Phase Pure CZTS Research Articles

Facile single step synthesis of Cu2ZnSnS4 thin films by sputtering from a single target and their electrical characterization

Facile deposition of ternary semiconducting Cu2ZnSnS4 (CZTS) thin films has been a challenge due to a very narrow range of single phase stability in the Cu-Zn-Sn-S system. While sputtering has been a very efficient method of growth of various optoelectronic thin films, the formation of secondary phases should be suppressed during sputter deposition of CZTS films. Differently with the reported studies on deposition of a precursor film followed by a delicately - controlled sulfurization process required for the formation of the kesterite CZTS phase, we demonstrate a single step synthesis route to grow phase pure CZTS thin films using a single elementary target with excess Cu by RF magnetron sputtering at a substrate temperature of 450 °C. The route did not require any post-deposition sulfurization. These films had an optical band gap of ~1.6 eV and white light sensitivity> 200% at a bias potential of 5 V, highly suitable for photovoltaic and photocatalytic activities. Detailed electro-impedance analyses from Nyquist, Bode and Mott-Schottky plots of a Mo/CZTS/Pt/electrolyte device revealed p-type conductivity with a flat band potential of 0.51 V (reversible hydrogen electrode), carrier concentration of 6.2 × 1017 cm−3 and a carrier life time as high as of ~17 μs.

Influencing mechanism of post-sulfurization with sulfur flakes on phase evolution and Schottky diode characteristic of Cu2ZnSnS4 thin films sputter deposited from a single target

While single target sputter deposition appears very attractive to prepare kesterite Cu2ZnSnS4 (CZTS) thin films for photovoltaic applications, the growth of secondary phases arising as a consequence of stoichiometric deviation stemming from variation in sputter yield of elements must be prevented. Here, we demonstrate growth of phase pure kesterite CZTS thin films by RF magnetron sputtering of a single target by intuitively manipulating the target composition and post-sulfurization process carried out in a quasi-open environment using sulfur flakes. The influencing mechanism of post-sulfurization process was elucidated from systematic variation in the dwell time, temperature and the sulfur amount. A high temperature or a shorter dwell time yielded a small-grained microstructure associated with the presence of secondary phases. Phase pure CZTS films with better microstructural features were obtained for sulfurization at 500 °C for 60 min with 1.0 g of sulfur flakes. This film exhibited an optical bandgap of ∼1.58 eV indicating its photovoltaic potential. A device in the Mo/CZTS/Ag configuration showed typical features of a Schottky junction. The obtained current–voltage characteristic was analyzed to estimate saturation current, ideality factor and series resistance in correlation with the properties of the CZTS film.

Characterization of Cu2ZnSnS4 thin films deposited by one-step thermal evaporation for a third generation solar cell

In this study, one-step thermal evaporation was employed for the fabrication of phase-pure CZTS thin films on glass substrates from single crystalline Cu-poor ingot grown by Bridgman technique. Structural analyses revealed the formation of a mono-phase CZTS kesterite structure for the thin films annealed at 450 °C under N2 atmosphere. The AFM measurements demonstrated significant morphological changes in films with increasing the annealing temperature from 200 °C to 450 °C, supporting the results obtained via XRD and Raman studies. From the optical measurements, the band gap was estimated to be 1.47 eV for the CZTS film annealed at 450 °C, which is close to the optimum value required for photovoltaic applications. As a device application of the optimized CZTS thin films, an attempt was made to construct a TiO2 NRs based and CdS-free FTO/TiO2 NRs/CZTS/Ag superstrate CZTS solar cells. TiO2 NRs were grown by hydrothermal technique, which offers a simple and a cost-effective route for large-scale production of one-dimensional nanostructures. The solar cell exhibited an open-circuit voltage of 0.35 V, a short-circuit current density of 7.28 mA/cm2, a fill factor of 23.9% and a power conversion efficiency of 0.61%, the highest efficiency recorded so far for a single-dimensional TiO2 nanostructures based superstrate CZTS solar cell.

