CZTS Films Research Articles

Effect of Sulfurization Temperature on Properties of Cu2ZnSnS4 Thin Films and Diffusion of Ti Substrate Elements

The addition of flexible Cu2ZnSnS4 (CZTS) thin film solar cells to titanium (Ti) substrates is an attractive way to achieve the low-cost manufacturing of photovoltaics. Prior research has indicated that the appropriate diffusion of Ti elements can enhance the crystalline growth of CZTS films. However, the excessive diffusion of Ti has been shown to adversely affect the photovoltaic performance of CZTS photovoltaic devices. Therefore, it is essential to regulate the diffusion of Ti elements within CZTS thin films to optimize their photovoltaic properties. The tendency for Ti substrate elements to diffuse into CZTS films is also influenced by the activation energy associated with these Ti elements. The sulfurization temperature is posited to be a critical factor in modulating the diffusion and activation energy of Ti elements within CZTS thin films. Consequently, this research investigates the alteration of the sulfurization temperature of CZTS thin films in order to enhance the properties of these thin films and to examine the diffusion behavior of titanium elements. The results reveal that as the sulfurization temperature increases, the diffusion of Ti elements within the CZTS thin films initially increases, then decreases, and subsequently increases again. This pattern suggests that the diffusion of Ti elements is affected not only by the activation energy of the Ti elements but also by the defect hopping distance within the CZTS thin films. Notably, at a sulfurization temperature of 550 °C, the grains at the base of the CZTS thin film demonstrate an increased density, which is associated with a reduced defect hopping distance, thereby hindering the diffusion of Ti elements within the CZTS thin films. Furthermore, at this specific sulfurization temperature, the slope of the current–voltage (I–V) curve for the CZTS/Ti structure reaches its maximum, indicating optimal ohmic contact characteristics.

Dark current in FTO/CZTS interface: a comprehensive comparison of practical Vs theoretical approach using SCAPS-1D

This study investigates the deposition of Cu2ZnSnS4 (CZTS) thin films on fluorine-doped tin oxide (FTO) and soda-lime glass (SLG) substrates using radio frequency (RF) sputtering at varying temperatures. A comprehensive characterization employing multiple analytical techniques was conducted. X-ray diffraction (XRD) analyses confirmed the amorphous nature of CZTS films being deposited up to 200 °C, while higher temperatures promoted increased crystallinity, with the presence of (112) and (220) planes observed at 300 °C and 400 °C. Rietveld refinement using Profex software revealed an increase in crystallite size with deposition temperature for films grown at 300 °C and 400 °C. Optical characterization through UV–vis spectroscopy unveiled a decrease in band gap energy with increasing deposition temperature, while the Urbach energy, associated with defects and imperfections, exhibited an inverse relationship with band gap and temperature. Experimental current–voltage (I-V) measurements using a Keithley source meter showed variations in the ideality factor with deposition temperature. SCAPS-1D simulations were performed to model the FTO/CZTS interface, incorporating experimental parameters. The simulated I-V behavior demonstrated a transition from recombination to diffusion-dominated current above 1.3 V forward bias. Simulations yielded higher ideality factors due to increased contributions from recombination and diffusion currents. Overall, this study provides insights into the growth, structural, optical, and electrical properties of CZTS thin films deposited by RF sputtering, enabling a comprehensive understanding of the FTO/CZTS heterojunction characteristics and their dependence on deposition temperature.

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.

8.36% Efficient CZTS Solar Cells on Transparent Electrode via Solution Processing

High‐bandgap Cu2ZnSnS4 (CZTS) thin film solar cells on transparent electrodes show favorable characteristics for new photovoltaic application scenarios including building‐integrated photovoltaics, vehicle‐integrated photovoltaics, and top cell for tandem structure. However, the efficiency of pure sulfide kesterite CZTS thin film solar cells on transparent substrates lags behind that on traditional Mo substrates. Herein, fabrication of high‐quality CZTS absorber films and efficient solar cells on fluorine‐doped tin oxide substrates from dimethyl sulfoxide solution is reported. The formation of harmful secondary phases in CZTS film is suppressed by simply adjusting the chemical stoichiometry in the precursor solution, leading to the development of 5.88% CZTS solar cells. Sodium (Na) doping further promotes grain growth and suppresses secondary phase, contributing to the reduced interface recombination and improved device performance. A champion device with an efficiency of 8.36% has been achieved with 1% Na doping, underscoring the significance of the solution process in achieving highly efficient kesterite solar cells on transparent electrodes.

