Ambipolar transport in solution-deposited pentacene transistors enhanced by molecular engineering of device contacts

We report ambipolar transport in bottom gold contact, pentacene field-effect transistors (FETs) fabricated by spin-coating and thermally converting its precursor on a benzocyclobutene/SiO2 gate dielectric with chemically modified source and drain electrodes. A wide range of aliphatic and aromatic self-assembled thiolate monolayers were used to derivatize the electrodes and all enhanced electron and hole currents, yet did not affect the observable thin film morphology. Hole and electron mobilities of 0.1–0.5 and 0.05–0.1 cm2 / V s are achieved, though the threshold for electron transport was \u3e80 V. These ambipolar FETs are used to demonstrate inverters with gains of up to 94

Ambipolar transport in solution deposited organic transistors

Ambipolar field effect transistors (FETs) based on a single organic semiconductor are interesting because of the unique platform they offer to study the transport physics of both holes and electrons simultaneously in a single FET and also because of novel multifunctional applications they promise such as CMOS-like inverters and light emitting FETs. However, fabrication of these devices has been challenging, much more so because devices based on high mobility semiconductors like pentacene have not shown ambipolar behavior until recently. In the first part of the thesis, the device engineering techniques that enabled the demonstration of ambipolar transport in pentacene devices are reported. Hydrophobic polymer dielectrics such as parylene and benzocylcobutene are used to support electron transport and source-drain electrodes are modified with thiolate and carbodithiolate self-assembled monolayers (SAMs) to lower the contact resistance and improve hole and electron injection. FETs with hole and electron mobilities of 0.1–0.5 and 0.05–0.1 cm2/V s are demonstrated using this method. The device scaling effects on the performance of these ambipolar FETs is analyzed and an estimation of the contact resistance limited channel lengths for both hole and electron transport is extracted. The second part of the thesis investigates the temperature dependence of ambipolar FET characteristics and provides new insights into the physics of charge transport of both electrons and holes. Electron and hole saturation mobilities show activated behavior consistent with transport by hopping through or trapping in a distribution of localized electronic states. The gate voltage dependent saturation mobility is modeled by a power law to reflect the exponential distribution of localized states extending into the energy gap of the organic semiconductor. The characteristic Meyer-Neldel temperature as a measure of the width of the localized distribution of states for both electrons and holes is extracted. It is larger for electrons than holes which indicate a greater density of localized states for electrons than holes which indicates a greater density of localized states for electrons than for holes. An analytical model and SPICE model to describe the current-voltage characteristics of ambipolar FETs and circuits is presented. Finally, performance of a few building blocks of more complex organic circuits such as CMOS-like inverters built from ambipolar pentacene FETs and diode-connected inverter and cascode amplifier built from unipolar (hole) pentacene FETs is reported