Kesterite absorber layers for thin-film solar cells

Abstract

The present disclosure relates to the preparation of kesterite (Cu2ZnSnS4, CZTS) absorber material for thin film solar cells The material is synthesized using microwave-assisted, solution-based, and melting–alloying methods from Cu, Zn, and Sn precursors. The process includes sintering under controlled sulphurisation, where the sulphur amount is maintained at 4–6 times the total metal precursors on a dry basis, and the sintering temperature is controlled between 350 °C and 850 °C. Raman and UV–Vis analysis confirm single-phase CZTS with a direct band gap of about 1.5 eV and high absorption coefficient (104–105 cm-1). Optimized films show a device efficiency up to 13.2% in a glass / ITO–ZnO / CdS–CZTS–Mo configuration. The method provides a scalable and cost-effective route for producing earth- abundant CZTS absorbers for high-performance thin film solar cells.

Description

[0001] TITLE

[0002] KESTERITE ABSORBER LAYERS FOR THIN-FILM SOLAR CELLS

[0003] CROSS REFERENCE TO RELATED APPLICATIONS

[0004] The present application is based upon and claims priority to India complete patent application number 202441066074 filed on August 31, 2025, which in turn claims priority to India provisional patent application number 202441066074 filed on August 31, 2024, the entire contents of which is herein incorporated by reference.

[0005] FIELD

[0006] The invention relates to kesterite (Cu2ZnSnS4 or CZTS) based semiconductor materials, and to methods for their synthesis, processing, and use in thin-film photovoltaic and other optoelectronic applications.

[0007] BACKGROUND

[0008] The increasing global demand for energy, estimated now-a-days at approximately ~4.76*1O20J / year, which is equivalent to world power consumption of ~15 terawatts (TW), which is expected to double by 2025. This demand is predominantly met by fossil fuels, which are limited and generate harmful emissions such as CO2, NO2, CO, and SO2. Therefore, the search for renewable, environmentally friendly, and cost-effective energy sources has become critical. Among various renewable sources, solar energy has its distinctive significance as it is abundant, clean, and sustainable.

[0009] Currently, more than 80% of the photovoltaic (PV) industry is based on crystalline silicon (c-Si) and polycrystalline silicon (pc-Si) technologies. While these technologies are prevalent, they rely on an indirect band gap absorber material that requires thick absorber layers and high-quality crystal growth, thereby increasing cost. As a result, recent advancements have shifted focus toward thin-film solar cell (TFSC) technologies based on direct band gap semiconductors such as copper indium gallium diselenide (CIGS), copper indium diselenide (CIS), and cadmium telluride (CdTe). These have reached commercialization, with reported efficiencies exceeding 11%. However, the toxicity of cadmium and selenium, along with the scarcity of indium and tellurium, limits large-scale adoption.

[0010] In response, significant research attention has turned to Cu2ZnSnS4 (CZTS), a quaternary I-II-IV-VI semiconductor material that is earth-abundant, non-toxic, and cost-effective. CZTS can be derived by replacing half of the indium in CuInSe2 with zinc and the other half with tin, resulting in a stable kesterite structure with an optimal direct band gap (1.4-1.5 eV) and high absorption coefficient (>104cm'1) in the visible range. Crystallographically, CZTS has two principal structures known as stannite type (Cu2FeSnS4) and kesterite type (Cu2ZnSnS4). The two structures are similar except different arrangements of Cu and Zn atoms. However, CZTS material usually appears in kesterite phase because it is thermodynamically more stable as compared to stannite type. Theoretical calculations predict a maximum power conversion efficiency of about 32.2% for CZTS-based solar cells, making it a promising absorber material.

[0011] The advantages of CZTS are reinforced by the abundance of its constituent elements. For example, zinc and tin are far more abundant in the earth’s crust compared to indium: zinc is present at around 79 ppm, which is roughly 1,500 times more abundant than indium, while tin (2.2 ppm) is about 45 times more abundant than indium. This abundance ensures scalability and low raw material cost, in contrast to indium and tellurium, which face supply limitations.

[0012] Progress in the synthesis of CZTS thin films has been achieved using a variety of physical and chemical techniques, including sputtering (atom beam, RF magnetron, hybrid), thermal evaporation, sulphurisation of electron beam evaporated precursors, pulsed laser deposition, spray pyrolysis, ultrasonic spray pyrolysis, electrodeposition, sol-gel spin coating, and successive ionic layer adsorption reaction (SILAR). Both vacuum-based and non-vacuum-based approaches have been explored with the goal of developing a high-efficiency, low-cost absorber layer.

[0013] Despite this progress, the reported highest efficiency of pure CZTS thin-film solar cells remains around 8.4% using vacuum evaporation techniques, and approximately 7.37% using non-vacuum based high temperature arrested precipitation methods. These values remain significantly lower than those achieved with CIS and CIGS solar cells that have already surpassed 15% efficiency and entered commercial production. Efficiency improvements in CZTS devices over time have largely resulted from optimization of process parameters, defect management, and device structure refinements.

[0014] Research trends also reflect the growing global interest in CZTS. Over the past decade, the number of scientific publications devoted to CZTS has increased steadily, overtaking earlier research on CIS and CIGS during their initial growth phase. This rise demonstrates the scientific community’s increasing interest in addressing existing limitations and pushing CZTS closer to commercial scale.

[0015] To achieve improved efficiency, a more detailed understanding of the nature of defects in the absorber layer as well as the intra- and interlayer interfaces must be achieved. To build the successful technology, the detailed understanding of this material synthesis is necessary. This can be obtained by studying material experimentally as well as theoretically. Theoretical simulation shows that the stable chemical potential region for the formation of stoichiometric compound in CZTS is small, and the growth of CZTS under a Cu poor / Zn rich condition would be optimal for maximising device performance. Also, the long-term durability of kesterite based CZTS thin film solar cell device under heat, light and moisture should be studied to establish a low cost, eco-friendly and high throughput approach for the deposition of CZTS thin films.

[0016] Yet, with its earth-abundant composition, non-toxic nature, optimal optoelectronic properties, and steadily improving performance, CZTS retains considerable potential as a sustainable absorber material for next-generation thin-film solar cells.

[0017] Accordingly, there remains a need for improved methods of CZTS synthesis and device architectures that overcome the current limitations of CZTS-based thin-film solar cells, while enhancing their efficiency, stability, and commercial viability. US9359222B2 discloses a colloidal, solution-phase synthesis of CZTS nanoparticles using a labile organothiol, which acts both as a sulfur source and a capping ligand

[0018] US20120219491A1 discloses method of synthesis of copper zinc tin sulfide, Cu2ZnSnS4 by a precursor mixture comprising a copper sulfide, a zinc sulfide, and a tin sulfide; a copper tin sulfide and a zinc sulfide; a copper zinc sulfide and a tin sulfide; or a zinc tin sulfide and a copper sulfide; wherein the precursor mixture has a total molar ratio of Cu:Zn: Sn: S of about 2: 1: 1 :4; and heating the precursor mixture in an inert atmosphere to a temperature of about 400° C. to about 800° C.

[0019] SUMMARY

[0020] The present disclosure relates to methods for the preparation of kesterite (Cu2ZnSnS4, CZTS) absorber layers suitable for thin film solar cell applications.

[0021] The disclosure provides three distinct synthetic approaches i.e. microwave-assisted synthesis, solution-based synthesis, and melting-and-alloying based synthesis, each followed by processing routes that enable formation of high-quality kesterite thin films.

[0022] In the microwave-based synthesis, individual metal sulphide precursors such as CuS, ZnS, and SnS are synthesized by precipitation and subjected to controlled microwave irradiation, wherein the irradiation power is modulated based on reflective power. The dried precursors are subsequently ground and formulated into a coating ink with alcoholic solvents, polymeric binders, and additives. The ink is coated on metal or glass substrates and subjected to sintering in an excess sulphur atmosphere.

[0023] In the solution-based synthesis, soluble precursors including CuCh, ZnCh, SnCh, and thiourea are combined with organic solvents and stabilizing agents to prepare a coating ink. The ink is deposited on substrates, followed by iterative baking to achieve desired thickness and final sintering in excess sulphur, thereby producing structurally pure kesterite films. In the melting-and-alloying synthesis, precursor mixtures of Cu, Zn, and Sn are melted, alloyed, powdered, and converted into an ink with solvent, a sugar-based binder and polyethylene glycol as additive. Subsequent coating and sintering in sulphur atmosphere enables formation of kesterite thin films.

