WO2020124160A1 - Cztse precursor inks and methods for preparing czts/se thin films and czts/se-based devices - Google Patents

Abstract

The present disclosure relates to compositions comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal-chalcogenide stabilizing agent, wherein the nanoparticles are dispersible in a polar solvent. More particularly, the disclosure relates to compositions of CZTSe nanoparticles. This disclosure provides processes for manufacturing these compositions. The disclosure also provides coated substrates, thin films and devices comprising the compositions, optionally including CZTS nanoparticles, and processes for manufacturing the same.

Description

CZTSe precursor inks and methods for preparing CZTS/Se thin films and

CZTS/Se-based devices

Cross reference to related applications

This application claims priority from Australian patent application 2018904917, the contents of which are incorporated herein by reference for all purposes. The contents of the applicant’s co-pending Australian patent application 2018282493 filed on the same date as Australian patent application 2018904917 and entitled “CZTS precursor inks and methods for preparing CZTS thin films and CZTS-based devices” are incorporated herein by reference for all purposes.

Field of the invention

The disclosure relates to quaternary metal chalcogenide nanoparticles that can be used as quaternary metal chalcogenide precursor inks and processes for manufacturing these inks. The disclosure also relates to coated substrates comprising quaternary metal chalcogenide nanoparticles and provides processes for manufacturing these coated substrates. This disclosure also relates to compositions of quaternary metal chalcogenide thin films and devices comprising such films, and processes for manufacturing the same.

Background of the invention

Chalcogenide photovoltaic (PV) materials have garnered growing interest over the years as robust and inexpensive inorganic alternatives to Si-based PV. Cu2ZnSnSe4 (CZTSe), Cu2ZnSnS4 (CZTS), and CZTSSe kesterites are promising absorber materials due to their elemental abundance, environmental benignity, high absorption coefficients exceeding 10⁴ cm^-1, p-type conductivity, and ideal direct band gaps of 1 .0 eV, 1 .5 eV and 1 .0-1 .5 eV, respectively. Of these, CZTSe and CZTSSe PV devices show the most promising photo-conversion efficiencies (PCE) of 1 1 .6% and 12.6%.

Various methods for the fabrication of CZTSSe thin films have been developed, including quinary sputtering and CZTSSe nanoparticle deposition, most commonly involving a two-step process of depositing pure CZTS or CZTSe films followed by selenization and/or sulfurization at high temperature under toxic atmospheres to form the mixed CZTSSe phase. The deposition of CZTS and CZTSe films can be achieved by the decomposition of molten salts, reactive sputtering, electroplating, vapor deposition, precursor solution deposition, and nanoparticle ink sintering. Of these, nanoparticle routes are among the most promising due to their potential to be inexpensive and scalable, while maintaining superior phase and compositional control when compared to non-solution based approaches. CZTSe nanoparticle syntheses typically involve the use of toxic solvents such as hydrazine, or hot injection reactions in non-polar solvents that require extensive post-processing. While “greener” CZTSe nanoparticle syntheses do exist, these typically do not have precise elemental ratio control and use bulky aliphatic ligands to stabilize nanoparticles via steric repulsion, which leaves organic impurities in the resultant film, even after thermal annealing, hindering device performance.

Selenization processes to date use temperatures of -500 ^QC, which may cause a multitude of problems selected from: (i) decomposition of CZTSSe into secondary phases and the potential loss of SnSe/SnS species if careful control of the atmosphere is not maintained; (ii) inefficient use of a large excess of Se, the rarest element in the composition, of which there is only 0.05 ppm in the earth’s crust; (iii) difficulty in controlling the exact S:Se ratio in the film and S:Se gradient with depth; (iv) formation of a thick MoSe2 layer of several hundred nanometers at the electrode interface that increases resistance and recombination; (v) increased cationic disorder, due to similar atomic radii, resulting in increased band tailing and trap states; and/or (vi) formation of toxic gases which makes scalability difficult and costly.

It would be advantageous to synthesize CZTSe nanoparticles that provide stoichiometric compositional control and phase purity using a low toxicity solvent, while also yielding negligible residual carbon impurities following thermal annealing. It would also be advantageous to provide a low temperature and scalable alternative to current selenization processes which offers homogeneous and stoichiometric control of the S:Se ratio in the film.

Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

Summary of the invention

In one aspect there is provided a composition comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal-chalcogenide stabilizing agent, optionally including a reducing agent, wherein the nanoparticles are dispersible in a polar solvent. Preferably, the quaternary metal chalcogenide nanoparticles are copper zinc tin selenide nanoparticles.

The composition comprising quaternary metal chalcogenide nanoparticles dispersed in a polar solvent can be used as a quaternary metal chalcogenide precursor ink.

In another aspect there is provided processes for manufacturing a composition comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal-chalcogenide stabilizing agent, optionally including a reducing agent, wherein the nanoparticles are dispersible in a polar solvent.

In another aspect there is provided coated substrates comprising a substrate and a coating, wherein the coating comprises one or more layers comprising the quaternary metal chalcogenide precursor inks. Preferably the quaternary metal chalcogenide precursor inks comprise CZTSe nanoparticles or mixtures of CZTS nanoparticles and CZTSe nanoparticles.

In another aspect there is provided processes for manufacturing coated substrates comprising a substrate and a coating, wherein the coating comprises the quaternary metal chalcogenide precursor inks.

In another aspect there is provided processes for manufacturing quaternary metal chalcogenide thin films using the quaternary metal chalcogenide precursor inks. The quaternary metal chalcogenide films can be used as absorbers in thin-film photovoltaic cells, gas sensors, photodetectors and/or photolytic systems.

As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings

Figure 1. a) Flydrothermal reaction of metallic tin and selenium to form Na6Sn2Se7-16Fl20 b) Redox reaction of metallic selenium to form Na2Se and then the subsequent reaction with SnSe2 to form Na4Sn2Se6- 13Fl20 c) A simplified process diagram for aqueous CZTSe ink preparation.

Figure 2. In-situ time resolved merged UV-Vis-NIR spectra of a 50:25:25 (Cu:Zn:Sn %) CZTSe nanoparticle ink with a concentration of 0.5 g/L. The FIR2000 detector signal and Flame NIR detector signals are merged at 950 nm, highlighted by the black vertical line. The purple curve shows the baseline of Milli-Q water and thiourea solution. The red curves highlight the addition of the SnSe-MCC precursor solution. Adding the zinc precursor solution exhibited no absorbance change from the equilibrated red curve at 1 s and as such spectra following the addition of Zn(NC>3) were omitted. The synthesis and growth of the CZTSe nanoparticles are shown by the blue curves over the first 639 ms after the addition of the 1 .25 M Cu(N03)2 solution with 71 ms resolution. The green curves highlight the stability of the resultant nanoparticle dispersion over the next 3 days on a variety of timescales.

Figure 3. a) TEM image of CZTSe nanoparticles with an elemental ratio of 2:1 :1 Cu:Zn:Sn produced at an original concentration of 10 g/L. Left Inset: histogram indicating a mean size of -25 nm b) High resolution TEM image showing the presence of nano-crystalline domains within the nanoparticle with lattice spacings of 0.34 nm (green) c) FTIR spectra showing the functional groups present in the dried CZTSe nanoparticle ink (black), twice washed and dried CZTSe nanoparticles (blue), and twice washed and annealed at 250 °C CZTSe nanoparticles (red).

Figure 4. a) TGA of a CZTSe nanoparticle powder with an elemental ratio of Cu:Zn:Sn 2:1 :1 , indicating most mass loss occurs at -187 °C. b) The corresponding FTIR spectra vs. temperature with color indicating the absorption (%) collected in tandem with the TGA in order to analyze the source of mass loss. Each set of peaks has been labelled with the corresponding functional groups and its source in the nanoparticle ink.

Figure 5. a) Experimental elemental ratios obtained by ICP-MS analysis (red dots) for CZTSe samples prepared for PXRD and Raman spectroscopy measurements were overlaid onto the corresponding theoretical elemental ratios (black dots). Variations between theory and experiment are within pipette and measurement error b) PXRD patterns for the 13 different elemental ratios shown in a) after annealing at 500 ^QC. The majority phase in each diffraction pattern can be seen to be CZTSe. For reference, the simulated diffraction pattern of stannite CZTSe is shown below c) Raman spectra for the 13 different elemental ratios shown in a) after annealing at 500 ^QC. The majority phase in each spectra is the characteristic CZTSe peak centered at 195 cm ¹ with a shoulder peak at 172 cm ¹ and broad peak at 238 cm ¹.

Figure 6. a) PXRD patterns of 2:1 :1 CZTSe nanoparticles annealed under N2 at 50, 100, 150, 200, 250, 300, 350, 400, 500, and 600 ^QC in order to capture the phase and size behavior of the crystals present with temperature. For reference, the simulated diffraction pattern of stannite CZTSe is shown below b) Calculated mean Cu2ZnSnSe4 crystallite size from fits to broadened PXRD peaks. SEM images at 35,000x of CZTSe nanoparticles annealed under N2 at c) 250 ^QC and d) 500 ^QC. e) Raman spectra of the same CZTSe nanoparticles annealed under N2 varying temperatures in order to confirm the phase behavior identified by PXRD.

