SYNTHESIS AND CHARACTERIZATIONS OF ZnO AND ZnO/CNT NANOCOMPOSITES FOR APPLICATION IN SOLAR CELL

ABSTRACT

Low power conversion efficiency resulting from recombination of photogenerated charges in TiO2 based photovoltaic devices has caused serious challenges in photoelectrode efficiency of such devices. Hence the need to replace TiO2 with ZnO and the composite of Zinc Oxide and Carbon Nanotube (ZnO/CNT) at optimised ratio to minimise the loss of photogenerated electrons during the photovoltaic activity is important. Hydrothermal synthesis was used to synthesis nanostructured ZnO and ZnO/CNT in the ratios of 0.1, 0.3 and 0.5 wt%. The synthesised nanostructured ZnO/CNT at different ratios were subjected to optical and structural characterisations using UV-Vis spectroscopy and X-ray diffraction spectroscopy (XRD). It was observed that the presence of CNTs enhanced the optical properties of ZnO with increasing concentration of the CNTs whilst the XRD data showed the presence of nanostructured crystalline phase of geometrical cubic and hexagonal planes of ZnO at (100), (002), (101), (110) and (102). The composite synthesised using hydrothermal synthesis revealed the suitability of the photo electrode to be used in solar cell application.

CHAPTER ONE

1.0    INTRODUCTION

1.1    Background of the Study

During the last two decades, the global  energy demands have rapidly increased and all predictions indicate that this growth will continue in both developed and developing nations (Mohammaznezhad et al., 2018). The effort to reduce energy crisis and global warming has given rise to high demand for highly performing solar energy conversion and energy storage devices (Chandu et al., 2017).

Different forms of fossil fuels including coal, oil and natural gas make up 80% of the world’s energy use, yet they have major environmental impacts starting from climate change. In this scenario, the development of renewable energy sources, for instance solar energy, is crucial and urgent (Mohammaznezhad et al., 2018).

Photovoltaic energy conversion could solve energy problems, as it is reliable, renewable and sustainable globally. Solar energy is one of the cleanest energy resources, with the potential to replace a significant percentage of fossil fuels use due to its availability and abundance. Solar energy can be converted into thermal or electrical energy, or into a liquid fuel like hydrogen (Chandu et al., 2017).

Solar energy is the most abundant source of energy on Earth which is pouring on our planet. The rate at which it is received every minute, suffices to cover the world’s energy demand for a year (Dey et al., 2016). Of all the energy produced, on a daily bases today, solar energy constitute about 1% while substantial fraction comes from the use of non-renewable energy sources which are not sustainable, environmental unfriendly and could even endanger the mere existence of life on Earth (Lee et al., 2015).

To convert solar energy to benefit humanity, suitable devices for such work are required. These devices are called photovoltaic devices. A photovoltaic device must have good photocatalytic capability in order to serve the purpose (Septiani et al., 2017).

Over the past decades, there is an enormous advancement in new materials along with their technological applications in various fields. Semiconducting metal oxide nanomaterials like: ZnO, SnO2, Fe2O3 & TiO2 have emerged as optimistic UV sensitive materials with applications in various optoelectronic devices (Lai et al., 2013). Among them, ZnO is one of the most researched transparent conductive oxides (TCOs). ZnO has wide energy gap of 3.37 eV and large binding energy of exciton of 60 meV. It is biocompatible, highly photostable and is used for the assemblage of blue LEDs and lasers. ZnO is an economically viable TCO to cover the UV spectral band ranging from 240 to 380 nm and is transparent to the visible light (Shkir et al., 2020).

Zinc oxide (ZnO) has received great attention in several fields of application due to its sensitivity to environmental factors such as humidity, gas species and light irradiation. Indeed, such factors can determine physico-chemical changes of ZnO that can be suitably used in photocatalytic apllications (Lee et al., 2016; Mei et al., 2018).

The morphology of ZnO is very interesting due to the rich nanostructures obtained under different methods and synthesis conditions. Hence, these nanostructures offer large potential to be applied in a wide range nanoscale devices (Luo et al., 2016). Along with this, carbon nanotubes (CNTs) were reported as a strong material, even stronger than steel and at the same time, CNTs are really light. Other than that, CNTs display extraordinary electrical, thermal and chemical properties which gives many advantages in optoelectronic devices (Septiani et al., 2017).

