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Self-assembling polymers could enable ultracheap solar panels

Photonics Spectra
Sep 2011
Dual probes reveal effect of molecular organization on solar efficiency.

Marie Freebody, Contributing Editor,

It is no surprise that many solar developers believe that polymer (plastic) cells that can cover huge areas could help bring us into a new age of renewable energy. Polymer solar cells are much cheaper to produce than conventional silicon types and have the potential to be produced in large quantities. But first, their efficiencies, which lag behind those of silicon solar cells, must be improved.

Scientists from the Universities of Sheffield and Cambridge in the UK have found that, when complex mixtures of molecules in solution are spread onto a surface, the various molecules separate to the top and bottom of the layer in a way that maximizes the efficiency of the resulting solar cell.

Solar cell with polymer structure. Courtesy of Dr. Andrew Parnell.

“Our results give important insights into how ultracheap solar energy panels for domestic and industrial use can be manufactured on a large scale,” said Dr. Andrew Parnell of the University of Sheffield. “Rather than using complex and expensive fabrication methods to create a specific semiconductor nanostructure, high-volume printing could be used to produce nano-scale (60-nm) films of solar cells that are more than a thousand times thinner than the width of a human hair.

“These films could then be used to make cost-effective, light and easily transportable plastic solar cell devices such as solar panels.”

The researchers discovered that the useful self-assembly nature of the molecules had taken place by probing the morphology and composition of the solar cells using the Diamond Light and ISIS neutron sources at the Science and Technology Facilities Council at Rutherford Appleton Laboratory in Oxfordshire.

The Diamond Light Source. Courtesy of the Science and Technology Facilities Council.

In the work, which was published in the July 4 issue of Advanced Energy Materials, two molecules – PCDTBT and PCBM – were blended together. The mixture was then deposited as a thin film onto a silicon substrate using spin-coating techniques. The silicon substrate provides an ultraflat surface from which the polymer solar cell layer can be characterized in an unambiguous way.

To reveal the inner distribution of molecules, the team shone bright beams from the Diamond x-ray source onto the solar cell’s surface to study the crystallinity of the material, while neutrons from the ISIS neutron source were used to examine the material’s composition profile.

The ISIS target station. Courtesy of the Science and Technology Facilities Council.

“The advantage of these two techniques is that they allow us to probe the polymer solar cell layer in a nondestructive way and allow us to peer below the surface and study the internal structure of the layer, which we already know has a big impact on the operation of a polymer solar cell,” Parnell said.

Currently, the polymer solar cell exhibits an efficiency of 4.9 percent; however, Parnell and his colleagues are hoping to boost this to 10 percent or more for commercial viability.

The research was funded with a grant from the Engineering and Physical Sciences Research Council, and the collaboration has just been allocated a new grant to carry out further studies. This includes more analysis into the structure and function of polymer solar cell materials as well as an examination of new materials and innovative processes for high-volume manufacture and future commercialization.

A material whose molecular structure consists of long chains made up by the repetition of many (usually thousands) of similar groups of atoms.
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