RESEARCHNEWS Golden window of opportunity
RESEARCHERS at the University of Warwick have developed a gold plated window as the transparent electrode for organic solar cells. Contrary to what one might expect, these electrodes have the potential to be relatively cheap since the thickness of gold used is only 8 billionths of a metre. This ultra-low thickness means that even at the current high gold price the cost of the gold needed to fabricate one square metre of this electrode is only around £4.5. It can also be readily recouped from the organic solar cell at the end of its life and since gold is already widely used to form reliable interconnects it is no stranger to the electronics industry.
Organic solar cells have long relied on Indium Tin Oxide (ITO) coated glass as the transparent electrode, although this is largely due to the absence of a suitable alternative. ITO is a complex, unstable material with a high surface roughness and tendency to crack upon bending if supported on a plastic substrate. If that wasn’t bad enough one of its key components, indium, is in short supply making it relatively expensive to use.
An ultra-thin film of air-stable metal like gold would offer a viable alternative to
ITO, but until now it has not proved possible to deposit a film thin enough to be transparent without being too fragile and electrically resistive to be useful.
Now research led by Dr Ross Hatton and Professor Tim Jones in the University of Warwick ’s department of Chemistry has developed a rapid method for the preparation of robust, ultra-thin gold films on glass. Importantly this method can be scaled up for large area applications like solar cells and the resulting electrodes are chemically very well-defined.
Dr Hatton says “This new method of creating gold based transparent electrodes is potentially widely applicable for a variety of large area applications, particularly where stable, chemically well- defined, ultra-smooth platform electrodes are required, such as in organic optoelectronics and the emerging fields of nanoelectronics and nanophotonics” The paper documents the team’s success in creating this simple, practical and effective method of depositing the films onto glass, and also reports how the optical properties can be fine tuned by perforating the film with tiny circular holes using something as simple as polystyrene balls. The University of Warwick research
Polymer solar thermal
A NEW polymer-based solar-thermal device is the first to generate power from both heat and visible sunlight, an advance that could shave the cost of heating a home by as much as 40 percent. Geothermal add-ons for heat pumps collect heat from the air or the ground. This new device uses a fluid that flows through a roof-mounted module to collect heat from the sun while an integrated solar cell generates electricity from the sun’s visible light.
“It’s a systems approach to making your home ultra-efficient because the device collects both solar energy and heat,” said David Carroll, Ph.D., director of the Center for Nanotechnology and Molecular Materials at Wake Forest University.
Research showing the effectiveness of the device appears in the journal Solar Energy Materials and Solar Cells. A
standard, rooftop solar cell will miss about 75 percent of the energy provided by the sun at any given time because it can’t collect the longest wavelengths of light, infrared heat. Such cells miss an even greater amount of the available daily solar power because they collect sunlight most efficiently between 10 a.m. and 2 p.m.
The design of the new solar-thermal device takes advantage of this heat through an integrated array of clear tubes, five millimeters in diameter. They lie flat, and an oil blended with a proprietary dye flows through them.
The visible sunlight shines into the tube and the oil inside, and is converted to electricity by a spray-on polymer photovoltaic on the back of the tubes. This process superheats the oil, which would then flow into the heat pump to transfer the heat inside a home.
Unlike the flat solar cells used today, the curve of the tubes inside the new device allows for the collection of both visible light and infrared heat from nearly sunrise to sunset. This means it provides power for a much greater part of the day than does a normal solar cell.
Because of the general structure and the ability to capture light at oblique angles, this is also the first solar-thermal device that can be truly building-integrated – it can be made to look nearly identical to roofing tiles used today.
team has also had some early success in depositing ultra-thin gold films directly on plastic substrates, an important step towards realising the holy grail of truly flexible solar cells. This innovation is set to be exploited by Molecular Solar Ltd, a Warwick spinout company dedicated to commercialising the discoveries of its academic founders in the area of organic solar cells.
This work was supported by the European Regional Development Fund (ERDF) / Advantage West Midlands Science City SCRA AM2 project, the Engineering and Physical Sciences Research Council (EPSRC) and the Royal Academy of Engineering.
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www.solar-pv-management.com Issue IV 2011
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