Charge Carrier Dynamics in Pulse-Irradiated Polyphenylenevinylenes: Effects of Broken Conjugation, Temperature, and Accumulated Dose

The transient conductivity resulting from nanosecond pulsed ionization of alkoxy-substituted phenylene?vinylene/ethylidene copolymers, “dMOM-PPV(n)”, with n the fractional vinylene content, has been studied using the pulse radiolysis time-resolved microwave conductivity (PR-TRMC) technique. Minimum values of the sum of the charge carrier mobilities within the bulk solids, ∑μ_min, have been estimated from the end-of-pulse conductivity. For the freshly precipitated materials at room temperature, ∑μ_min decreases gradually with decreasing n from 1.8 × 10^-7 m²/(V s) for n = 1 (full conjugation) to 0.4 × 10^-7 m²/(V s) for n = 0.57. After annealing dMOM-PPV(1) at 100 and 150 °C, ∑μ_min at room temperature increased to 3.2 × 10^-7 and 8.0 × 10^-7 m²/(V s), respectively. No significant effect of high-temperature annealing was found for n ≤ 0.87. On cooling dMOM-PPV(1) from 150 to ?50 °C, ∑μ_min decreased initially with an activation energy of approximately 0.07 eV but approached a plateau at the lowest temperatures. The after-pulse decay of the conductivity was disperse in all cases. First half-lives of several microseconds were found for n = 1. The decay kinetics were independent of the dose in the pulse. Large accumulated radiation doses (up to 1.2 MJ/kg) did not effect the end-of-pulse conductivity but did increase the decay rate. This effect could be reversed by high-temperature annealing.