Increasing Sulfurization Temperature Research Articles

Fabrication of phase-pure and large-grained Cu3BiS3 films by a two-stage process for thin film solar cells

Copper-bismuth-sulfide (Cu3BiS3) has gained great interest as solar cell absorber owing to the advantages of being cheap, abundant, and non-toxic. Fabrication of phase-pure and large-grain absorbers are desirable for their integration in solar cells. Herein, we have used a sequential evaporation and chalcogenization (350–450 oC) process to fabricate phase-pure and large-grain Cu3BiS3 films. X-ray diffraction indicated a CuS secondary phase at 350 °C and single-phase Cu3BiS3 at 400 and 450 °C in the sulfurized films. Elemental analysis revealed a near-stoichiometric Cu3BiS3 composition for all the films. X-ray photoelectron spectroscopy was used to assess the valence states of elements. Microstructural analysis confirmed the formation of compact and large-grained Cu3BiS3 films with an average grain size of 5 μm at 350 °C. The grain size decreased with increasing sulfurization temperature (400–450 °C). The direct bandgap decreases from 1.42 eV to 1.32 eV with increasing sulfurization temperature from 350 to 450 °C. Hall measurements indicated decreased electrical resistivity and increased hole mobility and carrier concentration with increasing the sulfurization temperature. From this study, the Cu3BiS3 absorbers produced at 400 oC exhibited superior phase purity and good quality suitable for the solar cells.

Structural and Optical Properties of Tungsten Disulfide Nanoscale Films Grown by Sulfurization from W and WO3.

Tungsten disulfide (WS2) was prepared from W metal and WO3 by ion beam sputtering and sulfurization in a different number of layers, including monolayer, bilayer, six-layer, and nine-layer. To obtain better crystallinity, the nine-layer of WS2 was also prepared from W metal and sulfurized in a furnace at different temperatures (800, 850, 900, and 950 °C). X-ray diffraction revealed that WS2 has a 2-H crystal structure and the crystallinity improved with increasing sulfurization temperature, while the crystallinity of WS2 sulfurized from WO3 (WS2-WO3) is better than that sulfurized from W-metal (WS2-W). Raman spectra show that the full-width at half maximum (FWHM) of WS2-WO3 is narrower than that of WS2-W. We demonstrate that high-quality monocrystalline WS2 thin films can be prepared at wafer scale by sulfurization of WO3. The photoluminescence of the WS2 monolayer is strongly enhanced and centered at 1.98 eV. The transmittance of the WS2 monolayer exceeds 80%, and the measured band gap is 1.9 eV, as shown by ultraviolet-visible-infrared spectroscopy.

High-Temperature Coefficient of Resistance in MoxW1−xS2 Thin Film

Despite the use of transition metal dichalcogenides being widespread in various applications, the knowledge and applications of MoxW1−xS2 compounds are relatively limited. In this study, we deposited a MoW alloy on a Si substrate using a sputter system. Consequently, we successfully utilized a furnace to sulfurize the MoW alloy from 800 to 950 °C, which transferred the alloy into a MoxW1−xS2 ternary compound. The Raman spectra of the MoxW1−xS2 samples indicated an additional hybridized Raman peak at 375 cm−1 not present in typical MoS2 and WS2. With increasing sulfurization temperature, the scanning electron microscopy images revealed the surface morphology of the MoxW1−xS2 gradually becoming a sheet-like structure. The X-ray diffraction results showed that the crystal structure of the MoxW1−xS2 tended toward a preferable (002) crystal orientation. The I–V results showed that the resistance of MoxW1−xS2 increased when the samples were sulfurized at a higher temperature due to the more porous structures generated within the thin film. Furthermore, a high-temperature coefficient of resistance for the MoxW1−xS2 thin film sulfurized at 950 °C was about −1.633%/K−1. This coefficient of resistance in a MoxW1−xS2 thin film indicates its suitability for use in thermal sensors.

Study of the Structure and Properties of ZnS Utilized in a Fluorescence Biosensor.

