Hisao Haku

Effects of very high hydrogen dilution at low temperature on hydrogenated amorphous silicon germanium

The effects of hydrogen dilution of up to 54:1 (=H2: SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) were investigated at substrate temperatures 10−5 Ω−1 cm−1 and silicon dihydride content (<2 at.%) of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties deteriorate with our conventional low hydrogen dilution conditions at a substrate temperature range <200°C. And this high-quality a-SiGe:H film was applied to the bottom photovoltaic layer of a glass superstrate type a-Si/a-SiGe tandem solar cell submodule (30 cm×40 cm), and a stabilized efficiency of 9.5% (light-soaked and measured at Japan Quality Assurance organization (JQA)) was achieved.

Efficiency evaluation of a-Si and c-Si solar cells for outdoor use

We have achieved the worlds highest stabilized efficiency of 8.9% for an a-Si single-junction solar cell (1 cm/sup 2/) and 10.6% for an a-Si/a-SiGe tandem solar cell (1 cm/sup 2/). To apply these results to practical outdoor power use, the annual output power of a-Si single-junction (a-Si) and tandem (a-Si/a-Si) solar cells and a crystalline silicon (c-Si) solar cell was calculated based on annual meteorological data and the solar spectrum in Tokyo, Japan, including the effect of the output power dependence on temperature, incident irradiance and solar spectrum. As a result, this simulation revealed that the annual change of efficiency for the c-Si solar cell is most affected by the solar spectrum among the three types of solar cells, and the annual fluctuations of the a-Si and a-Si/a-Si solar cells are mostly caused by recovery by the annealing effect.

Investigation of Hydrogenated Amorphous Silicon Germanium Fabricated under High Hydrogen Dilution and Low Deposition Temperature Conditions for Stable Solar Cells

The effects of hydrogen dilution of up to 54:1 (=H2:SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) were investigated while keeping the optical gap (Eopt) constant. It was found that deterioration of the film properties of a-SiGe:H due to a decrease in substrate temperature (Ts) can be compensated by the high hydrogen dilution method. As Ts decreases from 230°C to 180°C, the high photoconductivity [~1×10-5 (Ωcm)-1] and low silicon dihydride content (~1 at.%) of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties deteriorate with the conventional low hydrogen dilution ratio of 2.5:1. Probably, hydrogen radicals supply the energy required for the surface reaction during a-SiGe:H deposition which is lost when Ts is decreased. This tendency is useful for solar cell fabrication, especially for superstrate-type a-Si/a-SiGe tandem solar cells, because the decrease in the deposition temperature of a-SiGe:H for the bottom photovoltaic layer can reduce damage to the underlying layers caused by a high deposition temperature. As a result of applying this technique to the fabrication process of an a-Si/a-SiGe stacked solar cell submodule (area: 1200 cm2), the worlds highest stabilized efficiency of 9.5% (light-soaked and measured at JQA) was achieved.

A More Than 18% Efficiency Hit Structure a-Si/c-Si Solar Cell Using Artificially Constructed Junction (ACJ)

We have obtained the worlds highest total area conversion efficiency of 11.1% for a 100cm 2 integrated-type single-junction a-Si solar cell submodule. This was achieved by the development of various advanced technologies, such as a new ultra-thin i/n interface layer and a new laser patterning method using an ablation phenomenon. To acheive further improvement in the conversion efficiency of a-Si based solar cells, we focus on polycrystalline silicon (poly-Si) thin-film for a-Si/poly-Si tandem solar cells. As far as material technology is concerned, we have used a new solid phase crystallization (SPC) method from amorphous silicon (a-Si) films deposited by plasma-CVD. The maximum mobility of 623 cm 2 /V.s was achieved on textured substrates at a carrier concentration of 3.0 × 10 15 cm -3 . This film has been applied to the active layer of poly-Si solar cells on metal substrates and a conversion efficiency of 6.2% has been obtained with poly-Si film of 12 μm thickness made by SPC at 600°C. In the field of device technology, we have developed new artificially constructed junction (ACJ) solar cells using p-type a-Si/i-type a-Si/n-type crystalline silicon (c-Si). We call this a HIT (Heterojunction with Intrinsic Ihin-layer) structure, and we have achieved a conversion efficiency of 18.1% for this type of solar cells. This is the highest reported value for a cell with a junction fabricated at low temperature (∼ 120°C).

