Kesterite Crystal Structure Research Articles

Compact microstructured Cu2ZnSnS4 thin films with enhanced optoelectronic properties via (NH4)2S tunable hybrid colloidal ink coating and transformation

Kesterite Cu2ZnSnS4 (CZTS) is a quaternary semiconductor material composed of earth-abundant elements that is a promising material for sustainable thin film photovoltaics and other optoelectronic applications. In this work, CZTS thin films are deposited using aqueous colloidal suspensions of binary sulfides (Cu and Zn) and thiostannate complexes formed by in situ reaction of SnS2 with (NH4)2S. In a departure from previous works that used only dissolved thiostannate complexes, we utilize a hybrid state of tin sulfide as both nanoparticles and thiostannate complexes to improve thin film quality. This finding was made by adjusting the (NH4)2S concentration in our aqueous inks, which modified the relative abundance of thiostannate complexes vis-à-vis the amount of SnS2 nanoparticles. In the absence of (NH4)2S, the unstable ink yields nonuniform, partially oxidized thin films. Meanwhile, thiostannate complex formation with the use of (NH4)2S (2–10 vol%) can suppress the presence of secondary phases such as CuxS in the absorber layer and protects against oxidation (avoiding undesirable SnO2 formation). Lowering the aqueous (NH4)2S concentration to 2 vol% yielded the most stable ink formulation, featuring a hybrid state of tin sulfide as both colloidal particles and dissolved thiostannate complexes. The hybrid tin sulfide ink produced compact thin films and reduced the level of Cu-Zn antisite defects in the kesterite crystal structure. We attribute the compact microstructure attained with the hybrid tin sulfide formulation to thiostannate complexes acting as linkers among the three binary sulfides (Cu, Zn, and Sn), enabling their orderly packing into an amorphous, quaternary initial state which is then annealed to grow crystalline kesterite CZTS. UV-Vis absorption characterization shows the Tauc bandgap for 2 vol% hybrid ink coated films to be 1.40 eV with reduced absorption tails based on Urbach energy measurements, while photoluminescence studies showed significantly enhanced emission.

Disorder induced band gap lowering in kesterite type Cu2ZnSnSe4 and Ag2ZnSnSe4: a first-principles and special quasirandom structures investigation

Quaternary chalcogenides, i.e. Cu2ZnSnS4, crystallising in the kesterite crystal structure have already been demonstrated as potential building blocks of thin film solar cells, containing only abundant elements and exhibiting power conversion efficiencies of about 14.9% so far. However, due to the potential presence of several structurally similar polymorphs, the unequivocal identification of their ground state crystal structures required the application of more elaborate neutron diffraction experiments. One particular complication arose from the later identified Cu–Zn disorder, present in virtually all thin film samples. Subsequently, it has been shown experimentally that this unavoidable Cu–Zn disorder leads to a band gap lowering in the respective samples. Additional theoretical investigations, mostly based on Monte-Carlo methods, tried to understand the atomistic origin of this disorder induced band gap lowering. Here, we present theoretical results from first-principles calculations based on density functional theory for the disorder induced band gap lowering in kesterite Cu2ZnSnSe4 and Ag2ZnSnSe4, where the Cu–Zn and Ag–Zn disorder is modelled via a supercell approach and special quasirandom structures. Results of subsequent analyses of structural, electronic, and optical properties are discussed with respect to available experimental results, and will provide additional insight and knowledge towards the atomistic origin of the observed disorder induced band gap lowering in kesterite type materials.

