Masaki Shima

Sanyo's Challenges to the Development of High-efficiency HIT Solar Cells and the Expansion of HIT Business

The worlds highest conversion efficiency levels of 21.8% (Voc: 0.718 V, Isc: 3.852 A, FF: 79.0%, confirmed by AIST) with a practical size of 100.4 cm2 has been achieved by using the HIT (hetero-junction with intrinsic thin layer) structure. This high efficiency has been mainly realized by the excellent c-Si/a-Si hetero-interface property obtained by our optimized surface cleaning process and high-quality and low-damage a-Si deposition technologies. This excellent c-Si/a-Si hetero-interface of the HIT structure results in a relatively high open circuit voltage (Voc) over 710 mV. Recently, we have succeeded in achieving an outstanding Voc of 730 mV for other efficient HIT solar cells. This result indicates the possibility of further improvement in the conversion efficiency of HIT solar cells. The higher Voc results in not only a higher conversion efficiency but also an improved temperature coefficient, which is another practical advantage for outdoor use

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).

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.

Effects of high hydrogen dilution at low temperature on the film properties of hydrogenated amorphous silicon germanium

The effects of hydrogen dilution of up to 54:1 (=H2:SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) was investigated at a low substrate temperature, while keeping the optical gap (Eopt) constant. It was found that deterioration of the film properties, when substrate temperature decreases, can be compensated by the high hydrogen dilution method. As the substrate temperature decreases from 230 to 180 °C, the high photoconductivity, high photosensitivity, and low silicon dihydride content of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties becomes worse with conventional low hydrogen dilution ratios. Probably, hydrogen radicals substitute for the surface reaction energy lost by decreasing the temperature. Besides, a-SiGe:H films deposited under higher hydrogen dilution have more germanium and less hydrogen content than those of the conventional films, despite having the same Eopt. One possible explanation for why Eopt can be kept constant is the s...

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.

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.

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.

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.

Optical confinement and optical loss in high-efficiency a-Si solar cells

The worlds highest stabilized conversion efficiency of 9.5% has been achieved for a 30/spl times/40 cm/sup 2/ a-Si/a-SiGe glass superstrate solar cell submodule. However, significant optical loss still exists even in these high-efficiency a-Si solar cells. FEM numerical simulation has shown that the primary origin of the optical loss in textured a-Si solar cells at about /spl ges/800 nm is absorption in SnO/sub 2/ which is enhanced by the optical confinement effect. Optical confinement also results in increased absorption in the metal electrode, which is another source of optical loss.

Effect of Composition on the Properties of Amorphous Silicon Carbide at a Certain Optical Gap

The relationship between composition and optoelectric properties was investigated for a-SiC:H alloys with a constant optical gap (E opt) and different compositions. The compositions, hydrogen content (C H) and carbon content (C C), and the optical gap of a-SiC:H were successfully controlled independently. E opt of a-SiC:H can be expressed by a linear function of the compositions and a negative dependence of E opt on C C is observed for our samples. In the constant E opt system, C H increases with an increase in C C in spite of a rise in the substrate temperature. In particular, the increase in the Si–H2 density is much more significant than that in the C–H bond density. This result suggests that the incorporated carbon atoms affect the bonding configuration between silicon and hydrogen. The film properties, such as photoconductivity and defect density, and solar cell performance become inferior both before and after light-soaking with an increase in C C, namely the Si–H2 density. The Si–H2 bond is an important factor to consider when determining the stability of a-SiC:H as well as a-Si:H and a-SiGe:H.