Characterization of MoS2:Nb sputtered thin films. An application as hole transport layer in Cu2ZnSnS4/Si tandem solar cells

MoS2:Nb films deposited by radio-frequency magnetron sputtering are investigated in view of their application in infrared (IR)-transparent contacts for tandem photovoltaic devices.This material is already known to give a good electrical contact with p-type chalcogenide semiconductors, which are typically grown onto opaque molybdenum metallic contacts and a MoS2 layer spontaneously forms at the back interface during the on-top semiconductor growth. Our study explores a different approach, which involves the direct growth of IR-transparent MoS2:Nb films via sputtering inside complete photovoltaic devices, like Cu2ZnSnS4 (CZTS)-based single junction solar cells and CZTS/Si tandem devices. Films deposited at different sputtering pressures are compared by analysing their microstructure, morphology, chemical composition, optical and electrical properties. The effects of post-deposition sulfurization treatments are also investigated. We find that MoS2:Nb films deposited at around 0.1 Pa exhibit compactness but show a notable sulfur deficit ([S]/[Mo]≈1.4), a significant sub-bandgap optical absorptance and lack of crystallinity. Increasing the Ar pressure to 1 Pa raises the [S]/[Mo] ratio to 2.2, yielding crystalline films with good IR-transparency, although with a porous morphology. Despite 0.5 wt% Nb-doping of the sputtering target, the as-deposited films demonstrate n-type conductivity likely due to uncontrolled impurities and intrinsic defects. Ultraviolet Photoemission Spectroscopy measurements suggest that films’ work function higher than 5 eV can be obtained with a post-deposition sulfurization, making these materials suitable as Hole Transport Layer in photovoltaic applications. A similar increase in work function is expected in the CZTS/MoS2 junctions, since the sputtered MoS2:Nb films undergo a sulfurization process needed to obtain the overlying polycrystalline CZTS absorber.CZTS solar cells produced with sputtered MoS2 and Transparent Conductive Oxides contacts on glass substrates, despite plagued by severe adhesion problems, show the potentiality to give efficiencies comparable to reference devices with standard Mo back contact. Fabrication of CZTS/Si tandem devices on textured silicon bottom cells yields a maximum efficiency of 4.4 %, primarily hindered by the low quality of the CZTS film on textured substrates. Nonetheless, optoelectronic characterizations based on both spectrophotometric and quantum efficiency measurements confirm a good IR transparency of the MoS2-based intermediate contacts and the desired electrical behaviour.

Synthesis of ZnO/CZTS Hetero-Structure Junction by Sol–Gel Spin Coating Technique

Pertaining to the optimal band alignment of ZnO and CZTS, the heterostructure was fabricated with CZTS as the absorber layer and ZnO being the buffer layer coated on the FTO glass for photovoltaic application. Both the layers were deposited by using sol–gel spin coating technique. The results revealed the polycrystalline nature of both ZnO and CZTS films with distinct phase formations, with ZnO exhibiting prominent diffraction peaks at 31.77, 34.31, 36.20, 47.47, and 56.59° corresponding to (100), (002), (101), (102), and (110) planes, while CZTS showed diffraction peaks at (112), (220), and (312) planes. ZnO thin films demonstrated high optical transmittance (∼90%) and exhibited a wide band gap of 3.24 eV, while CZTS demonstrated a band gap of 1.44 eV. AFM images showed smooth surfaces with favorable topography for both films. Hall effect measurements indicated differences in electrical properties of the two films. Raman spectroscopy revealed vibrational energy patterns in both films. Additionally, current–voltage (I-V) characteristics revealed the electrical behavior of the ZnO/CZTS heterojunction, shedding light on its diode characteristics under both dark and illuminated conditions. With CZTS’s ideal band gap of 1.44 eV, sunlight is easily absorbed, and ZnO’s high electron mobility makes it an effective layer for electron transport. As a result, power conversion efficiency can be improved.