Figure 7. a) PXRD patterns of different ratios of mixed CZTS and CZTSe nanoparticles (varied bulk S:Se ratios) annealed under N2 at 500 ^QC. For reference, the simulated diffraction patterns of stannite CZTSe and kesterite CZTS are shown below. Black dashed lines are used as guides to better illustrate the shift of peaks due to S:Se ratio b) Raman spectra of the corresponding mixed nanoparticle powders. Black dashed lines are used as guides for the position of the center of the corresponding CZTS and CZTSe Raman peaks to better illustrate the shift due to S:Se ratio c) Plots of the three major PXRD peak shifts with varied S:Se ratio d) Plots of the CZTS and CZTSe peak shifts with varied S:Se ratio. Figure 8. a) UV-Vis-NIR reflectance spectra converted into equivalent absorbance spectra using the Kubelka-Munk function with BaS04 as reference, and b) corresponding Tauc plots of compacted discs containing different ratios of mixed CZTS and CZTSe nanoparticles (varied bulk S:Se ratios) annealed under N2 at 250 ^QC. c) Band diagram of the different S:Se ratio CZTSSe materials calculated from the valence band edge from PESA and band gap from Tauc plots d) Calculated band gap as a function of the S:Se ratio, showing the expected linear dependence with S:Se ratio.

Figure 9. a) PXRD patterns of mixed CZTS and CZTSe nanoparticles with a ratio of S:Se of 1 :1. All samples were annealed under N2 at their corresponding temperatures. The presence of Cu2ZnSnS2Se2 phase due to sintering of CZTS and CZTSe nanoparticles is evident above 200 ^QC, with the peaks sharpening as the crystals grow with temperature. For reference, the simulated diffraction patterns of stannite CZTSe and kesterite CZTS are shown below. Black dashed lines are used as guides to better illustrate the shift of peaks due to S:Se ratio b) Calculated mean Cu2ZnSnS2Se2 crystallite size from fits to broadened PXRD peaks. SEM images at 35,000x of mixed CZTS and CZTSe nanoparticles with a ratio of S:Se of 1 :1 annealed under N2 at c) 250 ^QC and d) 500 ^QC. e) The corresponding Raman spectra confirming the growth of Cu2ZnSnS2Se2 phase as seen in a) with black dashed lines as guides for the position of the center of the corresponding CZTS and CZTSe Raman peaks to better illustrate the shift due to S:Se ratio.

Figure 10. Valence band edges vs % sulfur content in CZTSSe showing an exponential trend.

Detailed description of the embodiments

Reference will now be made in detail to certain embodiments of the invention. While the invention will be described in conjunction with the embodiments, it will be understood that the intention is not to limit the invention to those embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalents, which may be included within the scope of the present invention as defined by the claims. One skilled in the art will recognize many methods and materials similar or equivalent to those described herein, which could be used in the practice of the present invention. The present invention is in no way limited to the methods and materials described. It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

All of the patents and publications referred to herein are incorporated by reference in their entirety. For purposes of interpreting this specification, terms used in the singular will also include the plural and vice versa.

As used herein, the term "chalcogen" refers to Group VI elements, and the terms "metal chalcogenides" or "chalcogenides" refer to materials that comprise metals and Group VI elements. Suitable Group VI elements include sulfur, selenium, and tellurium, preferably selenium. Herein, the term "quaternary-metal chalcogenide" refers to a chalcogenide composition comprising three metals in an approximate charge balance with a chalcogenide. The three metals may comprise X2-Y-Z compounds, wherein:

X is selected from the group consisting of Cu, Ag, Na, K, Li, Cs, and Au;

Y is selected from the group consisting of Zn, Cd, Fe, Ba, Mg, Ni, Co, Mn, Hg, Ca, and Sr; and

Z is selected from the group consisting of Sn, Ge, Si, Pb, and Zr.

Preferably, X is selected from Cu and Ag, Y is selected from Zn and Cd, and Z is selected from Sn and Ge.

Alternatively, the three metals may comprise X2-Qa-Qb compounds, wherein: X is selected from the group consisting of Cu, Ag, Na, K, Li, Cs and Au; and

Q_a is In and Qb is Ga. Suitable quaternary metal chalcogenides include, but are not limited to: CZTSe, Cu2ZnGeSe4, Cu2CoSnSe4, Cu2MnSnSe4, Cu2l\liSnSe4, Cu2FeSnSe4, Cu2MgSnSe4, Cu2CdSnSe4, Cu2SrSnSe4, Cu2BaSnSe4, Cu2MgSnSe4, Ag2ZnSnSe4, Ag2GeSnSe4, Ag2MnSnSe4, Ag2FeSnSe4, Ag2CdSnSe4, Ag2BaSnSe4, Li2ZnSnSe4, Li2GeSnSe4, Li2MnSnSe4, Li2FeSnSe4, Li2CdSnSe4, Li2CoSnSe4, Na2ZnSnSe4, and Na2CdSnSe4. In a preferred embodiment, the quaternary metal chalcogenide is copper zinc tin selenide (CZTSe).

Flerein, the term "CZTS" refers to Cu2ZnSnS4, “CZTSe” refers to Cu2ZnSnSe4, and “CZTS/Se” encompasses all possible combinations of Cu2ZnSn(S,Se)4 where Cu2ZnSnxSe₄-x and 0<x<4. The terms "CZTS", “CZTSe”, and “CZTS/Se” further encompass copper zinc tin sulfide/selenide semiconductors with fractional stoichiometries, e.g., Cu1.8Zm.2Sno.95S/Se4. That is, the stoichiometry of the elements can vary from strictly 2:1 :1 :4. Materials designated as CZTS/Se can also contain small amounts of other elements such as sodium. In addition, the Cu, Zn and Sn in CZTS/Se can be partially substituted by other metals. That is, Cu can be partially replaced by Ag, Na, K, Li, Cs, Au, and mixtures thereof; Zn by Cd, Fe, Ba, Mg, Ni, Co, Mn, Fig, Ca, Sr, and mixtures thereof; and Sn by Ge, Si, Pb, Zr, and mixtures thereof. Preferably, materials designated as CZTS consist essentially of Cu, Zn, Sn and S; materials designated as CZTSe consist essentially of Cu, Zn, Sn and Se; and materials designated as CZTS/Se consist essentially of Cu, Zn, Sn, S and Se.

The term "nanoparticle" is meant to include chalcogenide containing particles characterized by a longest dimension of about 1 nm to about 1000 nm, or about 1 nm to about 500 nm, or about 1 nm to about 100 nm, or about 1 nm to about 50 nm, or about 10 nm to about 40 nm. Nanoparticles can be globular or in the shape of spheres, platelets, rods, wires, disks, or prisms. Flerein, nanoparticle "size" or "size range" or "size distribution," refers to the average longest dimension of a plurality of nanoparticles that falls within the specified range. "Longest dimension" is defined herein as the measurement of a nanoparticle from end to end along the major axis of the projection. The "longest dimension" of a particle will depend on the shape of the particle. For example, for particles that are roughly or substantially spherical, the longest dimension will be a diameter of the particle. As defined herein, "coated particles" refers to quaternary metal chalcogenide nanoparticles that have organic or inorganic material, or a mixture thereof, bound to or associated with the surface. As defined herein, the terms "surface coating," "stabilizing agent," and "capping agent" are used interchangeably and refer to an adsorbed or chemically bonded monolayer of organic molecules, inorganic molecules, or mixtures thereof, at the surface of the particle(s). The stabilizing agent can aid in the dispersion of particles and can also inhibit their interaction and agglomeration in the precursor ink.

Quaternary Metal Chalcogenide Nanoparticles

In one embodiment, there is provided a composition comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal-chalcogenide stabilizing agent, optionally including a reducing agent, wherein the nanoparticles are dispersible in a polar solvent. Preferably, the quaternary metal chalcogenide nanoparticles are CZTSe nanoparticles.

Suitable inorganic metal-chalcogenide stabilizing agents include zintl ions, wherein zintl ion refers to a polyanionic compound containing 2 or more elements, wherein at least one element is a metalloid selected from Groups 14-17, wherein the zintl ion when dissolved in a polar solvent dissociates from a cationic species.

Preferably the inorganic metal-chalcogenide stabilizing agent is a ligand used in the synthesis of the quaternary metal chalcogenide nanoparticles. The inorganic metal- chalcogenide stabilizing agent may provide a source of metal and chalcogenide elements in the nanoparticle synthesis, as well as electrostatically stabilize the formed nanoparticles in solution. The inorganic metal-chalcogenide stabilizing agent may merge with the crystal phase of the core crystal structure of the nanoparticles upon heating. The composition of the inorganic metal-chalcogenide stabilizing agent may depend on the composition of the nanoparticles. For CZTSe nanoparticles, preferably the inorganic metal-chalcogenide stabilizing agent is selected from the group comprising, but not limited to: [Sn2Se6]⁴ , [SnSe4]⁴ , [Sn2Se3]² , [Sn2Se7]⁶ , [Sn4Sen]⁶ , [Sn3Se7]² , [SnSe2]² , [Sn4Seis]¹⁶ , [SnSe3]² , [Sn2Ses]² , and mixtures thereof, more preferably [Sn2Se7]⁶ , [Sn2Se6]⁴ , and mixtures thereof, most preferably [Sn2Se7]⁶ .

