Activation of CdTe-based thin films with zinc chloride and tetrachlorozincates

A series of CdTe thin film solar cells were prepared by applying aqueous solutions of ZnCl2 or tetrachlorozincates MnZnCl4 (M = Na or NH4 for n = 2; M = Mg or Ca for n = 1) as flux reagents in the activation process, instead of established but highly toxic CdCl2. The remaining process steps for the generation of solar cells have not been varied and activation-temperature and -duration were kept constant with 400 °C/25 min. The back contact comprised an etching step followed by deposition of ca. 150 nm gold. In contrast to Na2ZnCl4 and (NH4)2ZnCl4, the application of ZnCl2 or CaZnCl4 in 30% concentration gave promising results with current-voltage and quantum efficiency cell-data well comparable to CdCl2 standard activated reference cells. The aqueous solutions were either applied manually by dip-coating or sponge roller procedure. Microstructural and interfacial investigations were performed with high resolution field emission scanning electron microscopy imaging on polished cross sections of ZnCl2- or CaZnCl4-derived devices using ion beam preparation techniques. Moreover, the measurement of the electron beam induced current was achieved, yielding a similar appearance for a CaZnCl4-activated substrate compared to referential CdCl2 samples.