[0024] Across all processes, the ink coated on metal or glass substrates were subjected to sintering in an excess sulphur atmosphere, maintained at 350 °C to 850 °C, where the sulphur source is provided in quantities approximately 4 to 6 times the total dry weight of the precursors. This controlled sulphurisation ensures epitaxial growth of kesterite thin films with no detectable contamination from secondary phases such as wurtzite or Cu2SnS3.it has been observed that the amount of sulphur employed during sintering plays a crucial role in ensuring formation of a uniform phase of kesterite. Films sintered with insufficient sulphur show mixed phases including uSnSs and wurtzite, whereas those sintered in excess sulphur exhibit improved yield and crystallinity of the kesterite phase.

[0025] The resulting thin films with the above characteristics are advantageous as absorber layers in thin film solar cells.

[0026] BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0027] Fig. 1 illustrates XRD spectra of Microwave-based Kesterite film of Sample Micro- 1 before sintering

[0028] Fig. 2 illustrates XRD spectra of Microwave-based Kesterite film of Sample Micro-

[0029] 1 after sintering

[0030] Fig. 3 illustrates XRD spectra (smoothed) of Microwave-based Kesterite film of Sample Micro- 1 after sintering

[0031] Fig. 4 illustrates XRD spectra of Microwave-based Kesterite film of Sample Micro-

[0032] 2 after sintering

[0033] Fig. 5 illustrates XRD spectra (smoothed) of Microwave-based Kesterite film of Sample Micro-2 after sintering

[0034] Fig. 6 illustrates XRD spectra of Microwave-based Kesterite film of Sample Micro-

[0035] 3 after sintering Fig. 7 illustrates XRD spectra (smoothed) of Microwave-based Kesterite film of Sample Micro-3 after sintering

[0036] Fig. 8 illustrates SEM images and EDS spectra of Microwave-based Kesterite film of Sample Micro- 1 after sintering

[0037] Fig. 9 illustrates SEM images and EDS spectra of Microwave-based Kesterite film of Sample Micro-3 after sintering

[0038] Fig. 10 illustrates XRD spectra of Solution -based Kesterite film of Samples Sol-1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C, and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0039] Fig. 11 A illustrates SEM images of Solution-based Kesterite film of Samples Sol- 1, Sol-2, Sol-3, and Sol-4 after sintering at (a) 350°C (b) 450°C (c) 550°C and (d) 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0040] Fig. 1 IB illustrates EDS spectra of Solution-based Kesterite film of Samples Sol- 1, Sol-2, Sol-3, and Sol-4 after sintering at (a) 350°C (b) 450°C (c) 550°C and (d) 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0041] Fig. 11C illustrates Raman shift of Solution -based Kesterite film of Samples Sol-1, Sol-2, Sol-3, and Sol-4 after sintering at (a) 350°C (b) 450°C (c) 550°C and (d) 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0042] Fig. 1 ID illustrates band gap analysis using absorption coefficient (a) versus the photon energy (hv) plot and Tauc’s plot of Solution-based Kesterite film of Samples Sol-1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C, and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0043] Fig. 1 IE illustrates FTIR spectra of Solution-based Kesterite film of Samples Sol- 1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0044] Fig. 1 IF illustrates Cyclic voltammetry studies of Solution-based Kesterite film of Samples Sol-1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere. Fig. 11G illustrates Nyquist plots of Solution-based Kesterite film of Samples Sol- 1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0045] Fig. 11H illustrates Fitted circuit and equivalent circuits of Solution-based Kesterite film of Samples Sol-1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere. Fig. 12 illustrates XRD spectra of Melting -Alloying-based Kesterite film of Sample Alloy- 1 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0046] Fig. 13 illustrates XRD spectra (smoothed) of Melting- Alloying-based Kesterite film of Sample Alloy- 1 after sintering at 600°C in sulphur atmosphere for 4 hours. Fig. 14A and 14B illustrate SEM images and EDS spectra of Melting-Alloying- based Kesterite film of Sample Alloy- 1 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0047] Fig. 15 illustrates XRD spectra of Melting-Alloying-based Kesterite film of Sample Alloy -2 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0048] Fig. 16 illustrates XRD spectra (smoothed) of Melting-Alloying-based Kesterite film of Sample Alloy-2 after sintering at 600°C in sulphur atmosphere for 4 hours. Fig. 17A and 17B illustrate SEM images and EDS spectra of Melting-Alloying based Kesterite film of Sample Alloy -2 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0049] Fig. 18 illustrates XRD spectra of Melting-Alloying-based Kesterite film of Sample Alloy-3 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0050] Fig. 19 illustrates XRD spectra (smoothed) of Melting-Alloying-based Kesterite film of Sample Alloy-3 after sintering at 600°C in sulphur atmosphere for 4 hours. Fig. 20A and 20B illustrate SEM images and EDS spectra of Melting- Alloying based Kesterite film of Sample Alloy -3 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0051] Fig. 21 illustrates XRD spectra of Melting-Alloying-based Kesterite film of Sample Alloy -4 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0052] Fig. 22 illustrates XRD spectra (smoothed) of Melting-Alloying-based Kesterite film of Sample Alloy-4 after sintering at 600°C in sulphur atmosphere for 4 hours. Fig. 23 A and 23B illustrate SEM images and EDS spectra of Melting- Alloying based Kesterite film of Sample Alloy -4 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0053] Fig. 24 illustrates XRD spectra of Melting-Alloying-based Kesterite film of Sample Alloy-5 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0054] Fig. 25 illustrates XRD spectra (smoothed) of Melting-Alloying-based Kesterite film of Sample Alloy-5 after sintering at 600°C in sulphur atmosphere for 4 hours. Fig. 26A and 26B illustrate SEM images and EDS spectra of Melting- Alloying based Kesterite film of Sample Alloy-5 after sintering at 600°C in sulphur atmosphere for 4 hours.

[0055] DETAILED DESCRIPTION

[0056] The subject matter of the present disclosure is described in detail with reference to the accompanying drawings. Unless otherwise specified, all the technical and scientific terms used herein have the same meaning as is generally understood by a person skilled in the art pertaining to the present disclosure. Headings are used solely for organizational purposes, and are not intended to limit the disclosure in any way.

[0057] The use of the singular includes the plural unless specifically stated otherwise. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well. The use of “or” means “and / or” unless stated otherwise. Unless otherwise indicated, all numbers used herein to express quantities, dimensions, and so forth used should be understood as being modified in all instances by the term "about." It is to be understood that wherein a numerical range is recited, it includes all values within that range, and all narrower ranges within that range, whether specifically recited or not. As used herein, "including," "containing" and like terms are understood to be synonymous with "comprising" and are therefore open-ended and do not exclude the presence of additional undescribed or unrecited elements, materials, phases or method steps.

[0058] In addition, it should be appreciated that any figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings of them are not necessarily drawn to scale.

[0059] Any method / process steps and / or operations and / or instructions used in this disclosure, are for illustrative purposes in a particular order and / or grouping. Other orders and / or grouping of the process steps or its portions and / or operations or its portions and / or instructions or its portions are possible and, one or more of the process steps and / or operations and / or instructions can be combined and / or deleted.

[0060] The present disclosure relates to methods for the preparation of kesterite (Cu2ZnSnS4, CZTS) absorber layers suitable for thin film solar cell applications.

[0061] The disclosure provides three distinct synthetic approaches, microwave-assisted synthesis, solution-based synthesis, and melting-and-alloying based synthesis, each followed by processing routes that enable formation of high-quality kesterite thin films.

[0062] Microwave-assisted synthesis and processing of kesterite for thin film solar cells: In the Microwave-assisted synthesis method, initially kesterite precursors are synthesised, an ink is prepared from the precursors, subsequently, the ink is coated on a substrate, and the coated ink is sintered at an optimum temperature in an atmosphere containing excess sulphur. The process is provided in more detail here.

[0063] Preparing precursors for the kesterite:

[0064] Synthesis of CuS as a precursor for the kesterite:

[0065] - Equal volumes of IM CuCh and 2M Na2S are taken;

[0066] - 2M Na2S is added to IM CuCh in a dropwise manner with vigorous stirring, which results in a precipitate;

[0067] - The obtained precipitate is subjected to micro wave irradiation at a frequency of 2 to 3 GHz, with an input power of 2-5 kW for a duration of 100 to 120 minutes under an operating current of 3.5 A, and a voltage of 396 V. The irradiation is monitored at 10-minute intervals to assess the reflective power. It is to be noted that, if the reflective power increases during the course of the reaction, the input power can be accordingly adjusted. For example, 20% of 60 kW is applied for the first 30 minutes, followed by 40% of 6 kW for the subsequent 75-90 minutes. The resulting dried precipitate is then ground with acetone to obtain a fine powder, suitable for further characterization and application.

