Masahiko Sakamoto

Bifacial silicon solar cells with 21·3% front efficiency and 19·8% rear efficiency

We introduced a triode structure with p–n junctions on both sides into single-crystalline bifacial silicon solar cells in order to improve solar cell performance. These fabricated bifacial silicon solar cells have an energy conversion efficiency of 21·3% under front 1 sun illumination (the standard 1 kW/m2 AM 1·5 global spectrum at 25°C) and 19·8% under rear 1 sun illumination tested at the Japan Quality Assurance Organization. The total of the front and rear conversion efficiencies is the highest ever reported for bifacial silicon solar cells. Copyright

Optimization of thermal processing and device design for high-efficiency c-Si solar cells

Abstract To improve the cell performance of single-crystal silicon solar cells, the process conditions have been optimized by monitoring the bulk lifetime after each thermal step in the cell fabrication process. The emitter geometry, i.e., front and rear contact size and pitch were optimized, and the cells were fabricated through a set of environmentally considered processes, especially for surface treatment, oxidation, diffusion, and electrode fabrication. Conversion efficiency of 22.3% in a 4 cm 2 cell, and 22.6% in a 1 cm 2 cell, was attained, respectively, with structural features of SiO 2 single-AR, “inverted-pyramid” fron texture, point-contact with line-emitter for front electrodes, and locally diffused BSF for rear contacts.

Behaviour of oxides during friction stir welding of aluminium alloy and their effect on its mechanical properties

Friction stir welding (FSW) is expected to offer a wide range of advantages not available through diffusion welding or fusion welding, such as conventional arc welding, particularly for joining of aluminium alloys, and since development by the UK Welding Institute in 1991 has been used for no more than 7-9 short years to make railway rolling stock, aircraft, ships, etc. Numerous future applications are envisaged, including motor vehicles, semiconductors, public welfare facilities, etc. Within the context of the intended widespread industrial use of FSW, an important application feature is that the FSW joining mechanism entirely differs from that underlying conventional fusion welding. It is accordingly imperative to clarify the joining mechanism and joint properties of FSW. Over the brief 10-year history of FSW, the metallographic and mechanical properties of joints have been little studied. For example, most of the oxide film formed on the welding material surface in conventional arc welding floats to the weld surface as slag, while some decomposes to form gases discharged to the ambient air as gas bubbles, and welds have been shown to sustain few adverse effects as a result. On the other hand, by FSW, the aluminium alloy is joined below its melting point by plastic deformation and the heat of rotational friction between the rotation tool and work material, and the oxide film on the work material surface is predicted to be retained in the joint through being entrained inside. However, the oxide film of friction stir welds has so far been little documented.

Effect of light degradation on bifacial Si solar cells

We fabricated bifacial-type rear-floating-emitter solar cells and triode solar cells from several types of wafers and investigated the light degradation of solar-cell performance. Rear efficiency of a rear-floating-emitter bifacial solar cell degrades in proportion to bulk-minority carrier lifetime. This degradation of cell efficiency is much larger than that reported so far. Bifacial cells must therefore be made from wafers that are not degraded by light. And it was found that magnetic-field-applied CZ wafers or gallium-doped CZ wafers meet this requirement very well. Three-terminal measurement showed that light degradation does not have much influence on rear efficiency of triode cells compared with that of two-terminal bifacial cells.

Formulation of Power Loss Equation for an Emitter with Dot Contacts and Proposal of an Improved Emitter Structure in Silicon Solar Cells

A new two-dimensional emitter power-loss equation was formulated for a cell with a dot contact and verified numerically. Based on this formulation, the current crowding that occurs in two-dimensional geometry was evaluated. We propose a solar cell structure with line-shaped heavily diffused emitters under the dot contacts. The effectiveness of the proposed emitter geometry was verified by both calculation and measurement, and the formulation was found to be valid.

Development of Micro-Optics Using Total Internal Reflection for an Optical Tracking Sensor

A compact and lightweight optics that can be used as an optical tracking sensor for a large capacity floppy disk drive (FDD) is proposed. The developed micro-optics is featured with three points in construction. One is the total internal reflection (TIR) in the substrate which requires no coating material. Another is a refraction-type aperture which eliminates the unnecessary light. The last is a Fresnel lens with a blazed relief structure which realizes high diffraction efficiency. The optics is made of plastic molding that is suitable for mass production. The aperture and Fresnel lens patterns on the molding die can be easily fabricated by machining.