CZTS Films Research Articles

Experimental study and numerical simulation for the development of critical performance parameters of eco-friendly Cu2ZnSnS4-based solar cells

This work aims to study the basic Characteristics of CZTS thin films produced by chemical solution process in addition to a numerical study of the CZTS-based solar cells. In this context, eco-friendly and cost effective thin films of CZTS were coated on glass substrates by sol–gel dip-coating method. This method is used to grow crystalline CZTS thin films without sulfidation processes. The structural and optical properties have been carried out by X-ray Diffraction (XRD) and UV/Visible spectroscopy. The oxidation states of CZTS thin film have been studied by, X-ray photoelectron spectroscopy XPS analysis. The performance of CZTS solar cells has been investigated with the simulator SCAPS-1 D. The impact of different parameters like the CZTS absorber layer thickness, the carrier doping density and the operation temperature has been studied to get better understanding of the properties of our cells.

Integration of single layer graphene into CZTS thin film solar cells

In this study, CZTS samples were produced on Mo and graphene/Mo coated glass substrates using quaternary target. The CZTS thin films deposited by RF magnetron sputtering were annealed using rapid thermal processing (RTP) method in sulphur atmosphere at 500, 525, and 550 °C so as to obtain glass/Mo/CZTS and glass/Mo/graphene/CZTS (g-CZTS) structures. The obtained CZTS and g-CZTS thin films were then characterized by several methods such as EDX, XRD, Raman spectroscopy, SEM etc. The EDX data demonstrated that all CZTS thin films had Cu poor composition regardless of the sulfurization temperature and increasing the temperature led to Sn loss from the films. Diffraction peaks of kesterite CZTS phase were observed in all the samples; additionally, SnS and CuS secondary phases were also observed in CZTS samples annealed at 500 °C. The crystallite size of the CZTS thin films were found to be increasing with both increasing annealing temperature and use of graphene film as an inter-layer. The creation of kesterite phase with a very small CTS phase in all the samples were verified by the Raman spectroscopy measurement. The SEM images of the samples indicated that using graphene improves the crystalline quality of the CZTS films and contributes to forming more compact, homogenous and larger crystal structure. The determined optical band gap values varied from 1.41 to 1.44 eV depending on the Sn-content of the samples. The produced solar cells selected from the more promising absorber layers showed that implementing graphene in CZTS cell structure enhanced the conversion efficiency from 2.40% to 3.52% due to improvement of crystalline quality of the absorber layer.

Influence of annealing conditions on the performance of sputtered grown CZTS thin film solar cells

Kesterite Cu2ZnSnS4 (CZTS), with direct and tunable bandgap, high absorption coefficient, low-cost processing, earth-abundance, and non-toxicity, has become a promising absorber material for emerging thin film solar cells. The film growth, phase formation, and stoichiometry, as well as the electrical and optical properties of the sputtered grown CZTS thin films, are greatly influenced by the annealing temperature and pressure. In this work, we use a series of material characterization techniques followed by numerical simulations to determine how the annealing conditions affect the properties of CZTS thin films and the photovoltaic performance of the corresponding thin film solar cells. Sputtered grown CZTS samples were annealed at different temperatures (470 and 560 °C) and pressures (250 and 350 Torr). UV-visible spectroscopy was used to determine the optical bandgap and the absorption spectra. The experimentally determined values are then used as input parameters in the Solar Cell Capacitance Simulator-1 Dimension (SCAPS-1D) simulator tool to determine the solar cell performance. Simulation results show a maximum theoretical power conversion efficiency for the CZTS sample annealed at 560 °C and 350 Torr pressure. The results can lead to the optimization of sputtering conditions for obtaining high quality CZTS films that can yield excellent solar cell performance.

