Photovoltaic technology is making headlines in Europe as organizations explore new technologies and form joint ventures. The Fraunhofer Institute of Solar Energy Systems (ISE; Freiburg, Germany), for example, has developed monolithic tandem solar cells based on gallium indium phosphide (GaInP) and gallium indium arsenide (GaInAs) that have set new world records for efficiency. The driving force behind this work is researcher Andreas Bett. "For the first time in Europe, ISE has developed novel solar cells that yielded 28% efficiency for terrestrial and 24.5% for space applications," he says. "This is a single process, so it is less costly compared with two separately made cells mechanically stacked. The different materials provide different absorption characteristics that yield a more efficient sunlight conversion. We have added a Fresnel lens to concentrate the sunlight. With such a device, ISE has recently achieved a record efficiency of 32%."
Meanwhile, I.M. Dharmadasa's group at Sheffield Hallam University (Sheffield, UK) is also focused on improving efficiencies of solar cells and reducing costs (see figure). "These objectives can be achieved by the reduction of expensive semiconductors and the use of lower-cost process techniques," says Dharmadasa.
Sheffield's cells are only 2-µm thick compared with the 200 µm used for crystalline silicon cells. An electrochemical fabrication method also lowers cost. "In eight years of research, we have shown that these electro-deposited materials produced under the right conditions are as good, if not superior, to those made by the expensive growth methods," Dharmadasa says.
The II-VIs, such as cadmium sulfide (CdS) and zinc selenide (ZnSe), are being studied as window materials, and exotics, such as cadmium telluride (CdTe) or copper indium selenide (CuInSe2) for example, are being studied as absorbers. "We are most excited by our latest material, copper indium gallium selenide (CuInGaSe2)," Dharmadasa continues. "Our first structure is based on glass/FTO/n-ZnSe/p-CuInGaSe2/Au. It has shown 15% efficiency. To our knowledge, this is the best so far for one-step electro-deposited semiconductors." Moreover, it is not far from the 18.8% measurement for cells made by costly vacuum growth by the U.S. National Renewable Energy Laboratory (NREL; Golden, CO). "We are presently directing our research to see if we can match such efficiencies with our low-cost electro-deposited materials," he notes. heading to market
On the commercial front, energy companies Electrabel (Brussels, Belgium) and TotalFinaElf (Courbevoie, France) have joined forces with independent electronics research center IMEC (Leuven, Belgium) and IMEC spinoff Soltech (Heverlee, Belgium) to form Photovoltech. The new company will produce the photovoltaic cells and modules that form the basic components for photovoltaic systems. Photovoltech will begin construction of Belgium's first solar cell and modules factory in Tienen. The company expects to leverage IMEC production processes to manufacture higher-efficiency polycrystalline silicon solar cells at lower cost than by today's technology.
I. M. Dharmadasa of Sheffield Hallam University holds a solar panel in the lab. He is standing with teammates Anura Samantilleke, Nandu Chaure, and Tian Fang. (Sheffield Hallam University)
According to Reed Electronics Research (RER; Surrey, UK), the market for semiconductor solar cells in Europe will grow from $154 million in 2000 to reach $228 million by 2005. "This, like other opto device markets, suffered one of the first reversals of fortune in its history in 2001," says Andrew Fletcher, RER publisher. "The reasons for this were complex but had much to do with a lack of confidence in the telecom sector for which the higher-value solar-cell market segment has so much to contribute. There has also been a general decline in manufacturing worldwide, which shaved a few more points off the growth of opto and other devices in this period. Continuing downward unit price pressure is also affecting the market."
In terms of wafer area, solar cells are still one of the biggest consumers of wafers, the bulk of them germanium (Ge). Depending on the particular solar cell design, n- or p-type wafers are used. Single and dual GaAs-on-Ge cells use n-type Ge, the triple-junction and newer four-junction cells use p-type. Today's efficiencies average 25% for production cells with improved cells of up to 27% with four-junction cells capable of exceeding 30%.
"Apart from the material quality defined through its conductivity type, dislocation density and wafer finish are the most important customer criteria," says Ignace de Ruijter, business line manager for substrates at Umicore (previously UM Electro-Optic Materials; Olen, Belgium).
Still, interest in solar cells in Europe has never been stronger--not only in silicon and germanium, which provide the foundation for the bulk of the world's PV market, but also for a range of advanced semiconductors such as chalcogenides and multielement III-Vs. The field is evidently becoming ever more interesting via continual efficiency improvements and efforts to reduce costs. Companies are using a broader range of semiconductors and working with more efficient fabrication processes to produce high-efficiency devices with less material.