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Two phenylene–thiophene oligomer derivatives, 2,8-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (28HPTDBT) and 3,7-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (37HPTDBT), were used as active materials in thin-film organic field-effect transistors (OFETs). Although the two molecules have similar structures, they exhibited obvious differences in photophysical, crystal, π-stacking, and electrical properties. 28HPTDBT is an amorphous material and hence showed no semiconductor characteristics in its thin-film OFETs, while 37HPTDBT exhibited high crystallinity and strong π-stacking in the solid state, thus resulting in high charge carrier mobilities. The effects of gate insulators and annealing treatment on transistor performances were also investigated. Thin-film OFETs based on 37HPTDBT with an octadecanyltrichlorosilane (OTS)-treated SiO2 gate insulator exhibited excellent field-effect performances with a maximum mobility of 0.3 cm2 V-1 s-1 and a high I on/I off current ratio of 1.5×105. Although annealing treatment improved the crystallinity of the thin films, the appearance of voids (cracks) resulted in a decrease in the charge carrier mobilities in the OFETs.

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P-channel pentacene field effect transistorswith a Si/SiO2/pentacene/Au structure were fabricated, and were gamma-ray irradiated with a Co60source. The changes of the drain current IDvs. source/drain voltage VSD(ID- VSD) characteristics were measured after every 200 Gy in silicon (GySi) irradiations up to the total dose of 1200 GySi. The drain current IDcontinuously decreased to less than 10 % of that before irradiation after 1200 GySiirradiation. The threshold voltage Vthcontinuously decreased up to 800 GySi, started to saturate above 800 GySi,and recovered above1000 GySi. The mobility m continued to decrease up to 1200 GySi. Those behaviors were explained by accumulation of positive trapped charge within the gate insulator SiO2near the interface, continuous increase of interface traps near the interface between the SiO2and pentacene, and build up of electrons in the channel regions. These behaviors were discussed in comparisons with previously reported results on ultraviolet (UV) light irradiation experiments on similarly structured pentacene-based transistors.

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