[0068] Synthesis of ZnS as a precursor for the kesterite:

[0069] - Equal volumes of IM ZnCh and 2M Na2S are taken;

[0070] - 2M Na2S is added to IM ZnCh in a dropwise manner with vigorous stirring, which results in a precipitate;

[0071] - The obtained precipitate is subjected to micro wave irradiation at a frequency of 2 to 3 GHz, with an input power of 2-5 kW for a duration of 100 to 120 minutes under an operating current of 4.8 A, and a voltage of 396 V. The irradiation is monitored at 10-minute intervals to assess the reflective power.

[0072] As discussed before, if the reflective power increases during the course of the reaction, the input power can be accordingly adjusted. For example, 40% of 6 kW is applied. The resulting dried precipitate is then ground with acetone to obtain a fine powder, suitable for further characterization and application.

[0073] Synthesis of SnS as a precursor for the kesterite:

[0074] - Equal volumes of IM SnCh and 2M Na2S are taken;

[0075] - 2M Na2S is added to IM SnCh in a dropwise manner with vigorous stirring, which results in a precipitate;

[0076] - The obtained precipitate is subjected to micro wave irradiation at a frequency of 2 to 3 GHz, with an input power of 2-5 kW for 30 minutes under an operating current of 4.8 A, and a voltage of 396 V. The irradiation is monitored at 10-minute intervals to assess the reflective power.

[0077] As discussed before, if the reflective power increases during the course of the reaction, the input power can be accordingly adjusted. For example, 30% of 6 kW is applied for the first 10 minutes, followed by 40% of 6 kW for the subsequent 20 minutes. The resulting dried precipitate is then ground with acetone to obtain a fine powder, suitable for further characterization and application.

[0078] Preparation of ink from the precursors CuS, ZnS, and SnS.

[0079] - The three precursors CuS, ZnS, and SnS in their powder form are taken in a weight ratio of 2: 1 : 1.

[0080] - The precursor mixture is combined with an alcoholic solvent selected from methanol, ethanol, isoamyl alcohol, and isoamyl acetate; and about 4% by weight of a sugar-based binder and about 4% by weight of polyethylene glycol as additives, to form a coating ink.

[0081] The resulting ink is coated onto substrates selected from metals or metal alloys, such as aluminium, aluminium alloys, and copper, as well as glass to form a film, wherein the glass substrate can be selected from Mo coated glass or soda lime glass. The coating is repeated until a film thickness of approximately 30 pm is obtained. The coated samples are subsequently sintered at a temperature of about 350 °C to 850 °C for a duration of 3-5 hours in an atmosphere containing excess sulphur i.e. having a sulphur presence of 4 to 6 times by weight of the total weight of the precursor components of Cu, Zn, and Sn on a dry basis.

[0082] X-ray diffraction (XRD) analysis of the sintered coatings later reveals that a distinct kesterite (105) epitaxial peak, with no detectable contamination from wurtzite or Cu2SnS3.

[0083] Solution-based synthesis and processing of kesterite for thin film solar cells:

[0084] - The precursors 2M CuCl2, IM ZnCl2, IM SnCl2in a weight ratio of 2: 1 : 1 are combined with a composition containing 8 M thiourea in an organic solvent selected from 2-methoxyethanol or 2-butoxy ethanol. To this solution, a stabilizer selected from monoethanolamine or triethanolamine is added in an amount of 0.1-0.5 ml per 10 ml of solution. The resulting mixture is stirred for 2 hours at 45 °C to obtain a homogeneous coating ink.

[0085] - The coating ink prepared is coated onto substrates selected from metals or metal alloys, such as aluminium, aluminium alloys, and copper, as well as glass to form a film, wherein the glass substrate can be selected from Mo coated glass or soda lime glass.

[0086] - The coated film is baked for 5 min at 200°C and subsequently cooled to room temperature.

[0087] - The coating and baking steps are repeated until a film thickness of approximately 30 pm is obtained.

[0088] - The coated samples are subsequently sintered at a temperature of about 350 °C to 850 °C for a duration of 3-5 hours in an atmosphere containing excess sulphur i.e. having a sulphur presence of 4 to 6 times by weight of the total weight of the precursor components of Cu, Zn, and Sn on a dry basis.

[0089] Melting and alloying-based synthesis and processing of kesterite for thin film solar cells:

[0090] - 0.1-0.8 parts of Cu, 0.1-0.3 parts of Zn, and 0.1 to 0.6 parts of Sn precursor compositions are taken, melted, and alloyed, wherein the compositions of the precursors containing Cu, Zn, and Sn can be selected from Cuo.sZno.iSno.i; Cuo.7Zno.15Sno.15; Cuo.4Zno.3Sno,3; Cuo.3Zno.3Sno.4; Cuo.i5Zno.25Sno.6.

[0091] - The alloy is powdered;

[0092] - The powdered alloy is combined with an alcoholic solvent selected from methanol, ethanol, isoamyl alcohol, and isoamyl acetate; and about 4% by weight of a sugar-based binder and about 4% by weight of polyethylene glycol as additive, to form a coating ink.

[0093] - The resulting ink is coated onto substrates selected from metals or metal alloys, such as aluminium, aluminium alloys, and copper, as well as glass to form a film, wherein the glass substrate can be selected from Mo coated glass or soda lime glass.

[0094] - The coating is repeated until a film thickness of approximately 30 pm is obtained.

[0095] - The coated samples are subsequently sintered at a temperature of about 350 °C to 850 °C for a duration of 3-5 hours in an atmosphere containing excess sulphur i.e. having a sulphur presence of 4 to 6 times by weight of the total weight of the precursors. It has been observed that, the amount of suphur employed in sintering of the coating inks, plays a crucial role in the formation of kesterite form of the CZTS. Sintering the coating ink with lower amounts of sulphur leads to formation of different forms of CZTS such as Cu2SnSs and wurtzite together with kesterite; whereas the yield of the kesterite form of CZTS is higher with increasing amount of sulphur employed in sintering.

[0096] Due to the presence of excess sulphur in the system, sulphur act as both oxygen getter and sulphurisation source for kesterite synthesis.

[0097] In an aspect, the sugar-based binder can be selected from glucose, fructose, or sucrose.

[0098] For fabrication of solar cells, the synthesized kesterite absorber material may be coated on a substrate selected from metals, metal alloys, glass, or plastics such as polyimides (PI), polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyethersulfone (PES), polycarbonate (PC), or cyclic olefin polymers (COPs / COCs), as well as highly porous substrates of ceramic, metallic, or insulating origin, using suitable deposition methods including inkjet printing, 3D printing, tape casting, plasma spraying, spin coating, electrospinning, or electrodeposition. Following deposition, a buffer layer and a transparent conducting oxide layer, such as ZnO in bulk or as nanorods, is applied. Metal interconnects are subsequently formed, and the completed solar cell is characterized for currentvoltage (I-V) properties using a spectro-electrochemical workstation. ZnO nanorods, being piezoelectric in nature, can reduce carrier recombination at the p- n junction, modify the band structure, and enhance the overall efficiency of the solar cell.

[0099] Examples:

[0100] The present disclosure will now be explained in further detail by the following examples. These examples are illustrative of certain embodiments of the disclosure without limiting the scope of the present disclosure. Example 1

[0101] Microwave-assisted synthesis and processing of kesterite for thin film solar cells

[0102] Synthesis of CuS as a precursor for the kesterite:

[0103] - 500 ml each of IM CuCh and 2M Na2S were taken;

[0104] - 2M Na2S was added to IM CuCb in a dropwise manner with vigorous stirring, which resulted in a precipitate;

[0105] - The obtained precipitate was subj ected to microwave irradiation at a frequency of 2 to 3 GHz, with an input power of 3.5 kW for a duration of 105 minutes under an operating current of 3.5 A, and a voltage of 396 V. The irradiation was monitored at 10-minute intervals to assess the reflective power.

[0106] The initial reflective power was observed to be 6%. After 10 minutes, as the reflective power was increased to 60%, the input power was accordingly adjusted, i.e. 20% of 60 kW was applied for the next 30 minutes, followed by 40% of 6 kW for the subsequent 75 minutes. The resulting dried precipitate was then ground with acetone to obtain a fine powder, suitable for further characterization and application.

