As-deposited CZTS Research Articles

Secondary Phases, Morphology and Band Tails of Third Generation Photovoltaic Absorber Layer CZTS Annealed at Different Temperatures

Abstract: In this work, we have successfully fabricated Cu2ZnSnS4 (CZTS) thin films using the sol-gel spin-coating technique. Thin films were annealed in an argon atmosphere at different temperatures of 350, 375, 400, 425, and 450 ℃ for an hour. The influence of annealing temperature on structural, morphological, and optical properties was investigated. x-ray diffraction measurement confirmed the formation of secondary phases in as-deposited and annealed thin films at 350 and 375 ℃. A pure kesterite phase was obtained at an annealing temperature of 400 ℃, but degradation occurred at higher temperatures. The morphology study of the surface structure showed that thin films have homogeneous surfaces. The Volmer-Weber mode was the dominating growth mode. The as-deposited and annealed CZTS thin films exhibited a high absorption coefficient of the order of 104 cm-1, and the optical energy band gap Eg was red-shifted with increasing annealing temperature. The optical study showed a decrease in Urbach tail energy with an increase in annealing temperature and reached 0.39 eV at 400 ℃. The refractive index and dielectric constants of CZTS thin films were calculated. Keywords: Cu2ZnSnS4 thin films, Sol-Gel technique, Secondary phases, Roughness parameters, Band gap, Urbach tails.

Effect of volume of the solution and sulfurization on properties of Cu2ZnSnS4 thin films fabricated by spray assisted chemical vapour deposition method

ABSTRACT Cu2ZnSnS4 thin films were produced by spray assisted chemical vapor deposition process (SACVD) technique using different volumes of aqueous ionic solution, followed by sulfurization in sulfur vapors at 500°C and under Argon atmosphere. These CZTS thin films were analysed by X-ray diffraction and Raman scattering analysis that have confirmed the kesterite phase structure of CZTS films before and after sulfurization. The optimum band gap of the as-deposited CZTS films are between 1.40 and 1.43 eV. The cristallinity of the layers improve after sulfurization and their band gap varies between 1.44 and 1.50 eV. The surface morphology of the samples was examined by scanning electron microscopy. The chemical composition was determined by energy dispersive X-ray spectroscopy which shows the formation of Cu poor and Zn rich sulfurized CZTS thin films. Hall Effect measurements revealed that all the films before and after sulfurization are p-type. The sulfurized films obtained in our laboratory have carrier concentrations ranging from 3.1 to 6.8 x 1018 cm-3, a resistivity between 0.199 and 0.262 Ω.cm and mobility between 3.5 and 10.1 cm2/V.s.

Atmospheric Modified Thiol-Based Solution Deposition for Cu2ZnSn(S,Se)4 Absorber Layer

Abandoning the traditional deposition of as-prepared Cu2ZnSnS4 (CZTS) thin films in a glove box, a modified thiol-based solution system suitable for deposition in air is described to further reduce device production costs. In this modified approach, metal salts and thiourea are used as the starting materials and are dissolved in a mixed solution of thioglycolic acid and 2-methoxyethanol, forming a homogeneous precursor solution. The as-deposited CZTS thin films are obtained by spin-coating the precursor solution in air, followed by the selenization process to form large-grained Cu2ZnSn(S,Se)4 thin films. Combining the microstructural results with compositional analyses, the optimal selenization conditions for the Cu2ZnSn(S,Se)4 absorber layer were found to be 540°C and 10 min.

A green and low-cost synthetic approach based on deep eutectic choline-urea solvent toward synthesis of CZTS thin films

In this paper, a new, simple, and sustainable method of the Cu2ZnSnS4 (CZTS) thin-film synthesis is presented. The CZTS films have been electrochemically deposited by a single-step electrodeposition from deep eutectic (choline-urea) electrolyte, without sulfurization step, onto fluorine-doped tin oxide (FTO)-coated glass substrates. − 1.3 V/SCE has been selected as the optimum deposition potentials to grow the CZTS thin films. As-deposited CZTS films were characterized using a range of characterization techniques to study the structural, morphological, and compositional properties and confirmed the presence of the Cu2ZnSnS4 phases. The direct band gap energy for the CZTS thin films is found to be about 1.48 eV.