Improved carrier collection efficiency in CZTS solar cells by Li‐enhanced liquid‐phase‐assisted grain growth

AbstractThe liquid‐phase‐assisted grain growth (LGG) process is a promising strategy to fabricate large‐grain pure sulfide Cu2ZnSnS4 (CZTS) layers that span the absorber thickness and improve the carrier collection efficiency in photovoltaic devices. Li doping is an effective route to promote such LGG process of Cu2ZnSn(S,Se)4 (CZTSSe) as it can provide liquid Li‐Se phase facilitating the growth of large‐grain CZTSSe. However, the detailed function of the added Li in grain growth has rarely been investigated in both CZTS and CZTSSe, as the reported in situ, and pre‐deposition doping strategies usually suffer from substantial Li losses during the spin‐coating process and/or the high‐temperature sulfurization process. Herein, by monitoring the temperature‐dependent Li loss during the sulfurization process, we demonstrate that a small proportion of the added Li can remain at the CZTS film from the early sulfurization stage and provide Li‐S flux to promote the LGG process. An encouraging efficiency of 10.53%, with a remarkably high short‐circuit current density of 22.6 mA/cm2 and open‐circuit voltage of 0.744 V, is achieved by a significantly enlarged grain size of 3 μm with Li addition. This work could enhance the knowledge of employing Li‐S as flux for growing large‐grain chalcogenide absorbers for high performance devices with better carrier transport.

Deposition of nano-crystalline Cu2ZnSnS4 thin film in one step without sulfurization: Future prospects

AbstractCu2ZnSnS4 (CZTS) films were produced in a one-stage depositing spray pyrolysis technique instead of that needed post-sulfurization treatment. X-ray diffraction (XRD) has been used to identify crystal structure of studied films before and after E.B. irradiation. The presence of (112) as a preferred orientation indicates the kesterite phase structure of CZTS films. The energy-dispersive X-ray average data at several points of the film surface assured the homogeneous distribution of the constituent elements in the CZTS film composition. The optical behavior and the optical band-gap values of the studied CZTS films before and after E.B. irradiation have been checked by using the Tauc relation. The optical band-gap values reduced from 1.98 to 1.86 eV when the irradiation doses rose from 0 to 60 kGy. The electrochemical performance of CZTS films, on the two different conductive substrates, was tested by the cyclic voltammetry and electrochemical impedance spectroscopy analysis. Graphical abstract

Study of optical and structural properties of CZTS thin films using copper capping layer and sulfurization to correct stoichiometry

Non-toxic, environment friendly and low cost Cu2ZnSnS4 (CZTS) thin films were deposited on sodalime glass substrates by radio frequency magnetron sputter deposition technique. Stoichiometry analysis revealed that the as-deposited films were Cu deficient. Hence, Cu capping layer was deposited, to study its’ effect on optical and structural properties of the films. These films were subsequently sulfurized in a quartz setup. Ultimately, stoichiometric CZTS films with improved Cu content up to 27.4% were obtained. The kesterite CZTS phase was identified in the samples using x-ray diffraction measurements. The morphological studies revealed that, grain size varies as a function of the thickness of Cu capping layer. The formation of undesired Cu2S and ZnS phases were also observed in the film with Cu content above and below the optimum range. Optical bandgap (Eg) of the films could also be optimized by controlling the Cu content in the film. An optimal Cu content between 24% and 26% was found to be appropriate for achieving a desired bandgap within a range of 1.55 to 1.4 eV, suitable for application in thin film solar cells. The CZTS films with corrected stoichiometry have a charge carrier density in the rage of ∼1021 cm−3, which are much better than values reported so far in the literature.

