Takeo Fukatsu

Superlattice structure a-Si films fabricated by the photo-CVD method and their application to solar cells

Amorphous silicon superlattice structure films were fabricated by the photo-CVD method for the first time; also, the structural, optical and electrical properties of the films were investigated. A comparison of the photoluminescence intensities indicated that low damage to the interface was accomplished by using the photo-CVD method. A new type of solar cell was also developed using a superlattice structure as the p-layer of an a-Si solar cell. A conversion efficiency of 10.5% was obtained for a glass/TCO/p-superlattice structure/in/Metal a-Si solar cell.

Preparation and Photovoltaic Characteristics of a-Si Solar Cells Produced by a Consecutive, Separated Reaction Chamber Method

A new fabrication process of a-Si solar cells is presented. This new process is called the consecutive, separated reaction chamber method, and the p, i and n layers can be separately deposited in different reaction chambers. It was confirmed by IMA measurement that the codiffusion and undesirable mixing of donor and acceptor dopants, which always exist in the conventional single reaction chamber, and which result in a deterioration of the cell performance, are clearly removed by the separated reaction chambers. The best conversion efficiency of p-i-n a-Si solar cells prepared by this new method is 8.15%.

Amorphous Si Photovoltaic Cells and Cell Module (Integrated Cell Module)

A new type of the integrated a-Si: H photovoltaic cell module has been developed. The optimization of the cell parameters and dimensions were experimentally determined. The best conversion efficiency was 3.2%. The output power of the p-i-n cell was nearly equal to or larger than that of a crystal Si cell in fluorescent lighting.

Preparation and Properties of a-SiGe:H:F Films by a Glow Discharge Decomposition

As a high quality narrow bandgap material, hydrogenated and fluorinated amorphous silicon germanium (a-SiGe:H:F) by a glow discharge decomposition of SiF4+GeF4+H2 was systematically investigated. Microcrystalline (µc) Ge was found to be obtained when the GeF4 gas flow ratio is high, and µc-Si can be observed when the substrate temperature is high. These results indicate a strong influence of the formation of microcrystalline structures on film properties. More than 107 rectification ratio was obtained for a-SiGe:H:F Schottky diodes at a bias voltage of 1 V. The collection efficiency at the wavelength of 800 nm was improved to 32% with p-SiC:H (hydrogenated amorphous silicon carbide)/i-SiGe:H:F/n-Si:H(hydrogenated amorphous silicon) diodes.

Effects of Surface Conditions of Substrates upon Performance of a-Si Photovoltaic Cells

The growth process of a-Si film deposited onto an ITO substrate by a glow discharge reaction in SiH4 was investigated by transmission electron microscopy. The a-Si film grows to form an island structure at the first step (0–200A-thick) of the growth process. The p-layer of conventional ITO/p-i-n photovoltaic cells has an island structure with a thickness in the range of 50 to 200A. A multi-junction model in which a-Si photovoltaic cells consist of an ITO/p-i-n junction (hetero-face junction) and an ITO/i-n junction (hetero junction) is proposed. The radius of the island structure in the p-type a-Si is calculated with this model. The result of the calculation is in approximate agreement with that of the observation by transmission electron microscopy. The dependence of the open-circuit voltage of Glass/ITO/p-i-n photovoltaic cells upon the p-layer thickness can be explained by the multi-junction model.

Analysis of electrical properties of amorphous silicon films in distinct local environments by the ebic method

Abstract It has been confirmed that the Electron Beam Induced Current (EBIC) technique is effective for the investigation of amorphous Si (a-Si) properties. Appling the EBIC method to an analysis of a-Si properties, it is possible to investigate the fine property distribution with two-dimensional information at a distinct local environment. EBIC images of a-Si films on various kinds of substrates were obtained with the limit of the resolution being about 0.5 μm. The fine property distribution indicated by the EBIC image corresponds well with the unevenness of the surface of a-Si films. Therefore, it has been possible to carry out a morphological study and a property investigation of a-Si films by the EBIC technique.

Amorphous Silicon Solar Cells Produced by a Consecutive, Separated Reaction Chamber Method

A consecutive, separated reaction chamber method has been developed for the fabrication of a-Si solar cells. In this method, p. i, and n layers are deposited in different reaction chambers, and the undesirable doping caused by residual dopant gases which remain in the reaction chamber can be avoided. Following various kinds of fundamental experiments, Glass/SnO2/p(a-SiC)-i-n/Me type a-Si solar cells were fabricated by this method, and the best conversion efficiency was 8.15% with a size of 2mm x 2mm in sunlight of AM-1. The best conversion efficiency for integrated type a-Si solar cells fabricated by this method was 6.35% with a size of 10cm x 10cm. The integrated type a-Si solar cells are being experimentally used in battery chargers, radios, TV receivers, and a 2kW demonstration plant.