RENEWABLE ENERGY

The rising sun of photovoltaics

Researchers at the Fraunhofer Institute for Solar Energy Systems (ISE) in Germany accomplished their New Year’s resolution quicker than most. In January they achieved a world record for conversion efficiency in multi-junction photovoltaic cells, at 41.24%. Researchers think they can up the ante further using a special type of solar cell with a metamorphic multi-junction layer structure. Clearly, the sun has not had the last word.

Fraunhofer Institute’s metalorganic vapour phase epitaxy (MOVPE) reactor. Source: Fraunhofer ISE
Fraunhofer Institute’s metalorganic vapour phase epitaxy (MOVPE) reactor. Source: Fraunhofer ISE
Solar cell designed by the Fraunhofer Institute in 2009 using the metamorphic Ga0.35In0.65P and Ga0.83In0.17As/Ge material combination. In 2008, the researchers achieved a new world record with 35 % efficiency. They are expecting to achieve 42 % to 43 % efficiency before long. Source: Fraunhofer ISE
Solar cell designed by the Fraunhofer Institute in 2009 using the metamorphic Ga0.35In0.65P and Ga0.83In0.17As/Ge material combination. In 2008, the researchers achieved a new world record with 35 % efficiency. They are expecting to achieve 42 % to 43 % efficiency before long. Source: Fraunhofer ISE
© Shutterstock
© Shutterstock

In spite of their significant role, conventional silicon-based single junction photovoltaic (PV) cells have a major drawback: although they are sufficient to power wrist watches and desktop calculators, they are not efficient in converting the broad range of photon energy in the solar spectrum. Conventional industrial silicon PV cells convert only a fraction of the solar light spectrum, around 17%, while the record for a laboratory cell is 25%.

Research is therefore focusing on the development of multi-junction photovoltaic cells, which achieve higher efficiency. They are able to capture more sunlight energy for conversion into electricity, in part due to their composition of different materials, including gallium arsenide, gallium indium phosphide and germanium. The multi-junction structure is in principle a stack of single-junction cells but, because it makes use of several semiconductor energy band gaps, different segments of the solar spectrum can be converted by each junction at a higher efficiency. Fraunhofer ISE has been developing metamorphic multi-junction photovoltaic cells for a decade using III-V semiconductor compounds, which are semiconductors especially suitable for converting sunlight into energy.

The starting point of any photovoltaic cell is a semiconductor material with a P-N junction(1). This, however, is just the foundation. The basis for achieving high conversion efficiency is stacking cells of various materials on top of one another, with each material having its own P-N junction. In the case of Fraunhofer ISE’s record-breaking cells, the stacked materials are comprised of gallium indium phosphide as a top material, then gallium indium arsenide, and finally germanium as the third material. According to Andreas Bett, head of the solar cells and technology department at Fraunhofer ISE, although the premise of increasing efficiency by stacking cells is old, the key now is to use the right technology with high-quality materials.

The trick of the trade: metamorphic crystal growth

All semiconductors are made within a crystal. Inside this crystalline structure are layers of atoms with a specific distance between them, constituting what is known as the lattice constant. In this context, germanium is the bottom layer of the cake.

Traditionally, if another material is grown on top of germanium, it needs to be done in the same lattice constant and have lattice-matched material. This achieves high crystal quality, which poses fewer solar conversion problems. If the lattice constant differs even slightly, though, it will not “fit,” resulting in failures, called dislocations. These dislocations reduce solar cell efficiency significantly.

Herein lies the trick for record-breaking efficiency: Fraunhofer ISE has developed a specific, photovoltaic non-active layer – known as a buffer layer – in which all the dislocations in the crystals are confined. On top of the buffer layer is the material with a new lattice constant, which is relatively free of faults in the resulting crystals. In other words, all the defects are localised in an electrically inactive region of the solar cell, and the active regions remain relatively dislocation-free.

Metamorphic crystal growth also uses a larger scope of III-V semiconductors in multi-junction PV cells. To achieve high efficiency, the solar spectrum is divided into three equally sized regions by appropriate light-absorbing materials. The metamorphic Ga0.35In0.65P and Ga0.83In0.17As/Ge material combination, the solar cell structure, is perfectly current-matched in the solar spectrum (all three sub-cells of the triple-junction solar cell generate the same volume of current). In addition to metamorphic growth, this is essential to achieving high efficiency.