Spin-coated CZTS films prepared by two different precursor mixing regimes, at room temperature and at 150 °C

In this paper, we examine the impact of the precursor's mixing temperature and mixing protocol on the crystal structure and morphological and optical properties of Cu2ZnSnS4 (CZTS) thin films. Four samples of CZTS thin films were synthesized with the sol-gel spin coating technique by previously mixing precursors at (a) 150 °C and (b) room temperature (RT), either (i) all at once or (ii) through sequential adding the individual chemicals 30 min apart. SEM-EDX, XRD, Raman and Visible spectroscopy analysis showed that sample 150°C-ST (chemicals mixed at the same time at 150 °C) fulfilled all the theoretical stoichiometric criteria (poor in Cu, rich in Zn) for the high-quality CZTS absorbers. The larger grain size (850 nm) and crystallite size (73.96 nm), lower strain (0.49×10−3) and band gap Eg=1.44eV which is closest to the Shockley–Queisser limit for single junction solar cells (1.34 eV).

Improved Cu2ZnSnS4 Solar Cell Performance by Multimetallic Stacked Nanolayers

This study utilized a postdeposition sulfurization method to produce thin films of the Cu2SnSnS4 (CZTS) absorber layer. Initially, metal precursors were deposited onto a tin oxide‐coated substrate through thermal evaporation. Subsequently, sulfurization occurred in a mixed environment of sulfur vapor and argon gas. The sulfurization temperature was set at 500°C for a duration of 30 min. During the sulfurization process, the facile evaporation of tin compounds could lead to a deviation in the atomic ratio within the absorber layer and potentially result in the attachment of secondary phases to the surface of the absorber layer. Therefore, this study employed a multilayered metal precursor structure (with a constant total thickness for each metal and nonmetal sulfides as precursors) for sulfurization. This method effectively suppressed the formation of secondary phases, including ZnS within the absorber layer and SnS2 on the surface. From the quantification results, the ratio of ZnS to CZTS signal intensity decreased from 0.52 to 0, while the ratio of SnS2 to CZTS signal intensity dropped from 1.2 to 0. Additionally, the efficiency increased to 2.79%. In summary, this research introduced a novel preparation method to enhance the quality of CZTS films. The modification to a multilayered metal precursor structure reduced the evaporation of tin compounds, consequently minimizing the generation of secondary phases.

Facile growth of kesterite CZTS film by one-pot hydrothermal route using EDTA as complex agent for heterojunction solar cell applications: influence of zinc concentration

Facile growth of kesterite CZTS film by one-pot hydrothermal route using EDTA as complex agent for heterojunction solar cell applications: influence of zinc concentration

Synthesis and characterization of kesterite Cu2ZnSn(SxSe1-x)4 thin films with low-cost for efficient solar cells

In this study, we conducted an investigation on the Cu2ZnSnS4 (CZTS) kësterite compound, which is considered an attractive material for its favorable absorption properties, making it a suitable semiconductor for photovoltaic applications. However, its market potential is limited by its low efficiency of 12.6%, primarily caused by the presence of secondary phases that act as recombination centers. Theoretically, the efficiency limit for CZTS ranges from 31% to 33%.To enhance the photovoltaic performance of CZTS, we employed a doping strategy by incorporating selenium (Se) into the Cu2ZnSn(SxSe1-x)4 film, where the sulfur-to-selenium ratio (S/Se) was optimized within the range of 0< x <1. The CZTS films were deposited onto FTO substrates using the spray pyrolysis technique, which is a simple and cost-effective method. The characterization of these films involved X-ray diffraction (XRD) and UV–visible spectroscopy analysis. Additionally, the morphology and topography of the CZTSSe layers were examined using scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. Transmission electron microscopy (TEM) was also employed.The results obtained in this study indicate that CZTSSe films exhibit band gap values ranging from 1.54 eV to 1.68 eV and grain compactness, with larger agglomerated grains on the film surface when the S/Se ratio is approximately 0.5 or lower. Moreover, the resistivity measurements revealed significant variations when sulfur and selenium were combined, suggesting the influence of their composition on the electrical properties of the CZTS films.

