Development of field effect transistor using organic semiconductor

Abstract

Organic electronics have been an attractive research in the past decade. The driving force for the interest and development of organic-based devices is their mechanical flexibility and/or transparency, ease of chemical modification and processability at low temperature as compared to their inorganic counterpart, and its ability for large area device fabrication. Organic transistors, one of the fundamental structures of organic electronics, will probably never be as fast as their inorganic counterpart and unlikely to replace silicon in applications such as microprocessor. However, organic thin film transistors (OTFTs), also called organic field-effect transistors (OFETs), offer a great deal of potential for low-end market applications, such as disposable sensors, flat-panel displays, radio frequency identification (RFID) tags or memory applications for the future organic devices. The main focus of this thesis will be on the development of OTFT for possible future applications based on novel and smart arrangements of materials that would create an OTFT. The first objective of this thesis is to develop an optically transparent non-volatile memory (NVM) OTFT. Numerous studies have been reported on NVMOTFT based on novel materials, however no report has suggested successful development of a transparent NVM-OTFT. Transparent electronics can be beneficial in the future as it would provide more space by allowing the electronic devices to be consolidated and stacked in smaller clear spaces. In this study, the development of transparent NVM-OTFT begins with the selection of appropriate organic semiconductor as the active layer. A solution processable pentacene based on 13,6-N-Sulfinylacetamidopentacene as the precursor was chosen to be the active layer for the proposed NVM-OTFT due to its simple solution-processable fabrication method and its adequate performance. The next stage is to study the potential gate dielectric arrangement that would create a memory behaviour. The application of d.c. sputtered zinc oxide (ZnO) as a dielectric sandwiched between two low-k sol-gel methyl-silsesquioxane (MSQ) layers as the main charge storage system of NVM-OTFT has been demonstrated with ZnO as the main charge trapping layer. Together with the ease of processing and optical transparency of MSQ, this arrangement enables the development of transparent devices. The proposed NVM-OTPT was fabricated by integrating solution processable pentacene as the active layer and the novel triple layer structure as the gate dielectric. The optically transparent NVM-OTFT was successfully fabricated with a significant 4 V of threshold voltage shift between after programmed and erased state. Its optical transmittance is greater than 80% and its charge carrier mobility (μ) is about 0.08 cm2N.s. The second objective of this thesis is the integration of high-sensitive organic semiconductor molecules towards DNA molecules as the active layer of an OPET. Two types of organic semiconductor molecules were studied. A newly synthesised functionalised terthiophene monomer 3-((2':2",5":2"'-terthiophene)-3"-yl) acrylic acid (TAA) has been successfully electrodeposited as an active layer of an OFET. The polymer was oxidised in order to increase its conductivity. A mobility of 0.25 cm2N.s was achieved with an oxidising potential of 0.9 V after vacuum drying. A preliminary DNA sensing test was performed on the PTAA-OFET and a shift in threshold voltage was observed after DNA immobilisation and hybridisation, showing its potential as DNA sensor. Another molecule studied was polyaniline (P ANI) nanotubes. It was configured as OPET that exhibits a p-n junction diode behaviour. The forward-bias current can be modulated by the gate voltage. An energy band diagram model has been proposed to explain the rectifying effect of the P ANI-PET. The two main objectives of this thesis have been carefully studied and fulfilled. Several potential future studies would emerge from this thesis. The results also indicated that further research is required before the proposed devices are ready for implementation with other future organic electronics devices.