Length-dependent self-assembly of oligothiophene derivatives in thin films
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Jodi M. Szarko Department of Chemistry, Northwestern University, Evanston, Illinois 60208; and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208
Byeongdu Lee and Joseph Strzalka X-ray Sciences Division, Argonne National Laboratory, Argonne, Illinois 60439
Jianchang Guo Chemical Sciences and Engineering Division and Argonne National Laboratory, Argonne, Illinois 60439; and Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637
Yongye Liang Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637
Luping Yua) Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208; and Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637
Lin X. Chenb) Department of Chemistry, Northwestern University, Evanston, Illinois 60208; Chemical Sciences and Engineering Division and Argonne National Laboratory, Argonne, Illinois 60439; and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208 (Received 6 May 2010; accepted 4 August 2010)
Thin-film aggregation characteristics of a series of oligothiophenes with a central thieno[3,4-b] thiophene ester unit and 4 (M5), 8 (M9), and 16 (M17) regioregular hexylthiophene units were investigated. These oligomers exhibited length-dependent self-assembly characteristics upon spin coating. M9 formed long fibers, while M5 and M17 formed random domains. Grazing incidence x-ray diffraction was performed to understand the reason for this length dependence. The M5 had a dominant ester–ester interaction that disrupted long-range order. The M9 morphology was due to a balance of orthogonal backbone and ester effects, which imposed long-range order on the M9 aggregates. Meanwhile, the M17 ester chain had a smaller relative contribution to packing and functioned as a molecular defect, disrupting long-range order. As a result, though the local self-assembly between monomers was very similar for the molecules, backbone length dependent changes in intermolecular forces dominated long-range structure. The analysis of self-assembly characteristics in these materials provides guidance in the design of organic conjugated materials for use in semiconductor devices.
I. INTRODUCTION
The p-conjugated organic molecules are important building blocks for lightweight, highly tunable,1 costeffective,2 and easily processed3 materials used in devices such as organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaic (OPV) cells.4 However, one serious challenge in the Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2010.14 296
J. Mater. Res., Vol. 26, No. 2, Jan 28, 2011
http://journals.cambridge.org
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design of these materials lies in understanding how molecular structure
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