The Relationship between Molecular Structure and Field Effect Mobility in Organic Semiconductors

Short conjugated phenylene-thiophene oligomers with varying a and omega alkyl and alkoxy substitutions were synthesized to investigate the correlation between end-group structure and electronic properties on P2TP and P3TP conjugated cores. Several oligomers had high solubility and field-effect mobility, with 5,5'-Bis(4-n-octylphenyl)-2,2' -bithiophene showing the highest mobility (mu = 0.18 cm(2)/V.s) and on/off ratio of 10(7). Thin film morphologies analyzed using atomic force microscopy revealed higher nucleation densities for branched alkoxy side chain molecules compared to their straight alkyl side chain counterparts. Field effect mobility showed an inverse correlation to side-chain bulkiness, indicating a strong morphological component to charge transport. Novel methods of solution processing involving alignment under shear and the demonstration of these materials in low voltage aqueous sensors are highlighted.