Nanostructural Tailoring to Induce Flexibility in Thermoelectric Ca₃Co₄O₉ Thin Films

Because of their inherent rigidity and brittleness, inorganic materials have seen limited use in flexible thermoelectric applications. On the other hand, for high output power density and stability, the use of inorganic materials is required. Here, we demonstrate a concept of fully inorganic flexible thermoelectric thin films with Ca₃Co₄O₉-on-mica. Ca₃Co₄O₉ is promising not only because of its high Seebeck coefficient and good electrical conductivity but also because of the abundance, low cost, and nontoxicity of its constituent raw materials. We show a promising nanostructural tailoring approach to induce flexibility in inorganic thin-film materials, achieving flexibility in nanostructured Ca₃Co₄O₉ thin films. The films were grown by thermally induced phase transformation from CaO-CoO thin films deposited by reactive rf-magnetron cosputtering from metallic targets of Ca and Co to the final phase of Ca₃Co₄O₉ on a mica substrate. The pattern of nanostructural evolution during the solid-state phase transformation is determined by the surface energy and strain energy contributions, whereas different distributions of CaO and CoO phases in the as-deposited films promote different nanostructuring during the phase transformation. Another interesting fact is that the Ca₃Co₄O₉ film is transferable onto an arbitrary flexible platform from the parent mica substrate by etch-free dry transfer. The highest thermoelectric power factor obtained is above 1 × 10^-4 W m^-1 K^-2 in a wide temperature range, thus showing low-temperature applicability of this class of materials.