Kesterite Cu2ZnSnS4 Thin Films Research Articles

Fabrication of kesterite Cu2ZnSnS4 thin films using sequentially sputtered multilayers of Cu2SnS3 and Zinc.

Radio-frequency sputtered Cu2ZnSnS4 (CZTS) thin films deposited on glass substrates were characterized. The film deposition was carried out in an argon atmosphere with RF powers of 200 W for zinc and 300 W for the CTS films. After the deposition, the films were annealed at temperatures ranging from 350 °C to 500 °C in a nitrogen and sulfur atmosphere. By using X-ray diffraction, the CZTS films were observed to crystallize in a tetragonal structure with some minor secondary phases in some samples. The absorption coefficient and optical band gap are identified by the ultraviolet-visible-near-infrared optical transmission measurements and the calculated band gap was found to range between 1.4 eV and 1.51 eV. This research comprehensively addresses the effects of layer thickness and annealing temperature on our CTS/Zn films. The annealing process was oberved to result in a significant improvement in electrical conductivity, particularly in films without secondary phases.

Effects of incorporation of Ag into a kesterite Cu2ZnSnS4 thin film on its photoelectrochemical properties for water reduction.

Kesterite Cu2ZnSnS4 (CZTS) thin films in which the Cu site was partially replaced with Ag were prepared by spray deposition on an Mo-coated glass substrate. Successful replacement of Cu components in the CZTS lattice with Ag up to an Ag/(Cu + Ag) ratio of 0.20 was achieved. Samples with relatively low contents of Ag (Ag/(Cu + Ag) ratios of 0.05 and 0.10) showed obvious grain growth compared to that of bare CZTS, whereas samples with higher Ag contents showed an appreciable decrease in grain sizes. Photoelectrochemical properties for water reduction (H2 production), which was examined after surface modifications with an In2S3/CdS double layer and Pt catalyst for H2 evolution, depended strongly on such morphological differences; a maximum conversion efficiency, i.e., half-cell solar to hydrogen efficiency, of 2.4% was achieved by the photocathode based on the film with an Ag/(Cu + Ag) ratio of 0.10. Minority carrier dynamics examined by photoluminescence measurements indicated that such an active sample of PEC H2 production had a relatively long carrier lifetime, suggesting that the suppression of carrier recombination at grain boundaries in the bulk of these kesterite films is one of the important factors for enhancing PEC functions.

Impact of Solvent type on the Structural, Surface morphology and Optical energy gap of Kesterite Cu2ZnSnS4 thin films deposited by solvothermal method

In present work kesterite quaternary Cu2ZnSnS4 (CZTS) semiconductor thin films were prepared on fluorine-doped tin oxide (FTO) substrate by solvothermal method using different solvent types Ethylene glycol (EG), polyethylene glycol (PEG), and Ethylenediamine (ED). Crystal structure, surface morphology and optical characteristics of the prepared thin film were characterized by x-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS) and UV–Vis absorption spectroscopies. X-ray diffraction results reveal that the deposited films have polycrystalline nature and in a kesterite phase (tetragonal structure). the crystallite size is in the rang (20-27) nm and the largest one is for the prepared by using PEG solvent. The films surface morphology shows smooth and uniform surface with well distributed spherical particles with largest particles for the films deposited by EG solvent. The EDS spectra confirmed the the existence of the elements Cu, Zn, Sn and S of the films composition. The direct optical energy gap shows different values caused by using different solvent type, where the lower band gap is about 1.56 eV and the largest is about 1.9 eV for the films using ED and EG respectively.

