Eiji Maruyama

24.7% Record Efficiency HIT Solar Cell on Thin Silicon Wafer

A new record conversion efficiency of 24.7% was attained at the research level by using a heterojunction with intrinsic thin-layer structure of practical size (101.8 cm2, total area) at a 98-μm thickness. This is a world height record for any crystalline silicon-based solar cell of practical size (100 cm2 and above). Since we announced our former record of 23.7%, we have continued to reduce recombination losses at the hetero interface between a-Si and c-Si along with cutting down resistive losses by improving the silver paste with lower resistivity and optimization of the thicknesses in a-Si layers. Using a new technology that enables the formation of a-Si layer of even higher quality on the c-Si substrate, while limiting damage to the surface of the substrate, the Voc has been improved from 0.745 to 0.750 V. We also succeeded in improving the fill factor from 0.809 to 0.832.

Temperature Dependence of Amorphous/Crystalline Silicon Heterojunction Solar Cells

We evaluated the conduction mechanisms and temperature dependence of HIT (heterojunction with intrinsic thin layer) structure solar cells while changing the thickness of the undoped amorphous silicon layer. It was confirmed that the diffusion model determined the carrier transport property of this device at the high-forward-bias region (0.4<V<0.8 V), whereas the multistep tunneling model determined the current transport at the low-bias region (0.1<V<0.4 V). The insertion of the high-quality hydrogenated amorphous silicon (a-Si:H) i-layer is very important for suppressing the probabilities of tunneling through the localized states in a-Si:H and surface recombination velocity at the heterointerface. The better temperature dependence of output power of HIT structure solar cells than that of the crystalline silicon (c-Si) homojunction solar cell is caused mainly by the high-open circuit voltage that originates in the effectively suppressed saturation current with HIT structure, and also by the fill factor (F.F.), which is affected by the change in conductivity in the a-Si:H i-layer.

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

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.

Optical confinement in high-efficiency a-Si solar cells with textured surfaces

Abstract The experimental spectral response and reflectance of high-efficiency a-Si solar cells are systematically investigated by using an optical simulation based on realistic optical properties of the transparent conducting oxide (TCO), a-Si, and metal electrode, in order to improve the spectral response. It is shown that a practically important optical loss results from absorption by the TCO, which is enhanced by the optical confinement effect. This suggests that improvement in the spectral response is possible by suppressing the optical confinement in the TCO.

Excellent power-generating properties by using the HIT structure

We are developing HIT solar cells with high conversion efficiency, which was achieved the worlds highest conversion efficiency of 22.3% in a practical size solar cell in July 2007. We have four main approaches to reducing power-generating cost: improve the conversion efficiency, apply the HIT structure to a thin wafer, improve the temperature coefficient, and apply HIT solar cells to a bifacial solar module. Using these approaches, we have achieved the remarkably high conversion efficiency of 21.4% due to a high Voc of 0.739 V with an 85-μm cell, which was measured at SANYO. A thinner Si wafer brings not only high Voc but also generating more output power at high temperature for a better temperature coefficient. We have confirmed that the HIT structure is suitable for use in thinner wafers, allowing us to reduce power-generating cost.

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.

High-efficiency HIT solar cells with a very thin structure enabling a high Voc

To increase the competitiveness of HIT (Heterojunction with Intrinsic Thin-layer) solar cells, we have been working on the enhancing their conversion efficiency. This time, we improved the heterojunction of the HIT solar cell, which made it possible to enhance the cell conversion efficiency. In addition, we have developed module technologies such as a new tab design and anti-reflection coated glass. By combining these technologies, we have achieved 240-W model with module conversion efficiency of 19.0%. Those HIT solar cells have the worlds highest level of cell conversion efficiency 21.6 % at the mass-production stage. We have also been investigating the performance of thinner HIT solar cell using crystalline silicon (c-Si) wafers less than 100 μm in thickness. To minimize optical losses, such as the ultraviolet light absorption in the front transparent conductive oxide (TCO) layer and amorphous Si (a-Si) layers, and the near-infrared light absorption in the rear TCO layer, we have improved the deposition conditions of a-Si, and developed the TCO material respectively. To improve the surface passivation quality of the a-Si/c-Si heterointerface, we have examined our fabrication process from these three viewpoints: (1) the cleanliness of the c-Si surface, (2) the damage in the deposition process, and (3) the quality of the deposited a-Si layer. As a result, we have achieved an excellent Voc of 0.747 V with 58- and 75-μm-thick cells.

High-performance HIT solar cells for thinner silicon wafers

We have been researching and developing HIT (Heterojunction with Intrinsic Thin-layer) solar cells to obtain high conversion efficiency. Last year, we updated the worlds highest conversion efficiency, which was previously 22.3%, to 23.0% with a practical-sized HIT solar cell at the R&D stage. We have also been investigating the performance of thinner HIT solar cells using less than 100-µm-thick crystalline Si (c-Si) wafers in order to effectively reduce the production cost. By using improved technologies, we succeeded in gaining the high conversion efficiency of 22.8% in a HIT solar cell with a 98-µm-thick c-Si wafer and an excellent Voc of 743 mV at the R&D stage. The accomplishment of the 22.8% cell demonstrates that HIT solar cells are advantageous to the use of thinner Si wafers because of certain HIT solar cell features.

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.