, the present invention refers to a method for forming a copper-zinc- tin-X (Cu-Zn-Sn-X) thin film, wherein X is sulfur (S), selenium (Se), or a mixture of sulfur and selenium (S, Se).
[0023] Advantageously, the method of the present invention is a simple and low-cost method for the production of Cu-Zn-Sn-S, Cu-Zn-Sn-Se, and Cu-Zn-Sn-(S,Se) thin films. The use of readily available materials such as water and water-soluble metal salts including metal chlorides, sulfates, nitrates and acetates, coupled with low-cost solution deposition techniques such as spray coating, printing, drop-casting and dip-coating, translates into ease of fabrication of the films which may be easily scaled up for mass production. Furthermore, the method of the invention may be carried out at room temperature under ambient conditions which renders the process energy efficient. As no toxic byproduct is formed from the process, the synthetic route is environmentally friendly. By adjusting the molar ratio of the metal sulfide or metal selenide nanoparticles in the dispersion for example, the composition of the resulting film may be adjusted or controlled easily.
[0024] The Cu-Zn-Sn-X thin film formed by a method of the invention is also referred herein as a CZTX thin film or a CZTX film, where X is S, Se or SSe. Accordingly, the Cu- Zn-Sn-X thin film may be respectively termed as a CZTS, CZTSe, or CZTSSe film. In the context of the invention, the Cu-Zn-Sn-X thin film contains copper, zinc, tin and X, where X denotes sulfur, selenium, or a mixture of sulfur and selenium. In various embodiments, the CZTX thin film consists essentially of copper, zinc, thin and X, where X denotes sulfur, selenium, or a mixture of sulfur and selenium. In some embodiments, the thin film consists of copper, zinc, tin and X, where X denotes sulfur, selenium, or a mixture of sulfur and selenium. In embodiments in which X denotes sulfur or selenium or a mixture of sulfur and selenium, the thin film may also be described as a multi-component CZTX film, wherein term "multi-component" refers to a material that is composed of four or more different elements. In various embodiments of the above, X is sulfur or selenium or a mixture of sulfur and selenium.
[0025] The method comprises forming a dispersion comprising CupX (1 < p < 2) nanoparticles, ZnX nanoparticles and SnXq (1 <q <2) nanoparticles in a dispersing agent. Generally, each of the CupX nanoparticles, ZnX nanoparticles and SnXq nanoparticles are prepared individually, and subsequently mixed in together with a dispersing agent to form the dispersion. The CupX (1 <p 2) nanoparticles, ZnX nanoparticles and SnXq (1 <q <2) nanoparticles may each be prepared in an aqueous solution.
[0026] For example, the CupX (1 <p <2) nanoparticles may be prepared by reacting in an aqueous solution, a water-soluble salt of Cu and a water-soluble compound containing an anion of X. In one embodiment, p is 1, i.e. the CupX nanoparticles prepared are CuX nanoparticles. Examples of water-soluble salts of Cu that may be used include, but are not limited to, copper chloride, copper sulfate, copper nitrate, copper acetate, and mixture thereof.
[0027] Similarly, the ZnX nanoparticles may be prepared by reacting in an aqueous solution, a water-soluble salt of Zn and a water-soluble compound containing an anion of X. Examples of water-soluble salts of Zn that may be used include, but are not limited to, zinc chloride, zinc sulfate, zinc nitrate, zinc acetate, and mixture thereof.
[0028] The SnXq (1 <q <2) nanoparticles may also be prepared by reacting in an aqueous solution, a water-soluble salt of Sn and a water-soluble compound containing an anion of X. In one embodiment, q is 2, i.e. the SnXq nanoparticles prepared are SnX2 nanoparticles. Examples of water-soluble salts of Sn that may be used include, but are not limited to, tin chloride, tin sulfate, tin nitrate, tin acetate, and mixture thereof.