Compositions of nanoparticles stabilized with an inorganic metal-chalcogenide stabilizing agent may further include a reducing agent. Preferably the reducing agent is a mild reducing agent. In the context of this disclosure a mild reducing agent is a reducing agent which does not spontaneously react with water or oxygen and may only reduce a specific element or bond in a reaction by choosing an appropriate reduction potential. The reducing agent may be a stabilizing agent and adsorb or chemically bond to the nanoparticle surface. Preferably, the reducing agent decomposes and/or vaporizes at temperatures of less than about 300 °C, 290 °C, 280 °C, 270 °C, 260 °C, 250 °C, 240 °C, 230 °C, 220 °C, 210 °C, 200 °C, 190 °C, 180 °C, 170 °C, 160 °C, or 150 °C, preferably less than about 250 °C. A reducing agent that decomposes and/or vaporizes at low temperatures may vaporize during annealing of a thin film comprised of nanoparticles and therefore does not substantially contaminate the thin film, for example, by forming an insulating layer at interfaces following thermal annealing, and/or generating trap states that enhance carrier recombination. Typically, reducing agents that decompose and/or vaporize at temperatures of less than about 300 °C, have low- carbon and low-nitrogen content. In the context of this disclosure low-carbon and low- nitrogen content refers to molecules comprising about 5, 4, 3, 2, 1 or 0 atoms selected from C and N. Preferably the reducing agent comprises about 5, 4, 3, 2, 1 or 0 atoms selected from C and N. The composition of the reducing agent may depend on the nanoparticle composition. For CZTSe nanoparticles, preferably the reducing agent is selected from the group comprising thiourea, thiourea derivatives, selenourea, selenourea derivatives, diborane, ascorbic acid, formic acid, phosphites, hypophosphites, dithiols, and mixtures thereof. Preferably the reducing agent is thiourea or selenourea, more preferably thiourea.

In a particularly preferred embodiment, the quaternary metal chalcogenide nanoparticles are CZTSe nanoparticles, the inorganic metal-chalcogenide stabilizing agent is Sn2Sez⁶ , and the reducing agent is thiourea. Preferably, the inorganic metal- chalcogenide stabilizing agent and the reducing agent are derived from the synthesis of the CZTSe nanoparticles in a polar solvent.

In a preferred embodiment, the composition comprising quaternary metal chalcogenide nanoparticles is substantially free of non-vaporizable organic stabilizing agents. In the context of this disclosure, a non-vaporizable organic stabilizing agent refers to a carbon-containing molecule with a boiling/decomposition temperature of at least 220 °C, 230 °C, 240 °C, 250 °C, 260 °C, 270 °C, 280 °C, 290 °C, 300 °C, preferably at least 250 °C. Typically organic molecules with a boiling/decomposition temperature of at least 300 °C have carbon and/or nitrogen content sufficient to contaminate a thin film comprised of nanoparticles, for example, by forming an insulating layer at interfaces following thermal annealing, and/or generating trap states that enhance carrier recombination. Typically, organic molecules with a boiling/decomposition temperature of at least 300 °C have carbon and nitrogen content comprising at least 6 atoms selected from C and N.

Compositions comprising quaternary metal chalcogenide nanoparticles as described herein preferably contain essentially no carbon, nitrogen or oxygen which may contaminate a thin film comprised of the nanoparticles.

In a preferred embodiment, the amount of Cu, Zn, and Sn in the CZTSe nanoparticles is in a molar ratio of 50:25:25. In some embodiments, the amount of Cu, Zn, and Sn in the CZTSe nanoparticles can deviate from a 50:25:25 molar ratio by up to +/- 10 mole%, +/- 7.5 mole%, or +/- 5 mole% for each element. In some embodiments, the amount of Cu, Zn, and Sn in the CZTSe nanoparticles may be in a molar ratio of 52.5:27.5:20.0; 55.0:22.5:22.5; 52.5:20.0:27.5; 47.5:22.5:30.0; 45.0:27.5:27.5; 47.5:30.0:22.5; 50.0:27.5:22.5; 52.5:25.0:22.5; 52.5:22.5:25.0; 50.0:22.5:27.5; 47.5:25.0:27.5; 47.5:27.5:25.0; and 50.0:25.0:25.0. . Preferably, the amount of Cu, Zn, and Sn in the CZTSe nanoparticles is in a molar ratio of 50:25:25. In a particularly preferred embodiment, the amount of Cu, Zn, Sn and Se in the CZTSe nanoparticles is approximately in a 2:1 :1 :4 molar ratio.

The quaternary metal chalcogenide nanoparticles may be amorphous, semi crystalline, nanocrystalline, single crystals, or mixtures thereof. In one embodiment, the quaternary metal chalcogenide nanoparticles exhibit a substantially pure crystalline phase. The terms “pure crystalline phase” and “single crystalline phase” are used interchangeably. Preferably, the nanoparticles exhibit a substantially pure quaternary phase wherein the quaternary phase comprises greater than about 90 wt % of the total crystal phase wt %. In some embodiments, binary and/or ternary phases may also be present. Binary and ternary phases may include CuSe, Cu2Se, CuSe2, ZnSe, SnSe, SnSe2, Cu2SnSe3 and mixtures thereof. Preferably, binary and/or ternary phases comprise less than about 10 wt % of the total crystal phase wt %. In a particularly preferred embodiment, the nanoparticles exhibit a substantially pure quaternary phase, wherein the pure quaternary phase comprises greater than about 97 wt % of the total crystal phase wt %, and binary and/or ternary phases comprise less than about 3%.

The quaternary crystalline phase may be amorphous, sphalerite, kesterite, stannite, chalcopyrite wurtzite, kesterite-wurtzite, stannite-wurtzite, or mixtures thereof. Preferably the quaternary crystalline phase is sphalerite, kesterite, stannite, or mixtures thereof. In a particularly preferred embodiment, the quaternary phase is stannite.

The quaternary crystalline phase may be controlled by heating. Nanoparticles that exhibit a first crystalline phase may be heated to exhibit one or more alternative crystalline phases, or mixtures thereof. In one embodiment, the first crystalline phase is sphalerite and the alternative crystalline phase is kesterite. Typically temperatures of at least about 300 °C, 350 °C, 400 °C, 450 °C, 500 °C, 550 °C, or 600 °C, preferably at least about 400 °C, may be used to transition the crystalline phase from a first crystalline phase to one or more alternative crystalline phases. The phase transition temperature may depend on the molar ratio of Cu, Zn, and Sn in the CZTSe nanoparticles. Preferably, the nanoparticles may be heated to a temperature of at least about 300 °C, preferably 400 °C, without any phase transition.

In one embodiment, the quaternary metal chalcogenide nanoparticles exhibit a substantially globular spherical morphology. The morphology may be selected from the group consisting of globular, spherical, cubic, square and platelet morphologies, preferably spherical. The morphology may change upon heating.

The average nanoparticle size is preferably within the range of about 1 nm to 100 nm, about 1 nm to about 50 nm, about 10 nm to about 40 nm, with a size distribution of +/- 20%, +/- 15%, +/- 10%, or +/- 5%. The average nanoparticle size may increase upon heating.

Compositions comprising quaternary metal chalcogenide nanoparticles as described herein may be in powder form or dispersed in a polar solvent. Polar solvents include but are not limited to: water, deuterium oxide, water-soluble or water-miscible solvents, and mixtures thereof. Water-soluble or water-miscible solvents include but are not limited to: ammonia, alcohols, acetone, methyl ethyl ketone, acetonitrile, DMSO, and DMF. Examples of suitable alcohols include ethanol, methanol, isopropanol, and n- propanol. Preferably the polar solvent comprises water, optionally including ammonia. In embodiments wherein the polar solvent comprises water, the resultant aqueous solution may include buffering salts. The aqueous solution may include an ionic aqueous solution, including a high ionic strength aqueous solution.

Processes for Preparing Quaternary Metal Chalcogenide Nanoparticles

Herein, the term "metal salts" refers to compositions wherein metal cations and inorganic anions are joined by ionic bonding. Relevant classes of inorganic anions comprise oxides, carbonates, sulfates, selenates, nitrates, acetates, sulfides, selenides, and halides, preferably nitrates.

Herein, the term "metal complexes" refers to compositions wherein a metal is bonded to a surrounding array of molecules or anions, typically called "ligands" or "complexing agents." The atom within a ligand that is directly bonded to the metal atom or ion is called the "donor atom" and, herein, often comprises nitrogen, oxygen, selenium, or sulfur, preferably selenium.

In one aspect, there is provided a method of preparing a composition comprising quaternary metal chalcogenide nanoparticles as described herein. The method comprises reacting metal salts with a metal-chalcogenide complex, optionally including a reducing agent, in a polar solvent to form a polar dispersion of quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal-chalcogenide stabilizing agent.