ZnS materials have been widely used in fluorescence biosensors to characterize different types of stem cells due to their excellent fluorescence effect. In this study, ZnS was prepared by vulcanizing nano-Zn particles synthesized using a DC arc plasma. The composition and structure of the ZnS materials were studied by X-ray diffraction (XRD), and their functional group information and optical properties were investigated by using IR spectrophotometry and UV-vis spectrophotometry. It has been found that the synthesized materials consist of Zn, cubic ZnS, and hexagonal ZnS according to the vulcanization parameters. Crystalline ZnS was gradually transformed from a cubic to a hexagonal structure, and the cycling properties first increase, then decrease with increasing sulfurization temperature. There is an optimal curing temperature giving the best cycling performance and specific capacity: the material sulfurized thereat mainly consists of cubic β-ZnS phase with a small quantity of Zn and hexagonal α-ZnS. The cubic phase ZnS has better conductivity than hexagonal ZnS, as evinced by electrochemical impedance spectroscopy (EIS). The ZnS (as prepared) shows board absorption, which can be used in fluorescence biosensors in cell imaging systems.

Investigation of Zn/Sn ratio for improving the material quality of CZTS thin films with the reduction of Cu2-xS secondary phase

In this study, a sputtering system was adopted for the preparation of Cu2ZnSnS4 (CZTS) thin films with high material quality. The system precisely controlled the elemental ratio in a copper-zinc-tin (CZT) precursor for contributing to the grain growth of CZTS crystallites, and reduced the detrimental formation of the Cu2-xS secondary phase during the high-temperature sulfurization process.The CZT precursor was prepared in a layer sequence of Zn/Sn/Cu/Zn, as the Cu-capping layer could protect the underlying Zn/Sn bilayer from volatilization during the high-temperature sulfurization. The CZT precursors were subjected to the sulfurization process at varied temperatures ranging from 480 to 560 °C for 20 min to form the CZTS thin film. In this study, the Cu/(Zn + Sn) and Zn/Sn ratios in the CZTS film were studied by energy dispersive X-ray spectroscopy measurements, and both ratios revealed elevations with an increasing sulfurization temperature, owing to the thermal volatilization of Zn and Sn. Thus, the formation of the undesirable Cu2-xS secondary phase was enhanced, and the surface roughness and density of voids on the CZTS thin films were increased. For a reduced Zn/Sn ratio with a reduced sulfurization temperature, the improved CZTS thin-film quality was demonstrated by the increased X-ray diffraction (XRD) and Raman intensity, along with the reduction of the XRD linewidth of the (112) CZTS crystal phase. A CZTS thin film with a sulfurization condition of 500 °C for 20 min showed a low Zn/Sn ratio (at 1.1), along with the features of a dense and smooth thin-film surface. Moreover, a reduced photoluminescence spectra full width at half maximum (FWHM) value of 92.7 meV was observed.In addition, for further improving the grain growth of CZTS, a pre-annealing process of a CZT precursor was conducted at a temperature of 350 °C for 30 min. An enhancement of the CZTS crystallite size with the stoichiometric formation of a CZTS phase and a reduced Cu2-XS phase was observed, indicating the superior material quality of the CZTS thin films.

Investigation on the sulfurization temperature dependent phase and defect formation of sequentially evaporated Cu-rich CZTS thin films

We report synthesis of Copper Zinc Tin Sulfide (CZTS) thin films using sequential evaporation technique. The as-deposited films were subjected to various sulfurization temperatures ranging from 520 °C to 640 °C. This paper presents a detailed analysis of temperature dependent phase and defect formation in Cu-rich off-stoichiometric CZTS thin films. CZTS thin films moved from F-type to C-type with decreasing Cu/Zn disorder and defect concentration with increasing sulfurization temperature. The resulting films showed less off-stoichiometry in contrast to initial choice of intended off-stoichiometry exhibiting the structural flexibility within the compositional limits. Despite being off-stoichiometric, all the samples exhibited the tetragonal CZTS phase with preferred orientations. Raman analysis showed the disappearance and reappearance of secondary phases with increasing sulfurization temperature. EDS analysis suggested that composition of the films moved towards better stoichiometry till 580 °C before deteriorating again. Composition and lattice parameter values suggested Cu-rich off-stoichiometric nature of sulfurized CZTS thin films. AFM study showed dependence of roughness with sulfurization temperature. Band gap was obtained in the range of 1.28 eV to 1.48 eV from UV–Vis spectroscopy measurement. PL analysis revealed the asymmetric shape and blue shift of the peaks. Band-to-tail recombination becomes pronounced with increase in sulfurization temperature. XPS analysis showed elements are in expected oxidation states.