Synthesis of CZTS thin films from binary precursors stacking by chemical bath deposition for solar cell applications

Kesterite Cu2ZnSnS4 thin films were synthesized by chemical bath deposition without using the sulfurization/selenization process. The precursors were stacked following the deposition sequence: CuS/SnS/ZnS/substrate. The synthesized samples were annealed at 400, 450, and 550 °C in a vacuum condition. The structural analysis by XRD and Raman Spectroscopy confirmed the Cu2ZnSnS4 phase. The optoelectronic characteristics of the samples synthesized in this work were similar to those values of these kesterite-type semiconductors synthesized by other methods. The kesterite semiconductor bandgap was ranging from 1.45 to 1.49 eV with a conductivity of 10-3(Ω cm)-1, depending on the thermal treatment conditions. It is feasible to consider the synthesis of kesterite Cu2ZnSnS4 by chemical bath deposition method of binary precursors. The appropriate thermal treatment temperature is 450 °C for obtaining adequate optoelectronic characteristics for solar cell applications.

Fabrication of high-quality kesterite Cu2ZnSnS4 thin films deposited by an optimized sol–gel sulphurization technique for solar cells

Kesterite Cu2ZnSnS4 (CZTS) thin films were prepared by spin-coating deposition of sol–gel precursor followed by optimized sulphurization via custom-made home-built sulphurization setup. The effects of sulphurization on the structural, morphological, optical and electrical properties of CZTS thin films were investigated. X-ray diffraction and Raman spectroscopy studies revealed that the sulphurized CZTS thin films have a polycrystalline nature with a kesterite phase and the crystallite size of sulphurized CZTS thin film is higher as compare to as-synthesized CZTS thin films. The direct optical bandgap of sulphurized CZTS thin film is found to be 1.5 eV. Hall measurement indicates that the sulphurized CZTS thin films have p-type conductivity with charge carrier concentration of 9.47 × 1019 cm−3. Finally, a thin film solar cell was fabricated with the configuration—SLG/FTO/CdS/CZTS/Al. A photoelectric conversion efficiency of 1.5% was achieved with an open-circuit voltage of 0.32 V and a short-circuit current density of 11.9 mA cm−2. These findings clearly show that quality of CZTS samples and photovoltaic performance could be effectively improved by an optimized sol–gel sulphurization process.

Facile designing and assessment of photovoltaic performance of hydrothermally grown kesterite Cu2ZnSnS4 thin films: Influence of deposition time

Herein, low cost precursor source Cu2ZnSnS4 (CZTS) nanocrystalline thin films at various reaction time were successfully synthesized via one step hydrothermal route. Hydrothermal route was employed to achieve control over the size and grain growth of CZTS films. As deposited CZTS films were analyzed for its optoelectronic, structural, morphological and electrochemical properties to investigate the effect of hydrothermal reaction time on growth and photovoltaic performance. The hydrothermal synthesis promoted to high absorption (104 cm−1) of the CZTS film with a decrease in optical band gap energy from 1.52 eV to 1.41 eV. Structural study revealed that, improved crystallinity with A1 mode of vibration for pure phase kesterite CZTS structure. Morphological transition was observed from nanograins to well grown and compact nanospheres. Compositional analysis illustrates, stoichiometric CZTS film formation with the desired valence state of Cu+, Zn2+, Sn4+ and S2− elements. Current density-voltage (J-V) measurement of FTO/CZTS/(0.3 M Eu3+/Eu2+)/Graphite cell configuration shows, highest photocurrent of 2.60 mA/cm2 and open circuit voltage of 754 mV was observed for CZTS4 sample with best photoconversion efficiency (η) 3.21% under illumination of 30 mW/cm2 light intensity. Electron impedance spectroscopy (EIS) showed that, generation of lower charge transfer resistance (Rct) with increase in reaction time.

The phase optimization, optical and electrical properties of kesterite Cu2ZnSnS4 thin film prepared by single target RF magnetron sputtering technique for solar cell application

Cu2ZnSnS4 (CZTS) thin film was prepared by RF sputtering technique from a single quaternary target by optimizing RF power, in situ substrate temperature and post deposition annealing temperature. The single phase formation of crystalline CZTS thin film has been verified at a particular optimized condition by characterizing through x-ray diffraction, Raman spectrometer and x-ray photoelectron spectroscopy. The kesterite structure has been further confirmed from the lattice fringes of high resolution transmission electron microscope and selected area electron diffraction study. The surface properties such as particle size, shape and roughness and elemental composition of amorphous and crystalline phase of CZTS studied by atomic force microscope, scanning electron microscope and energy dispersive analysis of x-rays. The optical absorption and photoluminescence studies of post-deposition annealed film prove the defect free and single phase CZTS formation. The band gap calculation for crystalline CZTS showed band gap of 1.49 eV calculated from the Tauc plot. The electrical properties of optimized CZTS thin film has been studied from Hall effect measurement showing P-type conductivity with better carrier concentration and Hall mobility.

