Japan’s PV suppliers tout improved efficiencies

by Dr. Paula Doe, Contributing Editor, Solid-State Technology

Revealing the first details on its CIGS technology, Honda Soltec Co. Ltd. says it’s averaging 11.1% efficiency from its new solar module line. Other leading Japanese photovoltaic suppliers also described technologies for improved efficiencies now starting to move into production, reports SST partner Nikkei Microdevices, from the 17th International Photovoltaic Science and Engineering Conference in Fukuoka, Japan, in December.

Honda’s solar subsidiary said the key to its relatively high-efficiency volume CIGS thin-film production (see Fig. 1, below) is increasing the selenization temperature to more than 500°C for better crystal quality. This requires low-alkali, high-temperature glass, though it lacks the sodium that typically aids in crystallization. But, it turns out that the auto paint guns in Honda’s new plant are being used to spray a sodium solution on the glass, to add back the Na to enhance the crystallization process. Honda also bypasses Cd issues by replacing the usual CdS buffer layer with InS. The company notes best results from its pilot line are 12.2% efficiency.

The company developed its own PV technology in-house, initially aiming primarily to be able to efficiently generate the energy for making hydrogen to power its fuel cell vehicles, and to power its own factories. In fact, the solar cells have been installed at Honda’s demonstration hydrogen refueling station Los Angeles, and at some of its factories. Residential solar modules also are being marketed in Japan from its new plant in Kumamoto, slated to ramp production to 27.5MW by this spring.

Honda’s CIGS process relies on high temperature selenization, a sodium spray coat, and a buffer of InS. (Source: Honda Soltec, Nikkei Microdevices)

Mitsubishi Electric Corp., meanwhile, reported increasing its polycrystalline silicon cell efficiency to 18% with a cluster of innovations moving into production. It roughs up the surface to cut reflectivity and increase absorption of light by reactive ion etching through a quick and cheap mask layer of a coating of 3μm silica particles in solution that self assemble into the texture pattern. The company also terminates the dangling silicon bonds with hydrogen, and uses an undisclosed new circuit material and modified screens to reduce the size of the circuit lines on the cell surface, reportedly cutting metallization time about in half and reducing shading loss by 40%.

Sanyo Electric Co. Ltd. reported its cells made with its heterojunction with intrinsic thin layer technology (HIT) are now up to 19.7% efficiency in production, and 22.3% in the lab, and said it has developed a 20%-efficient version using 70μm thin wafers. These cells, made with a low-temperature 200°C process, coat a crystalline silicon wafer with thin amorphous silicon layers on both sides, which reportedly improves boundary characteristics and reduces power losses by forming impurity-free i-type silicon layers between the crystalline base and the n- and p-type amorphous silicon layers, while allowing use of thinner wafers. Sanyo said it plans to increase its production from the current 260MW to 650MW by 2010.

The company also noted progress on its thin-film deposition technology, claiming its localized plasma confinement CVD method was now depositing multicrystalline silicon film on 550mm x 650mm substrates at 2nm/sec, with +/-3.3% uniformity. Key was miniaturizing the pyramidal nozzles to create a uniform plasma. Though the company did not disclose efficiencies on the larger substrates, it has previously reported efficiencies of 7.6% in polysilicon films made by the process on smaller substrates.

Sharp Corp. showed off a 1cm2 organic cell rated at 3.8% efficiency by Japan’s National Institute of Advanced Industrial Science and Technology (AIST). Some other organic photovoltaics have reached 5% or so, but only over areas of <e;0.2cm2. The researchers used P3HT (poly (3-hexylthiophone) ) for the p-type semiconductor, PCBH ( [6,6]-phenyl C61 butyric acid methyl ester) for the n-type. Key to the improved performance, they said, was improving the alignment of the P3HT polymer chains.

Sanyo also released the first details of its work on organic solar cells, where it is getting 3.6% efficiencies using small molecules and fullerenes, albeit on tiny 0.033cm2 samples. Though much work on organic photovoltaics has focused on polymers that presumably could be very cheaply applied by wet coating, the Sanyo researchers argued that work in OLED displays had convinced them that small molecules will be the more effective material. It’s using its OLED material DBP (tetra- phenyldibenzoperiflanthene) for the p-type semiconductor and C60 for the n-type.

Sharp claimed a major improvement in a concentrating solar cell system as well, with 40% efficiency in a 1000x concentration system, using a 4.5mm2 InGaPAS heterojunction cell. It said 40% efficiency has previously been reported only up to 200x concentration.

Separately, Tokyo Electron chairman Tetsuro Higashi told Nikkei Microdevices that his company planned to enter the solar equipment business with a thin-film deposition system, rather than the turnkey lines currently supplied by most equipment makers. “The technology is changing so fast now that one really needs to work with the users to understand the product to improve the process,” he said. “We’ll work closely with users to move gradually into the market.” Higashi told NMD that the company “decided to focus first on strengthening our core business before investing in solar, “seeing an opportunity to grow the semiconductor business to catch up with AMAT.” — P.D.

PV suppliers planned capacity update

(Source: Nikkei Microdevices)


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