## 2 Materials and Methods  
Flexible polyethylene terephthalate (PET) is used as a substrate material for the piezoelectric harvesting devices. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) solution with 1.3% conductivity grade was spin-coated on PET at 2500 rpm per 30 s, and dried at 90 \({}^{\circ}\)C to remove the residual solvent. The resulting film has a thickness of 40 nm and exhibits a sheet resistance of 85 \(\Omega\)/sq. A ZnO film with a thickness of 550 nm was deposited directly on the polymer-coated PET without a seed layer by RF sputtering with additional oxidation of the target. The specific sputtering power was 405 mW/cm\({}^{2}\), the sputtering argon gas pressure was 2.10\({}^{-2}\) Torr, and the oxygen pressure initially was higher (8 \(\times\) 10\({}^{-4}\) Torr), with a continuous decrease down to 2 \(\times\) 10\({}^{-4}\) Torr by the end of the sputtering. Thus, the initial monolayers with high density served as seed for assistance of the ZnO growth. Further, the oxygen deficit resulted in vacations in the lattice and hollow formations between the nanobranches. PEDOT:PSS was spin-coated on the ZnO film at the same deposition conditions as for the bottom electrode, in order to make the contacting surface between ZnO and the top electrode smoother and flatter. As well, in this way, the diffusion of nanoparticles between the nanobranches, originating from the top electrode and moving toward the bottom electrode, is prevented. A top Au electrode with a thickness of 90 nm was deposited by low-temperature DC sputtering. Figure 1a presents the layered structure of the flexible nanobranched harvester.  
The ZnO surface morphology was analyzed by scanning electron microscopy (SEM, JSM-6390 LV, JEOL, Peabody, MA, USA). X-ray diffraction (XRD, D8 ADVANCE, Bruker, Karlsruhe, Germany) was used to determine the presence of piezoelectric features. Dielectric characterization of the fabricated prototype was made by resistance-inductance-capacitance RLC type measurements (Instek LCR 819, Good Will Instrument Co., Ltd., New Taipei City, Taiwan) for estimation of the structures' capacitance (relative permittivity is provided as plots, based on calculations, using the capacitance values), and dielectric losses in the frequency range 1 Hz-1 MHz for extracting information about the quality of the piezoelectric film and the polarization processes in the microdevice. Additionally, the same measurements were completed at different temperatures in the range of \(-10\) to 40 \({}^{\circ}\)C, which are the typical ambient temperatures for the intended potential application of the proposed harvester. The measurements were conducted for the two basic states of the samples: non-bended (flat), and continuously bended at a radius of 12 mm (curved), in order to provide data about the change in the dielectric behavior after mechanical loading. The piezoelectric response of the structure was recorded by digital oscilloscope (DQ2042 CN Shanghai MCP Corp., Shanghai, China) at an applied cyclic mechanical stimulus (tension/compression stress of the sample) of 350 g forces with tunable frequency. The electromechanical system for the stimulation of charges by bending is lab-made, where a motor is activated by a power source and a signal generator regulates the frequency of the vibrating cantilever. The samples were attached on the top of this cantilever. A photo of the setup for mechanical test is presented in Figure 1b,c which shows the stress zones over the samples at static bending. All measurements were conducted without preliminary poling of the samples.