The mobility and relaxation kinetics of charge carriers in molecular materials studied by means of pulse-radiolysis time-resolved microwave conductivity: dialkoxy-substituted phenylene-vinylene polymers

The information that can be obtained on the mobility and relaxation kinetics of electronic charge carriers in bulk molecular materials using the pulse-radiolysis time-resolved microwave conductivity technique is illustrated by results on several dialkoxy-substituted phenylene-vinylene polymers. The results demonstrate the sensitivity of the electronic properties of such conjugated polymers to their morphology. Thus, despite having the same conjugated backbone, the mobility and the relaxation kinetics (due to trapping and/or charge recombination) depend strongly on the nature of the alkyl-chain substituents, with in particular a marked difference between symmetrically and unsymmetrically dialkoxy-substituted compounds. For the latter, high-temperature annealing has a substantial positive effect on the mobility and lifetime of mobile carriers. The mobilities found for annealed materials range from a low of 0.0025 cm² V^-1 s^-1 for the methoxy, ethyl-hexoxy derivative, MEH-PPV, to a high of 0.036 cm² V^-1 s^-1 for the di-octadecoxy derivative, (OD)₂-PPV. The latter compound becomes a free-flowing liquid above 190ºC but still displays a high charge-carrier mobility of 0.017 cm² V^-1 s^-1. For all compounds the temperature dependence of the mobility after annealing is only slight over the range from −50 to +150ºC with an energy of activation ≤0.1 eV. Saturation of vinylene residues (breaking the conjugation) results in a marked decrease in the mobility. For very high accumulated doses of radiation the mobility on a nanosecond timescale remains unaffected but the decay of the mobile carriers at longer times becomes faster. This effect is completely reversed on annealing at 150ºC.