[0107] Synthesis of ZnS as a precursor for the kesterite:

[0108] - 250 ml each of IM ZnCb and 2M Na2S were taken;

[0109] - 2M Na2S was added to IM ZnCb in a dropwise manner with vigorous stirring, which results in a precipitate;

[0110] - The obtained precipitate was subj ected to microwave irradiation at a frequency of 2 to 3 GHz, with an input power of 3.5 kW for a duration of 100 to 120 minutes under an operating current of 4.8 A, and a voltage of 396 V. The irradiation was monitored at 10-minute intervals to assess the reflective power.

[0111] As the reflective power increased after 10 minutes to 80% from 40%, the input power was accordingly adjusted to bring it to 12%, i.e., 40% of 6 kW was applied. The resulting dried precipitate was then ground with acetone to obtain a fine powder, suitable for further characterization and application.

[0112] Synthesis of SnS as a precursor for the kesterite:

[0113] - 250 ml each of IM SnCb and 2M Na2S were taken; - 2M Na2S was added to IM SnCb in a dropwise manner with vigorous stirring, which results in a precipitate;

[0114] - The obtained precipitate was subj ected to microwave irradiation at a frequency of 2 to 3 GHz, with an input power of 3.5 kW for 30 minutes under an operating current of 4.8 A, and a voltage of 396 V. The irradiation was monitored at 10-minute intervals to assess the reflective power.

[0115] The input power applied being 40% of 6 kW and the reflective power was well below 12%. But after 17mins of microwave irradiation reflective power went upto 16% and hence the input power was reduced to 30% of 6 kW. After 10 mins, the material stopped conducting and hence the input power was again increased to 40% of 6 kW and irradiated for 30 mins. The resulting dried precipitate was then ground with acetone to obtain a fine powder, suitable for further characterization and application.

[0116] Preparation of ink from the precursors CuS, ZnS, and SnS.

[0117] - 10, 5, 5 grams each of the three precursors CuS, ZnS, and SnS in their powder form were taken.

[0118] - The coating ink was formed by combining the precursor mixture with 100ml of ethanol; and 5 grams of glucose (sugar-based binder) and 2 grams of polyethylene glycol as additives.

[0119] The resulting ink was coated onto different substrates, which formed a thin film. The coating was repeated to obtain a film thickness of 30 pm.

[0120] Multiple of such coated samples were subsequently sintered at different temperatures ranging from 350 °C to 850 °C for a duration of 3-5 hours in an atmosphere containing different amounts of sulphur and the sintered samples were subjected to characterisation.

[0121] Example 2

[0122] Solution-based synthesis and processing of kesterite for thin film solar cells: - 500, 250, 250 ml each of the precursors 2M CuCh, IM ZnCh, IM SnCh are combined with 50 ml of a composition containing 8 M thiourea in an organic solvent selected from 2-methoxyethanol or 2-butoxy ethanol. To this solution, 1 ml of monoethanolamine was added. The resulting mixture is stirred for 2 hours at 45 °C to obtain a homogeneous coating ink.

[0123] - The coating ink prepared was coated onto a Mo coated glass substrate which formed a thin film.

[0124] - The coated film was baked for 5 min at 200°C and subsequently cooled to room temperature.

[0125] - The coating and baking steps were repeated to obtain a film thickness of 30 pm.

[0126] - Multiple of such coated samples were subsequently sintered at temperatures 350 °C, 450 °C, 550 °C, and 650 °C for a duration of 3 hours in an atmosphere containing 60 grams of sulphur, and the sintered samples were subjected to characterisation.

[0127] Example 3

[0128] Melting and alloying-based synthesis and processing of kesterite for thin film solar cells:

[0129] - Five samples each comprising 100 grams of a mixture of Cu, Zn, and Sn were taken in the ratio of Cuo.sZno.iSno.i; Cuo.7Zno.15Sno.15; Cuo.4Zno.3Sno.3; Cuo.3Zno.3Sno .4; Cuo.i5Zno.25Sno.6, each of which was melted at 850 °C in a muffle furnace and alloyed,

[0130] - Each alloy was powdered by ball milling and grinding, and each powdered alloy was separately combined with 50 ml of isoamyl alcohol; and 5 gram of glucose (sugar-based binder) and 5 gram of polyethylene glycol as additives, which formed five samples of coating inks.

[0131] - Each resulting ink was coated onto soda lime glass substrate, which formed a thin film.

[0132] - The coating in each case was repeated until a film thickness of approximately 30 pm was obtained.

[0133] - Each of these coated samples were subsequently sintered at different temperatures ranging from 350 °C to 850 °C for a duration of 3-5 hours in an atmosphere containing different amounts of sulphur, and the sintered samples were subjected to characterisation.

[0134] Characterisation of Sintered Coated Samples from Example 1 (Microwave- assisted kesterite)

[0135] Sample Micro-1 (Microwave-assisted kesterite Ink): The ink was coated onto an Aluminium-Lithium alloy (Al -Li) substrate sintered at 350°C with 30 grams of Sulphur powder. The Kesterite film of Sample Micro- 1 before sintering was analysed by XRD as shown in Fig. 1. Table 1 presents the corresponding XRD data.

[0136] Table 1 cf (A) (%&) 28 (degree) Experimental XRD data for Sample Micro-1 after sintering

[0137] The Kesterite film of Sample Micro- 1 after sintering was analysed by XRD as shown in Fig. 2. Table 2 presents the corresponding XRD data. Table 2

[0138] The data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 22.4739 (4.35), 27.7079 (11.32), 31.0834 (10.88), 31.5779 (16.77), 36.0904 (6.36), 42.2552 (3.20), 43.7015 (2.94), 50.2445 (1.99), 71.1461 (5.91), 78.6120 (7.76) can be attributed to the presence of Cu2SnS3.

[0139] XRD spectra (smoothed) for Kesterite film of Sample Micro-1 after sintering is as shown in Fig. 3. Table 3 presents the corresponding XRD data (smoothed).

[0140] Table 3

[0141] The smoothed data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 31.4713 (16.50), 42.6383 (13.42), 54.5474 (20.40) can be attributed to the presence ofC SnSs. This comparison indicates that a polycrystalline wurtzite is formed along with large amount of polycrystalline kesterite.

[0142] Sample Micro-2 (Microwave-assisted kesterite Ink): The ink was coated onto an Aluminium substrate sintered at 350°C with 40 grams of Sulphur powder.

[0143] The Kesterite film of Sample Micro-2 after sintering was analysed by XRD as shown in Fig. 4. Table 4 presents the corresponding XRD data.

[0144] Table 4

[0145] The data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 20.8921 (10.48), 27.7142 (13.79), 30.8007 (4.61), 31.6363 (11.27), 32.0676 (14.47), 34.6522 (13.15), 34.8689 (8.65), 36.1898 (9.04), 42.6214 (7.01), 48.7632 (5.44), 54.5515 (18.56),

[0146] 54.7246 (20.43), 71.3083 (7.85), 78.6672 (10.75) can be attributed to the presence of Cu2SnS3.

[0147] XRD spectra (smoothed) for Kesterite film of Sample Micro-2 after sintering is as shown in Fig. 5. Table 5 presents the corresponding XRD data (smoothed).

[0148] Table 5

[0149] The smoothed data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 42.6550 (18.16), 54.6962 (26.42), 71.2650 (11.32) can be attributed to the presence of C SnSs. As can be observed, the major peaks are attributed to wurtzite crystal.

[0150] Sample Micro-3 (Microwave-assisted kesterite Ink): The ink was coated onto an Aluminium-Lithium alloy (Al -Li) substrate sintered at 350°C with 60 grams of Sulphur powder.

[0151] The Kesterite film of Sample Micro-3 after sintering was analysed by XRD as shown in Fig. 6. Table 6 presents the corresponding XRD data.

[0152] Table 6

[0153] The data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 27.5828 (0.81), 38.7738 (0.51), 42.9798 (0.63), 46.0567 (1.72), 54.5122 (1.15), 71.0824 (0.52), 78.0175 (3.02) can be attributed to the presence of Cu2SnS3.

[0154] XRD spectra (smoothed) for Kesterite film of Sample Micro-3 after sintering is as shown in Fig. 7. Table 7 presents the corresponding XRD data (smoothed). Table 7

[0155] The smoothed data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 22.1742 (0.32), 31.5865 (1.48), 43.0039 (0.57), 46.0910 (1.60), 54.5596 (0.99), 78.0124 (2.01) can be attributed to the presence of Cu2SnS3. As can be observed the peak at 44.4410 is 100% (highest peak), which shows the formation of epitaxy of kesetrite of 105 phase.

[0156] SEM and EDS analysis of the sintered Microwave-assisted kesterite Ink (Sample Micro-1, and Sample Micro-3).

