CZTS Films Research Articles

Novel Chemical Route for Deposition of Cu2ZnSnS4 Photovoltaic Absorbers

This work reports results of a study carried out to optimize the preparation conditions of Cu2ZnSnS4 (CZTS) thin films grown by sequential deposition of Cu2SnS3 (CTS) and ZnS layers, where the Cu2SnS3 compound was grown using a novel procedure consisting of simultaneous precipitation of Cu2S and SnS2 performed by diffusion membrane assisted chemical bath deposition (CBD) technique. The precipitation across the diffusion membranes allows achieving moderate control of release of metal ions into the work solution favoring the heterogeneous growth mainly through an ion-ion mechanism. Through a parameters study, conditions were found to grow Cu2SnS3 thin films which were used as precursors for the formation of Cu2ZnSnS4 films. The formation of CZTS thin films grown in the Cu2ZnSnS4 phase was verified through measurements of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Solar cells with efficiencies of 4.9% were obtained using CZTS films prepared by membrane assisted CBD technique as absorber layer.

Cu2ZnSnS4 thin films obtained by sulfurization of evaporated Cu2SnS3 and ZnS layers: Influence of the ternary precursor features

Cu2ZnSnS4 (CZTS) thin films have been grown by sulfurization of Cu2SnS3 (CTS) and ZnS layers evaporated on glass substrates. Four CTS precursor films have been tested, with two different atomic compositions (Cu/Sn=1.7 and Cu/Sn=2.1) and substrate temperatures (350 and 450°C), together with analogous ZnS layers deposited by maintaining the substrate at 200°C. The sulfurization of the CTS and ZnS stacked layers was performed at 500°C during 1h. The evolution of the crystalline structure, morphology, optical and electrical properties from each CTS precursor to the CZTS compound has been studied, especially the influence of the ternary precursor features on the quaternary film characteristics. The kesterite structure has been identified after sulfurization of the various samples, with main (112) orientation and mean crystallite sizes S112=40–56nm, being higher for the Cu-poor compositions. The CZTS average roughness has varied in a wide interval Ra=8–66nm, being directly related to the CTS precursor layer, which becomes rougher for a higher deposition temperature or Cu content. Besides, the band gap energy and the electrical resistivity of the CZTS films have changed in the ranges Eg=1.54–1.64eV and ρ=0.2–40Ωcm, both decreasing when the Cu content and/or the surface roughness increase.

Fabrication of Cu2ZnSnS4 thin film solar cells by annealing of reactively sputtered precursors

The Cu2ZnSnS4 (CZTS) absorbers for thin film solar cells were fabricated by reactive sputtering followed by post-annealing. The effects of annealing temperature and Cu/(Zn + Sn) ratio on the film microstructure and related device performance were systematically investigated. It was found that the higher annealing temperature facilitates Zn incorporation and grains growth, but leads to more Sn loss and thicker MoS2 layer. The CZTS absorbers with a lower Cu/(Zn + Sn) ratio show larger grain size and more ZnS particles on the surface, whereas the films with higher Cu content (Cu/Sn > 2) demonstrate small grain size, much Cu2−xS phases and cracks. The highest device efficiency of 3.72% was obtained from the CZTS film annealed at 570 °C with Cu/(Zn + Sn) ratio of 0.78. The performance of devices is limited by low Jsc and FF due to the interface recombination and the block of carrier transport by ZnS.

Sulfurization and annealing effects on thermally evaporated CZTS films

Thermally evaporated Cu2Zn1.5Sn1.2S4.4 (CZTS) films are annealed and sulfurized at different temperatures to study the structural modifications. The kesterite phase formation and phase purity of the CZTS films are compared and confirmed by X-ray diffraction and Raman spectroscopic technique. Surface oxidation state of the elements in the sulfurized film is studied by XPS. The calculated optical band gap of the 550°C sulfurized CZTS film is found to be 1.56eV; however getting modified due to annealing and sulfurization. The carrier concentration, resistance and mobility of the sulfurized films are found to be 2.8×1014cm−3, 2686Ω/square and 8.2cm2V−1s−1 respectively and the conduction type is p-type. This study sheds light on the effect of annealing and sulfurization on various phase-modifications and the light-harvesting capability of CZTS absorber layers for solar cell applications.

Identifying barriers to charge-carriers in the bulk and surface regions of Cu2ZnSnS4 nanocrystal films by x-ray absorption fine structures (XAFSs).

