Probing Charge Carrier Dynamics in Porphyrin-Based Organic Semiconductor Thin Films by Time-Resolved THz Spectroscopy

To improve the power conversion efficiency of solar cells, it is important to understand the underlying relaxation mechanisms of photogenerated charge carriers in organic semiconductors. In this work, we studied the charge carrier dynamics of diketopyrrolopyrrole-linked tetrabenzoporphyrin thin films where the diketopyrrolopyrrole unit has two n-butyl groups, abbreviated as C4-DPP-BP. We used time-resolved terahertz (THz) spectroscopy to track charge carrier dynamics with excitations at 800 and 400 nm. Compared with tetrabenzoporphyrin (BP), the extension of π-electron delocalization to the diketopyrrolopyrrole peripherals leads to an increase in absorption in the near-infrared region. Following the excitation at 800 nm, we found that the transient THz signals in C4-DPP-BP thin films decay with time constants of 0.5 and 9.1 ps, with small residual components. With excitation at 400 nm, we found that the transient THz signals decay with time constants of 0.4 and 7.5 ps. On the basis of the similarity of the decay profiles of the transient THz signals obtained with excitations at 400 and 800 nm, we considered that the decaying components are due to charge carrier recombination and/or trapping at defect sites, which do not depend on the excess energy of the photoexcitation. In contrast to BP, even without an electron acceptor, we observed the finite offset of the transient THz signals at 100 ps, demonstrating the existence of long-lived charge carriers. We also measured the photoconductivity spectra of C4-DPP-BP thin films with the excitation at both 800 and 400 nm. It was found that the spectra can be fitted by the Drude-Smith model. From these results, it was determined that the charge carriers are localized right after photoexcitation. At 0.4 ps, the product of the quantum yield of charge generation and mobility of charge carriers at 400 nm is approximately twice that obtained at 800 nm. We discuss the implications of the excess excitation energy in organic semiconductor-based devices.