Shinichi Muramatsu

Optical and electrical properties of amorphous silicon films prepared by photochemical vapor deposition

Amorphous silicon films have been prepared through mercury‐photosensitized decomposition of monosilane gas at low temperatures. The films show optical and electrical properties comparable with those of the best films prepared by plasma chemical vapor deposition. The feasibility of amorphous solar cells with short‐circuit current densities of more than 10 mA/cm2 has been demonstrated by fabrication of a Schottky barrier structure.

Static concentrator photovoltaic module with prism array

To obtain cost-effective photovoltaic modules, we have developed static prism-array concentrator modules consisting of prism concentrators about 4 mm thick assembled unidirectionally under a 3.2-mm-thick cover glass. Calculating the optical collection efficiency for the annual solar irradiation in Tokyo, we found that the theoretical efficiency of the modules is 94.4% when the geometrical concentration ratio is 1.88 and that it is 89.1% when that ratio is 2.66, respectively. Fabricating prism-array-concentrator modules with a geometrical concentration ratio of 2.66, we obtained a maximum optical collection efficiency of 82% with a flat reflector and 81.7% with a V-grooved reflector.

Thin-film c-Si solar cells prepared by metal- induced crystallization

We prepared a thin-film polycrystalline silicon solar cell using metal-induced crystallization (MIC) of an amorphous silicon film and a thin Ni layer. The MIC using a 0.6-nm-thick Ni layer produced a highly activated n-type crystalline layer at a 5501C annealing temperature. The Ni concentration in the i-layer of a solar cell prepared by successively depositing i- and p-layers on an MIC n-layer using plasma-enhanced CVD was lower than 1 10 16 /cm 3 . This solar cell was highly responsive in the long-wavelength region of its quantum efficiency, indicating that the n/i interface and i-layer region near the n-layer were of high quality. r 2002 Elsevier Science B.V. All rights reserved.

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.

Fabrication of -Oriented Si Film with a Ni/Ti Layer by Metal Induced Crystallization

We used metal-induced-crystallization (MIC) to crystallize amorphous silicon (a-Si) films over 1 µm thick. We employed a Ni/Ti layer for MIC. The a-Si films with the Ni/Ti layer were annealed at 550°C. We found that a 6-µm-thick Si film was oriented in the direction by inserting a Ti layer between Ni and the glass substrate. If the thicknesses of the Ni and Ti layer were nearly the same, the peak intensity ratio of the (220) to (111) planes was 3%. This oriented growth tendency is maintained from the beginning of crystallization to the full-crystallized stage.

Medium-Range Order of Amorphous Silicon Germanium Alloys: Small-Angle X-Ray Scattering Study

A medium-range order of hydrogenated amorphous silicon germanium (a-Si1-xGex:H) alloys is investigated using small-angle X-ray scattering. It is found that the scattering intensities I(h) of a-Si:H and a-Ge:H decrease with scattering vector h as represented by I(h)~h-3. However, in the case of a-Si1-xGex:H alloys, the scattering intensity decreases with h as represented by I(h)~h-P (P<3). This shows that the alloys take on a mass fractal structure. At around x=0.5, the fractal dimension tends to approach the value of 2.

Determination of electric field profiles in amorphous silicon solar cells

Time‐resolved photoconductivity measurements with subnanosecond time resolution are applied to study the electric field profile in amorphous silicon solar cells in the range from 0.3 V forward to 0.8 V reverse bias. The method is used for a comparison of state‐of‐the art devices with different junction design. Optical and electrical contributions to the device performance are discussed and the limitations in improving the performance by use of a‐SiC:H window layers are pointed out.

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.

Mobility-Lifetime Product in Hydrogenated Amorphous Silicon

We report an analysis of the carrier loss process in time-of-flight experiments on amorphous silicon solar cells. The electron mobility-lifetime ( µτ) product is determined by a transitory immobilization of the carriers in deep traps. The carriers can be recovered and collected when the experiment is extended into the ms time range. The collection efficiency is shown to depend on the occupation of gap states. We discuss the discrepancy between µτ values obtained from time-of-flight and photoconductivity measurements and present an analysis which relates the time-of-flight µτ products to the density of shallow and deep states in a-Si:H.

Scanning tunneling microscope study of microcrystalline silicon surfaces in air

Surfaces of microcrystalline silicon films prepared by the glow discharge method have been investigated by a scanning tunneling microscope (STM) in air. Grain‐like structures of 30–80 nm size which correspond to transmission electron microscope data have been observed. The film surface was found to be geometrically rather flat but the structure was observed electrically, that is, the resistivity seemed to be inhomogeneous due to preferential oxidation. Also, degradation of STM images of a HF‐etched microcrystalline silicon surface has been observed for the first time.