Charge-carrier trapping in polyfluorene-type conjugated polymers

Abstract

The trap spectrum of a fluorene-based conjugated polymer poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] (PF2∕6) and poly[9,9-bis(2-ethylhexyl)fluorene-2,7-diyl] end capped with hole-transporting moieties N , N -bis(4-methylphenyl)- N -phenylamine (PF2∕6am10) is investigated by means of thermally stimulated current (TSC) and thermally stimulated luminescence (TSL) techniques. A high-temperature TSC peak observed at 240 K in PF2∕6 is strongly affected by photooxidation and could be identified as electron trap with a depth of about 0.6 eV . It is ascribed to on-chain keto defects in the polymer. In contrast, end capping of PF2∕6 led to the appearance of a moderately deep trap for holes ( E a = 0.24 eV ) responsible for a TSC peak at 120 K . On the other hand, TSC data of this polymer reveal no keto-related traps for electrons implying much higher stability of PF2∕6am10 against oxidation. Besides the deep electron trap in PF2∕6, a very low-temperature TSC peak at about 60 K appears in PF2∕6 samples and correlates perfectly with thermally stimulated luminescence data. It is ascribed to shallow hole trapping most probably related to the tail states of the intrinsic density-of-state distribution. In general, it was found that the TSC in PF2∕6 samples is detectable only when a sufficiently high load voltage is applied during optical trap filling. The TSC spectra exhibit a striking difference with the polarity of load voltage for conjugated polymers, implying an important role of electrodes on the charge-carrier photogeneration in these polymers.