CZTS stoichiometry effects on the band gap energy (original) (raw)
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Stoichiometry effect on Cu2ZnSnS4 thin films morphological and optical properties
Journal of Renewable and Sustainable Energy, 2014
Thin films of Cu 2 ZnSnS 4 (CZTS) were prepared by sulfurization of multilayered precursors of ZnS, Cu, and Sn, changing the relative amounts to obtain CZTS layers with different compositions. X-Ray Diffraction (XRD), Energy Dispersive X-Ray spectroscopy, and SEM were used for structural, compositional, and morphological analyses, respectively. XRD quantitative phase analysis provides the amount of spurious phases and information on Sn-site occupancy. The optical properties were investigated by spectrophotometric measurements and Photothermal Deflection Spectroscopy. These films show a clear dependence of the optical and microstructural properties on the tin content. As the tin content increases we found: (i) an increase in both crystalline domain and grain size, (ii) an abrupt increase of the energy gap of about 150 meV, from 1.48 to 1.63 eV, and (iii) a decrease of sub-gap absorption up to two orders of magnitude. The results are interpreted assuming the formation of additional defects as the tin content is reduced. V C 2014 AIP Publishing LLC. [http://dx.
Characterization of combinatorial Cu 2 ZnSnS 4 thin films
2018
Characterization of combinatorial Cu 2 ZnSnS 4 thin films Joakim Adolfsson The kesterite Cu 2 ZnSnS 4 (CZTS) is considered as a possible absorber layer in future photovoltaic (PV) applications. The abundance of its constituent elements along with the material being non-toxic and having a direct band gap of around 1.5 eV make it an attractive material for solar cell applications. So far, cells with an efficiency of 12.6 % have been achieved. The limiting factor is the finished cells' open circuit voltage (V OC) deficit which has been attributed to structural defects in the material. Problems with sustaining a sulfur-enriched atmosphere during the annealing step of material production have been observed, and are thought to be partially responsible for the high density of structural defects. Elemental sulfur is commonly used for inducing a sulfur-enriched atmosphere in the anneal. In this study, four combinatorial and polycrystalline CZTS thin films were prepared and annealed in different conditions with regards to time, sulfur source and amount. The samples were characterized using Energy dispersive-, Raman-and Photoluminescence spectroscopy. The effect of the anneal on the different composition regions were analyzed and secondary phases were identified. Introducing CuS as the sulfur source during the anneal reduced the decomposition of the CZTS phase, and lowered the density of the defect complex [Zn Cu + Cu Zn ], while enlarging the single phase region. Strictly and highly Sn-rich compositions of CZTS was observed to yield both high cation order and photoluminescence intensity, and a link between the two parameters was observed.
2018
Solar cell absorber Kesteritetype Cu2ZnSnS4 (CZTS) thin films have been prepared by Chemical Bath Deposition (CBD). UV–vis absorption spectra measurement indicated that the band gap of as-synthesized CZTS was about1.68 eV, which was near the optimum value for photovoltaic solar conversion in a single-band-gap device. The polycrystalline CZTS thin films with kieserite crystal structure have been obtained by XRD. The average of crystalline size of
Influence of Cu 2 S, SnS and Cu 2 ZnSnSe 4 on optical properties of Cu 2 ZnSnS 4
Materials Letters, 2017
We have studied influence of Cu 2 ZnSnSe 4 (CZTSe) and secondary phases of Cu 2 S and SnS on optical properties of kesterite-type Cu 2 ZnSnS 4 (CZTS) by using the effective medium theory. We found that CZTSe and Cu 2 S cause band gap reduction and enhance light absorption of CZTS at all photon energies of the sunlight whereas SnS changes optical properties of CZTS only at large photon energies beyond the solar spectrum. Optical spectra of CZTS(Se) has been studied by first principles calculations within hybrid functional that was used as input for the effective medium theory.
KESTERITE THIN FILMS OF Cu 2 ZnSnS 4 OBTAINED BY SPRAY PYROLYSIS
2012
Thin films of Cu2ZnSnS4 (CZTS) were deposited using the spray pyrolysis method as relatively fast and vacuum-free method. Obtained samples were analyzed using the X-Ray Fluorescence, grazing incidence X-Ray Diffraction and Raman Spectroscopy techniques. Analysis showed close to stoichiometry composition of the films with kesterite type structure but poor crystalline quality and possible existence of secondary phases. To improve the quality of the films, the asprepared layers were annealed in the presence of elemental Sn and S. Comparison of the results before and after annealing showed a strong improvement of the crystalline quality and a significant reduction of concentration of secondary phases of the films without significant change of composition. The measured optical band gap is equal to 1.52 and 1.55 eV in the asprepared and annealed films, respectively. The optical absorption coefficient is found to be > 10 4 cm.
Study of optical and structural properties of Cu2ZnSnS4 thin films
Thin Solid Films, 2011
Cu 2 ZnSnS 4 is a promising semiconductor to be used as absorber in thin film solar cells. In this work, we investigated optical and structural properties of Cu 2 ZnSnS 4 thin films grown by sulphurization of metallic precursors deposited on soda lime glass substrates. The crystalline phases were studied by X-ray diffraction measurements showing the presence of only the Cu 2 ZnSnS 4 phase. The studied films were copper poor and zinc rich as shown by inductively coupled plasma mass spectroscopy. Scanning electron microscopy revealed a good crystallinity and compactness. An absorption coefficient varying between 3 and 4 × 10 4 cm −1 was measured in the energy range between 1.75 and 3.5 eV. The bad gap energy was estimated in 1.51 eV. Photoluminescence spectroscopy showed an asymmetric broad band emission. The dependence of this emission on the excitation power and temperature was investigated and compared to the predictions of the donor-acceptor-type transitions and radiative recombinations in the model of potential fluctuations. Experimental evidence was found to ascribe the observed emission to radiative transitions involving tail states created by potential fluctuations.