In a preferred embodiment, the method comprises:

(a) providing a first polar solution comprising a metal-chalcogenide complex, optionally including a reducing agent;

(b) adding a first metal salt to the first polar solution to form a second polar solution; and

(c) reacting a second metal salt with the second polar solution;

thereby forming a polar dispersion of quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal-chalcogenide stabilizing agent.

The first metal salt comprises a metal ion with an oxidation state of 2⁺. The second metal salt comprises a metal ion with an oxidation state of 1 ⁺ or 2⁺. Where the second metal salt comprises a metal ion with an oxidation state of 2⁺, preferably the method includes a reducing agent. Where the second metal salt comprises a metal ion with an oxidation state of 1 ⁺, preferably the method does not include a reducing agent.

For CZTSe nanoparticles, the first metal salt comprises metal salts of Zn(ll), and the second metal salt comprises metal salts of Cu(l) or Cu(ll). Suitable metal salts include Cu(l), Cu(ll), Zn(ll), oxides, carbonates, sulfates, selenates, nitrates, acetates, sulfides, selenides, and halides, preferably nitrates. Metal nitrate salts obviate the contamination of quaternary metal chalcogenide nanoparticles by halide ions and organic species, which can be difficult to remove during postprocessing. Advantageously, nitrate ions can be decomposed into nitric dioxide gas and water in the presence of an excess of ammonia above about 180 °C.

The metal-chalcogenide complex provides a source of metal and chalcogenide elements for nanoparticle formation, as well as electrostatically stabilizes the resultant nanoparticles in solution and merges with the crystal phase upon heating. Preferably, the metal-chalcogenide complex used in the nanoparticle synthesis is the same as the inorganic metal-chalcogenide stabilizing agent of the resultant quaternary metal chalcogenide nanoparticles. Suitable metal-chalcogenide complexes include [Sn2Se6]⁴ , [SnSe₄]⁴ , [Sn₂Se₃]² , [S^Sez]⁶ , [Sn₄Sen]⁶-, [SnaSez]² , [SnSe₂]²-, [Sn Sei₅]¹⁶ , [SnSe₃]² , [Sn₂Se5]² , and mixtures thereof, more preferably [Sn₂Se7]⁶ , [Sn₂Se6]⁴ , and mixtures thereof, most preferably [Sn₂Se7]⁶ .

Preferably, the reducing agent decomposes and/or vaporizes at temperatures of less than about 300 °C, 290 °C, 280 °C, 270 °C, 260 °C, 250 °C, 240 °C, 230 °C, 220 °C, 210 °C, 200 °C, 190 °C, 180 °C, 170 °C, 160 °C, or 150 °C, preferably less than about 250 °C. More preferably, the reducing agent has low-carbon and low-nitrogen content. Preferably the reducing agent comprises about 5, 4, 3, 2, 1 or 0 atoms selected from C and N. Suitable reducing agents include thiourea, thiourea derivatives, selenourea, selenourea derivatives, diborane, ascorbic acid, formic acid, phosphites, hypophosphites, dithiols, and mixtures thereof. Preferably the reducing agent is thiourea or selenourea, more preferably thiourea. Thiourea converts Cu(ll) to Cu(l). In aqueous solutions, thiourea decomposes above about 150 °C to cyanamide and isothiocyanic acid, which further decompose to ammonia and carbonyl sulphide and are released as gases. Thiourea therefore minimizes organic contaminants in thin films comprising the CZTSe nanoparticles.

Preferably, the molar ratio of the reducing agent to the second metal salt is greater than 1 , 1.1 , 1.2, 1.3, 1.4, or 1.5, 1.6, 1.7, 1.8, 1.9, or 2.0, preferably greater than 1.5.

Preferably, the metal-chalcogenide complex and the reducing agent simultaneously react with substantially all metal ions in the reaction mixture to form single-phase quaternary metal chalcogenide nanoparticles.

The first polar solution, second polar solution and polar dispersion comprise at least one polar solvent. The solvent composition of the first polar solution, the second polar solution and the polar dispersion may be the same or different. Polar solvents include but are not limited to: water, deuterium oxide, water-soluble or water-miscible solvents, and mixtures thereof. Water-soluble or water-miscible solvents include but are not limited to: ammonia, alcohols, acetone, methyl ethyl ketone, acetonitrile, DMSO, and DMF. Examples of suitable alcohols include ethanol, methanol, isopropanol, and n- propanol. Preferably the polar solvent comprises water, optionally including ammonia. In embodiments wherein the polar solvent comprises water, the resultant aqueous solution may include buffering salts. The aqueous solution may include an ionic aqueous solution, including a high ionic strength aqueous solution.

The reaction is typically conducted at a pH of greater than 7, 8, 9, 10, 1 1 , 12, 13 or 14, preferably greater than 1 1.

The reaction is typically conducted at a temperature between about 30 - 45 °C, preferably about 40 °C.

The reaction is typically conducted at atmospheric pressure.

The reaction may be conducted under an atmosphere comprising oxygen or under an inert atmosphere, preferably under an inert atmosphere.

The polar dispersion of nanoparticles may be used as-synthesized. Alternatively, the nanoparticles may be purified by precipitation using a non-solvent, centrifugation, and redispersing the precipitate in a polar solvent. The nanoparticles may be isolated for example, by precipitation using a non-solvent, centrifugation, and vacuum dried to give the nanoparticles in powder form.

The resultant quaternary metal chalcogenide nanoparticles obtained from this synthetic route are coated with the inorganic metal chalcogenide stabilizing agent. The nanoparticles may also be coated with the reducing agent.

Advantageously the coated quaternary metal chalcogenide nanoparticles may be used as-synthesized and do not require post-processing to alter the surface chemistry. The ligands used in the synthesis either merge with the crystal lattice (in the case of the metal-chalcogenide complex) or comprise one or more properties selected from low- carbon and low-nitrogen content, high volatility and low decomposition temperature. Nanoparticles coated with such stabilizing agents can lead to annealed films of high purity and favourable semiconductor properties. It is believed that films with lower levels of carbon impurities derived from the stabilizing agent(s) are desirable. Although the coated quaternary metal chalcogenide nanoparticles can be further treated with an alternative stabilizing agent to replace the initial stabilizing agent(s) with the alternative stabilizing agent, preferably the coated nanoparticles comprise stabilizing agents derived from their synthesis.

Precursor Inks

Polar dispersions comprising quaternary metal chalcogenide nanoparticles as described herein can be used as a quaternary metal chalcogenide precursor ink.

This ink is referred to as a quaternary metal chalcogenide precursor ink, as it contains the precursors for forming a quaternary metal chalcogenide thin film. In some embodiments, the precursor ink consists essentially of a polar dispersion comprising the coated quaternary metal chalcogenide nanoparticles. The precursor ink comprises a polar solvent fluid medium to carry the particles.

The fluid medium typically comprises 30-99 wt %, 50-95 wt %, 60-90 wt %, 50-98 wt %, 60-98 wt %, 70-98 wt %, 75-98 wt %, 80-98 wt %, 85-98 wt %, 75-95 wt %, 80-95 wt %, or 85-95 wt% of the total weight of the CZTSe precursor ink. Herein, all reference to wt % of particles is meant to include any surface coating that may be present. In addition to the fluid medium and the coated quaternary metal chalcogenide nanoparticles, the precursor ink can optionally further comprise additives. Preferably, the precursor ink is additive-free.

In embodiments whereby the precursor ink further comprises one or more additives, the additives may be selected from the group consisting of dispersants, surfactants, polymers, binders, cross-linking agents, emulsifiers, anti-foaming agents, dryers, fillers, extenders, thickening agents, film conditioners, anti-oxidants, flow agents, leveling agents, defoamers, plasticizers, thixotropic agents, viscosity modifiers, dopants, and corrosion inhibitors. Typically, the additives comprise less than 20 wt %, or less than 10 wt %, or less than 5 wt %, or less than 2 wt %, or less than 1 wt% of the CZTSe precursor ink. Preferably, the precursor ink does not include an additive. It will be clear to a skilled person that in the context of this disclosure an additive does not include the stabilizing agent, or optional reducing agent.

Coated Substrates

In another aspect, there is provided a process comprising depositing at least one precursor ink onto a substrate to form a coated substrate, wherein the precursor ink comprises a polar dispersion comprising quaternary metal chalcogenide nanoparticles. The precursor ink may comprise one or more compositions of quaternary metal chalcogenide nanoparticles. In a preferred embodiment, the precursor ink comprises CZTSe nanoparticles. In another preferred embodiment the precursor ink comprises a mixture of CZTS nanoparticles and CZTSe nanoparticles. Polar dispersions of CZTS nanoparticles and methods of preparation are described in the applicant’s co-pending Australian patent application 2018282493 filed on the same date as the present application and entitled“CZTS precursor inks and methods for preparing CZTS thin films and CZTS-based devices”, which is incorporated herein by reference. Alternatively, the process comprises depositing (i) a precursor ink comprising CZTS nanoparticles onto a substrate, and (ii) a precursor ink comprising CZTSe nanoparticles onto the substrate to form a coated substrate comprising CZTS nanoparticles and CZTSe nanoparticles.

