Fabrication and characterization of solution-processed methanofullerene-based organic field-effect transistors

The fabrication and characterization of high-mobility, n-channel organic field-effect transistors (OFET) based on methanofullerene [6,6]-phenyl C61-butyric acid methyl ester using various organic insulators as gate dielectrics is presented. Gate dielectrics not only influence the morphology of the active semiconductor, but also the distribution of the localized states at the semiconductor-dielectric interface. Spin-coated organic dielectrics with very smooth surfaces provide a well-defined interface for the formation of high quality organic semiconductor films. The charge transport and mobility in these OFET devices strongly depend on the choice of the gate dielectric. The electron mobilities obtained are in the range of 0.05-0.2 cm2 V-1 s-1. Most of the OFETs fabricated using organic dielectrics exhibit an inherent hysteresis due to charge trapping at the semiconductor-dielectric interface. Devices with a polymeric electret as gate dielectric show a very large and metastable hysteresis in its transfer characteristics. The observed hysteresis is found to be temperature dependent and has been used to develop a bistable memory element

Electrical response of highly ordered organic thin film metal-insulator-semiconductor devices

We report a detailed investigation of the electrical properties of organic field-effect transistors (OFETs) and metal-insulator-semiconductor (MIS) capacitors formed from highly ordered thin films of C60C60 as the active semiconductor and divinyltetramethyl disiloxane-bis(benzocyclobutene) (BCB) as the gate dielectric. Current-voltage measurements show the OFETs to be n-channel devices characterized by a high electron mobility (∼6 cm2/V s)(∼6 cm2/V s). An equivalent circuit model is developed which describes well both the frequency and voltage dependences of the small-signal admittance data obtained from the corresponding MIS capacitors. By fitting the circuit response to experimental data, we deduce that increasing gate voltages increases the injection of extrinsic charge carriers (electrons) into the C60C60. Simultaneously, the insulation resistance of the BCB decreases, presumably by electron injection into the insulator. Furthermore, the admittance spectra suggest that the capacitance-voltage (C-V)(C-V) behavior originates from a parasitic, lateral conduction effect occurring at the perimeter of the capacitor, rather than from the formation of a conventional depletion region