Flexible inorganic and hybrid thermoelectric thin films based on layered calcium cobaltate

Abstract: With the development of wearable and miniaturized electronics, self-sustaining energy sources have drawn extensive attention. Flexible thermoelectric materials and devices is an approach to convert waste heat into electricity as continuous power supply for such applications.Traditional inorganic thermoelectric materials, such as Bi2Te3, PbTe, and SnTe, exhibit high thermoelectric properties, but their disadvantages of toxicity and oxidation when exposed to high temperature in air, as well as the extreme rarity of tellurium, restrict them from widespread use in applications. Compared to conventional thermoelectric materials, oxides, especially misfit-layered Ca3Co4O9, have advantages as thermoelectric materials, not only has the typical advantages of oxides including low cost and good chemical stability at high temperatures, but they are also based on abundant raw materials and have relatively high thermoelectric properties due to the complex structure which composed of CoO2 conductive layers and rock-salt type Ca2CoO3 insulating layers. Many strategies have been used to enhance the thermoelectric performance of Ca3Co4O9. However, inorganic materials are generally rigid, limiting their use in flexible devices. Fully inorganic flexible thermoelectrics can be obtained through novel fabrication technologies, miniaturization, and structural design. Otherwise, organic/inorganic hybrids materials simultaneously combine the respective features of the good flexibility and low thermal conductivity from conducting polymers, and the electrical transport properties from inorganic materials.In order to explore flexible thermoelectric thin films based on layered calcium cobaltate, I have investigated the Ca3Co4O9 and CaxCoO2 systems. Nanoporous Ca3Co4O9 thin films were synthesized using sequential reactive magnetron sputtering and post annealing. The key factors, Ca(OH)2 content, bilayer thickness, and Ca elemental ratio in multilayers film, for the formation of nanoporous Ca3Co4O9 have been studied and can further tailor the porosity and morphology. Nanoporous Ca3Co4O9 with different porosity and discontinuous films with islands of highly textured Ca3Co4O9, effectively constituting distributed nanoparticles, have been obtained. Based on the nanoporous Ca3Co4O9 thin film, flexible double-layer nanoporous Ca3Co4O9/PEDOT:PSS thin films were synthesized by spin-coating PEDOT:PSS into the nanopores.