CZTS Films Research Articles

Intrinsic strain dependent redshift in optical band gap of Cu2ZnSnS4 nanostructured thin films

Single-phase Cu2ZnSnS4 (CZTS) thin films have been spray deposited on glass substrate, using uniform size oleic acid coated CZTS nanoparticles and the effect of heating temperature (75 °C to 300 °C) on the structural and optical properties have been investigated. X-ray diffraction results confirmed the formation of the single phase CZTS with a preferred orientation in (112) plane. The crystallite sizes of these films are ~7.0 ± 0.5 nm but the intrinsic strain present in the nanostructured films decreases from 9.1 × 10−3 to 1.3 × 10−3 with the increase of the heating temperature from room temperature to 300 °C. Optical band gap of the as deposited CZTS films is ~1.72 eV and it decreases to 1.45 eV for the CZTS films heated at 300 °C. The observed red shift in the band gap of CZTS films with the heating temperature has been attributed to a reduction in the intrinsic strain present in the film with the increase in the heating temperature.

A Precursor Stacking Strategy to Boost Open-Circuit Voltage of Cu2ZnSnS4Thin-Film Solar Cells

Solar cells based on Cu2ZnSnS4 (CZTS) thin-film light absorber are promising photovoltaic (PV) technologies to achieve cost-effective solar electricity. One of the challenges in CZTS PV technology is its low open-circuit voltage ( V oc), which is normally less than 600 mV. In this paper, we enhanced V oc of CZTS solar cells to above 700 mV by constructing a four-layer stacking precursor of Zn/Sn/Cu/Zn (ZTCZ). Compared with the ordinary three-layer Zn/Sn/Cu (ZTC) stacking precursor, the efficiency of CZTS solar cells prepared by the ZTCZ precursor of the same stoichiometry was significantly enhanced. Cross-sectional microscopic characterization revealed that a compact bilayer structured CZTS with Zn-rich surface was formed using the ZTCZ precursor while lots of voids were observed at the bottom of the CZTS film produced using the ZTC precursor. The mechanism is proposed for the high V oc achieved through a Zn-rich surface in the CZTS. Photoluminescence and time-resolved photoluminescence measurements confirmed that the device made from the four-layer ZTCZ stacking precursor possess higher charge separation efficiency and lower radiative recombination compared with the ZTC stacking precursor. These factors have greatly contributed for the high V oc and superior efficiency of the CZTS thin-film solar cells.

The role of pre-annealing in the sulfurization of Cu-Zn-Sn stacked metal layer prepared by magnetron sputtering

Cu2ZnSnS4 (CZTS) films were fabricated by sulfurizing the stacked metal layer (SML) prepared by DC magnetron sputtering. The influence of pre-annealing treatment of the SML precursor on the sulfurization process and the crystal growth characteristics of the CZTS film were investigated. It was found that the crystallinity of the CZTS film was improved with the increase of the pre-annealing temperature and duration, indicating that the pre-annealing treatment promoted the sulfurization. The formation and distribution of the metal compounds in the SML dominated the sulfurization reaction. And, the microstructure and elemental composition of the CZTS film depended very much on the properties of the SML. The role of pre-annealing treatment in the sulfurization of metal stacked precursors is proposed. A highly crystalline p-type kesterite Cu2ZnSnS4 film with carrier concentration of 2.38 × 1017 cm−3, mobility of 1.38 cm2/V s and optical bandgap of 1.46 eV was obtained. The CZTS layers are expected to fabricate high efficiency thin film solar cells.

Cost Effective Synthesis and Fabrication of Phase-Pure Kesterite Cu2-xZn1.3SnS4 P-type Absorber Layer Thin Films by Solvent Based Process Technique for Photovoltaic Solar Energy Applications

Solar has become one of the fastest growing renewable energy sources. With the push towards sustainability it is an excellent solution to resolve the issue of our diminishing finite resources. Alternative photovoltaic systems are of much importance to utilize solar energy efficiently. The Cu-chalcopyrite compounds CuInS2 and CuInSe2 and their alloys provide absorber material of high absorption coefficients of the order of 105 cm-1. Cu2ZnSnS4 (CZTS) is more promising material for photovoltaic applications as Zn and Sn are abundant materials of earth’s crust. Further, the preparation of CZTS-ink facilitates the production of flexible solar cells. The device can be designed with Al doped ZnO as the front contact, n-type window layer (e.g. intrinsic ZnO); an n-type thin film buffer layer (e.g. CdS) and a p-type CZTS absorber layer with Molybdenum (Mo) substrate as back contact. In this study, CZTS films were synthesized by a non-vaccum solvent based process technique from a molecular-ink using a non toxic eco-friendly solvent dimethyl sulfoxide (DMSO). The deposited CZTS films were optimized and characterized by XRD, UV-visible spectroscopy and SEM.