Microwave-induced band gap attenuation of novel manganese zinc aluminum sulfide kesterite nanomaterials for optimal photovoltaics

Recently, kesterite (copper zinc tin sulfide, CZTS) have emerged as a promising alternative active layer to cadmium telluride and copper indium gallium selenide for low-cost thin films for solar cell applications due to its high absorption coefficient, direct band gap, natural abundance, and low toxicity. The photovoltaic power conversion efficiency achieved with CZTS is still very low when compared to silicon, thereby hindering its commercialization. Developing new materials with improved optical and photovoltaic properties is, therefore, encouraged. Here, we report for the first time the microwave-assisted solvothermal synthesis of novel manganese zinc aluminum sulfide using chlorides, sulfates, nitrates and a mixture of chlorides, nitrates, and sulfates in ratio 1:1:1 as cation precursor salts. The influence of microwave reaction temperatures from 100 to 190 °C for 5–60 min were investigated on the crystal structure, morphology, composition, optical, and electrochemical properties. It has been found out that the microwave reaction temperature and time have potential effect on the overall reaction. The optimum temperature and time were found to be 190 °C for 5 min and 150 °C for 20 min for our studies. The obtained powder samples were analyzed using X-ray diffraction, Raman spectroscopy, energy-dispersive X-ray spectroscopy, high-resolution scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared, ultraviolet-visible, cyclic voltammetry, and electrochemical impedance spectroscopy. Structural characterization using X-ray diffraction revealed that the samples synthesized with chlorides, nitrates, and mixtures of the precursor salts were all amorphous while samples synthesized with sulfate precursor salts have a kesterite crystal structure. Raman spectroscopy shows major intense peaks between 326 and 331 cm for all the nanomaterials which could be assigned to the Raman A mode of kesterite CZTS. The morphological features revealed spherical shapes with little agglomeration for all the synthesized materials except for that synthesized with sulfate precursor salts at 190 °C revealing a rod-like morphology with a width of 0.780–1.200 μm, a length of 1.90–2.5 μm and attached to the surface are agglomerated nanospheres. Small-angle X-ray spectroscopy revealed a core-shell shape for most of the nanomaterials with an average particle size between 10 and 100 nm. Electrochemical impedance spectroscopy revealed that electron transfer is faster in the nanomaterials synthesized with the chloride precursor salts at both temperatures studied. The band gap falls within the range of 1.2–1.7 eV. The nanomaterials presented in this study possess the desirable properties for application in photoelectrochemical and solar cells.

Impact of 1,8-Diiodooctane (DIO) Additive on the Active Layer Properties of Cu2ZnSnS4 Kesterite Thin Films Prepared by Electrochemical Deposition for Photovoltaic Applications

Kesterite Cu2ZnSnS4 (CZTS) thin films using various 1,8-diiodooctane (DIO) polymer additive concentrations were fabricated by the electrochemical deposition method. The optical, electrical, morphological, and structural properties of the CZTS thin films synthesized using different concentrations of 5 mg/mL, 10 mg/mL, 15 mg/mL, and 20 mg/mL were investigated using different techniques. Cyclic voltammetry exhibited three cathodic peaks at -0.15 V, -0.54 V, and -0.73 V, corresponding to the reduction of Cu2+, Sn2+, Sn2+, and Zn2+ metal ions, respectively. The analysis of the X-ray diffraction (XRD) pattern indicated the formation of the pure kesterite crystal structure, and the Raman spectra showed pure CZTS with the A1 mode of vibration. Field emission scanning electron microscopy (FE-SEM) indicated that the films are well grown, with compact, crack-free, and uniform deposition and a grain size of approximately 4 µm. For sample DIO-20 mg/mL, the elemental composition of the CZTS thin film was modified to Cu:Zn:Sn: and S = 24.2:13.3:12.3:50.2, which indicates a zinc-rich and copper-poor composition. The X-ray photoelectron spectroscopy (XPS) results confirmed the existence of Cu, Sn, Zn, and S elements and revealed the element oxidation states. The electrochemical deposition synthesis increased the absorption of the CZTS film to more than 104 cm-1 with a band gap between 1.62 eV and 1.51 eV. Finally, the photovoltaic properties of glass/CZTS/CdS/n-ZnO/aluminum-doped zinc oxide (AZO)/Ag solar cells were investigated. The best-performing photovoltaic device, with a DIO concentration of 20 mg/mL, had a short-circuit current density of 16.44 mA/cm2, an open-circuit voltage of 0.465 V, and a fill factor of 64.3%, providing a conversion efficiency of 4.82%.

