Soluble Regioregular Polythiophene Derivatives as Semiconducting Materials for Field-Effect Transistors

Soluble regioregular polythiophene derivatives with chiral alkyl side chains, bulky side chains, and carboxylic side chains have been synthesized. Their thin-film field-effect transistor performance and structural characteristics have been investigated. The above properties were found to be strongly affected by the nature of the side chains. Poor molecular ordering and low crystallinity were found for regioregular polythiophenes with bulky or carboxylic-substituted side chains, and their resulting transistor devices showed poor field-effect mobilities (less than 10-5−10-4 cm2/(V s)). A chiral alkyl-substituted regioregular polythiophene showed high crystallinity. However, its π−π overlap distance between the polythiophene backbones increase substantially (i.e., to ∼4.3 vs 3.8 Å for regioregular poly(3-hexylthiophene) (PHT)) through the introduction of methyl branches in the side chains. The field-effect mobility of this polymer is reasonably high for solution-processable conjugated polymers (i.e., of the order of 10-3 cm2/(Vs)), but is still an order of magnitude lower than for regioregular PHT. These studies provide an initial basis for further structural design and molecular engineering of soluble regioregular polythiophenes with potentially improved transistor performance.