Hisaki Tarui

Preparation and Properties of High-Quality a-Si Films with a Super Chamber (Separated Ultra-High Vacuum Reaction Chamber)

A separated ultra-high vacuum (UHV) reaction chamber system, called the super chamber, has been newly developed. A background pressure of 10-9 Torr was obtained, and the impurity concentrations of oxygen, nitrogen and carbon in an a-Si film fabricated in the super chamber were 2×1018 cm-3, 1×1017 cm-3, and 2×1018 cm-3, respectively. The space charge density and the ESR spin density of the a-Si film were 5×1014 cm-3 and 2×1015 cm-3, respectively. These values were much lower than those for films fabricated in a conventional chamber. The ratio of the light-induced degradation in the photoconductivity of the a-Si film was also small compared with that of conventional a-Si films. A conversion efficiency of 11.7% was obtained for a glass/textured TCO/pin/Ag a-Si solar cell, whose i-layer was fabricated in the super chamber.

Optimization of a-SiGe:H Alloy Composition for Stable Solar Cells

The film properties and solar cell performance of amorphous SiGe:H (a-SiGe:H) samples have been systematically investigated, using constant optical gap and various compositions of hydrogen and germanium. It was found that the hydrogen content and bonding configurations play important roles in determining both the initial properties and stability. The optimum compositions were clarified for the minimum Urbach tail characteristic energy and defect density in the as-deposited film, and for the maximum conversion efficiency of the solar cells. The stability of a-SiGe single and a-Si/a-SiGe tandem solar cells becomes higher as the hydrogen content of the photovoltaic layer becomes lower. As a result, the optimum composition after light soaking shifts to the region of lower hydrogen content. Applying the above findings to the design of devices, the highest stabilized conversion efficiencies of 3.3% (initial 3.7%) under red light (λ>650 nm) for an a-SiGe single-junction solar cell and 10.6% (initial 11.6%) for an a-Si/a-SiGe tandem solar cell have been achieved (area: 1 cm2).

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.

A New Analytical Method of Amorphous Silicon Solar Cells

A new analytical method for amorphous silicon solar cells, called DICE (dynamic inner collection efficiency), has been developed. The depth profile of the photovoltaic characteristics of solar cells can be obtained by using the DICE method under any operating condition in a non-destructive manner for the first time. The DICE value is defined as the probability that an electron-hole pair generated at a certain depth in the generated region of an a-Si solar cell becomes an output current. In this paper the theory and the calculation method of DICE are described, and the results of applications to practical solar cells are reported. By using the DICE method it was found that carrier recombination at the p/i interface affects the open-circuit voltage.

Film Property Control of Hydrogenated Amorphous Silicon Germanium for Solar Cells

The optoelectric properties of a-SiGe:H alloys, deposited by the plasma chemical vapor deposition (plasma-CVD) method, were investigated with precise measurement of their germanium content (CGe) and hydrogen content (CH). This investigation revealed that the optical gap of a-SiGe:H alloys can be approximated by a linear function of CH and CGe and various combinations of CH and CGe resulted in identical optical gaps. For each optical gap, the optimum composition for the lowest defect density was derived by comparison with the subgap absorptions measured by the constant photocurrent method (CPM). Based on these, the highest conversion efficiency of 3.7% under red light illumination (>650 nm) for a 1 cm2 a-SiGe single-junction solar cell was achieved.

High-Quality p-Type a-SiC Films Obtained by Using a New Doping Gas of B(CH3)3

High-quality p-type a SiC films can be fabricated by using a new type of doping gas, B(CH3)3, instead of B2H6 in a photo-CVD method and a glow discharge method. The photoconductivity and doping efficiency of a-SiC films fabricated by the photo-CVD method are improved by using B(CH3)3. A reduction of tail state density and an increase in photoluminescence are also observed. Furthermore, a bandgap narrowing in highly B-doped a-SiC films fabricated by the glow discharge method can be prevented by using B(CH3)3. A conversion efficiency of 10.0% (total area efficiency of 9.02%) is obtained for a 100 cm2 integrated-type a-Si solar cell whose p-layer was fabricated by the glow discharge method with B(CH3)3.

Preparation and properties of amorphous silicon produced by a consecutive, separated reaction chamber method

Abstract A new fabrication apparatus was developed from the consecutive, separated reaction chamber method in order to fabricate the multi-gap amorphous solar cell. In this fabrication process, the different amorphous materials are deposited in different reaction chambers. It was confirmed by IMA measurement that the intermixing of different amorphous materials was clearly avoided. The space charge density (Ni) of the films, into which a slight amount of boron is doped in this method, was measured. The minimum Ni was about 2 × 1014 cm−3 at the gas ratio B2H6/SiH4 of 2 × 10−6. The best conversion efficiency of p-i-n amorphous solar cells fabricated by this method was 10.0%.

High efficiency a-Si solar cells with a superlattice structure p-layer and stable a-Si solar cells with reduced SiH2 bond density

Abstract In order to improve the conversion efficiency of a-Si solar cells, a superlattice structure p-layer and the use of B(CH 3 ) 3 as a dopant gas have been investigated for the first time. The collection efficiency spectrum of a glass/TCO/p-superlattice structure (a-SiC/a-Si)/in/Metal cell shows a remarkable increase in the short wavelength region. This result suggests that the superlattice structure p-layer is an active photovoltaic layer. It is also found that the photoconductivity of p-type a-Si:H films increases by using B(CH 3 ) 3 as a dopant gas. A conversion efficiency of 10.0 % (module efficiency 9.02 %) is obtained for 10 cm × 10 cm integrated single junction a-Si solar cell submodule using B(CH 3 ) 3 . As for reliability it is found that the light induced degradation of a-Si solar cells can be reduced by simultaneous reduction of the impurity concentrations in a-Si with the super chamber (separated UHV reaction chamber) and the SiH 2 bond density.

a-Si technologies for high efficiency solar cells

Abstract Conversion efficiencies of a-Si solar cells have been significantly improved in recent years, and 12.0% efficiency was achieved for a 10 cm × 10 cm a-Si solar cell submodule. As new trials, a-Si technologies are also applied to the fabrication of crystalline silicon solar cells. This paper summarizes our latest developments in a-Si technologies for high efficiency solar cells, and discusses our view of the future.

High Performance A–Si Solar Cells and Narrow Bandgap Materials

High performance a-Si solar cells were developed. A conversion efficiency of 11.5% was achieved for a textured TCO/p-SiC/in/Ag structure with a size of 1 cm 2 using the high quality i-layer fabricated by a new consecutive, separated reaction chamber apparatus. A conversion efficiency of 9.0% was obtained with a size of 10cm × 10cm. A high quality a-SiGe:H:F, which is a new narrow bandgap material for a-Si solar cells, was fabricated by a glow discharge decomposition of SiF 4 + GeF 4 + H 2 . A photo-CVD method was investigated in order to improve the interface properties of a–Si solar cells. A conversion efficiency of 11.0% was obtained with a solar cell in which the p-layer is fabricated by the photo-CVD method. a-SiGe:H films were fabricated by the photo-CVD method for the first time as a narrow bandgap material for multi-bandgap a-Si solar cells.