Evolution in crystal structure and electrical performance of thiophene-based polymer field effect transistors: A remarkable difference between thermal and solvent vapor annealing

Abstract

We report the various conformational structures of long pendant side-chains, and the effects of thermal and solvent vapor annealing (SVA) with the corresponding charge carrier mobilities of thiophene-based conjugated polymers, poly[5,5 '-bis(3-dodecyl-2-thienyl)-2,2 '- bithiophene] (PQT-12) and poly(4,4 '-bis-decyloxymethylquaterthiophene) (POQT), by correlated study of their extraordinary polymorphic crystal structures. In substitution for alkyl chains in polythiophenes, ether alkyl chains induce a favorable non-covalent interaction between the oxygen and sulfur atoms and help the polymer chains planar with lower torsion angles between conjugated backbone units showing a reduced p-p stacking distance. However, the flexibility and conformational freedom with such long side-chains dominantly induce polymorphic crystallites from bent and extended side-chains. Especially, POQT exhibit two polymorphic crystallite phases in a similar ratio probably due to the increased freedom of ether alkyl chains. Therefore, the field effect mobility of POQT is decreased gradually with the increase of annealing temperature from 0.024 (at 80 degrees C) to 3.96 x 10(-4) cm(2)/V s (at 170 degrees C). Contrary to the thermal annealing method, solventvapor- annealed POQT films show highly ordered and single-phase crystallites with edgeon orientation to the substrate, which ultimately provides an effectively improved charge carrier mobility from 0.023 (pristine) to 0.076 cm(2)/V s after adequate solvent vapor exposure. (C) 2014 Elsevier B.V. All rights reserved.