[0029] The reaction to form the nanoparticles may be carried out in the presence of a capping ligand. In various embodiments, the capping ligand comprises a water-soluble surfactant. Examples of the water-soluble surfactant that may be used include, but are not limited to dodecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, octadecyltrimethylammonium bromide, octadecyltrimethylammonium chloride, cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, cetylbenzyldimethylammonium chloride, cetyltriethylammmonium bromide, and mixture thereof. In various embodiments, the water-soluble surfactant comprises or consists of the above-mentioned compounds. In some embodiment, the water-soluble surfactant comprises cetyltrimethylammonium bromide (CTAB). In one embodiment, the water-soluble surfactant is CTAB.
[0030] In embodiments where X denotes sulfur, for example, where a quaternary Cu-Zn- Sn-S thin film is formed, the water-soluble compound containing an anion of S comprises or consists of a compound selected from the group consisting of lithium sulfide, sodium sulfide, potassium sulfide, rubidium sulfide, cesium sulfide, ammonium sulfide, lithium sulfide monohydrate, sodium sulfide nonahydrate, potassium sulfide nonahydrate, rubidium sulfide tetrahydrate, cesium sulfide tetrahydrate, and a mixture thereof. For example, the water- soluble compound containing an anion of S may comprise sodium sulfide nonahydrate or potassium sulfide. In some embodiments, the water-soluble compound containing an anion of S consists of sodium sulfide nonahydrate or potassium sulfide.
[0031 ] In embodiments where X denotes selenide, for example, where a quaternary Cu- Zn-Sn-Se thin film is formed, the water-soluble compound containing an anion of Se comprises or consists of a compound selected from the group consisting of lithium selenide, sodium selenide, potassium selenide, rubidium selenide, cesium selenide, ammonium selenide, and a mixture thereof. For example, the water-soluble compound containing an anion of Se may comprise sodium selenide or potassium selenide.
[0032] In embodiments where X denotes a mixture of sulfide and selenide, for example, where a Cu-Zn-Sn-(S, Se) thin film is formed, the water-soluble compound containing an anion of Se and S comprises or consists of a compound selected from the group consisting of lithium selenide sulfide, sodium selenide sulfide, potassium selenide sulfide, rubidium selenide sulfide, cesium selenide sulfide, ammonium selenide sulfide, and a mixture thereof. For example, the water-soluble compound containing an anion of Se and S may comprise sodium selenide sulfide or potassium selenide sulfide.
[0033] A Cu-Zn-Sn-(S, Se) thin film may also be formed using a mixture of water-soluble compounds containing an anion of Se or S. For example, lithium selenide may be mixed with sodium sulfide to form the mixture of water-soluble compounds. As another example, sodium selenide may be mixed with sodium sulfide to form the mixture. [0034] As mentioned above, the CupX (1 <p <2) nanoparticles, ZnX nanoparticles and SnXq (1 ≤q ≤2) nanoparticles may be prepared in an aqueous solution. Typically the aqueous solution consists essentially of water. In various embodiments, for example, to prepare a Cu-Zn-Sn-X thin film of high purity, the aqueous solution consists essentially of deionized water or distilled water.
[0035] Preparation of the nanoparticles may be carried out at any suitable temperature, such as between about 20 °C to about 50 °C, between about 20 °C to about 40 °C or between about 25 °C to about 30 °C. Generally, preparation of the nanoparticles is carried out at about 25 °C. In various embodiments, no external heating is required for preparing the nanoparticles, i.e. the process may be carried out at room temperature.
[0036] For example, in embodiments in which CuS nanoparticles are formed, a capping ligand such as CTAB may be added to deionized water to form the aqueous solution. Subsequently, a water-soluble salt such as copper chloride and a water-soluble compound containing an anion of S such as sodium sulfide nonahydrate may be added to the aqueous solution. As mentioned above, the reaction between copper chloride and sodium sulfide nonahydrate may be carried out at room temperature and under ambient conditions. In various embodiments, the reaction is carried out under constant stirring. As a result, CuS nanoparticles are formed.