Improved structural properties, morphological and optical behaviors of sprayed Cu2ZnSnS4 thin films induced by high gamma radiations for solar cells

Cu2ZnSnS4 (CZTS) thin films have been synthesized by spray pyrolysis technique, deposited on glass substrates and then irradiated by high gamma radiations. Six gamma radiation doses have been applied: 10, 20, 30, 40, 50 and 100 kGy. The main objective of this work was to study the physical properties behavior of CZTS thin films under high gamma irradiation. Structural, optical and morphological properties of CZTS thin films were explored by X-ray diffraction, spectrophotometer and Scanning Electron Microscope, respectively. Structural analysis has shown that no noticeable changes have been occurred in the preferred orientation (112) or diffraction angles after gamma irradiation. Nevertheless, a significant increase in crystallite size from 52 to 79 nm has been observed after irradiation with 100 kGy gamma dose, which indicates a clear enhancement in crystalline structure. Certain optical parameters such as absorption and extinction coefficients (α(λ),K(λ)) have been only slightly affected, which indicate a radiation hardness of the CZTS thin films within the ionizing radiation range studied in this paper. Band gap energy of the irradiated thin films have been increased with the irradiation and reached 1.6 eV at 100 kGy gamma dose. Other optical parameters such as refractive index n(λ) and lattice dielectric constant (ε) have been determined and analyzed. All these experimental results clearly showed that structural properties of CZTS films have been improved by gamma irradiation while the optical properties have been slightly changed, which is favorable for optoelectronic applications working near nuclear environments or even for outer space solar cells and instrumentation for high-altitude flight, where gamma radiations are abundant.

Pulsed current co-electrodeposition of kesterite Cu2ZnSnS4 absorber material on fluorinated tin oxide (FTO) glass substrate

The films of kesterite Cu2ZnSnS4 (CZTS) were prepared on a fluorinated tin oxide (FTO) glass substrate by a pulsed current electrodeposition to make copper zinc tin (CZT) precursor films and sulfurization to make CZTS films. The various duty cycles (33%, 50%, and 67%) were used to electrodepeposit the CZT precursor films. According to the experiments, a higher duty cycle resulted in both CZT precursor films and CZTS films became denser, more compact, more uniform, and smoother. Based upon the XRD and EDS, the sulfurized film's composition consisted of Cu, Zn, Sn, and S. The compound formulae were also proved to be the kesterite Cu2ZnSnS4 structure.

Influence of different sulfur sources on the phase formation of Cu2ZnSnS4 (CZTS) nanoparticles (NPs)

Wurtzite (Wz) and kesterite (Ks) phases of Cu2ZnSnS4 (CZTS) nanoparticles (NPs) have been selectively synthesized via hot injection method using 1-octadecene (1-ODE) as solvent. The solvents, 1-dodecanethiol (1-DDT) and tert-dodecanethiol (t-DDT) were utilized to control the reactivity of metal precursors and to tune the desirable crystallographic phases. The phase purity of the as synthesized CZTS NPs was confirmed using X-ray diffraction results. TEM images indicate that the developed nanoparticles consist of a mixture of triangular shaped (height 20 ± 3 nm, width 17 ± 2 nm) and sphere shaped NPs (13.4 ± 0.4 nm). These nanoparticles were formed due to the influence of thiols without any additional capping ligands. The band gap of as-synthesized CZTS NPs were calculated as 1.41 eV for wurtzite phase (Wz—1-DDT) and 1.47 eV for kesterite phase (Ks—t-DDT) from UV–Visible absorption results. CZTS thin films were prepared via spin coating and the electrical properties were analysed using Hall Effect measurements. Both the phases of CZTS films exhibit p-type conductivity. Wurtzite phase of CZTS has higher mobility (23.6 cm−3) and carrier concentration (2.64 × 1017) compared to kesterite phase of CZTS films.

