Effect of a-Si thin film on the performance of a-Si/ZnO-stacked piezoelectric energy harvesters

Abstract

In this letter, we present the fabrication and characterization of a zinc oxide (ZnO)-based nanogenerator for piezoelectric micro-energy harvesting by combining thin films of amorphous silicon (a-Si) and ZnO. We utilized the a-Si thin film as an interlayer to assemble several a-Si/ZnO-stacked piezoelectric nanogenerators (SZPNGs) on indium tin oxide (ITO)-coated polyethylene naphthalate substrates. We investigated the influence of the a-Si layer thickness on the output voltages of the SZPNGs and demonstrated the existence of an optimal a-Si thickness for maximizing the output voltage. Overall, the SZPNGs generated higher output voltages than a conventional ZnO-based piezoelectric nanogenerator (ZPNG) lacking an a-Si interlayer, indicating enhanced performance. In particular, the SZPNG based on the optimal a-Si thickness exhibited a sixfold higher output voltage compared with the conventional ZPNG. This improved performance was ascribed to a combination of the Schottky barrier at the ITO/a-Si interface, preventing the screening effect and the relatively high dielectric constant (εr≈13) of a-Si, minimizing the loss of the piezoelectric potential induced in the ZnO layer. The results herein are expected to assist the development of even more advanced ZnO-based piezoelectric nanogenerators in the future.

A Feasibility Study of Fabrication of Piezoelectric Energy Harvesters on Commercially Available Aluminum Foil

“…In particular, the sample B of the Ag/ZnO/AlN/Cr/ITO/Al-foil structure showed the highest output voltage. Similarly to other reports [12][13][14], we fabricated additional devices including each material (AlN, SiO 2 , or a-Si) used in our earlier researches for a further study, as shown in Figure 10. Here, each target of AlN (5N, iTASCO), n-type a-Si (5N, iTASCO), or SiO 2 (4N, iTASCO) were used to fabricate the devices shown in Figure 10.…”

“…For the ZnO deposition in particular, a mixture of 18 sccm Ar and 12 sccm N 2 O gas was flowing during the sputtering process to reduce the free-carrier concentration in the ZnO film, leading directly to the improvement of device performance [16]. Many previously reported devices fabricated on flexible plastic substrates [10,[12][13][14]16,17] have an intrinsic limitation to increasing process temperatures. Thus, in this work, instead of flexible plastic substrates, we have attempted to employ the Al-foil as a substrate to further increase the process temperatures, which helpfully allows for annealing heat treatments at higher temperatures.…”

“…However, lead-free ZnO-based piezoelectric energy harvesters produce lower output voltages than lead zirconate titanate (PZT)-based harvesters, containing toxic piezoelectric materials [7]. Many research results have reported on various piezoelectric energy harvesters using polyethylene naphthalate (PEN), polyethylene terephthalate (PET), various metal wires, various flexible substrates, etc., in order to enhance the output voltages [8][9][10][11][12][13][14][15][16]. Also, various piezoelectric energy harvesters have been fabricated which exploit insulators to inhibit the so-called screening effect of free carriers, compensating for the positively polarized parts of ZnO, thus reducing the piezoelectric effect [7,8,11,13,14].…”

“…Many research results have reported on various piezoelectric energy harvesters using polyethylene naphthalate (PEN), polyethylene terephthalate (PET), various metal wires, various flexible substrates, etc., in order to enhance the output voltages [8][9][10][11][12][13][14][15][16]. Also, various piezoelectric energy harvesters have been fabricated which exploit insulators to inhibit the so-called screening effect of free carriers, compensating for the positively polarized parts of ZnO, thus reducing the piezoelectric effect [7,8,11,13,14]. To suppress the screening effect, wide-band-gap or p-type semiconductor materials have been combined with ZnO [7,8,11,12,[15][16][17].…”