Research projects
In recent years, energy and environmental problems such as global warming and the exhaustion of fossil fuel resources have come to prominence. The legally binding Kyoto Protocol, of which everyone will be well aware, was adopted at the third conference of the parties to the United Nations Framework Convention on Climate Change in 1997, at which Japan served as the chairman country. It obliges advanced countries to reduce emissions of carbon dioxide, which is the primary cause of the greenhouse effect. In this respect, photovoltaic generation, which converts the limitless supply of energy from the sun into electrical power, is expected to become more and more important since it is an environmentally friendly and clean energy source. It is said that the amount of solar energy that streams down onto the earth's surface in one hour corresponds to the total energy consumed on the earth in one year. Development of photovoltaic generation technology, which started in earnest with the first oil crisis in 1973, has showed steady results up to today; worldwide annual energy production using this technology reached 1.7GW in 2005. Japan is among the top 50% of photovoltaic generating countries in terms of output but still has a long way to go before solar energy becomes the principal energy source. The “Photovoltaic generation roadmap”, PV2030, settled on by the New Energy and Industrial Technology Development Organization of Japan (NEDO), aims at production of 100GW, i.e. about 10% of all power generation, using photovoltaic generation by the year 2030, and it is envisaged that this will account for 50% of domestic electric power.
The materials used in solar cells can be classified into crystalline silicon, amorphous silicon and compound semiconductors. Currently, crystalline silicon solar cells account for 90% or more of the total production, and of these, about 50% are polycrystalline silicon (“polysilicon”). A polycrystalline material is composed a large number of single crystals with different crystal orientations. The boundaries between these individual crystals, or grains, are known as grain boundaries. Under research conditions, the efficiency of light-to-electrical energy conversion of single-crystal silicon solar cells reaches approximately 24%, whereas polysilicon cells only have a conversion efficiency of approximately 19%. Thus, considered merely from the point of view of efficiency, polysilicon is inferior to single-crystal silicon. However, polysilicon costs less to produce and is more suitable for large-scale production, so for widespread application of photovoltaic generation and to meet production targets, continuous research and development, aimed at improving the conversion efficiency of polysilicon solar cells and reducing costs, is both necessary and important.
Most of the research so far on solar cells can be classed into two approaches, one from the point of view of system design and one from a materials point of view. Our research on increasing the efficiency of solar cells falls into the materials science class, and particularly focuses on the role of grain boundaries. When sunlight irradiates the surface of a silicon crystal, light energy causes negatively charged electrons and positively charged holes to be produced (photoexcitation). In solar cells that use p-type silicon, the surface layer becomes an n-type layer and a p-n junction is formed. The electron-hole pairs formed by photoexcitation migrate in opposite directions as a result of the internal electric field of the p-n junction, and photovoltaic power is produced. However, the grain boundaries present in polysilicon act as preferential sites for recombination of electrons and holes, resulting in a loss of carriers (bulk recombination loss). In addition, grain boundaries act as potential barriers; the resistance that they exert on the flow of carriers causes energy to be lost as Joule heat (resistance loss). These two loss processes are the main reason for the reduced conversion efficiency of solar cells made of polysilicon, as compared to those made of single-crystal silicon.
The structure and properties of individual grain boundaries depend on the geometrical orientation relationship between the two grains meeting at the boundary. Ongoing basic research has demonstrated that various mechanical and functional properties depend markedly on grain boundary structure and character. Thus, all grain boundaries do not necessarily contribute equally to the energy conversion losses described above. To improve the conversion efficiency of polysilicon, it is important to elucidate the relationship of grain boundary character and structure to the elecrical properties involved in recombination and resistance losses. A further important task is to design, based on these findings, crystal growth and processing techniques capable of controlling the distribution of grain boundaries to produce an optimised microstructure in the polysilicon substrate. In this research group, state-of-the-art research equipment, including the latest electron microscopes, will be used to design, control and characterise polysilicon microstructures with the aim of improving the efficiency of polysilicon solar cells.
This is a new, endowed research laboratory set up thanks to the financial support of Fuji Electric Systems Ltd., which began the production of amorphous silicon solar cells in Nankan-cho, Kumamoto Prefecture, in 2006. Kumamoto Prefecture, as part of its environmental initiatives, is promoting the solar industry and research and education in the solar energy field with the aim of becoming a base of research into solar energy production.
Principal Researcher
Prof. Sadahiro TSUREKAWA (Kumamoto University, Faculty of Engineering)Term of project
Academic years 2007－2011 (April 2007 - March 2011)Outline of survey
The mechanical and functional properties of materials depend significantly on their microstructure, so it is important to develop optimised microstructures to obtain desired materials properties. Traditionally, thermomechanical treatments have been applied to control microstructures in metallic materials. A new strategy for more precise control of microstructure by the application of a magnetic field during processing has recently been proposed (electromagnetic processing of materials, EPM). Extensive studies have demonstrated the effect of applied magnetic fields on many metallurgical phenomena such as recrystallization, phase transformation and precipitation. However, the origin of the magnetic field effects observed is not necessarily fully understood, and few reliable data are available on the influence of magnetic fields on fundamental phenomena such as diffusion and the energies of grain boundaries and interfaces. One main motivation of the current work is therefore to investigate the mechanism by which a magnetic field can exert an influence on such fundamental phenomena, and to generate basic data to help to elucidate the origin of the magnetic-field effects observed. A second is the application of EPM to practical materials processing.
Expected results
A key issue is to establish an “electromagnetic science of materials”, which will act as a guiding principle for EPM to enable innovation in this research field. The results obtained in the current study will contribute to this goal, and give some points of departure for the application of magnetic fields to practical materials processing to achieve enhanced properties and performance of materials through precise control of microstructures.
References by the principal researcher
- H. Fujii, S. Tsurekawa, T. Matsuzaki and T. Watanabe: Evolution of a sharp {110} texture in microcrystalline Fe78Si9B13 during magnetic crystallization from the amorphous phase, Phil. Mag. Lett., 86 (2006), 113 – 122.
- S. Tsurekawa, K. Okamoto, K. Kawahara and T. Watanabe: The Control of Grain Boundary Segregation and Segregation-Induced Brittleness in Iron by the Application of a Magnetic Field, J. Mater. Sci., 40 (2005), 895 – 901.