| Over the past two decades, organic thin film transistors (OTFTs) have developed from the research laboratory scale to the scale of commercially feasible technology.;The most attractive features of OTFTs are their low-cost manufacturing procedures and their compatibility with flexible substrates.;The characteristics of the organic semiconductor/metal electrode interface dramatically affect the performance of OTFTs. For instance, charge carrier injection efficiency can be limited by the offset between the HOMO and LUMO energy levels of the organic semiconductor and the Fermi level of the metal electrode, since this offset leads to the formation of an energy barrier for charge carrier injection.;This MSc work focuses on the use of single walled carbon nanotube (SWCNT) array electrodes, to improve the injection efficiency in OTFTs. A comparative approach has been adopted, based on the systematic comparison between SWCNT array electrode-based OTFTs with their, well investigated, Au electrode-based counterparts.;An improvement in the injection characteristics of OTFTs making use of SWCNT array electrodes has been previously demonstrated in OTFTs based on organic semiconductors such as (p-type) pentacene and copper phthalocyanine and (n-type) phenyl-C61-butyric acid methyl ester (PCBM). In these previous studies, it has been hypothesized that the improvement of the charge carrier injection characteristics is mainly due to the one dimensional (1D) structure of SWCNTs, which favors tunneling injection across the injection barrier.;To assess the general validity of this hypothesis, a large number of experiments need to be carried out with different organic semiconductors, belonging to different classes of materials (e.g., polymers and small molecules) and differently processed (e.g. solution processed and vacuum processed).;In this project, the performance of OTFTs making use of SWCNT array electrodes has been investigated for thin films of Titanyl-phthalocyanine (TiOPc), Poly-3 (hexylthiophene) (P3HT), and P3HT/PCBM blends. TiOPc belongs to metal phthalocyanines (MPcs), which, due to their low solubility in organic solvents, are typically deposited by vacuum-based techniques. P3HT is a solution processable polythiophene. P3HT/PCBM blends, which are processed from solution, are benchmark materials in organic photovoltaics.;SWCNT array electrodes were patterned on a SiO2 layer thermally grown on highly doped (n-type) Si (100) wafer. SWCNT array electrode fabrication started with the deposition of SWCNT networks by vacuum filtration. Subsequently, metallic (Ti) contacts were patterned on the SWCNT network by photholithography and lift-off. Finally, SWCNT array electrodes were obtained via a sonication in a stripper solution, which removes the SWCNTs not directly attached to the metal contacts and cuts the remaining SWCNTs to a length of a few hundred nm.;For TiOPc TFTs, SWCNT array electrodes led to improved injection efficiency, higher charge carrier mobility, especially in the linear regime, and higher ION/IOFF compared with benchmark Au TiOPc TFTs. The injection efficiency of TiOPc TFTs with SWCNT array electrodes was two orders of magnitude higher than for TiOPc TFTs made with Au electrodes. Conversion of unipolar TiOPc TFTs to ambipolar TFTs upon thermal annealing under vacuum was also observed when using SWCNT array electrodes.;For P3HT TFTs, P3HT was deposited on bare SiO2 and on SiO 2 treated with self assembled monolayers of hexamethyldisilazane (HMDS) and octadecylthrichlorosilane (OTS). In all cases, SWCNT array electrodes provided higher injection efficiency and mobility in comparison with benchmark Au OTFTs.;For P3HT/PCBM blends, quasi linear output characteristics in SWCNT P3HT/PCBM TFTs were observed for both electrons and holes, as opposed to the non linear behavior found in P3HT/PCBM TFTs making use of Au electrodes. Electron and hole injection efficiency of P3HT/PCBM TFTs made with SWCNT array electrodes could be as high as three orders of magnitude and four times higher than those of P3HT/PCBM TFTs made with Au electrodes, respectively.;The results presented in this MSc work are an important contribution to the demonstration of the possibility to improve the injection efficiency in OTFTs based on SWCNT array electrodes over Au electrodes, independently of the type of organic semiconductor and processing procedure employed. |
