Organic semiconductors for field-effect transistors (FETs): tuning of spectroscopic, electrochemical, electronic and structural properties of naphthalene bisimides via substituents containing alkylthienyl moieties

摘要

A series of new naphthalene bisimides containing alkylthienyl groups as a part of their N-substituents have been prepared. Two synthetic routes were used. In the first one appropriate alkylthienyl anilines were synthesized from bromoalkylthiophene derivatives and then condensed with 1,4,5,8-naphthalenetetracarboxylic acid bisanhydride. The second route involved condensation of thienylmethylamine with the same bisanhydride, followed by bromination of the product and its Suzuki-type coupling with alkylthiophene. UV-vis and FTIR spectroscopic investigations showed that the extent of the conjugation in the substituents is very sensitive to their regiochemical arrangement. The presence of alkylthienyl groups lowers the bisimide LUMO level to -3.94 eV with respect to the vacuum level. This property combined with their solution processibility makes the new bisimides excellent candidates for their application as active layers in all-organic field-effect transistors, fabricated by solution processing (spin coating or printing techniques). X-Ray diffraction measurements have shown that the bisimide molecules, deposited on either polyethylene naphthalate (PEN) or untreated Si (100) substrates, adopt regular arrangements giving rise to crystalline domains, which, in their overwhelming part, are highly oriented respectively to the substrate with the long period of their structure being in a plane perpendicular to the substrate. As a result a tendency to align it-stacked moieties in a direction parallel to the substrate can clearly be observed. Bottom contact staggered organic field-effect transistors have been fabricated and tested in air environment. All layers, except the source and drain electrodes, have been deposited from solution. The obtained transistors exhibit charge carriers mobility approaching 10~(-2) cm~2 V~(-1) s~(-1) and the on/off ratio exceeding 10~5, indicating that this new class of bisimide semiconductors can be considered as promising solution processible materials for n-channel transistors.