The best values of Tsub for Bi2Te3 and for Sb2Te3 thin-films were about 270 ºC and
220ºC, respectively. Finally, it must be noted that all thin-films were deposited on a
polyimide (kapton) foil with a thickness of 25 μm. The Figure 13 shows two SEM
cross-section and surface images of both Bi2Te3 and Sb2Te3 thin-films, where their
polycrystalline structure can be confirmed. Also, the former temperatures and the
optimal evaporation flow rate ratios, R, were those which resulted in thin-films with
larger grain size. This is of major concern, because a crystalline structure with an
increased grain size is less resistive, whose consequence is a thin-film with an increased
thermoelectric figures-of-merit, ZT.