Three-dimensional solar cells

Using zinc-oxide nanostructures grown on optical fibres and coated with dye-sensitised solar-cell materials, researchers at the Georgia Institute of Technology have developed a new type of three-dimensional photovoltaic (PV) system.

The approach could allow PV systems to be hidden from view and located away from traditional locations such as rooftops.

'Using this technology, we can make photovoltaic generators that are foldable, concealed and mobile,' said Zhong Lin Wang, a Regents professor in the Georgia Tech School of Materials Science and Engineering.

'Optical fibre could conduct sunlight into a building’s walls, where the nanostructures would convert it to electricity.

'This is truly a three-dimensional solar cell.'

Dye-sensitised solar cells use a photochemical system to generate electricity.

They are inexpensive to manufacture, flexible and mechanically robust, but their conversion efficiency is lower than that of silicon-based cells.

But using nanostructure arrays to increase the surface area available to convert light could help reduce the efficiency disadvantage, while giving architects and designers new options for incorporating PV into buildings, vehicles and even military equipment.

Fabrication of the new Georgia Tech PV system begins with taking optical fibre and removing the cladding layer.

Then a conductive coating is applied to the surface of the fibre before the surface is seeded with zinc oxide.

Next, aligned zinc-oxide nanowires are grown around the fibre.

The nanowires are then coated with the dye-sensitised materials that convert light to electricity.

Sunlight entering the optical fibre passes into the nanowires, where it interacts with the dye molecules to produce electrical current.

A liquid electrolyte between the nanowires collects the electrical charges.

The result is a hybrid nanowire/optical fibre system that can be up to six times as efficient as planar zinc-oxide cells with the same surface area.

'In each reflection within the fibre, the light has the opportunity to interact with the nanostructures that are coated with the dye molecules,' Wang explained.

'You have multiple light reflections within the fibre and multiple reflections within the nanostructures.

'These interactions increase the likelihood that the light will interact with the dye molecules, and that increases the efficiency.'

Wang and his research team have reached an efficiency of 3.3 per cent and hope to reach seven to eight per cent after surface modification.

While lower than silicon solar cells, this efficiency would be useful for practical energy harvesting.

By providing a larger area for gathering light, the technique would maximise the amount of energy produced from strong sunlight, as well as generate respectable power levels even in weak light.

The amount of light entering the optical fibre could be increased by using lenses to focus the incoming light.

Wang believes this new structure will offer architects and product designers an alternative PV format for incorporating into other applications.

'This will really provide some new options for photovoltaic systems,' Wang added.

'We could eliminate the aesthetic issues of PV arrays on buildings.

'We can also envision PV systems for providing energy to parked vehicles and for charging mobile military equipment where traditional arrays aren’t practical or you wouldn’t want to use them.'

Wang and his research team, which includes Benjamin Weintraub and Yaguang Wei, have produced generators on optical fibre up to 20cm in length.

'The longer the better because the longer the light can travel along the fibre, the more bounces it will make and the more it will be absorbed,' said Wang.

Traditional quartz optical fibre has been used so far, but Wang would like to use less-expensive polymer fibre to reduce the cost.

He is also considering other improvements, such as a better method for collecting the charges and a titanium-oxide surface coating that could further boost efficiency.

Though it could be used for large PV systems, Wang doesn’t expect his solar cells to replace silicon devices soon.

But he does believe they will broaden the potential applications for photovoltaic energy.