Hetero Structured Single Wall Carbon NanoTubes for Solution-Processed Field Effect Transistors

Abstract

In recent years, the demand for flexible/stretchable devices for the rapid spread of mobile devices such as smartphones and wearable devices have increased. The semiconductor layer for the stretchable devices requires high mechanical properties together with excellent charge carrier mobility and a random network structure. Also a solution process is required for a large-area and low cost application. Single wall carbon nanotubes (SWNT) are one of the suitable materials for stretchable devices because they are solutions processed, have random network structure when they form films, exhibit high carrier mobility and excellent mechanical properties. However, they have both metallic and semi-conductivity properties in the synthesis, so it is necessary to sort off only the semiconducting properties from the SWNTs. There have been several ways to separate this mixture, but they usually result in low purity and yield. We tried to solve this issue. In Chapter 1, we will introduce basic transistors knowledge such as transistors history, the operating principles, characteristic parameters, and type of tfts. Also we will introduce basic SWNT knowledge include Carbon allotropes, electric band structure of SWNT, and synthesis method of single wall carbon nanotubes. In Chapter 2, we sort off high-purity semiconducting single-walled carbon nanotubes (s-SWNT) by polymer wrapping method. We demonstrate the selection of s-SWNTs in SWNTs grown by the high pressure carbon mono oxide (HiPCO) process using poly-9,9-di-n-octyl-fluorenyl-2,7-diyl (PFO) and poly (3-dodecylthiophene-2, 5-diyl) (P3DDT), the wrapping polymer was used poly (9, 9-di-n-dodecylfluorene) (PFDD) for plasma discharge process (PD).We analyzed the purity, concentration and random network surface with Ultraviolet-visible spectroscopy, Raman spectroscopy, Atomic force microscope, Field emission scanning electron microscopy, and Transmission electron microscopy. Based on the UV-vis-NIR absorption spectra, Raman spectroscopy, and electrical parameter of the resulting devices, the purity of s-SWNT in P3DDT-HiPCO is estimated to be > 99% and the purity of s-SWNT in PFDD-PD is estimated 98-99%. In Chapter 3, we will talk about method, we fabricated transistors by using high purity s-SWNTs ink. The P3DDT-HiPCO transistors show high on/off ratio, but low mobility, while the PFDD-PD exhibit high mobility with high off current level. A hetero structure system was adopted to solve these challenges. And by this way, we achieved a hole mobility of 7 cm^2/VS and on/off ratio of 1.5×10^7. And FeCl3-doping hetero structure system, we achieved a hole mobility of over than 100 cm^2/VS (Vd = -20V) and on/off ratio of 10^6 (Vd=-5V).