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More Solar Efficiency Marks: SoloPower, Suntech, ISC, Alta Devices, Panasonic

'Tis the season for announcements about solar cells with improved conversion efficiencies. Today's solar acronym stew ingredients: Zebra, Pluto, CIGS, GaAs, and HIT.

Early in the new year, we've seen a crop of new performance records spanning the spectrum of solar cell technologies: thin-film (including CIGS, CdTe, and other), c-Si, some unknown combination, several involving polymer/plastic materials, and even some with some nanoscale assistance.

In the past couple of weeks we've received more conversion-efficiency marks to add to the tally:

  • The International Solar Energy Research Center (ISC Konstanz) says it's developed solar cells topping 21 percent conversion efficiency, with potential to exceed 24 percent efficiency, an improvement over 19 to 22 percent estimates made last summer. The n-type monocrystalline (Cz) silicon "Zebra" cells (156 x 156 mm2) employ interdigitated back-contacts (IBC), meaning the metallization is laid out on the back side of the cell instead of the front, which enables the cells to be made thinner on the side facing the sun. (imec and partners recently described a 23 percent efficient IBC cell, though on a much smaller scale [2x2 cm2], and a German coalition also pursuing back-contact cell technology recently announced a big jump in efficiency to 20 percent.) The Zebra process sequence also is said to be "a new and smart combination of single process steps that are already implemented in mass production," so the IBC could be made on current standard cell manufacturing equipment and lines. Silfab SpA will help solar cell and module producers to upgrade their lines for the new technology. More info on the new IBC cells is being presented this week at the Photon Terawatt-hours conference in Berlin, and in May at the nPV Workshop in Amsterdam.
  • SoloPower says it's built a 13.4 percent efficient (aperture) flexible CIGS-based panels, confirmed by NREL. The cells were manufactured in the company's San Jose plant "so we know we have the systems in place," said CEO Tim Harris. SoloPower chief technical officer Mustafa Pinarbasi also credited a two-percentage-point jump (11.2 to 13.4) as a result of unspecified "new processes" implemented into its roll-to-roll production line, and hinted at "a strong pipeline of improvements yet to come this year." (We've inquired into details about both.) The company continues to beef up its new facility in Portland, Ore., which it says is on schedule to open later this year, and could eventually top 400 megawatts of output with 450 workers.
  • Suntech says it's new Pluto cell technology topped 20.3 percent efficiency on a production cell, using standard commercial-grade p-type monocrystalline silicon wafers. The R&D work done with the U. of New South Wales, confirmed by the Solar Energy Research Institute of Singapore, is a big step up from Pluto's initial 19 percent mark set in mid-2009, and the company says it should be able to hit 21 percent within the next 6-12 months. Key to upping the efficiency is incorporating "similar characteristics" of the record-holding "passivated emitter and rear locally diffused" (PERL) solar cells, including reducing the metal/silicon interface area on the Pluto cells' rear surface, while keeping noncontacted area well-passivated. Another key change is tweaking processes to minimize the use of high temperatures, which will enable more high-efficiency processes in commonly used commercial wafers, the company says.

    Update 3/27: A Suntech rep confirms the 20.3 percent efficiency is aperture area, not full-cell. And minimizing high temperatures is achieved by using sputtered aluminium for rear contact formation, which can be sintered at low temperatures compared to screen printed contacts.

  • Last month Alta Devices jumped into the fray with its gallium arsenide (GaAs) offering and an NREL-confirmed 23.5 percent efficiency. GaAs and related "compound semiconductor" materials offer unique advantages in higher temperatures and ambient light performance vs. silicon-based and other thin-film solar technologies. The company's cells are also significantly thinner, at just one micron thick using a thin-slicing method — versions of which are becoming increasingly popular among solar manufacturers — and they're flexible which makes them applicable for unique applications, for example roofing and other building-integrated materials. The company says it is "making substantial progress" to build a pilot manufacturing line using "mostly off-the-shelf" equipment with some proprietary tweaks, and "is starting to plan for full-scale production" including seeking manufacturing partnerships and a bigger production site. ( contributor Ucilia Wang has a good review and tour of Alta Devices' technology.)
  • Looking further down the road, Panasonic is targeting an improved version of its heterojunction with intrinsic thin layer (HIT) solar cells at around 22 percent by the end of fiscal 2012. Last summer Panasonic's HIT cells trailed SunPower's cells in efficiency, coming in around 19 percent, with reports of 20 percent cells in the field — but the company has demonstrated nearly 24 percent efficient cells, and reportedly is readying 22 percent efficient cells for release later this fiscal year, to come from its new factory now being built in Malaysia. The plant, which reportedly will operate with 10 percent lower production costs than typical facilities, will start off with 300 megawatts (MW) of production capacity, boosting the company's overall solar output by 50 percent to 900 MW.
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