Tuesday, September 23, 2008

Suniva's Light-Trapping Pushes Effiiciency to Over 20%

Suniva, an Atlanta-based startup, announced that is has produced high efficiency monocrystalline solar cells with a conversion efficiency of 20%, representing a significant improvement over the efficiency of 18.5% of its current line of ARTisun cells and close to the industry's leading efficiencies (23.4% by SunPower).

According to its website, Suniva has a three-prong approach to making efficient cells:

  • An improved set of screen-printed contacts. While screen-printing of solar cell gridlines is now a standard practice in the industry, Suniva has adjusted processing parameters and paste to improve contact performance.
  • An improved high sheet-resistance emitter to increase response from the blue end of the solar spectrum (where photons are more energetic) and raise the current level of the device.
  • An improved dielectric passivation layer to minimize recombination of electrons with holes and reflect light for a second pass through the active layer. By improving this passivation, fewer photogenerated carriers are lost at the surfaces and the power output of the cell is increased.

Translating the above bullet points into English, Technology Review ran an excellent piece discussing in greater detail how Suniva achieves higher efficiencies by "light-trapping" through the use of additional texturing on the surface of the silicon layer coupled with the addition of a reflective layer at the back of the silicon surface. This results in the ability to halve the thickness of the solar cell while achieving the same level of light absorption, which in turn allows Suniva to make do with not only a reduced amount of expensive silicon material but also with a lower quality, less pure and cheaper grade of silicon.

In a conventional solar cell, which can have a silicon layer 200 micrometers thick, impurities within the material can easily trap electrons before they reach the surface and escape to generate a current. In a layer of silicon just 100 micrometers thick, however, the electrons have a shorter distance to travel, so they're less likely to encounter an impurity before they escape.

Suniva’s website currently has stated goals of achieving 20% conversion efficiencies by, depending on which specific webpage you are on, 2010 or 2011. It seems that they have already handily outdone themselves. On the other hand, perhaps these are dates by which they hope to have 20% efficient cells fully commercialized, in which case some challenges lie ahead, according to Technology Review:

The results [of Suniva’s cells achieving 20% conversion efficiency] have been confirmed by the National Renewable Energy Laboratory, in Golden, CO. But for those tests, Suniva used cells with 200-micrometer-thick silicon wafers, and reaching 8 cents a kilowatt [i.e. grid-parity] will require 100-micrometer wafers. That this is technically possible has been established. The challenge lies in acquiring large amounts of such silicon, since wafers that thin aren't commercially available, [Founder and CTO Dr. Ajeet] Rohatgi says. What's more, factories will need to be retooled to handle 100-micrometer cells, which machines designed to handle thicker wafers could break.

Suniva's approach thus seems to be dispelling the myth that high conversions efficiencies and lower production costs are necessary trade-offs. (As an aside, the silicon-based solar guys aren’t the only kids in the block trying to get wafer thickness down. Utah scientists have devised cutting edge methods to slice germanium-based wafers resulting in less waste, and thus more usable wafers and ultimately lower costs per watt.)

Suniva was spun out of Georgia Institute of Technology's Center for Excellence in Photovoltaics, a research group founded in 1992 by the university. According to Venture Beat, the company received US$50 million in a second round of financing earlier this year, and expects to achieve the holy grail of producing cells at US$1 per watt (generally agreed as the cost level corresponding to grid parity) in two to three years time. Suniva ranked #9 in GreentechMedia's list of Top Ten Startups this year.

Suniva has been wheeling and dealing quite a bit for a two-year old (and not heavily capitilized) startup. In June, it announced it would begin production on a 32 MW pilot plant (and expects to add another 100 MW over the next two years) and signed up to a US$300 million wafer supply deal with REC. Just last month, it sealed a US$500 million deal to supply Solon AG, Europe's largest solar photovoltaic module manufacturer, with high-efficiency monocrystalline solar cells through 2012. In the same month, it tappws its India connections through its founder and CTO, Dr. Ajeet Rohatgi (pictured), Suniva entered into a long-term supply contract to supply Titan Energy Systems, one of India's largest and longest-standing manufacturers of solar modules, more than US$480 million worth of high-efficiency monocrystalline silicon solar cells through 2013 to be used in Titan's highest efficiency product lines.

We'll try to keep tabs on the progress of Suniva's ongoing RD&D.

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