Probing Charge Carrier Dynamics in Porphyrin-Based Organic Semiconductor Thin Films by Time-Resolved THz Spectroscopy Kaoru Ohta, †,‡ Shunrou Tokonami, ‡ Kotaro Takahashi, § Yuto Tamura, § Hiroko Yamada,* ,§ and Keisuke Tominaga* ,†,‡ † Molecular Photoscience Research Center, Kobe University, 1-1 Rokkodai-cho, Nada, Kobe 657-8501, Japan ‡ Graduate School of Science, Kobe University, 1-1 Rokkodai-cho, Nada, Kobe 657-8501, Japan § Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5, Takayama-cho, Ikoma, Nara 630-0192, Japan * S Supporting Information ABSTRACT: 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 tetrabenzopor- phyrin 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 delocal- ization 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. 1. INTRODUCTION Organic semiconductors, such as conjugated polymers and small-molecule crystals, are important ingredients for field- effect transistors and solar cells. 1-3 These materials have great advantages over inorganic-based ones because of the potential for producing cost-effective and flexible devices by using solution processing. For organic photovoltaics, conjugated polymers, such as poly(3-hexylthiophene) (P3HT), are commonly used as electron donors. 1,2 Since nature has utilized porphyrin chromophores, such as chlorophylls and bacterio- chlorophylls, for light harvesting, porphyrin-based molecules are a natural choice as an artificial counterpart. 4,5 These molecules have a number of unique photophysical properties. As is well-known, they show very intense absorptions in the visible region, which are called Soret and Q bands. Even though porphyrins have been frequently used as a photosensitizer for dye sensitized solar cells, 6-8 their application in solution- processed bulk heterojunction (BHJ) solar cells is still under development. Tetrabenzoporphrin (BP) is a well-known p-type organic semiconductor that has excellent photophysical properties such as a strong absorption in the visible region and high hole mobility. However, because of the extended π-framework, it exhibits a low solubility in common organic solvents. To overcome this drawback, soluble precursors were synthesized and thermally converted into target molecules. 9 By using a soluble precursor of BP, it was demonstrated that BP-based field-effect transistors exhibit comparable performance to Received: July 17, 2017 Revised: October 12, 2017 Published: October 12, 2017 Article pubs.acs.org/JPCB © XXXX American Chemical Society A DOI: 10.1021/acs.jpcb.7b07025 J. Phys. Chem. B XXXX, XXX, XXX-XXX Cite This: J. Phys. Chem. B XXXX, XXX, XXX-XXX