Solar cell performance is most affected by the quality of the light absorber layer. For thin-film devices, this becomes a two-fold problem of maintaining a low-cost design with well-ordered nanocrystal (NC) structure. The use of Cu2ZnSnS4 (CZTS) NCs as the light absorber films forms an ideal low-cost design, but the quaternary structure makes it difficult to maintain a well-ordered layer without the use of high-temperature treatments. There is little understanding of how CZTS NC structures affect the photoconversion efficiency, the charge-carriers, and therefore the performance of the device manufactured from it. To examine these relationships, the measured photoresponse from the photo-generation of charge-carrier electron-hole pairs was compared against the crystal structure, as short-range and long-range crystal orders for the films. The photoresponse simplifies the electronic properties into three basic steps that can be associated with changes in energy levels within the band structure. These changes result in the formation of barriers to charge-carrier flow. The extent of these barriers was determined using synchrotron-based X-ray absorbance fine structure to probe the individual metal centers in the film, and comparing these to molecular simulations of the ideal extended x-ray absorbance fine structure scattering. This allowed for the quantification of bond lengths, and thus an interpretation of the distortions in the crystal lattice. The various characteristics of the photoresponse were then correlated to the crystallographic order and used to gain physical insight into barriers to charge-carriers in the bulk and surface regions of CZTS films.

Enhancement of photosensitivity in bismuth doped Cu2ZnSnS4 thin films

In this paper, the effect of bismuth doping on the structural, morphological, optical and electrical properties of Cu2ZnSnS4 (CZTS) films has been investigated. The undoped and bismuth doped CZTS films (0, 0.5, 1, 1.5 and 2 mol%) were deposited on glass substrates by solution based method. The XRD result shows a significant improvement in the crystallinity of the films with increase in bismuth concentration. The Raman spectra of the films show the dominant peak at 334 cm–1 corresponding to A1 vibrational mode of CZTS kesterite phase. The FESEM micrographs of the films show an enhancement in the grain size and densification with the addition of bismuth ion concentration. The optical bandgap of the films was found to vary (1.59–1.40 eV) with the doping of bismuth ions. The I –V characteristics indicate twofold increment in the photoconductivity for the bismuth doped CZTS films under 100 mW/cm2 illumination suggesting their potential application in photovoltaic devices. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

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.

Synthesis and optical characterization of n-ZnO and p-Cu2ZnSnS4 nanocrystalline thin films for low cost solar cells

High quality ZnO/Cu2ZnSnS4 thin films as a window/absorber layers were successfully synthesized via spin coating the sol-gel precursor of each composition without using any vacuum facilities. In this study, the impact of annealing temperature (400 °C, 3 h) on the ZnO window layer and different thickness (3 and 5 layers) of the Cu2ZnSnS4 (CZTS) absorber layer were investigated. X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM) and UV–vis–NIR spectroscopy were used for the structural, compositional, morphological and optical absorption analysis of each layer. ZnO exhibits wurtzite hexagonal crystal structure with particle size equals to 8.60 and 28.59 nm for fresh and annealed films, respectively. Micro-strain and dislocations density decreased with the annealing temperature. X-ray diffraction patterns for CZTS films show small peak at (112) according to the kesterite structure with particle size in nano-scale for the two thicknesses. ZnO films demonstrated direct optical band gap of 3.23 and 3.21 eV for fresh and annealed films, respectively. CZTS films (3 and 5 layers) also have direct optical band with optimum value (1.51 eV) for thickness of 5 layers. The J-V characteristics of the CZTS-based thin film solar cells (CZTS/ZnO/ZnO:Ag) were measured under air mass AM 1.5 and 100 mW/cm2 illumination. The values of the short circuit current (Jsc), open circuit voltage (Voc) and fill factor (FF) also have been obtained.

Effects of temperature-time profile on Cu2ZnSnS4 films and cells based on sulfur-contained precursors

Cu2ZnSnS4 (CZTS) films with high-quality are necessary to the high-performance CZTS solar cells. The influence of different temperature-time profiles during sulfurization on CZTS films were investigated by characterizing the compactness, roughness and grain size of CZTS films. With slowing the heating-rate during sulfurization process, the grain size of the film is becoming larger and larger, but the large grain size degrades the flatness of the CZTS films at the same time. The formation of films having the target composition (EDX) and phase purity (Raman) was attributed to the optimum choice of composition ration of the precursor and the thermal profile during sulfurization. The optical and electrical properties of corresponding CZTS films were detected by using UV–vis–INR Spectrophotometer and Hall measurement, respectively. Finally, the CZTS film solar cells were fabricated and the best cell demonstrated 3.91% efficiency. A high Rs is the main reason to inhibit the performance of the cells.