Tuning the Properties of CZTS Films by Controlling the Process Parameters in Cost-Effective Non-vacuum Technique

Highly dispersive Cu2ZnSnS4 (CZTS) nanoparticles were successfully synthesized by a simple solvothermal route. A low cost, non-vacuum method was used to deposit CZTS nanoparticle ink on glass substrates by a doctor blade process followed by selenization in a tube furnace to form Cu2ZnSn (S,Se)4 (CZTSSe) layers. Different selenization conditions and particle concentrations were considered in order to improve the crystallinity and surface morphology; the annealing temperature was varied between 400°C and 550°C and the annealing time was varied between 5 min and 20 min in a selenium-nitrogen atmosphere. The influence of annealing conditions on structural, compositional, optical and electrical properties of CZTSSe thin films was studied. An improvement in the structural and surface morphology was observed with increasing of annealing temperature (up to 500°C). An enhancement in the crystallinity and surface morphology were observed for thin films annealed for 10–15 min. Absorption study revealed that the band gap energy of as-deposited CZTS thin film was approximately 1.43 eV, while for CZTSSe thin films it ranged from 1.15 eV to 1.34 eV at different annealing temperatures, and from 1.33 eV to 1.38 eV for different annealing times.

Preparation of Cu2ZnSnS4 thin films with high carrier concentration and high carrier mobility by optimized annealing

The Cu2ZnSnS4 (CZTS) thin film with both high carrier concentration of 3.10 × 1020 cm−3 and high mobility of 19.8 cm2/V/s was prepared with optimization of heat treatment technique. In this work, different heat treatment techniques have been applied for the as-deposited CZTS by electrodeposition method. The as-deposited CZTS thin films have suitable Cu-poor and Zn-rich chemical composition, determining the high carrier concentration. The grain boundaries were designed for enhancement of carrier mobility of the CZTS thin films. Multi-step annealing refined the grains and increased the area of the grain boundaries in the films, resulting in the slight decrease of carrier mobility. Based on multi-step annealing, long and narrow particles and the preferential orientation of the grain boundaries formed by the decrease of Ar flow rate in 550 °C annealing. With such structure, the direct transportation of the holes along the vicinity of the grain boundaries increased carrier mobility.

Beyond 10% efficient CZTSSe thin-film solar cells fabricated by a two-step CdS deposition process

The CZTSSe/CdS interface qualities are vital aspects and factors that would greatly decide the photovoltaic performance of CZTSSe solar cells. High-quality CZTSSe/CdS interfaces could reduce defect density and improve CZTSSe device performance. However, due to the inhomogeneous CZTSSe grain growth in local micro-regions during selenization, void defects would emerge at the CZTSSe/CdS interfaces by traditional CdS deposition method. In order to obtain high-quality CZTSSe/CdS interfaces, a novel method of two-step CdS deposition, with which the two CdS layers were deposited before and after the selenization of CZTS absorbers, was firstly introduced in the work. The first CdS layer with a certain designed thickness was deposited onto the as-deposited CZTS thin films by CBD. Then the thin films were annealed in selenium-containing atmosphere to obtain CZTSSe thin films of large grain sizes. The second CdS layers with different thicknesses were deposited onto the annealed CZTSSe thin films by CBD again. Comparing with that by the traditional one-step CdS deposition method, the efficiencies of CZTSSe solar cells by two-step CdS deposition method show a remarkable increase from 7.25% to 10.19%. An action mechanism for improving device performance by two-step CdS deposition was proposed: The first CdS layer on CZTS precursors could consistently keep a well coverage for the CZTSSe grains on the surfaces during selenization; The second CdS layer could ensure p-n junctions in good condition, which would reduce recombination centers and shunting paths. The results suggest that the two-step CdS deposition process is an effective method to obtain high-quality CZTSSe/CdS interfaces and high-performance solar cell devices.