CZTS thin film solar cells on flexible Molybdenum foil by electrodeposition-annealing route

Earth-abundant and non-toxic Kesterite-based Cu2ZnSnS4 (CZTS) thin film solar cells are successfully fabricated on flexible Molybdenum (Mo) foil substrates by an electrodeposition-annealing route. A well-adherent, densely packed, homogeneous, compact, and mirror-like CZT precursor is initially produced through electrodeposition by using a rotating working electrode. Subsequently, the co-electrodeposited CuZnSn (CZT) precursor is sulfurized in quartz tube furnace at 550 °C for 2 h in N2 atmosphere with the presence of elemental sulfur in order to form CZTS. Different characterization techniques like XRD, SEM, HR-TEM, Raman, and Photoluminescence demonstrate that almost phase-pure CZTS formed after sulfurization. A flexible Al/Al-ZnO/i-ZnO/CdS/CZTS/Mo foil solar cell is produced, where CdS is deposited by chemical bath deposition and transparent conducting oxide (TCO) is deposited by DC sputtering. The CZTS solar device shows a 0.55% power conversion efficiency on flexible Mo foil substrate and it constitutes the first prototype of this kind of solar cell produced by electrodeposition-annealing route without any surface modification of the Mo substrate.Graphic abstract

A thin Cu interlayer-mediated control of phase evolution of Cu2ZnSnS4 thin films grown by RF magnetron sputtering of a single elementary target with high white light sensitivity

Although sputter deposition from a single target to prepare Cu2ZnSnS4 (CZTS) thin films appears attractive, it must suppress the growth of secondary phases that detrimentally affect performance of CZTS based thin film solar cells. Here we demonstrate a rational strategy for deposition of phase pure kesterite CZTS films by RF magnetron sputtering using a single target prepared from elemental powders via a thin Cu interlayer-mediated control of phase evolution. The films grown from a target of stoichiometric elemental proportion were Cu poor that determined the reaction pathway during sulfurization and resulted in growth of spurious phases despite a large variation in the sulfurization configuration. Inserting a 195 nm thick Cu interlayer between successively sputtered precursor layers provided improved compositional stability through thermally activated diffusion of Cu from the intermediate layer towards both the sides and resulted in the formation of kesterite CZTS even at a modest sulfurization temperature of 500 °C. The obtained phase-pure films have a bandgap of ~1.58 eV. The films show excellent photoresponse behavior characterized by increase in current by three orders of magnitude at a bias of 3 V upon white light illumination, typically that required for a potential absorber layer in thin film solar cells.

The preparation of Cu2ZnSnS4 thin film solar cell based on oxygen containing precursor

In this paper, we reported a new method to fabricate Cu2ZnSnS4 (CZTS) thin film solar cells. Oxygen containing precursor thin films were deposited on the Mo-coated soda lime glass (SLG) by sputtering ZnO, SnO2 and Cu targets. For getting higher quality CZTS thin films, the effects of different annealing temperatures of 380 °C, 480 °C, 580 °C and 650 °C on CZTS films were systematically investigated. The results showed that the optimum annealing temperature was 580 °C. Through optimizing the sulfurization process, phase-pure CZTS thin films with compact and large grains were obtained. Composition analysis showed that O was not detected after sulfurizing process which indicated all of O was replaced by S. And due to the stability of SnO2, the loss of tin can be avoided. In addition, ZnO not only can provide zinc sources, but also can decrease the thickness of MoS2 as a barrier layer. Finally, CZTS solar cells based on sputtering oxides targets were obtained and the preliminary fabricated CZTS thin film solar cells had an efficiency of 5.08%, with an open-circuit voltage of 645 mV, a short-circuit current density of 21.74 mA/cm2 and a fill factor of 36.23%.