Precursors’ order effect on the properties of sulfurized Cu2ZnSnS4 thin films
A dc magnetron sputtering-based method to grow high-quality Cu2ZnSnS4 (CZTS) thin films, to be used as an absorber layer in solar cells, is being developed. This method combines dc sputtering of metallic precursors with sulfurization in S vapour and with post-growth KCN treatment for removal of possible undesired Cu2−xS phases. In this work, we report the results of a study of the effects of changing the precursors’ deposition order on the final CZTS films’ morphological and structural properties. The effect of KCN treatment on the optical properties was also analysed through diffuse reflectance measurements. Morphological, compositional and structural analyses of the various stages of the growth have been performed using stylus profilometry, SEM/EDS analysis, XRD and Raman Spectroscopy. Diffuse reflectance studies have been done in order to estimate the band gap energy of the CZTS films. We tested two different deposition orders for the copper precursor, namely Mo/Zn/Cu/Sn and Mo/Zn/Sn/Cu. The stylus profilometry analysis shows high average surface roughness in the ranges 300–550 nm and 230–250 nm before and after KCN treatment, respectively. All XRD spectra show preferential growth orientation along (1 1 2) at 28.45◦. Raman spectroscopy shows main peaks at 338 cm−1 and 287 cm−1 which are attributed to Cu2ZnSnS4. These measurements also confirm the effectiveness of KCN treatment in removing Cu2−xS phases. From the analysis of the diffuse reflectance measurements the band gap energy for both precursors’ sequences is estimated to be close to 1.43 eV. The KCN-treated films show a better defined absorption edge; however, the band gap values are not significantly affected. Hot point probe measurements confirmed that CZTS had p-type semiconductor behaviour and C–V analysis was used to estimate the majority carrier density giving a value of 3.3 × 1018 cm−3.
2015
We prepared Cu2ZnSnS4 (CZTS) films on Mocoated glass substrates by using electrodeposition in an acidic electrolyte containing Cu, Zn, and Sn species. We examined how the substrate temperature influenced the compositional, structural, morphological, and electrical properties of the CZTS films by using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), Raman spectroscopy, energy dispersive spectroscopy, and ultraviolet–visible absorption spectroscopy. The film sulfurized at a substrate temperature of 580 C exhibited the best characteristics because of its large grain size and low number of voids. At a sufficiently high temperature, this fabrication process yielded single-phase CZTS with no secondary phases. The grain size increased with deposition temperature: the size at 580 C was about twice that at 460 C. All the CZTS films were Cu-rich and S-poor, with the composition changing at 580 C. XRD revealed that the films had nanocrystalline kesterite structures, indicated by intense, sharp (112), (200), (220), and (312) diffraction peaks. The crystallite size was 34.6–57.3 nm, increasing with substrate temperature. FESEM indicated that the morphology improved with su
Growth and characterization of Cu2ZnSn(S,Se)4 thin films for solar cells
Solar Energy Materials and Solar Cells, 2012
Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSnSe 4 (CZTSe) with their band gap energies around 1.45 eV and 1.0 eV, respectively, can be used as the absorber layer in thin film solar cells. By using a mixture of both compounds, Cu 2 ZnSn(S,Se) 4 (CZTSSe), a band gap tuning may be possible. The latter material has already shown promising results such as solar cell efficiencies up to 10.1%. In this work, CZTSSe thin films were grown in order to study its structure and to establish the best growth precursors. SEM micrographs reveal an open columnar structure for most samples and EDS composition profiling of the cross sections show different selenium gradients. X-ray diffractograms show different shifts of the kesterite/stannite (1 1 2) peak, which indicate the presence of CZTSSe. From Raman scattering analysis, it was concluded that all samples had traces of CZTS and CZTSSe. The composition of the CZTSSe layer was estimated using X-ray diffraction and Raman scattering and both results were compared. It was concluded that Se diffused more easily in precursors with ternary Cu-Sn-S phases and metallic Zn than in precursors with ZnS and/or CZTS already formed. It was also showed that a combination of X-ray diffraction and Raman scattering can be used to estimate the ratio of S per Se in CZTSSe samples.
AIP Advances
A potential solar absorber material, sputtered kesterite Cu 2 ZnSnS 4 (CZTS) thin film, has been extensively studied in recent years due to its advantageous properties, including the earth abundance of its constituent elements, nontoxicity, suitable band gap, and high absorption coefficient. 2000 nm CZTS thin films were deposited on soda lime glass by a sputtering technique. The prepared films underwent a postannealing treatment for crystallization in which different temperatures and pressures were applied to understand its impact on film growth, phase formation, and stoichiometry. The annealed samples were subsequently characterized by Raman and UV-visible (UV-Vis) spectroscopy, energy-dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The thickness of each film was measured using a surface profilometer and from a cross-sectional image obtained by SEM. The XRD pattern for each film showed characteristic (112), (220), and (312) peaks, and the phase purity was confirmed via Raman studies. Film surface morphology and roughness were studied by AFM. The root mean square roughness was found to increase with annealing temperature and base pressure. The chemical compositions of the prepared samples were analyzed by EDX, and the films showed desired stoichiometry. UV-Vis absorption spectroscopy indicated that the direct band gap energies (Eg) of the films were 1.47 eV-1.51 eV, within the optimum range for use in solar cells. These attractive properties of the sputtered CZTS thin film should heighten interest in its use as a solar absorber layer in the next-generation photovoltaic cells, suggesting that it possesses substantial commercial promise.