In one embodiment there is provided a coated substrate comprising: a) a substrate; and b) at least one layer deposited on the substrate comprising a precursor ink comprising quaternary metal chalcogenide nanoparticles.

The precursor ink is deposited on a surface of a substrate by any one of several conventional coating or printing techniques, e.g., spin-coating, doctor blade coating, spray coating, dip-coating, rod-coating, drop-cast coating, wet coating, roller coating, slot-die coating, meyerbar coating, capillary coating, ink -jet printing, draw -down coating, contact printing, gravure printing, flexographic printing, screen printing and electrophoretic deposition. The coating can be dried by evaporation, by applying vacuum, by heating, or by combinations thereof. In some embodiments, the coated substrate is heated at a temperature from about 40-550 °C, about 80-400 °C., about 80- 350 °C., about 100-300 °C., about 150-250 °C, about 180-250 °C, or about 180-220 °C to remove at least a portion of the solvent, if present, by-products, and volatile capping agents. In some embodiments, the drying step is carried out under an inert atmosphere. In some embodiments, the drying step is carried out under an atmosphere comprising oxygen. The drying step can be a separate, distinct step, or can occur as the coated substrate is heated in an annealing step. The substrate can be rigid or flexible. In one embodiment, the substrate comprises: (i) a base; and (ii) optionally, an electrically conductive coating on the base. The base material is selected from the group consisting of glass, metals, ceramics, and polymeric films. Suitable base materials include metal foils, plastics, polymers, metalized plastics, glass, solar glass, low-iron glass, green glass, soda-lime glass, metalized glass, steel, stainless steel, aluminium, ceramics, metal plates, metalized ceramic plates, and metalized polymer plates. In some embodiments, the base material comprises a filled polymer (e.g., a polyimide and an inorganic filler). In some embodiments, the base material is coated with a thin insulating layer (e.g., alumina or zirconia). Suitable electrically conductive coatings include metal conductors, transparent conducting oxides, and organic conductors.

Of particular interest are substrates of molybdenum-coated-soda-lime glass and molybdenum-coated polyimide films.

In some embodiments, the molar ratio of Cu:Zn:Sn in the coating on the substrate is 2:1 :1. In other embodiments, the molar ratio of Cu:(Zn+Sn) is less than one, and the molar ratio of Zn:Sn is greater than one (e.g., Cu1.8Zm.2Sno.95S/Se4). The molar ratio of S:Se in the coating can be varied by mixing the CZTS nanoparticle precursor ink and the CZTSe nanoparticle precursor ink in controlled ratios. In a preferred embodiment, the ratio of S:Se in a precursor ink comprising CZTS and CZTSe is substantially the same as the ratio of S:Se in an annealed film of CZTS/Se derived from a coating of that ink.

By varying the precursor ink concentration, solvent, additives, and/or coating technique and temperature, layers of varying thickness can be coated in a single coating step. In some embodiments, the coating thickness can be increased by repeating the coating and drying steps.

Formation of Thin Films

In another aspect, there is provided a thin film comprising a coated substrate as described herein, wherein the layer of the coated substrate comprising the precursor ink comprising quaternary metal chalcogenide nanoparticles comprises substantially annealed nanoparticles. In a preferred embodiment, the molar ratio of metal and chalcogenide elements in the thin film is substantially similar to the molar ratio of metal and chalcogenide elements of the nanoparticles in the ink.

In another aspect, there is provided a process comprising annealing the coated substrate to form an annealed thin film. The annealing step comprises heating the coated substrate to remove residual solvent, and if present, by-products, and volatile capping agents, and to improve at least one film characteristic selected from reducing grain boundaries, trap states, and pinholes. The annealed film typically has an increased density and/or reduced thickness compared to that of the unannealed coated substrate.

Advantageously, quaternary metal chalcogenide nanoparticles as described herein are amenable to low annealing temperatures. The reducing agent decomposes at annealing temperatures of less than about 250 °C leaving substantially pure inorganic nanoparticles, which enables direct contact, and thus charge transport, between particles in a continuous film. In some embodiments, the coated substrate is heated at about 100-550 °C., about 100-300 °C., about 150-250 °C., about 180-250 °C, or about 180-220 °C. More particularly, the coated substrate may be heated using annealing temperatures of less than about 300 °C, 290 °C. 280 °C, 270 °C, 260 °C, 250 °C, 240 °C, 230 °C, 220 °C, 210 °C, 200 °C, 190 °C, or 180 °C. Preferably, low annealing temperatures may be used, such as about 180-250 °C.

Low annealing temperatures also advantageously maintain control over the stoichiometry of the elements in the annealed film. At higher temperatures for example (eg above 300°C) tin-loss may be observed due to the formation of SnS and/or SnSe.

Preferbaly, the molar ratio of metal and chalcogenide elements in the thin film is substantially similar to the molar ratio of metal and chalcogenide elements of the nanoparticles in the ink. The ratio of Cu:Zn:Sn in the quaternary metal chalcogenide nanoparticles may be tuned during the synthesis of the nanoparticles. In a preferred embodiment, the ratio of Cu:Zn:Sn in a CZTS/Se precursor ink is substantially the same as the ratio of Cu:Zn:Sn in an annealed film of CZTS/Se derived from a coating of that ink. In some embodiments, the molar ratio of Cu:Zn:Sn is 2:1 :1 in the annealed film. In some embodiments, the molar ratio of Cu:(Zn+Sn) is less than one and the molar ratio of Zn:Sn is greater than one in an annealed film comprising CZTS/Se nanoparticles.

The molar ratio of S:Se can be varied by mixing the CZTS nanoparticle precursor ink and the CZTSe nanoparticle precursor ink in stoichiometric ratios. In a preferred embodiment, the ratio of S:Se in a precursor ink comprising CZTS nanoparticles and CZTSe nanoparticles is substantially the same as the ratio of S:Se in an annealed film of CZTS/Se derived from a coating of that ink.

Preferably, the annealing is carried out in the absence of a chalcogen vapor source.

Controlling the molar ratio of S:Se in the annealed film by mixing stoichiometric ratios of CZTS and CZTSe precursor inks and annealing at low temperatures in the absence of a chalcogen vapor enables the band-gap of the annealed film to be tuned between about 0.95 eV and about 1.55 eV. Preferably, the band gap of the thin film is between about 0.95 eV and about 1.55 eV, more preferably between about 1.0 eV and about 1.5 eV.

In some embodiments, the coated substrate is heated for a time in the range of about 1 min to about 48 h; 1 min to about 30 min; 10 min to about 10 h; 15 min to about 5 h; 20 min to about 3 h; or, 30 min to about 2 h. Typically, annealing may be conducted using thermal processing, rapid thermal processing (RTP), rapid thermal annealing (RTA), pulsed thermal processing (PTP), laser beam exposure, heating via IR lamps, electron beam exposure, pulsed electron beam processing, heating via microwave irradiation, flash light annealing, or combinations thereof. Preferably annealing may be conducted using thermal processing.

The annealed film typically has an increased density and/or reduced thickness compared to that of the unannealed coated substrate. In some embodiments, the film thicknesses of the dried and annealed coatings are 2.5 nm -200 microns; 0.1 -100 microns; 0.1 - 50 microns; 0.1 -25 microns; 0.1 -10 microns; 0.1 -5 microns; 0.1 -3 microns; 0.3-3 microns; or 0.5-2 microns.

Photovoltaic cells

In another embodiment, there is provided processes for forming photovoltaic cells comprising: a) coating a photovoltaic cell substrate with a polar dispersion comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal chalcogenide stabilizing agent; b) heating the coated photovoltaic cell substrate to form an annealed CZTS/Se thin film on the photovoltaic cell substrate; c) optionally repeating steps a) and b) to form a CZTS/Se film of the desired thickness; d) optionally depositing a buffer layer onto the CZTS/Se layer; e) depositing an N-type layer onto the CZTS/Se layer or buffer layer; f) depositing at least one top contact layer onto the N-type layer; g) depositing an electrode onto the top contact layer.

In another embodiment, there is provided a photovoltaic cell comprising a photovoltaic cell substrate comprising an annealed CZTS/Se thin film, wherein the annealed CZTS/Se thin film is derived from a polar dispersion comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal chalcogenide stabilizing agent.

Suitable substrate materials for the photovoltaic cell substrate include glass, metals or polymers. The substrate can be rigid or flexible. If the substrate material is not itself a conductor (e.g., a metal), the substrate comprises a conductive coating. Suitable substrate materials include soda-lime glass, polyimide films, solar glass, low-iron glass, green glass, steel, stainless steel, aluminium, and ceramics. Suitable photovoltaic cell substrates include molybdenum-coated soda-lime glass, molybdenum-coated polyimide films, metalized ceramic plates, metalized polymer plates, and metalized glass plates.

Typical photovoltaic cell substrates are glass or plastic, coated on one side with a conductive material, e.g., a metal. In one embodiment, the substrate is molybdenum- coated glass.