An investigation on the phase purity of iron pyrite (FeS2) thin films obtained from the sulfurization of hematite (Fe2O3) thin films

Iron pyrite (FeS2) thin films were obtained by sulfurization of hematite (Fe2O3) thin films at different sulfurization temperatures (TS) and sulfurization time (tS). Phase purity of sulfurized films was investigated with X-ray diffraction, Raman Spectroscopy and Vibrating sample magnetometry (VSM). Sulfurization temperature and time were found to have a complementary effect on the phase purity of pyrite films. Secondary phase formation in the pyrite films decreased with increasing sulfurization temperature or time. Pyrite films confirmed to be phase pure with XRD and Raman spectroscopy showed the presence of other phase(s) when analyzed with VSM. Merely XRD measurements were not sufficient to claim the phase purity of pyrite films.

Sulfur treated 1D anodic TiO2 nanotube layers for significant photo- and electroactivity enhancement

In this work, we show that sulfur treated 1D anodic TiO2 nanotube layers lead to improved photo-electrochemical and catalytic properties compared to the blank nanotube layers. This treatment was performed in the evacuated quartz ampoules in the temperature range from 250 to 450 °C. Inspection of the sulfurized nanotube layers via scanning electron microscopy (SEM) and X-ray diffraction (XRD) has disclosed a gradual crystal growth within nanotube walls, represented by TiS2 or TiS3 phases. Optimally sulfurized TiO2 nanotube layers exhibit 3 times enhanced photocurrent in the UV spectral region, compared to the blank counterpart, with a shift of the light absorption up to the wavelength of 550 nm. In addition, the photocatalytic decomposition of a methylene blue aqueous solution using a wavelength of 365 nm is gradually improved with increasing sulfurization temperature. The highest photocatalytic decomposition rate is 2.3 times larger compared to the blank TiO2 nanotube layer. The application of sulfurized TiO2 nanotube layers for the electrocatalytic hydrogen evolution is also discussed.

Sulfurization of planar MoO3 optical crystals: Enhanced Raman response and surface porosity

Surface properties of planar crystals can be controlled for multi-purpose application. In this regards, we demonstrate a successful MoO3 planar crystal growth proposed for a wide tunability in surface characteristics. Such planar crystals are exposed systematically in sulfur gas at different temperatures. A large enhancement in Raman intensity was observed by sulfurization at 300 °C with the maximum of 46 times w.r.t. the non-sulfurized crystals due to surface oxygen vacancy. During sulfurization at higher temperatures, an MoS2 phase was observed to form at the surface of the crystals. Moreover, systematic imaging of surface evolution and optical response with increasing sulfurization temperature show systematic increase in surface porosity, decrease in optical band gap together with significant crystal color changes. Our findings can open pathways towards the growth of planar crystals at large scales with tunable characteristics for application in the fields of optics, sensor and energy.

Sulfurization temperature dependent physical properties of Cu2SnS3 films grown by a two-stage process

Cu2SnS3 (CTS) is a promising absorber for thin film solar cells because of its suitable opto-electronic properties. This article reports the effect of sulfurization temperature (Ts) on the physical properties of CTS thin films deposited by two-stage process. X-ray diffraction and Raman analyses revealed that sulfurized CTS films exhibited different polymorphic forms, such as triclinic structure (at Ts = 350 °C), tetragonal structure (at Ts = 400 °C), and monoclinic structure (at Ts = 450 °C). A phase change from monoclinic CTS to orthorhombic Cu3SnS4 was observed at Ts = 500 °C. The AFM results confirmed that the sulfurized films had the smooth surface without pinholes. The optical band gap was varied in the range, 2.34–1.49 eV with increasing sulfurization temperature from 150 °C to 500 °C. All the sulfurized films showed p-type conducting nature. The obtained results indicated that single phase CTS films prepared in the temperature range of 400–450 °C could be used as an absorber layer for the application of thin film solar cells.

X-ray diffraction and X-ray photoelectron spectroscopy characterization of sulfurized tin thin films deposited by thermal evaporation

In this paper, we report the structural properties of tin sulfide (SnS) thin films deposited by a technique involving thermal evaporation and sulfurization. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to determine the crystallinity and elemental composition of the sulfurized tin thin films. With increasing sulfurization temperature, Sn and its native oxide reacted with S, yielding SnS. XRD showed evidence of crystal growth along the SnS [040] direction in films sulfurized at 260 and 300°C. The growth of SnS crystals was completed at 300°C, and stoichiometry was the highest. Depth profiles of the composition measured by XPS showed a flat sulfur concentration towards the interior of the film sulfurized at 300°C. The hole concentration was 1.6×1015cm−3. The films showed a band gap of 1.2eV and an absorption coefficient of >104cm−1.