[0037] The size of the nanoparticle depends on factors such as concentration of the reagents used, time for reaction, temperature used and stirring speed. The size of the nanoparticles may be characterized by their mean diameter. The term "diameter" as used herein refers to the maximal length of a straight line segment passing through the center of a figure and terminating at the periphery. The term "mean diameter" refers to an average diameter of the nanoparticles, and may be calculated by dividing the sum of the diameter of each nanoparticle by the total number of nanoparticles. Although the term "diameter" is used normally to refer to the maximal length of a line segment passing through the centre and connecting two points on the periphery of a nanosphere, it is also used herein to refer to the maximal length of a line segment passing through the centre and connecting two points on the periphery of nanoparticles having other shapes, such as a nanocube or a nanotetrahedra, or an irregular shape. The mean diameter of the nanoparticles may range from about 20 nm to about 50 nm. This is an ideal size range wherein the nanoparticles are small enough to allow for homogeneous mixing, and the reduction in surface energy provides the driving force for the solid state reaction between neighboring particles during annealing. In various embodiments, the nanoparticles are essentially monodisperse. Generally, smaller sized and/or monodisperse nanoparticles are desirable as they result in formation of a more homogeneous CZTX film.
[0038] Prior to forming the dispersion comprising CupX (1 <p 2) nanoparticles, ZnX nanoparticles and SnXq (1 < q < 2) nanoparticles, the nanoparticles, which may be individually prepared, may be subjected to an optional washing step. For example, the CupX (1 p <2) nanoparticles, ZnX nanoparticles and SnXq (1 <q ≤2) nanoparticles may be washed in a washing reagent. In various embodiments, the washing reagent comprises an alcohol. In some embodiments, the washing reagent consists of an alcohol. Examples of alcohols that may be used include, but are not limited to, methanol, ethanol, propanol, isopropyl alcohol, butanol, amyl alcohol, hexanol, 2-ethoxyethanol, benzyl alcohol, optionally substituted cyclopentanol, optionally substituted cyclohexanol, optionally substituted cycloheptanol, glycerol, and mixtures thereof.
[0039] The method of the invention comprises dispersing the nanoparticles in a dispersing agent or dispersant. The dispersing agent may include, but not limited to, water, an alcohol or a mixture of alcohols. Types of water that may be used have already been described herein and include, but are not limited to deionized water and distilled water. Examples of alcohols that may be used have already been described herein and include, but are not limited to methanol, ethanol, 1 -propanol, iso-propanol, 1 -butanol, 2-butanol, tert-butanol, and glycol. In embodiments where the nanoparticles are washed in a washing reagent prior to forming the dispersion, the washing reagent and the dispersing agent may comprise, or be formed from the same alcohol. For example, the washing reagent and the dispersing agent may both comprise ethanol. The washing reagent and the dispersing agent, may alternatively, comprise a different alcohol. For example, the washing reagent may comprise ethanol, and the dispersing agent may comprise isopropyl alcohol.
[0040] In various embodiments, the alcohol that is used to form the washing reagent and/or the dispersing agent comprises ethanol, isopropyl alcohol, or mixtures thereof. In some embodiments, the washing reagent and/or the dispersing agent consists of ethanol, isopropyl alcohol, or mixtures thereof.
[0041] The dispersion containing the nanoparticles may be essentially homogeneous. The term "essentially homogeneous" as used herein refers to a condition or state in which nanoparticles that are dispersed or suspended in a liquid are uniformly or essentially uniformly distributed throughout the liquid.
[0042] As mentioned herein, the method for forming a copper-zinc-tin-sulfur thin film, a copper-zinc-tin-selenium thin film or a copper-zinc-tin-sulfur-selenium thin film according to the invention is advantageous in that the composition of the film may be adjusted or controlled easily by adjusting the molar ratio of the metal sulfide or metal selenide nanoparticles in the dispersion. Depending on the type of application, different composition of the film may be formed. This translates into different molar ratios of the CupX (1 <p <2), ZnX and SnXq (1 <q <2) nanoparticles used in the dispersion.