Spectroscopic ellipsometry and photovoltaic characteristics for n-CdS/p-Cu2ZnSnS4 heterojunction by annealing for solar cells

Owing to its direct bandgap in the range to be used as an absorbent material and due to its high absorption rate, kesterite Cu2ZnSnS4 (CZTS) is a p-type prospective absorber material (with thickness 500 nm) for solar cell applications. Kesterite Cu2ZnSnS4 (CZTS) thin films were prepared using the thermal evaporation technique. The (CZTS) thin films were annealed at different annealing temperatures chosen according to TGA analysis in the range of (400o C, 450o C, 475o C, and 500o C). The influence of annealing temperatures on the structural, morphology and optical properties of the CZTS films was investigated. The XRD patterns and Raman spectra have revealed the formation of CZTS thin with a high-quality crystal structure. The optical constants refractive index n, and extinction coefficient, k consequently band gap are estimated from SE via construction an optical model. The refractive index n of the CZTS /glass films received from SE model increases with the annealing temperature that is credited to the rise of the size of the crystal. It was also found that when the annealing temperature of the CZTS layer increases, the general behavior of the extinction coefficient k of the CZTS /glass film increases. In addition, it is found that the direct optical transition with energy band gap is compact from 1.75 eV at RT to 1.49 eV at maximum crystallization 500 o C. The Ni/n-CdSe/p-CdTe/Pt heterojunction has been successfully assembled. The dark and illumination (currentvoltage) behavior of fabricated heterojunctions had been suggested at distinctive different annealing of CZTS layer, as well as for voltages ranging from -2 to 2 volts.

Constructing of Cu2ZnSnS4 thin films with enhanced optical properties for solar cell application

Thin films of Cu2ZnSnS4 (CZTS) were created utilizing the thermal evaporation method. For the (CZTS) nanoparticles, thermogravimetric analysis (TGA) was initially carried out. According to TGA analysis, different annealing temperatures (400, 450, 475, and 500 °C) were used to anneal the (CZTS) thin films. Investigations were done into how different annealing temperatures affected the structural, morphological, and optical characteristics of the CZTS films. The development of a thin CZTS crystal with a high-quality crystal structure has been demonstrated by XRD patterns and Raman spectra. Using Debye-Scherrer's equation, the crystallite size for CZTS thin films was determined to be between 8 and 69.9 nm. As the annealing temperature rises, crystallite size increases. Using a double-beam computer-controlled spectrophotometer with a 400–2500 nm wavelength range, the optical characteristics were examined. While the reflectance was discovered to increase with an increase in annealing temperature, the transmittance was found to decrease. Calculating the extinction coefficient and refractive index was done using Kramers-Kronig relations. Using Tauc's relation, the optical energy gap was estimated, and it was discovered that it shrank as annealing temperature was raised. The (CZTS) thin film that was annealed at 500 °C had the highest power conversion efficiency (PCE).

A new approach for alkali incorporation in Cu2ZnSnS4 solar cells

The addition of alkali elements has become mandatory for boosting solar cell performance in chalcogenide thin films based on kesterites (Cu2ZnSnS4, CZTS). A novel doping process is presented here, that consists in the incorporation of sodium or lithium during the deposition of the CdS buffer layer, followed by a post-deposition annealing (PDA). As the doping route leads to more efficient devices in comparison with the undoped reference sample, the influence of PDA temperature was also investigated. Compositional profiling techniques, time-of-flight secondary ion mass spectrometry (TOF-SIMS) and glow discharge optical mission spectroscopy (GDOES), revealed a dependence of the alkaline distribution in kesterites with the PDA temperature. Although the doping process is effective in that it increases the alkaline concentration compared to the undoped sample, the compositional profiles indicate that a significant proportion of Li and Na remains ‘trapped’ within the CdS layer. In the 200 °C–300 °C range the alkali profiles registered the higher concentration inside the kesterite. Despite this, an additional alkali accumulation close to the molybdenum/fluorine doped tin oxide substrate was found for all the samples, which is frequently related to alkali segregation at interfaces. The addition of both, lithium and sodium, improves the photovoltaic response compared to the undoped reference device. This is mainly explained by a substantial improvement in the open-circuit potential (V oc) of the cells, with best devices achieving efficiencies of 4.5% and 3% for lithium and sodium, respectively. Scanning-electron microscopy images depicted a ‘bilayer structure’ with larger grains at the top and small grains at the bottom in all samples. Moreover, the calculated bandgap energies of the CZTS films account for changes in the crystallographic order-disorder of the kesterites, more related to the PDA treatment rather than alkali incorporation. Even if further optimization of the absorber synthesis and doping process will be required, this investigation allowed the evaluation of a novel strategy for alkali incorporation in kesterite based solar cells.

