Akira Terakawa

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.

An approach for the higher efficiency in the HIT cells

The highest conversion efficiency to date of 21.5% (confirmed by AIST) with a size of 100.3 cm/sup 2/ has been achieved in an HIT cell. Because of this high efficiency and the cells superior temperature characteristics, HIT cells are highly regarded by consumers. Sanyo will increase the production volume of cells and modules to meet the demand both inside and outside of Japan. We have been investigating suitable materials based on Sanyos technology for fabricating high-quality a-Si solar cells to obtain higher build-in potential and control the junction properties, and have been studying how to treat the surface to create a good interface without introducing any damage. We will continue our efforts to obtain even higher levels of conversion efficiency by using the high potential that this structure has.

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.

Applications of laser patterning to fabricate innovative thin-film silicon solar cells

In view of the need to obtain high-efficiency and low-cost photovoltaic power generation systems, the electrical series connection of multiple solar cells by laser patterning is a key issue for thin-film silicon solar cells. For a series connection with no thermal damage to the photovoltaic layers, a theoretical analysis of glass-side laser patterning, in which a laser beam is irradiated from the side of a glass substrate, and the optimization of the structure of the solar cells are conducted for a-Si:H/a-SiGe:H stacked solar cells deposited on glass substrates. As a result, an a-Si:H/a-SiGe:H module with both a large area (8,252 cm2) and a conversion efficiency of 11.2% is obtained. Then, to improve efficiency and to reduce cost, the minute structure of microcrystalline silicon (μ c-Si:H) and film-side laser patterning, in which a laser beam is irradiated from the side of the deposited film, are investigated for a-Si:H/μ c-Si:H stacked solar cells deposited on insulator/metal substrates. It is proved that the discontinuity of the doped and photovoltaic layer may cause a reduction in the path density of the leak current, and that this contributes to an improvement in the efficiency of the solar cells. Based on the developed structure, an initial efficiency of 12.6% is obtained in a small-size solar cell. An a-Si:H/μ c-Si:H module (Aperture area = 56.1cm2) with three segments has also been fabricated with an initial efficiency of 11.7% as a first try.

Effect of optical gap on the stability of a-SiGe solar cells

Abstract The light-induced degradation and thermal recovery behaviors of a-SiGe single-junction solar cells with i-layers having various optical gaps ( E opt ) have been investigated. The narrower E opt cells show slower degradation under light-soaking than the wider E opt cells. However, they recover by thermal annealing at 150°C as fast as the wider E opt cells. The results indicate that the light-induced defect creation processes are suppressed in the narrower E opt materials, but the thermally induced process does not depend on E opt . From a comparison among several samples, the E opt dependence of the light-induced processes is attributable to the difference of E opt itself, i.e., the band-to-band recombination energy, rather than to the difference of the compositions.

Efficient production technology for microcrystalline silicon solar cells using a Localized Plasma Confinement (LPC) CVD method

A very high deposition rate for high-quality microcrystalline silicon (μc-Si) films has been achieved under very high-pressure conditions (≫ 1,000 Pa) using a Localized Plasma Confinement (LPC)-CVD method which has a special cathode. The uniformity of the μc-Si film thickness was 2.4% on a 55×65 cm² glass substrate with a deposition rate of 2.7 nm/s. We also achieved maximum conversion efficiency of 11.4% for an a-Si/μc-Si tandem solar cell (1 cm²) on a 20×20 cm² glass substrate and average conversion efficiency of 9.84% for a-Si/μc-Si tandem solar cells (1 cm²) on a 55×65 cm² glass substrate with a deposition rate (Rd) of 1.8 nm/s. These results indicate that LPC-CVD method is a good candidate as an effective production technology for large-area, high-performance μc-Si thin-film solar cells.

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

Hydrogenated Amorphous Silicon Germanium Alloy for Stable Solar Cells

The film properties and solar cell performance of a-SiGe:H samples with the same optical gap and different combinations of hydrogen content (C H ) and germanium content (C Ge ) have been compared. The optimum composition for the initial properties, such as the tail characteristic energy, defect density and conversion efficiency of the solar cell, was determined, and the differences could be explained by the difference in H bonding configuration. The degradation ratio of the conversion efficiency becomes larger in higher C H samples. This suggests that hydrogen or Si-H 2 participates in light-induced degradation. As a result, the optimum C H for an efficient solar cell is believed to shift to the lower C H region after light soaking. Based on these findings, the stabilized conversion efficiency of 3.3% under red light (γ>650nm) for an a-SiGe:H single-junction solar cell (1cm 2 ) and 10.6% under lsun light for an a-Si/a-SiGe double-junction stacked solar cell (1cm 2 ) have been achieved. The degradation ratio is only 8.6% for the double-junction solar cell.

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

Progress in the development of high-conversion-efficiency a-Si/μc-Si tandem solar module using μc-Si thin film with high deposition rate on Gen. 5.5 large-area glass substrate

The technology to make high-quality, high-reliability solar modules with a high deposition rate for μc-Si thin-film is a problem for the industrialization of low-cost, high-conversion-efficiency a-Si/μc-Si tandem structure solar modules. Sanyo has solved this problem by developing an original CVD technique called Localized Plasma Confinement CVD and a new evaluation method for μc-Si thin film. A stabilized conversion efficiency of 10.0% was achieved for an a-Si/μc-Si tandem structure solar module, and a deposition rate of 2.4 nm/s for μc-Si thin-film was attained on a Gen. 5.5 full-size glass substrate. To obtain a higher conversion-efficiency a-Si/μc-Si tandem structure solar module, fundamental studies of μc-Si thin-film have been performed, and a stabilized conversion efficiency of 10.5% (Initial solar module conversion efficiency: 12.0%) has been achieved on a large-area glass substrate. Furthermore, in the study of this development, the highest stabilized conversion efficiency of 12.0% (Initial conversion-efficiency: 13.5%) was attained. Module reliability tests confirmed by IEC 61646 Ed. 2 revealed that the performance of the module is adapted. These high-performance a-Si/μc-Si tandem structure solar modules were prepared by using the knowledge of our thin-film and module technologies.