Collections > Electronic Theses and Dissertations > A Study of Optical and Electronic Properties of Organic Thin Film Transistors Based on Naphthalenetetracarboxylic Diimide Derivatives
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The optical properties of spin cast thin films of N,N'-bis(3-phenoxy-3-phenoxyphenoxy)- 1,4,5,8-naphthalene-tetracarboxylic diimide (NDA-n2) and N,N'-bis(3-phenoxyphenoxy)- 1,4,5,8-naphthalene-tetracarboxylic diimide (NDA-n1) were investigated using spectroscopic ellipsometry (SE) complimented by optical absorption spectroscopy in the visible-near UV optical range and atomic force microscopy (AFM) for surface roughness. A combination of Tauc-Lorentzian and Gaussian oscillators model was used to fit the measurements obtained from SE. Film roughness results were also evaluated in the optical model using Bruggman Effective Medium Approximation (BEMA). The effect of different spin deposit conditions including spin speed, concentration of solution and deposition ambient on the NDA's film thickness, surface roughness, optical properties and optical anisotropy have been investigated. No anisotropy has been found for the spin cast film and moderate temperature annealing in high vacuum leads to film densification. Organic thin film transistors (OTFT's) were fabricated with NDA'sas the active semiconductor layer, silicon dioxide (SiO2) as the gate dielectric, heavily doped silicon as the substrate, and vacuum evaporated gold lines as the source and drain contacts. The electronic properties were characterized using a custom built probe station. The custom probe station was automated with software program written in LabVIEW™. NDA'syielded a P-channel device. From transfer characteristic and turn-on plot, the charge mobility was calculated which was in the range of about 10-2 cm2 V-1 s-1. Various post fabrication processes were carried out to optimize the device performance. Bottom contact configuration has shown higher charge mobility than top contact in this study. Moderate temperature annealing in high vacuum has improved the device mobility by several orders, yielding evidence for a hopping mechanism for charge transport in NDA's. The high mobility of NDA-n1 compared with NDA-n2 demonstrated that aryl-ether tail group hindered the charge transport in the film. Two alternate gate dielectric layers for the OTFT were also considered; while a non-polar low-K dielectric, polyethylene improved mobility; polar high-K dielectric, copolymer of vinylidene fluoride with trifluoroethylene had an adverse effect on mobility.