High-Performance Organic Thin-Film Transistors of J-Stacked Squaraine Dyes

Abstract

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-C12) 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-C4, 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-C8 are of particular interest with regard to the structure-functionality relationship: The first is the slipped and antiparallel π-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···Se noncovalent bonds which show similarities to the more common halogen-bonding interactions and which interconnect the individual one-dimensional slipped π-stacks, thus leading to two-dimensional percolation pathways along the source-drain direction.

Influence of Solid-State Packing of Dipolar Merocyanine Dyes on Transistor and Solar Cell Performances

“…The reason why these dyes tend to pack in staircase-like arrangement instead of forming dimers is still unclear, however selenium is known to have a strong Lewis acidic character and its interactions with halogen and nitrile groups have been previously reported. 55, 56 The electrostatic potential surface in Figure 6d shows indeed a reddish area at the selenium position near the nitrile group, thus suggesting some kind of electronic density lying on the Se atom. If we take into account the small intermolecular N-Se distance (3.60 Å) found in the crystal, it seems reasonable to explain the adopted packing structure by CN-Se electrostatic interactions.…”

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…Further, for benzothiazoland benzoselenazol-derived SQB-type squaraine dyes very high charge-carrier mobilities of >1 cm 2 V −1 s −1 could be achieved for properly aligned thin films of J-aggregated dyes. [241] …”

Surface engineering to achieve organic ternary memory with a high device yield and improved performance

“…Squaraines show resonance-stabilized zwitterionic characteristics originating from their squaric acid rings and amine moieties. Therefore, they exhibit strong absorption and emission in the near-IR range, low band gaps and high photostability, which can lead to wide applications in optical recording, 22 solar cells, 23 electrophotography, chrom/fluorogenic sensing, 24 thin film transistors 25 and gas sensors. 26 These properties guarantee that SA-Bu has unique electronic properties and is thus likely to be appropriate for RRAM device fabrication.…”