Self-Assembly, Molecular Ordering, and Charge Mobility in Solution-Processed Ultrathin Oligothiophene Films

Abstract

Symmetrical α,ω-substituted quarter-( T4 ), penta-( T5 ), sexi-( T6 ), and heptathiophene ( T7 ) oligomers containing thermally removable aliphatic ester solubilizing groups were synthesized, and their UV−vis and thermal characteristics were compared. Spun-cast thin films of each oligomer were examined with atomic force microscopy and near-edge X-ray absorption fine structure spectroscopy to evaluate the ability of the material to self-assemble from a solution-based process while maintaining complete surface coverage. Films of the T5 − T7 oligomers self-assemble into crystalline terraces after thermal annealing with higher temperatures required to affect this transformation as the size of the oligomers increases. A symmetrical α,ω-substituted sexithiophene ( T6-acid ) that reveals carboxylic acids after thermolysis was also prepared to evaluate the effect of the presence of hydrogen-bonding moieties. The charge transport properties for these materials evaluated in top-contact thin film transistor devices were found to correlate with the observed morphology of the films. Therefore, the T4 and the T6-acid performed poorly because of incomplete surface coverage after thermolysis, while T5 − T7 exhibited much higher performance as a result of molecular ordering. Increases in charge mobility correlated to increasing conjugation length with measured mobilities ranging from 0.02 to 0.06 cm 2 /(V·s). The highest mobilities were measured when films of each oligomer had an average thickness between one and two monolayers, indicating that the molecules become exceptionally well-ordered during the thermolysis process. This unprecedented ordering of the solution-cast molecules results in efficient charge mobility rarely seen in such ultrathin films.