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

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 and, mainly, to save raw materials. At a early stage, the research in thin film solar cells has been dominated by absorber materials based on CdTe and Cu(In,Ga)Se2. However, both the toxicity of cadmium and the scarce availability of In, Ga and Te lead towards the development of materials baased on earth-abundant and relatively harmless elements.

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.
Further alternatives are copper manganese tin sulfide (CMTS) and copper iron tin sulfide (CFTS), which both show an important advantage with respect to CZTS. As a matter of fact, not only the abundance in the Earth’s crust of Fe and Mn is orders of magnitude higher than that of Zn, but the amount of Zn annually produced is definetely lower than that of both Mn and Fe. Therefore, since Fe and Mn are definitely cheaper than Zn, optimized CMTS and CFTS could potentially provide Wp cost definitely lower than CZTS, which is crucial for thin film PV applications. Furthermore, they consist of non-toxic elements and show high absorption coefficient and direct band gap suitable for PV applications.

At MIB-SOLAR labs, CZTS thin films can be prepared 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, a comprehensive structural and spectroscopical characterization, including scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS), Raman spectroscopy, X-ray diffraction (XRD) and photoluminescence (PL) of the CZTS films was carried out. Electrical characterization of the solar cells demonstrates reproducible efficiency around 4%.
In the chemical method, the precursor solution is prepared by dissolving copper (II) acetate hydrate, tin (II) chloride dihydrate, zinc (II) acetate dihydrate and thiourea into DMSO. The solution is then drop-casted on Mo coated soda lime substrates. After 30 min of gelation process, a 10 min annealing in tubular oven under Ar atmosphere at 550 °C is performed. The drop-casting, gelation and annealing sequence is then repeated leading to a total film thickness around 2 μm. The so produced CZTS thin films are analyzed before and after each step with SEM/EDS, Raman, XRD and Photoluminescence. A similar procedure was used to obtain CFTS thin films with promising features.

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 MIB-SOLAR labs. The relationships between material properties and PV device performance are established through a careful characterization of the absorber layers by Raman, XRD and Photoluminescence.

Research team: Profs. Simona Binetti and Maurizio Acciarri (Associate Professors), PhD Alessia Le Donne (Research Technologist), PhD Giorgio Tesberlidis (Post-doctoral fellow), Luigi Frioni (PhD student)

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