Paper solar cells can be folded away and used when required

At present, the paper solar cells have an efficiency of about one per cent — enough to power ‘small electric gizmos’ — but the team believes this can be increased significantly with the further fine-tuning of the materials.

‘We have demonstrated quite thoroughly the robustness of this technology,’ said collaborator Prof Vladimir Bulović of MIT. ‘We think we can fabricate scalable solar cells that can reach record-high watts-per-kilogramme performance.’

The new printing process uses vapours, not liquids, and temperatures of less than 120ºC. These ‘gentle’ conditions make it possible to use ordinary untreated paper, cloth or plastic as the substrate on which the solar cells can be printed.

However, in order to create an array of photovoltaic cells on the paper, five layers of material need to be deposited onto the same sheet of paper in successive passes, using a mask (also made of paper) to form the patterns of cells on the surface. In addition, the process has to take place in a vacuum chamber.

But the basic process is essentially the same as the one used to make the silver lining in crisp packets: a vapour deposition process that can be carried out inexpensively on a vast commercial scale.

In conventional solar cells, the costs of the inactive components — the substrate (usually glass) that supports the active photovoltaic material, the structures to support that substrate and the installation costs — are typically greater than the cost of the active films of the cells themselves, sometimes twice as much.

Being able to print solar cells directly onto inexpensive, easily available materials such as paper or cloth, and then easily fasten that paper to a wall for support, could ultimately make it possible to drastically reduce the costs of solar installations. For example, paper solar cells could be made into window shades or wallpaper — and paper costs one thousandth as much as glass for a given area, according to the researchers.

For outdoor uses, the researchers demonstrated that the paper could be coated with standard lamination materials to protect it from the elements.