Heteroatom Engineering Of Organic Semiconductors For Organic Thin Film Transistor Applications

Organic thin-film transistors?OTFTs?have drawn great attention due to their extensive applications in flexible displays,organic sensors,logical circuits and many others.However,high threshold voltages in OTFTs is a major bottleneck in the commercialization of these devices.On the other hand,organic light-emitting transistors?OLETs?,one of the multi-functional OTFTs,have difficulty in finding a suitable organic semiconductor which combines a high carrier mobility together with a high luminescence efficiency.In order to address these issues,we modified organic molecules with heteroatoms such as sulfur and oxygen atoms,to develop a molecule which could demonstrate a high mobility with low threshold voltage or high luminescence efficiency.Moreover,OTFTs were fabricated under different conditions to investigate their performance at different temperatures as well as to explore the relationship between device performance and liquid crystalline properties.Our main findings are as follows:Chapter 1:Background and development of OTFTs,their basic structures,working principles and important parameters are first highlighted in this chapter.Then,we analyzed the reasons for the existing challenges in OTFTs and evaluated the current research status of organic semiconductors,based on which the main research contents of this thesis are obtained.Chapter 2:The influence of alkoxy or alkyl chain substituted BTBT on OTFTs,especially in terms of threshold voltage is investigated.C8-OPh-BTBT is designed and successfully synthesized by introducing an oxygen atom to side chain of BTBT core,and compared with the counterparts without an oxygen atom,C12-Ph-BTBT and C6-Ph-BTBT.C8-OPh-BTBT-based OTFTs exhibited a low threshold voltage which is attributed to the introduction of oxygen atoms that lowers the contact resistance between the electrodes and the organic semiconductor.The lower threshold voltage for C8-OPh-BTBT-based OTFTs compared with C12-Ph-BTBT and C6-Ph-BTBT based equivalents was confirmed by the device data.The liquid crystalline phases were observed for these three molecules,therefore,we investigated the device performance by fabricating or annealing the OTFTs at substrate temperature of liquid crystal phase.C8-OPh-BTBT based OTFTs fabricated at substrate temperature of liquid crystal phase showed a higher mobility of8.35 cm² V^-1 s^-1 compared to those fabricated at 25 ^oC,which is ascribed to the formation of a single-crystal-like bilayer structure from the monolayer.Also,the influence on the properties of molecules and OTFTs by changing the linear chain with a side chain modified with heteroatoms is investigated.Side chain molecule,C6C2-OPh-BTBT is designed and successfully synthesized,and compared with the linear chain molecule,C8-OPh-BTBT.Both of these molecules have similar electrical and thermal properties,however,C6C2-OPh-BTBT demonstrated worse carrier transporting properties because of the disorder in molecular arrangement caused by the weakened intermolecular interactions due to the existence of side chain.This research provides guidance for the design of high-performance organic semiconductors.Chapter 3:Investigation of fluorescence properties and performance of fluorescence OTFTs via furan substitution.BTBT-DPh have been reported to possess good mobility,but its fluorescence performance is not so good.In this work,BTBF-DPh is designed and successfully synthesized by replacing one of the sulfur atom with oxygen atom in BTBT-DPh.The investigation of the optical properties revealed that BTBF-DPh exhibits a strong fluorescence intensity and a high photoluminescence quantum yield of 50.0%.These superior optical properties are attributed to the suppression of exciton quenching because of the smaller atomic size of oxygen and intermolecular distance between the heteroatoms which lead to a weak aromaticity.BTBF-DPh based OTFTs showed a good mobility of 0.181 cm² V^-1 s^-1.The single crystal analysis demonstrated that the short intermolecular distance contributes to a strong intermolecular interaction,facilitating the charge transport through adjacent molecules.This work provides an effective route for the design of new organic semiconductor for OLETs.