Mechanism of Photocarrier Generation and Transport in Poly(3-Alkylthiophene) Films

Abstract

Carrier mobilities in poly(3-alkylthiophene) (PAT) films (alkyl = 6, 12 and 18), synthesized by chemical oxidation of 3-alkylthiophenes with FeCl3, have been studied using the time of flight (TOF) method. The response of photocurrents indicates the hole as the carrier sign and the highly dispersive transport particularly in the films substituted with long alkyl chains. Using regioregular poly(3-dodecylthiophene) (PAT12) and indium tin oxide (ITO) in the ITO/PAT12/Al sandwich cell, the depletion layer width at the Al/PAT12 interface was estimated to be 60 nm. It has been proposed that photocarriers are effectively generated at the interface and the high internal field helps in the separation of photocarriers. The drift mobilities in PAT12 and PAT18 are 8 ×10-6 cm2/V.s and 3 ×10-6 cm2/V.s, respectively, at the bias field, E = 1.0 MV/cm. They depend on the field as exp (βE1/2/kT), indicating the Poole-Frenkel mechanism for the carrier transport. On the other hand, the mobility of PAT6 is 1.1 ×10-4 cm2/V.s at E = 0.1 MV/cm, shows very weak positive field dependence. The mechanisms of photocarrier generation and transport are discussed taking the polymer structure into consideration and are compared with the results of regioregular PAT films.