Cross-linkable random copolymers as dielectrics for low-voltage organic field-effect transistors

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2015-01, Vol.3 (35), p.9217-9223
• Main Authors: Reis Simas, E, Kang, E. S. H, Gassmann, A, Katholing, E, Janietz, S, von Seggern, H
• Format: Article
• Language: English
• Subjects: Addition polymerization; Azides (organic); Copolymers; Crosslinking; Dielectrics; Field effect transistors; Polymethyl methacrylates; Semiconductor devices
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/c5tc00352k

A large number of cross-linkable dielectrics with good dielectric properties have been reported; however, all of them suffer from disadvantages like uncontrolled pre-crosslinking, necessity of high process temperatures and the need for an additional cross-linking compound. In this contribution, two new poly(methyl methacrylate) polymers are introduced which can be cross-linked due to the attached benzyl azide (N 3 ) monomer units making the addition of hardeners or initiators obsolete. The synthesis of the copolymers as well as their successful characterization and usage as gate dielectrics for organic field-effect transistors is demonstrated. The investigated polymers have been labeled PAZ 12 and PAZ 14 according to their azide content in mol%. The additional building blocks of the polymers are methyl methacrylate for PAZ 12 and methyl methacrylate and styrene monomer units in about an equal ratio for PAZ 14. Spin-coated thin films were cross-linked by a thermal treatment at 110 °C followed by an UV exposure at a wavelength of 254 nm yielding insoluble, smooth and electrically dense polymeric networks. Optimal cross-linking parameters were obtained using infrared spectroscopy to follow the N 3 vibrational mode. Its disappearance confirms a complete cross-linking reaction, and thus fully reacted azide groups facilitate the analytics. The dielectric properties of the cross-linked thin films have been studied by impedance spectroscopy. The application of double layer dielectrics results in lower dielectric losses and lower leakage currents in the subsequently produced pentacene-based field-effect transistors. These devices operate at voltages below −6 V and show hysteresis-free current-voltage characteristics with hole mobilities up to 0.16 cm 2 V −1 s −1 . PAZ 12 appears to be superior to PAZ 14 due to a lower total layer thickness of down to 92 nm still providing good insulation in the transistor presumably related to a lower free volume that arises in the cross-linked network of the two-component containing copolymer PAZ 12. A novel co- and terpolymer utilizing cross-linkable azide groups has been proven to be applicable as thin-film dielectric in organic field effect transistors. Reliable operation with saturation currents above V g = 4 to 5 V can be achieved for a dielectric thickness of smaller than 100 nm.