Effects of regioregularity on carrier transport in poly(alkylthiophene) films with various alkyl chain lengths

Transport properties in regioregular poly(3-alkylthiophene) (PAT) films (alkyl=butyl, hexyl, decyl, dodecyl and octadecyl) have been studied by the time-of-flight (TOF) method for the sandwich cell of indium tin oxide (ITO)/film (spin-coated)/Al. It is found that the photocarriers are the holes, and their mobilities decrease and activation energies increase with increasing alkyl chain length. Poly(3-butylthiophene) exhibits the largest mobility of 1.8×10−2 cm2/V s at a field (E) of around 10 kV/cm. The mobility of poly(3-octadecylthiophene) with a value of 1.0×10−6 cm2/V s at E=0.2 mV/cm shows slightly different behavior in field dependence. The TOF mobilities at low field regions show negative field dependence apparently. The negative field dependence of mobilities is explained by proposing a model of heterogeneous field distribution for the Schottky-type cell. The field dependences of the mobilities in regioregular PAT are discussed, comparing them with the results of regiorandom PAT films.