Date of Award

Fall 2005

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Yi Su

Abstract

Organic field effect transistors (OFETs) with poly(3-hexylthiophene) (P3HT) as the active layer are developed and studied. The device characteristics are significantly affected by source/drain contact resistance, and P3HT-SiO 2 interface and the traps. These results are verified by the numerical device simulations. The temperature dependence of device mobility is studied, which indicates that the carrier transport is either heat-assisted or heat-limited at different temperature ranges. The on/off ratio and threshold voltage are found to be dependent on the temperature. Hysteresis effect due to gate electric stress is investigated. The silanol groups present at the SiO2 surface are thought to be the key factor, which could trap the gate-induced electrons forming immobile negative ions, and shift the device threshold voltage.

Replacing gold with modified poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT-PSS) for the source/drain electrodes, reduces contact resistance and leads to an improved device performance. The SiO2 surface is also improved. Annealing the SiO2surface prior to the deposition of the P3HT layer is found to improve the performance of the device significantly. The device mobility is increased from 0.01 to 0.026 cm2/Vs, the on/off ratio increased from 2.3 × 103 to 8.2 × 103, and subthreshold slope decreased from 3.6 to 2 V/dec. The enhanced device performance is attributed to the possible reduction of physically adsorbed water molecules and hydroxyl groups at the SiO2 surface upon annealing.

Polymer heterostructure OFETs are also developed for establishing a method to fabricate new devices and the possibility to increase the device performance. This idea stems from the conventional inorganic modulation doped field effect transistors (MODFETs) that have shown strikingly high carrier mobility. The operation of conventional MODFETs is based on the technique of "modulation doping" which provides a good means of introducing carriers into the conduction layer without the adverse effects of donors. A polymer heterojunction structure is made of P3HT and poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) and is integrated into a field effect transistor. The resulting device characteristic shows the "modulation doping" effect. To our knowledge, the modulation doping effect with a polymer heterojunction has not been reported so far. This finding opens a potential pathway to improve the OFETs' device performance.

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