Improvement of CZTSSe film quality and superstrate solar cell performance through optimized post-deposition annealing

Improving the performance of kesterite solar cells requires high-quality, defect-free CZTS(Se) films with a reduced number of secondary phases and impurities. Post-annealing of the CZTS films at high temperatures in a sulfur or selenium atmosphere is commonly used to improve the quality of the absorbing material. However, annealing at high-temperatures can promote material decomposition, mainly due to the loss of volatile elements such as tin or sulfur. In this work, we investigate how the additional step of sulfurization at reduced temperatures affects the quality and performance of CZTSSe based solar cells. A comprehensive structural analysis using conventional and high resolution XRD as well as Raman spectroscopy revealed that the highest CZTSSe material quality with the lowest structural disorder and defect densities was obtained from the CZTS films pre-sulfurized at 420 °C. Furthermore, we demonstrate the possibility of using Sb2Se3 as a buffer layer in the superstrate configuration of CZTSSe solar cells, which is possible alternative to replace commonly employed toxic CdS as a buffer layer. We show that the additional low-temperature selenization process and the successful use of Sb2Se3 as a buffer layer could improve the performance of CZTSSe-based solar cells by up to 3.48%, with an average efficiency of 3.1%.

Non-stoichiometric effect and disorder in as-prepared Cu2ZnSnS4 films deposited at different temperatures by ultrasonic spray pyrolysis

A better understanding of its crystal structure, the formation of possible secondary phases, defects and Cu–Zn disorder effects is needed to improve the photovoltaic device performance of CZTS films. In this direction, the effect of deposition temperature on the structural inhomogeneities, such as secondary phases and Cu–Zn disorder, etc. as well as opto-electrical properties of CZTS were experimentally examined. For this purpose, a non-stoichiometric spraying solution was prepared and ultrasonically sprayed onto glass substrates at different deposition temperatures (350 °C, 400 °C, 450 °C, and 500 °C) to obtain CZTS films. Afterward, the structural, morphological, elemental, optical, and electrical properties of the deposited films were investigated in detail. By Lorentzian deconvolution of Raman spectra, 14 Raman vibrational modes were detected and seven of these were assigned to the secondary phases. Also, the ordered-kesterite phase (337 cm−1) of CZTS was found to crystallize along with the disordered-kesterite phase (329 cm−1) due to the disorder of the cation (Cu–Zn) sublattice. Optical band gaps for CZTS films decreased from 1.89 eV to 1.42 eV with increasing in deposition temperature. It was seen that not only optical band gaps but also Cu–Zn disorder and the amount of secondary phases in CZTS films tightly depend on the deposition temperature.

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.

XPS analysis and structural characterization of CZTS thin films deposited by one-step thermal evaporation

Earth abundant and non-toxic CZTS films were grown by one-step thermal evaporation from as prepared and calcined powders followed by a sulfurization process at 400 °C.400 °C. X-ray diffraction and Raman studies have shown polycrystalline quality of the samples with kesterite structure, which are preferentially oriented along the (112) plane. The structural parameters of all CZTS samples were estimated before and after sulfurization. The structural properties indicate an enhancement of crystallinity after the sulfurization process. Scanning electron microscopy demonstrates significant morphological changes in the films after the sulfurization. Compositional analysis made by energy dispersive spectroscopy shows that the composition of the films moves towards the theoretical 2:1:1:4 stoichiometry due to sulfurization process. Finally, we have investigated the oxidation state of all the elements in CZTS after sulfurization by using XPS. We found that all the films were formed with the valence state of Cu+, Zn2+, Sn4+ and S2−.

