Solar power: An organic tin approach

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  • Published: Oct 1, 2014
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Solar power: An organic tin approach

Tin integration

Highly Tin-Selective Stille Coupling: Synthesis of a Polymer Containing a Stannole in the Main Chain Credit: Staubitz et al/Angewandte Chemie/Wiley-VCH

German researchers have successfully integrated an organic tin moiety into a semiconducting polymer for the first time to make a new class of solar energy absorption material that can absorb a broad band across the electromagnetic spectrum from the sun and might one day lead to new solar power materials.

Julian Linshoeft and Anne Staubitz of the Otto Diels Institute for Organic Chemistry at the University of Kiel, Germany, and their colleagues explain the potential of semiconducting polymers in solar energy conversion. In contrast to electrical conductors such as metals, semiconductors conduct electricity only under certain conditions, such as under irradiation with light. The conducting polymer variant are much cheaper than conventional semiconductors, such as silicon and gallium arsenic, are also less dense, flexible and have the benefit of being simpler to process.

Band gap

"However, organic solar cells still do not achieve the same efficiencies as inorganic solar cells based on silicon so that there is a substantial need for research in this area," explains Staubitz. One critical consideration in using semiconducting polymers the efficiency with which the absorption of sun light can excite the negative electrons and leave behind a band of positive holes at a lower energy leading to a percolation of charges to the opposite electrical poles and giving rise to a current. The closer together the energy levels, the smaller the band gap, in other words, the greater the number of photons that might be absorbed and thus the larger the amount of solar energy that can be rendered as an electrical current. Polymers with suitably small band gaps are commonly red and rarely even purple in colour because of the colours they absorb.

One aim of synthetic organic semiconductor research is therefore to produce organic polymers with such small band gaps. Unfortunately, the development of such strongly light absorbing, highly coloured materials is very difficult.

The researchers turned to a novel concept to make their low band gap semiconducting polymers. They incorporated an organic tin group, a cyclic molecule known as a stannole into the more conventional organic polymer backbone. Tin, of course, shares group 14 of the Periodic Table with carbon (and silicon and germanium) and has some similarities in its chemical and physical properties. Nevertheless, the electronic properties of the stannole molecules and their carbon equivalent, the cyclopentadienes are very different. "Tin is not just an overweight carbon atom," Staubitz explains. "It can lower the energy levels in its organic compounds dramatically."

High risk polymers

Joining the individual stannole molecular building blocks together to form a viable polymer was no easy task for the researchers. The presence of tin in the stannole units themselves and in the reactive coupling groups that were necessary for joining the monomers together to form the polymer meant that there were two sites of reactivity one of which would lead to polymerization, the other to unwanted side products and shorter, useless, polymer chains. "This was a high risk project, because coupling reactions that can select between two different organic tin groups were not known in chemistry before," Staubitz adds. Graduate student Julian Linshoeft was charged with the job of having to develop the requisite highly selective cross-coupling reaction. "The first difficulty was to find the correct reactivity patterns for the monomers," Linshoeft recalls. "For this, there was no lead in the chemical literature."

His experiments were a success. The team was able to prepare the desired plastic using a palladium catalyst. The resulting product can be processed readily into thin film form. "With the new material from our laboratories, it is visible to the naked eye that we were successful in developing such plastics," says Staubitz. The polymer is deep purple in solution and almost black when processed into a thin film.

"At the moment, the central focus of the work was to make these compounds accessible and in this sense you could say that it is, at this stage, of more fundamental than applied character," Staubitz told SpectroscopyNOW. "However, now this problem has been solved and we need to look at what this material can really do. So the next step is making more of it, which sounds trivial, but it isn't," she confesses. Viable quantities of around 200 milligrams would be necessary for further studies especially if the material shows promose. "Another step is a much more complete physical characterisation," Staubitz adds. "For solar cells, many more aspects play a role than just the absorption spectrum. For example, we need to look at charge carrier mobility, which we would measure in an organic field effect transistor. We will also need to build prototype solar cells and look at their performance. Another thing we do not know at this moment in time is how stable the material will prove to be under irradiation."

Ultimately, the team would like to see this material move to a useful application, but although solar cells are of course a very attractive target, they are also open towards other applications such as field effect transistors (FETs) or polymer light emitting diodes (LEDs).

Related Links

Angew Chem Int Edn, 2014, online: "Highly Tin-Selective Stille Coupling: Synthesis of a Polymer Containing a Stannole in the Main Chain"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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