Preparation of highly transparent conductive aluminum-doped zinc oxide thin films using a low-temperature aqueous solution process for thin-film solar cells applications

In this work, we developed a facile route to fabricate highly transparent conductive AZO thin films by using an aqueous solution process followed by an ultraviolet (UV) exposure technique at an annealing temperature as low as 200 °C, where the aluminum citrate complex was used as the Al doping source. Various of deposition parameters in the AZO thin films fabrication process were studied and optimized. The mechanisms on UV exposure and heat treatment to the conductivity improvement and conductivity stability of the AZO films were analysed. Results showed that the AZO thin films under the best parameters conditions demonstrated an optical transmittance higher than 85% in the visible spectra region and a lowest electrical resistivity of 4.8 × 10−3 Ω cm, while exhibited densely oriented columnar grains and uniform surface morphology as well as uniform composition distribution. The UV exposure could remove carbon species from the surface of the AZO thin films to reduce oxygen-related defects and release free carriers at the boundaries and interfaces, thereby improving the conductivity of the AZO thin films. The simultaneous treatment of the AZO thin film by ultraviolet exposure and heat treatment could remove carbon species at a deeper thickness, thereby improving the conductive stability of the AZO thin films. Kesterite Cu2ZnSnS4 thin film solar cells incorporating the optimal AZO thin films as top electrodes demonstrated a best power conversion efficiency (PCE) of 7.15%, which was comparable to the PCE value obtained by using the sputtering deposited AZO thin films as top electrodes.

Fabrication of 4.9% efficient Cu2ZnSnS4 solar cell using electron-beam evaporated CdS buffer layer

The chemical bath deposition process of depositing CdS buffer layer was replaced with electron-beam evaporation method, full vacuum preparation process, which is more environmentally friendly and non-solution contact, was achieved to prepare Cu2ZnSnS4 (CZTS) solar cells. First, the influence of growth rate on the structural, optical, morphological, electrical properties of electron-beam evaporated CdS films was investigated. CdS thin films were deposited at growth rates of about 3, 6, 9, 12 Å/s, respectively. The results revealed that CdS thin films deposited at lower rate showed better crystallinity, higher transmittance, lower carrier concentrations, preferred for the application in CZTS solar cells. Then, kesterite Cu2ZnSnS4 thin film solar cells using the optimal CdS thin films as a buffer layer are fabricated, and a best efficiency of 4.87% was demonstrated. The statistical distribution of device performance parameters of 150 solar cells using electron-beam evaporated CdS buffer layer was also demonstrated.

Effect of sulfur quantity in post-treatment on physicals and electrical properties of electrodeposited Kesterite Cu2ZnSnS4 thin films

In this work, thin films of Cu2ZnSnS4 (CZTS) have been successfully electrodeposited onto FTO substrate. The films are post-sulfurized with different amount of sulfur varied from 0 to 0.5 g at 550 °C under N2 atmosphere. Effect of sulfur quantity during the annealing treatment on structural, morphological, optical and electrical properties of CZTS films have been investigated using various techniques. XRD analysis shows that CZTS thin film annealed at 550 °C in the presence of 0.3 g of sulfur element exhibited the best crystalline quality of kesterite phase with (112) preferential orientation. This result was confirmed by Raman spectroscopy. Surface morphology studied by scanning electron microscopy revealed that the post-sulfurization and especially the quantity of sulfur have a significant effect on the surface and roughness of thin films. According to the UV–Vis measurements, all films show high absorption in the visible region (~ 104 cm− 1) with a direct energy band gap ranging from 2.18 to 1.65 eV, which depends on the sulfur amount during the post-sulfurization. Finally, the electrical characterization by means of electrochemical impedance spectroscopy was carried out to analyze the capacitive nature of CZTS/Na2SO4 interface. This interface was modeled by an equivalent circuit approach. From the Mott–Schottky plot, the flat band potential and the acceptor density for CZTS thin films are determined. All the films demonstrated a p-type semiconductor character with a carrier density varying between 1016 and 1020 cm− 3.

