www.afm-journal.de FULL PAPER © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.MaterialsViews.com wileyonlinelibrary.com Nayool Shin, Jihoon Kang, Lee J. Richter, Vivek M. Prabhu, R. Joseph Kline, Daniel A. Fischer, Dean M. DeLongchamp,* Michael F. Toney, Sushil K. Satija, David J. Gundlach, Balaji Purushothaman, John E. Anthony, and Do Y. Yoon* Vertically Segregated Structure and Properties of Small Molecule–Polymer Blend Semiconductors for Organic Thin-Film Transistors 1. Introduction Organic thin-film transistors (OTFTs) have recently emerged as a promising low-cost technology for manufacturing flexible, large-area electronics because they provide high field-effect mobility together with the desired solution processability. Recent advances in the molecular design and synthesis of small molecule organic semi- conductors have achieved materials with field-effect charge mobilities equal to or exceeding that of amorphous silicon. [1–3] Among small molecule semiconductors, the highest mobilities have typically been demonstrated with single crystals, where the structural defects are minimized. [4,5] Although the electrical performance of single crystal organic semiconductors is excellent, the large-scale fabrication of device circuitry employing single crystals faces enormous processing challenges due to the difficulties of growing uni- form crystals and positioning them in a regular manner across a large number of devices. [4] Therefore, current strategies for fabricating large-area organic electronics are focused on solution processing, such as inkjet printing, to achieve thin semiconducting films with high crystallinity and controlled crystal size and orientation. While solution-coating processes of polymer semiconductors readily result in satisfactory thin films, it is far more challenging to prepare thin uniform films by solution coating of small mol- ecule semiconductors, due to the low viscosity and poor wet- ting. [6] A promising new approach to improve the solution processability of small molecule semiconductors is the addi- tion of an insulating binder polymer as a thickening and wet- ting agent. [7–9] This approach has been shown to result in blend films that do not sacrifice the inherent excellent charge mobility of small-molecule semiconductors. [10–13] In blend films, the seg- regation of the semiconducting small molecules to the gate-die- lectric interface is critical to device performance, [14,15] since the charge transport occurs within a very narrow ( <10 nm) region adjacent to the dielectric layer; preferential segregation of the insulating binder polymer to the gate dielectric would greatly A comprehensive structure and performance study of thin blend films of the small-molecule semiconductor, 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT), with various insulating binder polymers in organic thin-film transistors is reported. The vertically segregated com- position profile and nanostructure in the blend films are characterized by a combination of complementary experimental methods including grazing incidence X-ray diffraction, neutron reflectivity, variable angle spectroscopic ellipsometry, and near edge X-ray absorption fine structure spectroscopy. Three polymer binders are considered: atactic poly( α α-methylstyrene), atactic poly(methylmethacrylate), and syndiotactic polystyrene. The choice of polymer can strongly affect the vertical composition profile and the extent of crystalline order in blend films due to the competing effects of confinement entropy, interaction energy with substrate surfaces, and solidification kinetics. The variations in the vertically segregated composi- tion profile and crystalline order in thin blend films explain the significant impacts of binder polymer choice on the charge carrier mobility of these films in the solution-processed bottom-gate/bottom-contact thin-film transistors. DOI: 10.1002/adfm.201201389 Dr. N. Shin, Dr. J. Kang, Prof. D. Y. Yoon Department of Chemistry Seoul National University Seoul, 151-747, Korea E-mail: dyyoon@snu.ac.kr Dr. L. J. Richter, Dr. V. M. Prabhu, Dr. R. J. Kline, Dr. D. A. Fischer, Dr. D. M. DeLongchamp, Dr. S. K. Satija, Dr. D. J. Gundlach National Institute of Standards and Technology Gaithersburg, MD 20899, USA E-mail: dean.delongchamp@nist.gov Dr. M. F. Toney Stanford Synchrotron Radiation Laboratory Melon Park, CA 94025, USA Dr. B. Purushothaman, Prof. J. E. Anthony Department of Chemistry University of Kentucky Lexington, KY 40506, USA Adv. Funct. Mater. 2012, DOI: 10.1002/adfm.201201389