High‐Mobility Organic Transistors Based on Single‐Crystalline Microribbons of Triisopropylsilylethynyl Pentacene via Solution‐Phase Self‐Assembly

Abstract

There has been significant interest in the fabrication of 1D single-crystalline building blocks from p-conjugated organic molecules for use in high-performance supramolecular electronics. Control over the single-crystalline supramolecular self-assembly of p-conjugated organic molecules provides great opportunities to fine-tune the molecular ordering of 1D nanoor microstructured building blocks grown by strong p–p interactions, and thus to optimize their electrical properties for applications in organic field-effect transistors (OFETs). Obtaining high-quality single crystals with p-conjugated organic molecules through solution processing is required more than obtaining them through complex vacuum processes. Some research has recently shown that self-assembly through strong p–p stacking is an effective approach to producing well-defined single-crystalline nanoor microstructures for p-conjugated organic molecules in the solution phase. Among p-conjugated organic materials, pentacene, a fused-ring polycyclic aromatic hydrocarbon, is regarded as one of the most promising materials for use in organic electronics, because of its excellent semiconducting behavior, which is comparable to that of hydrogenated amorphous silicon (a-Si:H). However, the structural analysis of p-conjugated organic materials such as acenes has shown that their crystal structures adopt the so-called “herringbone” motif in which the molecules are packed more or less edge-to-face in 2D layers. However, this edge-to-face packing minimizes the p-overlap between adjacent molecules, resulting in relatively low mobilities. It has been suggested that it might be possible to achieve higher mobilities by designing p-conjugated molecules that stack face-to-face (p-stacking) in the solid state, thus increasing the intermolecular interactions. In other words, a cofacial p-stacked structure is expected to provide more efficient p-orbital overlap, thereby facilitating charge transport. Simulations have shown that halogen groups promote p-stacking and that one or more substitutions of relatively bulky groups into the peripositions of the polyacenes disrupts the herringbone structure of these compounds. In this study, therefore, the properties of triisopropylsilylethynyl pentacene (TIPS-PEN) were investigated, as suggested by Anthony and co-workers, because of its solution processability, significantly greater p-orbital overlap, and lower interplanar spacing, than unsubstituted pentacene. Some previous research has focused on the fabrication of thin films and bulk materials using TIPS-PEN molecules, however, the 1D self-assembled single-crystalline nanoor microstructures (i.e., the intermediate state between free molecules and bulk materials) of TIPS-PEN molecules have not yet been studied. In this study, 1D single-crystalline microribbons of TIPS-PEN with structural perfectness comparable to that of inorganic single crystals were easily prepared by using the specific-solvent-exchange method in the solution phase, and OFETs based on individual microribbons with hitherto unreported high performance were fabricated. The TIPS-PEN microribbons were prepared from TIPSPEN powder as the starting material by using the solvent-exchange method in the solution phase (for details, see the Experimental section). The conformational flexibility of the triisopropylsilyl side group of the TIPS-PEN molecule (Fig. 1) gives it sufficient solubility in hydrophobic solvents such as toluene, and the increased density of the bulky groups enables the tight packing of the pentacene backbones to maximize the p–p interactions. As a result, the TIPS-PEN molecule becomes insoluble in more polar solvents such as acetonitrile. As shown in Figure 1B, the injection of a minimum volume of concentrated toluene solution of TIPS-PEN into acetonitrile led to the formation of nanocrystals through self-assembly and the growth of a microribbon in the closed chamber (Fig. 1C). Figure S1 (Supporting Information) shows the C O M M U N IC A TI O N