Solution processible Cu2SnS3 thin films for cost effective photovoltaics: Characterization
Graphical abstract
Introduction
Although the progress of Cu(InGa)Se2 and CdTe solar cells over the last decade showed promising efficiencies of 21.7% and 21% respectively [1], [2], the pilot scale commercialization is limited due to the fact that In and Ga are expensive [3], Te being a scarcely available element [4] and Cd being environmentally toxic [5]. Hence most of the research in the recent years is focussed on alternate energy materials for thin film photovoltaics. In the group of Cu based chalcogenides, Cu2ZnSnSxSe4–x (CZTS) based absorbers have reached the photon conversion efficiency of 12.6% [6]. Note that CZTS consisting of earth abundant and non-toxic elements, suffers from the formation of secondary phases such as CTS and ZnS and hence controlling the growth of Cu2ZnSnSxSe4–x is difficult. CTS is a p-type ternary semiconductor, consisting of non-toxic, earth-abundant elements, with a tunable direct bandgap from 0.93 eV to 1.51 eV and a high absorption coefficient of 104 cm−1 to 105 cm−1 which are optimal for solar cell applications [7], [8], [9], [10]. Its high conductivity of 0.5–10 Scm−1, hole concentration of 1018 cm−3 and a hole mobility of 1–80 cm2 V−1s−1 is an added advantage to improve the transport properties for photovoltaic applications [10], [11].
Various methods have been used for the deposition of CTS films. Kuku and Fakolujo thermally evaporated CTS films and made the first solar cell device of CTS [9]. P.A. Fernandes et al. have deposited CTS films by sulphurizing sputtered stacked metal precursors at temperatures of 350 °C and 400 °C [8]. CTS thin films deposited by using physical vapour deposition techniques like thermal evaporation and sputtering involve expensive equipment [9]. Alternate chemical routes for deposition have also been studied. Q. Chen et al. fabricated a solar cell device by coating a paste of CTS powders in propylene glycol using doctor blade technique [12]. Avellaneda et al. synthesized CTS films by heating stacked layers of CuS and SnS deposited via chemical bath deposition, at 315 °C and 350 °C in nitrogen atmosphere [11]. Z. Su et al. synthesized CTS films by the successive ionic layer adsorption and reaction (SILAR) method. The Cu–Sn–S precursor film was sulphurized in a nitrogen and sulphur atmosphere at 400 °C to yield cubic CTS films [13]. Adelifard et al. deposited triclinic CTS films via spray pyrolysis technique at a temperature of 285 °C for different Sn/Cu ratios (0.0–1.0) [14]. Note that the above techniques involve several steps, high-temperature annealing procedure and few approaches may also lead to the formation of secondary phases like binary sulphides of Cu and Sn. Therefore, a single step CTS synthesis is essential for affordable photovoltaics. Herein we demonstrate a simple modified method of directly spin coating the precursor solution of CTS on a substrate followed by a low-temperature heat treatment. Such a method not only ensures the complete phase formation but is also applicable for large area depositions.
Section snippets
Synthesis
The CTS precursor solution was prepared by dissolving CuCl2 (1M, 99.999% from Sigma Aldrich), SnCl2 (0.5M, 99.99% from Sigma Aldrich) and thiourea (3M, 99.0% from Sigma Aldrich) in anhydrous 2-methoxyethanol (5 ml, 99.8% from Sigma Aldrich). Thiourea was taken in excess to prevent formation of secondary phases and to compensate for the sulphur loss during annealing. Addition of SnCl2 to anhydrous 2-methoxyethanol gave a clear solution. On adding CuCl2, the solution became white in colour.
Analysis of the Cu–Sn-thiourea complex precursor
To explain the mechanism of metal-thiourea complex formation, the Cu–Sn-thiourea complex precursor and thiourea were analysed using FTIR. Fig. 1 shows the FTIR spectra of thiourea and the Cu–Sn-thiourea complex precursor. The peak at 730 cm−1 in the thiourea spectrum assigned to CS stretching shifts to 700 cm−1 in the Cu–Sn-thiourea complex spectrum. This shift to lower wavenumber is ascribed to the bonding of the Cu2+ and Sn2+ ions to the S atom of thiourea aided by the electron transfer from
Conclusions
Thin films of CTS were synthesized by the solution processible route of spin coating of the CTS precursor solution followed by a low-temperature annealing. The mechanism of formation of the Cu–Sn-thiourea complex was deduced using FTIR. The phase transformations and deposition temperature were determined by thermal analysis of the CTS precursor. The annealed CTS films were further characterized. The phase formation and tetragonal crystal structure was inferred from XRD. The formation of
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2021, Journal of Materials Research and TechnologyCitation Excerpt :Six peaks at 2θ = 21.23°, 28.39°, 33.77°, 47.42°, 51.58° and 57.24° were noticed. The peaks at 28.39° [23], 33.77°, 47.42°, 51.58° corresponding to the crystal planes (112), (200), (220/204) and (006) indicate the tetragonal crystal structure of CCTS. The peak at 57.24° corresponds to (312) plane of Cu2SnS3.