A general water-based precursor solution approach to deposit earth abundant Cu2ZnSn(S,Se)4 thin film solar cells

Earth abundant Cu2ZnSn(S,Se)4 (CZTSSe) has been considered as one of the most promising thin film solar cell absorber candidates. Here, we develop a facile water-based precursor solution approach for depositing high-efficiency Cu2ZnSn(S,Se)4 thin film solar cells. In this environmentally friendly approach, inexpensive elemental Cu, Zn, Sn and S powders are used as the starting materials and are dissolved in the aqueous solution of thioglycolic acid and methylamine, forming a homogeneous precursor solution for depositing Cu2ZnSnS4 nanocrystal thin film. As-deposited CZTS nanocrystal thin films are selenized to form the large-grain CZTSSe absorber layers. It was found that Na doping plays an important role in the formation of the extremely dense and flat CZTSSe absorber layer, and fill factor can be significantly improved for Na-doped CZTSSe solar cells, which lead to a photoelectric conversion efficiency of 6.96% with an open-circuit voltage of 378 mV, a short current density of 28.17 mA cm−2, and a fill factor of 65.4%.

Enhanced Conductivity in CZTS/Cu(2-x)Se Nanocrystal Thin Films: Growth of a Conductive Shell.

Poor charge transport in Cu2ZnSnS4 (CZTS) nanocrystal (NC) thin films presents a great challenge in the fabrication of solar cells without postannealing treatments. We introduce a novel approach to facilitate the charge carrier hopping between CZTS NCs by growing a stoichiometric Cu2Se shell that can be oxidized to form a conductive Cu2-xSe phase when exposed to air. The CZTS/Cu2Se core/shell NCs with varying numbers of shell monolayers were synthesized by the successive ionic layer adsorption and reaction (SILAR) method, and the variation in structural and optical properties of the CZTS NCs with varying shell thicknesses was investigated. Solid-phase sulfide ligand exchange was employed to fabricate NC thin films by layer-by-layer dip coating and a 2 orders of magnitude rise in dark conductivity (∼10(-3) S cm(-1) at 0 monolayer and ∼10(-1) S cm(-1) at 1.5 monolayers) was observed with an increase in the number of shell monolayers. The approach described herein is the first key step in achieving a significant increase in the photoconductivity of as-deposited CZTS NC thin films.

Electrochemical Synthesis and Characterization of Cu2ZnSnS4 Thin Films

Cu2ZnSnS4 (CZTS) thin films have been electrochemically deposited from aqueous electrolyte containing CuCl2, ZnCl2, SnCl4 and Na2S2O3 onto fluorine doped tin oxide (FTO) coated glass substrates. A conventional three-electrode geometry consisting working, counter and reference electrodes was used to perform the electrochemical experiments. The films were deposited at - 1.1 V with respect to Ag/AgCl reference, which was optimized by cyclic voltammetry. CZTS layers were annealed in tubular furnace at 400°C for 15 minutes in vacuum. As-deposited and annealed CZTS films were characterized using range of characterization techniques to study the structural, optical, morphological, and compositional and optoelectronic properties. Annealed sample revealed (112), (220) and (312) planes corresponds to tetragonal kesterite CZTS structure and secondary peaks of CuZn alloy. The optical study shows that the band gap of the as-deposited CZTS film was found to be 1.68 eV. Upon annealing the optical band gap ~ 1.49 eV corresponds to CZTS were estimated from UV-Visible Spectroscopy and photoluminescence. Densely packed, void free and relatively uniform thin films were deposited by electrodeposition technique. The grain size has been increased upon the heat treatment. Copper and zinc rich off-stoichiometric films were deposited at -1.1 V. Current density-Voltage (J-V) measurements showed Schottkey behavior. The flat band potential and carrier concentration estimated by C-V measurement for annealed CZTS sample was 0.30 V and ~ 2.4 x 1016 cm-3 respectively.