[0157] Fig- 8 illustrates SEM images and EDS spectra of Microwave-assisted Kesterite film of Sample Micro- 1 after sintering

[0158] Fig- 9 illustrates SEM images and EDS spectra of Microwave-assisted Kesterite film of Sample Micro-3 after sintering

[0159] The corresponding chemical composition data of Fig. 8 is provided in Table 8.

[0160] The corresponding chemical composition data of Fig. 9 is provided in Table 9.

[0161] Table 8

[0162] As observed from the above data, the EDS chemical composition analysis of Sample Micro- 1 sintered in the presence of less sulphur leads to formation of mixed oxides of Cu, Sn and Zn along with sulphides thus leading to more wurtzite, less kesterite and more Cu2SnS3 and formation of ZnO, SnO and CuO. Table 9

[0163] As observed from the above data, the EDS chemical composition analysis of Sample Micro-3 sintered in the presence of excess sulphur leads to formation of Cu, Sn and Zn sulphides thus leading to less wurtzite, more kesterite and very less Cu2SnS3. The kesterite is of lattice parameters 105, seen as epitaxy in XRD.

[0164] Characterisation of Sintered Coated Samples from Example 2 (Solution based Kesterite)

[0165] Samples (Solution-based kesterite Ink): The ink was coated onto four glass substrates to prepare Samples Sol-1, Sol -2, Sol-3, and Sol-4.

[0166] Sample Sol-1 : Sintered at 350 °C with 60 grams of Sulphur powder.

[0167] Sample Sol-2: Sintered at 450 °C with 60 grams of Sulphur powder.

[0168] Sample Sol-3: Sintered at 550 °C with 60 grams of Sulphur powder.

[0169] Sample Sol-4: Sintered at 650 °C with 60 grams of Sulphur powder.

[0170] Note: 60 grams of sulphur powder is about 5-6 times the dry weight of the precursors.

[0171] The Kesterite film of Samples Sol-1 to Sol -4 after sintering were analysed by XRD as shown in Fig. 10. Table 10 presents the corresponding XRD data.

[0172] Table 10

[0173] All XRD patterns show good agreement with the literature JCPDS No. 26-0575 database and lattice constants.

[0174] As observed, from the XRD spectra, diffraction peaks with corresponding crystal planes of 28.47° for (112), 31.89° for (200), 47.59° for (220) and 56.24° for (312) consistent with the kesterite (CZTS) phase. The (112) peak intensity increased with increased sulfurization temperatures, indicating enhanced crystallinity at 550 °C and 650 °C. A decrease in FWHM values with increasing temperature was observed, accompanied by an increase in crystallite size, confirming improved crystal quality.

[0175] The average crystallite sizes were ~15 nm at 350 °C, ~22 nm at 450 °C, ~29 nm at 550 °C, and ~31 nm at 650 °C. Structural analysis indicated presence of secondary phases including SnS2(011) at 33.02° (JCPDS-83-1701), SnS (200) at 45.8° (JCPDS-831758) and Cu3SnS4at 37.27° (212), 39.81° (2 0 12), 51.8° (2 0 18), 56.36° (3 2 12), 58.39° (406) consistent with structure of orthorhombic Cu3SnS4(ICDD-00-036-0217) were observed at 350° and 450 °C, but these secondary phases disappeared at higher sulfurization temperatures (550 °C and 650 °C).

[0176] SEM and EDS analysis of the sintered Solution-based kesterite Ink (Samples Sol-1, Sol-2, Sol-3, and Sol-4).

[0177] Fig. 11A illustrates SEM images of Solution-based Kesterite film of Samples Sol- 1, Sol-2, Sol-3, and Sol-4 after sintering at (a) 350°C (b) 450°C (c) 550°C and (d) 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere. SEM analysis of CZTS samples sulfurized at 350 °C, 450 °C, 550 °C, and 650 °C revealed non-uniform particle distribution and mixed morphology with large, agglomerated crystallites. An increase in sulfurization temperature was observed to enhance the crystallite size and improve overall particle uniformity.

[0178] Fig. 11B illustrates EDS spectra of Solution-based Kesterite film of Samples Sol- 1, Sol-2, Sol-3, and Sol-4 after sintering at (a) 350°C (b) 450°C (c) 550°C and (d) 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere. The corresponding chemical composition data is provided in Table 11.

[0179] Table 11

[0180] According to literature guidelines, the Zn / Sn ratio serves as an indicator of phase purity in CZTS: (i) Zn / Sn = 1 predicts formation of 100% CZTS, (ii) Zn / Sn < 1 indicates predominant CZTS formation with excess Sn leading to C SnSs formation as a secondary phase, and (iii) Zn / Sn > 1 suggests major CZTS formation with residual Zn forming ZnS.

[0181] EDS chemical composition analysis of Samples Sol-1 to Sol-4 indicates the following:

[0182] - for CZTS-350 °C and CZTS-450 °C, the Zn / Sn ratios of 3.41 and 2.58 respectively suggest Zn-rich compositions with ZnS presence, and Cu / (Zn+Sn) ratios of 2.71 and 0.73 for CZTS-350 °C and CZTS-450 °C respectively. - For CZTS-550 °C, the Cu / (Zn+Sn) ratio of 1.09, Zn / Sn ratio of 0.87, and Cu / Sn ratio of 2.05 indicate near-stoichiometric CZTS formation.

[0183] - In CZTS-650 °C, approximately 30% excess zinc was observed, indicating a Zn-rich composition.

[0184] Raman Spectroscopy

[0185] Fig. 11C illustrates Raman shift of Solution-based Kesterite film of Samples Sol- 1, Sol-2, Sol-3, and Sol-4 after sintering at (a) 350°C (b) 450°C (c) 550°C and (d) 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0186] Identification of secondary phases such as ZnS, CusSnS4 (CTS), and SnS2, that overlap with the diffraction peaks of quaternary CZTS, was performed using Raman spectroscopy.

[0187] For CZTS-350 °C, 2° phases were observed with peaks at 150 and 222 cm'1(SnS), the strongest peak at 314 cm'1(SnS2 or CusSnS4 orthorhombic) corresponding to a Cu-rich composition, and CuS at 464 cm'1. CZTS-450 °C exhibited minor peaks of SnS (100 and 190 cm'1) and ZnS (257 and 346 cm'1) due to Zn-rich composition, along with small unidentified peaks at 406 and 484 These observations of binary and ternary secondary phases are consistent with XRD and EDS analyses at 350 °C and 450 °C. CZTS-550 °C displayed an intense peak at 337 cm'1confirming the CZTS kesterite structure, with a shoulder peak at 285 cm'1attributed to ZnS surface vibrations. Similarly, CZTS-650 °C showed the intense peak at 336 cm'1with a shoulder at 285 cm'1. These results indicate that CZTS-550 °C exhibits single-phase kesterite formation, consistent with the EDS-derived elemental composition ratio of Cu:Zn:Sn:S = 2: 1 : 1 :4.

[0188] UV-Vis DRS studies: Band-gap analysis

[0189] Fig. 11D illustrates band gap analysis using absorption coefficient (a) versus the photon energy (hv) plot and Tauc’s plot of Solution-based Kesterite film of Samples Sol-1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C, and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere. The energy band gap of Cu2ZnSnS4 is calculated by Tauc’s plot where a represents absorption coefficient, h being Planck constant, v represents frequency, hv being energy of photons, Eg represents energy band gap, A being absorbance and n is a constant. Using Fresnel’s equation to calculate the absorption coefficient

[0040] , where the refractive index = 2.85 and that of air is 1.0003

[0190] Hence

[0191] Rearranging the above equation

[0192] 1 , 9.1225(1 - 7?)2a = - In d T l - T?2) where d is the thickness of the film (6pm), Tf represents transmission coefficient of the film (VIo) and R represents reflectance coefficient of CZTS surface, a ~ 104to 105cm-1for CZTS-350°, 450°, 550° and 650°C is satisfactory agreement with previous literature [41-44], Fig. 11D shows the Eg value of 1.42 eV for 350° and 450°C and 1.51 eV for 550° and 650°C. This band gap values are in good agreement with reported values. Shift in lower band gap for 350° and 450°C is due to presence of secondary phase such as C SnSs and CTS. However, Eg shift towards expected optimum value (1.5 eV) of CZTS with increase in sulfurization temperature. The narrower band gap of 1.51 eV for 550° and 650°C is nearest optimal Eg value of kesterite CZTS structure was confirmed and is suitable for solar cell absorber layer.