Optical and Structural Study of the CZTS (Cu2ZnSnS4) Thin Film for Solar Cell Derived from the Chloride-Based sol-gel Precursor Solution

Fabrication of environmentally safe C2ZnSnS4 (CZTS) photovoltaic thin films of pure kesterite state is crucial. We have preparedCZTS thin films by sol-gel dip-coating process from chloride-based chemicals. After vacuum-annealed at 550°C, the optical andstructural characters of the films were further examined by UV–vis spectroscopy, X-ray diffraction (XRD), scanning electronmicroscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) methods. The CZTS films offered high optical absorption(0.4x104cm-1) and nearly optimum bandgap energy (1.65e V). X-ray diffraction examination confirmed the kesterite structure offilms. Scanning electron micrograph affirmed the creation of jam-packed, condensed and granulated CZTS films. The thin filmdisplayed intermittent disposal of agglomerated particles with clear-cut edges. The energy dispersive X-ray spectroscopy study gavethe stoichiometric ratio as Cu: Zn: Sn: S= 2.2: 1.4: 1: 5.1.
 Dhaka Univ. J. Sci. 70(1): 1-7, 2022 (January)

Identification of structural inhomogeneity on spray pyrolyzed Cu2ZnSnS4 thin film using micro-Raman spectroscopy

The study on the structural inhomogeneity of Cu2ZnSnS4 (CZTS) thin film is imperative to enhance the absorber layer performance in photovoltaic devices (PV). Herein, we have examined the structural inhomogeneity of spray pyrolyzed CZTS thin film using micro-Raman spectroscopy. The shift of major A-mode i.e., 337 cm−1, 334 cm−1 and 332 cm−1, in Raman spectra recorded at different spots, corroborated the presence of ordered and disordered kesterite structure. The observed shift of A-mode in different spots is ascribed to phase segregation and local inhomogeneity that existed on the surface of CZTS film. The respective distribution of the different acquired Raman spectral regions has been presented using the Raman mapping. Scanning electron microscopy was employed to confirm the phase segregation in the surface of CZTS film. The attained composition ratio of CZTS film i.e., Cu/(Zn+Sn)=0.89 and Zn/Sn=1.49, in the elemental analysis are expedient.

Favorable Bonding and Band Structures of Cu2ZnSnS4 and CdS Films and Their Photovoltaic Interfaces.

Thin-film photovoltaic cells using Cu2ZnSnS4 (CZTS, p-type) have many advantages, such as high photoconversion, low cost, and great tunability with earth-abundant, nontoxic elements, all of which are necessary to be long-term contributors to next-generation solar energy harvesting. Accurate measurements of bonding and band structures of both the thin-film materials and their interfaces are paramount to designing the solar devices layer-by-layer. Here, finely tuned 1 μm thick CZTS films, 50 nm thick CdS layers (n-type), and their 1 μm/2 nm p-n junction were fabricated inexpensively using our previously studied methods and investigated extensively for maximizing the key interface in the CZTS solar devices. Synthesized bulk CZTS and CdS were analyzed for structural deviations and crystal defects using synchrotron-based (SR) X-ray absorption fine structure (XAFS) along with simulated XAFS patterns. The structural properties of the two materials were designed to favor photovoltaic activity. Interface valence band structures of the CZTS/CdS p-n junction were measured through SR X-ray photoelectron spectroscopy (SR-XPS) and compared with the ones simulated using density functional theory. A full band diagram was constructed from XPS of the bulk films and SR-XPS of the interface, providing guidelines in optimizing charge-carrier extraction from the CZTS absorber to CdS buffer layer. It turns out that a small spike-like interface in the conduction band overlap was formed, maintaining a strong internal bias, while favoring a small energy barrier to prevent large-scale recombination from occurring. A large open-circuit voltage was obtained in the preliminary solar cell devices built on the small spike-like interface.

