Organic electronics has potential for use in a number of fields. In order to take full advantage of this new technology, however, scientists need to gain a better understanding of what exactly goes on at the semiconductor interface. This was the objective behind a recent EU-funded project.
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Organic electronics is a rapidly growing field that relies on the use of organic conjugated materials (rather than inorganic semiconductors like silicon) as active components in devices such as light-emitting diodes, field effect transistors, biochemical sensors, storage devices or solar cells. It is called 'organic' electronics because the polymers and small molecules are carbon-based.
As these devices are usually based on a sandwich-type structure, interfaces between layers of a different chemical nature play a crucial role in the overall working mechanism. Indeed, many key electronic processes - such as charge injection from metallic electrodes, charge recombination into light or light conversion into charges, spin injection, etc. - occur at interfaces.
The EU-supported Minotor project worked on developing a deeper understanding of the electronic processes taking place at these interfaces. This work is important, because understanding what goes on here could significantly increase the number of potential applications for organic electronic devices.
Poor and rich
"To cite just one example, charge separation in organic photovoltaic cells occurs at hetero-junctions between electron-rich and electron-poor materials," explains Dr David Beljonne, one of three project coordinators. "It is therefore of the utmost importance to account for the reshuffling in electronic density at the interfaces, when designing the donor and acceptor materials with appropriate energy-level alignments."
Furthermore, although a reasonable amount of study has been carried out on the characterisation of such interfaces - especially morphological issues a detailed and unified understanding of the electronic processes occurring at these interfaces has remained elusive.
Discoveries such as this have enabled the international team behind Minotor to identify possible new material combinations with tailored interfacial characteristics. "There are lots of advantages to organic semiconductors," says Dr Jérôme Cornil. "The equipment is less expensive, they are easier to process, and you can even print them."
The project sought to assess the electronic processes occurring at interfaces through theoretical modelling tools, supported by surface-sensitive characterisation techniques. One major outcome was the development of a multi-scale theoretical approach capable of modelling such interfaces in a highly realistic manner. This enabled the scientists to pull together a more unified view of the electronic phenomena taking place at the interfaces.
Theoretical predictions were then compared to experimental investigations performed in the consortium, thereby allowing direct feedback between theory and experiment.
"The benefit of this project to Europe is twofold," continues Dr Beljonne. "First, the basic knowledge we've accumulated during the project will be a stepping stone in the development of a new generation of material and device architectures with improved efficiencies. Secondly, Minotor has been able to train a young generation of researchers on multi-scale modelling, which we hope and believe will become a ubiquitous toolkit in emerging technologies."
Another major success of Minotor, says Prof. Roberto Lazzaroni, has been the fact the project has facilitated both theoretical and experimental research activities on joint objectives related to interfaces. This, he believes, has led to the publication of some excellent work, and will ensure that the legacy of Minotor will be felt for years to come.
The final workshop, which was organised in Mons, Belgium earlier this year, gave the Minotor consortium the opportunity to show the scientific community just how far our understanding of organic electronic interfaces has come.