Exciton Strings and Exciton Mediated Carrier Transport in a Quasi-one-dimensional Organic Charge-Transfer Crystal

Abstract

We discuss two types of photoinduced phenomena in which Coulomb attraction between ionized donor and acceptor molecules are important in a quasi-one-dimensional weak charge transfer crystal of anthracene-PMDA. Firstly, we report the formation of multi-excitons by a strong photo-excitation. High density excitation of charge transfer excitons by sub-picosecond light pulses generate multi-exciton chains along the stack axis of the crystal. Secondly, we report a new type of photo-induced carrier transport phenomena. We apply a static electric field parallel to the stack axis of a thin single crystal at liquid helium temperature and examine the DC Stark splitting of the zero phonon line of the CT-exciton under the additional photoexcitation at the CT exciton band. With the assistance of the photo-excitation, carriers move along the stack axis and stay at the edge of the illuminated region. They eventually cancel the external field. A self trapped electron or hole attracts a photogenerated CT-exciton and forms a coupled state A D⁺A or D⁺AD⁺. Successive annihilation and creation of the CT exciton result in the charge transport along the stack axis.