[0193] Vibrational analysis

[0194] Fig. HE illustrates FTIR spectra of Solution-based Kesterite film of Samples Sol- 1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0195] CZTS samples was recorded in FTIR spectrum (500 to 4000 cm'1) as shown in Fig. HE. For CZTS-3500and 450°C, broad peak at 3434 and 3432cm'1hydroxyl group (O-H) stretching mode, 900-1600cm'1arises from oxygen stretching frequency. Peak at 1626cm'1symmetrical (C-O) stretching mode of vibration and 1366cm'1antisymmetric (C-O) stretching vibration, C-0 symmetric stretching frequency (1119cm'1) and ZnS band (619 and 618cm-1) due to functionalized surfaces of Cu2SnSs attracts Zn2+ions from aqueous zinc acetate solution. Thus, zinc hydroxide layers were formed on the Cu2SnSs surface. This formation of Cu2ZnSnS4 happens through replacement of OH' by S2' ions.

[0196] For CZTS samples formed at 550° and 650°C, 618 and 616cm'1ZnS band, 3438 and 3433cm'1sulphur rich composition, 1639 and 1638cm'1and 1385 and 1366cm'1represents symmetrical and antisymmetric (C-O bond) stretching vibrations, 1117 and 1123 cm'1C-O symmetric stretching frequency. As CZTS is hygroscopic in nature, the surface H2O involves in formation of O-H and C-O bands as seen in FTIR.

[0197] Electrochemical studies

[0198] Cyclic voltammetry (CV) analysis

[0199] Fig. 11F illustrates Cyclic voltammetry studies of Solution-based Kesterite film of Samples Sol-1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere. Three-electrode system was done in 0.1M NaCl electrolyte. CZTS was coated on platinum (Pt) electrode via drop casting method, Pt wire is used as counter and (Ag / AgCl) reference electrode and following electrochemical reactions of Copper, Zinc, Tin and Sulphur with respect to the Ag / AgCl.

[0200] From the above equations, standard electrode potential difference of Copper-Zinc- Tin-Sulphur. (i) CZTS (350°C), peaks at 0.31V and 0.45V indicating Cu2+— > Cu+—> Cu and -0.33 V corresponding to Cu2+—> Cu in the scan rate of 5mVs-1. At scan rate of 50mVs-1, reduction peaks at 0.16V and 0.6V indicates Cu2+—> Cu+ions; Cu+—> Cu. Reverse peak around -0.39V represents Cu2+to Cu attributed to the formation of CZTS along with CusSnS4 indicates copper-rich composition as shown in the Fig. 11F.

[0201] (ii) CZTS- 450°C, slew rate (5mVs-1) at 0.1V, 0.27V corresponds to the Cu2+— > Cu+ions and Cu2+—> Cu. Small hump near -0.52V accounts for Cu+—> Cu. Only ion and mass transfer (do not show any redox peak) was observed at the 50mVs-1as shown in Fig. 11F.

[0202] (iii) whereas CZTS-550°C, forward scan of 50mVs-1around 0.53 V corresponds to Cu+reduction to Cu and 0.09V reducing to Cu2+— > Cu+ions in reverse scan. At 50mVs-1, 0.34V corresponds to Cu2+to Cu and 0.65V due to indicates the Zn. More enhanced peak at 0.07V reduced to Cu2+ions to Cu+ions as shown in Fig. 11F.

[0203] (iv) CZTS- 650°C, the slew rate (5mVs-1) reduction peak 0.36V indicating Cu2+to Cu and 0.65V denotes S2O32' S. At 50mVs-1, 0.32V peak is attributed to the reduction of Cu2+to copper, 0.6V corresponding to reduction of Cu+to copper and 0.17V is due to Cu2+reduction to Cu+respectively as shown in Fig. 11F. Thus, CV concluded that reduction peaks of Cu2+— > Cu+and Cu+—> Cu showing copper-rich compound at 350° and 450°C, sulphur rich compound at 550° and 650°C.

[0204] EIS measurements

[0205] Fig. 11G illustrates Nyquist plots of Solution-based Kesterite film of Samples Sol- 1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0206] Fig. 11H illustrates Fitted circuit and equivalent circuits of Solution-based Kesterite film of Samples Sol-1, Sol-2, Sol-3, and Sol-4 after sintering at 350°C, 450°C, 550°C and 650°C respectively each for 3 hours in 60 grams of sulphur atmosphere.

[0207] Performance of EIS analysis was studied in the range of 10Hz tolOOkHz under dark conditions and Nyquist plots as shown in Fig. 11G and 11H. Suitable fitted equivalent circuit data using ZMAN 2.3 Software consists ohmic resistance Rs, diffusion resistance Ri, charge transfer resistance R2 or Ret, Qyi, and Qy2 denote psedocapacitance constant phase elements (CPE). Rs act as electrode / electrolyte interface indicates interaction between electrolyte of Na+Cl" ions and surface of CZTS (Pt working electrode). Ri diffusion resistance indicates charge diffusion from CZTS surface to bulk electrolyte. High Ri, low diffusion charges at CZTS / electrolyte interface leads to recombination in bulk. Hence recombination at the interface for CZTS-350°C and 450°C, whereas recombination at the bulk for CZTS-550°C and 650°C takes place. The three-electrode assembly with fitted circuit is analogous to those reported in the literature. The R2 or Ret for CZTS - 550°C and 650°C indicates interfacial charge transfer, ideal capacitors has both Qyi and Qy corresponding to a > 0.5 indicating two interfaces such as (i) CZTS / NaCl and (ii) Pt / CZTS / NaCl. In the case of CZTS (350° and 450°C); Qy2 > Qyi indicates separation of charges at electric double layer greater at interface than at bulk. Whereas CZTS (550° and 650°C) charge separation at CZTS / NaCl is higher than Pt / CZTS is noticed, indicating good adhesion of the Pt surface of CZTS. Hence CZTS (550° and 650°C) are suitable materials for solar cell absorber. Solar cell studies

[0208] The solar cell architecture using kesterite Sample Sol-4 as absorber layer is as follows:

[0209] Configuration: Superstate

[0210] Substrate: Glass

[0211] Transparent conducting oxide: ITO (indium tin oxide), FTO (fluorine-doped tin oxide)

[0212] Window layer: ZnO or TiO2

[0213] Buffer layer: CdS or ZnS or Cd doped ZnS

[0214] Absorber: CZTS Sample Sol-4

[0215] Back (Metal) contact: Mo or Ag or Au

[0216] The best device efficiency (power conversion efficiency) obtained was 13.2%.

[0217] Characterisation of Sintered Coated Samples from Example 3 (Melting and alloying-based Kesterite)

[0218] Sample Alloy-1 (Melting-Alloying-based kesterite Ink):

[0219] - Input proportion of metals melted: Cuo.sZno.iSno.i

[0220] - After alloying the EDS indicated the composition to be Cu0.s5Zn0.07Sn0.077

[0221] - The ink was coated onto a glass substrate sintered at 600 °C with 30 grams of Sulphur powder for four hours.

[0222] The Kesterite film of Sample Alloy-1 after sintering was analysed by XRD as shown in Fig. 12. Table 12 presents the corresponding XRD data.

[0223] Table 12

[0224] The data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 33.9859 (7.96), 46.0792 (100), 48.6846 (14.76), 54.6054 (21.41) can be attributed to the presence of C SnSs.

[0225] XRD spectra (smoothed) for Kesterite film of Sample Alloy- 1 after sintering is as shown in Fig. 13. Table 13 presents the corresponding XRD data (smoothed).

[0226] Table 13

[0227] The smoothed data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 46.0785 (100), 54.4654 (31.80) can be attributed to the presence of C SnSs. As can be observed the sample contains both C SnSs and Kesetrite phases in equal proportions, one major peak is for Cu2SnSs but other high intensity peaks are attributed to kesterite phase. SEM and EDS analysis of the sintered Melting-Alloy-based kesterite Ink (Sample Alloy-1).

[0228] Fig. 14A and 14B illustrate SEM images and EDS spectra of Melting- Alloy based Kesterite film of Sample Alloy- 1 after sintering at 600°C in sulphur atmosphere for 4 hours. The corresponding chemical composition data is provided in Table 14.

[0229] Table 14

[0230] The input proportion of Cuo.sZno.iSno.i, after alloying, the EDS indicated the composition to be Cu0.s5Zn0.07Sn0.077.

[0231] Sample Alloy-2 (Melting-Alloying-based kesterite Ink):

[0232] - Input proportion of metals melted: Cuo.7Zno.15Sno.15

[0233] - After alloying the EDS indicated the composition to be Cuo.76Zno.12Sno.12

[0234] - The ink was coated onto a glass substrate sintered at 600 °C with 30 grams of Sulphur powder for four hours.

