Recombination of long-lived photoexcitations in regioregular polyalkylthiophenes

We have measured the recombination dynamics of long-lived polarons in two regioregular poly(3-alkylthiophenes) (RR-PATs) using cw- and transient (in the ms time regime) photoinduced absorption (PA). We find that the recombination of the long-lived polarons in the millisecond time regime can be fitted with a combination of one (fast) hyperbolic decay component and a (slow) modified hyperbolic. The fast component is almost temperature independent at low temperatures, while the slow component shows an exponential temperature dependence in the whole measured temperature range. We have modeled the transient PA decay using bimolecular recombination of polarons in a continuum band, including multiple trapping and retrapping in an exponential distribution of localized states. We show that this model fairly well approximates the transient decay components and the theoretical temperature dependence of the slow decay component is in good agreement with the measured values in RR-PATs.