Effect of metal layer stacking order on the growth of Cu2ZnSnS4thin films

In this paper we have presented an in-depth study of effect of metallic precursor stacking order on the growth of the Cu2ZnSnS4 (CZTS) thin films. The CZTS films were prepared by employing a two-step process comprising of sequential sputtering of the metal precursors followed by sulfurization. An optimized stacking sequence as well as growth mechanism for obtaining the single phase CZTS has been proposed based on the results of XRD, Raman, XPS, UV–Vis and electrical studies. A combination of Raman analysis and XPS has been carried out to confirm the CZTS phase formation and to detect any minor phases, if present. The occurrence of Raman modes at around 286 and 336cm−1 for the Zn/Cu/Sn/Cu stack sulfurized at 500°C indicated the existence of prominent Kesterite CZTS phase. The perfect homogeneous mixing of sequential precursors together with the elemental sulfur was observed in the case of sulfurized stack order of Zn/Cu/Sn/Cu, which yielded single phase CZTS films, and further confirmed by high resolution core level XPS measurements. Stack dependent micro structural features and elemental analysis were also carried out using FESEM attached to EDS. The p-type charge carriers as detected using hot-probe measurement technique and the band-gap energy of ∼1.52eV as estimated from the absorbance spectrum, suggested that the Zn/Cu/Sn/Cu stack order is most appropriate for realizing single phase CZTS growth using two step method.

Fabrication Of Cost Effective Pt And FTO-free Counter Electrode For ZnO Based Dye Sensitized Solar Cell Using Thermally Decomposed Cu2ZnSnS4 Nanoparticles

Use of Pt and fluorine-doped tin oxide (FTO) substrate together as a counter electrode (CE) significantly increases the fabrication cost of dye-sensitized solar cells (DSSCs). In the present study, we report the cost effective Pt and FTO-free (Cu2ZnSnS4 (CZTS) nanoparticle coated on W substrate) CE for ZnO-based DSSCs. Phase purity, the morphology and elemental composition of synthesized ZnO, CZTS nanoparticles and films were confirmed by physical characteristics such as XRD, FEGTEM, and FEG-SEM respectively. Cyclic voltammetry study confirmed electrochemical catalytic activity of CZTS films. DSSCs fabricated with CZTS film on W and FTO substrate as CE exhibited efficiencies of 2% and 2.8%, respectively. Conventionally used Pt-based CE demonstrated an efficiency of 3.8 %. Copyright © 2016 VBRI Press.

Towards phase pure kesterite CZTS films via Cu-Zn-Sn electrodeposition followed by sulfurization

Electrodeposition of Cu-Zn-Sn alloy precursors followed by sulfurization has been utilized to form phase pure CZTS absorber layers for photovoltaic applications. In this study, we have varied the deposition potential in order to grow Cu-Zn-Sn metallic precursor layers with a fixed Cu fraction while varying the Zn/Sn ratio within the ideal composition range to obtain high quality CZTS films. The phase purity and crystal quality of CZTS films were investigated as a function of Zn/Sn ratio. Phase purity and photoelectrochemical response are both enhanced with increasing Zn/Sn ratio after sulfurization at 500°C or 550°C. The highest photoelectrochemical response at a liquid junction is 0.99mA/cm2 for CZTS layer with Zn/Sn=0.98 after sulfurization at 550°C. Raman spectroscopy and X-ray diffraction studies suggest that the enhanced photoresponse is correlated to a high crystallinity together with improved phase purity.

Influence of sulfurization conditions on properties of Cu2ZnSnS4 films and cells

The influences of sulfurization process on Cu2ZnSnS4(CZTS) film characters were investigated through soft-annealing and sulfurization precursors, which were prepared by sputtering at room temperature with metal targets and sulfide targets. The Sn-loss of CZTS film was researched by comparing different CZTS films based on metallic precursors and sulfur-contained precursors. As a result, the CZTS film based on metallic precursors showed more serious Sn-loss than that from sulfur-contained precursors. Moreover, the experimental results also demonstrated that an appropriate temperature-time profile during sulfuring process can act as a crucial factor on the morphologies and Sn-loss of CZTS films, and the CZTS films with fine morphologies and appropriate composition were obtained by modifying the time-temperature profile during sulfurization process. Finally, the best solar cell with efficiency of 2.81% was obtained based on the metallic precursor.