Controlling hydrogen contents and bond configurations for high-quality and high-reliability a-Si films

Abstract We previously reported that a reduction of SiH 2 bond density in the i-layer effectively prevents light-induced degradation of a-Si solar cells. For further reduction of SiH 2 bond density, we fabricate a-Si films at higher substrate temperature (Ts:250 ∼ 400°C) with a glow discharge method by using super chamber. SiH 2 bond density can be reduced to about 10 20 cm −3 , and highly photoconductive and stable a-Si films are obtained. As a new attempt to reduce SiH 2 bond density, we have investigated an ion-gun CVD method. Hydrogen bond configurations are found to be strongly affected by the accelerating voltage of ions.

Approach from a-Ge Films for Development of High-Quality a-SiGe Films

a-Ge:H films fabricated by means of a separated ultrahigh-vacuum reaction chamber, called the super chamber, were systematically studied. In the conventional glow-discharge method, there is a very narrow substrate-temperature region for fabricating high-density a-Ge:H films; that is, a minimum deposition rate and a maximum refractive index were obtained at about 250°C. From the point of view of optoelectrical properties, it was clear that not only rigidity of the film network but also total hydrogen content are important. In order to satisfy the two above-mentioned factors simultaneously, a low substrate-temperature high hydrogen-dilution method was effective, and film properties of a-Ge:H were largely improved; δd~4.0×10-5 Ω-1 cm-1, δph~1.5×10-4 Ω-1 cm-1, B value ~803 (eVcm)-1/2, and the ESR spin density ~1.5×1017 cm-3.

Preparation and Properties of a-SiGe:H Films Fabricated with a Super Chamber (Separated Ultra-High Vacuum Reaction Chamber)

High-quality a-SiGe:H films with low impurity concentrations were studied using a separated ultra-high vacuum reaction chamber system called the super chamber. The ESR spin density and the tail characteristic energy of the a-SiGe films (Eopt 1.5 eV) were 6.5×1015 cm-3 and 46 meV, respectively. These values were much lower than those for films fabricated in a conventional chamber as well as those for a-Si films. Structural properties, such as the refractive indices and thermal effusion of hydrogen, were also measured. The results suggest that impurity reduction contributed not only to an improvement in the optoelectrical properties, but also the formation of a rigid a-SiGe network.

High quality a-SiGe alloys prepared by new methods - a-SiGe:H:F and superlattice structure fabricated by photo-CVD -

Abstract For the purpose of improving the conversion efficiency of multi-bandgap a-Si solar cells, high quality amorphous silicon alloy materials were systematically investigated. a-SiGe:H:F films were fabricated by glow discharge decomposition and their structures were analyzed by Raman spectroscopy. a-SiGe:H films and a-SiGe:H:F films were also fabricated by the photo-CVD method for the first time. Furthermore, as a new trial for alloy material, an amorphous superlattice structure was fabricated by the photo-CVD method for the first time.

Development of Stable a-Si/a-SiGe Tandem Solar Cell Submodules Deposited by a Very High Hydrogen Dilution at Low Temperature

The worlds highest stabilized efficiency of 9.5% (light-soaked and measured by the Japan Quality Assurance Organization (JQA)) for an a-Si/a-SiGe superstrate-type solar cell submodule (area: 1200 cm 2 ) has been achieved. This value was obtained by investigating the effects of very-high hydrogen dilution of up to 54:1 (= H 2 : SiH 4 ) on hydrogenated amorphous silicon germanium (a-SiGe:H) deposition at a low substrate temperature (T s ). It was found that deterioration of the film properties of a-SiGe:H when T s decreases under low hydrogen dilution conditions can be suppressed by the high hydrogen dilution. This finding probably indicates that the energy provided by hydrogen radicals substitutes for the lost energy caused by the decrease in T s and that sufficient surface reactions can occur. In addition, results from an estimation of the hydrogen and germanium contents of a-SiGe:H suggest the occurrence of some kinds of structural variations by the high hydrogen dilution. A guideline for optimization of a-SiGe:H films for solar cells can be presented on the basis of the experimental results. The possibility of a-SiGe:H as a narrow gap material for a-Si stacked solar cells in contrast with microcrystalline silicon (μ c-Si:H) will also be discussed from various standpoints. At present, a-SiGe:H is considered to have an advantage over μ1 c-Si:H.

Development of Localized Plasma Confinement (LPC) CVD method for high rate and uniform deposition of thin-film crystalline Si

A Localized Plasma Confinement (LPC) CVD method was newly developed. The special cathode, which has periodically arranged pyramid-nozzles and pumping holes, enables stable plasma generation under very high-pressure (1,000-2,000Pa) conditions. We could fabricate uniform and high quality muc-Si films with very high deposition rates and very high gas utilization efficiencies by using LPC-CVD. The maximum deposition rates of 4.1 nm/s for muc-Si and 5.7 nm/s for a-Si have been also achieved. This method is expected to be effective for larger-area deposition