Investigation of nucleation and growth mechanism of Cu2ZnSnS4 absorber layer electrodeposition on Indium Tin Oxide coated glass

In this paper, earth-abundant and environmentally friendly kesterite Cu2ZnSnS4 (CZTS) thin films are successfully synthesized by electrodeposition on Indium Tin Oxide (ITO) coated glass for photovoltaic application. For the first time, the electrochemical nucleation and growth mechanisms of the kesterite electrodeposition are investigated. Here, the electrochemical growth mechanism is studied and the diffusion coefficient D is calculated for different applied potentials. Our findings show that kesterite deposition occurs through progressive nucleation followed by the limited growth of diffusion of 3D hemispheric islands. Furthermore, it has been proven that the type of nucleation and growth is not influenced by the applied potential. Scanning electron microscopy, X-ray diffraction and UV–vis are used to characterize the physicochemical properties of the prepared samples. The morphological and crystallographic investigations reveal a different morphology and structure of the CZTS film deposited at different potentials for different times. Among the samples studied, the CZTS thin film prepared at −1.1 V for 60 min offers the best properties with 2.4 µm thickness after annealing at 450 °C.

Dependence of CZTS thin film properties and photovoltaic performance on heating rate and sulfurization time

In this study, effect of heating rate and sulfurization time on the properties of CZTS thin films and their photovoltaic performances was investigated. CZTS layers were prepared by a two-stage technique comprising deposition of precursor films by sputtering on Mo-foil substrates (Mo-foil/ZnS/CuSn/Cu) and a sulfurization process utilizing Rapid Thermal Annealing (RTA) approach. Heating rates and sulfurization times at 550 °C were changed to determine the best conditions. CZTS films were characterized by several techniques. All samples showed Cu-poor and Zn-rich chemical composition regardless of the heating rate and annealing time of the sulfurization process. XRD diffraction patterns showed that only kesterite CZTS phase was formed in all samples. Although Raman spectroscopy measurements confirmed formation of kesterite CZTS phase in all samples, forming of Cu2SnS3 (CTS) secondary phase was detected in all films except for the sample prepared utilizing a relatively low heating rate of 1 °C/s and 60 s sulfurization time (CZTS-I-60). The largest crystallite size and the less micro strain and dislocation density were also found in this sample. All CZTS thin films displayed dense and compact polycrystalline surface microstructure. Optical characterizations showed that the optical band gaps of the films were in the range of 1.48–1.58 eV. PL emission spectra of the films exhibited transition from conduction band to acceptor/defect levels instead of band-to-band transition. Solar cells were fabricated and the highest Voc and conversion efficiency (4.39%) were achieved employing the film obtained with ramp rate of 1 °C/s and the sulfurization time of 60 s, because of better structural properties and purer crystal structure determined for this sample.

Effect of preheating and annealing temperature on the microstructure and optoelectronic properties of CZTS films prepared by sol-gel method

Effect of preheating and annealing temperature on the microstructure and optoelectronic properties of CZTS films prepared by sol-gel method

Fabrication of CZTS Films through a Combined Electrodeposition and Solution Process: An Experimental and First‐Principles Study

AbstractSulfurization has a critical role in preparation of CZTS films since the main drawback of CZTS is due to the presence of S‐based impurities. In general, sulfurization could be performed during or following annealing, or the entire CZTS structure could be prepared through sol‐gel. However, toxic gases are used for the former, and the latter takes prolonged times. Alternatively, a combined electrodeposition and solution process was proposed in the present work to prepare CZTS. Briefly, we electrodeposited CZT layers on a metal substrate, and then immersed the as‐prepared coating in a S containing solution for the sulfurization. The effects of electrodeposition time, electrodeposition order, current density, electrolyte compositions, sulfurization time, and annealing pressure were studied. Overall, the results show that S content of the films varied between 27 and 52 % depending on process parameters, thus, suggesting the effectiveness of the proposed approach regarding sulfurization. However, various secondary phases were also detected as well as considerable amount of oxygen impurity. Accordingly, we opted for a computational approach to elucidate the effect of oxygen impurities, and report that the band gap of the films varied between 0.59 and 0.05 eV depending on the presence and which sites oxygen atoms occupy.

