In this work, we used solar energy converted via photosynthesis into chemical energy of the biomass of the fastgrowing perennial herb Miscanthus x giganteus for the manufacture of nanocellulose (NC) films, which are biodegradable alternative to common petroleum-based polymer substrates used in flexible electronics. To create the NC substrates, we applied an environmentally friendly method of organosolv delignification of plant raw materials carried out at a low temperature and in a relatively short time. Then by means of the low-temperature cheap and scalable method Successive Ionic Layer Adsorption and Reaction (SILAR) we deposited copper iodide (CuI) film of 0.72 mu m thickness on both sides of the 12 mu m thick NC substrate, and thus obtained light-weight and flexible biodegradable nontoxic thermoelectric material CuI/NC. Crystal structure, morphology, chemical composition, and optical, electrical and thermoelectric properties of the CuI/NC have been researched. Studies have shown that nanostructured p-type semiconductor CuI film in the CuI/NC TE material is quite dense and completely covers the NC surface. It has typical optical direct band gap approximate to 3.0 eV, is single-phase gamma-CuI with crystallite sizes in the 19-25 nm range, with moderate dislocation density of (1.6-2.8) x 10(15) lines/m(2), and tolerable microstrains e of (4-9) x 10(-3) a.u. The determined value of the Seebeck coefficient S is similar to 228 mu V K-1, at that, S is constant in the temperature range 290-335 K. Together with the thermoelectric power factor similar to 36 mu W.m(-1).K-2 it is favorable for the use of CuI/NC as new thermoelectric material for an in-plane design of biodegradable flexible thin film thermoelectric generator (TEG). At temperature gradient of 50 K, the single p-CuI thermoelectric leg made from CuI/NC strip of 3 cm long and 0.5 cm wide generates open circuit voltage 8.4 mV, short circuit current 0.7 mu A and maximum output power 1.5 nW. It corresponds to the output power density 10 mu W/m(2), and thus confirms the suitability of CuI/NC to obtain electricity by the harvesting the waste environmental heat.