Depositing and annealing the CZTS/Se layer on the photovoltaic cell substrate can be carried out as described above.

The buffer layer typically comprises an inorganic material such as CdS, ZnS, zinc hydroxide, Zn (S, O, OH), cadmium zinc sulfides, ln(OH)3, ln2S3 , ZnSe, zinc indium selenides, indium selenides, zinc magnesium oxides, Sn02, T1O2, or n-type organic materials, or combinations thereof. Layers of these materials can be deposited by chemical bath deposition, atomic layer deposition, coevaporation, sputtering or chemical surface deposition to a thickness of about 1 nm to about 1000 nm, or from about 5 nm to about 500 nm, or from about 10 nm to about 300 nm, or 40 nm to 100 nm, or 50 nm to 80 nm.

The N-type layer typically comprises an inorganic material such as i-ZnO, zinc magnesium oxides, Zn (S, O, OH) or n-type organic materials, or combinations thereof. Layers of these materials can be deposited by chemical bath deposition, atomic layer deposition, coevaporation, sputtering or chemical surface deposition to a thickness of about 2 nm to about 1000 nm, or from about 5 nm to about 500 nm, or from about 10 nm to about 300 nm, or 40 nm to 100 nm, or 50 nm to 80 nm. The top contact layer is typically a transparent conducting oxide, e.g., indium tin oxide, aluminum-doped zinc oxide, graphene, cadmium stannate, or silver/gold nanowires. Suitable deposition techniques include sputtering, evaporation, chemical bath deposition, chemical surface deposition, electroplating, chemical vapor deposition, physical vapor deposition, and atomic layer deposition. Alternatively, the top contact layer can comprise a transparent conductive polymeric layer, e.g., poly-3, 4- ethylenedioxythiophene (PEDOT) doped with poly(styrenesulfonate) (PSS), which can be deposited by standard methods, including spin coating, dip-coating or spray coating.

One advantage of using a polar dispersion comprising single-phase quaternary metal chalcogenide nanoparticles as the precursor ink is that the nanoparticles are easily prepared. Another advantage is that the overall ratios of copper, zinc, tin and chalcogenide in the precursor ink can be easily varied to achieve optimum performance of the photovoltaic cell. Another advantage is that the nanoparticles can be annealed at low temperatures, allowing the use of a wider range of substrates for the photovoltaic cells. Another advantage is that the dense packing of the nanoparticles leads to a dense and smooth film.

Gas sensors

In another embodiment, there is provided a gas sensor comprising a gas sensor substrate comprising an annealed CZTS/Se thin film, wherein the annealed CZTS/Se thin film is derived from a polar dispersion comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal chalcogenide stabilizing agent.

Depositing and annealing a CZTS/Se layer on a gas sensor substrate can be carried out as described above. Preferably, the gas sensor substrate is a porous film. Preferably, the CZTS/Se layer is deposited on the gas sensor substrate by spray coating.

Photodetectors

In another embodiment, there is provided a photodetector comprising a photodetector substrate comprising an annealed CZTS/Se thin film, wherein the annealed CZTS/Se thin film is derived from a polar dispersion comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal chalcogenide stabilizing agent.

Depositing and annealing a CZTS/Se layer on a photodetector substrate can be carried out as described above. Preferably, the CZTS/Se layer is deposited into an array by electrophoretic deposition.

Examples

All metal salts and reagents were obtained from commercial sources and used as received unless otherwise noted. Any solvents used under inert gas were first degassed on a Schlenk line under vacuum (1 mPa) using an ultrasonic bath on a degas cycle with a power of 120 W. A cold trap was also employed to prevent any vaporized solvent reaching the vacuum pump. Tin powder and selenium powder were stored in a glove box under a nitrogen atmosphere with >1 ppm H2O to prevent oxidation. Deionized water was obtained from a Milli-Q system (18.2 MW.ah resistivity). Reaction containers were polypropylene specimen containers with polypropylene screw caps purchased from Techno Plas.

Preparation of Tin Selenide Chalcogenide

Tin selenide chalcogenide is exemplary of the inorganic metal chalcogenide complex described herein.

Tin selenide chalcogenide (SnSe-MCC) solutions were prepared by two routes: (i) a hydrothermal route, and (ii) a redox route. These are shown schematically in Figure 1 a) & b). Preferably, SnSe-MCC is prepared via the hydrothermal route.

Hydrothermal route: Tin powder (50 mmol, 5.936 g) and selenium powder (175 mmol, 13.825 g) were added to a high pressure Teflon lined autoclave and sealed under a nitrogen atmosphere to prevent their premature oxidation. Sodium hydroxide (200 mmol, 7.999 g), Milli-Q water (30 ml_), and methanol (60 ml_) were quickly added to the high pressure Teflon lined autoclave in air before filling with argon and resealing. The original turbid mixture was allowed to react for 3 days with the temperature maintained at 140 °C using a chamber furnace. This yielded a transparent 0.2778 M Sn2Se7^6- solution (25 mmol in 2:1 methanol-water, 90 ml_) with a deep-red color. This solution was then stored at 4 °C for 3 days to form crystals, which were then filtered and washed with cold methanol in a N2 environment to give red Na6Sn2Se7-16H20 needles (23.6 mmol, 31.115 g, 94% yield).

A colorless ammonium selenide solution was then produced by reacting selenium (2.5 mmol, 0.198 g), and sodium borohydride (5 mmol, 0.189 g) in ammonia (6.94 ml_, 12.8 % in Milli-Q water) under a N2 atmosphere with stirring until bubbling stopped. Na6Sn2Se7-16H20 (2.5 mmol, 3.296 g) was then dissolved in this ammonium-selenide solution to give the final 0.36 M SnSe-MCC precursor solution (2.5 mmol in 6.94 ml_ of 12.8% aqueous ammonium selenide).

Redox route: Sodium hydroxide (15 mmol, 0.600 g) was dissolved in degassed Milli-Q water (150 ml_) and placed under N2 at 1 ^QC. Sodium borohydride (30 mmol, 1.135 g) and selenium powder (15 mmol, 1.184 g) were added with stirring and allowed to react (~3 hours) to give a clear 0.1 M Na2Se solution. To this solution SnSe2 (5 mmol, 1.383 g) was added and left to react with stirring (~12 hours) to give 16.7 mM Sn2Se6^4- and 66.7 mM Na2Se, which was a transparent green solution under N2 and a transparent red solution under vacuum or air. The solution was then vacuum dried to give an orange-red mix of Na4Sn2Se6-13H20 (2.5 mmol, 2.593 g) and Na2Se (10 mmol, 1.249 g) crystals with a final combined yield of 3.546 g (92.3%). These crystals were dissolved in ammonia (6.94 ml_, 12.8% in Milli-Q water) under N2 atmosphere to give the final 0.36 M SnSe-MCC precursor solution (2.5 mmol in 6.94 ml_ of 12.8% ammonium-selenide water). A 3-fold excess of selenium powder relative to tin diselenide is used to ensure complete reaction to form Sn2Se6^4-.

The Na6Sn2Se7-16H20 and Na4Sn2Se6-13H20 crystals may be stored in a sealed container under N2 for long periods (>30 days) and dissolved when needed.

Synthesis of CZTSe Nanoparticles

A typical synthesis of CZTSe nanoparticles with an elemental ratio of 2:1 :1 Cu:Zn:Sn and total metal concentration of 40 g/L is as follows: a 1 M thiourea solution (15 mmol in 15 mL of Milli-Q water) and a 0.36 M SnSe-MCC solution (2.5 mmol in 6.94 ml_ of 12.8% aqueous ammonium selenide) were added to Milli-Q water (42 mL, to adjust the final ink concentration to 40 g/L) at 40 °C with stirring. A 0.76 M Zn(N03)2 solution (5 mmol in 6.54 mL of 18% ammonia solution) was then added, causing the formation of a pale orange precipitate, which redissolved within 3 minutes to return the solution to its original transparent red appearance. Once the solution was completely transparent, a 1.25 M Cu(N03)2 solution (10 mmol in 8 ml_ of Milli-Q water) was added quickly with stirring, causing the immediate formation of a black CZTSe nanoparticle ink. The yield by mass was > 98% (3.1 g of dried CZTSe nanoparticles). This synthetic method was successfully scaled-up to produce 10 g of dried CZTS nanoparticle powder. The reaction was performed inside a N2 glove box with degassed solvents to eliminate oxygen and carbon dioxide.

CZTSe nanoparticles of different elemental ratios and concentrations can be synthesized stoichiometrically by the addition of different amounts of copper, zinc, and tin precursor solutions, while keeping a pH >1 1 and ~1.5-fold excess of thiourea to copper ions in solution to ensure reduction of Cu(ll) to Cu(l) by thiourea. This synthesis can be varied to make any of the nanoparticle inks described herein at concentrations of up to ~90 g/L.

It should be noted that selenourea may be substituted for thiourea in the above synthesis. No difference in the resultant CZTSe nanoparticles was identified aside from the presence of the relevant urea derivative on the nanoparticle surface. Specifically, no sulfur was detected in the CZTSe crystals when using thiourea even with annealing. As such, based on cost and toxicity considerations, thiourea was chosen over selenourea.