Effect of sulfurization temperature on the property of Cu2ZnSnS4 thin film by eco-friendly nanoparticle ink method

Cu2ZnSnS4 (CZTS) thin films were fabricated by a low-cost nanoparticle ink method. The eco-friendly hydrophilic CZTS nanoparticles were mixed with low-cost n-propanol to form nanoparticle ink. To improve crystallinity and remove oxygen element, the CZTS thin films were sulfurized further. The effects of sulfurization temperature on the structure, morphologies, and photovoltaic performances of CZTS thin films were investigated. The results showed that the crystallinity of CZTS thin film was improved with increasing sulfurization temperature. The surface morphology studies demonstrated the formation of compact and homogenous CZTS thin film at a sulfurization temperature of 600 °C. By optimizing thickness of CZTS thin film, the CZTS thin-film solar cell with an optimal efficiency of 2.1% was obtained.

Synthesis and characterization of Cu2FeSnS4 thin films prepared by electrochemical deposition

Quaternary compound Cu2FeSnS4 (CFTS) thin films, a potential absorber layer candidate in thin film solar cells, were successfully synthesized for the first time by electrochemical deposition method with a post-sulfurization treatment at 500°C and 550°C. The samples were analyzed by X-ray diffraction (XRD), Raman spectroscopy (Raman), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible-near-infrared spectrophotometer (UV–vis-NIR spectrophotometer). XRD data and Raman spectra revealed that the CFTS thin films possess a stannite structure and the crystallinity of the samples was improved with the increasing sulfurization temperature. X-ray photoemission spectroscopy analysis of the four constituent elements indicated Cu(I), Fe(II), Sn(IV) and sulfide ions. The direct band gap values of samples with post-sulfurization treatment at 500°C and 550°C were evaluated to be 1.35eV and 1.40eV by UV–vis-NIR absorption spectra respectively, which show that CFTS is promising for photovoltaic applications.

Preparations of Cu2ZnSnS4 thin films and Cu2ZnSnS4/Si heterojunctions on silicon substrates by sputtering

The single-crystalline silicon substrate has close lattice constant with Cu2ZnSnS4 (CZTS), which is beneficial to the crystalline growth of CZTS thin films. In this work, the CZTS thin films were deposited on silicon substrates by sputtering and post-sulfurization to form CZTS/Si heterojunctions. The experimental results indicate the formation of CZTS with preferred orientation of (112) plane. The crystallinity of CZTS enhances and the secondary phases reduce with increasing sulfurization temperature. The fabricated CZTS/Si heterojunctions show rectifying and photovoltaic properties. The photovoltaic properties of CZTS/Si heterojunctions are further analyzed by the absorption of incident photons in the CZTS thin films and silicon substrates.

Sulfurization temperature dependence of the structural transition in Cu2FeSnS4-based thin films

Cu2FeSnS4 (CFTS)-based thin films have been prepared on Mo-coated glass substrates by magnetron sputtering technique combined with a post-sulfurization processing. Energy dispersive X-ray measurement indicates that the Sn content decreases with the increasing sulfurization temperature, and all samples are in Cu-poor and Fe-rich states. X-ray photoelectron spectroscopy and X-ray diffraction show that all phases belong to the CFTS and no impure phase, e.g. Cu2SnS3, can be discovered in thin films. However, samples exhibit a phase transition behavior from rhodostannite to stannite structure with the increasing sulfurization temperature. The frequency of A1 mode of CFTS with rhodostannite structure is estimated to be 323.8cm−1 by using the empirical model, which coincides with the experimental value as measured by Raman spectra analysis. These results are helpful to understand the properties of CFTS-based thin films.

Surface sulfurization on MBE-grown Cu(In1−x,Gax)Se2 thin films and devices

Molecular beam epitaxy (MBE) grown Cu(In1−x,Gax)Se2 (CIGS) thin films were sulfurized at temperatures of 450–550 °C for 30 min in a 10% H2S–N2 mixture gas. The micro-roughness together with the S diffusion in the CIGS surfaces increased with increasing sulfurization temperature. Both near-band-edge PL intensity and decay time of the CIGS absorber layer enhanced after sulfurization. PL sub-peak around 80 meV below the main peak almost disappeared after sulfurization above 500 °C, which is expected due to the occupation of Se vacancies (Vse) with S. The open-circuit voltage (Voc), hence conversion efficiency, improved after sulfurization. The photovoltaic performance of the solar cells was consistent with PL intensity. Moreover, it is found for the first time from the SIMS analysis that the Cu atoms were depleted at the surface of CIGS layer after sulfurization, which could result in the improved Voc.

