Charge Transport in Covalently Functionalized Single-Walled Carbon Nanotubes for Tunable Electroluminescent Devices

The controlled covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent <i>sp</i>³ defects gives rise to tunable emission in the near-infrared with increased photoluminescence quantum yields and single-photon emission at room temperature [<i>ACS Nano </i><b>2019</b>, <i>13</i>, 9259]. To enable their application in electrically driven light sources, a detailed understanding of the impact of <i>sp</i>³ defects on charge transport within individual nanotubes and nanotube networks is required. Here, we demonstrate that <i>sp</i>³-functionalized, polymer-sorted (6,5) SWCNTs still support efficient ambipolar charge transport in nanotube network field-effect transistors (FETs). While both hole and electron mobilities moderately decrease with increasing degree of functionalization, the transistors remain fully operational with high on/off current ratios. Charge-modulated photoluminescence spectroscopy, which exclusively probes the mobile charge carriers in SWCNT networks, confirms that the defects are efficiently sampled by mobile carriers and consequently, <i>sp</i>³-functionalized SWCNTs actively participate in the charge transport within the networks [<i>ACS Nano </i><b>2021</b>, <i>15</i>, 10451]. We further employ optical-pump terahertz-probe (OPTP) spectroscopy, which probes the intrinsic and local charge transport properties (<i>e.g.</i>, carrier mobility) in a quantitative and contact-free fashion, to investigate the ultrafast carrier dynamics in <i>sp</i>³-functionalized (6,5) SWCNT dispersions and thin films. The maximum photoconductivity and lifetime decrease upon functionalization, corroborating the direct impact of <i>sp</i>³ defects on intra-nanotube charge transport. For low levels of functionalization (4-10 defects per micrometer), which are relevant for applications, the reduction of intrinsic carrier mobilities is still low. Through combination of (temperature-dependent) OPTP measurements of <i>sp</i>³-functionalized SWCNT dispersions and films with electrical measurements of nanotube network FETs, we can separate the contributions of <i>sp</i>³ defects (intra-nanotube) and nanotube junctions (inter-nanotube) to charge transport in SWCNT networks [<i>ACS Nano </i><b>2022</b>, DOI 10.1021/acsnano.2c02199]. When operated in the ambipolar regime, FETs with <i>sp</i>³-functionalized SWCNT networks exhibit red-shifted electroluminescence from the defect states that is tunable via the defect concentration. The selective introduction of defects with even further red-shifted emission [<i>Nat. Commun. </i><b>2021</b>, <i>12</i>, 2119] or the functionalization of other nanotube species such as (7,5) SWCNTs enables us to fabricate electroluminescent devices with tunable emission in the near-infrared and paves the way towards electrically pumped single-photon sources at telecom wavelengths.