Organische Dünnschichtdielektrika für high-performance OFETs

• Applicant: Professor Dr.-Ing. Heinz von Seggern
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Synthesis and Properties of Functional Materials
• Term: from 2011 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 198315737

• Final Report Year: 2017

Final Report Abstract

Flexible and inexpensive organic devices with properties comparable to inorganic counterparts have attracted attention to its future application for flexible and wearable electronics. For commercialization of organic field-effect transistors (OFETs), low-voltage operating devices with not only high charge carrier mobility semiconductors but also densely packed dielectric layers with high capacitance are essential. For this reason, a large number of cross-linkable dielectrics have been reported with good dielectric properties, however, all of them suffer from disadvantages like uncontrolled pre-crosslinking, necessity of high process temperatures or the need for an additional cross-linking compound. In this study, a variety of cross-linking materials were systematically investigated. Among them, two new poly(methyl methacrylate) polymers (PAZ 12 and PAZ 14) showed superior dielectric properties, which can be cross-linked due to attached benzyl azide (N3) monomer units making the addition of hardeners or initiators obsolete. The synthesis of these polymers as well as their successful characterization and usage as gate dielectric for organic field-effect transistors is demonstrated. Spin-coated thin films were cross-linked either by an UV exposure or by a thermal treatment, yielding insoluble, smooth, moderately hydrophobic and electrically dense polymeric networks. Disappearance of the N3 vibrational mode in the infrared spectroscopy confirms a complete cross-linking reaction. The dielectric properties of the cross-linked thin films have been studied by impedance spectroscopy. The removal of ionic residues and the application of double layer dielectrics result in lower dielectric losses and lower leakage currents in the subsequently produced pentacene-based field-effect transistors. These devices operate at voltages less than -6 V and show hysteresis-free current-voltage-characteristics with hole mobilities up to 0.16 cm² V-1s-1. PAZ 12 appears to be superior to PAZ 14 due to a smaller total layer thickness of down to 92 nm still providing good insulation in the transistor. This is presumably related to a smaller free volume that arises in the cross-linked network of the two component containing copolymer PAZ 12 compared to terpolymer PAZ 14 with a more complex chemical structure. Since the cross-linked PAZ layers allow spin-coating of subsequent organic layers, these materials facilitate the fabrication of various types of multilayer OFET structures. In addition, because the UV curing does not require the usage of any additive (hardener and/or photoinitiator) or severe thermal treatment at high temperatures, the resultant polymer network is free of impurities in the sense of residual not reacted or degraded compounds that could deteriorate the device performance which makes the UV route applicable to plastic substrates. Therefore, based on these merits of PAZ materials, a wide application in low-voltage OFETs is highly expected.

Publications

• „Verfahren zur Herstellung durchschlagsfester ultradünner Dielektrika in elektronischen Bauteilen unter Verwendung vernetzbarer polymerer dielektrischer Materialien“. ICP: B05D 7/24, Nr. 102010034577
  T. Egorov-Brening, S. Janietz, A., E. Katholing, K. Schulze, H. von Seggern
  
• “Cross-linkable random copolymers as dielectrics for low-voltage organic field-effect Transistors”, J. Mater. Chem. C 3, 9217-9223 (2015)
  E. Reis Simas, E. S. H. Kang, A. Gassmann, E. Katholing, S. Janietz, H. von Seggern
  
• „Azide containing random copolymers as dielectrics for low-voltage field effect transistors”, 8th International Symposium on flexible organic Electronics (ISFOE15), 6- 9 July 2015, Thessaloniki, Greece
  E. Reis Simas, A. Gassmann, E. Katholing, S. Janietz, H. von Seggern
  
• „Electrical and structural origin of selfhealing phenomena in pentacene thin-film transistors”, ECS Prime 2016, Honolulu, Hawaii (USA)
  E. S.H. Kang, H. Zhang, W. Donner, H. von Seggern
  
• “Thin-Film Dielectrics for Low- Voltage Organic Field-Effect Transistors”, 81st Prague Meeting on Macromolecules (PMM): Polymers and Organic Materials, 10-14.Sept. 2017, Prague
  E. Reis Simas, E. S. H. Kang, S. Janietz, H. von Seggern