Nanorods for solar cells

A solar project team is investigating cost-effective ways to grow nanorods on large substrates

Commercial solar cells could have double the efficiency in five years’ time with novel research into nanomaterials.

The EU-funded project, called ROD-SOL (Rods for Novel Solar Cells), aims to create large-scale methods for growing arrays of small, 3D silicon cylinders known as nanorods. This would replace conventional silicon-growing techniques that involve casting ingots and cutting wafers.

A solar cell based on arrays of 3D nanorods will absorb more photons than regular 2D wafers because it scatters less light, said Juhana Kostamo, managing director of Picosun, an atomic layer deposition (ALD) systems manufacturer based in Finland. The company is one of several industrial partners involved in ROD-SOL. ‘Current solar cells in mass production have an efficiency typically ranging from five per cent to 10 per cent. With this technology it is possible to double that efficiency.’

While several companies and research groups have produced working prototypes of solar cells based on silicon nanorods, the technology has yet to be commercialised. One reason is cost. Silicon nanorods are currently grown on silicon wafers, but the ROD-SOL team hopes to optimise a method for growing silicon nanorods on larger, less expensive substrates, such as glass or synthetic foil that could be fashioned into thin-film solar cells. Silicon nanorods are typically grown using a process common in the semiconductor industry known as chemical vapour deposition (CVD). In this process, a substrate is exposed to vapourising compounds that either react or decompose on its surface to produce a deposit. Once the nanorods are grown, they are joined together to form a matrix using atomic layer deposition (ALD).

The deposition technique, which was developed in Finland in the 1970s for memory chips and microprocessors, uses sequential gas phase chemical processes to deposit a transparent conductive oxide film that connects the nanorods together.

Kostamo said that the ROD-SOL team hopes to develop a feasible method for using these processes to produce arrays of nanorods on a mass scale in three years. A commercial product, he added, could be available as soon as five years.

‘The area of solar energy is moving very fast,’ said Kostamo. ‘We have proved that the basic idea of having 3D solar cells is possible and we hope to complete a record efficiency with this technology.’

Picosun estimates for illustration purposes that the world’s electric power needs could be met with a photovoltaic square panel with 380km-long sides.

This corresponds to the surface area equivalent to roughly half the size of Italy.

The more effective that photovoltaics can be made, the more likely they will find future applications, said Kostamo. ‘We hope in the future everyone can enjoy cheaper solar energy because of better efficiency,’ he added.

ROD-SOL will last three years and the EU has allocated €2.9m (£2.6m) to it.

It includes scientific partners from Austria, Finland, Germany, Hungary, Switzerland and the US and industrial collaborators from Finland, Germany and Slovenia.

Siobhan Wagner