Journal of the American Chemical Society, Vol.136, No.6, 2351-2362, 2014
High-Performance Organic Thin-Film Transistors of J-Stacked Squaraine Dyes
We have synthesized a series of dipolar squaraine dyes that contain dicyanovinyl groups as acceptor and benzannulated five-membered ring heterocycles with alkyl chains of varied length as donor moieties. Based on these squaraines, thin-film transistors (TFT) were fabricated by spin coating and solution shearing. Moreover, with one of these squaraine derivatives vacuum-deposited TFTs were prepared as well. Our detailed studies revealed that the transistor performance of the present series of squaraines is strongly dependent on their structural features as well as on the processing method of thin films. Thus, solution-sheared OTFTs of selenium squaraine bearing dodecyl substituents (denoted as Se-SQ-C-12) performed best with a maximum hole mobility of 0.45 cm(2) V-1 s(-1), which is by far the highest value yet reported for OTFTs based on squaraines. This value was even surpassed by vacuum-deposited thin films of n-butyl-substituted selenium squaraine Se-SQ-C-4, the only sublimable compound in this series, exhibiting a record hole mobility of 1.3 cm(2) V-1 s(-1). Furthermore, we have investigated the morphology of the thin films and the molecular packing of these squaraine dyes by optical spectroscopy, atomic force microscopy, and X-ray diffraction. These studies revealed a relationship between the molecular structure, packing motif, thin-film morphology, and transistor performance of the squaraine dyes. From the supramolecular point of view two packing features discovered in the single crystal structure of Se-SQ-C-8 are of particular interest with regard to the structure-functionality relationship: The first is the slipped and antiparallel pi-stacking motif which ensures cancellation of the molecules' dipole moments and J-type absorption band formation in thin films. The second is the presence of CN center dot center dot center dot Se noncovalent bonds which show similarities to the more common halogen-bonding interactions and which interconnect the individual one-dimensional slipped pi-stacks, thus leading to two-dimensional percolation pathways along the source-drain direction.