Emerging Cu2MnSnS4 thin film solar cells grown by sulfurization of evaporated precursors

As it is well known, in the last decade much attention has been focused on I2-II-IV-VI4 thin films as an attractive possibility for the synthesis of In and Ga free chalcogenides which may allow terawatt range photovoltaic (PV) applications. A further alternative to copper zinc tin sulfide/selenide belonging to this class of materials is Cu2MnSnS4 (CMTS), a p-type semiconductor fully based on Earth-abundant and low-cost elements which shows an important advantage with respect to copper zinc tin sulfide/selenide. 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 copper zinc tin sulfide/selenide, which is crucial for thin film PV applications. CMTS, which crystallizes into a stannite structure (space group: I-42m), shows high absorption coefficient (104 cm-1) and direct band gap suitable for PV applications. So far, CMTS was mainly studied as bulk magnetic semiconductor, while, only in the last two years, few works , , reported on CMTS thin films for PV applications. Our work deals with CMTS thin films grown by a two-step vacuum process. The metal precursors deposited on Mo-coated soda lime glass (SLG) by thermal evaporation are annealed in sulfur vapors for 1h at 585°C with an initial 1h step at 115°C to enhance the metal intermixing. Of the many possible stoichiometries, Cu-poor/Mn-rich CMTS films with Mn/Sn ratio around 1 were chosen in order to prevent the development of both highly conductive (e.g. Cu2-xS) and insulating (e.g. MnS) secondary phases. Several techniques, including Scanning Electron Microscopy, Energy Dispersive Spectroscopy (EDS), Raman and Photoluminescence have been used to test the quality of CMTS thin films, while CMTS/CdS/iZnO+AZO solar cells were inspected both by External Quantum Efficiency and current density-voltage (J-V) measurements under 1 sun illumination. The beneficial effects of low temperature post-deposition annealing either in air or inert atmosphere between 200 and 275°C on CMTS solar cells were also investigated, both in terms of electrical performance and modification of the material properties. In particular, a 40 min 225°C annealing in air provided the best overall performance enhancement, leading to the present efficiency record in the literature : efficiency 0.83%, open-circuit voltage 354 mV, short-circuit current density 5.8 mA/cm2, fill factor 40%. The reasons behind these beneficial modifications of the device parameters were investigated by Raman and PL spectroscopies. Results showed that post-deposition thermal treatments in air between 200 and 275°C generally reduce generally reduces the density of the bulk defect, thus reducing recombination losses, and increase the CdS crystalline quality, while red-shifting its absorption edge. A 225°C annealing lead to a significant reduction of recombination losses without a strong increase of CdS absorption, allowing for the best overall performance enhancement.