Structural, morphological and optical properties of Cu–Fe–Sn–S thin films prepared by electrodeposition at fixed applied potential
Graphical abstract
Introduction
Polycrystalline thin-film solar cells based on absorbers such as CdTe [1], [2], [3], CuInGa(S, Se)2 (CIGS) [4], [5], [6] or organo-halide perovskites [7] have attracted the interest of many research groups due to their promising power conversion efficiencies (PCE). The presence of some toxic elements in these photovoltaic devices and/or their high production price limited the mass production are regarded as disadvantageous. Therefore, some other materials which have the same opto-electrical properties, but being less toxic and less expensive are also investigated and studied [8,9]. The quaternary chalcogenide CZTS(Se) (Cu2ZnSnSxSe1-x) is considered as an emerging p-type semiconductor for thin films photovoltaic cells being already used as an absorber layer [10,11]. The maximum reported PCE of CZTS(Se) is 12.6% [12], a value which is well below the targeted efficiency for critical raw materials - free technologies of 20% [13], needed be granted as a mature photovoltaic technology (such as CIGS ones) and to economically compete with commercially mass-produced Silicon cells which are typically only 13–14% efficient [14]. Since CuZn and ZnCu anti-site defects [15], [16], [17] are detrimental to solar cells performances, the substitution of Zn2+ with other chemical elements with the same valence was explored, the quaternary class of materials, Cu2MSnS4 (CMTS), (where M = Ca2+; Mg2+; Ba2+; Mn2+; Cd2+; Ni2+; Co2+; Fe2+) being recently investigated [18], [19], [20], [21], [22], [23].
The Cu2FeSnS4 (CFTS) quaternary material is one of the promising candidates for thin-film photovoltaic applications owing to their non-toxic elements, their suitable band gap (which is about 1.2 eV to 1.5 eV) and their high absorption coefficient (104 cm−1) in the band-edge region [24]. However, a CFTS based photovoltaic device obtained by thermolysis had an extremely low PCE as compared with other absorbers due to low mobility and high carrier concentration [25]. Therefore, CFTS has been used as counter electrode (photo-cathode) in dye-sensitized solar cells [26]. Several physical and chemical approaches have been also developed for the synthesis of the CFTS quaternary semiconductors, such as chemical spray pyrolysis [27], sputtering metallic precursors followed by rapid thermal annealing sulfurization process [28], ultrasound assisted microwave irradiation [29], simple hot injection method [30], or solvothermal synthesis [31].
Electrodeposition is considered an alternative cheaper and reliable method to obtain CMTS layers. Various parameters, such as the pH of the bath, the post electrodeposition treatments parameters, the concentration of the complexing agent or the used solvent, influence the physico-chemical properties of the obtained films [32], [33], [34], [35]. Published studies on CFTS obtained using electrodeposition are only a few. Zhou et al. [36] used an extremely high concentration of FeCl2 with respect to the other cation salts, in acid solution (pH~3), and a relative lower potential of approximately −1.1 V vs. the saturated calomel electrode (SCE), showing that the addition of ascorbic acid can increase more than 10 times the concentration of Fe in the electrodeposited layer, since it reduced the concentration of oxidized Fe3+ species in the aqueous electrolyte. The authors varied the CuCl2 concentration to finally achieve a single nearly stoichiometric CFTS phase, while no discussion on the stannite microstructure and the corresponding stoichiometry is given. Miao et al. [37] synthesized CFTS by electrochemical deposition followed by a annealing in sulphur environment, the electrodeposition being performed at −1.265 V vs. SCE using a 2:1:1 cation sulphates concentration ratio. However, in the mentioned paper the final stoichiometry of the obtained coatings are not given. The deposition potential is reported also as an important factor which affects the stoichiometry and phase composition of kesterites [38], [39], [40].
In this work Cu–Fe–Sn–S films were synthesized and characterized using single-step electrodeposition at different applied potentials followed by high-temperature sulfurization (500 °C) in argon flow.
Section snippets
Materials and methods
The Cu–Fe–Sn–S films were deposited on ITO (indium tin oxide) covered glass substrates using a three-electrode cell (VersaSTAT 4 potentiostat) consisting of an Ag/AgCl/KCl reference electrode, a Pt wire counter electrode, while the working electrode was the ITO substrate surface. First, the substrates were rinsed manually using alkaline solution and then ultrasonically cleaned with acetone, ethanol and distilled water. Prior to electrodeposition a cyclic voltammetry study was conducted to
Cyclic voltammetry
Cyclic voltammetry (CV) was used as a method to study and to understand the electrochemical behavior of the CFTS four elements components. Fig. 1a–d) shows the CV plots recorded on ITO glass substrates for the individual precursors electrolytic solutions in distilled water, with (a) 0.02 M copper sulphate CuSO4, (b) 0.02 M iron sulphate FeSO4, (c) 0.02 M tin sulphate SnSO4, and (d) 0.02 M thiosulfate sodium Na2S2O3, respectively, while Fig. 1e) represents the CV plot of all the previous
Conclusion
In summary, CFTS films have been deposited by electrochemical route on ITO substrates starting from a single solution containing the Cu–Fe–Sn–S precursors, with Cu:Fe:Sn = 1:1:1, followed by high-temperature heat treatment in sulphur environment. The nearly single CFTS phase films were obtained by applying −1.25 V. However, the relative concentration of iron is almost half than in a stoichiometric compound, while all films are copper and tin reach. Thus, different concentrations of cation salts
CRediT authorship contribution statement
Outman El Khouja: Conceptualization, Formal analysis, Investigation, Project administration, Validation, Visualization, Writing - original draft, Writing - review & editing. Aurelian Catalin Galca: Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing. Khalid Nouneh: Funding acquisition, Methodology, Validation. Mohamed Yassine Zaki: Investigation. Mohamed Ebn Touhami:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
O.E.K. acknowledges Physics and Culture at Magurele Foundation for the received financial support during COVID-19 lockdown period in 2020, and the receipt of the OEA grant AF-15/20-01 from the Abdus Salam International Centre for Theoretical Physics, Trieste, Italy. All authors acknowledge the Moroccan Ministry of Higher Education and Research and Centre National pour la Recherche Scientifique et Technique in the framework of PPR/37/2015 project, and the Romanian Ministry of Research,
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