Solution processed copper zinc tin sulfide thin films for thermoelectric device applications

In this work, Cu2ZnSnS4 (CZTS) thin film has been fabricated on a soda-lime glass substrate through an economical green chemical route employing nebulizer assisted spray pyrolysis technique. The deposition is carried out under ambient atmospheric pressure without annealing and sulphurization. An investigation was done to reveal the temperature dependence of various characteristics especially the thermoelectric property of this Earth-abundant, nontoxic CZTS films in the range of 300 °C–375 °C. This study reveals the thermoelectric conversion capability of the material in addition to its photovoltaic conversion application. XRD spectra revealed the formation of tetragonal structured kesterite CZTS peaks preferentially oriented along [112] direction. The film thickness and size of the crystallites is found to increase with temperature up to 350 °C while the dislocation density and microstrain decreased. Raman spectra exhibit a phase pure CZTS layer free from secondary and tertiary phases at a substrate temperature of 300 °C whose composition seems to be off stoichiometric with a Cu/(Zn + Sn) ratio of 0.78. The surface morphological analysis shows a dense, smooth and compact surface for the 350 °C sample. The bandgap was found to vary in the range of 1.52–1.74 eV. Refractive index, extinction coefficient and electrical characteristics of the prepared films endorse its usage as an absorber material for thin-film photovoltaic energy conversion. The Seebeck coefficient of 350 °C grown film is found to be 153 μV/K and the corresponding power factor increases to 5.76 × 10−3 W/mK−2 which recommends its utility in the thermoelectric devices.

CZTS solar cell with non-toxic buffer layer: A study on the sulphurization temperature and absorber layer thickness

Cu2ZnSnS4 solar cell has been fabricated with a non-toxic buffer layer and Al doped ZnO transparent conducting layer. A detailed material study on each layer of the solar cell has been carried out independently. The precursor films prepared by spin coating were sulphurized at different temperatures to optimize the conditions to obtain phase pure CZTS films. X-ray diffraction, Raman spectroscopy and rietveld refinement studies confirmed the phase purity of the film sulphurized at 500 °C. The optimum CZTS properties like band gap of 1.45 eV, high absorption coefficient ~2 × 105 cm−1 and dense surface morphology were obtained for films sulphurized at 500 °C. Carrier concentration, mobility and resistivity of these films were ~1 × 1019 cm−3, 0.23 cm2 V−1 s−1 and 2.7 Ωcm, respectively. The efficiency measurements of the cells with device structure SLG/Mo/CZTS/ZnS/AZO/Ag were carried out using absorber films with three different thicknesses. The optimized CZTS absorber layer with thickness of ~1.8 µm exhibited solar cell conversion efficiency of 3.02% for an active area of 0.21 cm2 withopen-circuit voltageof 0.38 V,short-circuitcurrent density of 17.19 mA/cm2 and fill factor of 46%.

Towards phase pure CZTS thin films by SILAR method with augmented Zn adsorption for photovoltaic applications

Cu2ZnSnS4 (CZTS) thin films were deposited from a single cationic bath by Successive Ionic Layer Adsorption and Reaction (SILAR) method. Regular SILAR route for CZTS had the drawback of preferential adsorption of copper and tin cations in comparison with zinc. This resulted in Cu3SnS4 (CTS) and Cu2S phases rather than phase pure CZTS films. A modified SILAR route, with a separate bath for Zn2+ ions, circumvented the difficulty and hence led to phase pure CZTS thin films. UV–visible absorption spectra of the CZTS thin films showed absorption coefficients of ~ 104 cm−1 and a band gap of 1.5 eV. Combined van der Pauw and Hall measurements of CZTS thin films showed a resistivity of approximately 1.51 × 102 Ωcm, carrier density of ~ 1.28 × 1017 cm−3, and mobility ~ 0.32 cm2 V−1s−1. A completely solution processed P–N junction was fabricated and characterized by forming glass/FTO/TiO2/CdS/CZTS multilayer.