Raising the bar again?

Fraunhofer ISE researchers are no strangers to record-breaking. Last year they upped the European record for solar efficiency from 37.6% to 39.7% over a span of three months. They developed the triple-junction cell in the context of an FP6 project called FullSpectrum, which was part of Fraunhofer ISE’s long-term strategy to attain record efficiency. When the FullSpectrum project began in 2003 the researchers had an efficiency of 32%. From there they went on to surpass each new goal set, and were well ahead of their 35% efficiency mark when FullSpectrum concluded in September 2008.

By building on their approach, Andreas Bett believes that an efficiency of 42% or 43% is not far off. “The next step is to add more junctions. Now we have three junctions, and if we go to four, five or six junctions we can increase the efficiency by as much as 50%. However, this next phase will also require the development of new materials and will need some time,” Bett says. On the other hand, III-V semiconductor triple-junction solar cells are already in industrial production and used in photovoltaic concentrator systems for solar power stations. In fact, the record-breaking metamorphic technology has been transferred to Fraunhofer ISE’s industry partner AZUR Space Solar Power in Heilbronn (DE). AZUR Space also participated in the FullSpectrum project and is already on track to produce these PV cells in large quantities.

“We are confident that our cells will help the young concentrating PV technology to become market competitive and to bring down the costs of generating electricity from the sun even further in the future,” Bett says.

Amy Shifflette

  1. (1) See research*eu, Special Issue “Extracting ourselves from oil,” April 2008, page 23, “Photovoltomania.”


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A bright future

Record-breaking conversion efficiency isn’t the only good news in modern photovoltaics. Despite gloomy economic forecasts, manufacturers of solar panels are poised to help the European Union (EU) make good on its goal of sourcing 20% of energy needs from renewable energy by 2020. Consumers seem to be equally receptive. Germany saw a turnover of €5.7 billion in 2007 and the country had more than 100 000 homes with solar panels installed.

In December 2008, the latest Photovoltaics Status Report published by the Joint Research Centre (JRC) reported an annual growth rate of solar photovoltaic production averaging 40% over the past five years.

The EU has reason to be proud of its renewable energy role: world electricity production with photovoltaic systems is around 10 billion KWh, of which half comes from the EU. The net effect is 4 million fewer tonnes of CO2 emissions. However, solar energy still accounts for only 0.2% of total electricity consumption in Europe. The impact of solar energy, from both environmental and market-driven perspectives, is clearly on the rise. Technical advances and growing incentive schemes are making photovoltaics more cost-effective. By 2010 the market value is expected to reach €40 billion, which will mean lower prices for consumers.

www.jrc.ec.europa.eu


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At the cutting edge

Israel has successfully integrated renewable energy for practically as long as the country has existed. By 1967, one in 20 households heated their water using solar energy and, with the onset of the 1970s oil crisis, solar heaters were installed in over 90% of Israeli homes. Photovoltaic technology in the country is nearly as competitive as fossil fuels. In fact, Israel has a photovoltaic obligation: solar technology is now compulsory for all new buildings in Israel. Economies of scale, widespread awareness and training have led to substantial cost reductions. Last year the Israel Public Utility authority approved a feed-in tariff for solar plants. This is an incentive structure to encourage the adoption of renewable energy through government legislation. Under this tariff scheme, the utility companies are compelled to buy renewable electricity at above market rates set by the government. A higher price therefore helps offset the cost disadvantages of renewable energy sources.


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Green-powered oasis... in the desert?

Soon to be built just outside the bustling city of Abu Dhabi in the United Arab Emirates is a city with its sights on raising the bar on ecological power and living. The city, called Masdar (meaning “the source” in Arabic), will rely entirely on solar energy and other renewable resources, with zero-emission, zero-waste ecology. Begun in 2006, and with a projected cost of US$ 22 billion, the city will cover 6 square kilometres and be home to some 50 000 inhabitants. Among the renewable innovations will be a large solar power plant facility, with additional photovoltaic cells on rooftops to provide supplemental energy. Wind farms will be set up outside the city walls (it will be walled to keep out the hot desert wind), and Masdar planners intend to capitalise on geothermal and hydrogen power as well. The project is supported by the environmental charity WWF.

www.masdaruae.com


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