Efficient biodegradable flexible hydrophobic thermoelectric material based on biomass-derived nanocellulose film and copper iodide thin nanostructured layer

• Solar energy converted into biomass to produce thermoelectric (TE) material. • Nanocellulose (NC) with stable monoclinic structure and high crystallinity index. • Successive Ionic Layer Adsorption and Reaction method used to deposit CuI films. • Non-toxic water-repellent thermoelectric material CuI/NC obtained. • CuI/NC with high electrical conductivity and record thermoelectric power factor. • Output thermoelectric parameters of single thermoelectric leg CuI/NC presented. Here we applied solar energy converted into biomass to produce efficient biodegradable flexible hydrophobic thermoelectric (TE) material with nanocellulose (NC) film as environmentally friendly functional substrate. We used fast-growing perennial herb Miscanthus × giganteus to manufacture flexible 12 µm thick NC film with stable monoclinic cellulose structure (I β ), high crystallinity index ( CI = 78%) and average crystallite size 3 – 4 nm. Through the low-temperature cheap and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR) we deposited copper iodide (CuI) films on NC substrates and thus obtained non-toxic TE materials CuI/NC, which can be water-repellent, as their contact angles reach 140°. In the most efficient TE sample CuI/NC, the obtained via SILAR 0.39 µm thick nanostructured CuI film consists of cubic (1 1 1)-oriented γ-CuI crystals with faceted surfaces of ~200–300 nm. The high electrical conductivity (σ) and shape of the σ vs. temperature ( T ) curve of this CuI/NC sample is realized through suppression of grain boundary scattering due to tunneling currents in CuI. The CuI/NC material has large thermoelectric power factor ( PF ) that grows with increasing temperature and reaches value 140 μW·m −1 ·K −2 at T = 333 K. This PF is the record for biodegradable flexible thermoelectric materials. At Δ T = 40 K the CuI/NC-based single p -CuI thermoelectric leg generates open circuit voltage 3.5 mV, short circuit current 4 µA , and power 3.8 nW, and these output parameters can be further improved through a thickening the CuI film in CuI/NC by increasing the number of SILAR cycles.