Yoshinori Yukimoto

Tandem type amorphous solar cells

Abstract An efficiency of 8.6 % by the three-stacked tandem type amorphous solar cell which consists of a-Si:H and a-SiGe:H with a size of 10 by 10 cm was obtained. This tandem type solar cell showed stable characteristics for the prolonged light exposure.

High speed high voltage static induction thyristor

We have developed a new thyristor called Static Induction Thyristor (SI Thyristor) which showed a short fall time of current (0.1 µsec) and a high forward blocking voltage (700 V). The SI thyristor has a combined structure of an N channel Static Induction Transistor and a PNP transistor. It has both capabilities of gate turn-on and gate turn-off according to the value of gate biasing voltage. We have investigated the turn-off mechanism for the high voltage device and have clarified the relation between the device parameters and carrier decaying time responsible for current fall time. From these investigation, we have improved the switching characteristics of high voltage device.

Study of Tin Diffusion into Silicon by Backscattering Analysis

Diffusion of tin into Si from tin-doped oxide is studied by the backscattering and channeling analyses. The depth distribution of tin in Si can be fitted to complementary error functions. The diffusion coefficient can be expressed as D=0.054exp (-3.5eV/kT). The solubility limit in Si and the segregation constant between Si and SiO2 are estimated to be 6~8×1019/cm3 and 0.066 at 1100°C~1200°C, respectively. The lattice location of tin in Si is also determined by angular scan, and it is found that more than 90% of tin atoms occupy substitutional sites. Uniformity of the concentration over a silicon wafer is also checked with the microanalysis system by using backscattering and is found to be good. Effect of post-diffusion of phosphorous or boron on the depth profile of tin is also studied.

Amorphous SiGe: H for High Performance Solar Cells

A preliminary study has been carried out on electrical and optical properties of amorphous silicon-germanium mixed films (a-SiGe: H) deposited by a glow discharge plasma reaction. As a germanium content in the film increases, an optical absorption coefficient increases and an optical gap energy Egopt. decreases. Growth rate of a-SiGe: H films increases with the germanium content in the film and reaches to the value for pure a-Ge: H films about 3 times larger than that of a pure a-Si: H film. Film properties were examined by dark- and photoconductivity, MOS FET transistor characteristics, and photoluminescence. Photoconductivity and carrier mobilities decreases drastically when the mole ratio GeH4/(GeH4+SiH4) exceeds 25%. Solar cells of various structures using a-SiGe: H and a-Si: H films were fabricated and compared their photovoltaic properties with that of a simple p-i-n type a-Si: H solar cell. Tandem structure with a-Si: H p-i-n and a-SiGe: H p-i-n cells shows a broadened spectral response at long wave length region and an open circuit voltage of 1.13 V, about twice of a single cell.

Staebler-Wronski effects in hydrogenated amorphous Si1−xGex

Abstract Photo-induced defects in hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous Si1−xGex (a-Si1−xGex:H) created by prolonged light exposure and their effecton opto-electronic properties are studied. The nearly constant carrier lifetime in a-Si1−xGex:H with increasing germanium content shows that the states created by germanium dangling bonds in as-deposited a-Si1−xGex:H are situated near midband and that the spread of their distribution within the band gap may be small compared with that of the photo-induced metastable defect states. A relatively small Staebler-Wronski effect is observed in a-Si1−xGex:H. However, the density of photo-induced states in a-Si1−xGex:H is of about the same order of magnitude as that in a-Si:H. The reason for this seems to be that the large density of states created by germanium dangling bonds masks the effect of light-induced defects.

The Origin and Structures of the Anomalous Defects Observed in Silicon Crystals

The structure of defects in silicon crystals, which cause anomalous etching phenomenon and X-ray double images, is studied in detail by X-ray diffraction topography and etching technique. If one uses a particular etching method, the etching phenomenon is a sensitive reflection of the defect structure. Generation of these defects is related to the content of hydrogen gas flow used as the ambient and not to the thermal gradient in the crystal during growth. The images of the defects resemble to those of inclusions with spherical symmetry. When a specimen was annealed in the range from 1000°C to 1200°C, the anomalous etching phenomenon diminishes considerably and the double image changes to various structures such as disc, pair of dot arrays, and concentric multi-loops. A model is proposed to explain the observed experimental results. The differences in the images between the floating zoned crystals and the pulled crystals and between the prismatic loops and the concentric loops are explained in terms of the cooling rates and the vacancy concentration, respectively.

Amorphous Solar Cells Using a-Si: H and a-SiGe: H Films

The electric properties of optimized a-SiGe: H films deposited by an R.F. glow discharge in (SiH4+GeH4) and application of these films to the tandem type solar cells were investigated. The dark-conductivity of a-SiGe: H films increase with increasing Ge content and follows quite well to the Meyer-Neldel rule with measured activation energies. Fermi level which was deduced from the activation energy shifts toward mid-band gap under the optimized growth condition. The ratio σph/µn which is related to the lifetime of photogenerated carriers was found nearly constant for various Ge contents in the a-SiGe: H films. Inverted P-I-N type amorphous solar cell with an intrinsic a-SiGe: H film shows higher spectral response in the red region than other kind of a-Si: H aolar cells. The highest conversion efficiency of 8.5% has been obtained by the three junctions stacked cell composed of two inverted a-Si: H P-I-N cells and one inverted P-I-N cell with an intrinsic a-SiGe: H film.

Defects Induced by Deep Diffusion of Phosphorus into Silicon

Defects induced by diffusion of phosphorus in silicon have been found to develop to considerable distances beyond the diffusion front. The extension of dislocations into silicon has been found to depend on the surface treatment before diffusion. The X-ray diffraction topography has been used to study the structures and distribution of the lattice imperfections. When phosphorus is diffused in dislocation free silicon crystals, separated dislocation loops and precipitates as well as the ordinary dislocations are observed. Dislocation loops consist of hexagonal loops and disc shaped ones which have 1/3 type Burgers vectors. The precipitate has an elastic strain with circular symmetry and may be considered to be phosphorus or phosphorus rich silicon. The mechanisms of the generation of these defects are briefly discussed.

High reliability tandem type amorphous solar cells fabricated by multi reaction chamber system

Abstract Mechanical stress and hydrogen bonding states are investigated in hydrogenated amorphous silicon binary alloys used for tandem type amorphous solar cells. Magnitude of compressive stress is found to be proportional to the concentration of SiH bonds, content of germanium, and volume fraction of μc-Si in the amorphous network, and inversely proportional to the concentration of SiH 2 bonds, (SiH 2 ) n bonds, phosphorus atoms and boron atoms.

Design and Performances of a Triple (GaAs, Si, and Ge)- Solar-cell System with Multi-layered Spectrum Splitters

Calculation has been performed to predict the ultimate conversion efficiency of various split-spectrum multiple-solar cell systems. Based on this analysis, a triple-cell system is designed and fabricated, in which the solar radiation is divided by multi-layered filters into three beams of different spectral bands; the beams are then converted separately by GaAs-, Si-, and Ge-cell. The total conversion efficiency of 22.7% is achieved. The factors determining the efficiency are examined to clarify possible methods of attaining higher efficiencies.