Organic tin-based semiconducting polymers show solar cell promise
By incorporating organic tin into the plastic, light can be absorbed across a wide range of the solar spectrum. The new polymer has been detailed by project leader Professor Anne Staubitz and the Ph. D. student Julian Linshöft in the journal ‘Angewandte Chemie, International Edition’.
Contrary to electric conductors such as metals, semiconductors are materials that conduct electricity only under certain circumstances, for example under irradiation with light. Because of this property semiconducting polymers are promising materials for the latest generation of solar cells – organic solar cells. Compared to the classical inorganic variants, their fabrication can be cheaper and they are very light weight materials, which can be advantageous for many applications, for example in the transport sector. “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,” explained Anne Staubitz from the Otto Diels-Institute.
An important criterion of such semiconductors is how efficiently they absorb sun light in order to convert it into electricity. When sun light is converted into electricity, negatively charged electrons in the semiconductor are being lifted from one energy level into a higher energy level. This process leaves behind a positively charged ‘hole’ in the lower energy level. Then the charges are percolating separately to the different electrical poles: A current can be observed. Sun light is able to initiate this process. The closer these energy levels are together, the more facile this process is: More photons can be absorbed and thus more solar energy can be used. Polymers, in which this band gap between the energy levels is small, have a red, in rare cases even a purple colour.
One aim of synthetic organic semiconductor research is to produce organic polymers with small energy gaps (or band gaps). However, the development of such strongly light absorbing, deeply coloured plastics is difficult. “With the new material from our laboratories, it is visible to the naked eye that we were successful in developing such plastics!” said Staubitz. The polymer is deep purple in solution and almost black when processed into a thin film.
In order to achieve very small energy gaps, the scientists from Kiel used a new concept. They incorporated organic tin in the form of cyclic molecules (‘stannoles’) into the carbon-polymer backbone. Tin belongs to the same chemical group as carbon and is therefore similar in some of its properties. The electronic properties however between stannoles and the corresponding carbon congeners (cyclopentadienes) are different. “Tin is not just an overweight carbon atom,” explained Staubitz. “It can lower the energetic levels in its organic compounds dramatically.” But until now, nobody was able to use these special properties of tin in polymeric materials".
Joining these individual molecular building blocks (the monomers) together was a difficult task for the researchers: The monomers did not only contain the desired tin in the stannole-units themselves; organic tin was also present in the reactive coupling groups that were necessary for joining the monomers together to form the polymer. Only these groups were supposed to react, whereas the stannole rings should not be attacked. This was vital, because any undesired side reaction would lead to a significant shortening of the polymer chain, leading to a substantial deterioration of the polymer’s quality. “This was a high risk project, because coupling reactions that can select between two different organic tin groups were not known in chemistry before,” said Staubitz. Ph. D. student Julian Linshöft did not just have to develop a selective, but a highly selective cross-coupling reaction. “The first difficulty was to find the correct reactivity patterns for the monomers,” recalled Linshöft.
The experiment was a success. The team was able to prepare the desired plastic using palladium as a reaction catalyst. The material can be processed easily into thin films, which are gleaming black and whose application in solar cells can now be tested.
Reference: Julian Linshoeft, Evan J. Baum, Andreas Hussain, Paul J. Gates, Christian Näther and Anne Staubitz. Highly Tin Selective Stille Coupling: Synthesis of a Polymer Containing a Stannole in the Main Chain. Angewandte Chemie, International Edition DOI: 10.1002/anie.201407377
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