Photovoltaics (electric power generation directly from sunlight using solar cells) have attracted worldwide attention as a new energy source because of its cleanness and potential contributing to energy supply. For widespread introduction of photovoltaic power generation, further improvements in the conversion efficiency and reducing the cost of the solar cells are necessary. This paper reviews potential and technical problems of silicon solar cells as one of the most promising next generation solar cells. As next generation solar cells, silicon thin films (amorphous, microcrystalline and poly-crystalline) and solar cells should be researched and developed actively for widespread applications of photovoltaics.
We achieved the highest conversion efficiency of 9.5% in a large area amorphous silicon (a-Si) solar cell (40 cm×30 cm) for the first time in the world. It was achieved by employing surface technologies, such as, the optical confinement structure, the high quality a-Si fabrication technique, and high quality a-Si germanium technology. Using the a-Si technology HIT solar cell that is a hybrid of a-Si and c-Si was also developed. The conversion efficiency of a module using HIT solar cells is 15.2% that is the highest for the products. In addition to high conversion efficiency the HIT structure solar cell has an improved temperature dependence compared to conventional c-Si cells.
Photovoltaic devices are recognized as a prospective electricity source in the 21st century. Microcrystalline silicon has attained intense interest because of its application to stable, high-efficiency and low cost solar cells. The microcrystalline silicon is prepared by nonequilibrium processes at low temperatures. In this article, we review the low temperature growth of crystalline silicon including both the microcrystalline and homoepitaxial growth, which are dominated not only by surface reactions on the growing surface but also by gaseous phase reactions among electrons, source gases and radicals. A novel aspect in the nonequilibrium process is that the homogeneous termination on the surface promotes the crystal growth unlike the MBE case. We also demonstrate a successful application of the low temperature process to the manufacture of high efficiency solar cells.
Research and development of our thin film Si solar cells are reviewed. By combining a poly-Si cell with an a-Si cell both of which are fabricated by plasma chemical vapor deposition (CVD) at low temperatures, a stabilized efficiency of 12% has been achieved for a-Si : H/poly-Si/poly-Si cell structure. The stabilized efficiency of 11.3% has been achieved for 10 segments of a-Si/poly-Si stacked cell. The segments are monolithically interconnected in series and have an area of 25 cm2.
Advanced excimer-laser-annealing (ELA) technologies of Si thin films on glass have been reviewed. ELA-induced lateral grain growth seems attractive from an application view point since grains more than several microns in length can be grown by single shot of an excimer-laser light pulse. There are two technological ways for enhancing lateral growth kinetics. The one is introduction of non-uniform light intensity on the sample surface. Different values of thermal energy density stored in the molten Si film results in its solidification time delay along the sample surface, which triggers the long lateral grain growth. The other is an application of a non-uniform sample structure, which causes a non-uniform heat removal rate, resulting in the solidification time delay. An underlayer plays also very important role in ELA. By using organic SOG as the underlayer, grains more than 20 microns in length could be grown, and by using porous silica the grain size became more than 0.1 mm. These lateral growth technologies seem applicable not only to thin-film transistors but also to solar cells.
In order to achieve high energy conversion efficiency of the ruthenium-dye (Dye 1)-sensitized TiO2 solar cells, it is required to control each interface among meso-porous-TiO2 film as an electron transport layer, dye molecules as a charge carrier generator, and I-/I3-redox solution as a hole transport layer. This review article deals with our researches on the electronic control of the interface structures of the dye-sensitized solar cells.
We describe the features of a polarization-dependent total-reflection fluorescence EXAFS technique (TPRF EXAFS) and its application to Mo/TiO2(110) systems. This technique uses the polarization dependence of EXAFS expressed by the following equation, χ(k)=3Σicos2θi · χi(k), where θi and χi are an angle between electric polarization vector and the i-th bond direction and an EXAFS oscillation accompanying i-th bond, respectively. By changing the orientation of the sample against polarization vector, we can obtain 3-dimensional information of the chemical species dispersed on surface. TPRF EXAFS has revealed how Mo species on TiO2(110) changes its structure depending on the preparation conditions. Mo dimer species is stabilized under oxidative conditions while Mo chain is present under reductive conditions. We would like to stress that the TPRF EXAFS has become a practical tool for a characterization of 3 dimensional local structures of active sites on model supports.
Printing has been one of the most important technologies in thousand years because of the reproduction of the Bible. Printing plates called pre-sensitized plate are not familiar compared with ink, paper and printing machines, but have an important role in printing technology. In offset printing, two different zones, ink-acceptant and water-acceptant areas, produce images. Water and ink are supplied to the plate in printing simultaneously and then transferred to paper. This is the essence of offset printing technololgy. Since the thickness of image area is about 1 micron (1/1000 mm), the offset printing is said to be a subtle surface technology. Usually surface-treated aluminum is used as non-image area and photo-sensitive polymers are used as image area. Advances of computer and laser technologies increase dynamically the demand of new plates called CTP (=Computer To Plate) in the printing market.