High quality Cu2ZnSnS4 thin films for photovoltaic applications

In the photovoltaic field, the demand for high efficiency devices with reduced costs has addressed to the development of new materials as light absorbers, in replacement of the most used silicon. Photovoltaic cells based on thin film semiconductors, such as chalcogenide-based solar cells, are one of the most promising technology for inexpensive photovoltaic electricity generation. In this contest, Cu2ZnSnS4 (CZTS), in the structure of kesterite, as absorber layer is a hopeful candidate due to the high adsorption coefficient (α ~ 104 cm-1), good optic band value (1,4-1,6 eV), earth abundance and low toxicity of the main elements (copper, zinc, tin), especially if compared with other inorganic semiconductor materials as CdTe, whose devices are already available on the market. Many approaches have been developed to fabricate kesterite thin films: on one side the conventional methods make use of vacuum processes, e.g. co-evaporation and sputtering. On the other side, non-vacuum processes have also been studied, because of the lower production costs, the higher productivity and the improved uniformity of the final stoichiometry composition. Nevertheless, the synthesis of CZTS films, still has some problems related to the presence of secondary phases, surface dishomogeneity and the difficult to achieve suitable thickness (1-1,5 µm), responsible for the low cell efficiency1. In this context, we present a new chemical procedure to obtain superior quality Cu2ZnSnS4 films, composed by highly soluble and inexpensive precursors in a non-toxic and environmentally friendly solvent. Therefore, the films were prepared by a sol-gel method, based on hydrolysis and condensation process of metal inorganic salts in an organic solvent (dimethylsulfoxide); thiourea was used as a source of sulphur. Both the solvent and thiourea are able to coordinate the metal centers, giving rise to an ordered network in solution, that could favourably lead to the ordered structure inside the film. The influence of the composition of the starting solutions containing metal ions was studied: solutions of metal salts containing different anions were compared, to evaluate the role of anions and their coordination with metal ions in the formation of a pure and homogeneous phase (sulphate and acetate anions were tested). Moreover, the effect of the addition of an organic ligand to the precursors solution was studied: acetylacetone (acacH) was added to the solution of the acetate salts, in order to further improve the metals coordination in solution and to better control the kinetic of the sol-gel reaction and the uniformity of the films. The sol-gel transition was monitored for all the solutions, measuring the viscosity with time (25°C). The film deposition was carried out by spin coating on fluorine doped tin oxide coated glass (FTO). The subsequent heat treatment guarantees the formation of the correct crystalline phase; it’s worth noting that the treatment was made without sulfurization, to avoid toxic by-products, using inert atmosphere [and at temperature suitable also for flexible and plastic substrates]. The films were characterized structurally by X-ray diffraction (XRD), Raman spectroscopy and energy dispersive X-ray analysis (EDX), morphologically by electronic scanning electron microscopy (SEM). Photoluminescence (PL) spectroscopy was employed for investigating the electronic structure of realized layers. First results have demonstrated that the solutions were stable within a week since their preparation, without transforming into gel; moreover, the addition of acacH led to stable solutions for longer time, as expected from the further coordination of the organic ligand to metal centers. Regarding the CZTS films, we have demonstrated that the coordinating effect of acetate ions, employed as precursors instead of the sulphate ions, plays a primary role in ensuring homogeneity in solution and consequently in the film. Only the films formed by the acetate showed the absence of secondary phases (XRD and Raman), while EDX and SEM analyses showed the right stoichiometry and excellent homogeneity, respectively. The presence of acacH ligand improved the morphological properties of the film, as a consequence of the higher coordination of metal ions observed in the solution. Therefore, the developed methodology has successfully identified an innovative way to achieve high quality Cu2ZnSnS4 thin films for photovoltaic applications; relate devices optimization is currently underway. References 1. Wang W. et al., Advanced energy materials, 2014, 4 (7), 1301465.