Shingo Okamoto

Toward stabilized 10% efficiency of large-area (>5000 cm2) a-Si/a-SiGe tandem solar cells using high-rate deposition

Abstract This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Optimized hydrogen dilution conditions for high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films and thinner i-layer structures have been systematically investigated for improving both the stabilized efficiency and the process throughput. As a result, a high photosensitivity of 10 6 for a-Si:H films has been maintained up to the deposition rate of 15xa0A/s. Furthermore, the worlds highest initial conversion efficiency of 11.2% which corresponds to a stabilized efficiency of about 10% has been achieved for a 8252xa0cm 2 a-Si/a-SiGe tandem solar cell by combining the optimized hydrogen dilution and other successful technologies.

Towards large-area, high-efficiency a-Si/a-SiGe tandem solar cells

Abstract This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Considerable efforts have been devoted to improving both the stabilized efficiency and the production throughput. High-speed patterning using plasma chemical vaporization machining (CVM) has been successfully applied to the a-Si patterning of a 1200xa0cm 2 a-Si integrated-type submodule. The optimization of the hydrogen dilution has led to a high stabilized efficiency of 9.2% for a 1200xa0cm 2 a-Si/a-SiGe tandem submodule whose photovoltaic layers are deposited at 3xa0A/s. A scale-up of the submodule size from 1200 to 5400xa0cm 2 is now being done in a pilot plant, which has been operating since April 1999.

Effects of the i-layer properties and impurity on the performance of a-Si solar cells

Abstract The relation between the opto-electric properties of an a-Si : H i-layer and the performance of a-Si solar cells are extensively investigated, paying careful attention to the controllable range of the opto-electric properties and the effects of the impurities. It is shown that even trace amounts (10 19 cm −3 or less) of oxygen impurity in the i-layer affect the film properties and the solar cell performance. When the impurity concentration and other undesirable factors are suppressed, the mutual relationship among the i-layer properties becomes clear. Although there is a limitation in the controllable range of the properties of device-quality a-Si : H, techniques such as a hydrogen plasma treatment can improve the controllable range. Guidelines to design and optimize the i-layer for solar cells are discussed on the basis of these experimental results. A total-area conversion efficiency of 12.0% is achieved for a 10 cm × 10 cm integrated a-Si solar cell submodule.

Characterization of the defect density and band tail of an a-Si:H i-layer for solar cells by improved CPM measurements

Abstract A new CPM (Constant Photocurrent Method) system, which improves the accuracy of measurement by suppressing the effect of optical interference, has been developed. This system enables determination of the defect density (Nd) and the slope of the Urbach tail (Ech) of a-Si:H films for solar cells with improved accuracy. The system has shown that Nd and Ech are minimized when Eopt = 1.55–1.61 eV. Here, Eopt is the optical gap determined from ( αhv 1 3 versus hv plots. The conversion efficiency of a-Si:H solar cells can be optimized by using the i-layer in this range of Eopt.

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

Abstract We previously reported that a reduction of Siue5f8H 2 bond density in the i-layer effectively prevents light-induced degradation of a-Si solar cells. For further reduction of Siue5f8H 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. Siue5f8H 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 Siue5f8H 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.

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.

Preparation of High-Quality poly-Si and μc-Si Films by the SPC Method

We have achieved the highest total area conversion efficiency for an integrated type 10cm × 10cm a-Si solar cell at 10.2%. This value is the world record for a 10cm × 10cm a-Si solar cell. For further improvement of conversion efficiency in a-Si solar cells, it is necessary to develop materials with high-photosensitivity in the long wavelength region and materials with high conductivity. We have developed a Solid Phase Crystallization (SPC) method of growing a Si crystal at temperatures as low as 600°C. Using this method, thin-film polycrystalline silicon (poly-Si) with higP-photosensitivity in the long wavelength region and Hall mobility of 70cm 2 /V sec was obtained and quantum efficiency in the range of 800,∼ lO00nm was achieved up to 80% in the n-type poly-Si with grain size of about 2μm. We also succeeded in preparing a device-quality p-type microcrystalline silicon (μc-Si) using the SPC method at 620°C for 3 hours from the conventional plasma-CVD p-type amorphous silicon (a-5i) withoul using any post-doping process. Obtained properties of μd=2 × 10 3 (.cm) and a high optical transmittance in the 2.0 ∼ 3.0 eV range are better as a window material than the conventional p-type μc-Si:H. Therefore, it was concluded that the SPC method is better as a new technique to prepare high-quality solar cell materials.

Very wide-gap and device-quality a-Si:H from highly H{sub 2} diluted SiH{sub 4} plasma decomposed by high rf power

The H{sub 2} dilution technique at a high deposition rate (R{sub D}) was investigated by depositing hydrogenated amorphous silicon (a-Si:H) under a high rf power density of 750 mW/cm{sup 2}, which is 20 times as large as that of conventional conditions. It was found that the H{sub 2} dilution ratio {gamma}(=[H{sub 2} gas flow rate]/[SiH{sub 4} gas flow rate]) tendency of the film properties, such as the H content (C{sub H}), optical gap (E{sub opt}), SiH{sub 2}/SiH and photoconductivity ({sigma}{sub ph}) of a-Si:H is different for the high rf power (750 mW/cm{sup 2}) and the medium rf power (75 mW/cm{sup 2}) conditions. Under medium rf power, the C{sub H}, E{sub opt} and SiH{sub 2}/SiH decrease as {gamma} increases. Under the high rf power, on the contrary, the C{sub H} and E{sub opt} monotonously increase while maintaining a low SiH{sub 2}/SiH and a high {sigma}{sub ph} of 10{sup {minus}6} S/cm as {gamma} increases. These results suggest that increasing the rf power enhances the H incorporation reactions due to H{sub 2} dilution. It is thought that a high rf power causes the depletion of SiH{sub 4} and hence the extinction of H radicals, expressed by SiH{sub 4} + H* {yields} SiH{sub 3}*morexa0» + H{sub 2}, is suppressed. A high H radical density enhances the incorporation of H into a-Si:H, resulting in very wide-gap a-Si:H with a high C{sub H}. Consequently, very wide-gap a-Si:H with device-quality (E{sub opt} of 1.82 eV with an ({alpha}h{nu}){sup 1/3} plot, corresponding to > 2.1 eV with Taucs plot, and {sigma}{sub ph} of 10{sup {minus}6} S/cm) can be obtained at a high R{sub D} of 12 {angstrom}/s without carbon alloying.«xa0less

Correlation between material properties and photovoltaic performance in amorphous silicon solar cells

We have focused on the i-layer material in our efforts to improve in the conversion efficiency of a-Si:H solar cells. Reductions in the defect density has been also investigated from the viewpoints of extrinsic (impurities) and intrinsic effects. The main incorporated impurity in a- Si:H is oxygen, which affects the conversion efficiency of a-Si:H solar cells by increasing the defect density and its donorlike behavior. A unified relationship can be observed among the properties of intrinsic (pure) a-Si:H. The film deposition rate plays an essential role in controlling the properties. A lower or higher deposition rate results in a narrower or wider bandgap, respectively. Therefore, the properties of a-Si:H can be controlled independent of the substrate temperature in a certain range by varying the film deposition rate. The controllability of the a-Si:H properties can be improved by applying vibrational / rotational energy to SiH4 molecules or related radicals by heating the source gas, and a-Si:H with the same properties as the best conventional one can be deposited at a lower substrate temperature and/or a higher film deposition rate. The highest conversion efficiency of 12% for an integrated a-Si solar cell submodule of 100 cm2 has been achieved by combining the high-quality i-layer and other technologies.