Mitigating dynamic disorder in crystalline organic semiconductors

Non-technical summary

Organic semiconductors form a special class of soluble inks that can be used to create new forms of electronic components such as transistors, diodes, and solar cells. As the performance of these materials improves, there are ready markets for their unique properties: For example, Organic Light Emitting Diodes (OLEDs), where the organic semiconductor is used for its light-emitting properties, are now a mainstream high-end display technology.

For organic semiconductors to have impact in other key innovative technologies - such as transistors for sensing, computation, and control elements for flexible electronics, the performance metric that must be optimized is charge transport. Charge transport is related to the speed that charges (positive or negative) can move through these solids - the faster the movement, the better the performance.

It is well known that when electric current moves through a material, the material tends to get hotter (think: clothes iron or electric stove burner). When things get hotter, the molecules in those objects begin to move around. Scientists researching organic semiconductors have proposed that this movement of molecules can have a very negative impact on charge transport - however, no definitive evidence of this impact has been shown.

Supported by the Solid State and Materials Chemistry program in the Division of Materials Research at the NSF, the object of this proposal is to make a series of molecules that can each move around in different ways as the material heats up. By studying the impact of these movements on charge transport, we will be able to create design rules for the development of next-generation charge transport materials, a key enabling step in the use of organic semiconductors in technologies for lower-cost display, communication, and sensing applications.

In the process, students from a wide array of educational stages (from high school through post-doctoral) and backgrounds (particularly students from rural regions of Kentucky) will have hands-on experiences making and measuring cutting-edge materials for flexible electronic technologies. Furthermore, outreach to researchers at Eastern Kentucky University will assist faculty there to kickstart their research efforts through workshops on proposal submission to the NSF.

Technical summary

Recent studies suggested that in crystalline organic semiconductors there are certain highly disruptive (killer) phonon modes – vibrations particularly disruptive to charge transport – that limit maximum performance in devices such as transistors. While it has been known for many years that thermal dynamic disorder is detrimental to charge transport, the degree to which these motions disrupt transport in organic semiconductors has not been demonstrated.

Through the synthesis and study of three new classes of organic semiconductors, each designed to inhibit particular phonon modes, and in collaboration with physicists, spectroscopists and device engineers across the U.S., we will elucidate the relationships between phonon modes and charge transport (as measured through field-effect mobility in organic transistors). Along with providing critical data to advance our understanding of organic semiconductors, the students from these collaborative research groups will receive training in the analysis of previously unknown electronic materials.

From these data, we will determine the real degree of impact of, in particular, long molecular axis vibrations on charge transport, and then develop design rules to mitigate the impact of thermal disorder. The simplest of the synthetic targets will be prepared by high school students, introducing them at this critical early stage to collaborative research in organic electronics.

Along with outreach to recruit students from Appalachian-serving institutions in Kentucky, workshops on preparing and submitting NSF proposals will also be presented at Eastern Kentucky University, as they begin to start a program in manufacturing engineering, and desire to reignite their long-dormant research programs in the basic sciences.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.