Revisiting the Cu-Zn Disorder in Kesterite Type Cu2ZnSnSe4 Employing a Novel Approach to Hybrid Functional Calculations

In recent years, the search for more efficient and environmentally friendly materials to be employed in the next generation of thin film solar cell devices has seen a shift towards hybrid halide perovskites and chalcogenide materials crystallising in the kesterite crystal structure. Prime examples for the latter are Cu2ZnSnS4, Cu2ZnSnSe4, and their solid solution Cu2ZnSn(SxSe1−x)4, where actual devices already demonstrated power conversion efficiencies of about 13 %. However, in their naturally occurring kesterite crystal structure, the so-called Cu-Zn disorder plays an important role and impacts the structural, electronic, and optical properties. To understand the influence of Cu-Zn disorder, we perform first-principles calculations based on density functional theory combined with special quasirandom structures to accurately model the cation disorder. Since the electronic band gaps and derived optical properties are severely underestimated by (semi)local exchange and correlation functionals, supplementary hybrid functional calculations have been performed. Concerning the latter, we additionally employ a recently devised technique to speed up structural relaxations for hybrid functional calculations. Our calculations show that the Cu-Zn disorder leads to a slight increase in the unit cell volume compared to the conventional kesterite structure showing full cation order, and that the band gap gets reduced by about 0.2 eV, which is in very good agreement with earlier experimental and theoretical findings. Our detailed results on structural, electronic, and optical properties will be discussed with respect to available experimental data, and will provide further insights into the atomistic origin of the disorder-induced band gap lowering in these promising kesterite type materials.

Fabrication of Cu2ZnSnS4 Light Absorber Using a Cost-Effective Mechanochemical Method for Photovoltaic Applications.

In the present study, we adopt an easy and cost-effective route for preparing Cu2ZnSnS4 (CZTS)-absorber nanoparticles by a mechanochemical method using non-toxic and environmentally benign solvents (butanol, methyl ethyl ketone, and ethanol). The gram-scale synthesis of absorber nanoparticles was achieved in a non-hazardous, zero-waste process without using high-vacuum equipment. The effects of annealing and Na incorporation on the properties of spin-coated CZTS thin films were scrutinized. The deposited samples showed kesterite crystal structure and single phase. The morphological results revealed an improvement in the surface morphology after annealing. The optical bandgaps of the thin films lied in the range of 1.50–1.57 eV with p-type nature. Finally, photovoltaic devices were fabricated, and their cell performance parameters were studied. An efficiency of 0.16% was observed. The present study provides a potential route for the cost-effective fabrication of CZTS-based photovoltaic devices.

Structural and Optoelectronic Characterization of Synthesized Undoped CZTS and Cd-doped CZTS Thin Films

Copper zinc tin sulfide (CZTS) thin films with different doping ratios of Cadmium (Cd) were successfully fabricated using the sol-gel method by dip-coating technique. The surface morphology, the crystal structure properties as well the optical properties of undoped CZTS thin film and Cd- doped CZTS thin films were investigated using scanning electron microscopy (SEM), x-ray diffraction (XRD), and UV-Vis spectrophotometer. SEM micrographs demonstrated that the size and morphology of the particles improve due to increasing the Cd concentration in CZTS thin films. In addition, the XRD patterns exhibited the crystalline nature for CZTS thin films with kesterite crystal structure and showed improvement in some crystal structure properties such as crystal size and volume of unit cell with the incorporation of Cd into CZTS thin films. Moreover, optical bandgap energy