Characterization

As-synthesized CZTSe powders were obtained by drying at 40 °C under N2. Purified CZTSe nanoparticles were obtained by adding ethanol or isopropanol as an antisolvent, centrifuging at > 4000 ref (~5 mins), disposing of the supernatant and redispersing the precipitant in Milli-Q water. Powder samples were prepared at a concentration of 40 g/L, dried at 40 °C under N2 for 3 days, then under vacuum (1 mPa for 1 hour) to yield a dry nanoparticle powder. Such powders were made into compacted discs with a diameter of 1.3 cm using a KBr disc manual hydraulic press with 9 tons applied for 2 minutes.

In order to observe the nucleation and growth of the CZTSe nanoparticles, and their subsequent colloidal stability, in-situ temporal resolved UV-Vis-NIR measurements were carried out. The purple baseline curve in Figure 2 was Milli-Q water and thiourea solution, while the red curves were collected after the addition and mixing of the SnSe- MCC solution. Adding the zinc precursor solution exhibited no absorbance change from the equilibrated red curve at 1 s and as such the series of curves were omitted. In the blue curves at 0 ms the Cu(NC>3)2 solution was quickly added resulting in an increasing red shift in the absorption up until -639 ms indicating rapid CZTSe nanoparticle formation. Following this time, the spectra remains stable with only a slight shift in absorbance over the next 3 days.

TEM was carried out to identify the size distribution and morphology of the CZTSe nanoparticles produced. Figure 3 a) shows globular spherical nanoparticles with a mean diameter of -25 nm with a standard deviation of 3.6 nm. The high resolution TEM (FIR-TEM) in Figure 3 b) reveals the presence of nano-crystalline regions in the CZTSe nanoparticle with lattice spacing of -0.347 nm, consistent with the inter-planar dioo spacing of CZTSe; however, a large fraction of the nanoparticles appear amorphous.

FTIR was used to identify the surface bound molecules. In the unwashed ink, all of the expected functional groups stemming from the original reaction chemistry were present as seen in Figure 3 c). After purification, the surface chemistry is revealed to be composed of just Sn2Se7^6- anions and thiourea molecules. Upon heating at 250 ^QC all surface bound molecules are removed.

Thermal Decomposition Characteristics of CZTSe Nanoparticles

Nanoparticles that require minimal costly post-synthesis processing, such as centrifugation, filtering or precipitation steps, and have no impurities upon mild annealing, are ideal for thin film applications. In order to identify at what temperatures the different components of the unwashed CZTSe nanoparticle ink vapourise, TGA- FTIR was employed, the results are shown in Figure 4.

The initial slow mass loss evident in the TGA curve below 150 ^QC is due to evaporation of residual ammonia (-4.5%), followed by some loss of thiourea as cyanamide and isothiocyanic acid, which starts at -150 °C.

In the presence of water released from other decomposition reactions at -180 °C, isothiocyanic acid further decomposes to carbonyl sulfide. The sharp mass decrease between 170-187 °C (-43.6%) largely corresponds to the continued loss of thiourea (21.5%), then the subsequent loss of nitrate (22.1 %). Between 190-280 °C the loss of borohydride (0.4%) is observed. No loss of carbonates was observed. Conducting the reaction inside a N2 filled glovebox using degassed solvents, substantially eliminated naturally dissolved carbonate in solvents from atmospheric CO2.

Preferably, the CZTSe nanoparticle ink may be heated to temperatures of at least about 200 °C to remove substantially all of the undesirable components in the nanocrystal ink through either decomposition and/or vaporization to produce a nitrogen- , oxygen-, and carbon-free CZTSe bulk material.

Phase Map of Annealed Compositionally-Variant CZTSe Nanoparticles

13 different compositions of CZTSe nanoparticles were analyzed. Samples were prepared with theoretical elemental ratios as shown in Figure 5 a) at a concentration of 40 g/L. ICP-MS was performed on small portions of washed and dried powders to compare the theoretical and experimental elemental ratios. As seen in Figure 5 a), the values were found to be within measurement and pipetting errors (<0.5%), thus confirming that the synthetic method yields stoichiometric precipitation of the metallic ions into nanoparticles.

Each of the 13 sample powders were prepared for PXRD and Raman by annealing at 500 °C for 10 minutes under N2 in a tube furnace. The PXRD patterns shown in Figure 5 b) indicate that the majority phase present in all 13 different compositions is stannite CZTSe (JCPDS 52-0868), however a small proportion of kesterite CZTSe (JCPDS 70-8930) may also be present. Stannite CZTSe is differentiated from kesterite CZTSe by the presence of two additional peaks at -31 ^Q and 35^Q. Small tin selenide peaks were expected due to the elevated annealing temperatures employed; however, these were either not present or not large enough to be observable. It is well known that PXRD alone cannot differentiate between ZnSe, Cu2SnSe3 and Cu2ZnSnSe4 phases due to the similarity in their diffraction patterns. In order to confirm that CZTSe is indeed the majority phase, Raman analysis was performed. The Raman spectra of the 13 samples in Figure 5 c) show the characteristic CZTSe peak centered at 191 cm^-1 with a shoulder peak at 173 cm^-1 and broad peak at 235 cm^-1. On the other hand, no compositions exhibited a Cu2SnSe3 or ZnSe peak, even when using a 325 nm laser to excite. This suggests the majority phase present in each of the crystals is pure-phase CZTSe without any ZnSe contributions. The crystal phases observed by Raman spectroscopy are consistent with the phases observed by PXRD. This confirms that the synthesis is stoichiometric and allows for CZTSe nanoparticles to be synthesized with tuneable composition.

Crystal phase vs. temperature in unwashed CZTSe nanoparticle inks was also investigated for 50:25:25 (Cu:Zn:Sn %) CZTSe. These CZTSe powders were annealed at varied temperatures between 50 and 600 ^QC for 10 minutes under N2 in a tube furnace and the resulting crystal phases were then probed by PXRD and Raman spectroscopy. The pattern at 200 ^QC shows a clear sharpening of the CZTSe peaks and a reduction in the additional impurity contributions. Between 200 ^QC and 250 ^QC additional ink components are substantially removed and the CZTSe nanoparticles sinter to yield crystallites with a mean size of >240 nm at 250 °C, determined via the Scherrer equation (Figure 6 b). At 250 ^QC a range of grain sizes are observed via SEM (Figure 6 c) consisting of small grains of -90 nm adhered to surfaces of large >1 pm grains interspersed with intermediate -300 nm grains. Between 350 ^QC and 400 ^QC two addition peaks around the <1 12> stannite CZTSe peak at 27.2^Q are observed, which correspond to the characteristic triple peak structure of wurtzite-stannite CZTSe. These peaks account for -2 and 5 wt% of the crystal phase at 350 and 400 ^QC, respectively, and are not present in significant amounts (<0.5 wt%) outside this range. The CZTSe grains further grow to mean sizes >500 nm by 500 °C as reported in Figure 6 b). The mean crystallite size calculated from the PXRD was confirmed via SEM in Figure 6 d) where large grains between ~400 nm and ~2.5 pm are observed.

Figure 6 e) contains the corresponding Raman spectra of samples heated to >200 ^QC. A clear sharpening of the characteristic Ai, A2 and B2 (TO) CZTSe Raman mode peaks at 195, 172, and 238 cm^-1, respectively, was observed with increasing temperature. ZnSe was not present in the Raman at any temperature. This supports the PXRD assignments of the CZTSe phase and the gradual grain growth. By 250-300 ^QC in both PXRD patterns and Raman spectra, distinct and sharp CZTSe peaks with crystallite sizes ranging from ~240 to ~290 nm are observed with the lack of any secondary phase peaks.

Annealing of CZTS & CZTSe Nanoparticles - Temperature and S:Se Ratio Dependence

In order to map out the processes occurring during annealing of mixtures of CZTS and CZTSe nanoparticles, a series of temperature and composition ratio dependent PXRD and Raman spectroscopy studies were conducted. CZTSe nanoparticles were synthesized as described herein. CZTS nanoparticles were synthesized as described in the applicant’s co-pending Australian patent application filed on the same date as the present application and entitled“CZTS precursor inks and methods for preparing CZTS thin films and CZTS-based devices”. Both CZTSe and CZTS nanoparticles were prepared in the same N2 glovebox. The nanoparticle inks were then mixed in controlled ratios, dried under N2 and vacuum, and finally heated to the designated temperature for 10 mins under N2 in a tube furnace.

In Figure 7 a) the PXRD patterns collected from three mixed S:Se ratios and pure CZTS and CZTSe are shown. A linear shift in the scattering peaks with composition is observed between the pure CZTS and CZTSe phases (Figure 7 c), and there is only a single phase present. A comparison of the Raman spectra across the alloyed samples reveals a progressive change in the ratios and positions of the strong A kesterite CZTS (332 cm^-1) and Ai stannite CZTSe (195 cm^-1) Raman mode peaks as a function of composition. These correspond to vibrations of the anion lattice of S and Se, respectively (Figure 7 b, d). The relative intensities of the CZTS peaks are lower than that for CZTSe, which creates a noted disparity when comparing samples with different compositions. Moreover, shifts in the main Raman peaks are also noted for the mixed compositions, with the CZTSe and CZTS peaks shifting toward higher and lower wavenumbers, respectively. This is consistent with the formation of an alloyed CZTSSe phase due to the strong A kesterite CZTS and Ai stannite CZTSe Raman modes being stretched or contracted by changes in the anionic lattice spacing.

