Sulfurization time effects on the growth of Cu2ZnSnS4 thin films by solution method

Cu₂ZnSnS₄ (CZTS) films were obtained by sulfurizing (Cu, Sn) S/ZnS structured precursors prepared by a combination of the successive ionic layer absorption and reaction method and the chemical bath deposition method, respectively. The effect of sulfurization time on structure, composition and optical properties of these CZTS thin films was studied. The results of energy dispersive spectroscopy indicate that the annealed CZTS thin films are of Cu-poor and Zn-rich states. The X-ray diffraction studies reveal that Cu_2?x S phase exists in the annealed CZTS thin film prepared by sulfurization for 20 min, while the Raman spectroscopy analysis shows that there is a small Cu₂SnS₃ phase existing in those by sulfurization for 20 and 40 min. The band gap (E _g ) of the annealed CZTS thin films, which are determined by reflection spectroscopy, varies from 1.49 to 1.56 eV depending on sulfurization time. The best CZTS thin film is the one prepared by sulfurization for 80 min, exhibiting a single kesterite structure, dense morphology, ideal band gap (E _g  = 1.55 eV) and high optical absorption coefficient (>10⁴ cm^?1).     

Fabrication of Cu<Subscript>2</Subscript>SnS<Subscript>3</Subscript> thin-film solar cells with oxide precursor by pulsed laser deposition

In this paper, Cu₂SnS₃ (CTS) thin film is fabricated through sulfurization of oxide precursor which is deposited by pulsed laser deposition with a mixed CuO/SnO₂ target. XRD and Raman analyses indicate a pure monoclinic Cu₂SnS₃ phase has been obtained by sulfurization at temperature from 500 to 600 °C. A compact and smooth film with polycrystalline structure is observed through SEM result. In addition, the CTS films show excellent absorbance with the band gap around 0.91 eV estimated by UV–Vis, which is suitable for the absorption layer of solar cells. Final devices were fabricated with a SLG/Mo/CTS/CdS/i-ZnO/AZO/Al structure. Device performance is improved with the temperature increasing. The best efficiency of CTS-based solar cells is 0.69% with an open-circuit voltage of 144 mV and a short-circuit current density of 18.30 mA/cm^?2.     

Phase transformation in Cu2SnS3 (CTS) thin films through pre-treatment in sulfur atmosphere

In this study, Cu₂SnS₃ (CTS) thin films prepared by a two-step sulfurization process were characterized. Cu and Sn metallic layers were first deposited on glass substrates by sputtering and then annealed in-situ while in the sputtering chamber to obtain CuSn (CT) alloys. This was followed by a pre-treatment step at temperatures between 200 and 350 °C in presence of S vapors. Finally, a full sulfurization step was performed at 525 °C to obtain the desired CTS phase. CTS films were characterized using EDX, XRD, Raman spectroscopy, SEM, optical transmission and Van der Pauw methods. It was found that all CTS samples had Cu-poor chemical composition. XRD data revealed only diffraction peaks belonging to CTS structure after the full sulfurization step. Raman spectra of the samples showed that except for the CTS sample pre-treated at 250 °C (CTS-250), which displayed the tetragonal crystal system, the films were dominated by the monoclinic structure. SEM surface images showed dense and polycrystalline microstructure, CTS-200 sample exhibiting a more uniform morphology. Optical band gap values were found to be ranging from 0.92 to 1.19 eV. All samples showed p-type conductivity but the sample pre-treated at 350 °C had higher resistivity and lower carrier concentration values. Overall, the CTS layer prepared using the pre-treatment step at 200 °C exhibited more promising structural and optical properties for potential photovoltaic applications. This work demonstrated that it is possible to change the crystal structure of sulfurized CTS thin films through a pre-treatment step.