On the ground state crystal structure of (Ag0.5Cu0.5)2ZnSnSe4

In our quest to improve the device performance of kesterite based thin film solar cells a recent combined neutron and x-ray diffraction study on the structural properties of powder samples of (Ag1-xCux)2ZnSnSe4 yielded surprising results. Although both end members, Ag2ZnSnSe4 and Cu2ZnSnSe4 crystallise in the kesterite crystal structure, in the intermediate region x∼0.5 the structurally similar stannite crystal structure has been identified as the prevalent ground state crystal structure. Here, in order to understand this behaviour, we perform first-principles calculations based on density functional theory, combined with evolutionary crystal structure prediction investigation as implemented in the USPEX code. Our results show that (Ag0.5Cu0.5)2ZnSnSe4 crystallises in the kesterite crystal structure as well, with the Ag and Cu atoms being exclusively placed at the (2a) and (2c) Wyckoff positions, with no apparent mixing. A detailed structural analysis combined with electronic and optical properties will be discussed alongside experimental results.

Highly stable ethanol-based Cu2ZnSnS4 (CZTS) low-cost thin film absorber: Effect of solution aging

Aging is an important factor that can affect the grain size, surface morphology and optical band gap of the thin films. Especially, it is thought that these important parameters will have a strong impact on the solar cell energy conversion ratio in Cu2ZnSnS4 (CZTS) absorber based photovoltaic devices. With this aim the presented study reports the effect of the aging time on the structural, morphological and optical characteristics of p-type quaternary CZTS absorber materials, produced by using a solution assisted dip-coating process on the glass substrate and annealed at 515 °C without using any dangerous environment (such as sulphurization or selenization). The kesterite crystal structure was determined by X-ray diffraction (XRD) analyses for all the CZTS samples. To support the observed kesterite crystal structure of the CZTS, the main vibration modes and chemical composition of the samples were investigated by Raman and XPS tools. Besides, the comprehensive surface analyses were conducted to better understanding of the morphology variation within the CZTS thin films depending on the aging time of the CZTS precursor solution by using SEM and AFM tools. Generally, large grain size and solution stability of absorber material have important impacts on the solar cell energy conversion solar cells. Furthermore, the band gap of the CZTS thin film shifts toward the optimal energy value (from 1.82 eV to 1.56 eV), observed from the optical studies. This last result is highly attractive for the CZTS based solar cell applications.

Efficient and stoichiometric controlled oleic acid assisted hydrothermal synthesis of Ag incorporated Cu2ZnSnS4 chalcogenide nanoparticles

Ag incorporated CZTS (Ag-CZTS) nanoparticles with an average crystallite size of 13−17 nm were successfully synthesized by using simple oleic acid assisted hydrothermal method. X-ray diffraction, Raman spectrum and electron microscopical studies indicate that Ag-CZTS nanoparticles with pure tetragonal kesterite crystal structure. The prepared Ag- CZTS nanoparticles exhibits surface area about 34.14 m2/g with pore size volume 0.1210 cm3/g and average pore size was found to be in 10 nm. The optical band gap value is 1.51 eV, suggesting that Ag-CZTS nanoparticles are apt for solar cell applications. This work paves a way for utilizing these Ag-CZTS nanoparticles as a potential absorber layer in photovoltaic devices.

Novel hydrothermal route for synthesis of photoactive Cu2ZnSn(S,Se)4 nanocrystalline thin film: efficient photovoltaic performance

This work reports single-step deposition of Cu2ZnSn(S,Se)4 (CZTSSe) nanocrystalline film via simple and non-toxic hydrothermal route. The hydrothermal growth of CZTSSe multinary nanocrystalline film formation based on Ostwald ripening law which results in highly oriented nanocrystalline nanoflakes. The UV–Vis spectroscopy illustrates that maximum absorption was observed in the range of 650–700 nm with band gap energy of 1.48 eV. Structural studies confirm the formation of pure-phase kesterite crystal structure. Further, Raman shift at 174 cm−1, 196 cm−1 and 236 cm−1 shows A1 mode of vibration corresponding to kesterite structure. Densely packed and compact nanoflake formation was observed in SEM studies. Compositional analysis confirms the stoichiometric film formation with expected valence states of CZTSSe stoichiometry: Cu+, Zn2+, Sn4+, S2− and Se2−. The photoelectrochemical cell performance of CZTSSe film photoelectrode was investigated using simple two-electrode system. In dark condition, J–V measurement demonstrates semiconductor behavior and under illumination shows generation of photocurrent of 3.64 mA/cm2 at a photovoltage of 400 mV. The CZTSSe films show photoconversion efficiency (η) of 3.41%, indicating that the hydrothermally grown CZTSSe photocathode is a promising material for film solar cell technology. Electron impedance spectroscopy illustrated generation of charge transport resistance of 465 Ω.

