Fabrication of Flexible and Transparent Carbon Nanotube Network Transistors on Polyethylene Terephthalate Substrates

Abstract

Fabricating uniform carbon nanotube networks (NTN) on polymer substrates has attracted much attention due to their possible application as transparent and flexible thin-film transistors (TFT). For reliable TFT operation, low density single-wall NTNs with reduced bundle concentration on flexible substrates is necessary to realize semiconducting thin-films with reduced leakage. The lower nanotube density and reduced bundle concentration decreases the probability of having metallic nanotube conduction paths between source and drain, which is a major cause of leakage and reduced On/Off current ratio in nanotube network transistors. We presented here a method to fabricate low density NTN-TFT on flexible and transparent polyethylene teraphthalate (PET) substrates. The single-wall nanotubes were prepared on glass substrates by CVD. The nanotube density was controlled by controlling the catalyst density and growth time. The NTN was directly transferred from the glass surface to the PET surface by simply contacting two surfaces at elevated temperatures. This process allows the NTN to be immersed within the PET surface polymer matrix and be transferred uniformly from the growth substrate. Contact electrodes (Ti/Au) were deposited by thermal evaporation using stencil masking technique. Since the NTN surface had varying surface textures, we used spin-coated polymer poly-4-vinylphenol (PVP) thin film (470nm) as the gate insulator which showed relatively isotropic surface coverage and was also mechanically compatible with the PET substrate. The transfer characteristics showed that the On/Off ratio increased by more than a factor of 10 compared to NTN-TFTs fabricated with nanotube networks formed from nanotubes dispersed in solution.