Charge Localization and Transport at Interfaces

Nontechnical Description: Carbon-based (organic) electronics are light-weight and relatively inexpensive, but have been more difficult to realize commercially. Their electrical performance depends crucially on how spread out electrical charges become at the interfaces between different materials. However, the most common analyses report on properties through the bulk, overlooking the underlying reasons for both good and bad electrical behaviors. This project employs a measurement technique that delivers information about how molecules are arranged at material interfaces and the number of molecules over which electrical charges are shared. The project focuses on materials that work well and then introduces defects that diminish their performance, monitoring the effect of these defects on conductivity and the number of molecules on which charges reside. In parallel with these research directions, the research team designs and leads the chemistry activities at an annual three-day outreach event called University on the Prairie. This program hosts 60 students (grades 6-11) from rural communities in the Midwest and introduces many of them to the role of chemistry in agriculture, energy conversion, and in many cases, begins the process by which they envision themselves studying science at a college level.

Technical Description: The extent of localization of electrical charges at material interfaces has an enormous influence on their mobility, yet there are few characterization methods applied to organic electronic films that are interface specific. Of these surface specific approaches, only vibrational sum frequency generation has the ability to simultaneously report the molecular ordering and orientation as well as the extent of charge localization. This project seeks to directly measure the influence of molecular packing and charge localization on the efficiency of charge transport at organic material interfaces. The research team applies vibrational sum frequency generation spectroscopy to field-effect transistors constructed with a family of perylene diimide molecular films and observes the changes in interfacial structure, mobility, and localization as the molecular structure is varied. They also introduce planar thickness gradients of dielectric materials between charge donor and acceptor layers to progressively decrease the extent of localization, while carrying out the same spectroscopic analysis. Finally, photoinduced charge transfer is used to prepare charge delocalized states while vibrational sum frequency generation is performed. Overall, the project identifies the role of charge localization in transport at organic thin film interfaces.