[0043] The molar ratio of the respective elements Cu, Zn and Sn in the dispersion comprising CupX (1 p <2), ZnX and SnXq (1 <q ≤2) nanoparticles may differ slightly from that of the prepared Cu-Zn-Sn-X thin film. Accordingly, the amount of CupX (1 <p <2), ZnX and SnXq (1 q <2) nanoparticles in the dispersion used may be varied to obtain a desired molar ratio of the prepared Cu-Zn-Sn-X thin film. In various embodiments, the molar ratio of Cu:(Zn+Sn) in the Cu-Zn-Sn-X thin film is in the range of about 0.75:1 to about 1.25: 1, such as about 0.75: 1 to about 1 : 1 or about 1 : 1 to about 1.25: 1. The molar ratio of Zn:Sn in the Cu-Zn-Sn-X thin film may be in the range of about 1.5: 1 to about 0.75:1, such as about 1.5: 1 to about 1 :1, about 1.5:1 to about 1.25: 1, or about 1 : 1 to about 0.75: 1. In various embodiments, the molar ratio of Cu:Zn:Sn in the Cu-Zn-Sn-X thin film is about 2.025: 1.25: 1. In one embodiment, the molar ratio of Cu:Zn:Sn in the Cu-Zn-Sn-X thin film is about 2: 1 :1.
[0044] The dispersion containing the nanoparticles may be deposited on a substrate to form a precursor thin film. Any suitable thin film forming method may be used. For example, the dispersion may be deposited on the substrate by spray coating, inkjet printing, drop- casting, dip-coating, painting, or spin coating. In one embodiment, the dispersion is deposited on the substrate by spray coating.
[0045] By changing the composition of the dispersion and depositing the dispersion layer by layer on the substrate during each deposition cycle, a graded precursor thin film may be formed. The term "graded" refers to a change in composition of the thin film across the thickness of the film. The graded precursor thin film may be formed from a stack of layers, with each layer having a composition that is different from a layer that is adjacent to it. In case the layer is a middle layer, it may have a composition that is different from either layer that is adjacent to it. In the context of the invention, the term "graded" also encompasses thin film structures having one or more thickness portions having a different grading profile, and one or more other thickness portions having a substantially constant composition, i.e. the change in composition of the thin film is not gradual or in a step wise fashion across the thickness of the film. In line with the above, the thickness of each layer comprised in the graded precursor thin film may be the same or different. The thickness of each layer may be controlled by varying, for example, the deposition time of the dispersion. In some embodiments, the graded precursor thin film comprises a "A-B-A-B" configuration, where A and B respectively denotes two different compositions. In other embodiments, the graded precursor thin film comprises a "A-A-B-B" or "A-B-A-A" configuration.
[0046] Grading of the precursor thin film in turn results in formation of a graded CZTX thin film. The ability to obtain a graded CZTX thin film may be important for certain applications, for example, to vary the bandgap and/or to tune the optical properties of the CZTX thin film for better light absorption.
[0047] Depending on the type of application, different substrates may be used. For example, suitable substrates that may be used for forming the CZTX thin film include glass, silicon, metal foil or ceramic. In various embodiments, the substrate comprises glass or silicon. In some embodiments, the substrate consists of glass or silicon.
[0048] The precursor thin film that is formed by a method according to the invention may be of any suitable thickness. In some embodiments the thickness of the precursor thin film may be from about 100 nm to about 10 μηι, such as about 100 nm to about 1 μη , or about 1 μιη to about 10 μπι.
[0049] The method of the invention includes heating the substrate comprising the precursor thin film in an inert atmosphere at a temperature in the range of about 300 °C to about 600 °C to form the Cu-Zn-Sn-X thin film. The term "inert atmosphere" as used herein refers to an environment that is at least substantially oxygen-free and/or moisture-free. In various embodiments, the term "inert atmosphere" refers to an environment having less than 1 % oxygen. The inert atmosphere may comprise an inert gas. Examples of inert gas include, but are not limited to, nitrogen, helium, hydrogen, argon, xenon, krypton, radon, and mixtures thereof. In various embodiments, the inert gas environment comprises argon gas.
