Development of zinc oxide nanowire arrays on flexible conductive substrates for energy applications

Abstract Nano/micro scale devices have attracted a lot of interest due to the emergence of wearable/portable devices. One of the challenging tasks in the miniaturization is to reduce the size and weight of the powering unit. Harvesting mechanical energy and making the device a self-powered one, not only helps in reducing the size/weight ratio but also in designing a maintenance free and sustainable device. Piezoelectric energy harvesting research has gained new momentum with the discovery of piezoelectric charges in semiconducting zinc oxide nanowires (ZnO NWs). Semiconducting ZnO NWs provide an opportunity to integrate with electronic devices and circuits directly unlike non-conducting traditional piezoelectric materials. The coupling of piezoelectric and semiconducting properties was used to design energy generating devices called nanogenerators (NGs). The basic working principle involves application of a mechanical force to create a piezopotential across the wurtzite structured NWs and this piezopotential is channelled employing metal-semiconducting pathways such as p-n junctions. These junctions also play a key role in various other devices such as solar cells, capacitors, fuel cells and water splitting devices. This thesis concentrates mainly on the fabrication of semiconducting piezoelectric nanowires on functionalised flexible substrates and the junctions thereby obtained. It is based on the idea that ZnO NWs can be grown directly on poly(3,4-ethylendioxydithiophene) (PEDOT) or graphene-functionalised substrates using low temperature aqueous synthesis. ZnO NWs can be fabricated using a low temperature aqueous processing route on flexible substrates and fibres. ZnO creates a wide variety of nanostructures due to the polar terminating layers and the surface chemistry of the substrate. The position of the substrate in the growth solution was therefore investigated and found to dictate the morphology and aspect ratio of the nanostructure in seed mediated low temperature aqueous synthesis on polyethersulfone (PES)-based flexible substrates. Vapour phase polymerisation was used to fabricate PEDOT coated 2-D and 3-D PES. To produce graphene-coated flexible substrates, colloidal graphene was synthesized and functionalised onto 2-D and 3-D PES using layer by layer technique (LbL) with polyelectrolytes such as polyallylamine hydrochloride (PAH) and polystyrenesulfonate (PSS). The LbL modification was achieved by exploiting the surface functional groups in the colloidal graphene. Various surface treatments and heat treatments were carried out to tune the system to obtain higher conductivity. ZnO seed solution was coated and NWs were grown on the functionalized substrates. The newly formed junctions were characterised for their I-V characteristics to determine if they have similar function to junctions formed with ZnO on ITO or metals. ZnO NWs grown on PEDOT shows an ohmic contact and gives linear I-V characteristics. On the other hand when a PEDOT coated substrate was made to form a junction at the top of the ZnO NWs, it forms a Schottky contact and gives rectification. However the ZnO-graphene interface shows a Schottky contact. When a top graphene electrode was made to form a junction with ZnO NWs grown on graphene, the I-V characteristics shows a symmetrical and rectifying junction on both sides. Nanogenerators were designed and tested using ZnO NWs grown on PEDOT coated 2-D and 3-D PES. Thus, the fabricated PEDOT-NGs produced a higher current by a factor of 106 and a 102 times increase in the voltage compared to the traditional ITO grown NG design. Vapour phase polymerised PEDOT on flexible substrates eliminated the use of expensive and less efficient electrodes such as ITO and Au. It has also been shown that this approach can be extended to fibre substrates by sandwiching them between PEDOT sheets which make them more suitable for wearable energy harvesting with 102 times improved efficiency compared to ITO sandwiched fibre NG. The higher performance of PEDOT NGs was accounted by the new junctions formed at the interfaces which reduce the screening of free charge carriers in the system. Graphene NGs were fabricated using gold top electrodes. The NG fabricated on surface treated PES was found to outperform the NG fabricated without surface treatment due to the higher conductivity of the surface treated electrode. The output of the surface treated NG was found to be much less than the ITO based or PEDOT based NGs.