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Solar Cell Efficiency Round-Up: Thin Film Closing the Gap with Silicon

A flurry of new solar-cell efficiency records — including a quiet surprise — are putting thin-film solar PV technologies tantalizingly close to silicon rivals.

The latest results (except Empa, which is awaiting official confirmation from the overseas lab) are in NREL's updated multistrand chart (below) and in the solar cell efficiency tables published by Progress in Photovoltaics

CdTe: 18.3 percent. There's a new record holder in thin-film cadmium telluride (CdTe): GE Research is now on top with an 18.3 percent efficient cell, a full percentage point higher than the 17.3 percent mark achieved by First Solar last year. In the world solar cell efficiency ladder where fractions of a percent improvements are typical, a full point improvement is actually pretty remarkable — but maybe not a surprise. GE has been building on the CdTe technology it acquired from PrimeStar, and believes the technology "hasn't really been explored as much as it could be," said Anil Duggal, GE Research's solar technology platform leader. Three years ago "we were making 10 percent cells," he said; their internal goal is to match the ~20 percent efficiency of today's multicrystalline silicon cells. Duggal wouldn't describe the specifics of GE's CdTe process or what it tweaked to raise the efficiency bar, except to say that it was an equal achievement between work on the materials, device design, and processing. He also noted that GE is already evaluating how the new cell performs on a pilot line, and that the goal is CdTe modules with ~15 percent efficiencies.

CIGS (on polymer): 20.4 percent. The Swiss Federal Laboratories for Materials Science and Technology (Empa) say they have created 20.4 percent-efficient solar cells based on CIGS (copper indium gallium (di)selenide), on flexible polymer substrate foils. The mark is verified by the Fraunhofer Institute for Solar Energy Systems (ISE); the same Empa group notched 18.7 percent about 20 months ago. The latest mark, they say, comes from modifying the properties of the CIGS layer grown at low temperatures.

Empa's new efficiency mark exceeds the current CIGS record holder (NREL itself at 20.3 percent) which used a glass substrate. It's also right on par with champion multicrystalline cells. "We have now — finally — managed to close the 'efficiency gap' to solar cells based on polycrystalline silicon wafers or CIGS thin film cells on glass", stated Ayodhya Tiwari, team leader of Empa's CIGS efforts. Next step, with partner Flisom (an Empa research spinoff) is to scale up the technology to work in large-area, and cost-efficient, roll-to-roll manufacturing processes on an industrial scale to churn out flexible CIGS cells for applications from solar farms to roofs and facades to portable electronics. Empa is partnering with startup Flisom in this direction.

CIS: 19.7 percent. Solar Frontier has hit 19.7 percent efficiency for its copper-indium-selenium (CIS) thin-film cells, measured by Japan's National Institute of Advanced Industrial Science and Technology (AIST), beating a decade-long mark of 18.6 percent. The cells, cut from a 30 &times' 30 cm substrate (instead of individually developed small-area cells) were made using a sputtering-followed-by-selenization method that Solar Frontier says can translate into greater efficiencies in mass production vs. a co-evaporation process, which currently tops out at 20.3 percent efficiency. The company also touts its technology as "cadmium-free" to emphasize its more ecologically-friendly nature vs. other thin-film solar technologies. Modules produced at the company's Kunitomo plant exceed 13 percent efficiency.

NREL's solar cell efficiency chart doesn't currently track CIS as a standalone solar cell technology. Keith Emery, who manages NREL's cell and module performance characterization group (and updates that chart), acknowledges it's a running debate within research circles whether to add a strand to the chart for CIS.

Organic tandem cells: 12.0 percent. Germany's Heliatek says its organic solar photovoltaic (OPV) cells have topped 12.0 percent efficiency, with help from the University of Ulm and TU Dresden. The mark was measured by SGS. The 1.1 cm2 standard-size cell combines two patented absorber materials that convert light from different wavelengths, absorbing more photons and improving the material's performance. An OPV cell efficiency of 12 percent, the company notes, is roughly equivalent to 14-15 percent efficiency for crystalline silicon or thin-film PV (both of which have champion efficiencies currently at 20 percent). It also "is a clear validation of Heliatek's choice not to focus on printed polymers but to go with vacuum deposited oligomers," technology already used for organic light-emitting diode (OLED) displays, stated Martin Pfeiffer, co-founder and chief technology officer. The technology can lay down superthin layers (down to 5 nanometers), meaning a lot of layers can be stacked to create tandem or triple-junction cells to absorb more of the light spectrum.

Heliatek, which set the previous OPV cell record of 10.7 percent just nine months ago, is aiming for 15 percent efficient OPV cells by 2015. An in-house roll-to-roll production line was launched last spring churning out products for evaluation (it's currently seeking to raise €60 million from current investors for a new line), and the company hopes to commercialize partner applications later this year.

Raising the efficiency bar for solar technologies represents the leading edge of product development that ultimately will help improve solar energy generation. It's important to remember, though, that these technologies first have to become manufacturable at cost and scale, and perform in the field at least on par with what's already out there. Translating these champion cell numbers to module performance, silicon cells typically lose about 10 percent of their efficiency; for thin-film it's more like 20 percent, according to the PIP charts referenced above. Thin films can achieve manufacturing costs comparably well vs. silicon (dollar per watt), but their lower efficiency has to be offset either in field performance (e.g. better degradation rate) or with higher balance-of-system considerations, Emery explained.

Lead image (c) Heliatek GmbH

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