Effect of calcination temperature on the properties of CZTS absorber layer prepared by RF sputtering for solar cell applications
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In present work, we report synthesis of nanocrystalline Kesterite copper zinc tin sulfide (CZTS) films by RF magnetron sputtering method. Influence of calcination temperature on structural, morphology, optical, and electrical properties has been investigated. Formation of CZTS has been confirmed by XPS, whereas formation of Kesterite-CZTS films has been confirmed by XRD, TEM, and Raman spectroscopy. It has been observed that crystallinity and average grain size increase with increase in calcination temperature and CZTS crystallites have preferred orientation in (112) direction. NC-AFM analysis revealed the formation of uniform, densely packed, and highly interconnected network of grains of CZTS over the large area. Furthermore, surface roughness of CZTS films increases with increase in calcination temperature. Optical bandgap estimated using UV–Visible spectroscopy decreases from 1.91 eV for as-deposited CZTS film to 1.59 eV for the film calcinated at 400 °C which is quite close to optimum value of bandgap for energy conversion in visible region. The photo response shows a significant improvement with increase in calcinations temperature. The employment these films in solar cells can improve the conversion efficiency by reducing recombination rate of photo-generated charge carriers due to larger grain size. However, further detail study is needed before its realization in the solar cells.
KeywordsCZTS films RF sputtering Kesterite Calcination XRD, XPS, NC-AFM
As on today, silicon (Si) has the lion’s share in the photovoltaic industry. The main reason behind it is the huge availability of Si on the earth and a developed and established industry for making high-quality Si solar cells. However, the cost of Si solar cells is still high due to the high production cost of device quality Si. The photovoltaic market nowadays is demanding low production cost of material and hence of solar cells . Therefore, it is necessary to reduce the material cost of solar cells which effectively reduces the cost of solar cells . Several other direct bandgap semiconductor materials, such as copper indium gallium sulfides (CIGS), cadmium telluride (CdTe), etc, have been tried for solar cell application. However, these materials have their own problems like Cd and Te which are toxic, while Ga and In are expensive, which restrict the future development of solar cells. Copper zinc tin sulfide (Cu2ZnSnS4) or simply CZTS is one of the promising absorber materials in thin-film solar cell because of its excellent material properties for obtaining high efficiency such as direct bandgap (1.45 eV) , which is very close to optimum bandgap for solar energy conversion, high absorption coefficients (>104 cm−1) , etc. In addition, CZTS does not contain any toxic and expensive element, resulting in realizing of solar cell with less environmentally damaging and low cost. It is composed of naturally abundant and nontoxic elements . The maximum theoretical power conversion efficiency of CZTS solar cells reported was 29.4% . Wang and his group fabricated laboratory scale CZTSSe solar cell having area 0.42 cm2 with efficiency 12.6%, which is the highest conversion efficiency achieved until today .
There are two methods used for the preparation of CZTS films, chemical methods, and physical/vacuum-based methods. The chemical methods include several techniques, such as chemical spray pyrolysis , photochemical depositions , sol–gel technique , spin coating , electrodeposition , electro-spinning , and successive ionic layer adsorption and reaction (SILAR) , etc. The physical or vacuum-based method includes atom beam sputtering , e-beam and thermal evaporation , pulsed laser deposition , etc. Each method has its own advantages and limitations. Among these methods, RF magnetron sputtering has received considerable attention in recent years owing to its capability to synthesize device quality CZTS films. It permits deposition at low substrate temperature, gives the good adhesion, possibility of large area deposition, maximum uniformity, controllable thickness, precise in chemical composition control, matching with tradition solar cell production line, as well as easy scale-up than other CZTS thin-film deposition methods .
Properties of CZTS thin films are greatly influenced by pre- and post-annealing or calcinations treatment in various gas atmospheres. Recently, small work has been done on effect of pre- and post-annealing or calcinations treatment in various gas atmospheres on structural, optical, and electrical properties of CZTS thin films deposited by various methods. Recently, Seboui et al.  investigated post growth effect on properties of CZTS thin films prepared by spray pyrolysis and reported that the post-annealing effect reduces the optical transmission and increases the bandgap of CZTS films. Secondary phases may remain in the film after heat treatment. Ericson et al.  obtained highly crystalline CZTS films after annealing in H2S atmosphere. Surgina et al.  have also studied the annealing effect on structural and optical properties of CZTS films grown by pulsed laser deposition in N2 atmosphere. Vanalakar et al.  explained the post-annealing effect on grain size and surface morphology of CZTS thin films in the different gas atmospheres. Recently, Liu et al.  reported preheating effect on CZTS film properties. Most of the authors reported the effect of pre- or post-annealing of CZTS films either at high temperature or in presence of toxic or hazardous gases. To best of our knowledge, low temperature calcination of CZTS in inert gas atmosphere is missing till date. With this motivation an attempt has been made to investigate low post calcination effect (>400 °C) in inert gas (Ar) atmosphere on structural, optical, morphology and electrical properties of CZTS thin films deposited by RF magnetron sputtering. It has been observed that by increasing calcination temperature in Ar atmosphere, it is possible to grow highly uniform, large area (~4 cm2) nanocrystalline kesterite-CZTS films with optimum bandgap (~1.59 eV) which can be useful for enhancing the efficiency of CZTS solar cells.