Evaluating the role of precursor concentration in facile conformal coating of sub-micrometer thick Cu2ZnSnS4 films using non-toxic ethanol based solutions

Although non-toxic solution processing offers a scalable approach for growth of Cu2ZnSnS4 (CZTS) thin films, it must suppress the formation of secondary phases and yield facile conformal large grained films with minimum process steps. While growing CZTS films from environment-friendly ethanol based solutions, here we demonstrate how precursor concentration and post sulfurization critically affect the reaction pathway and morphology of the films. Deposition from lower ionic concentration precursor solution and sulfurization with nominal sulfur amount yielded phase-pure films. However, the thickness and grain size of these films were far lower than that desired for solar cell application, thus necessitating overlay depositions to achieve desired thickness and grain size. This issue was alleviated by increasing the ionic concentration in the precursor solution and the sulfur amount during sulfurization. It was found that coating from 1 M Cu-solution in just two dipping cycles followed by sulfurization at 500 °C yielded phase pure kesterite CZTS films of ~850 nm thickness. In addition, differently with the reported bilayered structure, the films exhibit a uniform microstructure with reasonably large grains. The resulting films showed the bandgap of 1.42 eV and very high photosensitivity upon illumination of white light.

Improving the optoelectrical properties of Cu2ZnSnS4 using gold and graphene nano-fillers

We have investigated photo-response of Cu2ZnSnS4 (CZTS) and have explored ways for enhancing the photo-response and conductivity using suitable nano-compositing strategies. In phase pure CZTS, gold nanoparticles and graphene flakes are used as nanofillers to make nano-composite samples. Photo-response of as-grown CZTS is improved by compositing gold with the observation of Ilight/Idark=1.29. With increasing gold nanoparticle size conductivity improved for CZTS gold composite. Compositing 1% graphene by weight has enhanced the current flow in the CTZS by 125 times. The graphene weight ratio in the composite is limited by absorbance reduction. The demonstrated high-quality CZTS nano-composite samples with improved optoelectrical properties holds tremendous promise for photovoltaic applications.

Eliminating secondary phases: Understanding kesterite phase evolution of Cu2ZnSnS4 thin films grown from ethanol based solutions with high photosensitivity

Non-toxic solution based approaches need to suppress the formation of secondary phases in the growth of Cu2ZnSnS4 (CZTS) thin films, which otherwise detrimentally affect the device performance especially by limiting the open circuit voltage. While growing CZTS films from ethanol based solutions containing common metal salts, we have conclusively established the reaction pathway leading to the formation of the single phase kesterite CZTS from the precursor solution via intermediate solid state binary and ternary compounds during deposition and the subsequent sulfurization treatment. Differently with the previous reports, we found that the presence of monoethanolamine (MEA) - a routinely used complexing agent - in the initial stage of solution mixing, inhibits complete reduction (oxidation) of Cu2+ (Sn2+), which subsequently affected the reaction pathway resulting in undesirable secondary phases. The activation energy for the decomposition of the precursor complex was found to be smaller, surmounted in the early formation of the single phase CZTS in the absence of MEA. Having a bandgap of 1.44 eV, these phase pure CZTS films exhibited very high photoresponse behavior (about 275% change in current upon white light illumination at a bias of 6 V).

Tuning copper sulfide nanosheets by cation exchange reactions to realize two-dimensional CZTS dielectric layers

Colloidal synthesis of ultrathin phase-pure CZTS nanosheets has been demonstrated, which exhibit a nearly temperature and frequency independent dielectric constant (>6.1) with a low dielectric loss over broad temperature and frequency ranges.

Evolution of the microstructural, electrical and optical characteristics of sol-gel derived Cu2ZnSnS4 thin films during sulfurization