[0235] The Kesterite film of Sample Alloy-2 after sintering was analysed by XRD as shown in Fig. 15. Table 15 presents the corresponding XRD data.

[0236] Table 15

[0237] The data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 33.1517 (7.68), 34.5374 (39.67), 42.9949 (6.46), 43.8720 (4.58), 48.1111 (18.32), 52.3799 (30.61), 54.1373 (9.17), 68.6848 (18.54), 72.9082 (5.74), 77.2187 (2.48), 79.2456 (3.45) can be attributed to the presence of C SnSs. XRD spectra (smoothed) for Kesterite film of Sample Alloy -2 after sintering is as shown in Fig. 16. Table 16 presents the corresponding XRD data (smoothed).

[0238] Table 16

[0239] The smoothed data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 34.6032 (43.94), 49.5102 (19.72) can be attributed to the presence of C SnSs. As can be observed the sample has major wurtzite CZTS peaks. Major peaks are not mapped.

[0240] SEM and EDS analysis of the sintered Melting-Alloy-based kesterite Ink (Sample Alloy-2).

[0241] Fig. 17A and 17B illustrate SEM images and EDS spectra of Melting- Alloy based Kesterite film of Sample Alloy -2 after sintering at 600°C in sulphur atmosphere for

[0242] 4 hours. The corresponding chemical composition data is provided in Table 17.

[0243] Table 17

[0244] The input proportion of Cuo.7Zno.15Sno.15, after alloying, the EDS indicated the composition to be Cuo.76Zno.12Sno.12. Sample Alloy-3 (Melting-Alloying-based kesterite Ink):

[0245] - Input proportion of metals melted: Cuo.4Zno.3Sno.3

[0246] - After alloying the EDS indicated the composition to be Cuo.53Zno.27Sno,2

[0247] - The ink was coated onto a glass substrate sintered at 600 °C with 30 grams of Sulphur powder for four hours.

[0248] The Kesterite film of Sample Alloy-3 after sintering was analysed by XRD as shown in Fig. 18. Table 18 presents the corresponding XRD data.

[0249] Table 18

[0250]

[0251] The data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 25.0644 (3.17), 34.4965 (42.74), 36.1189 (12.81), 42.3797 (17.57), 54.1437 (3.26), 68.5166 (24.92), 71.8792 (2.57), 77.7119 (11.32) can be attributed to the presence of C SnSs.

[0252] XRD spectra (smoothed) for Kesterite film of Sample Alloy-3 after sintering is as shown in Fig. 19. Table 19 presents the corresponding XRD data (smoothed). Table 19

[0253] The smoothed data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 34.7915 (82.49), 43.8595 (100), 48.2215 (44.72) can be attributed to the presence of Cu2SnS3. SEM and EDS analysis of the sintered Melting-Alloy-based kesterite Ink (Sample Alloy-3).

[0254] Fig. 20A and 20B illustrate SEM images and EDS spectra of Melting- Alloy based Kesterite film of Sample Alloy-3 after sintering at 600°C in sulphur atmosphere for 4 hours. The corresponding chemical composition data is provided in Table 20.

[0255] Table 20

[0256] The input proportion of Cuo.4Zno.3Sno.3, after alloying, the EDS indicated the composition to be Cuo.53Zno.27Sno,2.

[0257] Sample Alloy-4 (Melting-Alloying-based kesterite Ink):

[0258] - Input proportion of metals melted: CuojZno Sncu

[0259] - After alloying the EDS indicated the composition to be Cuo.33Zno.2sSno.39

[0260] - The ink was coated onto a glass substrate sintered at 600 °C with 30 grams of Sulphur powder for four hours.

[0261] The Kesterite film of Sample Alloy-4 after sintering was analysed by XRD as shown in Fig. 21. Table 21 presents the corresponding XRD data.

[0262] Table 21

[0263] The data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 33.7087 (5.29), 34.5623 (29.76), 36.9256 (100), 42.3595 (4.86), 43.8191 (72.99), 46.6428 (1.76), 54.0697 (7.12), 68.5084 (20.81), 71.6027 (5.55), 74.7887 (3.02) can be attributed to the presence of Cu2SnS3.

[0264] XRD spectra (smoothed) for Kesterite film of Sample Alloy-4 after sintering is as shown in Fig. 22. Table 22 presents the corresponding XRD data (smoothed).

[0265] Table 22

[0266]

[0267] The smoothed data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 34.8518 (17.65), 43.8586 (100), 48.2134 (61.72), 54.0901 (8.15), 57.1821 (54.41), 68.5249 (15.25), 71.6463 (10.95), 76.8655 (18.15) can be attributed to the presence of C SnSs.

[0268] SEM and EDS analysis of the sintered Melting-Alloy-based kesterite Ink (Sample Alloy-4). Fig. 23A and 23B illustrate SEM images and EDS spectra of Melting- Alloy based Kesterite film of Sample Alloy -4 after sintering at 600°C in sulphur atmosphere for 4 hours. The corresponding chemical composition data is provided in Table 23.

[0269] Table 23

[0270] The input proportion of Cuo.3Zno.3Sno .4, after alloying, the EDS indicated the composition to be Cuo.33Zno.2sSno.39. Sample Alloy-5 (Melting-Alloying-based kesterite Ink):

[0271] - Input proportion of metals melted: Cuo.i5Zno.25Sno.6

[0272] - After alloying the EDS indicated the composition to be Cuo.11Zno.2Sno.69

[0273] - The ink was coated onto a glass substrate sintered at 600 °C with 30 grams of Sulphur powder for four hours.

[0274] The Kesterite film of Sample Alloy-5 after sintering was analysed by XRD as shown in Fig. 24. Table 24 presents the corresponding XRD data.

[0275] Table 24

[0276]

[0277] The data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 20.9840 (3.53), 31.4077 (74.72), 43.7950 (91.84), 45.6335 (76.36), 68.5603 (13.43), 73.0787 (12.75) can be attributed to the presence of C SnSs.

[0278] XRD spectra (smoothed) for Kesterite film of Sample Alloy-5 after sintering is as shown in Fig. 25. Table 25 presents the corresponding XRD data (smoothed). Table 25

[0279] The smoothed data shows good agreement with the JCPDS database 26-0575. Certain peaks that did not find a close match with the literature i.e. 31.2250 (51.93), 35.1341 (36.13), 43.9055 (54.78), 45.7127 (45.29), 48.3167 (25.46), 73.4383 (8.96) can be attributed to the presence of C SnSs.

[0280] SEM and EDS analysis of the sintered Melting-Alloy-based kesterite Ink (Sample Alloy-5).

[0281] Fig. 26A and 26B illustrate SEM images and EDS spectra of Melting- Alloy based Kesterite film of Sample Alloy-5 after sintering at 600°C in sulphur atmosphere for 4 hours. The corresponding chemical composition data is provided in Table 26.

[0282] Table 26

[0283] The input proportion of Cuo.i5Zno.25Sno.6, after alloying, the EDS indicated the composition to be Cuo.11Zno.2Sno.69.

[0284] Advantages:

[0285] The kesterite absorber layers of the present disclosure has the following nonlimiting advantages.

[0286] - Microwave-assisted method enables fastest route for formation of kesterite and also scale up is much easier

[0287] - Solution-based method enables uniform particle size of the keseterite

[0288] - Melting-alloying method enables simple, direct synthesis route with high yield of bulk material.

[0289] - Excess sulphur during sintering suppresses the formation of secondary phases (SnS, SnS2, Cu2SnSs, CusSnS4, ZnS) that typically coexist with CZTS, thereby yielding predominantly single-phase kesterite CZTS. - Sintering at the optimum temperature promotes larger crystallite size and reduced lattice defects.

[0290] - By minimizing secondary phases improved crystallinity.

[0291] - The controlled excess sulphur sintering process provides consistent phase formation and material properties, which is crucial for large-scale fabrication of thin-film solar devices.

