Carrier dynamics of rubrene single-crystals revealed by transient broadband terahertz spectroscopy

Carrier dynamics of an organic molecular semiconductor, rubrene, was investigated by optical-pump terahertz-probe spectroscopy from 1 to 15 THz. At 294,K, a Drude-like response due to photogenerated hole carriers is observed below 8 THz. The real part ?1 (?) of the optical conductivity is suppressed below 2 THz, indicating the presence of a localization effect. Such a spectral feature was reproduced by a Drude-Anderson model including the effect of dynamical disorder due to intermolecular vibrations. At 50,K, the spectral weight of ?1 (?) due to photocarriers shifts to lower frequency below 4 THz and the suppression of ?1 (?) is hardly observed, which we associate with a reduction of thermal molecular motions. The overall photocarrier generation and recombination dynamics is also discussed.