Morphological characterization and microstructural study of Cu2ZnSnSe4 thin films with compositional variation

In second generation thin film solar cell, Cu2ZnSnSe4 (CZTSe) is used as an absorber layer due to some remarkable properties like non-toxicity and earth abundancy. In this paper we have presented the preparation of Cu2ZnSnSe4 thin films in five different elemental ratios employing the RF-DC sputtering and E-beam deposition technique. In order to study the effects of elemental compositional variation on structural and morphological properties, thin films were characterized using SEM-EDX, XRD and AFM technique. Elemental compositional variation is studied using EDX technique. Numerical analysis of the surface parameters affected by compositional variations are presented against the data processed by AFM images. Watershed based segmentation algorithm is employed to present a quantitative grain size distribution. Variation in growth exponent (β) and roughness critical exponent (α) due to compositional variation in CZTSe thin films is studied using height-height correlation function. Auto correlation length varied from 3.65 nm to 4.44 nm while Rku is found greater than three for all thin films. A critical analysis of fractal dimension and auto correlation length with surface parameters like amplitude parameters, spacing parameters and hybrid parameters is also presented. Optoelectronic properties are studied with respect to compositional variation. Sample with lowest Cu/(Zn+Sn) ratio showed the lowest value of band gap of 0.909 eV . EDX results confirmed that the atomic ratio of Cu-Zn-Sn-Se in thin films of all CZTSe samples remained controlled. Films showed tetragonal kesterite crystal structure with the space group of I −4 2 m with preferential orientation along the (105) plane.

Facile synthesis of sulfide-based chalcogenide as hole-transporting materials for cost-effective efficient perovskite solar cells

Sulfide-based chalcogenide quaternary semiconductors Cu2MSnS4 where M = Zn, Mn, Ni and, Co have emerged as a promising material for photovoltaic application due to their opto-electrical properties, friendly environment and earth-abundant composition. Herein, we report on the surfactant-assisted, solvent-free, hydrothermal synthesis of Cu2MSnS4 (M = Zn, Mn, Ni, Co) nanoparticles and their application in photovoltaics as well. The kesterite and stannite crystal structure of the investigated compounds were confirmed by powder X-ray diffraction. Raman, Photoluminescence and UV–Vis reflectance spectroscopies were employed to study the lattice vibrational and the optical properties. High-resolution transmittance electron microscopy was performed to study the morphology of nanoparticles size, as well as the crystallinity of chalcogenide compounds. In the presence of Cu2MSnS4 with different divalent transition metal ions (M) = Mn2+, Co2+, Ni2+ and Zn2+ as a hole transport material, the perovskite-sensitized TiO2 films showed power conversion efficiencies of 8.35, 6.24, 7.55 and 4.16%, respectively.

