n-Type Doping of Triethylenetetramine on Single-Wall Carbon Nanotubes for Transparent Conducting Cathodes

Transparent conductive electrodes (TCEs) are fundamental components for designing flexible electronics and displays. TCEs should exhibit high electrical conductivity, optical transparency, mechanical flexibility, and a suitable work function (WF) for efficient performance. Because of their unique mechanical, electrical, and optical properties, sparse single-wall carbon nanotube (SWCNT) networks are attractive candidates for TCEs. However, to achieve a highly conductive sparse network, a reduction of the junctions’ resistances between the SWCNTs is required. In addition, SWCNTs inherently possess a high WF, which is fundamental for functional anodes but not suitable for cathodes. In this work, n-type doping of SWCNTs via coordinative bonding of triethylenetetramine (TETA) to their surface is introduced to tune both the WF and the junctions’ resistance. A self-developed conductive atomic force microscopy (cAFM) technique is used to investigate the same individual junctions in SWCNT networks before and after exposure to TETA fumes and post heating. The mechanisms by which TETA doping modifies the “global” properties of SWCNT networks are studied by Kelvin probe microscopy, X-ray photoemission spectroscopy (XPS), Raman spectroscopy, and ultraviolet–visible spectroscopy. Following TETA doping, improved conductivity and reduced WF are achieved, implying n-type charge-transfer doping. These results provide a significant step toward the use of SWCNTs as transparent cathodes in organic-based electronic devices.