[0292] - The continuous excess sulphur environment also facilitates kesterite formation in atmospheric conditions i.e., non-vacuum, where the excess sulphur exists as oxygen getter and does not allow the precursors to react with oxygen forming undesired oxides in the mixture

[0293] - This also enables to achieve efficiency at solar device level of close to 12% (very close to theoretical efficiency of (90% of 13.2%)

[0294] Although the present disclosure is described in terms of one or more embodiments, it is to be understood that they have been presented by way of example, and are not limiting. Thus, the present disclosure should not be limited by any of the abovedescribed exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

CLAIMS1. A method for preparing kesterite (Cu2ZnSnS4, CZTS) thin film absorber layers for solar cells, the method comprising:- synthesising precursors of Cu, Zn, and Sn by a process selected from:• microwave-assisted synthesis, wherein aqueous or alcoholic solutions of Cu, Zn, and Sn salts are subjected to microwave irradiation;• solution-based synthesis, wherein CuCh, ZnCh, and SnCh in a weight ratio of 2: 1 : 1 are reacted with thiourea in an organic solvent selected from 2-methoxyethanol or 2-butoxyethanol, in the presence of 0.1-0.5 ml of a stabilizer selected from monoethanolamine or triethanolamine per 10 ml of solution, with stirring for about 2 hours at 45 °C to form a coating ink; or• melting-alloying synthesis, wherein metallic Cu, Zn, and Sn are melted together in amounts of 0.1 to 0.8 parts Cu, 0.1 to 0.3 parts Zn, and 0.1 to 0.6 parts Sn to form an alloy precursor;- preparing a coating ink by dispersing the precursors in an organic medium containing about 4% by weight of a sugar-based binder and 4% by weight of polyethylene glycol;- coating the ink on a substrate selected from metal, metal alloy, or glass; and- sintering the coated substrate at a temperature between 350 °C and 850 °C under an atmosphere containing sulphur in an amount 4-6 times the total dry weight of the precursors, thereby obtaining a kesterite CZTS thin film substantially free from secondary phases including Cu2SnSs and wurtzite.

2. The method as claimed in claim 1, wherein the microwave-assisted synthesis is carried out by irradiating precursor solutions at a frequency of 2-3 GHz with an input power of 2-5 kW.

3. The method as claimed in claim 1, wherein the X-ray diffraction of the sintered films shows a distinct (105) epitaxial peak corresponding to CZTS structure.

4. The method as claimed in claim 1, wherein the substrate is molybdenum-coated glass, stainless steel, or soda lime glass, and wherein the film thickness is about 30 gm.

5. The method as claimed in claim 1, wherein for melting-alloying synthesis, metallic Cu, Zn, and Sn are melted together in atomic ratios selected from Cuo.sZno.iSno.i; Cuo.7Zno.15Sno.15; Cuo.4Zno.3Sno,3; Cuo Zno Sncu; Cuo.i5Zno.25Sno.6 to form the alloy precursor.

6. The method as claimed in claim 1, wherein Raman spectroscopy of the sintered CZTS thin film exhibits:- peaks at 337 cm'1and 336 cm'1confirming kesterite CZTS formation at 550 °C and 650 °C, respectively, and- shoulder peaks at 285 cm'1corresponding to ZnS surface vibrations,- wherein the films sintered at 550 °C are substantially single-phase CZTS free of secondary phases.

7. The method as claimed in claim 1, wherein FTIR spectra of the CZTS thin films exhibit absorption bands at about 3434 cm'1and 3432 cm'1(O-H stretching), 1626 cm'1(C-0 stretching), 1366 cm'1(antisymmetric C-0 stretching), 1119cm' 1 (symmetric C-0 stretching), and 618 cm'1and 619 cm'1(ZnS bands), confirming ZnS functionalization and sulphur substitution in the CZTS structure.

8. The method as claimed in claim 1, wherein the sintered CZTS thin film exhibits an optical band gap of about 1.42 eV when sintered at 350 °C and 450 °C under an atmosphere containing sulphur in an amount 5-6 times the dry weight of the precursors, and an optical band gap of about 1.51 eV when sintered at 550 °C and 650 °C under an atmosphere containing sulphur in an amount 5-6 times the dry weight of the precursors.

9. The method as claimed in claim 1, wherein cyclic voltammetry studies of the CZTS thin films exhibit reduction peaks corresponding to Cu2+—> Cu+and Cu+Cu transitions, with copper-rich phases observed at 350 °C and 450 °C, and sulphur-rich compositions observed at 550 °C and 650 °C.

10. A solar cell comprising:- a substrate selected from metal, metal alloy, or glass;- a kesterite CZTS absorber layer prepared by solution-based synthesis as claimed in claim 1, which is sintered at 650 °C under an atmosphere containing sulphur in an amount 5-6 times the dry weight of the precursors CuCh, ZnCl2, and SnCh;- a buffer layer selected from CdS, ZnS, or Cd-doped ZnS,- a window layer selected from ZnO or TiO2,- a transparent conducting oxide layer selected from ITO or FTO; and- a back contact selected from Mo, Ag, or Au.

11. The solar cell as claimed in claim 10, wherein the device exhibits a device efficiency of about 13.2%.

12. A microwave-assisted method for synthesizing and processing kesterite for thin film solar cells, the method comprising:- preparing precursors of CuS, ZnS, and SnS by reacting respective chlorides with sodium sulphide to form precipitates, followed by subjecting the precipitates to microwave irradiation at a frequency of 2 to 3 GHz, with an input power of 2 to 5 kW for a duration between 100 and 120 minutes under an operating current of 4.8 A, and a voltage of about 396 V, wherein the input power is adjusted based on reflective power during irradiation, and grinding the dried precipitates to obtain fine powders;- preparing an ink by mixing the precursor powders in a weight ratio of 2: 1 : 1 of CuS:ZnS:SnS, combining the mixture with an alcoholic solvent selected from methanol, ethanol, isoamyl alcohol, and isoamyl acetate, and further incorporating about 4% by weight of a sugar-based binder and about 4% by weight of polyethylene glycol as additives;- coating the ink onto a substrate selected from metals, metal alloys, aluminium, aluminium alloys, copper, Mo-coated glass, or soda lime glass to obtain a film of about 30 pm thickness; and- sintering the coated film at a temperature of about 350 °C to 850 °C for 3 to 5 hours in an atmosphere containing excess sulphur, wherein the sulphur is present at 4 to 6 times by weight of the total precursor components of Cu, Zn, and Sn on a dry basis.

13. A solution-based method for synthesizing and processing kesterite for thin film solar cells, the method comprising:- preparing a coating ink by combining precursors comprising 2M CuCh, IM ZnCh, and IM SnCh in a weight ratio of 2: 1 : 1 with 8M thiourea in an organic solvent selected from 2-methoxyethanol or 2-butoxyethanol, and adding a stabilizer selected from monoethanolamine or triethanolamine in an amount of 0.1 to 0.5 ml per 10 ml of solution, followed by stirring the mixture for about 2 hours at a temperature of about 45 °C to obtain a homogeneous ink;- coating the ink onto a substrate selected from metals, metal alloys, or glass, wherein the glass substrate is selected from molybdenum-coated glass or soda lime glass, to form a film;- baking the coated film at a temperature of about 200 °C for about 5 minutes and subsequently cooling to room temperature;- repeating the coating and baking steps until the film attains a thickness of about 30 pm; and- sintering the coated film at a temperature of about 350 °C to 850 °C for a duration of 3 to 5 hours in an atmosphere containing excess sulphur, wherein the sulphur is present in an amount of 4 to 6 times by weight of the total weight of the precursor components of Cu, Zn, and Sn on a dry basis.

14. A method for preparing a kesterite thin film for use in solar cells, the method comprising:- providing precursor compositions of copper (Cu), zinc (Zn), and tin (Sn) in amounts of 0.1 to 0.8 parts Cu, 0.1 to 0.3 parts Zn, and 0.1 to 0.6 parts Sn;- melting and alloying the precursors to form an alloy;- powdering the alloy;- combining the powdered alloy with an alcoholic solvent selected from methanol, ethanol, isoamyl alcohol, or isoamyl acetate, and further adding about 4% by weight of a sugar-based binder and about 4% by weight of polyethylene glycol to form a coating ink;- coating the ink onto a substrate selected from metals, metal alloys, or glass, wherein the metal or metal alloy is aluminium, aluminium alloy, or copper, and the glass is selected from Mo-coated glass or soda lime glass;- repeating the coating until a film thickness of about 30 pm is obtained; and- sintering the coated film at a temperature of about 350 °C to 850 °C for 3 to 5 hours in an atmosphere containing excess sulphur, wherein the sulphur is present in an amount 4 to 6 times by weight relative to the total weight of Cu, Zn, and Sn precursors.

15. The method as claimed in claim 14, wherein metallic Cu, Zn, and Sn are melted together in atomic ratios selected from Cuo.sZno.iSno.i; Cuo.7Zno.15Sno.15; Cuo.4Zno.3Sno,3; Cuo.3Zno.3Sno.4; Cuo.i5Zno.25Sno.6 to form the alloy precursor.

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