The role of sulphur in the sulfurization of CZTS layer prepared by DC magnetron sputtering from a single quaternary ceramic target

The influence of sulphur vapour pressure, controlled by its mass, on the grain growth and optoelectronic properties of Cu2ZnSnS4 (CZTS) films prepared by a two-step procedure based on sputtering was studied. It was found that both the crystallinity and grain size of the films were promoted with the increase of the sulphur vapour pressure, indicating that the crystal growth was controlled by the sulphur vapour pressure. In addition, the crystal growth process of CZTS was investigated by analysing the microstructure and elemental composition of the sulfurized films with different masses of sulphur. It was also found that the content of Sn in the sulfurized films decreased after high-temperature annealing. However, the second phase SnS2 was observed on the sample surface, which led to the increase of the optical band gap of the film. Moreover, we proposed the regulatory mechanism of sulphur vapour pressure in the grain growth of CZTS film. Finally, a highly crystalline p-type kieserite Cu2ZnSnS4 film with carrier concentration of 8.16 × 1017 cm−3, mobility of 1.24 cm2/V s and optical bandgap of 1.54 eV was obtained. This CZTS layers are expected to fabricate high efficiency thin film solar cells.

Compositional disparity in Cu2ZnSnS4 (CZTS) thin film deposited by RF-sputtering from a single quaternary compound target

Cu2ZnSnS4 (CZTS) is highly researched photovoltaic (PV) material in recent years because of resource availability and non-toxicity of its constituents. Radio frequency (RF) magnetron sputtering is one of the potential methods of fabricating CZTS thin film due to its suitability for large scale industrial implementation. Of the three methods of depositing CZTS films by RF sputtering, one step single quaternary target sputtering is by far the simplest and straightforward one. However, deposition profile plays a crucial role in maintaining the integrity of pre-defined target composition transferred to glass substrates. In this study, several deposition parameters, such as RF power, working pressure, and target to sample distance were varied and their influences on corresponding compositional variation in deposited samples were studied. A quaternary CZTS target with pre-fixed composition (Cu = 22%, Zn = 15%, Sn = 13%, S = 50%) was used to deposit CZTS thin films at various depositing conditions. Among all deposition profiles, high RF power was found to be more detrimental with high Cu loss up to 53% from the original atomic composition.

INFLUENCE OF ANNEALING TEMPERATURE ON SOME OPTICAL AND STRUCTURAL PROPERTIES OF Cu2ZnSnS4 DEPOSITED BY CZT CO-ELECTRODEPOSITION COUPLED WITH CHEMICAL BATH TECHNIQUE

Copper indium gallium selenide (CIGS) is currently most efficient thin film solar technology in use but it is faced with problems of material scarcity and toxicity. An alternative earth abundant and non-toxic materials consisting of Cu2ZnSnS4 (CZTS) have been investigated as a replacement for CIGS. In this work, CZTS thin films deposited by low cost co-electrodeposition, at a potential of [Formula: see text]1.2[Formula: see text]V, coupled with chemical bath techniques at room temperature and then annealed under sulphur rich atmosphere were investigated. CZTS thin film quality determination was carried out using Raman spectroscopy which confirmed formation of quality CZTS film, main Raman peaks at 288[Formula: see text]cm[Formula: see text] and 338[Formula: see text]cm[Formula: see text] were observed. Electrical characterization was carried out using four-point probe instrument and the resistivity was in the order of [Formula: see text]-cm. The optical characterization was done using UV-VIS-NIR spectrophotometer. The bandgaps of the annealed CZTS film ranged from 1.45 to 1.94[Formula: see text]eV with absorption coefficient of order [Formula: see text][Formula: see text]cm[Formula: see text] in the visible and near infrared range of the solar spectrum were observed.