Film preparation and calcination
Process parameters employed during the deposition of CZTS films
6 × 10−3 bar
Distance between substrate holder and target electrode
Ar gas flow rate
Before each deposition the substrates were cleaned using a standard cleaning procedure using piranha solution. Prior to deposition, the substrate holder and deposition chamber were baked for two hours at 100 °C to remove any water vapor absorbed on the substrates and to reduce the oxygen contamination in the film. Sputter-etch of 10 min were used to remove the target surface contamination. As-deposited CZTS films then calcinated at different temperatures in argon atmosphere for 90 min in a cylindrical stainless steel chamber without air-break. During calcination the argon flow rate and pressure were kept constant at 50 sccm and 20 mTorr respectively. After calcination films were allowed to cool to room temperature in vacuum and then taken out for characterization.
X-ray diffraction patterns were obtained by X-ray diffractometer (Bruker D8 Advance, Germany) using CuKα line (λ = 1.54 Å) at a grazing angle of 1°. Raman spectra were recorded in the range of 100–600 cm−1. The spectrometer has the back-scattering geometry for detection of Raman spectrum with the resolution of 1 cm−1. The excitation source was 532 nm line of He–Ne laser. The power of the Raman laser was kept less than 5 mW to avoid laser-induced crystallization of the film. The HR-TEM and SAED patterns were recorded using TECNAI G2-20-TWIN, transmission electron microscope operating at 200 kV. The optical bandgap of CZTS was deduced from absorbance spectra and was measured using a JASCO, V-670 UV–Visible spectrophotometer. The surface topology of the films was investigated NC-AFM (JEOL, JSPM-5200). The XPS spectra were recorded using a VSW ESCA instrument with a total energy resolution ~0.9 eV fitted with an Al Kα source (soft X-ray source at 1486.6 eV) at base vacuum >10−9 Torr. The XPS signal was obtained after several scans in the acquisition process. The spectra were recorded for the specific elements (Cu, Zn, Sn, S). The photo response measurement of the CZ TS films was studied using a Keithley 2401 system. For light illumination, a PEC-L01 Portable Solar Simulator was used. Thickness of films was determined by profilometer (KLA Tencor, P-16+).
Results and discussion
X-ray diffraction (XRD) analysis
Structural parameters, average grain size (d x-ray), full-width half-maxima (FWHM), inter-planar spacing (d hkl), and microstrain (ϵ) for CZTS films
Calcination temperature (°C)
d x-ray (nm)
d hkl (Å)
4.23 × 10−3
4.52 × 10−3
3.29 × 10−3
2.06 × 10−3
Raman spectroscopy analysis
X-ray photoelectron spectroscopy (XPS) analysis
Transmission electron microscopy (TEM) analysis
Atomic force microscopy (AFM) analysis
UV–Visible spectroscopy analysis
Photo response measurement
In summary, nanocrystalline CZTS films have been prepared by home-made RF magnetron sputtering technique. Influence of calcination temperature in Ar atmosphere on structural, morphological, electrical and optical properties on CZTS films has been investigated. Formation of CZTS has been confirmed by x-ray photoelectron spectroscopy (XPS) whereas formation of Kesterite-CZTS films has been confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. We found that the calcination process has a great influence on growth and nucleation of grains. XRD analysis revealed that the crystallinity and average grain size increases with increase in calcination temperature. Raman spectroscopy analysis show shifting of Raman peak shift towards lower wavenumber with increase in calcination temperature. The presence of internal compressive stress and shrinking of substrate during cooling may responsible for shifting of Raman peak towards lower wavenumber. However, shrinking of substrate while cooling has not been verified experimentally. Detail surface study (morphology and topology) reveal that CZTS thin films have densely packed and a highly interconnected network of grains with large area (4 cm2). AFM show significant difference in surface topography of CZTS films with change in calcination temperature. Increase in calcination temperature show increase in rms and average surface roughness of the CZTS films. UV–Visible spectroscopy analysis revealed that the absorption coefficient of as-deposited and calcinated CZTS films are in the range 104–105 cm−1 in the visible region. The bandgap show decreasing trend with increase in calcination temperature (1.91–1.59 eV). The bandgap of CZTS film annealed at 400 °C was found ~1.59 eV which is quite close to the optimum value for photovoltaic solar conversion in the visible region of solar spectrum. It is found that the photo response depends upon the grain size effect, whereas photo response increases with the increase of the grain size. Employment these films as an absorber layer in CZTS solar cells can improve the conversion efficiency by reducing recombination rate of photo-generated charge carriers due to increased grain size.
Mr. Sachin Rondiya is grateful to Dr. Babasaheb Ambedkar Research and Training Institute (BARTI), Pune for research fellowship and financial assistance and INUP IITB project sponsored by DeitY, MCIT, Government of India. Mr. Avinash Rokade is grateful to MNRE, New Delhi for National Renewable Energy (NRE) fellowship. One of the authors Dr. Sandesh Jadkar is thankful to University Grants Commission, New Delhi for special financial support under UPE program. Mr. Ashok Jadhavar is thankful to BARC-SSPU program for financial support.
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