Fully organic ITO replacement through acid doping of double-walled carbon nanotube thin film assemblies

Transparent electrodes made from metal oxides, of which indium tin oxide (ITO) is most common, suffer from poor mechanical flexibility and electrochemical stability. Highly transparent and electrically conductive thin films based on double-walled carbon nanotubes(DWNTs) were assembled layer-by-layer (LbL) as a potential replacement for ITO. The alternate deposition of positively charged poly(diallyldimethyl ammonium chloride) [PDDA] and DWNTs, stabilized with negatively charged deoxycholate (DOC), exhibits linear film growth. A five bilayer (BL) assembly exhibits a sheet resistance of 309 Ω sq⁻¹, and visible light transmittance of 84%. In an effort to further reduce sheet resistance, these thin films were exposed to HNO₃ vapor. Sheet resistance of the 5 BL film was reduced to 104 Ω sq⁻¹ (4200 S cm⁻¹ conductivity, 22.9 nm thickness), with no change in transparency. Single-walled carbon nanotube assemblies show an even greater change when exposed to HNO₃ vapor, with a 5 BL assembly (%T > 85) decreasing from 1236 to 237 Ω sq⁻¹. The DWNT-based assemblies maintained their low sheet resistance after repeated bending and also showed excellent electrochemical stability relative to ITO. This work demonstrates the excellent optoelectronic performance, mechanical flexibility, and electrochemical stability of nanotube-based assemblies, which are potentially useful as transparent electrodes for a variety of flexible electronics.