[0050] In heating or annealing the substrate comprising the precursor thin film in an inert gas atmosphere at a temperature of about 300 °C to 600 °C, the precursor thin film undergoes annealing to form the Cu-Zn-Sn-X thin film. The substrate comprising the precursor thin film may be annealed in a furnace, such as a quartz tube furnace operable in the range of about 25 °C to about 1000 °C. In various embodiments, the quartz tube is connected to an inert gas supply at one end and a rotary pump at the other end. The quartz tube may first be purged with an inert gas, before heating of the precursor films at a specified reaction temperature in the range of between about 300 °C to about 600 °C. The heating rate may be up to 20 °C/min. Subsequently, the samples may be allowed to react for about 30 minutes in this inert gas environment. At high temperatures, the Cu, Zn, Sn and X atoms are able to diffuse between different particles, and for the formation of Cu-Zn-Sn-X thin film from binary sulfides precursors, the process is driven by a reduction in surface energy of nanoparticles. In embodiments in which X is sulfur, the quaternary Cu-Zn-Sn-S thin film may be obtained by heating the substrate comprising the precursor thin film in an inert gas atmosphere at a temperature of about 400 °C to 600 °C.
[0051] The thickness of the resultant Cu-Zn-Sn-X thin film may depend on the thickness of the precursor thin film formed. In various embodiments, the thickness of the Cu-Zn-Sn-X thin film is about 100 nm to about 10 μπι. The average grain size of the Cu-Zn-Sn-X thin film, without selenization may range from about 20 nm to about 200 nm. In one embodiment, the average grain size of CZTS thin film is about 100 nm.
[0052] As mentioned above, the molar ratio of the respective elements Cu, Zn and Sn in the prepared Cu-Zn-Sn-X thin film may differ slightly from that contained in the dispersion comprising CupX (1 ≤p 2), ZnX and SnXq (1 <q 2) nanoparticles. This difference may come about as a result of the heating process. Accordingly, the molar ratio of the resultant
Cu-Zn-Sn-X thin film may be varied or controlled by varying the amount of CupX (1 <p <2),
ZnX and SnXq (1 <q≤2) nanoparticles in the dispersion used.
[0053] The Cu-Zn-Sn-X thin film formed by a method of the invention may comprise
Cu2ZnSnS4. In various embodiments, the Cu-Zn-Sn-X thin film consists essentially of
Cu2ZnSnS4.
[0054] In a second aspect, the invention relates to a Cu-Zn-Sn-X thin film, wherein X is S, Se, or a mixture of S and Se, formed by a method according to the first aspect.
[0055] As mentioned above, by changing the composition of the dispersion and depositing the dispersion layer by layer on the substrate during each deposition cycle, the resultant CZTS film may have a graded profile or structure along the thickness of the film. [0056] In a third aspect, the invention relates to a device comprising the Cu-Zn-Sn-X thin film, wherein X is S, Se or a mixture of S and Se, according to the second aspect. In various embodiments, the device is a solar cell. The CZTS, CZTSe, or CZTSSe films formed may be used as a photovoltaic absorber materials in solar cells, in view that it is a direct band gap material with a band gap of about 1.5 eV and a very high absorption coefficient in the order of l04 cmTl.
[0057] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.
[0058] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.
[0059] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.
EXPERIMENTAL SECTION
[0060] Example 1: Synthesis of Cu-, Zn- and Sn- Binary Sulfide Nanoparticles
[0061] CuS, ZnS and SnS2 nanoparticles were synthesized through precipitate reactions in water between S2" and Cu2+, Zn2+, Sn4+ respectively, (n-hexadycyl) Cetyl trimethylammonium bromide (CTAB) was used as the capping ligand for all three types of nanoparticles. The synthesis experiments were performed at room temperature (25 °C) in ambient condition. The as-synthesized CuS, ZnS and SnS2 nanoparticles were washed thoroughly and undergo the same centrifugation process using water and ethanol. After the final wash, the collected CuS, ZnS and SnS2 nanoparticles are re-suspended separately into ethanol to form respective CuS, ZnS and SnS2 stock solutions.