The post-growth effect on the properties of Cu2ZnSnS4 thin films

Cu2ZnSnS4 (CZTS) thin film was deposited on glass substrate by spray pyrolysis technique using a methanolic solution. This film was annealed under nitrogen atmosphere for an hour at different annealing temperatures. The effect of the annealing temperature on composition, structure, morphology, electrical, and optical properties of CZTS films was investigated. Energy dispersive x-ray analysis showed that after the annealing process, the atomic ratio values tend to their stoichiometric values. Moreover, the X-ray diffraction (XRD) and Raman spectroscopy revealed a kesterite structure with (112) preferential orientation for both as-deposited and annealed CZTS films. Moreover, they exhibited an improvement in crystallinity after annealing which was confirmed by SEM analysis. The conductivity of the heated CZTS films showed also an important increase in crystallinity. The absorption edge shifted toward lower wavelengths leading to an increase in band gap energy for the annealed CZTS films.

Effect of annealing process on the properties of Cu2ZnSnS4 thin films

Sprayed Cu2ZnSnS4 (CZTS) thin film prepared from an aqueous solution is annealed in nitrogen atmosphere at 450, 500 and 550°C during 60min. The effect of nitrogen annealing temperature Ta on the physical properties of CZTS thin films has been investigated. As-deposited film is Cu rich and Sn poor. After heat treatment process, CZTS thin films become Cu poor and Sn rich. Moreover, upon nitrogen annealing, a higher crystallite size of (112) diffraction peak, of kesterite crystal structure, is observed at Ta=500°C. The annealing temperature Ta significantly influences the surface morphology of the thin film annealed at 500°C. Electrical properties exhibit an enhancement also after thermal annealing process. Optical analysis shows that the band gap energy of as-deposited CZTS thin film is about 1.67eV; whereas it is improved reaching 1.52eV after thermal treatment at 500°C.

Aqueous synthesis of visible-light photoactive cuboid Cu2ZnSnS4 nanocrystals using rotary evaporation

In this work, cuboid kesterite Cu2ZnSnS4 (CZTS) nanocrystals were synthesized via a rotary evaporation method, using thiorea as a sulfur source. The effects of varying the rotation rate on the properties of the cuboid Cu2ZnSnS4 nanocrystals were investigated. The samples were characterized using X-ray diffraction, Raman spectroscopy, transmission electron microscopy (TEM), and UV–vis spectroscopy. The X-ray diffraction (XRD) patterns indicated that the as-deposited CZTS nanocrystals were transformed from an amorphous state into a crystalline state with a kesterite structure at a rotation rate of 100rpm. Results from the Raman spectroscopy experiments agreed with the XRD results. TEM and high-resolution transmission electron microscopy (HRTEM) images showed that the as-prepared samples were polycrystalline, and had a uniform nanocrystal morphology. UV–vis absorption spectra revealed strong absorption in the visible light region, with a direct band gap of 1.48eV and an absorption coefficient on the order of 104cm−1, which is suitable for photovoltaic applications.

Fabrication and Evaluation of Low-cost Cu2ZnSn(S,Se)4 Counter Electrodes for Dye-sensitized Solar Cells

