1.1 Significance of solar cells
One of the most critical issues of today’s society is how to satisfy the ever growing demand with decreasing resource. Nuclear power and thermal power are two main energy sources for regular usage. Nuclear power provides long term and steady energy supplementation; however, its nuclear waste poses serious threat to general health of the environment and society. Thermal power uses large amount of fossil fuel. As a result, excess carbon dioxide and other green house gases are produced as wasteful byproducts of thermal power.
The goal of the Kyoto Protocol of 1997 was to reduce green house gas emissions, or to practice emission trading among the pledged nations in order to achieve emission reduction. To ease the energy crisis, intensive researches are put into exploring alternative energy sources with less pollution and affordable cost. As a result, solar power could be the solution to today’s energy crisis.
The efficiency and capacity of solar power depend on sunlight exposure on the solar cell, and the degree of exposure is affected by climate and latitude.
1.1.1 Current status of solar cells
The current status of solar cells is shown in Fig 1-1. The Si single crystalline is the most popular solar cell for our living. The conversion efficiency of Si single
crystalline is up to 24%. However, a dye-sensitized solar cell could possibly offer better efficiency than Si single crystalline. Such a feature of a dye-sensitized solar cell (DSSC) makes it possible for DSSCs to use low- to medium-purity materials through low-cost processes, and provides commercially realistic energy-conversion efficiency. In addition, it is of great importance that the materials used in DSSCs are eco-friendly.
Fig. 1-1 Current status of solar cells
1.1.2 Working principle of DSSC
In 1991, dye-sensitized nanocrystalline TiO2 solar cells (DSSCs) based on the mechanism of a fast regenerative photoelectrochemical process were first reported by Grätzel et al [1]. The working principle of DSSC is shown in Fig. 1-2. When the
solar cell was excited by incoming light, the original state of the dye is restored by electron donation from the electrolyte, which is usually an organic solvent containing iodide/triioide redox system. The electrons poured into the conduction band of the nanocrystalline TiO2 will eventually reach the anode. The electrons reach the cathode through the load, where they were then absorbed through redox reaction into electrolytic liquid.
Fig. 1-2 Working principle of DSSC [1]
1.2 Motivation
How to produce solar cell at reduced cost has become a critical challenge.
Some new ideas for solar-to-electric energy conversion have challenged the traditional devices based on the p-n junction diode for several years [2]. The efficiencies of the TiO2 nanoparticle-based DSSCs has exceeded 10% [3]. A very important feature of DSSCs is its photoelectrode, which includes mesoporous wide-bandgap oxide semiconductor films with an enormous internal surface area, typically a thousand times larger than that of bulk films. To date, the highest solar-to-electric conversion efficiency has been achieved with films that consist of 20 nm TiO2 nanocrystallites sensitized by ruthenium-based dyes. It is advantageous to replace the TiO2 nanoparticles with dense array of wide-band-gap semiconductor nanowires. The morphology of nanowires provides direct conduction paths for the electrons from the dye to the electrode [4]. The semiconductor ZnO has wide gap (3.37 eV) which is similar to TiO2 and very high electron mobility which is about 155 cm2V–1 s–1 for high quality thin film. The ZnO material has a very high UV emission efficiency at room temperature (free exciton binding energy is 60 meV). It is possible that ZnO may be a new material to construct dye sensitized solar cell [5].
However, to significantly improve energy conversion efficiency of DSSCs remains a challenge. In order to increase efficiency of the nanowire cell, higher dye
loading could be achieved by increasing surface area. Competition between generation and recombination of photoexcited carriers in DSSCs is a technical bottleneck for developing higher conversion efficiency. We are now extending our synthetic strategy to design nanowire electrodes with much larger areas available for dye adsorption.
A solution is to incorporate nanoparticles with original one-dimensional nanostructures [6][7]. The nanowires/nanoparticles composite is mainly ascribed to enrich the light harvesting without sacrificing the electron transport efficiency. Even so, the excess electron hopping through the interparticle barriers still has the chance for charge recombination.
In the present work, we suggested the tree-like ZnO nanowire structures (one-dimensional branches directly attached the main nanowire backbone) could simultaneously afford a direct conduction pathway and achieve higher dye adsorption, therefore significantly enhance the overall efficiency of the DSSC.
The resulting high temperature of fabricating 1-D ZnO nanostructures by the vapor-liquid-solid (VLS) method would create too much resistance within the substrate. Therefore, the aqueous solution method will be first adopted to fabricate ZnO nanowires. Second, the tree-like ZnO nanowire structure was suggested, such that it could afford a direct conduction pathway and could simultaneously achieve
higher dye adsorption. As a result, the overall efficiency of the DSSC was enhanced significantly. It was a considerable success to improve the original conversion efficiency from 0.18% by VLS method up to 1.5% with the tree-like ZnO nanowire structures by chemical bath method.
1.3 Organization of thesis
After the introduction, the thesis includes four other chapters. Chapter 2 discusses the theoretical background of the experiments such as the vapor-liquid-solid method and the chemical bath process, the device operation principles, the electrochemical impedance spectroscopy theory (EIS), and the scanning electron microscopy (SEM), respectively. In Chapter 3, the experimental processes including VLS and solution-liquid-solid (SLS) methods are explored. In Chapter 4, the morphology of ZnO nanowires and I-V curve, EIS, and Photon to Current conversion Efficiency (IPCE) will be investigated. Finally, the final chapter discusses potential future development and concludes the thesis.