Study of transient effects in photo-excited semiconducting polymers

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Peng, Yi-Hsing
Lee, Chi H
Semiconducting conjugated polymers regioregular poly(3-hexylthiophene) (RR-P3HT) and poly[2-methoxy-5-(2'ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) have received considerable interest for prospective applications in organic photovoltaics (OPV). Understanding the photo-initiated processes and carrier transport in those materials is essential to improve the performance of the polymer OPV devices. Optical pump-Terahertz (THz) probe time domain spectroscopy (OPTP-TDS) is a noncontact technique which combines THz time domain spectroscopy and the pump-probe method. OPTP-TDS provides the ability to study the transient properties of photoexcited semiconducting materials. In this thesis, we establish new standard experimental and analysis procedures for OPTP-TDS by adopting the analysis method suggested by Nienhuys and Sundstrom for investigating the transient events that are faster than the duration of THz probe pulses. We observed experimentally artifacts in the conductivity of photoexcited GaAs, as predicted by Nienhuys and Sundstrom, when we apply the conventional analysis method. For the first time, we successfully remove the artifacts and recover the true transient conductivity of photoexcited GaAs using the correction transformation. P3HT/PCBM blends are investigated using OPTP-TDS. The new analysis process enables us to obtain the time resolved frequency dependent complex photoconductivity in subpicosecond resolution. The time resolved conductivity is analyzed by the Drude-Smith model to describe the behavior of localized charge carriers in the polymer. Transient mobility drop at subpicosecond time resolution in the photoexcited polymer is observed for the first time. The mobility drop can be explained by the polaron formation in the polymer, and is the main cause of the transient real conductivity drop in the first picosecond after photoexcitation. Semiconducting polymer MEH-PPV is investigated using OPTP-TDS, DC-bias transient photoconductivity, and photo-induced reflectivity change with high time resolution to get the transient conductivities at electrical, THz, and optical frequencies. The data are fitted with the Drude-Smith model and the Lorentzian oscillation model to describe free and bound carriers. The quantum efficiency of excitons was estimated to be less than 0.01, which is lower than previous reports. The imaginary conductivity at THz frequencies is attributed not to excitons but bound carriers with one tenth energy of excitons, which are possibly phonons.