Abstract

Ultrafast electronic dynamics of high purity molecular crystals are studied by transient photomodulation spectroscopy. Free and trapped excitons are characterized by their unique transient absorption and stimulated emission bands. Our results provide several insights into the gain dynamics of these materials and help the realization of an electrically driven organic laser. We study ultrafast relaxation between different excited states in molecular semiconductors and conjugated polymers using a novel ultrafast spectroscopy. The new technique utilizes two excitation pulses and one probe pulse, which are independently tunable across the entire visible and near infrared spectral range. We find that strongly bound excitons are the primary photoexcitations and intramolecular internal conversion is the dominant relaxation mechanism in the organic semiconductors. Due to the large binding energy and fast internal conversion, exciton dissociation with subsequent charge carrier generation is usually an inefficient process. We find that efficient exciton dissociation requires either the assistance of high electric field or optical re-excitation into so-called charge transfer states. We show that the latter states mediate charge separation, so that exciton dissociation occurs even without the applied electric field.

© 2001 Optical Society of America