The present study reports the evolution of microstructural, electrical and optical characteristics of sol-gel derived Cu2ZnSnS4 (CZTS) thin-films during sulfurization process performed at 530 °C. The precursor films prior to sulfurization were largely amorphous although the presence of CZTS phase was apparent in the corresponding X-ray diffraction (XRD) pattern. For the sulfurization process, as the temperature of the film arrived at 300 °C during ramp-up stage the secondary phase copper sulfide (CuxS) (x = 1–2) was detected in the corresponding XRD pattern. This phase was subsequently assimilated into the CZTS phase as the film reached at 450 °C. When the film arrived at 530 °C, the sulfurization temperature, it had completely converted into a phase pure CZTS with a close to stoichiometric composition and uniform surface morphology. Beyond this stage the structural characteristics did not show significant changes. The grain size increased from 4 to 200 nm during the sulfurization process. The electrical conductivity of the films was p-type at all the stages of the sulfurization process, while the resistivity first increased to 165 Ω⋅cm as the film reached 530 °C and from there it decreased continuously to a value of 0.66 Ω⋅cm at the end of the sulfurization process. The optical band gap and the Urbach energy decreased from 2.0 to 1.5 eV and from 1.2 to 0.25 eV, respectively. The disorder in CZTS was studied using Raman spectroscopy and its implications on the electrical properties have been discussed. Further, a post sulfurization heating of the CZTS film at 300 °C for 1 h in an inert ambient resulted in two orders of magnitude decrease in the charge carrier density from 2.1 × 1019 cm–3 to 1.35 × 1017 cm–3 which can be attributed to an increase in the density of donor-type defects.

Synthesis of quaternary chalcogenide CZTS nanoparticles by a hydrothermal route

Cu2ZnSnS4 (CZTS) has emerged as a potential absorber towards inorganic photovoltaic device application for its outstanding properties like non toxicity, earth abundancy nature, optimal band gap matched with solar spectrum (1.45- 1.65eV), high absorption coefficient (104cm−1). Here, a low cost, environment friendly facile hydrothermal route to synthesize phase pure CZTS nanoparticles using Cu (II), Zn (II), Sn (II) inorganic metal salts and thiourea as Sulphur source in distilled water solution as precursor is reported. The as synthesized samples characterized by X-Ray diffraction (XRD) and RAMAN confirmed structure and phase of CZTS nanocrystals. The morphology of the prepared CZTS have been characterized by scanning electron microscopy (SEM). The particle size is found in the range 4-5 nm with crystalline nature have been characterized by transmission electron microscope (TEM). The optical band gap of the as prepared samples is calculated to be 1.65eV from UV-Visible analysis which proves it can be used towards photovoltaic applications.

Detailed investigations on influence of precursor stacking and sulfurization for Cu2ZnSnS4 film formation

Cu2ZnSnS4 (CZTS) is a suitable absorber material in thin film hetero-junction solar cell. In the present study, CZTS has been successfully fabricated by electron beam evaporation and the influence of precursor stacking sequences and sulfurization conditions on the formation of secondary phases have been investigated in detail. CZTS thin films are prepared by two stage process: first stage, precursor layers with two different sequences glass/Zn/Sn/Cu and glass/Sn/Zn/Cu are deposited on glass substrates at room temperature using electron beam evaporation of elemental metallic precursors; followed by the second stage, the precursor films are sulfurized in two different conditions, in presence of elemental sulfur powder and in 5% H2S + N2 environment, at 500 °C for 1 h of annealing time for the formation of CZTS films. Effects of sulfurization environment (sulfur powder and H2S) and ramping rate (1 K/min, 5 K/min and 10 K/min) on the formation of CZTS and other probable secondary phases for different stacking sequences are investigated using X-ray diffraction, Raman spectroscopy, Field emission gun scanning electron microscopy, energy dispersive X-ray spectroscopy and UV–vis spectroscopy. Deconvoluted Raman spectra revealed high crystallinity in films sulfurized at low ramping rate (1 K/min). CZTS formation is noticed to be enhanced by sulfurizing the films in the presence of sulfur powder when compared with H2S gas. Phase pure CZTS film with a bandgap of 1.53 eV is prepared using sulfurization of glass/Sn/Zn/Cu stacked film in elemental sulfur powder.