CZTSSe Solar Cells from Nanoparticle Inks

Future energy demand can be addressed by using renewable and inexhaustible solar energy, providing clean, unlimited, economical and green energy. Electricity generation from the sun employing PV technology is currently dominated by Si-based PV and requires expensive equipment and process and schemes for cost reduction on a large scale are limited. Thin film technologies such as CdTe and Cu(In,Ga)Se2, provide a lower cost alternative primarily due to the use of in-line and low-temperature processes. While considerable efforts have been made to increase efficiency and reduce costs, thin film PV currently relies on scarce and therefore expensive resources and/or toxic elements. Alternative thin film materials would therefore provide routes to reduce PV cost-per-watt while still exhibiting lower input energy requirements. Solar cells based on Cu2ZnSn(S,Se)4 (CZTSSe) absorber layers offer such an alternative. Despite its young history CZTSSe record efficiency stands at 12.6% and the major limitations are (i) a lower than expected open circuit voltage accompanied by a low efficiency at converting and collecting carriers from low energy photons; (ii) the difficulty in controlling the kesterite crystal structure throughout the fabrication process; and (iii) the use of hydrazine, a highly toxic chemical, in the fabrication process to achieve the record efficiencies. This project uses nanocrystal dispersions (inks) of CZTS synthesised from hot injection as the starting material. This technique can reliably control crystal structure, composition and doping and does not present any environmental risks. Inks are easily spin coated or sprayed on substrates and a heat treatment under selenium rich atmosphere promotes grain growth without loss of the crystal structure. In order to fabricate record efficiencies using this technique the microstructure of the absorber and back contact layers need to be engineered to provide large grains extending the full thickness of the absorber combined with a small interfacial layer to ensure a good ohmic contact.

Optimization of Cu2ZnSnS4 thin film absorber layer growth without sulphurization using triethanolamine as complexing agent for thin film solar cells applications

Quaternary kesterite Cu2ZnSnS4 (CZTS) thin films have been prepared via a simple spin-coating technique based on a sol–gel precursor of 2-methoxyethanol solution with metal salts and thiourea. Solution processed CZTS thin film growth parameters using complexing agent triethanolamine (TEA) have been investigated. Effects of complexing agent TEA on structural, morphological, optical, electrical and photovoltaic properties of CZTS thin films were systematically investigated. X-ray diffraction and Raman spectroscopy studies reveal that amorphous nature of CZTS thin film changes into polycrystalline with kesterite crystal structure with optimized TEA concentartion. Surface morphology of CZTS films were analyzed by field emission scanning electron microscope and atomic force microscope, which revealed the smooth, uniform, homogeneous and densely packed grains and systematic grain growth formation with varying TEA concentrations. UV–Vis spectra revealed a direct energy band gap ranging from 1.78 to 1.50 eV, which was found to depend upon the TEA concentration. X-ray photoelectron spectroscopy demonstrated stoichiometric atomic ratios of multicationic quaternary CZTS thin film grown without sulphurization. p-type conductivity was confirmed using Hall measurements and the effect of varying concentartion of TEA on electrical and photovoltaic properties are studied. The SLG/FTO/ZnO/CZTS/Al thin film solar cell is fabricated with the CZTS absorber layer grown at optimized TAE concentration of 0.06 M. It shows a power conversion efficiency of 0.87% for a 0.16 cm2 area with Voc = 0.257 mV, Jsc = 8.95 mA/cm2 and FF = 38%.

Cationic and Anionic Disorder in CZTSSe Kesterite Compounds: A Chemical Crystallography Study

The cationic and anionic disorder in the Cu2ZnSnSe4-Cu2ZnSnS4 (CZTSe-CZTS) system has been investigated through a chemical crystallography approach including X-ray diffraction (in conventional and resonant setup), 119Sn and 77Se NMR spectroscopy, and high-resolution transmission electron microscopy (HRTEM) techniques. Single-crystal XRD analysis demonstrates that the studied compounds behave as a solid solution with the kesterite crystal structure in the whole S/(S + Se) composition range. As previously reported for pure sulfide and pure selenide compounds, the 119Sn NMR spectroscopy study gives clear evidence that the level of Cu/Zn disorder in mixed S/Se compounds depends on the thermal history of the samples (slow cooled or quenched). This conclusion is also supported by the investigation of the 77Se NMR spectra. The resonant single-crystal XRD technique shows that regardless of the duration of annealing step below the order-disorder critical temperature the ordering is not a long-range phenomenon. Finally, for the very first time, HREM images of pure selenide and mixed S/Se crystals clearly show that these compounds have different microstructures. Indeed, only the mixed S/Se compound exhibits a mosaic-type contrast which could be the sign of short-range anionic order. Calculated images corroborate that HRTEM contrast is highly dependent on the nature of the anion as well as on the local anionic order.

