Alkyl Chain Length Dependence of Field-Effect Mobilities in Regioregular Poly(3-Alkylthiophene) Films

Abstract

Carrier mobilities have been studied using the field effect in regioregular poly(3-alkylthiphene) (PAT, alkyl=butyl, hexyl, decyl, dodecyl and octadecyl) cast films at a field of around 10⁴ V/cm. It was found that the carrier mobility of poly(3-butylthiophene) is 1.0×10^-2 cm²/V·s, which is the largest one to date in the polythiophene family and three to four orders of magnitude larger than that of poly(3-octadecylthiophene). This result indicates that the substituted alkyl chain plays the role of a barrier to carrier migration between π-conjugated main chains, and primarily determines the mobility. The residual carrier densities are 10¹⁵ to 10¹⁶ cm^-3 for all PATs and slightly lower for PATs with shorter alkyl chains. The field-effect mobilities are nearly the same as the values estimated by the time-of-flight method.