Cu2XSnY4 (X= Zn, Mn, Fe; Y= S, Se): earth-abundant materials for thin film solar cells deposited by physical and chemical methods (S. Binetti, M. Acciarri)

The development of photovoltaic (PV) absorbers proper for thin films based devices has become fundamental in the last decades to increase the efficiency to cost ratio of solar energy. The research in thin film solar cells has been dominated by absorber materials based on chalcogenides CdTe and Cu(In,Ga)Se2 in the last decades. Both the toxicity of cadmium and the scarce availability of indium lead towards the development of materials which constituents are earth-abundant and relatively harmless.
The most studied alternatives to CIGS are Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), where more abundant and less expensive elements like Zn and Sn are used in place of In and Ga. CZTS and CZTSe, whose most stable crystalline form is kesterite, have a direct band gap of 1.4-1.5 eV and 1.0 eV, respectively, as well as an absorption coefficient higher than 104 cm-1, which make them suitable for PV applications. They share similar structure with the chalcopyrite CuInS2 except that half of the In is replaced with Zn and another half with Sn.
A further alternative is copper manganese tin sulfide (CMTS), which shows an important advantage with respect to CZTS. As a matter of fact, not only the abundance in the Earth’s crust of Mn is two order of magnitude higher than that of Zn (1100 ppm vs 79 ppm), but the amount of Zn produced in 2015 was 4’600’000 tons lower than that of Mn (13’400’000 Zn tons vs 18’000’000 Mn tons). Therefore, since Mn is definitely cheaper than Zn, optimized CMTS could potentially provide Wp cost definitely lower than CZTS, which is crucial for thin film PV applications. Furthermore, CMTS consists of non-toxic elements and shows high absorption coefficient and direct band gap (1.1–1.3 eV) suitable for PV applications.

At MIBSOLAR laboratories we grow CZTS both by sputtering and by a soft-chemical route. Using RF sputtering, tin, copper and zinc are deposited on a molydbenum thin layer and the CZTS phase is obtained by a thermal treatment of the stacked metals in sulfur vapours. To optimise the growth process, we carry out a comprehensive structural and spectroscopical characterization, including scanning electron microscopy, Raman spectroscopy, X-ray diffraction and photoluminescence of the CZTS films. Electrical characterization of the solar cells demonstrates reproducible efficiency around 4%.
In the chemical method, the precursor solution is prepared by dissolving cupric acetate, zinc acetate and stannous chloride in methanol. Thiourea is added to form a colourless solution containing the metal-ligand complexes. The solution is then dip-coated or drop-casted on the thin layer of Mo. Between each deposition, the layers are treated at temperature above 180° C in air for 10 minutes, so that the thermolysis of thiourea occurs, resulting in a grayish film composed of mixed sulfides. The films are finally annealed in argon flux at high temperature to remove any organic residual and crystallize the kesterite phase. The thin films are analyzed before and after thermal annealing with EDX, Raman, XRD, FTIR and SEM.

Thin film solar cells based on both CZTSe grown by Politecnico di Milano and CMTS grown by RSE SpA are also prepared and tested at MIBSOLAR laboratories. The relationship between material properties and PV device performance are established through a careful characterization of the absorber layers by Raman, FTIR and Photoluminescence spectroscopies.

Post-doctoral fellow: Vanira Trifiletti, Alessia Le Donne

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