Dioctylbenzothieno[2,3-b]benzothiophene (C 8 -BTBT)-based organic field-effect transistors (OFETs) with a top-gate configuration, having fluoropolymer gate insulators, are fabricated by means of a spin-coating technique. The device fabrication is simple, and it enables us to obtain C 8 -BTBT FETs having high field-effect mobility ( FET ), low threshold voltage (V th ), and high electrical stability. We fabricate 116 top-gate C 8 -BTBT FETs having FET of 1:59 AE 0:40 cm 2 V À1 s À1 and V th of À1:48 AE 3:02 V, and the maximum FET is approximately 3 cm 2 V À1 s À1 . No changes in the devices characteristics are observed after applying a negative gate bias stress of À1:2 MV/cm for 10 4 s.
The excited-state structure of ordered ͑-phase͒ poly͑9,9-dioctylfluorene͒ ͑F8͒ thin films has been investigated by electroabsorption ͑EA͒ spectroscopy. The EA spectra of a number of conjugated polymers, including disordered ͑glassy͒ F8, are well explained by a theoretical essential-state model, which consists of three characteristic excited states and the ground state ͓Y. Kawabe et al., Phys. Rev. B 44, 6530 ͑1991͔͒. However, it is found that the essential-state model fails to explain the EA spectrum of -phase F8 because of the appearance of an oscillatory EA signal near the bottom of the continuum state. From the applied electric-field dependence and the directional dependence of the field and polarization of the light, it has been revealed that the oscillation results from several intrachain excited states, some of which are not observed in glassy F8 because of considerable inhomogeneous broadening. To reproduce the EA spectrum of -phase F8 by a sum-over-states calculation, at least seven excited states, in addition to the ground state, have to be taken into account.
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