     

Formation of Cu2SnS3 thin film by the heat treatment of electrodeposited SnS–Cu layers

Thin films of copper tin sulfide (Cu₂SnS₃) were obtained by sulfurizing a stack of thin layers of Cu and SnS in nitrogen atmosphere. The film stack was obtained by the sequential electrodeposition of SnS and Cu. The Cu₂SnS₃ film was characterized for structural, morphological, composition, optical, spectroscopic, and electrical properties. The optimum condition for the formation of Cu₂SnS₃ was developed after testing different sulfurization temperatures. The films were polycrystalline with monoclinic structure which was confirmed by Raman and transmission electron microscopy analysis. The interplanar spacings estimated from the high resolution transmission electron microscopy images are 2.74, 2.19, and 2.06 Å. The average crystallite size is 13 nm, and the band gap of the film is in the range of 1 eV. The surface chemical composition determined by X-ray photoelectron spectroscopy showed the Cu:Sn:S ratio as 1.9:1:2.85 which is close to the stoichiometric Cu₂SnS₃. The films are p-type, photosensitive, and the conductivity measured in dark was in the range of 4 × 10^?3 Ω^?1 cm^?1. The comprehensive characterization presented in this paper will update the knowledge on this material.     

Structural and optical properties of layer-by-layer solution deposited Cu2SnS3 films

Cu₂SnS₃ (CTS) is a simple and potential material for low-cost thin film solar cells. The present work incorporates the study of changes in structural and optical properties of layer-by-layer solution deposited CTS films with annealing. Raman spectroscopy is used to ascertain structural modification upon annealing. Increase in annealing temperature leads to a structural transition from tetragonal to cubic phase. Effect of annealing on optical properties of the films is evaluated in the wavelength range of 400–2,400 nm. It is proposed that layer-by-layer growth method fundamentally defines the optical behaviour of these films. Optical constants and parameters such as refractive indices, dielectric constants and electron energy loss function are calculated from transmittance and reflectance data. The refractive indices, n and k are determined to be in ranges of 1.8–2.2 and 0.18–1.2, respectively. The real and imaginary dielectric constants vary from 1.5 to 4.6 and 0.7 to 5, respectively. Dispersion of refractive index is analyzed using two different theoretical models of Wemple–diDomenico and Spitzer–Fan.     

The influence of annealing atmosphere on the phase formation of Cu–Sn–S ternary compound by SILAR method

The influence of annealing atmosphere on the phase formation of Cu–Sn–S ternary compound by SILAR method was studied. Structural, optical and electrical properties of the compound were studied for the samples annealed at 420 °C for 1 h in different atmosphere. X-ray diffraction and Raman spectra showed that Cu₂SnS₃ cubic phase was obtained in an atmosphere of nitrogen and sulfur vapor mixture, while Cu₄SnS₄ orthorhombic phase was obtained in H₂S atmosphere. An optical band-gap of 0.98 eV was obtained for Cu₂SnS₃ and 0.93 eV for Cu₄SnS₄ phase. The activation energies are about 0.1 eV for Cu₂SnS₃ phase and 0.06 eV for the Cu₄SnS₄ phase in high temperature region, but those are about 0.007 and 0.009 eV for them in low temperature region respectively.     

Toward Ultrahigh Thermoelectric Performance of Cu2SnS3-Based Materials by Analog Alloying

Cu₂SnS₃ is a promising thermoelectric candidate for power generation at medium temperature due to its low-cost and environmental-benign features. However, the high electrical resistivity due to low hole concentration severely restricts its final thermoelectric performance. Here, analog alloying with CuInSe₂ is first adopted to optimize the electrical resistivity by promoting the formation of Sn vacancies and the precipitation of In, and optimize lattice thermal conductivity through the formation of stacking faults and nanotwins. Such analog alloying enables a greatly enhanced power factor of 8.03 µW cm⁻¹ K⁻² and a largely reduced lattice thermal conductivity of 0.38 W m⁻¹ K⁻¹ for Cu₂SnS₃ – 9 mol.% CuInSe₂. Eventually, a peak ZT as high as 1.14 at 773 K is achieved for Cu₂SnS₃ – 9 mol.% CuInSe₂, which is one of the highest ZT among the researches on Cu₂SnS₃-based thermoelectric materials. The work implies analog alloying with CuInSe₂ is a very effective route to unleash superior thermoelectric performance of Cu₂SnS₃.     