[0062] Prior to film deposition, CuS, ZnS and SnS2 stock solutions are mixed together according to stoichiometry of CZTS under constant stirring, as described in Figure 2. The solution containing homogeneous inter-mixed CuS, ZnS and SnS2 nanoparticles is used as CZTS precursor solution. In the following step, CZTS precursor solution is sprayed onto sodium lime glass (SLG) or Si substrate to form precursor film. After that, the CZTS precursor film is annealed to above 400 °C under Ar gas protection. There is obvious color change from dark green to black.
[0063] Example 2: Characterization
[0064] The X-ray diffraction (XRD) patterns were obtained on Bruker D8 Advance equipped with Cu Ka radiation (λ = 1.54 A). SEM images were acquired on a JOEL JSM 7600F field emission scanning electron microscope (FESEM) at an accelerating voltage of 5 kV. The FESEM was equipped with an Oxford X-MAX EDS detector, which was used to estimate the atomic ratio of CZTS thin film. The TEM analysis was performed using JOEL JSM - 21 OOF equipped with ED AX EDS detector. The voltage used for imaging and elemental mapping was 200 kV. The UV-vis-NIR absorption spectrum was measured using Shimadzu UV-3600 spectrometer.
[0065] Example 3: CuS, ZnS and SnS2 nanoparticles
[0066] The as-prepared CuS, ZnS and SnS2 nanoparticles were investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM) in order to study the phase and morphology of each binary sulfide material. The XRD patterns of CuS, ZnS and SnS2 nanoparticles are listed in Figures 3 to 5 respectively. It is found that CuS covellite, ZnS sphalerite and SnS2 berndtite-2/ITT/RG have been successfully synthesized. Figure 6a-c shows the TEM images of each binary sulfide material.
[0067] Example 4: CZTS thin film
[0068] Figure 7 shows the X-ray diffraction (XRD) pattern of as-prepared thin film. It matches with kesterite CZTS database (JCPDS 26-575 and ICSD 171983). Rietveld refinement was performed to obtain accurate lattice parameters (a = b = c = 5.4336 ± 0.0006A and c = 10.827 ± 0.002 A).
[0069] Figure 8 shows the (a) topographical and (b) cross-sectional scanning electron microscopy (SEM) images of as-prepared CZTS film. CZTS shares very similar unit cell structure with ZnS (JCPDS 5-0566) and Cu2SnS3 (JCPDS 1-089-4714). As a result, the three compounds have very similar XRD patterns. In order to rule out ZnS and Cu2SnS3, scanning transmission electron microscopy - energy dispersive X-ray spectroscopy (STEM-EDS) elemental mapping were performed (Figure 9). The TEM sample was prepared by re- dispersing CZTS nanocrystals film into ethanol and drop-casting the solution onto holey carbon-coated Ni-grid. It was found that all the four elements Cu, Zn, Sn and S were distributed in the inspected nanocrystals, which exclude the existence of ZnS and Cu2SnS3.
[0070] The average composition of various areas of CZTS thin film determined by EDS is Cu1.98Zn1.o2Sni.o4S4.i8, close to stoichiometric ratio of 2:1 :1 :4. Figure 10 displays the UV-vis- NIR absorption spectroscopy and the band gap is estimated to be 1.48 eV. Using this method the control and variation of composition of Cu, Zn and Sn is easily carried out by simply adjusting the amount of CuS, ZnS and SnS2 added. The band gap of CZTS can be controlled by varying the ratio of Cu2Sn and Zn cations, making it possible to tune the optical properties. It would also be convenient to produce composition graded film structures (meaning the composition of CZTS film changed gradually from bottom to the top of film) for better light absorption. This could be achieved by changing the composition of the precursors during each spray cycle. Figure 11 shows the photoelectrochemical response of as prepared CZTS film coated on a fluorine-doped tin oxide (FTO) substrate. The measurement was performed with an AM 1.5 light source. The results reveal a good photoresponse of as prepared CZTS film with high stability.
