High Mobility Solution Processed Organic Thin Film Transistors

Open Access
Park, Sung Kyu
Graduate Program:
Electrical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
June 04, 2007
Committee Members:
  • Thomas Nelson Jackson, Committee Chair
  • Christopher Roman Wronski, Committee Member
  • Jerzy Ruzyllo, Committee Member
  • Qing Wang, Committee Member
  • William Kenneth Jenkins, Committee Member
  • Organic Thin Film Transistors
  • Solution Processed OTFT
  • Organic circuits
  • Molecular electronics
To date, most high mobility organic thin film transistors (OTFTs) have used vapor-deposited organic semiconductors as the active material. The OTFT fabrication processes for vapor deposited organic materials are not so different from conventional inorganic TFT fabrication. Therefore they are constrained by similar production costs with some savings related to reduced processing temperatures and low cost substrates. Solution- processed OTFTs are of interest because of their compatibility with roll-to-roll processing which may allow simplified device fabrication and further reduced processing costs. In the project outlined for this thesis, high performance solution processed small molecule OTFTs were developed for their application in integrated circuits and flat panel displays. 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and fluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (F-TES ADT) were used as high performance solution processible small molecules. Using TIPS-pentacene and F-TES ADT in combination with a variety of device fabrication techniques, mobilities greater than 1.2 cm2/V•s and 3 cm2/V•s have been obtained, respectively. These devices were made using a drop casting process and represent the best mobility for solution processed OTFTs to date. Additionally, using the F-TES ADT, spin cast OTFTs which show mobilities of greater than 1.0 cm2 / V×s with relatively good film uniformity have been obtained. Film growth which is considerably more ordered on pentafluorobenzene (PFBT) treated Au surfaces, and on samples with patterned PFBT-Au structures grains appear to grow out from the PFBT-Au areas into the oxide areas were observed. This results in a substantial variation in field effect mobility with gate length as grains growing from the source and drain electrodes meet and overlap. Spun F-TES ADT OTFTs fabricated with films deposited on PFBT-treated Au electrodes show mobilities of 0.1 - 0.5 cm2/V×s from a toluene solution and 0.2 - 1.0 cm2 / V×s from a cholobenzene solution. The potential application for solution processed OTFTs was examined by performing operational and environmental stability tests. In addition, 7 and 15-stage ring oscillators were fabricated on both glass and plastic substrates. The 7-stage ring oscillators on a plastic substrate operate at more than 22 kHz. The circuits also operate at low voltages with oscillation beginning at a supply voltage of about - 3 V and operate at 2.8 kHz at - 5 V. In an effort to optimally design a soluble small molecule organic semiconductor, we investigated molecular ordering, orientation, packing structure, and surface morphology of several different soluble small molecule organic semiconductors and compared their electrical performance. A patterning technology of solution processed organic films which could be applied to OTFT circuits and flat panel displays was developed. Using an octadecyltrichlorosilane (OTS) self assembled monolayer (SAM) patterned by deep ultra-violet light (185 and 254 nm), 7-stage ring oscillators with patterned solution processed organic films were fabricated. For future researches, solution processed OTFT integrated circuits such as novel logic gates, D-flip flop and a 5-stage frequency divider were designed and simulated. Also an entirely solution processible flat panel display were designed and demonstrated by using waveform simulations for practical feasibility of solution processed OTFT driven liquid crystal displays.