Structural Evolution of Multilayer SnS/Cu/ZnS Stack to Phase-Pure Cu₂ZnSnS₄Thin Films by Thermal Processing

Abstract

In this work, thin films of phase-pure Cu 2 ZnSnS 4 (CZTS) were developed from a stack of binary metal sulfides by post-deposition thermal processing. The precursor stack SnS/Cu/ZnS was grown by sequential electrodeposition of SnS and Cu layers followed by thermal evaporation of ZnS layer. The transformation from binary/ternary composition to phase-pure CZTS was studied using different experimental tools such as X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The TEM images revealed the formation of tetragonal crystals with interplanar spacing 0.312 nm. Raman spectra of the films confirmed that annealing at 550 • C for 30 min under N 2 /S ambient resulted in the formation of phase-pure CZTS film. The bandgap estimated from the optical transmittance and reflectance spectra showed a direct transition at 1.59 eV. The films are photosensitive and the photo electrochemical measurements showed the p-type conductivity of the films. and a theoretical power conversion efficiency of 32.2%. [2][3][4][5] Moreover, the absence of toxic and expensive elements highlights CZTS as an alternative to CdTe and CIGS, which is important in high volume manufacturing of photovoltaic panels and its deployment.CZTS thin films have been deposited by several methods such as thermal evaporation, 6 sputtering, 7 RF magnetron sputtering, 8 electrodeposition, 9-12 sulfurization of the metals, binary sulfide or CZTS layers deposited by different methods 1,[13][14][15][16] etc. In majority of the cases, the films were subjected to post deposition thermal treatment to attain stoichiometry and improve the structural and optical properties.Devices with CZTS and CZTS/Se absorber layers developed by both chemical and physical methods have been reported earlier. [17][18][19][20][21][22] Among the vaccum based techniques, co-evaporated Cu 2 ZnSnSe 4 (CZTSe) and sputter deposited CZTS have attained device efficiencies of 9.15% and 9.3% respectively. 18,19 The films developed with chemical methods such as electrodeposition and nanoparticle based suspension are equally competent and economically attractive. Solar cells with 5.5% and 7% efficiencies were reported with CZTS and CZTS/Se absorber layers developed by electrodeposition technique.21,22 All these aforementioned results suggest a potential commercial interest behind sulfide/selenide based kesterite solar cells which need further optimization in the film synthesis and device fabrication steps to compete with established CdTe and CIGS technologies.In the present work we developed the CZTS films on SnO 2 :F substrates by the sulfurization of a stack of SnS/Cu/ZnS films under N 2 /S atmosphere. The SnS and Cu films were developed by electrodeposition whereas ZnS film was deposited by evaporation in the order shown for the stack. The films were annealed at different temperatures in presence of S to identify the optimum annealing temperature for the formation of phase pure CZTS. The cryst...

“Green” Aqueous Synthesis and Advanced Spectral Characterization of Size-Selected Cu2ZnSnS4 Nanocrystal Inks

“…2d). The observed Sn 3d 5/2 peak centered at 486.3 eV and the spin-orbit splitting of 8.5 eV are typical for Sn(IV) 24,43,47,61,67–69,73 . This assignment is supported by an analysis of the Sn M 4,5 N 4,5 N 4,5 Auger electron energy spectrum (Fig.…”

“…This broadening is a result of an additional relaxation path for the L 2 (2p 1/2 ) hole due to L 2 L 3 M 4,5 Coster-Cronig transition, which reduces the lifetime of the hole and broadens the Cu2p 1/2 peak 71 . The positions of peaks, the distance between them (19.9 eV), the absence of an additional contribution at around 934/954 eV, as well as the absence of paramagnetic Cu(II) satellite peaks at ~940–945 eV 72 indicate that copper is present in the sample solely as Cu(I) typically found in CZTS NCs 24,43,47,61,67–69,73 . Nevertheless, as the differences reported in the literature between the positions of Cu(I) and Cu(II) compounds (oxides and chalcogenides) are rather small, we examined additionally with high-resolution XPS spectra the range of the Auger electron peak of copper, which is more sensitive to the chemical surrounding than the XPS peaks.…”

“…2a) shows a characteristic doublet at 1021 eV and 1044 eV with a spin-orbit splitting of 23 eV typical for Zn 2+ in chalcogenide lattices 24,43,47,61,67–69 . The high quality of the fit attests the absence of other forms of Zn(II) in the system under scrutiny.

Figure 2( a – d ) High-resolution XPS spectra in the range of Zn 2p ( a ), S 2p ( b ), Cu 2p ( c ), and Sn 3d ( d ).

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Cross‐Section Analysis of the Composition of Sprayed Cu2ZnSnS4 Thin Films by XPS, EDS, and Multi‐Wavelength Raman Spectroscopy

“…Figure a presents the Cu 2p core level spectra which exhibits binding energies for the Cu 2p 3/2 and Cu 2p 1/2 peaks at ranges between 931.8–932.3 and 952–951.1 eV, respectively. There is also a peak splitting of 19.8 eV, indicating the formation of Cu(I) . Furthermore, the satellite peak of Cu 2p 3/2 which is characteristic of Cu 2+ is not present, indicating the absence of any Cu 2+ compound .…”

Impact of pre-alloying before sulfurization of electroplated Cu/Sn bilayers

“…The possibility could be ternary phase such as Cu 2 Sn 3 S 7 and Cu 3 SnS 4 or binary phases such as SnS 2 . The presence of Cu 2 Sn 3 S 7 and Cu 3 SnS 4 phases was ruled out, because no peaks at 375 cm -1 and 268 cm -1 , 348 cm -1 and 295 cm -1 were observed, respectively[18][19]. Hence, SnS 2 was the main second phase in the referenced CTS.…”