The Disperse Charge-Carrier Kinetics in Regioregular Poly(3-hexylthiophene)

The pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) is an electrodeless technique to measure the transient conductivity in bulk samples induced by a nanosecond high-energy electron pulse. By using the PR-TRMC technique, two commercial samples of regioregular poly(3-hexylthiophene) (P3HT), obtained from Merck and Sigma-Aldrich, were measured as a function of temperature (between 170 and 380 K) and radiation dose. The real part of the high-frequency GHz charge-carrier mobility sum was found to be 0.014 cm²/V s at room temperature with an activation energy of 28 meV. The conductivity in the Aldrich sample decayed rapidly, with a half-life of 4 ns, while the conductivity of the Merck sample had a half-life of 0.2 μs at room temperature. From measurements of the background conductivity under atmospheric conditions and using the charge-carrier mobility of 0.014 cm²/V s, a hole doping concentration of 5 × 10¹⁷ cm^-3 (with an activation energy of 61 meV) was found for the Aldrich sample, while it was only 2 × 10¹⁶ cm^-3 (with an activation energy of 98 meV) for the Merck sample. For radiation pulses generating a higher initial electron−hole pair concentration than the doping level, second-order electron−hole recombination was observed in the Merck sample, while in the Aldrich sample, the decay was first-order at all applied doses. This is attributed to the high doping concentration in the latter sample, which exceeded the highest possible pulse-generated electron−hole pair concentration. All transients were of the stretched exponential type (Kohlrausch law). The stretch parameter β increased linearly with temperature in both samples, according to β = T/T₀ with T₀ = 930 and 670 K for Merck and Aldrich P3HT, respectively. The linear increase of β with temperature is in accordance with a model of dispersive hole transport with an exponential distribution of the activation energy of the hopping rates. A generalized version of the Kohlrausch law is derived to include both first- and second-order recombination processes at high radiation doses.