Extended and Modulated Thienothiophenes for Thermally Durable and Solution-Processable Organic Semiconductors

Herein,
we report the rational design of practical small-molecule
organic semiconductors based on a π-electron skeleton of benzothieno­[3,2-b]­naphtho­[2,3-b]­thiophene (BTNT) whose
layered herringbone (LHB) packing is intentionally modulated by the
designated asymmetric substitutions with the phenyl group and normal
alkyl chains. The thermal stability of the hybrid BTNT core is high
enough, as it lies between those of dinaphtho­[2,3-b:2′,3′-f]­thieno­[3,2-b]­thiophene (DNTT) and benzothieno­[3,2-b]­benzothiophene
(BTBT), although the solvent solubility for the substituted BTNT at
ordinary 2,8-substituting positions by the alkyl chain and phenyl
group remains extremely low. We show in the BTBT and BTNT derivatives
that the tuning of the substituting position works to slightly bend
the rodlike organic semiconductor molecules and thus to decrease the
cohesive energy of the crystals with retention of the bilayer-type
herringbone (b-LHB) packing for the asymmetric rodlike
molecules. This modification eventually leads to an increase in solvent
solubility, a decrease in phase transition temperature, and the suppression
of liquid-crystalline phases at high temperatures. By using the substituting
effect, we successfully achieve the organic semiconductors with modulated
alkylated Ph-BTNT that exhibits both a sufficiently high solvent solubility
and a sufficiently high thermal stability. The variation in the crystal
packing also enhances the intermolecular transfer integrals along
the T-shaped contacts within the intralayer herringbone packing. Spin
coating of the material under ambient conditions affords high-performance
bottom-gate, bottom-contact organic thin-film transistors, exhibiting
high thermal durability in the device characteristics below 150 °C.
The obtained devices also exhibit a higher mobility, a lower threshold
voltage, and a smaller subthreshold swing, by initial thermal treatment
at 140 °C, composed to those of the as-prepared films, because
the thermal treatment stabilizes the b-LHB packing
and thus suppresses the residual minority holes and shallow traps.
These findings should be crucial in the design and development of
organic semiconductor materials for practical printed electronics
applications.