Nanotechnology Now - Harnessing the energy of the sun: Developing 3-D nano-scale solutions

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Dr Ville Miikkulainen
Project Manager
Picosun Oy

Abstract:
Theoretical potential of solar power covers current global energy needs 2,850 times over. The real promise of renewable energy lies with the sun. According to industry estimates, thin film solar cells will dominate the photovoltaic market in the future and replace bulk silicon from its current leading position. Photovoltaics is entering the wholesale electricity market and the following decade will see it explode. Only the best thin-film technologies will be able to spearhead such an explosion. Finnish-based leading global manufacturer of Atomic Layer Deposition (ALD) reactors Picosun Oy (www.picosun.com) is one of only four industry partners of a European Union funded solar energy project called ROD-SOL. The aim of the project is to revolutionize solar energy markets with a new concept of nano-scale thin-film light traps.

June 4th, 2009

Harnessing the energy of the sun: Developing 3-D nano-scale solutions

There is much controversy in defining the probable date when global fossil fuel resources are completely depleted: the very last morsel of the hardest-to-utilize field of arctic oil shale has been squeezed dry. Some people say they already know the year, if not the date. Some say that technology and yet unknown new resources may solve the problem for us.

This controversy has overshadowed the much more important fact that the mere existence of a realistic possibility of such a date becoming reality one day should as such be enough to direct all available interest towards finding energy solutions which are based on sustained renewable resources and can be utilized as economically as possible. If, by a miracle, oil reserves do not dry up, the worst case scenario would be that human race might be swimming in alternative energy options.

RENEWABLE ENERGY EXISTS in plentiful amounts.

Compared to the current global energy needs, there is enough potential hydropower available in the world to cover those needs completely. However, only 15 percent of that potential is currently technically accessible.

Theoretical potential of solar power covers current global energy needs 2,850 times over. Already, there is technically accessible solar power to cover those needs 3.8 times. One must note, however, that the capacity to capture solar power lags way behind what could be accessed today.

Theoretically, there is enough wind power resources for the global energy consumption to grow 200 times from today's level, although only half of the current consumption can be covered with today's accessible wind power technologies.

Theoretically, biomass gives us 20 times current consumption worth of energy, but only 40 percent of current usage can be accessed through today's technology. Geothermal solutions give us the ration of five times current needs, with full coverage accessible already. And finally, waves and tidal energy offers us two times current needs, but only five percent of today's energy consumption could be covered with current ocean power technologies.

All and all, the amount of renewable energy that is accessible through the use of current technologies adds up to 5.9 times current needs (2007), writes a recent report co-authored by Greenpeace and the European Renewable Energy Council.

TODAY, WIND IS THE PREFERRED SOURCE of renewable energy. Between 2000 and 2007, each year on average, 13,300 megawatts of new wind power was connected to the global grid. Between 2005 and 2007, wind power capacity grew from 59 gigawatts to 95 gigawatts. However, in 2007, 80 percent of global energy consumption continued to be covered by fossil fuels, a full third by the use of oil.

Studying the figures above, it becomes clear that, despite the obvious popularity of wind power, the real promise of renewable energy lies with the sun. In 2007, almost half of global investments in new renewable power and heating capacity were for wind power, less than a third for solar photovoltaics (PV). An estimate puts the total global renewable energy investment in 2007, R+D included, to $100 billion.

Solar photovoltaics is one of many means used to harness the energy of the sun. PV has great promise because it converts light directly to electricity and continues to do so in daylight even when no direct sunlight is available. This is a clear advantage compared to solar thermal "concentrating" power stations which can only use direct sunlight. Solar thermal technologies are very promising for those parts of the world with high average peak sun hour record. These areas are typically found in what constitutes most of world's deserts.

Photo: Dr. Andreas Vogler

THE EUROPEAN UNION has examined available facts and figures, and is increasingly investing into research and development aimed at solving tomorrow's energy problems. EU's Seventh Framework Programme for research and technological development (FP7) runs from 2007 to 2013 with a programme budget of 53.2 billion euro. One part of the programme is aimed at energy issues, another part towards funding research and development in the field of nanotechnologies. In this story nano meets renewable energy.

INCREASING THE ACTUAL, PHYSICAL SURFACE of a square meter, with no change in the dimensions of the sides of the square, sounds like a real estate agent's wet dream.

