Probing charge transfer dynamics in porphyrin-based organic semiconductors by time-resolved THz spectroscopy

We carried out optical pump-terahertz (THz) probe measurements to investigate the charge carrier dynamics of diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) thin films. In contrast to tetrabenzoporphyrin, DPP-BP exhibits broad and intense absorption, ranging from the visible and near infrared region. After the photoexcitation at 400 nm, we observed an instantaneous appearance of transient THz signals which are due to formation of free charge carriers. Transient signals decay with the time constants of 0.3 ps and 4 ps, meaning that charge carrier recombination and/or trapping take place on this time scale. For DPP-BP thin films, we found that near-infrared photoexcitation produces mobile charge carriers efficiently even without significant excess excitation energy. This will help to improve the power conversion efficiency for DPP-BP based solar cells.