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

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 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.

The Necessity of Periodic Boundary Conditions for the Accurate Calculation of Crystalline Terahertz Spectra

“…Terahertz vibrational spectroscopy is a powerful technique for studying intermolecular interactions in the condensed phase, having been demonstrated to be useful for a wide-array of applications. [1][2][3][4][5][6] At terahertz frequencies (0.1 -10 THz or 3 -333 cm −1 ) large-amplitude complex motions of entire molecules are excited and sampled, and thus terahertz spectroscopy is a highly-sensitive probe of weak, often non-covalent, forces. The unique combination of sampling weak forces while probing long-range collective dynamics has spurred the growth of two, often related, lines of study.…”

Electrochemical Deposition of a Single‐Crystalline Nanorod Polycyclic Aromatic Hydrocarbon Film with Efficient Charge and Exciton Transport

“…Based on the calculated reduced effective mass m* =0.60 m 0 (Figure S22), the scattering time τ =36±7 fs and c parameter obtained from the fit to the data in Figure 4b, the charge carrier mobility of the film to be 30.9±6.0 cm 2 V −1 s −1 following μ (= e*τ*(1+c)/m* ). The obtained charge carrier mobility is one of the highest among organic semiconductors, [34–37] which we attribute to the perfect molecular arrangement. In the past, carrier‐transport properties of organic single crystals have been extensively investigated as a function of the prevailing packing modes.…”

Near-infrared emissive bacteriochlorin-diketopyrrolopyrrole triads: Synthesis and photophysical properties

“…Thus, in order to simplify the preparation of high-performance engineered NIR fluorophores based on a (bacterio)chlorin scaffold without resorting to tedious de novo syntheses, we plan to explore a chromophore merging-based strategy leading to unprecedented hybrids structures between hydroporphyrin and diketopyrrolopyrrole (DPP) photoactive units (Fig. 1) [39][40][41][42][43][44][45][46][47][48]. Indeed, in view of their oustanding properties (high fluorescence quantum yields, large two-photon absorption cross-sections and exceptional chemical, light and thermal stability) and their facile synthesis and functionalization, DPP dyes have recently emerged as valuable alternatives to conventional visible fluorophores [49,50].…”