Dependence of the properties of copper zinc tin sulfide thin films prepared by electrochemical deposition on sulfurization temperature

Copper zinc tin sulfide (CZTS, Cu₂ZnSnS₄) is a low band gap semiconductor that is attractive for use in solar cells. We investigated the dependence of the structure and properties of CZTS thin films on the temperature used to sulfurize precursor thin films composed of copper, zinc and tin fabricated by electrochemical deposition. The precursor films were sulfurized in a furnace with three zones, which allowed fine control of the sulfurization temperature between 250 and 400 °C. X-ray diffraction and Raman spectroscopic measurements confirmed that the films were composed of CZTS following sulfurization. The grain size and crystallinity of the films increased with sulfurization temperature. The composition of CZTS also varied with sulfurization temperature. The proportions of Cu and Zn increased while that of Sn decreased with increasing sulfurization temperature. Absorption and reflectance spectra revealed that the absorption coefficients and band gaps of the CZTS films varied with sulfurization temperature between 3–4.1 × 10⁴ cm^?1 and 1.4–1.53 eV, respectively. Solar cells containing CZTS sulfurized at 400 °C showed a maximum efficiency of 2.04 %, which was attributed to the higher crystallinity and larger grain size of CTZS compared with thin films sulfurized at lower temperatures. Our results show that control of sulfurization temperature is an important factor in optimizing the performance of CZTS thin films in solar cells.     

A study of the structural,optical, and electrical properties of SnS<Subscript>2</Subscript>:Cu optical semiconductor thin films deposited by the spray pyrolysis technique

Cu-doped tin-sulfide thin films were deposited onto glass substrates at T = 400 °C through spray pyrolysis. The effects of Cu doping on the structural, optical, and electrical properties of the thin films were investigated. The precursor solution was prepared by dissolving tin chloride (SnCl₄·5H₂O) and thiourea (CS(NH₃)₂) in deionized water and then adding copper chloride (Cl₂Cu₂H₂O). SnS₂:Cu thin films were prepared with \(\frac{{\left[ {Cu} \right]}}{{\left[ {Sn} \right]}}\% = 0, 1, 2, 3, 4 \,{\text{at}}.\%\). X-ray diffraction analysis showed that the thin films had a preferred (001) orientation of the SnS₂ phase and that the intensity of the (001) peak decreased with increased doping concentration from 1–4 at.%. Scanning electron microscopy studies indicated that the thin films had spherical grains. Characterization results of thin films showed that single-crystal grains, average grain size, optical band gap, carrier concentration, Hall mobility, and electrical resistance varied within 5–14 nm, 46–104 nm, 2.81–2.99 eV, 2.42 × 10¹⁶–26.73 × 10¹⁶ cm^?3, 2.41 × 10^?3–20.04 × 10^?3 cm²/v.s, and 9.05–12.89 Ω cm, respectively. Hall effect studies further revealed that the films exhibited n-type conductivity.     

The crystallisation of Cu2ZnSnS4 thin film solar cell absorbers from co-electroplated Cu-Zn-Sn precursors

The best CZTS solar cell so far was produced by co-sputtering continued with vapour phase sulfurization method. Efficiencies of up to 5.74% were reached by Katagiri et al. The one step electrochemical deposition of copper, zinc, tin and subsequent sulfurization is an alternative fabrication technique for the production of Cu₂ZnSnS₄ based thin film solar cells. A kesterite based solar cell (size 0.5 cm²) with a conversion efficiency of 3.4% (AM1.5) was produced by vapour phase sulfurization of co-electroplated Cu-Zn-Sn films. We report on results of in-situ X-ray diffraction (XRD) experiments during crystallisation of kesterite thin films from electrochemically co-deposited metal films. The kesterite crystallisation is completed by the solid state reaction of Cu₂SnS₃ and ZnS. The measurements show two different reaction paths depending on the metal ratios in the as deposited films. In copper-rich metal films Cu₃Sn and CuZn were found after electrodeposition. In copper-poor or near stoichiometric precursors additional Cu₆Sn₅ and Sn phases were detected. The formation mechanism of Cu₂SnS₃ involves the binary sulphides Cu_2 − xS and SnS₂ in the absence of the binary precursor phase Cu₆Sn₅. The presence of Cu₆Sn₅ leads to a preferred formation of Cu₂SnS₃ via the reaction educts Cu_2 − xS and SnS₂ in the presence of a SnS₂(Cu₄SnS₆) melt. The melt phase may be advantageous in crystallising the kesterite, leading to enhanced grain growth in the presence of a liquid phase.     