Structural, optical and electrical properties of Cu2ZnSnS4 films prepared from a non-toxic DMSO-based sol-gel and synthesized in low vacuum

This paper reports a complex analysis of structural, optical and electrical properties of Cu2ZnSnS4 (CZTS) films, prepared by spin-coating of a sol-gel based on the low-cost and environmentally friendly solvent dimethyl sulfoxide (DMSO) and synthesized in low vacuum (0.1Pa). The effect of a short-term low-temperature treatment in air and final annealing under low vacuum (0.1Pa) on the synthesis of CZTS films was tested and analyzed. XRD and Raman spectra have shown the polycrystalline nature of obtained CZTS films with almost a pure kesterite phase content. The average size of crystallites D~60nm in the films was estimated. The value of the band gap Eg≈1.53eV was determined from the analysis of optical characteristics. The analysis of electrical characteristics was carried out in the scope of the model of current transport in polycrystalline materials with electrically active grain boundaries. The determined values of the specific resistance ρ≈2.38Ωcm, the hole density p0≈4.2×1018cm−3 and the effective mobility μp≈0.63cm2/(Vs) demonstrate that the CZTS films under investigation are prospective for the fabrication of highly efficient solar cells.

A CZTSe solar cell with 8.2% power conversion efficiency fabricated using electrodeposited Cu/Sn/Zn precursor and a three-step selenization process at low Se pressure

In preparing Cu2ZnSnSe4 (CZTSe) thin films, selenization at high Se pressure is likely to form a thick molybdenum diselenide (MoSe2) layer. An excessively thick MoSe2 layer degrades the performance of CZTSe solar cells. Annealing under low Se pressure has been suggested to prevent the formation of such a thick MoSe2 layer. However, the potential loss of Sn metal from CZTSe thin films at low Se pressure or high temperature raises the additional difficulty of controlling the compositions and phases of the films. Carefully controlling these selenization parameters to yield high-quality CZTSe thin films is important. This study investigates the loss of Sn and the formation of molybdenum diselenide in CZTSe thin films at low Se pressure. Three-step selenization at low Se pressure is performed on an electrodeposited Cu/Sn/Zn precursor to prepare high-quality CZTSe thin films. In the first step, rapid thermal annealing with a substrate temperature of 500°C and a low Se vapor pressure of 4.6Pa was performed to reduce the loss of Sn and the thickness of the MoSe2 layer, and thus to obtain a Cu-poor and Zn-rich film. In the second step, a high annealing temperature of 570°C and an inert gas at high pressure was used to improve elemental diffusion and increase the uniformity of the elemental distribution. In the third step, thermal annealing with a substrate temperature of 500°C and a low Se vapor pressure of 4.6Pa was performed to reduce the loss of Sn and the thickness of the MoSe2 layer, and to eliminate the impurity phase of SnSe (which was formed in the second step of annealing) on the surface of the CZTS films. The loss of Sn and the formation of molybdenum diselenide in the CZTSe thin films are negligible when this three-step selenization at low Se pressure is used. When the selenization parameters are properly controlled, CZTSe solar cells with an efficiency of as high as 8.2% can be obtained.

Studies on the disorder in DC magnetron sputtered Cu2ZnSnS4 (CZTS) thin films grown in sulfide plasma

Cu2ZnSnS4 (CZTS) is a widely studied material in thin film photovoltaics. The desirable property of copper poor and zinc rich in CZTS thin films introduces several point defects and secondary and tertiary phases irrespective of growth techniques and growth conditions; these defects and the phases introduce disorder in the lattice. In the present study, CZTS thin films are prepared at 520K with DC magnetron sputtering technique using CZT alloy target in the presence of hydrogen sulfide plasma. The CZTS thin films have been prepared at 30W and 40W magnetron powers. X-ray diffraction and Raman spectroscopy show that the CZTS films are polycrystalline and have partially disordered kesterite (PD-KS) phase. The CZTS films grown are copper poor and zinc rich as indicated by Energy Dispersive X-ray (EDX) analysis. The optical band gap of the films is 1.56eV. The disorder of PD-KS CZTS films has been studied by the low temperature (300−20K) electrical resistivity and Hall effect measurements. The transport of charge carriers in disordered p-CZTS takes place via hopping mechanisms. CZTS films prepared with 30W magnetron power are more disordered structure due to the copper vacancy (VCu). The surface work function of CZTS thin films measured by Kelvin probe technique is 4.7eV.

