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Surface Ordering of High-Performance Semiconducting Polymer Films

Conjugated (alternating single and double bonds) organic polymers are increasingly being developed as functional materials for various electrical and optical devices including organic thin film transistors, photovoltaic power sources, and organic light emitting diodes (OLEDs). Practical devices require high electrical charge carrier mobilities and polarized optical photon emission. Such characteristics are obtainable when the conjugated polymer films are macroscopically ordered. For certain device architectures the alignment of the polymer surface plays a large role in the device performance. For instance, in OLEDs and photovoltaic sources, the alignment affects charge injection and separation at interfaces, and for non-inverted transistor structures the top surface of the semiconductor is crucial to carrier mobility, as this is the region of current flow in transistor operation.

Fluorene-thiophene semiconducting polymers exemplify the strong correlation of polymer chain alignment with charge carrier mobility. When polymer chains are aligned parallel to the transistor channel length, the field-effect mobility values are two to three times greater than mobility for as-deposited (amorphous) films, and four to six times greater than values obtained for devices with polymer chain alignment perpendicular to the channel length. Strong optical evidence of bulk fluorene-thiophene alignment on rubbed polyimide alignment layers exists, yet it was not known whether this alignment persists throughout the film to the surface

To answer this question, researchers from MML and the University of California, Santa Barbara, measured the surface reorientation of fluorene-thiophene polymers induced by deposition on a rubbed polyimide alignment layer after various thermal treatments. The researchers used Near-Edge X-ray Absorption Fine Structure measurements at the NIST low energy x-ray spectroscopy beamline, U7A, at the National Synchrotron Light Source, to determine the molecular orientation at the surface. The measurements revealed strong surface alignment of the fluorene-thiophene main-chain axis with the rubbing direction of the alignment layer, and the temperature stability of the alignment. Further measurements also showed that rubbing of the polyimide substrate aligns the surface molecules along the rubbing direction, creating an asymmetry in the molecular bonds at the surface. The unidirectional molecular alignment at the surface of the rubbed polyimide provides a template for the fluorene-thiophene orientation when heated and validates a paradigm for possible fluorene-thiophene high performance semiconductor device fabrication.


Daniel A. Fischer
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