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PhD Defense - Paul Cunningham

Paul successfully defended his dissertation on April 16, 2010.

Optical Pump Terahertz Probe Studies of Semiconducting Polymers

Optical-pump terahertz-probe spectroscopy (OPTP) has been applied to study charge generation, transport and the evolution of the photo-induced excited states in thin film organic semiconductors, with emphasis on their relevance to photovoltaic technology. In these experiments the response of the photoexcited material to the AC electric field of a terahertz (THz) pulse was measured. From this response, the evolution of the complex conductivity in the far-infrared was monitored. OPTP presents advantages over other techniques by being an all-optical probe of the complex conductivity over nanometer scale distances with sub-picosecond resolution and exhibits particular sensitivity to carrier scattering rates, which typically lay in the THz range. Conductivity models were applied to the extracted conductivity curves in order to determine technologically relevant quantities like the charge carrier mobility and external quantum yield of charge carrier generation.

We observed charge carriers generated on a subpicosecond time scale in thin films of polyhexylthiophene (P3HT). Through application of the Drude-Smith model (DSM) over the 0-2 THz band, we determined a room temperature intrinsic mobility of about 30 cm2/Vs. The temperature dependence of the conductivity dynamics showed signs of thermally activated polaron hopping influenced by torsional disorder. Above and below gap excitation resulted in similar dynamics, both showing that the majority of carriers recombine within 1 ps. We were able to observe charge transfer occurring on a sub-ps timescale to the soluble fullerene, PCBM, for both excited states, demonstrating that narrow gap polymers can be blended with PCBM for photovoltaic applications.

We observed charge carrier generated on a sub-ps time scale in thin amorphous films of metalated polymers. The time evolution of the conductivity showed that charge carriers recombine and only excitons persist after 100 ps. This characteristic appears to be common to amorphous systems. An intrinsic mobility of 20 cm2/Vs was found for the most promising material.

Broadband (0-6 THz ) studies of the photoconductivity in P3HT suggest that the hole mobility is lower than initially determined. They also bring into question whether the DSM can describe the conductivity effectively or whether delocalized polaron transitions at higher frequencies are the origin of the observed features.