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

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

Electrical bistability of sol-gel derived Cu2ZnSnS4 thin films

Kesterite type CZTS (Cu2ZnSnS4) thin films were deposited on Indium Tin Oxide (ITO) coated glass substrates by simple sol-gel method. X-ray diffraction (XRD) pattern showed the presence of crystalline kesterite phase, whereas the electron microscopic (FESEM) analysis established the 1D morphology of the CZTS films. Electrical studies revealed the interesting bistability phenomena of CZTS films. The current-voltage (I-V) characteristics showed two distinct paths for forward and reverse sweep directions. Conducting AFM (C-AFM) studies also supported the bistability phenomena of the CZTS films. Present study on electrical bi-stability of the CZTS thin films will help to understand the defects in the absorber materials and the photovoltaic performance of the CZTS device.

Structural properties of CZTS thin films on glass and Mo coated glass substrates: a Rietveld refinement study

Due to the difficulty in analyzing a few properties of CZTS (Cu2ZnSnS4) films coated on opaque substrates, the process conditions are often optimized on normal soda lime glass substrates and then transfer the process to Mo coated soda lime glass (SLG) substrates for device fabrication. This study concentrates on the analysis of structural properties of CZTS films on normal glass and Mo coated SLG substrates. CZTS films were coated on these two substrates by spin coating followed by sulphurization in N2 + H2S atmosphere. Phase purity of the films was analyzed by GIXRD and Raman spectroscopy. W–H analysis was used for estimating crystallite size and lattice strain of the films on the two substrates. Detailed structural studies were carried out with Rietveld refinement technique using GSAS package. Kesterite model with space group I4 (SG No: 82) was chosen for the refinement. Formation of kesterite phase on both substrates was confirmed from the refined lattice constant values. Deviation in bond angle with respect to kesterite structure was also analyzed. Different structural parameters were observed for the films on these substrates. The study indicates that it is important to optimize the properties of CZTS film on Mo coated SLG substrate separately, without following the same preparation conditions used for coating film on glass substrates.

The fabrication of Cd-free Cu2ZnSnS4-Ag2ZnSnS4 heterojunction photovoltaic devices

Cu2ZnSnS4 (CZTS) is a low cost, non-toxic material composed of earth-abundant elements with a large optical absorption coefficient. However, in CZTS solar cells, CdS buffer layers are widely used, which contain toxic elements and lead to lattice mismatch with CZTS thin films. In this paper, we have systematically investigated a I-II-IV-VI semiconductor material to replace the CdS buffer layer with a non-toxic material that offers good lattice matching with the CZTS film. For the first time, Cd-free Cu2ZnSnS4-Ag2ZnSnS4 heterojunction photovoltaic device is fabricated, where Cu2ZnSnS4 and Ag2ZnSnS4 layers are obtained by co-sputtering method. The cell exhibits a 4.51% efficiency which is higher than those of CZTS solar cells with ZnS and In2S3 buffer layers. This work offers the possibility to realize environmentally benign, scalable, low-cost, and high-efficiency solar cells.

Structural and Solar Cell Properties of a Ag-Containing Cu2ZnSnS4 Thin Film Derived from Spray Pyrolysis.

A silver (Ag)-incorporated kesterite Cu2ZnSnS4 (CZTS) thin film was fabricated by a facile spray pyrolysis method. Crystallographic analyses indicated successful incorporation of various amounts of Ag up to a Ag/(Ag + Cu) ratio of ca. 0.1 into the crystal lattice of CZTS in a homogeneous manner without formation of other impurity compounds. From the results of morphological investigations, Ag-incorporated films had larger crystal grains than the CZTS film. The sample with a relatively low Ag content (Ag/(Ag + Cu) of ca. 0.02) had a compact morphology without appreciable voids and pinholes. However, an increase in the Ag content in the CZTS film (Ag/(Ag + Cu) ca. 0.10) induced the formation of a large number of pinholes. As can be expected from these morphological properties, the best sunlight conversion efficiency was obtained by the solar cell based on the film with Ag/(Ag + Cu) of ca. 0.02. Electrostructural analyses of the devices suggested that the Ag-incorporated film in the device achieved reduction in the amounts of unfavorable copper on zinc antisite defects compared to the bare CZTS film. Moreover, the use of a Ag-incorporated film improved band alignment at the CdS(buffer)-CZTS interface. These alterations should also contribute to enhancement of device properties.