Phase evolution pathways of kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 thin films during the annealing of sputtered Cu-Sn-Zn metallic precursors

The formation of secondary phases in kesterite-based solar cells plays a critical role, affecting the performances of devices primarily due to the open circuit voltage deficient characteristics. Systematic studies of the phase evolution pathways and the formation of secondary phases in kesterite thin films are desirable. In this paper, the phase evolution pathways and the formation of secondary phases in kesterite Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) thin films during thermal treatment of sputtered Cu-Sn-Zn metallic precursors have been studied using high-resolution X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). A possible phase evolution process for the kesterite thin films is proposed based on the experimental observations. The Cu-Sn-Zn metallic precursor thin films were prepared using the sputtering technique followed by thermal treatments at different temperatures ranging from 250°C to 580°C in a chalcogen atmosphere. XRD and Raman characterizations of the thin films annealed under chalcogen atmospheres indicated that the precursor thin films were completely transformed to the CZTS and CZTSe kesterite phases above 500°C. However, the TEM analysis of the thin films annealed above 450°C revealed Cu-based secondary phases. The formation of phase pure CZTS and CZTSe kesterites was observed at annealing temperatures above 500°C. Based on the experimental observations, the phase evolution pathways for the formation of kesterite and secondary phases from metallic precursors are proposed in this paper.

Towards phase pure kesterite CZTS films via Cu-Zn-Sn electrodeposition followed by sulfurization

Electrodeposition of Cu-Zn-Sn alloy precursors followed by sulfurization has been utilized to form phase pure CZTS absorber layers for photovoltaic applications. In this study, we have varied the deposition potential in order to grow Cu-Zn-Sn metallic precursor layers with a fixed Cu fraction while varying the Zn/Sn ratio within the ideal composition range to obtain high quality CZTS films. The phase purity and crystal quality of CZTS films were investigated as a function of Zn/Sn ratio. Phase purity and photoelectrochemical response are both enhanced with increasing Zn/Sn ratio after sulfurization at 500°C or 550°C. The highest photoelectrochemical response at a liquid junction is 0.99mA/cm2 for CZTS layer with Zn/Sn=0.98 after sulfurization at 550°C. Raman spectroscopy and X-ray diffraction studies suggest that the enhanced photoresponse is correlated to a high crystallinity together with improved phase purity.

Effect of sulfurization time on the properties of copper zinc tin sulfide thin films grown by electrochemical deposition

We report growth of quaternary Cu2 ZnSnS4 (CZTS) thin films prepared by the electrochemical deposition from salt precursors containing Cu (II), Zn (II) and Sn (IV) metals. The influence of different sulfurization times t (t = 75, 90, 105, and 120 min) on the structural, compositional, morphological, and optical properties, as well as on the electrical properties is studied. The films sulfurized 2 hours showed a prominent kesterite phase with a nearly stoichiometric composition. Samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and Raman and UV-VIS-NIR spectrometer at different stages of work. X-ray diffraction and Raman spectroscopy analyses confirmed the formation of phase-pure CZTS films. (FESEM) shows that compact and dense morphology and enhanced photo-sensitivity. STEM - EDS elemental map of CZTS cross-section confirms homogeneous distribution. From optical study, energy gap was enlarged with a changed sulfurization times in the range of 1.37–1.47 eV.

Investigation of optimum annealing parameters for formation of dip coated Cu2ZnSnS4 thin film

Cu2ZnSnS4 (CZTS) is most attractive absorber material for inorganic solar cell applications because of its cost effective and ecofriendly nature. To obtain phase pure CZTS film, effects of annealing parameters on synthesis of CZTS thin film are investigated. CZTS films are deposited through dip coating method followed by heat treatment to form crystalline CZTS thin films. Factors influencing the crystallinity, morphology and composition of the films such as annealing temperature, time, rate and atmosphere are studied through X-Ray Diffraction, Raman Spectroscopy, Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy. After numerous experiments of synthesis of CZTS in different annealing conditions and its characterization, it is observed that 1.4eV band gap CZTS thin film of kesterite structure is obtained by annealing the film in nitrogen atmosphere for 60min at 300°C with 10°C/min ramping rate.