Structural and optical properties of the Cu2ZnSnSe4 thin films grown by nano-ink coating and selenization

Quaternary semiconductor Cu₂ZnSnSe₄ (CZTSe) is a very promising alternative to semiconductors based on indium (In) and gallium (Ga) as solar absorber material due to its direct band gap, inherent high absorption coefficient (>10⁴ cm^?1) and abundance of cheap elements zinc (Zn) and tin (Sn). In this study, high quality CZTSe thin films were successfully synthesized by a green and low-cost solution based non-vacuum method, which involves spin coating non-toxic solvent-based CZTSe nano-inks onto Mo coated soda lime glass substrates followed by selenization with elemental Se vapor. The effect of selenization temperature on structural, morphological, compositional and optical properties of CZTSe films are investigated using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and photoluminescence spectroscopy. XRD and Raman analysis indicates that a tetragonal stannite-type structured CZTSe is formed. Depend on the selenization temperature, the dense and compact films with grain sizes from 200 nm (500 °C) up to about 1 μm (580 °C) are obtained. EDS measurement indicates that the composition ratios of the prepared CZTSe films are copper-poor and zinc-rich nature. The CZTSe films are p-type conductivity confirmed by a hot point probe method. Photoluminescence spectrum shows slightly asymmetric narrow bands with a maximum of intensity at 0.92 eV. The dependence of the photoluminescence on the excitation temperature reveals a decrease in the intensity of the photoluminescence bands. An absorption coefficient exceeding 10⁴ cm^?1 and the band gap energy about 0.87 eV of the studied films are determined by an absorption spectroscopy.     

Structural and optical properties of sulfurized Cu2ZnSnS4 thin films from Cu–Zn–Sn alloy precursors

This study reports the preparation of Cu₂ZnSnS₄ (CZTS) thin films by magnetron sputtering deposition with a Cu–Zn–Sn ternary alloy target and sequential sulfurization. The effects of substrate temperatures on the structural, morphological, compositional as well as optical and electrical properties were characterized. The results showed the CZTS thin films prepared by sulfurization at substrate temperature of 570 °C yielded secondary phases along with CZTS compound. The relatively good properties of CZTS thin film were obtained after sulfurization at substrate temperature of 550 °C. This CZTS film showed compact structure with large grain size of 900 nm, direct optical band gap of 1.47 eV, optical absorption coefficient over 10⁴ cm^?1, resistivity of 4.05 Ω cm, carrier concentration of 8.22 × 10¹⁸ cm^?3, and mobility of 43.38 cm² V^?1 S^?1.     

Growth and properties of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin films prepared by multiple metallic layer stacks as a function of sulfurization time

In this paper, we report the two stage growth of Cu₂ZnSnS₄ (CZTS) thin films as a function of sulfurization time. First, magnetron sputtered metallic precursors were deposited sequentially (Zn/Cu/Sn/Cu) over rotating glass substrates held at 230?°C. Later, the sputtered precursors were heat treated at 500?°C in the ambiance of sulfur for various time durations in the range, 10–120 min. The sulfur treated samples were examined using various analytical tools to understand the role of sulfurization time on the CZTS growth and properties. From composition and structural analysis, Zn/Cu/Sn/Cu precursors sulfurized for shorter duration (10 and 20 min) revealed severe deficiency of sulfur that resulted in several metallic, bi-metallic and metal sulfide phases. With the increase of sulfurization time to 30 min, sulfur incorporation was enhanced and reached stoichiometric ratio (~50% S) for CZTS growth, however, samples were poorly crystalline in nature and consisted of prominent Cu_2?xS phase as well. The Zn/Cu/Sn/Cu precursors sulfurized for 60 min exhibited prominent CZTS phase without Cu_2?xS phase. Further, rise in sulfurization time to 120 min enabled drastic improvement in crystallinity of CZTS phase. Raman mapping over 60 µm × 60 µm for these films confirmed the homogeneous phase growth of CZTS. XPS study revealed the oxidation states of Cu¹⁺, Zn²⁺, Sn⁴⁺ and S^2? in CZTS films. The optimized films showed high absorption coefficient of 10⁵ cm^?1 with an optical band gap of 1.51 eV. These films showed leaf like grain morphology with high mobility and low resistivity of 18.2 cm²/V-s and 0.7 Ω-cm, respectively.     