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%.

Detailed investigations on influence of precursor stacking and sulfurization for Cu2ZnSnS4 film formation

Cu2ZnSnS4 (CZTS) is a suitable absorber material in thin film hetero-junction solar cell. In the present study, CZTS has been successfully fabricated by electron beam evaporation and the influence of precursor stacking sequences and sulfurization conditions on the formation of secondary phases have been investigated in detail. CZTS thin films are prepared by two stage process: first stage, precursor layers with two different sequences glass/Zn/Sn/Cu and glass/Sn/Zn/Cu are deposited on glass substrates at room temperature using electron beam evaporation of elemental metallic precursors; followed by the second stage, the precursor films are sulfurized in two different conditions, in presence of elemental sulfur powder and in 5% H2S + N2 environment, at 500 °C for 1 h of annealing time for the formation of CZTS films. Effects of sulfurization environment (sulfur powder and H2S) and ramping rate (1 K/min, 5 K/min and 10 K/min) on the formation of CZTS and other probable secondary phases for different stacking sequences are investigated using X-ray diffraction, Raman spectroscopy, Field emission gun scanning electron microscopy, energy dispersive X-ray spectroscopy and UV–vis spectroscopy. Deconvoluted Raman spectra revealed high crystallinity in films sulfurized at low ramping rate (1 K/min). CZTS formation is noticed to be enhanced by sulfurizing the films in the presence of sulfur powder when compared with H2S gas. Phase pure CZTS film with a bandgap of 1.53 eV is prepared using sulfurization of glass/Sn/Zn/Cu stacked film in elemental sulfur powder.

Influence of sulfurization temperature on Cu2ZnSnS4 absorber layer on flexible titanium substrates for thin film solar cells

In this study, the effect of sulfurization temperature on the morphology, composition and structure of Cu2ZnSnS4 (CZTS) thin films grown on titanium (Ti) substrates has been investigated. Since Ti foils are flexible, they were preferred as a substrate. As a result of their flexibility, they allow large area manufacturing and roll-to-roll processes. To understand the effects of sulfurization temperature on the CZTS formation on Ti foils, CZTS films fabricated with various sulfurization temperatures were investigated with several analyses including x-ray diffraction (XRD), scanning electron microscopy (SEM), x-ray photoelectron spectroscopy and Raman scattering. XRD measurements showed a sharp and intense peak coming from the (112) planes of the kesterite type lattice structure (KS), which is strong evidence for good crystallinity. The surface morphologies of our thin films were investigated using SEM. Electron dispersive spectroscopy was also used for the compositional analysis of the thin films. According to these analysis, it is observed that Ti foils were suitable as substrates for the growth of CZTS thin films with desired properties and the sulfurization temperature plays a crucial role for producing good quality CZTS thin films on Ti foil substrates.

Secondary phases in Cu2ZnSnS4 films obtained by spray pyrolysis at different substrate temperatures and Cu contents

Cu2ZnSnS4 (CZTS) thin films prepared by the spray pyrolysis method were studied. This deposition method does not require sophisticated technological setup and allows to easily varying different technological parameters, which is perspective in terms of mass application. Here we show how the variation of the substrate temperature and of the Cu/Zn stoichiometry ratio in the precursor solutions influence the optical and structural properties of CZTS film. The Raman spectra reveal that an increase in the substrate temperature and Cu content leads to a higher disorder of the crystal structure and to the secondary phase formation, in particular, Cu2-xS. In turn, the formation of secondary phases significantly affected the CZTS bandgap. We have found that the films with the substrate temperature 289 °C and the Cu/Zn stoichiometric ratio equal to 2.0 comprise a minimum content of secondary phases and having the best crystal structure.

Secondary Phases in CZTS Thin Films Grown Using Direct Liquid Coating Approach

The CZTS films are made by direct liquid coating of solution precursors. A benign solution of propanol and polyethylene glycol as solvent is used. Films of CZTS are prepared by drop casting and spin coating followed by annealing and sulfurisation. The present synthesis need as low as 350°C to form the kesterite phase CZTS films. The presence of secondary phase in CZTS films are not conclusively detectable using X-ray diffraction and hence the role of Raman spectroscopy for the same is emphasized.