[0071] Example 5: Advantages of Aqueous Solution Synthesized Binary Sulfides Nanoparticles Route
[0072] A new synthetic route for quaternary Cu2ZnSnS4 (CZTS) thin films from binary CuS, ZnS and SnS2 nanoparticles synthesized in aqueous solution has been demonstrated. The method according to the invention offers a low-cost fabrication method using for example, water as solvent for synthesis of binary metal sulfides nanoparticles, commonly available, inexpensive starting chemicals, and low cost solution deposition techniques. For example, common water-soluble metal compounds such as metal chlorides, sulfates, nitrates and acetates may be used. Furthermore, the synthesis of binary metal sulfides nanoparticles is typically carried out at room temperature under ambient conditions, which is very user friendly. Low-cost solution deposition techniques such as spray coating, printing, drop- casting and dip-coating may be used. Advantageously, this fabrication method allows the replacement of costly vacuum deposition techniques including sputtering, and evaporation for the fabrication of CZTS, CZTSe, or CZTSSe. This synthetic route is environmentally friendly as no toxic byproduct is produced.
[0073] Another advantage of a method according to the invention relates to the ease at which composition of the film may be adjusted and controlled. By adjusting the molar ratio of different binary metal sulfides, composition of the film may be adjusted and controlled easily. Furthermore, this synthetic route can be easily upscale for mass production.

Claims

A method for forming a copper-zinc-tin-X (Cu-Zn-Sn-X) thin film, wherein X is sulfur, selenium, or a mixture of sulfur and selenium, the method comprising a) forming a dispersion comprising CupX (1 < p ≤ 2) nanoparticles, ZnX nanoparticles and SnXq (1 q 2) nanoparticles in a dispersing agent;
b) depositing the dispersion on a substrate to form a precursor thin film; and c) heating the substrate comprising the precursor thin film in an inert atmosphere at a temperature in the range of about 300 °C to about 600 °C to form the Cu-Zn-Sn-X thin film.
The method according to claim 1, wherein the molar ratio of Cu:(Zn+Sn) in the Cu- Zn-Sn-X thin film is in the range of about 0.75: 1 to about 1.25: 1.
The method according to claim 1 or 2, wherein the molar ratio of Zn:Sn in the Cu-Zn- Sn-X thin film is in the range of about 1.5: 1 to about 0.75: 1.
The method according to any one of claims 1 to 3, wherein the molar ratio of Cu:Zn:Sn in the Cu-Zn-Sn-X thin film is in the range of about 2.5: 1.5: 1 to about 1.9:0.9:1.
The method according to claim 4, wherein the molar ratio of Cu:Zn:Sn in the Cu-Zn- Sn-X thin film is about 2.025: 1.25: 1.
The method according to any one of claims 1 to 5, wherein the nanoparticles are prepared in an aqueous solution.
The method according to claim 6, wherein the CupX (1 ≤p ≤2) nanoparticles are prepared by reacting in an aqueous solution, a water-soluble salt of Cu and a water- soluble compound containing an anion of X. The method according to claim 7, wherein the water-soluble salt of Cu is a member selected from the group consisting of copper chloride, copper sulfate, copper nitrate, copper acetate, and mixture thereof.
The method according to any one of claims 6 to 8, wherein the ZnX nanoparticles are prepared by reacting in an aqueous solution, a water-soluble salt of Zn and a water- soluble compound containing an anion of X.
The method according to claim 9, wherein the water-soluble salt of Zn is a member selected from the group consisting of zinc chloride, zinc sulfate, zinc nitrate, zinc acetate, and mixture thereof.
The method according to any one of claims 6 to 10, wherein the SnXq (1 <q <2) nanoparticles are prepared by reacting in an aqueous solution, a water-soluble salt of Sn and a water-soluble compound containing an anion of X.
The method according to claim 1 1 , wherein the water-soluble salt of Sn is a member selected from the group consisting of tin chloride, tin sulfate, tin nitrate, tin acetate, and mixture thereof.
The method according to any one of claims 6 to 12, wherein the reaction is carried out in the presence of a capping ligand for the nanoparticles.
The method according to claim 13, wherein the capping ligand comprises a water- soluble surfactant.
