Probing Steady‐State Carrier Properties and Charge Transport in Covalent Organic Framework by Frequency‐Domain Terahertz Spectroscopy

Terahertz (THz) spectroscopy is an emerging tool for probing charge transport and optical properties in covalent organic frameworks (COFs). Existing studies have predominantly relied on time-resolved THz spectroscopy (TRTS) to investigate photoexcited carriers, with only one report on time-dependent THz spectroscopy (TDTS) to understand ground-state carriers within a narrow spectral window. Frequency-domain THz spectroscopy (FDTS), which offers high spectral resolution across the far-infrared-THz range remains unexplored. Herein, we employ FDTS to investigate steady-state carrier transport in a COF (TTC-PD) and amorphous frameworks (TTC-DTO and TTC-PD (amor)), along with their molecular analogues. Frequency-dependent optical constants and complex conductivity were extracted using Kramers-Kronig transformations (KKT) and validated by TDTS. Despite stronger carrier localization, TTC-DTO exhibits higher intrinsic conductivity due to increased carrier density, whereas TTC-PD shows lower conductivity but higher mobility arising from more delocalized transport pathways. This conceptual study demonstrates the potential of FDTS and TDTS as a combined and complementary platform for comprehensive analysis of the ground state charge transport properties of frameworks across the extended THz regime.