"Organic semiconductors have recently attracted much attention and show great promise in the ongoing effort to lower the cost of solar cells," Jim McLeskey of Virginia Commonwealth University told nanotechweb.org. "These materials are usually soluble in common solvents, which leads to the possibility of making large-area thin-film solar cells using inexpensive liquid-based processing techniques."

Unlike other polymers used to make solar cells, sodium poly[2-(3-thienyl)-ethoxy-4-butylsulphonate] (PTEBS) is water-soluble. Solutions of the polymer in water have a low viscosity, so the scientists used a doctor blade technique rather than spin coating to deposit a film of the polymer onto a titanium dioxide-coated substrate. The doctor blade technique sweeps a glass rod across the polymer film as it dries.

The presence of a nanocrystalline film of titanium dioxide helps to enhance charge separation in solar cells. To lay down the film, the team spin coated a solution of anatase titanium dioxide powder in acetic acid onto a layer of fluorinated tin oxide on a glass substrate. Then they sintered the material at 500 °C for one hour. This resulted in a porous nanocrystalline network of titanium dioxide particles. The average particle size was 80 nm and the pores were more than 20 nm across – large enough for the polymer to penetrate.

Once they had deposited the polymer coating onto the titanium dioxide layer the team dried the structure at 150 °C overnight to remove any remaining water. Then they applied a mask and a thin gold layer to act as an electrode. The resulting devices had an energy-conversion effficiency of 0.13%, an open circuit voltage of 0.81 V, a short-circuit current density of 0.35 mA/sq. cm and a fill factor of 0.4.

"Using water as a solvent has several benefits," said McLeskey. "It is an environmentally friendly, low-cost solvent that allows safe processing. In addition, using water allows the fabricator to use heat to adjust the evaporation rate. This leads to greater control over device morphology and electronic properties. The water-soluble polymer is also less sensitive to moisture and oxygen in the air."

It’s also possible to tune PTEBS' absorption spectrum by changing the pH of the aqueous solution. In water the material has an absorption band of 400–650 nm. Adding acid to the solution creates an absorption band of 600–800 nm. The researchers say that this means that they could harvest a greater portion of the solar spectrum by building tandem junction cells containing PTEBS layers made using both acidic and basic solutions.

The researchers say that the water-soluble polymer may have applications in photodetectors, light-emitting diodes, displays and sensors, as well as in solar cells.

"We are working to optimize our devices by improving our fabrication techniques," said McLeskey. "In addition, our best devices to date have been in a bilayer configuration where we deposit a layer of TiO2 and then deposit a layer of polymer on top. We are working on building efficient bulk heterojunction devices where the TiO2 (the electon acceptor) and the polymer (the electron donor) are deposited simultaneously."

The researchers reported their work in Applied Physics Letters.