Photocatalytic degradation of methylene blue by hydrothermally synthesized CZTS nanoparticles

Due to the toxic effect of Methylene blue (MB) dye, its removal from water and water waste has gained significant attention in concern with green environment. In present work, a simple and low cost hydrothermal method has been used for the synthesis of copper zinc tin sulfide (CZTS) nanoparticles. Structural, morphological and optical properties of CZTS nanoparticles were characterized and it reveals that the synthesized powder has kesterite crystal structure with agglomerated morphology and shows a band gap of 1.53 eV leading to good potential to degrade methylene blue dye. When CZTS was introduced in aqueous MB solution as a catalyst and exposed to visible light for 45 min, about 50% degradation of MB was observed.

Structural and optical properties of sol gel derived Cu2ZnSnS4 nanoparticles

The spherical Cu2ZnSnS4 nanoparticles with the average diameters (∼8–10nm) have been synthesized by sol gel method. The effects of solvents and reaction temperatures on the properties of the as-synthesized nanoparticles were investigated. The X-ray diffraction shows as grown Cu2ZnSnS4 nanoparticles exhibit kesterite crystal structure along preferential orientation (112) plane. The crystalline nature of nanoparticles was improved in ethylene glycol solvent with the increase in reaction temperature. Rietveld refinement study was performed and structural parameters were determined for the Cu2ZnSnS4 nanoparticles. The Raman spectra show the main characteristic peak of A1 vibrational mode which confirmed the formation of Cu2ZnSnS4 phase in all the samples. Scanning electron micrographs depict the irregular aggregate formation of nanoparticles in methanol solvent and uniformly distributed aggregates of nanoparticles with ethylene glycol solvent. Transmission electron microscopy results show the synthesis of polycrystalline porous nanostructures and uniform spherical nanoparticles in methanol and ethylene glycol solvents respectively at the temperature of 250°C. UV–vis absorption spectra indicated the broad absorption in visible range and the band gap of the nanoparticles was found to 1.38 and 1.45eV which is suitable for absorbing the solar radiation. The obtained results revealed ethylene glycol as a suitable solvent and 250°C as the favorable synthesis temperature.

Effects of copper content on properties of CZTS thin films grown on flexible substrate

In this work, Cu2ZnSnS4 (CZTS) thin films on the flexible molybdenum foils were fabricated by vacuum-based magnetron sputtering and followed by annealing in the sulfur vapor. The effects of copper content on CZTS films’ structures and properties were studied using X-ray diffraction (XRD), Raman scattering, energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and UV–Vis spectroscopy techniques. According to the experimental results, the strong peak in XRD pattern along (112) plane confirms the kesterite crystal structure of CZTS thin films. A suitable increase of the copper content can improve the crystallinity of the CZTS thin films. However, the continued increase in copper content can lead to the presence of Cu2S and Cu3SnS4 phases in sulfurized CZTS thin films. Furthermore, the performances of flexible CZTS thin films after repeated bending were also explored. In our case, no cracks were observed by SEM, but a fold appeared in the surface. Also, increasing copper content makes the fold less obvious.

Influence of the Cu Content in Cu2ZnSn(S,Se)4 solar cell absorbers on order-disorder related band gap changes

We investigate the electronic structure and the radiative recombination in wet-chemically fabricated Cu2ZnSn(S,Se)4 solar cell absorbers utilizing photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy, focusing especially on the effects of varying Cu content. This includes the impact of the latter on the band gap energy and the change in band gap energy related to the order-disorder transition. Characteristic PL and PLE parameters like the energetic position of the PL maximum and the PL yield as a function of the excitation power as well as the PLE tailing parameter do not depend on composition indicating that the nature of the radiative transition is not altered by the Cu content. However, the band gap energy Eg significantly increases as a function of decreasing Cu content. This increase is more pronounced in the disordered than in the ordered atomic arrangement of Cu and Zn atoms in the Cu–Zn planes of the kesterite crystal structure.