Crystalline Organic Semiconducting Thin Films Cast from a Novel Thermolytic Thiophene Oligomer
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Crystalline Organic Semiconducting Thin Films Cast from a Novel Thermolytic Thiophene Oligomer Paul C. Chang1, Amanda R. Murphy2, Josephine B. Lee1, Jean M. J. Fréchet2, and Vivek Subramanian1 Department of Electrical Engineering & Computer Sciences1, Department of Chemistry2, University of California, Berkeley ABSTRACT Here we report on OTFTs made from a novel thermoresponsive thiophene based oligomer. The functionalized oligomer is soluble in common solvents, but loses its solubility upon thermolysis. Devices with channel lengths from 5 to 40 µm were fabricated from solution then subjected to thermolysis at temperatures ranging from 150-250 ºC. The initially amorphous materials reorganize into highly crystalline films upon thermolysis. Crystallinity was characterized by x-ray diffractometry and atomic force imaging. AFM studies depicted the nucleation of molecular terraces, where the formation and overall height of terraces was observed to be dependent upon the thermolysis temperature and solvent choice. These factors correlated to overall performance. For both dip-cast and spin-cast samples, devices were fabricated at a process temperature of 180ºC with mobilities of 0.07 cm2/Vs and on/off ratios > 105. The devices were relatively stable in both air and common solvents, with their mobility degrading only by ~25% upon immersion in the original casting solvents. The solubility characteristics and process temperatures for these devices may be particularly suitable for solution processing on flexible substrates. INTRODUCTION One of the main attractions of organic semiconducting materials is the potential for lowcost solution processing [1]. In addition to allowing print-based additive patterning, the low process temperatures required for these materials may enable the use of flexible substrates. Potential applications include disposable RFID tags or large area sensors [2,3]. Peak performing organic semiconductors exhibit field-effect mobilities between 1-10 cm2/Vs, rivaling that of amorphous silicon [4]. However, these materials have generally been small molecules with limited solubility, deposited in thin films via thermal evaporation or grown into single crystals through sublimation. Soluble organic semiconductors typically exhibit field-effect mobilities one to two orders lower in magnitude, between 10-3-10-1 cm2/Vs [1]. Indeed the functionality that imparts solubility typically interrupts the pi-stacking between molecules, reducing the piorbital overlap thought desirable for high charge mobility. An approach for addressing this intrinsic tradeoff between solubility and pi-stacking is to utilize solubilizing functional groups that can be removed following solution based thin-film deposition. This technique has been demonstrated with success using pentacene, where the solublizing group is removable via thermolysis, resulting in films exhibiting strong semiconducting characterisitics [5]. We recently used a similar approach with EtB12T6, a sexithiophene end-substituted with thermally removable solubilizing
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