Towards understanding poor performances in spray-deposited Cu2ZnSnS4 thin film solar cells

Cu2ZnSnS4 (CZTS) thin films deposited by spray pyrolysis technique are quite attractive for solar cell applications. However, low efficiency values have been achieved up to date. Among the different optoelectronic parameters, relatively low open-circuit voltage and short-circuit current density values are the common feature in sprayed-CZTS solar cells. Nevertheless, there is no report focused on explaining the main sources of such low performances. One of the most important parameters governing the behavior of a solar cell is minority carrier diffusion length. Furthermore, a transport mechanisms study is required to understand main recombination paths. In this work, sprayed-CZTS thin film solar cells are fabricated in order to study the main limiting factors regarding this technology. It is found that CZTS films are characterized by low minority carrier diffusion length values, emphasizing the need for improving sprayed-CZTS crystalline quality. Finally, current-voltage measurements at different temperatures for solar cell with maximum efficiency were carried out in order to find out the dominant transport mechanisms. It is discovered that tunneling enhanced recombination at junction interface gives a quantitative description of the electronic loss mechanisms in the sprayed-CZTS based heterojunction solar cells.

Effects of different precursors on Cu2ZnSnS4 thin film solar cells prepared by sputtering method

In this study, the impacts of different precursors on Cu2ZnSnS4 thin film solar cells were investigated. The two kinds of precursors of (Cu+Sn)/Zn and (Cu+Sn)/ZnS were deposited on Mo-coated soda lime glasses by magnetron sputtering. Cu2ZnSnS4 (CZTS) films based on different precursors were fabricated by soft annealing and following two-step sulfurization in sulphur vapour. The crystal structure, phase purity, surface morphology, composition and optical properties of CZTS films from different precursors were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), energy dispersive spectrometry (EDS) and UV–vis–NIR spectroscopy, respectively. As a result, the CZTS thin films with smooth surface and uniform compositional ratio distribution were obtained from the precursors of (Cu+Sn)/ZnS. The best conversion efficiency of the fabricated CZTS film solar cell based on (Cu+Sn)/ZnS precursors was 3.36%.

Effect of sulfurization time on the properties of copper zinc tin sulfide thin films grown by electrochemical deposition

We report growth of quaternary Cu2 ZnSnS4 (CZTS) thin films prepared by the electrochemical deposition from salt precursors containing Cu (II), Zn (II) and Sn (IV) metals. The influence of different sulfurization times t (t = 75, 90, 105, and 120 min) on the structural, compositional, morphological, and optical properties, as well as on the electrical properties is studied. The films sulfurized 2 hours showed a prominent kesterite phase with a nearly stoichiometric composition. Samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and Raman and UV-VIS-NIR spectrometer at different stages of work. X-ray diffraction and Raman spectroscopy analyses confirmed the formation of phase-pure CZTS films. (FESEM) shows that compact and dense morphology and enhanced photo-sensitivity. STEM - EDS elemental map of CZTS cross-section confirms homogeneous distribution. From optical study, energy gap was enlarged with a changed sulfurization times in the range of 1.37–1.47 eV.

High‐efficiency Cu2ZnSn(S,Se)4solar cells fabricated through a low‐cost solution process and a two‐step heat treatment

AbstractA method for fabricating high‐efficiency Cu2ZnSn(S,Se)4(CZTSSe) solar cells is presented, and it is based on a non‐explosive, low‐cost, and simple solution process followed by a two‐step heat treatment. 2‐Methoxyethanol was used as a solvent, and Cu, Zn, Sn, chloride salts, and thiourea were used as solutes. A CZTSSe absorber was prepared by sulfurising and then selenising an as‐coated Cu2ZnSnS4(CZTS) film. Sulfurisation in a sulfur vapour filled furnace for a long time (2 h) enhanced the crystallisation of the as‐coated CZTS film and improved the stability of the CZTS precursor, and selenisation promoted further grain growth to yield a void‐free CZTSSe film. Segregation of Cu and S at the grain boundaries, the absence of a fine‐grain bottom layer, and the large grain size of the CZTSSe absorber were the main factors that enhanced the grain‐to‐grain transport of carriers and consequently the short‐circuit current (Jsc) and efficiency. The efficiency of the CZTS solar cell was 5.0%, which increased to 10.1% after selenisation. For the 10.1% CZTSSe solar cell, the external quantum efficiency was approximately 80%, the open‐circuit voltage was 450 mV, the short‐circuit current was 36.5 mA/cm2, and the fill factor was 61.9%. Copyright © 2016 John Wiley & Sons, Ltd.