The method according to claim 14, wherein the water-soluble surfactant comprises or consists of a compound selected from the group consisting of dodecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, octadecyltrimethylammonium bromide, octadecyltrimethylammonium chloride, cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, cetylbenzyldimethylammonium chloride, cetyltriethylammmonium bromide, and mixture thereof.
The method according to claim 15, wherein the water-soluble surfactant comprises or consists of cetyltrimethylammonium bromide.
The method according to any one of claims 1 to 16, wherein X is S.
The method according to claim 17, wherein the water-soluble compound containing an anion of S comprises or consists of a compound selected from the group consisting of lithium sulfide, sodium sulfide, potassium sulfide, rubidium sulfide, cesium sulfide, ammonium sulfide, lithium sulfide monohydrate, sodium sulfide nonahydrate, potassium sulfide nonahydrate, rubidium sulfide tetrahydrate, cesium sulfide tetrahydrate, and a mixture thereof.
The method according to claim 17 or 18, wherein the water-soluble compound containing an anion of S comprises or consists of sodium sulfide nonahydrate or potassium sulfide.
The method according to any one of claims 6 to 19, wherein the aqueous solution consists essentially of deionized water or distilled water.
The method according to any one of claims 6 to 20, wherein preparation of the nanoparticles is carried out at about 25 °C.
The method according to any one of claims 6 to 21, further comprising washing nanoparticles in a washing reagent prior to step a).
23. The method according to claim 22, wherein the washing reagent comprises or consists of an alcohol.
24. The method according to any one of claims 1 to 23, wherein the dispersing agent comprises or consists of water, an alcohol, or a mixture of alcohols.
25. The method according to claim 24, wherein the water is deionized water or distilled water.
26. The method according to claim 23 or 24, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, isopropyl alcohol, butanol, amyl alcohol, hexanol, 2-ethoxyethanol, benzyl alcohol, optionally substituted cyclopentanol, optionally substituted cyclohexanol, optionally substituted cycloheptanol, glycerol, and mixtures thereof.
27. The method according to any one of claims 23 to 26, wherein the alcohol comprises or consists of ethanol, isopropyl alcohol, or mixtures thereof.
28. The method according to any one of claims 1 to 27, wherein the dispersion is essentially homogeneous.
29. The method according to any one of claims 1 to 28, wherein the dispersion is deposited on the substrate by spray coating, inkjet printing, drop-casting, dip-coating, sputtering, painting or spin coating.
30. The method according to claim 29, wherein the dispersion is deposited on the substrate by spray coating.
31. The method according to any one of claims 1 to 30, wherein the dispersion is deposited in a layer by layer fashion on the substrate to form a graded precursor thin film.
32. The method according to any one of claims 1 to 31, wherein the substrate comprises a material selected from the group consisting of glass, silicon, metal foil, and ceramic.
33. The method according to claim 32, wherein the substrate comprises or consists of glass or silicon.
34. The method according to any one of claims 1 to 33, wherein the thickness of the precursor thin film is about 100 nm to about 10 μιη.
35. The method according to any one of claims 1 to 34, wherein the Cu-Zn-Sn-X thin film has an average grain size of about 20 nm to about 200 nm.
36. The method according to any one of claims 1 to 35, wherein the thickness of the Cu- Zn-Sn-X thin film is about 100 nm to about 10 μη .
37. The method according to any one of claims 1 to 36, wherein the Cu-Zn-Sn-X thin film comprises or consists essentially of Cu2ZnSnS4.
-
38. A Cu-Zn-Sn-X thin film, wherein X is S, Se, or a mixture of S and Se, formed by a method according to any one of claims 1 to 37.
39. The Cu-Zn-Sn-X thin film of claim 38, wherein the film is a graded Cu-Zn-Sn-X thin film.
40. A device comprising the Cu-Zn-Sn-X thin film according to claim 38 or 39.
41. The device of claim 40, wherein the device is a solar cell.
PCT/SG2012/000270 2011-07-25 2012-07-25 Cu-zn-sn-s/se thin film and methods of forming the same WO2013015745A1 (en)

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