Fabrication of Cu2ZnSnS4 absorber layers with adjustable Zn/Sn and Cu/Zn+Sn ratios

In this work Cu₂ZnSnS₄ (CZTS) thin films were successfully prepared by sulfurization of spin coated CuO + ZnO precursor films under Sn and S ambience with different time. Precursor films were synthesized using air-stable inks consist of carboxylate-capped metal oxide nanoparticles. The composition, microstructure and properties of CZTS thin films prepared with different sulfurization time were investigated using inductively coupled plasma-mass spectrometry, X-ray diffraction, scanning electron microscopy, Raman spectroscopy and UV–vis–NIR spectroscopy. The inductively coupled plasma-mass spectrometry results show that mole ratios of Zn/Sn and Cu/(Zn + Sn) in the films can be adjusted by controlling sulfurization time. A composition of Cu/Zn + Sn = ~0.8, and Zn/Sn = ~1.2 can be reached after sulfurizating with proper time. The influence of element composition change was also studied in our work using X-ray diffraction and Raman scattering. Two laser sources of 325 and 514 nm were involved in the Raman scattering analyze in order to identify secondary phases such as ZnS and Cu_2?xS. The as-prepared CZTS films with a composition of Cu/Zn + Sn = ~0.8, and Zn/Sn = ~1.2 exhibit a direct optical band gap about 1.45 eV.     

Substrate influences on the properties of SnS thin films deposited by chemical spray pyrolysis technique for photovoltaic applications

Herein, we report on tin monosulfide (SnS) thin films elaborated by the Chemical Spray Pyrolysis (CSP) technique onto various substrates as simple glass, ITO-, and Mo-coated glasses in order to study the influence of substrates on the physical and chemical properties of Sns thin films. Structural analysis revealed that all films crystallize in orthorhombic structure with (111) as the sole preferential direction without secondary phases. In addition, film prepared onto pure glass exhibits a better crystallization compared to films deposited onto coated glass substrates. Raman spectroscopy analysis confirms the results obtained by X-ray diffraction with modes corresponding well to SnS single-crystal orthorhombic ones (47, 65, 94, 160, 186, and 219 cm ^?1) without any additional parasite secondary phase like Sn₂S₃ or SnS₂. Field emission scanning electron microscope revealed that all films have a cornflake-like particles surface morphology, and energy dispersive X-ray spectroscopy analysis showed the presence of sulfur and tin with a nearly stoichiometric ratio in films deposited onto pure glass. High surface roughness and large grains are observable in film deposited onto glass. From optical spectroscopy, it is inferred that band gap energy of SnS/glass and SnS/ITO were 1.64 and 1.82 eV, respectively.     

Synthesis and characterization of nanocrystalline TiO<Subscript>2</Subscript> thin films

Nanocrystalline thin films of TiO₂ have been synthesized by sol gel spin coating technique Thin films of TiO₂ annealed at 700 °C were characterized by X-ray diffraction(XRD), Atomic Force Microscopy, High resolution TEM and Scanning Electron Microscopy (SEM), The XRD shows formation of tetragonal anatase and rutile phases with lattice parameters a = 3.7837 Å and c = 9.5087 Å. The surface morphology of the TiO₂ films showed that the nanoparticles are fine with an average grain size of about 60 nm. Optical studies revealed a high absorption coefficient (10⁴ cm^?1) with a direct band gap of 3.24 eV. The films are of the n type conduction with room temperature electrical conductivity of 10^?6 (Ω cm)^?1.     

