Characterization of Cu 2 ZnSn(SSe) 4 monograin powders by FE-SEM

The design and synthesis of high-efficiency materials to convert solar to electrical energy is an increasingly important research field. Within the photovoltaic technologies, crystalline Si have an 80% share while the remaining 20% are mostly thin film solar cells based on Cu(In,Ga)(S,Se) 2 (CIGSSe) and CdTe. However, the cost, the abundance and the environmental impact of the elemental components cannot be neglected. For these reasons, Cu 2 ZnSnS 4 (CZTS), Cu 2 ZnSnSe 4 (CZTSe) and their solid solutions CZTSSe has attracted much attention recently since they can provide the development of cost competitive solar cells. The CZTS-based solar cells consist of earth abundant and relatively inexpensive elements and represent an environmentally friendly alternative compared to the above mentioned systems. The energy conversion efficiency of the CZTS-based solar cells has increased from 0.66% in 1996 to 11.1% recently. The present work shows preliminary results that are related to the characterization of CZTSSe monograin powders by scanning electron microscopy. High purity metal compounds, S and Se powders were used as precursors for the synthesis of the Cu 2 ZnSn(SSe) 4 monograin powders. The precursor powders were mixed to the desired composition and, additionally, KI was added as a flux material. Afterwards, the powders were blended in a mixer and encapsulated in quartz ampoules. The blended powders were degassed under dynamic vacuum at room temperature, sealed and annealed isothermally between 700 ºC and 780 ºC for a time ranging between 44 h and 136 h. After synthesis the flux material was removed with deionized water and the powders were sieved into several fractions. The morphology, microstructure and chemical composition of the synthesized powders was obtained with a Philips XL30 field-emission scanning electron microscope (FE-SEM) equipped with a backscattered electron (BSE) detector and an integrated EDAX energy dispersive X-ray spectroscopy (EDS) microanalysis system. The typical morphology obtained for the CZTSSe powders can be seen in Figure 1. Basically, the particles show a polyhedral morphology with some of them showing a needle shape, i.e. a large shape factor (L/D>>1). Moreover, it was also observed a slight increase of the median particle size with the increase of the synthesis temperature. Due to the complexity of the synthesis of CZTSSe monograins, the formation of binary or ternary phases is a common feature. A very good control over the syn