The covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent sp³-hybridized defects gives rise to tunable, narrowband emission in the near-infrared and increases their photoluminescence quantum yields [ACS Nano 2019, 13, 9259]. To enable their application in electrically driven light sources, a detailed understanding of the impact of sp³ defects on charge transport within individual nanotubes and nanotube networks is required. Here, we investigate the ultrafast carrier dynamics in sp³-functionalized, polymer-sorted (6,5) SWCNT dispersions and thin films using optical-pump terahertz-probe (OPTP) spectroscopy. OPTP spectroscopy relies on the photo-generation of charge carriers that are subsequently probed by a single-cycle terahertz pulse, rendering it a highly suitable technique for the determination of intrinsic charge transport properties (e.g., carrier mobility) in a quantitative and contact-free fashion. We observe a large transient photoconductivity of pristine and functionalized nanotubes that is dominated by the contribution of free charge carriers. The photoconductivity intensity and lifetime decrease upon functionalization, corroborating the impact of sp³ defects on intra-nanotube charge transport. Through combination of (temperature-dependent) OPTP measurements of sp³-functionalized SWCNT dispersions and films with electrical measurements of ambipolar, light-emitting field-effect transistors based on functionalized SWCNT networks [ACS Nano 2021, 15, 10451], we can separate the contributions of sp³ defects (intra-nanotube) and nanotube junctions (inter-nanotube) to charge transport in such networks and distinguish the short- and long-range transport characteristics.

Fig. 1: Ultrafast optical-pump terahertz-probe spectroscopy is a highly suitable technique to study the intrinsic charge transport properties of sp³-functionalized (6,5) SWCNTs in dispersions and thin films.