To a group of European and American science institutions and companies the above is not a dream, but something which they will accomplish within a set time frame of three years. The aim of the group is to revolutionize solar energy markets with a new concept of nano-scale thin-film light traps. These three-dimensional light traps add, through their third dimension, to the surface per square meter capability of thin-film photovoltaic solar cells. The nano-rod carpet layer, at most a few 痠 thick, shows a strongly increased optical absorption capacity in comparison to traditional thin-film solar cell structures.

Finnish-based leading global manufacturer of Atomic Layer Deposition (ALD) reactors Picosun Oy (www.picosun.com) is one of only four industry partners of this European Union funded solar energy project called ROD-SOL (short for All-inorganic nano-rod based thin-film solar cells).

ROD-SOL aims at the synthesis of silicon (Si) nano-rods, densely packed at sufficiently large diameters (few 100 nm) and lengths (<1痠 for sufficient light absorption in indirect semiconductors) directly on low-cost substrates such as glass or flexible metal foils.

The idea of the project is to grow inherently defect free silicon nano-rods from the gas phase, with a wrapped around pn-junction that bares the potential to decouple absorption of light from charge transport by allowing lateral diffusion of minority carriers to the pn-junction, which is at most a few hundred nm away, rather than a few 痠 as in conventional thin film solar cells.

ACCORDING TO INDUSTRY ESTIMATES, thin film solar cells will dominate the photovoltaic market in the future and replace bulk silicon from its current leading position. The increase in the capabilities of the photovoltaic energy market will add to the global capacity for renewable energy.

iSuppli, a world-leader in PV market research has recently counted 115 companies worldwide either developing or building production lines for thin-film cells. In 2012, thin-film cells could represent more than 30 percent of the global PV module production. iSuppli estimates that the market for thin-film cells could reach $6.5 billion in 2012.

Photovoltaics is entering the wholesale electricity market and the following decade will see it explode. Only the best thin-film technologies will be able to spearhead such an explosion. Criteria for success of thin-film in the long term are low cost, high efficiency, non-toxicity, abundance and durability.

The ROD-SOL silicon nano-rod based solar cell material on glass self-evidently fulfills most of these criteria with the exception of high efficiency. A solution to this problem has to be demonstrated experimentally within the course of the ROD-SOL project.

PICOSUN'S RESPONSIBILITY IN THE PROJECT is, together with the German AIXTRON AG, to build equipment to allow for industrial processing of the novel solar cell materials. Picosun is a world leader in ALD technologies and carries the responsibility to crack the ALD nut of the equation: industrial level metal oxide contact layers for the structures of the film. AIXTRON, a leading provider of deposition equipment to the semiconductor industry and world's leading manufacturer of Metal Organic Chemical Vapor Deposition (MOCVD) equipment, will concentrate on developing industrial scale Chemical Vapor Deposition (CVD) techniques.

Of the eleven ROD-SOL partners, measured in terms of development costs, Picosun is the second largest, preceded only by the coordinator of the project, the German Institute of Photonic Technology. The European Union invests 2.9 million euro towards the total ROD-SOL cost of 4 million.

THIS SOLAR ENERGY PROJECT EXPLAINED ABOVE aims at radically changing the use and abilities of an established PV material, silicon. Simultaneously, there is activity to go beyond silicon, and find other, better or more cost-effective PV materials. A recent article in the Environmental Science & Technology magazine (Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment) studies alternative materials.

Cyrus Wadia, A. Paul Alivisatos and Daniel M. Kammen, scientists from the University of California at Berkeley (Berkeley's Marvell Microlab is a Picosun Partner and runs a recently installed Picosun SUNALE(TM) ALD reactor) studied 23 promising semiconductor materials and found twelve composite materials systems to have the abundance and capacity to meet or exceed the annual worldwide electricity consumption of 17,000 terawatt hours. Of these twelve nine were found to have the potential for a significant cost reduction over crystalline silicon.

Obviously, Berkeley scientists cannot yet study the abilities of ROD-SOL PV material systems which, as stated by ROD-SOL -partners, already aim, through the introduction of the novel material, to significantly reduce production costs and double the efficiency in comparison to the current thin-film PV solar systems.

The science of ALD, having the ability to build composite materials one atomic layer at a time, could at some point in the future very well add to the selection of semiconductor materials available for PV use. ALD has the capability of producing combinations of materials which do not exist in nature.

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