Numerical approach to design efficient mechanical energy harvesting system based on piezo-semiconducting nanowires
Résumé
Nanoscale mechanical energy harvesting using PiezoElectric NanoGenerators (PENGs) has been highlighted
as a potential candidate for the realization of self-powered biomedical devices such as pacemakers and
wearable communication devices. A PENG can be described as a stack of five layers (Figure 1) : a rigid or
flexible substrate, a bottom electrode, an electroactive layer (piezoelectric semiconducting nanowires (NWs)
surrounded by an insulating polymer), a top layer of insulating material and a top electrode [1]. For the
continual performance improvement of PENG devices, optimization of the piezoelectric semiconducting
nanomaterial, including morphology, density and quality, is of great interest and crucial. For example, it has
been theoretically shown that the output potential and efficiency of PENGs strongly depend on the electrical
quality of the nanomaterial [2]. Among several nanomaterials, zinc oxide (ZnO) NWs can be synthesized by
hydrothermal method, which is low temperature, industrially scalable and compatible with flexible substrates
[3]. Different approaches have been used to model ZnO NW based PENGs including analytical models and
numerical models using finite element method (FEM) [1].
In the present work, COMSOL software is used to model ZnO NW based PENGs. First, the piezoelectric
potential distribution in ZnO NWs is studied for different intrinsic doping concentrations. In terms of
piezopotential magnitude, cylindrical shaped NWs with low doping completely outperformed their homologue
NWs being highly doped leading to significant piezoelectric potential screening effect. Figure 2 shows that, at
the applied static pressure of 6.25 MPa, a cylindrical NW with a 1010 (/cm3) charge density presents a
maximum piezoelectric potential of 2.9 V compared to 0.25 V reached by a 1015 (/cm3) doped NW. Moreover,
in order to predict the complete PENG electrical characteristics and to reduce computation time, a new
approach using FEM simulation and analytical modelling has been developed [1]. This method combines FEM
simulation of a PENG unit cell (including one NW) in order to estimate the open-circuit voltage (without any
external electrical circuit), and an analytical model of the full PENG in order to predict the maximum power
and the corresponding optimal load. Further works will consist in studying the effect of the geometrical
characteristics and electromechanical properties of the nanowire-polymer composite on the complete PENG
performance.
References
[1] Doumit N and Poulin-Vittrant G 2018 A new simulation approach for performance prediction of
vertically integrated nanogenerators Adv. Theory Simul. 1800033 1–8
[2] Gao Y and Wang Z L 2009 Equilibrium potential of free charge carriers in a bent piezoelectric
semiconductive nanowire Nano Lett. 9 1103–10
[3] Dahiya A S, Morini F, Boubenia S, Nadaud K, Alquier D, Poulin-Vittrant G 2017 Organic / inorganic
hybrid stretchable piezoelectric nanogenerators for self-powered wearable electronics Advanced
Materials Technologies 1700249 11 pp.