Kesterite Cu2ZnSnS4 thin films by drop-on-demand inkjet printing from molecular ink

Inkjet printing of kesterite Cu2ZnSnS4 (CZTS) thin films on glass from molecular ink is described. CZTS ink consists of copper acetate, zinc acetate, tin chloride and thiourea dissolved in a mixture of ethylene glycol and isopropyl alcohol. The printed precursor films are vacuum dried and thermolysed at 200 °C in air to obtain CZTS films. X-ray diffraction and Raman spectroscopy of films confirm the formation of kesterite CZTS without any secondary phases. The band gap of the films is 1.48 eV as deduced from transmission spectrum using Tauc plot. The films are p-type with hole density and mobility of 2.65 × 1019 cm−3 and 0.3 cm2V−1s−1, respectively. Measurement of electrical conductivity of films in the temperature range from 77 to 300 K show that dominant mechanisms of conduction are Mott-Variable Range Hopping, Nearest Neighbour Hopping and Thermally Activated Band Conduction in the temperature ranges of 77–155 K, 180–240 K and 250–300 K, respectively.

Novel development of nanocrystalline kesterite Cu2ZnSnS4 thin film with high photocatalytic activity under visible light illumination

Cu2ZnSnS4 (CZTS) represents a promising p-type direct band gap semiconductor with large absorption coefficient in the visible region of solar light. In the present study, a kesterite CZTS nanocrystalline film, with high purity, was successfully synthesized via the combination of successive ionic layer adsorption and reaction (SILAR) and chemical bath deposition (CBD) technique. The morphology and structural properties of the CZTS films were characterized by FE-SEM microscopy, porosimetry in terms of Brunauer-Emmett-Teller (BET) technique, X-ray diffraction and Raman spectroscopy. The as-prepared films under mild heat treatment at 250 °C in the presence of sulfur atmosphere exhibited fine nanostructure with 35 nm average particle size, high specific surface area of 53 m2/g and 9 nm pore diameter. The photocatalytic activity of the films was examined to the degradation of Basic Blue 41 (BB-41) and Acid Orange 8 (AO-8) organic azo dyes under visible light irradiation, demonstrating 97.5% and 70% discoloration for BB-41 and AO-8 respectively. Reusability of the CZTS films was also tested proving good stability over several repetitions. The reduction of photocatalyst's efficiency after three successive repetitions didn't exceed 5.6% and 8.5% for BB-41 and AO-8 respectively.

Precisely Controlled Synthesis of High Quality Kesterite Cu2ZnSnS4 Thin Film via Co-Electrodeposited CuZnSn Alloy Film.

In this work, a facile co-electrodeposition method was used to fabricate CuZnSn alloy films where the content of copper, zinc and tin could be precisely controlled through manipulating the mass transfer process in the electrochemical deposition. By finely tuning the concentration of the cations of Cu2+, Zn2+ and Sn2+ in the electrochemical bath solution, uniform CuZnSn film with desired composition of copper poor and zinc rich was made. Sulphurisation of the CuZnSn alloy film led to the formation of compact and large grains Cu2ZnSnS4 thin film absorber with an optimum composition of Cu/(Zn+Sn) ≈ 0.8, Zn/Sn ≈ 1.2. Both SEM morphology and EDS mapping results confirmed the uniformity of the CuZnSn and Cu2ZnSnS4 films and the homogeneous distribution of Cu, Zn, Sn and S elements in the bulk films. The XRD and Raman measurements indicated that the synthesized Cu2ZnSnS4 film was kesterite phase without impurities detected. Photoelectrochemical tests were carried out to evaluate the CZTS film's photocurrent response under illumination of green light.