A Comparative Study: Sequential and Single‐Step‐Electrodeposited CZTS Thin Films

CZTS (Cu2ZnSnS4) is a relatively new and promising semiconductor material suitable for photovoltaic applications due to its favorable optoelectronic properties. Of the many techniques available for growing these films, a comparative study on sequential and single‐step electrodeposition methods to grow CZTS films is carried out in this investigation to explore the possibility of improving the quality of the films using the inexpensive electrodeposition technique. Mainly in both methods, potentiostatic electrodeposition technique is adopted for growing CZTS thin films. In both methods, growth conditions of the CZTS films are optimized after measuring the photoresponses in a photoelectrochemical (PEC) cell of the films that resulted at the end of each deposition step. The observed structural and optoelectronic properties of the films reveal that, in general, structurally good and photoactive CZTS films can be prepared using both methods. Moreover, photoresponse and Mott–Schottky measurements on CZTS films in a PEC reveal that CZTS films prepared using the single‐step electrodeposition have better photoactive properties and improved doping densities. This important finding shows that when developing CZTS‐based solar cells using the inexpensive electrodeposition technique, single‐step electrodeposition is more advantageous.

Electrodeposition assisted sol-gel process to prepare CZTS thin films

The traditional sol-gel process usually takes a long time to form a stable Cu2ZnSnS4 (CZTS) sol-gel solution, which increases the whole process time. In this paper, an electrodeposition assisted sol-gel process was employed to reduce the solution preparation time. Cu was electrodeposited on Mo surface and ZnSnS sol-gel was rotated on Cu surface to prepare CZTS films. The solution preparation time was reduced from 12h to 30min. The mechanism of CZTS phase formation with sol-gel method assisted by electrodeposition was studied. Cu3SnS4 phase was formed during the electrodeposition assisted sol-gel process, while Cu2SnS3 phase was formed when the CZTS precursor during sol-gel process. Cu content was rich at the beginning of the reaction to form Cu3SnS4 during the electrodeposited assisted sol-gel process. CZTS films with Cu/(Zn + Sn) of 0.84 and Zn/Sn of 1.03 were prepared by sol-gel method assisted by electrodeposition. Compared with CZTS films prepared by sol-gel method, CZTS films prepared by this method have larger and more uniform grains. It is also found that the thickness of MoS2 layer was thinner than the one prepared by sol-gel method.

Investigation of broad-band optical absorption and electrical properties in vacuum annealed CZTS/Ag multi-layered stack structure for plasmonic solar cell application

As there is only a few research work carried out in thin film plasmonic Cu2ZnSnS4 (CZTS) based solar cells, it is exceedingly inspiring to advance the optical and electrical properties through incorporating Ag nanoparticles without affecting its structural properties. In this work, CZTS and Ag were stacked with the help of sequential magnetron sputtering method using RF and DC power supplies. Homogeneous distribution of Ag nanoparticles in CZTS matrix has been achieved via post deposition annealing at 500 °C. The concentration of Ag has been tailored by varying deposition time from 30 to 90 s. The morphological properties of Ag incorporated CZTS thin films has been investigated by using FE-SEM technique. The optical property of CZTS films has been enhanced by increasing the optical path length through multiple scattering due to the localized surface plasmon resonance (LSPR) of Ag nanoparticles. In Hall effect study, a gradual drop in the resistivity is observed with increasing Ag deposition time however it is within the tolerable limit of CZTS film.