We explore a simple and eco-friendly approach for preparing CZTS powders and a screen-printing process for Cu2ZnSn(S,Se)4 (CZTSSe) counter electrodes (CEs) in dye-sensitized solar cells (DSCs). Cu2ZnSnS4 (CZTS) nanoparticles have been synthesized via a hydrazine-free solvothermal approach without the assistance of organic ligands. CZTS has been prepared by directly drop-casting the CZTS ink on the cleaned FTO glass, while CZTSSe CEs have been fabricated by screen-printing CZTS pastes, followed by post selenization using Se vapor obtained from elemental Se pellets. The crystal structure, composition and morphology of the as-deposited CZTS nanoparticles and CZTSSe electrodes are characterized by X-ray diffractometer, energy dispersive spectrometer, field emission scanning electron microscopy and transmission electron microscopy. The electrochemical properties of CZTS, CZTSSe and Pt CE based DSCs are examined and analyzed by electrochemical impedance spectroscopy. The prepared CZTS and CZTSSe CEs exhibit a cellular structure with high porosity. DSCs fabricated with CZTSSe CEs achieve a power conversion efficiency of 5.75% under AM 1.5 G illumination with an intensity of 100 mW/cm2, which is higher than that (3.22%) of the cell using the CZTS CE. The results demonstrate that the CZTSSe CE possesses good electrocatalytic activity for the reduction of charge carriers in electrolyte. The comprehensive CZTSSe CE process is cheap and scalable. It can make large-scale electro-catalytic film fabrication cost competitive for both energy harvesting and storage applications.

Fabrication and Evaluation of Low-cost Cu2ZnSn(S,Se)4 Counter Electrodes for Dye-sensitized Solar Cells

Abstract We explore a simple and eco-friendly approach for preparing CZTS powders and a screen-printing process for Cu2ZnSn(S,Se)4 (CZTSSe) counter electrodes (CEs) in dye-sensitized solar cells (DSCs). Cu2ZnSnS4 (CZTS) nanoparticles have been synthesized via a hydrazine-free solvothermal approach without the assistance of organic ligands. CZTS has been prepared by directly drop-casting the CZTS ink on the cleaned FTO glass, while CZTSSe CEs have been fabricated by screen-printing CZTS pastes, followed by post selenization using Se vapor obtained from elemental Se pellets. The crystal structure, composition and morphology of the as-deposited CZTS nanoparticles and CZTSSe electrodes are characterized by X-ray diffractometer, energy dispersive spectrometer, field emission scanning electron microscopy and transmission electron microscopy. The electrochemical properties of CZTS, CZTSSe and Pt CE based DSCs are examined and analyzed by electrochemical impedance spectroscopy. The prepared CZTS and CZTSSe CEs exhibit a cellular structure with high porosity. DSCs fabricated with CZTSSe CEs achieve a power conversion efficiency of 5.75% under AM 1.5 G illumination with an intensity of 100 mW/cm2, which is higher than that (3.22%) of the cell using the CZTS CE. The results demonstrate that the CZTSSe CE possesses good electrocatalytic activity for the reduction of charge carriers in electrolyte. The comprehensive CZTSSe CE process is cheap and scalable. It can make large-scale electro-catalytic film fabrication cost competitive for both energy harvesting and storage applications.

Re-Crystallization of CZTS Solar Cell Materials Prepared by Galvanostat Electro-Deposition

Quaternary Cu2ZnSnS4 (CZTS) films were successfully obtained on Mo/(soda lime glass) substrates by one-step electro-deposition. The process was more efficient than other electro-deposit processes. Electrolytic aqueous was prepared using CuSO4, ZnSO4, SnSO4 and Na2S2O3 mixing solution. A galvanostat was applied for the deposition using a double-electrode scheme. The as-deposited CZTS was re-crystallized and then treated with sulfurization. Kesterite-type CZTS with nano-particle sizes ~17 nm on Mo/SLG were identified by x-ray diffraction (XRD). Different CZTS granules with sizes of 0.5 ~ 5 um were aggregated on surface morphologies as analyzed by scanning electron microscopy (SEM). The CZTS crystallinity was improved through the re-crystallization process. The absorption coefficients are over 1x104 cm-1 in the visible range. The spectrum of square absorption coefficient versus photon energy spectra depicts that the obtained CZTS has a band gap energy of ~1.25 eV. The optical band gap depicts that the obtained CZTS is suitable for thin film solar cell application.