On the other hand, the favourable optoelectronic properties of carbon nanotubes (CNTs) have attracted more research interest for photonic applications. CNTs are known for their high electrical conduction, carrier mobility and a wide optical transmittance which can be tunable by the process of doping with other metal oxide materials like ZnO (Xolani et al., 2017).

1.2    Statement of the Research Problem

Photoelectrode in DSSCs made from TiO2  nanoparticles (NPs) films coated onto fluorine- doped tin oxide (FTO) is suitable for its chemical affinity, large surface area for dye adsorption and high energy band gap for good charge transfer between the electrolytes and dye (Fang et al., 2015).

However, the persistent problem of DSSCs is that not all of the photo-generated electrons can arrive at the collecting electrode, because electron transport within the photo-anode nanoparticle film of TiO2 network takes place via a series of jumps to adjacent particles, and the energy damage that occurs during charge transport processes results in reduced power conversion efficiency (PCE) of the DSSC (Fang et al., 2015). However, researchers have tried to solve the recombination problem in TiO2  based DSSCs by increasing the size of the photoanode film thickness but it rather consequently resulted in aggravating the problem by extending the distance which the electrons diffuse through to the TCO collector. In view of this, it is proposed in this study to replace TiO2 with ZnO because unlike TiO2, the life span of electrons in ZnO is generally extended with minimised recombination losses especially if nanostructured ZnO is hydrothermally synthesised (Laurent et al., 2015). This is because electrons can easily be led through a direct path within nanostructures of ZnO rather than by multiple-scattering transport between nanoparticles in TiO2.The electron transport is tens to hundreds of times faster in ZnO based DSSCs than in TiO2 based DSSCs.

Introducing CNTs into the ZnO lattices initiates modification in the density of outermost orbital state and the Fermi level. This causes an enhancement of photo activity of the material thereby increasing its transparency to visible light and conductivity. This consequently addresses the problem of dye anchorage on ZnO film when used as photoanode film in DSSCs. When 1D ZnO nanostructures are hydrothermally synthesised, doped with CNTs, and its ZnO/CNT composite used for photoanode in DSSC, the surface activity of the composite film is greatly enhanced and it in turn boosts the PCE of the DSSC (Deepti et al., 2017).

1.3    Aim and Objectives of the Study

The aim of this study is to hydrothermally synthesise ZnO and use ZnO/CNT nanocomposites for photoanode in photovoltaic devices.

The objectives of the study is to:

i.      synthesise nanostructured ZnO using hydrothermal synthesis.

ii.      fabricate the photoelectrodes of ZnO and ZnO/CNTs composites.

iii.       characterise the prepared samples using x-ray diffraction spectroscopy (XRD) and ultraviolet-visible spectroscopy (UV-Vis).

1.4    Significance of the Study

Combining the bio-safety and environmental friendliness of ZnO with the simplicity and affordability of its synthesis has pave way for entrepreneurial opportunities in production of cosmetics in our immediate environment. The conversion from solar energy to electricity is fulfilled by photovaltaic devices based on the photovoltaic effect. Many photovoltaic devices have already been developed over the past few decades. However, conversion efficiency and cost are the drawbacks limiting its wide-spread use. ZnO photoelectrode can be used in photovoltaic devices, especially when enhanced with CNT, to solve the conversion efficiency problem. The results acquired from this study will also provide additional knowledge for manufacturing photovoltaic devices through the use of cost-effective methods such as hydrothermal method.

1.5    Scope and Limitation of the Study

1.5.1    Scope of the Study

This study considered the synthesis of nanostructured ZnO using hydrothermal synthesis. The study also covered the enhancement of the as-synthesized ZnO with CNTs using direct mixing method of doping. Furthermore, the study also included the fabrication of ZnO and ZnO/CNT photoanodes with 0.1 wt%, 0.3 wt% and 0.5 wt% concentrations of CNT. The fabricated photoanodes were as well characterised using XRD and UV-Visible Spectroscopy.

1.5.2    Limitation of the Study

Scanning Electron Microscopy/Energy Dispersive Spectroscopy (SEM/EDS) was carried out twice at different laboratories but the magnification of the images were poor hence SEM/EDS not presented in here.

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