A New Structural View Of Organic Electronic Devices

In the on-line issue of Advanced Materials,* researchers from theNational Institute of Standards and Technology (NIST) and theUniversity of California at Berkeley report success in using anon-destructive measurement method to detail three structuralproperties crucial to making reliable electronic devices with thinfilms of the carbon-rich (organic) semiconductors. The new capabilitycould help industry clear hurdles responsible for high manufacturingdevelopment costs that stand in the way of widespread commercialapplication of the materials.

With the technique called near-edge X-ray absorptionfine-structure spectroscopy, or NEXAFS, the team tracked chemicalreactions, molecular reordering and defect formation over a range ofprocessing temperatures.

They then evaluated how process-induced changes in thin-filmcomposition and structure affected the movement of charge carriers(either electrons or electron "holes") in organic field effecttransistors, devices basic to electronic circuits. With NEXAFSmeasurements taken over the range from room temperature to 300 degreesCelsius, the team monitored the conversion of a precursor chemical toan oligothiophene, an organic semiconductor. The molecular organizationand composition achieved at 250 degrees Celsius yielded the highestlevels of charge carrier movement and, consequently, maximumelectric-current flow.

As chemical conversion progressed, the researchers calculatedhow the molecules arranged themselves on top of an electricalinsulator. Top transistor performance corresponded to a verticalalignment of molecules. In addition, they used NEXAFS to determine theangles of chemical bonds and to assess the thickness and uniformity offilm coverage, also critical to performance.

NEXAFS has the potential to be the "ideal measurement platformfor systematic investigation" of organic electronic materials, sayslead investigator Dean DeLongchamp, a NIST materials scientist. "Astraightforward means of correlating chemical and physical structure tothe electronic performance of organic semiconductor films is amuch-needed tool."

The research was conducted at the NIST/Dow Chemical materialscharacterization facility at the National Synchrotron Light Source.Funding providers included the U.S. Department of Energy, DefenseAdvanced Research Projects Agency and the Microelectronics AdvancedResearch Corporation.

*D.M. DeLongchamp, S. Sambasivan, D.A. Fischer, E.K. Lin, P.Chang, A.R. Murphy, J.M.J. Frechet, and V. Subramanian, "DirectCorrelation of Organic semiconductor film structure to field-effectmobility," Advanced Materials, published online Aug. 30, 2005, DOInumber (10.1002/adma.200500253).