Investigation of Solubility-Field Effect Mobility Orthogonality in Substituted Phenylene Thiophene Co-Oligomers

We report here on our investigations of structure-property correlations in soluble organic semiconductors. In order to develop formulations for mass-printing of semiconductor films, a series of alkoxy- and alkyl- disubstituted symmetric phenylene thiophene co-oligomers (disubstituted-PTTP) were systematically synthesized to achieve high solubility in common organic solvents. Solubilities of up to 40% in tetrahydrofuran at room temperature were achieved by introducing branching at various positions on the terminal chains, and by silyloxy or hydroxy functionalization of the chains. Concomitant with the solubility increase, mobilities and on/off ratios of these materials in TFT devices fall several orders of magnitude. X-ray diffraction of the semiconductor films reveals increasing tilt of the molecular orientation on the substrate with an increase in steric crowding near the PTTP core. This indicates that pi-stacking of these molecules along the semiconductor channel between the source and drain is hampered, and leads to the observed inverse interdependence of solubility and the field effect mobility.