The effect of nanoscale dielectric morphology on the microstructure and carrier mobility of high-performance polythiophene semiconductors
Poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b]thiophene) (pBTTT), a high performance polymer semiconductor with charge carrier mobilities of 0.2 - 0.6 cm2/Vs, forms large and well-oriented terraced domains after it is cast upon flat, low surface energy substrates and heated to a mesophase. Upon dielectrics with root-mean square (RMS) roughness greater than 0.5 nm, we find significant detrimental morphology changes in the pBTTT active layer. This critical RMS roughness implies that there exists a condition at which pBTTT domains can no longer remain intact while conforming to the substrate. This result has important implications for the application of pBTTT to plastic substrates and polymer dielectrics, because it places stringent constraints on the acceptable dielectric morphology. However, the nanoscale curvature of the dielectric features may play a more important role in pBTTT terrace disruption than the simple value of the RMS roughness.
To precisely determine this critical nanoscale curvature, we developed model dielectrics using imprint lithography, with (10 to 20) nm deep line patterns etched into 200 nm silicon oxide dielectrics. The nanoscale radii of curvature at the convex line edges were controlled by varying etch depth. Characterization by atomic force microscopy (AFM), scanning electron microscopy (SEM), and field-effect carrier mobility measurements reveals that sharp line edges with small radii of curvature may interrupt domain growth during terrace formation. Interestingly, this interruption results in a significant 5x anisotropy in in-plane carrier mobility atop the line-patterned dielectrics. The critical nanoscale radius provides a precise guideline for acceptable dielectric characteristics, to facilitate the design of high-performance flexible circuits.
Mentor: Dean M. DeLongchamp
Materials science and Engineering Laboratory
National Institute of Standards and Technology
100 Bureau Drive, MS 8541
Gaithersburg , MD 20899-8541, USA
Building 224, Room B312
Tel: (301) 975-4769
Fax: (301) 975-3928