A Study on the Growth of ZnO Nanorods for the Application to Dye-sensitized Solar Cells

Abstract

Silicon-based solar cell is a type of solar cell that is widely used today. Although these solar cells are now dominated by silicon material, high production cost difficult fabrication process become one of the obstacles. Besides that, the conventional type of silicon solar cells has a limited supply of silicon raw materials. This is understandable since the price increase due to the increasing demand from silicon semiconductor industry. It becomes worse with the limited supply of silicon. Base on this condition, the manufacture of solar cells made from silicon are considered less efficient in the fabrication process and less affordable in price. To overcome this problem, a creative idea on making a variety of solar cells made from dye-sensitized is needed, which alternatives are expected to be more efficient and affordable than solar cells made from silicon. Previously in 1991 Gratzel and Brian have introduced a solar cell which uses a combination of organic semiconductor materials with inorganic semiconductors known as Dye-sensitized Solar Cell (DSSC). The most apparent advantage from DSSC application is a simple fabrication process without using sophisticated and expensive device so that manufacturing costs will be more affordable. DSSC-base material can be easily obtained. In this study, a more simple and optimum method will be investigated which make DSSC can become more prominent as an alternative energy source in the future. There are many fabrication methods for fabricating DSSC such as vapor and solution phase approaches. However, this research primarily focuses on a solution phase approach. Solution phase approaches based on zinc nitrate and hexamine through waterbath and hydrothermal methods can be carried out under relatively low temperature and pressure. The main advantages of using these methods are simple fabrication method, shorter fabrication duration and low-cost fabrication cost. Through these methods, zinc oxide thin film which consists of vertically aligns zinc oxide nanorods will be synthesized. In this experiment, the role of seed layer on synthesizing vertically align nanorods will be investigated. Based on the comparison from two samples (with seed layer and without seed layer), it can be proven that seed layer can improve the alignment of the zinc oxide nanorods arrays. Without seed layer, the zinc oxide structure will grow on the solution and fall down to the bottom. When the FTO substrate is put on the bottom of precursor solution and facing up, it will have a thin film made from falling zinc oxide nanostructure. The thin film deposited on the substrate coated with seed layer will show well align nanorods arrays. By applying different parameters on seed layer deposition process, the alignment and the morphology of zinc oxide nanorods can be altered. These parameters can be changed by applying different annealing temperature, changing the solution concentration, or changing the thickness of the seed layer. Annealing temperature affect the adhesion strength of the seed layer. Based on this experiment, the optimum annealing temperature is 350 o C. Lower than 350 o C can reduce the adhesion of seed layer which will cause the seed layer fail during the nanorods synthesizing process. On the other side, increasing the temperature higher than 350 o C can cause the zinc oxide particle on the seed layer becomes crystalized and forming nanostructure. The seed layer solution concentration can be changed to control the size and the density of the nanorods. By having more nucleation site (higher concentration), the distance from one rod to neighboring rod will become really close which can initiate coarsening effect. Some rods will grow and diffused together become 1 larger rods. Fewer nucleation sites (lower concentration) will result in smaller diameter nanorod. This condition will also result in many empty areas between the rods. These empty areas between the nanorods will allow nanorods to grow with more inclination which resulting in less alignment. Not only condition of the seed layer which can modify the morphology of nanorods, the substrate position in the growth solution during the synthesizing process can also cause some difference on the morphology of nanorods. Vertically align nanorods were not successfully synthesized through hydrothermal method. It seems that the seed layer failed during the synthesizing process due to high temperature and high pressure. Instead of vertically align nanorods, nano-flower were formed. Waterbath method was successfully used in this research to grow vertically align nanorods. Two different type of zinc (zinc acetate and zinc nitrate) were used to investigate the effect on the nanorods growth. Both type of zinc were managed to grow nanorods with different length. The reaction time from each type of zinc can be the cause of the length difference. The zinc nitrate reaction time is slower the zinc acetate reaction time. To grow nanorods with the same length, zinc nitrate needs longer process. By applying vertically align nanorods of DSSC application, a small efficiency improvement has been achieved. The photovoltaic efficiency has been improved from 1.0% to 1.06%. Using the waterbath method, a shorter fabrication process has been achieved. By applying low temperature synthesizing process, thin film can be grown on many substrates including PTE. Both advantages can reduce the production cost.