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 x giganteus to manufacture flexible 12 mu m thick NC film with stable monoclinic cellulose structure (I beta), 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 degrees. In the most efficient TE sample CuI/NC, the obtained via SILAR 0.39 mu m thick nanostructured CuI film consists of cubic (1 1 1)-oriented gamma-CuI crystals with faceted surfaces of similar to 200-300 nm. The high electrical conductivity (sigma) and shape of the sigma 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 mu W.m(-1).K-2 at T = 333 K. This PF is the record for biodegradable flexible thermoelectric materials. At Delta T = 40 K the CuI/NC-based single p-CuI thermoelectric leg generates open circuit voltage 3.5 mV, short circuit current 4 mu A, 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.