Carrier Dynamics Resulting from Above and Below Gap Excitation of P3HT and P3HT/PCBM Investigated by Optical-Pump Terahertz-Probe Spectroscopy

Abstract

We report subpicosecond carrier dynamics in photoexcited solid films of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and a blend of P3HT/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) using optical-pump terahertz-probe spectroscopy for excitations well above and below the generally accepted energy gap in P3HT. Both 400 nm excitation and 800 nm excitation yield similar results, suggesting the same photoproduct results from these two different excitation energies. The time evolution of the frequency-dependent complex conductivity after optical-pumping reveals characteristics of strong carrier localization. Carrier mobilities and photon-to-carrier yields are estimated based on the application of the Drude−Smith model. Low carrier yields (≤1.5%) suggest that free carriers are not the primary photogenerated species. High hole mobilities (30−40 cm2/Vs) are consistent with the THz probing frequency, which selectively probes short-range highly ordered domains. Low-temperature measurements show a slight increase in mobility. Inclusion of PCBM led to an increase in carrier yield for both excitation wavelengths.