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AXT inc. - Germanium solar cells fuel greener future



Use of germanium substrates for photovoltaic solar cells offers higher efficiency than with silicon substrates according Dr Philip Yin of US semiconductor substrate specialist AXT.

Dr Yin is Chairman of the Board and Chief Executive Officer of AXT Inc., a major manufacturer of high-performance compound semiconductor substrates for integrated circuit production based on gallium arsenide (GaAs), indium phosphide (InP) and germanium (Ge). AXT was the commercial pioneer of the vertical gradient freeze (VGF) process that provides very low defect substrates for electronic devices. Dr Yin believes strongly in the use of Ge substrates for photovoltaic (PV) solar cells as important alternative energy source.

Why is solar power so important?

By now, almost everyone has heard the term ‘solar power’ and we all know that this comes from the sun’s rays. The concept that we may all someday benefit from free electricity from the sun is indeed intriguing. Approximately 1 000 Watts of energy per square meter of the Earth’s surface comes from the sun on a clear, sunny day. The potential of powering our homes, offices, manufacturing plants and maybe even cars for free is very alluring.

The use of solar cells also dramatically reduces our dependence on fossil fuels and makes for a much greener Earth. If we want to harness renewable sources of energy for the future, we must begin planning today by developing clean, green, sustainable energy sources such as solar to benefit future generations. 


What are the commercial opportunities for PV technology?

From solar-powered street lights in Venezuela to PV summits and conferences scheduled around the world and the wealth of information available on the Internet about solar cells and PV technology, companies around the world are not overlooking the opportunities to invest in this eco-technology. Returns on investments are indeed great, and dozens of start-up companies are being established every year together with large, multinational companies already involved in solar cell technology. Moreover, one should not neglect the role of public authorities in this enthusiasm. In the USA, thanks to individual state legislation and federally-funded programmes, companies invest at lower risks.

National research centres and companies are working closely to get innovative products on the market. The US National Renewable Energy Laboratory (NREL) is working on photovoltaic research and is focused on decreasing American reliance on fossil fuel-generated electricity by lowering the cost of delivered electricity and improving the efficiency of PV modules and systems. NREL's PV research contributes to these goals through fundamental research, advanced materials and devices, and technology development.

What materials are used for such cells?

The two primary applications for photovoltaic solar cells are space-based, for satellites, and terrestrial or Earth-based, for solar power generation. The vast majority of solar cell devices used commercially today are based on silicon substrates, mostly for terrestrial applications. Major manufacturers include GE Energy (USA), Schott Solar (Germany) and SHARP (Japan) – the latter claims to be the largest producer of solar cells globally.

However, there is another fast emerging substrate with a much higher efficiency rating called germanium. Germanium-based solar cells already account for more than 80% of satellite applications. Ge-based solar cells are used for their power outputs, higher efficiencies and better conversion rate. Efficiency for commercial silicon-based solar cells is around 19 to 21%, while efficiency for Ge-based solar cells is approximately 30 to 35%.

Technically, the growth for Ge substrates is being fuelled by concentrating photovoltaics (CPVs). The use of mirrors or lens concentrators increases the solar flux by 100 to 500 times normal. Market research firm Strategy Analyti is predicting that the overall CPV market – now only about 1% of the total – will grow at a compound annual rate of more than 350% until 2012

Nowadays, major economic actors worldwide are pushing forward the use of Ge substrates whether in the private or public sector. Major international companies such as Emcore and Spectrolab – owned by Boeing – are key users of germanium and there are few doubts that together with environmental concerns, these are using Ge-based solar cells for their competitiveness in terms of price per Watt when considering overall efficiencies. Recently, both companies have been running large terrestrial power stations.

In Europe, Emcore is scheduled to install a CPV system in Castillo La Mancha (ES) by December 2008 and is building an 850-kW solar power park in Extremadura (ES), to be completed by July 2008. This company has also stated it would supply a CPV system for the Canadian-Korean market and expect to achieve an efficiency rate for its CPV system of more than 45% by 2010. And Spanish company Acciona is due to inaugurate ‘Nevada Solar One’, not to far from Las Vegas in the USA.

In addition, the US Government understands the importance of technical innovation – or eco-innovation – in that sector and is injecting massive amounts of money in solar programmes to improve the overall efficiencies of Ge. The US Defense Advanced Research Projects Agency  (DARPA) has for example a 48-month programme, valued at approximately $50 million, to obtain Ge-based solar cells efficiencies of up to 50%.

How is AXT contributing?

AXT is one of the few companies in the world having the infrastructure to offer germanium substrates in commercial-grade quantities. We are using an innovative technology called ‘vertical gradient freeze’ (VGF), which offers very low defect density. With the price of germanium growing sharply in the past months and knowing the threat of metal scarcity, we give full priority to technical innovation which reduces direct and indirect – environmental – costs while ameliorating performances. Indeed, AXT recognises that sustaining natural resources goes hand in hand with sustaining business, a business that becomes greener by day.

These are very exciting times indeed to be involved with this promising, green technology. Only time will tell what new breakthroughs will occur in the near future, which will further launch solar cell technology to the next level. One thing is for sure, the earth and its people will benefit from the dedication and hard work of the people working environmental-technologies today.

Contact: Dr Philip Yin, AXT – philip.yin@axt.com

VGF technology

Vertical gradient freeze (VGF) technology was pioneered by Bell Labs in the early 1980s for growing semiconductors composed of multiple elements that include gallium, arsenic, indium and phosphorus. Such semiconductors offer capabilities that exceed those of wafers created on a single crystal silicon substrate. AXT was first company to market the process independently. The process involves growing uniform crystals to a certain diameter, thinly slicing the crystal to very precise tolerances, edge rounding the form factors, polishing them into smooth wafers and then carefully packaging the wafers in a clean room for shipping to customers. The main advantages of the VGF process include scalability, low stress, high mechanical strength and a defect rate that is much lower than conventional compound semiconductor crystal growth processes.

European Examples

European germanium manufacturers such as Umicore have been involved in photovoltaics for some time. And the IMEC nanoelectronics research centre in Belgium recently announced record conversion efficiencies of 24.7% in a single-junction GaAs solar cell on a Ge substrate. The efficiency was measured and confirmed by the NREL in the USA. IMEC sees improving the efficiency of this single-junction cell as a further step in the development of a hybrid monolithic/mechanically stacked triple-junction solar cell for use in satellite solar panels and Earth-based solar concentrators. 

The European Photovoltaic Technology Platform is focusing research on improving performance of silicon PVs, germanium-based PVs are also being studied as part of efforts on emerging PV technologies.

High-efficiency solar cells

While silicon substrates play a major role in photovoltaics, germanium substrates have been used extensively in space applications where their smaller size, higher efficiency and greater power output are crucial. Germanium-based PVs are now also being used in terrestrial applications where, combined with optical systems that concentrate solar beams on tiny wafers, they offer much higher efficiencies than silicon-based PVs, outweighing the cost difference. And, while germanium is a rarer material than silicon, considerable reserves exist in coal and zinc-processing waste.