Flexible Environmentally Friendly Thermoelectric Material Made of Copper (1) Iodide and Nanocellulose for Green Energy

Herein, we have developed a new efficient flexible hydrophobic thermoelectric (TE) material with increased thermal stability and mechanical strength for green energy production. The 8 µm thick flexible nanocellulose (NC) substrate was obtained from stalks of common reed by TEMPO-mediated oxidation. It has a high crystallinity index of 88 % and the amount of coherent scattering of nanocrystals in the range from 2 to 3 nm. A CuI film with a thickness of 4.3 μm was deposited on the NC substrate by the Successive Ionic Layer Adsorption and Reaction (SILAR), and thus a CuI/NC thin film thermoelectric material was obtained. The average size of the CuI coherent scattering regions is 25 nm. CuI nanocrystals contain dislocations (1.6x10¹⁵ lines/m²) and tensile microstrains of 6x10³ arb. units. The optical band gap Eg for direct allowed transitions in the CuI film is about 3.0 eV. The high hydrophobicity of the CuI/NC material is a useful feature for applications in humid environments. The CuI/NC material contains the same number of copper and iodine atoms. In addition, a small amount of sulfur (<1 at.%) is present in the copper iodide film, which increases the thermopower coefficient of CuI/NC to 6.7 μW/(m·K²) at T = 340 K. Its positive Seebeck coefficient S = 108 μV/K confirmed the p-type conductivity of CuI. The change in the resistivity upon initial heating and subsequent cooling demonstrated the electrical conductivity crossover typical of films of nanocrystalline degenerate semiconductor CuI. A CuI/NC sample in the form of a 3 cm x 1 cm strip was tested as a planar thin film TE element. The output power density of a CuI/NC TE element 0.123 W/m2 was experimentally recorded at a difference between its hot and cold edges of 40 K.