Spray pyrolysed Cu2ZnSnS4 thin film photovoltaic cell fabricated using cost effective materials

The kesterite Cu2ZnSnS4 (CZTS) absorber material was prepared using 2-methoxyethanol solvent without formation of any secondary phases and the same was confirmed using Glancing Incidence X-Ray Diffraction and Raman studies. The surface analysis was carried out by Field Emission Scanning Electron Microscopy and Atomic Force Microscope and the thickness of the deposited films was calculated using FIB-SEM analysis. The precursor concentration of CZTS with 2:1:1:10 exhibits Cu-poor and Zn-rich CZTS film were achieved via sulfurization-free approach and was confirmed by EDX analysis. The optical absorbance, transmittance and bandgap of the films were characterized by UV–Visible spectrometer. The fluorescence spectroscopy was used to detect the electronic structural defects of the films and Hall measurements revealed that the prepared CZTS film exhibits p-type semiconductor behavior. As an attempt towards the preparation of nontoxic and low cost fabrication, the superstrate device structure glass/FTO/ZnO/ZnS/CZTS/Al was deposited using earth abundant materials in a cost effective method. The photovoltaic response was attained with a short circuit current density of 5.6 μA cm−2, open circuit voltage of 307 mV. The interface quality and internal charge transfer resistance of the CZTS device were investigated using Electrochemical Impedance Spectroscopy.

Enhancement of solution-processed Cu2ZnSnS4 film properties via a facile approach of sodium incorporation

Properties of solution-processed thin-films of Cu2ZnSnS4 (CZTS), a promising photo-absorber material for the photovoltaic application, were investigated as a function of sodium (Na) incorporation. This was achieved via a facile approach in which Na source was introduced at the bottom of the CZTS precursor as well as at the top. A significant increase in the grain size, from 319 to 1125 nm, and a large drop in the resistivity from 7.9 × 103 to 5.0 × 101 Ω cm have been observed. Large grains are desirable for reducing the recombination loss of the carriers at the defect sites at grain boundaries. Additionally, the approach incorporated in this work was also found beneficial for the suppression of tin loss. The effect of Na incorporation was also measured on the optical band gap, and the results indicate that Nai is the dominant defect present in these CZTS films.

Fabrication of Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS) thin films by ultrasonic spray pyrolysis at a low substrate temperature and effect of tin concentration on the characteristics of the CZTS thin films

Cu2ZnSnS4 (CZTS) thin films are fabricated on glass substrates using the spray pyrolysis method with different concentrations of Sn content were studied in this research work. All the CZTS thin films are fabricated at substrate temperature 200°C to minimize the formation of secondary phases. Here, we show how the variation in Sn content concentration influences the optical and structural properties of the CZTS thin films. The XRD patterns reveal that the concentration of Sn content has to be optimized to minimize the formation of secondary phases for a fixed substrate temperature. In turn, the band gap of the CZTS films is highly influenced by the formation of secondary phases. We have found that the films prepared from the precursor solution with 1.8 mM concentration of Sn content have the best crystal structure and an optical band gap of 1.55 eV. The CZTS thin films also have good carrier concentrations ranging from 4.2×1019 to 22.9×1020 cm-3. Bangladesh J. Sci. Ind. Res. 57(1), 1-6, 2022

Mapping the Energetics of Defect States in Cu2ZnSnS4 films and the Impact of Sb Doping.

The sub-bandgap levels associated with defect states in Cu2ZnSnS4 (CZTS) thin films are investigated by correlating the temperature dependence of the absorber photoluminescence (PL) with the device admittance spectroscopy. CZTS thin films are prepared by thermolysis of molecular precursors incorporating chloride salts of the cations and thiourea. Na and Sb are introduced as dopants in the precursor layers to assess their impact on Cu/Zn and Sn site disorder, respectively. Systematic analysis of PL spectra as a function of excitation power and temperature show that radiative recombination is dominated by quasi-donor–acceptor pairs (QDAP) with a maximum between 1.03 and 1.18 eV. It is noteworthy that Sb doping leads to a transition from localized to delocalized QDAP. The activation energies obtained associated with QDAP emission closely correlate with the activation energies of the admittance responses in a temperature range between 150 K and room temperature in films with or without added dopants. Admittance data of CZTS films with no added dopants also have a strong contribution from a deeper state associated with Sn disorder. The ensemble of PL and admittance data, in addition to energy-filtered photoemission of electron microscopy (EF-PEEM), shows a detailed picture of the distribution of sub-bandgap states in CZTS and the impact of doping on their energetics and device performance.