Greatly Enhanced Thermoelectric Performance of Flexible Cu_2-xS Composite Film on Nylon by Se Doping

In this work, flexible Cu_2-xS films on nylon membranes are prepared by combining a simple hydrothermal synthesis and vacuum filtration followed by hot pressing. The films consist of Cu₂S and Cu_1.96S two phases with grain sizes from nano to submicron. Doping Se on the S site not only increases the Cu_1.96S content in the Cu_2-xS to increase carrier concentration but also modifies electronic structure, thereby greatly improves the electrical properties of the Cu_2-xS. Specifically, an optimal composite film with a nominal composition of Cu_2-xS_0.98Se_0.02 exhibits a high power factor of ~150.1 μW m^-1 K^-2 at 300 K, which increases by ~138% compared to that of the pristine Cu_2-xS film. Meanwhile, the composite film shows outstanding flexibility (~97.2% of the original electrical conductivity is maintained after 1500 bending cycles with a bending radius of 4 mm). A four-leg flexible thermoelectric (TE) generator assembled with the optimal film generates a maximum power of 329.6 nW (corresponding power density of 1.70 W m^-2) at a temperature difference of 31.1 K. This work provides a simple route to the preparation of high TE performance Cu_2-xS-based films.