Over 9% Efficient Kesterite Cu2ZnSnS4 Solar Cell Fabricated by Using Zn1–xCdxS Buffer Layer

A kesterite Cu2ZnSnS4 thin film solar cell with efficiency of over 9% is obtained by utilizing Zn1–xCdxS film as a replacement to traditional CdS buffer layer. Zn1–xCdxS film can optimize the conduction band offset between Cu2ZnSnS4 absorber and buffer, forming a minor positive conduction band alignment, thereby alleviating the recombination significantly and improving the open circuit voltage and fill factor efficiently.

Galvanostatically-electrodeposited Cu–Zn–Sn multilayers as precursors for crystallising kesterite Cu2ZnSnS4 thin films

Metallic stacked layers containing Cu, Zn and Sn on Mo-coated glass substrates were prepared by galvanostatic electrodeposition as precursors for crystallising kesterite Cu2ZnSnS4 films. Due to the nature of the MoOx passivating layer on the Mo surface, initial trials were particularly addressed to deposit strong adhesive precursors by firstly depositing Cu layers on Mo-coated substrates from acidic as well as alkaline solutions. It was found that the acidic Cu solutions were not appropriate for the Cu deposition on Mo-coated glass substrates, since they led to an inhomogeneous coverage of the Cu deposits in addition to extremely poor adhesion. In contrast, utilising an alkaline Cu solution yielded a homogeneous coverage of the Cu deposits, having strong adhesion to the Mo-coated glass substrates, which was essential for the subsequent acidic Zn or Sn galvanostatic electrodepositions. The crystallisation of kesterite Cu2ZnSnS4 from metallic stacked Cu/Sn/Zn precursors was investigated as well. The sulfurisation process crystallises the Cu–Zn–Sn precursor films into films exhibiting predominant kesterite phase. This result demonstrates that the established galvanostatic electrodeposition technique for metals has a technological potential for preparing kesterite precursors.

Crystallization of kesterite Cu2ZnSnS4 prepared by the sulfurization of sputtered Cu–Zn–Sn precursors

The one step deposition of a single layer of a Cu–Zn–Sn (CZT) precursor by radio frequency magnetron sputtering from an alloy target was performed to prepare kesterite Cu2ZnSnS4 thin film compound. The crystallization of Cu2ZnSnS4 after sulfurization at elevated temperatures was studied. Chemical reactions at 250°C to 350°C resulted in the formation of binary sulfides (CuS, SnS and ZnS), as well as a ternary sulfide (Cu2SnS3). Subsequently, the crystallization of kesterite Cu2ZnSnS4 was achieved from a reaction between ZnS and Cu2SnS3 at temperatures ranging from 400 to 550°C. At 400°C, Cu2ZnSnS4 was formed in the presence of secondary phases. These secondary phases disappeared gradually as the temperature was increased with a single Cu2ZnSnS4 phase observed at 500°C and above. The optical properties of Cu2ZnSnS4 films were characterized and the direct energy band gap for Cu2ZnSnS4 films was estimated to be 1.51eV.

Determination of secondary phases in kesterite Cu2ZnSnS4 thin films by x-ray absorption near edge structure analysis

Secondary phases in Cu2ZnSnS4 (CZTS) are investigated by x-ray absorption spectroscopy. Evaluating the x-ray absorption near edge structure at the sulfur K-edge, we show that secondary phases exhibit sufficiently distinct features to allow their quantitative determination with high accuracy. We are able to quantify the ZnS fraction with an absolute accuracy of ±3%, by applying linear combination analysis using reference spectra. We find that even in CZTS thin films with [Sn]/[Zn] ≈ 1, a significant amount of ZnS can be present. A strong correlation of the ZnS-content with the degradation of the electrical performance of solar cells is observed.