We use a systematic approach that combines experimental X-ray diffraction (XRD) and computational modeling based on molecular mechanics and two-dimensional XRD simulations to develop a detailed model of the molecular-scale packing structure of poly(2,5-bis (3-tetradecylthiophene-2-yl) thieno[3,2-b]thiophene) (PBTTT-C 14) films. Both uniaxially and biaxially aligned films are used in this comparison and lead to an improved understanding of the molecular-scale orientation and crystal structure. We then examine how individual polymer components (i.e., conjugated backbone and alkyl side chains) contribute to the complete diffraction pattern, and how modest changes to a particular component orientation (e.g., backbone or side-chain tilt) influence the diffraction pattern. The effects on the polymer crystal structure of varying the alkyl side-chain length from C 12 to C 14 and C 16 are also studied. The accurate determination of the three-dimensional polymer structure allows us to examine the PBTTT electronic band structure and intermolecular electronic couplings (transfer integrals) as a function of alkyl side-chain length. This combination of theoretical and experimental techniques proves to be an important tool to help establish the relationship between the structural and electronic properties of polymer thin films. © 2012 American Chemical Society.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledged KAUST grant number(s): KUS-C1-015-21
Acknowledgements: This work has been supported primarily by the Center for Advanced Molecular Photovoltaics (Award No. KUS-C1-015-21 made by King Abdullah University of Science and Technology, KAUST) as well as by the National Science Foundation under the STC program (Award DMR-0120967) and under the CRIF Program (Award CHE-0946869). Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The authors are grateful to Drs. Veaceslav Coropceanu and Lingyun Zhu for fruitful discussions. We also thank Drs. Martin Heeney and Iain McCulloch at Imperial College London for providing PBTTT samples.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.