Compacted discs of the heat treated powders were used to investigate the optoelectronic properties of three mixed alloyed CZTSSe with different S:Se ratios and pure CZTS and CZTSe. Reflectance UV-Vis-NIR spectra were converted to equivalent absorption using the Kubelka-Munk function with BaSC as a reference. Clear red-shifts in equivalent absorbance UV-Vis-NIR spectra are observed for higher Se (see Figure 8 a). These spectra were then converted into the corresponding Tauc plots in Figure 8 b) and the band gaps for each S:Se ratio calculated. Mixing stoichiometric amounts of CZTS and CZTSe and sintering at 250 ^QC results in linear shifts in the band gaps of CZTSSe between 1 .51 eV and 1 .05 eV for pure CZTS and pure CZTSe, respectively, as seen in Figure 8 d). By utilizing PESA the valence band edge of semiconductors can be obtained, which when coupled with the bandgap of the material, can be used to determine the conduction band edge. The valence band edge energies for the pure CZTS and CZTSe are within the expected error for PESA of 0.05 eV to their theoretical and experimental values of -5.1 eV and -4.95 eV respectively. Based on these valence band energies, and the corresponding band gaps, band diagrams have been constructed for different S:Se ratios and are presented in Figure 8 c). These show the relative positions of the valence band edge and conduction band edge and exhibit the expected broadening profile with decreasing valence band edge and increasing conduction band edge with increased sulfur doping.

To further probe the alloying process, in Figure 9 a) we show the PXRD following annealing at different temperatures of 1 :1 S:Se ratio (Cu2ZnSnS2Se2) powder samples. This sample is taken as a representative across the composition range, as all mixed CZTS/CZTSe powders show the same phase and crystallite size dependence with temperature. The presence of the Cu2ZnSnS2Se2 phase is evident at 200 ^QC, with the emergence of distinct CZTSSe scattering contributions appearing between the CZTS and CZTSe peaks. This corresponds to the temperature where additional ink components are substantially removed allowing particles to more easily contact each other. By 250 ^QC, the kesterite/stannite Cu2ZnSnS2Se2 phase with a mean crystallite size of -90 nm is observed, determined via the Scherrer equation (Figure 9 b). At 250 ^QC a range of grain sizes are observed via SEM (Figure 9 c) from small grains of -30 nm to larger agglomerates of ~300 nm grains spanning >1 pm. At progressively higher temperatures, the PXRD patterns exhibit a continued sharpening of the CZTSSe peaks, with the crystallites reaching a size of >400 nm by 500 °C determined via PXRD (Figure 9 b). We confirmed this calculated mean grain size via SEM in Figure 9 d) where large grains between ~250 nm and ~2 pm are observed. As shown in Figure 9 e), Raman measurements of the CZTSSe powders corroborate the PXRD. These PXRD and Raman spectroscopy results illustrate that by mixing stoichiometric quantities of CZTS and CZTSe nanoparticles, then heating to only 250 ^QC, an alloyed CZTSSe phase with controlled S:Se ratio can be achieved.

Valence Band Edge Determination Photoelectron Spectroscopy in Air (PESA) measurements were performed on compacted discs with 1 :0, 2:1 , 1 :1 , 1 :2, and 0:1 S:Se ratios prepared as described in the main text. The valence band edge energies were calculated by fitting the two linear sections of each plot and identifying the intercept energy value. The values for the pure CZTS and CZTSe are within the expected error for PESA of 0.05 eV to their theoretical and experimental valence band edge values of ~5.1 eV and ~4.95 eV respectively. The PESA plots also suggest an exponential increase in valence band energy from the CZTSe value of 4.90 eV with increasing sulfur content to the CZTS value of 5.11 eV, as seen in Figure 10.

Claims

1 . A composition comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal-chalcogenide stabilizing agent, wherein: i) the quaternary metal chalcogenide nanoparticles substantially comprise CZTSe nanoparticles; and ii) the nanoparticles are dispersible in a polar solvent.

2. A composition according to claim 1 , wherein the polar solvent comprises water.

3. A composition according to claim 1 or 2, wherein the inorganic metal- chalcogenide stabilizing agent is selected from the group consisting of: [Sn2Se6]⁴ ,

[SnSe₄]⁴ , [Sn₂Se₃]² , [S^Sez]⁶ , [Sn₄Sen]⁶-, [SnsSez]² , [SnSe₂]²-, [Sn Sei₅]¹⁶ , [SnSes]² , [Sn₂Se5]² , and mixtures thereof.

4. A composition according to claim 3, wherein the inorganic metal-chalcogenide stabilizing agent is [Sn₂Se7]⁶ , [Sn₂Se6]⁴, or mixtures thereof.

5. A composition according to any one of claims 1 - 4, wherein the composition includes a reducing agent.

6. A composition according to claim 5, wherein the reducing agent decomposes and/or vaporize at temperatures of less than about 220 °C.

7. A composition according to claim 5 or 6, wherein the reducing agent is selected from the group consisting of: thiourea, thiourea derivatives, selenourea, selenourea derivatives, diborane, ascorbic acid, formic acid, phosphites, hypophosphites, dithiols, and mixtures thereof.

8. A composition according to any one of claims 1 - 7, wherein the composition is substantially free of non-vaporizable organic stabilizing agents.

9. A precursor ink comprising a composition according to any one of claims 1 - 8 and at least one polar solvent, wherein the composition is dispersed in the solvent thereby forming the precursor ink.

10. A precursor ink according to claim 9, further comprising CZTS nanoparticles.

1 1 . A method of preparing a composition comprising quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal-chalcogenide stabilizing agent, wherein the method comprises: (a) providing a first polar solution comprising a metal-chalcogenide complex;

(b) adding a first metal salt to the first polar solution to form a second polar solution; and

(c) reacting a second metal salt with the second polar solution;

thereby forming a polar dispersion of quaternary metal chalcogenide nanoparticles stabilized by an inorganic metal-chalcogenide stabilizing agent, wherein the quaternary metal chalcogenide nanoparticles substantially comprise CZTSe nanoparticles.

12. A method according to claim 1 1 , wherein the polar dispersion comprises water.

13. A method according to claim 1 1 or 12, wherein the first metal salt comprises metal salts of Zn(ll), and the second metal salt comprises metal salts of Cu(l) or Cu(ll).

14. A method according to claim 13, wherein the first metal salt and the second metal salt are nitrate metal salts.

15. A method according to claim to any one of claims 1 1 - 14, wherein the inorganic metal-chalcogenide stabilizing agent is selected from the group consisting of: [Sn2Se6]⁴ ,

[SnSe₄]⁴ , [Sn₂Se₃]² , [S^Sez]⁶ , [Sn₄Sen]⁶-, [SnsSez]² , [SnSe₂]²-, [Sn Sei₅]¹⁶ , [SnSe₃]² , [Sn₂Se5]² , and mixtures thereof.

16. A method according to claim 15, wherein the inorganic metal-chalcogenide stabilizing agent is [Sn₂Se7]⁶ , [Sn₂Se6]⁴, or mixtures thereof.

17. A method according to any one of claims 1 1 - 16, wherein the first solution includes a reducing agent.

18. A method according to claim 17, wherein the reducing agent decomposes and/or vaporize at temperatures of less than about 220 °C.

19. A method according to claim 17 or 18, wherein the reducing agent is selected from the group consisting of: thiourea, thiourea derivatives, selenourea, selenourea derivatives, diborane, ascorbic acid, formic acid, phosphites, hypophosphites, dithiols, and mixtures thereof.

20. A method according to any one of claims 17 - 19, wherein the molar ratio of the reducing agent to the second metal salt is greater than 1.5.

21. A method according to any one of claims 1 1 - 20, wherein the polar dispersion of nanoparticles is substantially free of non-vaporizable organic stabilizing agents.

22. A method according to any one of claims 1 1 - 21 , wherein the reaction is conducted at a temperature of about 40 °C.

23. A coated substrate comprising: a) a substrate; and b) at least one layer deposited on the substrate comprising a precursor ink according to claim 9 or 10.

24. A thin film comprising a coated substrate according to claim 23, wherein the layer comprises substantially annealed nanoparticles.

25. A thin film according to claim 24, wherein the molar ratio of metal and

chalcogenide elements in the thin film is substantially similar to the molar ratio of metal and chalcogenide elements of the nanoparticles in the precursor ink.

26. A method of preparing a thin film, wherein the method comprises heating a coated substrate according to claim 23 to a temperature of at about 180 - 250 °C to form the annealed thin film.

27. A method according to claim 26, wherein the annealing is carried out in the absence of a chalcogen vapor source.