Cauliflower-like copper zinc tin sulfur for ppb-level NO2 sensing at room temperature

Structure nanocrystallization dramatically enhances the specific surface area and is one of the most popular methods for preparing high-performance gas sensing materials. However, most of the preparation methods are suffering high-cost and long time-consuming. This work synthesized Cu2ZnSnS4 (CZTS), a cost-effective and stable direct-bandgap semiconductor with cauliflower-like morphology by facile solution drop coating and low-temperature annealing. The morphology and gas sensing performance of CZTS films could be modulated by controlling the annealing time. The gas sensors showed a significant response toward ppb-level NO2 (1.287 toward 50 ppb) with a short response, recovery time, and high selectivity at room temperature (25 °C). It is worth mentioning that the limit of detection is 50 ppb, which is the first report on the ppb-level detection ability of CZTS-based NO2 gas sensors. The enhanced sensitivity of the cauliflower-like CZTS nanocrystal-based gas sensor can be ascribed to the small grain size, large specific surface area for effective gas diffusion, and excellent conductivity of the spherical packed structure.

The effect of step cooling treatment on the microstructure of Cu2ZnSnS4 film and its electric property

Flexible Cu2ZnSnS4 (CZTS) thin-film solar cells have the advantages of high flexibility, lightweight, and easy storage, which can provide portable and clean energy for various portable electronic and communication devices and vehicles. In this work, CZTS thin film deposited on flexible aluminum foil were prepared by vacuum magnetron sputtering and sulfide annealing in a tubular furnace. A step-by-step cooling way was applied to the annealing stage aiming at tuning the formation of CZTS, that is, when the slow cooling reached a target temperature (400 °C, 350 °C, 300 °C, 250 °C), a rapid cooling was triggered by opening the tube furnace lid. The effects of step cooling on the micro structure and property of CZTS film were studied by X-ray diffraction, Raman scattering, energy dispersive spectroscopy, scanning electron microscopy and current-voltage (I-V) characteristic curve test. The experimental results showed that all samples have CZTS phase and the Al/CZTS interface exhibited ohmic contact. And the formation of secondary phases like SnS, Sn2S3 and Cu31S16 in the sample film can be tuned by the step cooling treatments. The sample treated at 300 °C had the highest CZTS content, the average grain size of CZTS was 45 nm and the number of secondary phases was fewer. Beside, the I-V curve revealed that this sample had the lowest resistance. The experimental results in this work indicate the feasibility of Al as a substrate and back electrode for CZTS thin film.

The CZTS Thin Films Grown by Sulfurization of Electrodeposited Metallic Precursors: The Effect of Increasing Tin Content of the Metallic Precursors on the Structure, Morphology and Optical Properties of the Thin Films

A study has been carried out to investigate the influence of the amount of Sn in the precursor solution, on some physical properties of CZTS films grown by sulfurization of electrodeposited metallic precursors. The growth of the CZTS samples was achieved by sequential electrodepositon of constituent metallic layers on ITO glass substrates using a 3-electrode electrochemical cell with graphite as a counter electrode and Ag/AgCl as the reference electrode. The Sn-content in the metallic precursor was varied by varying the deposition time of Sn. The stacked elemental layer was then soft annealed in Argon at 350 °C, and subsequently sulfurized at 550° C to grow the CZTS thin films. The structure, morphology and optical properties were investigated. X-ray diffraction studies revealed that, irrespective of the Sn content all the films were polycrystalline and exhibited the Kesterite CZTS structure with preferred orientation along the (112) plane. However, there was an increase in the amount of peaks indexed to the undesirable secondary phases, as the Sn content in metallic precursor was increased. Optical absorption measurements revealed the existence of a direct transition with band gap values decreasing from 1.74 eV to 1.25 eV with increasing amounts of Sn. The lower value for the band gap was attributed to the presence of secondary phases formed in addition to the CZTS film. Morphology of the sulfurized films showed a compact and rocky texture with good coverage across the entire substrate. However, CZTS films with a higher Sn content appeared to have a molten metallic surface with deep cracks which could have adverse effects on the electrical properties of the film. EDAX analysis showed all the films were consistent with the formation of CZTS. It is evident from all the characterization techniques that increasing the Sn content of the stacked metallic precursors beyond stoichiometric amounts had an adverse effect on the structural and optical properties of CZTS films grown by this technique.