Polymer progress

Polymer photovoltaic devices could potentially provide a less expensive means of producing solar energy than their inorganic photovoltaic counterparts, which are made from materials including silicon and gallium arsenide.

Yet the widespread deployment of solar systems based on polymer photovoltaics has been limited, partly due to their low efficiency at converting solar power into useful electric power.

Now, however, a new type of high-efficiency organic polymer, developed by the department of chemistry at Strathclyde University, could greatly improve the efficiency of such solar-powered systems.

Dubbed 'polythiophene-tetrathiafulvalene', or PTV-TTF for short by its developers, it could have a significant impact on the solar business not least because it could be produced using reasonably unsophisticated and inexpensive means.

The structure of the new polymer, and more specifically the alternating lengths of the carbon-carbon bonds in it, creates an energy gap that gives rise to a conduction band and a valence band. Furthermore, the material presents a low band gap due to the introduction of the fused tetrathiafulvalene (TTF) to the thiophene of the main chain of the polymer where polarons and biopolarons are stabilised upon charge transfer from the TTF to the acceptor.

Because of this, the polymer acts like a semiconductor. When photons of light fall on the material, they create electron-hole pairs in the material that then separate, resulting in the production of an electric current, as charge from the PTV-TTF donor molecule is transferred to the C60 acceptor in a photovoltaic device.

The electron-hole pairs are only produced if the energy of the photon is higher than the value of the band gap of the material. Hence, the band gap of the polymer determines what portion of the solar spectrum it can absorb.

Many current photoactive polymer materials have relatively large band gaps between 1.8 to 2.4 electron volts (eV). Because of this, the polymers can only absorb light of low wavelengths in the visible spectrum and the power created by such devices is constricted by the number of photons that can be collected in that range.

Materials with low band gaps from 1.3-1.4eV are a desirable target for developers of organic photovoltaics, because such materials can then absorb light in the most lucrative part of the solar spectrum, which means the photocurrent from a solar cell manufactured from it will be a lot greater.

That is certainly the case with one of the PTV-TTF polymers developed by Peter Skabara, a professor of materials chemistry at Strathclyde University. The polymer has a band gap of 1.44eV, unlike the most common polymers used in organic photovoltaics, which sport a band gap of approximately 2eV. The low band gap means that the photocurrent observed at a wavelength of light of 650nm has around three per cent efficiency.

The polymers were synthesised via well-established conventional palladium-catalysed Stille cross-coupling protocols, and the devices fabricated were solution-processed, or spin-cast, to create a thin film.

There is a patent filed on the technology and several companies have demonstrated interest in developing it further, although there are no commercial partners yet.

According to Dr Filipe Vilela, a research fellow at Strathclyde, ongoing work involves investigating important properties of the specific polymers that influence the efficiency of the photovoltaic device. Along with others, Vilela is working to tune the band gap and the morphology of the polymer further.

While the Strathclyde polymer represents one approach to increasing the number of photons that can be harvested from a material, it is by no means the only polymer that is being investigated by researchers across the world.

Numerous other polymers, as well as alternative manufacturing techniques, are under development that may turn out to be equally effective for polymer photovoltaic devices.

These include multiple-layer solar cells with materials of different band gaps, the use of light-scattering particles embedded in optically active layers of a polymer to enhance its performance through light scattering, as well as dye-sensitised solar cells.

David Wilson