Effect of sulfurization temperature on properties of Cu2SnS3 thin films and solar cells prepared by sulfurization of stacked metallic precursors

Cu2SnS3 (CTS) thin films were synthesized by the sulfurization of stacked metallic precursor fabricated with a radio frequency magnetron sputtering method. The compositional, structural, morphological, optical and electrical properties of CTS thin films grown on soda lime glass substrates depend on the sulfurization temperature. The compositional analysis shows that all samples are in Sn-rich state. The ratio of Cu/Sn is close to the stoichiometric composition of CTS with increasing sulfurization temperature. The CTS thin films sulfurized at higher sulfurization temperature have better crystalline quality from the results of structural characterization. The CTS thin film obtained at sulfurization temperature 550°C exhibits higher degree of crystallization with larger average grain size. The transmission and reflectance spectra demonstrate the band gap is around 1.0eV, which is consistent with the estimation extrapolated from external quantum efficiency. The photoelectric properties of solar cell are enhanced with the increase of sulfurization temperature. The solar cell fabricated with the CTS absorber layer annealed at 550°C has the efficiency up to 0.28% for a 0.16cm2 area.

Effects of sulfurization temperature on phases and opto-electrical properties of Cu2ZnSnS4 films prepared by sol–gel deposition

Cu2ZnSnS4 (CZTS) thin films were prepared by sol–gel method and sulfurization process. The effects of the sulfurization temperature on the structural, morphological, compositional, and opto-electrical properties of the CZTS films were investigated. X-ray diffraction and Raman spectroscopy analyses confirmed the formation of CZTS films. With increasing sulfurization temperature, the crystallinity of the films was enhanced, which was accompanied by metallic deficiency, especially tin loss. When the sulfurization temperature was increased from 460 to 540°C, the optical band-gap value decreased from 1.63 to 1.38eV, while the resistivity and mobility increased from 1.415 to 1313Ω·cm and from 0.372 to 7.231cm2/V·s, respectively. The best CZTS film properties with a bandgap of 1.47eV, resistivity of 581.5Ω·cm, carrier concentration of 2.165×1016cm−3 and mobility of 1.411cm2/(V·s) were achieved at a sulfurization temperature of 500°C, and make the films suitable as absorbers for solar cells.

Fabrication of Cu2ZnSnS4 thin films on flexible polyimide substrates by sputtering and post-sulfurization

Flexible Cu2ZnSnS4 (CZTS) thin films are more advantageous than those on rigid glass substrates. In this study, vacuum-based magnetron sputtering was utilized to fabricate CZTS thin films on flexible polyimide substrates. Zn/Sn/Cu precursors were sputtered and then sulfurized. The influences of sulfurization temperature on the structural, compositional, morphological, electrical, and optical properties of the fabricated thin films were analyzed. The experimental results show that the CZTS structures form on the polyimide substrates after sulfurization. The crystallinity of CZTS enhances and the secondary phases in the thin films decrease with increasing sulfurization temperature. Single-phase CZTS thin films are obtained for sulfurization temperatures reaching 450 °C. The compositions of the fabricated thin films are Cu-poor and Zn-rich. The fabricated CZTS thin films show p-type conductivity. The direct optical band gaps of the thin films range from 1.51 eV to 1.55 eV. The absorption coefficients of these films are larger than 1 × 104 cm−1 above the band gap edge. The experimental results reveal the feasibility of the deposition of CZTS thin films on polyimide substrates by vacuum-based methods. The fabricated thin films can suitably function as absorbers for solar cell applications.

Influence of post-treatment on properties of Cu(In,Ga)Se2 thin films deposited by RF magnetron sputtering using a quaternary single target for photovoltaic devices

The deposition of Cu(In,Ga)Se2 (CIGS) thin films has been performed by one-step RF sputtering using a single quaternary target and followed by sulfurization to incorporate S into CIGS films. The effect of sulfurization temperature and time on the properties of the films was studied. The sulfurized Cu(In,Ga)(Se,S)2 (CIGSeS) films show that the chalcopyrite peaks shifted to high diffraction angles and the CuS and InS second phases could be formed at low sulfurization temperature. These indicate possible incorporation of S into the films. The formation and disappearance of these second phases depended on the sulfurization temperature and time. The band gap increased with increasing sulfurization temperature and time because of the shift of the absorption edge due to the increase of S/(S+Se) ratio. It was revealed that the resistivity of the as-deposited CIGS film increased after sulfurization while the carrier concentration and mobility decreased. It is believed that the sulfurization process of CIGS films can be utilized as a method to control the properties of the films.