Highly dispersible and uniform size Cu2ZnSnS4 nanoparticles for photocatalytic application

Highly dispersible, uniform size (～7 nm) single-phase Cu₂ZnSnS₄ nanoparticles have been synthesized by hydrothermal method using non-toxic surfactant (oleic acid). High resolution transmission electron microscopy image indicates good crystallinity of the Cu₂ZnSnS₄ nanoparticles with the growth along (1 1 2) plane. X-ray photoelectron spectroscopy analyses suggested that the formation of with Cu, Zn, and Sn in +1, +2 and +4 oxidation states. The optical absorption spectrum of Cu₂ZnSnS₄ nanoparticles exhibits an absorption in the visible region and its optical band gap was found to be ～1.72 eV, which could be much more appropriate for photocatalytic application under visible light irradiation. These Cu₂ZnSnS₄ nanoparticles have been shown high photocatalytic degradation activity of methylene blue (MB) dye in the presence of visible light irradiation. The rate constant (k) value of Cu₂ZnSnS₄ nanoparticles is found to be 0.0144 min^?1. We have discussed the mechanism of dye degradation process that drives the photocatalytic degradation process. The reusability of the Cu₂ZnSnS₄ nanoparticles for the dye degradation is also demonstrated.     

Structural and optical properties of SnS nanoparticles and electron-beam-evaporated SnS thin films

SnS nanoparticles were synthesised by the precipitation method using SnCl₂.2H₂O and Na₂S.xH₂O and the nanoparticles were characterised by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analysis. From the particles’ XRD pattern, a strong peak at 2θ = 31.5? was observed, which confirms the Herzenbergite orthorhombic crystal structure of SnS. The FTIR result also confirmed the SnS nanoparticles at 2354 cm^?1 and 615 cm^?1. Second, thin SnS films were prepared on a glass substrate by the electron beam evaporation technique at room temperature and annealed at 100°C, 200°C and 300°C. The effect of the annealing temperature on structural and optical properties of the SnS films was characterised by XRD and ultraviolet–visible (UV–Vis) analysis. From the experimental studies, optical absorption of SnS films increases with respect to the annealing temperature, while the values of band gap energy (E_g) get reduced from 1.77 to 1.57 eV.     

Cu2ZnSnS4 films by paste coating and their optoelectronic properties

Cu₂ZnSnS₄ (CZTS) thin films were prepared by a paste coating method as the absorb layer of solar cells. This method is more eco-friendly using ethanol as solvent and more convenient than traditional sol–gel method. The effects of sulfurization temperature on properties of thin film were studied. The results of X-ray diffraction and Raman spectroscopy showed the formation of kesterite structure of CZTS films. The scanning electron microscopy images revealed that CZTS thin film obtained at 550 °C were compact and uniform. The optical band gap of the CZTS film was about 1.5 eV, and the CZTS film had an obvious optoelectronic response. Moreover, CZTS solar cell was prepared with a conversion efficiency of 0.47 %.     

Influence of Ag concentration on the structure,optical and electrical properties of SnS<Subscript>2</Subscript>:Ag thin films prepared by spray pyrolysis deposition

Ag-doped tin-sulfide thin films were deposited with in spray pyrolysis method at T = 425 °C on soda lime glass substrates. The effects of Ag doping were investigated on the structural, optical, and electrical properties of thin films. Double deionized water was used as a precursor solution in which tin chloride (SnCl₄5H₂O) and thiourea (CS(NH₃)₂) in addition to silver acetate (AgC₂H₃O₂) were dissolved. All in all resulted to preparation of SnS₂:Ag thin films with \(\frac{{\left[ {\text{Ag}} \right]}}{{\left[ {\text{Sn}} \right]}}\% = 0, \,1, \,2, \,3\, {\text{and}} \,4\,{\text{at}}.\%\). The (001) plane is the preferred orientation of the SnS₂ phase which is analyzed by X-ray diffraction (XRD). The intensity of mentioned peak has an increasing trend, generally, with increasing Ag doping concentration. Thin films have spherical grains as is shown in SEM images. Increasing doping concentration from 1 to 4%, causes decrease in: single-crystal grains from 14.68 to 6.31 nm, optical band gap from 2.75 to 2.62 eV, carrier concentration from 3.11 × 10¹⁷ to 2.58 × 10¹⁷ cm^?3, and Hall mobility from 1.81 to 0.13 cm²/v s, as well as increase in: average grain size, generally, from 70 to 79 nm and electrical resistance from 11.11 to 181.26 Ω cm, respectively. The majority carriers are electrons for these films as is concluded from Hall Effect measurements.