Scanned-Probe Characterization of Charge Trapping and Fluctuations in Organic Semiconductors

Cornell University, Ithaca NY

Investigators

Abstract

TECHNICAL SUMMARY: A microscopic understanding of the mechanisms of charge trapping, transport, injection, and charge generation in organic semiconductors is presently lacking. The development of organic circuits and solar cells could be greatly accelerated if a better basic understanding of these fundamental processes were available. Improving our basic understanding of fundamental processes in organic semiconductor devices is challenging. Nearly all organic semiconductor devices show significant device-to-device variation, and the most promising devices are often comprised of complex multicomponent blends. To build up a microscopic picture of charge trapping and transport in organic semiconductors, we will study organic devices in situ using vacuum, variable-temperature electric force microscopy. We will use light-enhanced electric force microscopy as a tool to spectroscopically identify impurities, study charge generation, and probe trapping mechanisms in a wide range of organic semiconductors. In a second set of experiments, a high-compliance silicon microcantilever will be used to measure minute electric field gradient fluctuations near the surface of an organic semiconductor. From these electric field fluctuations we propose to deduce (and image) the diffusion constant of charges beneath the cantilever tip. We expect these microscopic studies will open up exciting possibilities for advancing our understanding of charge generation, transport, trapping, and injection in organic semiconductor materials and devices. This project will train graduate students in the arts of advanced scanned probe microscopy and nanofabrication. These students will broaden their training by working on collaborative projects with scientists at academic, federal, and industrial laboratories. This work is funded by the Solid State and Materials Chemistry program. NON-TECHNICAL SUMMARY: In order for our nation to obtain energy independence, we must be able to manufacture solar cells that can convert sunlight efficiently into electricity. Many materials are being examined for use in solar cells, and none work as well as we need. One promising class of materials is semiconducting polymers, plastics that have the remarkable property of being able to both absorb light and conduct electricity. In order to get these materials to work well in solar cells, the materials need to absorb light, the absorbed light must be converted into an electrical current, and the current must be carried through the material and extracted into a wire. These last two processes - the conversion of light to current and the transport of charge - are not well understood in these materials. Without a better understanding of these processes, it is not clear how to manufacture improved solar cells from semiconducting polymers. Characterizing these materials is challenging, because their properties show large variations across distances separated by only 10 billionths to 100 billionths of a meter - distances hundreds to thousands of atoms across. To advance our understanding of semiconducting polymers, we will develop new kinds of microscopes that can take pictures of both moving and stationary charges at this length scale in working solar cells. This work will promote the general welfare by training PhD and undergraduate students to do research in energy-related materials and nanotechnology. This work will involve collaboration and knowledge sharing among multiple universities, government laboratories, and industrial laboratories. This work is funded by the Solid State and Materials Chemistry program of the U.S. National Science Foundation.