Fabrication of Cu2ZnSnS4 Thin Film Solar Cell Using Single Step Electrodeposition Method

Cu2ZnSnS4 (CZTS) thin films were deposited onto Mo-coated and tin-doped indium oxide (ITO) coated glass substrates by using single step electrodeposition technique followed by annealing in N2 + H2S atmosphere. Subsequently, they were applied to the fabrication of thin film solar cells. Upon annealing, the amorphous nature of as-deposited precursor film changes into polycrystalline kesterite crystal structure with uniform and densely packed surface morphology. Energy dispersive X-ray spectroscopy (EDS) study reveals that the deposited thin films are nearly stoichiometric. Optical absorption study shows the band gap energy of as-deposited CZTS thin films is 2.7 eV whereas, after annealing, it is found to be 1.53 eV. The solar cell fabricated with CZTS absorber layer, showed the best conversion efficiency (η) 1.21% for 0.44 cm2 with open-circuit voltage (Voc) = 315 mV, short-circuit current density (Jsc) = 12.27 mA/cm2 and fill factor (FF) = 0.31.

Deposition and Characterization of Low-Cost Spray Pyrolyzed Cu2ZnSnS4 (CZTS) Thin-Films for Large-Area High-Efficiency Heterojunction Solar Cells

Cu2ZnSnS4 (CZTS) thin films have been prepared on Mo coated soda-lime glass (SLG) substrates using a low-cost spray pyrolysis technique. Aqueous solutions of CuCl, ZnCl2, SnCl4 and thiourea were used as precursors. Spray deposition was carried out at substrate temperatures of 300-400C with varying precursor concentrations. Structural, morphological and compositional characterization of as-deposited and annealed CZTS films were carried out by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). Optical and electrical properties were measured by UV-Vis spectroscopy, van der Pauw, and Hall-effect measurements. Heterojunction solar cells have been fabricated by deposition of n-CdS window layer on spray deposited p-CZTS films. Details of film fabrication, processing and characterization are presented. Our results show that CZTS-based solar cells can be designed and fabricated by a low-cost spraying technique for roll-to-roll commercial production.

Deposition of Cu2ZnSnS4 thin films by vacuum thermal evaporation from single quaternary compound source

Cu2ZnSnS4 (CZTS) thin films were successfully deposited using vacuum thermal evaporation from single quaternary CZTS semiconducting material powder source, followed by annealing at 300°C for 40min under high purity N2 atmosphere. X-ray diffraction (XRD) patterns indicated that as-deposited CZTS thin films transformed from amorphous state into crystalline state with kesterite structure after annealing. Energy dispersive X-ray spectroscopy (EDS) determined the compositions of the CZTS thin films were Zn-poor, Sn-rich and S-rich. Scanning electron microscope (SEM) images, ultraviolet–visible–near infrared (UV–Vis–NIR) spectra and the Hall measurements showed the annealed CZTS thin film exhibited a smooth, densely packed and homogeneous surface, a direct band gap of 1.55eV and a p-type conductivity. The fabricated photovoltaic device obtained a conversion efficiency of 0.36%, open-circuit voltage of 493mV, short-circuit current density of 1.76mA·cm−2, and fill factor of 0.42.

Effect of Annealing Atmosphere on the Properties of Electrochemically Deposited Cu2ZnSnS4 (CZTS) Thin Films

The Cu2ZnSnS4 (CZTS) thin films have been electrochemically deposited from a weak acidic medium (pH 4.50~5.00) onto Mo- coated and ITO-coated glass substrate by using single-step electrodeposition method. Trisodium citrate was used as a complexing agent. The effect of annealing atmospheres such as Ar, N2, N2+H2S on the structural, morphological, compositional, and optical properties of CZTS thin films has been investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and optical absorption techniques, respectively. XRD studies reveal that the as-deposited CZTS film is amorphous in nature. Upon annealing in different atmospheres, a relatively more intense and sharper diffraction peaks (112), (200), (220), and (312) of kesterite crystal structure with uniform and densely packed surface morphology are observed in N2+H2S atmosphere. Absorption study shows that the band gap energy of as-deposited CZTS thin film is 2.8 eV whereas after annealing, it is found to be 1.48, 1.76, and 1.53 eV for Ar, N2, N2+H2S atmospheres.