Mechanism of photocarrier generation and transport in poly(3-alkylthiophene) films

Carrier mobilities in poly(3-alkylthiophene) (PAT) films (alkyl = 6, 12 and 18), synthesized by chemical oxidation of 3-alkylthiophenes with FeCl3, ha ve been studied using the time of flight (TOF) method. The response of phot ocurrents indicates the hole as the carrier sign and the highly dispersive transport particularly in the films substituted with long alkyl chains. Usi ng regioregular poly(3-dodecylthiophene) (PAT12) and indium tin oxide (ITO) in the ITO/PAT12/Al sandwich cell, the depletion layer width at the A1/PAT 12 interface was estimated to be 60 nm. It has been proposed that photocarr iers are effectively generated at the interface and the high internal field helps in the separation of photocarriers. The drift mobilities in PAT12 an d PAT18 are 8 x 10(-6) cm(2)/V.s and 3 x 10(-6) cm(2)/V.s, respectively, at the bias field, E = 1.0 MV/cm. They depend on the field as exp(betaE(1/2)/ kT), indicating the Poole-Frenkel mechanism for the carrier transport. On t he other hand, the mobility of FATS is 1.1 x 10(-4) cm(2)/V.s at E = 0.1 MV /cm, shows very weak positive field dependence. The mechanisms of photocarr ier generation and transport are discussed taking the polymer structure int o consideration and are compared with the results of regioregular PAT films .