Browsing by Subject "Density of states"

Now showing 1 - 3 of 3
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    Characterization of pi-conjugated polymers for transistor and photovoltaic applications
    (2012-12) Paulsen, Bryan D.
    pi-Conjugated polymers represent a unique class of optoelectronic materials. Being polymers, they are solution processable and inherently "soft" materials. This makes them attractive candidates for the production of roll-to-roll printed electronic devices on flexible substrates. The optical and electronic properties of pi-conjugated polymers are synthetically tunable allowing material sets to be tailored to specific applications. Two of the most heavily researched applications are the thin film transistor, the building block of electronic circuits, and the bulk heterojunction solar cell, which holds great potential as a renewable energy source. Key to developing commercially feasible pi-conjugated polymer devices is a thorough understanding of the electronic structure and charge transport behavior of these materials in relationship with polymer structure. Here this structure property relationship has been investigated through electrical and electrochemical means in concert with a variety of other characterization techniques and device test beds. The tunability of polymer optical band gap and frontier molecular orbital energy level was investigated in systems of vinyl incorporating statistical copolymers. Energy levels and band gaps are crucial parameters in developing efficient photovoltaic devices, with control of these parameters being highly desirable. Additionally, charge transport and density of electronic states were investigated in pi-conjugated polymers at extremely high electrochemically induced charge density. Finally, the effects of molecular weight on pi-conjugated polymer optical properties, energy levels, charge transport, morphology, and photovoltaic device performance was examined.
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    Lanczos Approximation of Joint Spectral Quantities and Applications
    (2021-04) Loudon, Tyson
    The Lanczos process is a method to approximate quadratic forms (f(A)v, v) for a symmetric matrix A, vector v, and smooth function f. In this thesis we focus on utilizing the Lanczos process to approximate joint spectral quantities determined by the spectra of two distinct operators. Joint spectral quantities are relevant in many areas of physics. The application we focus on is modeling the optical properties of semiconductors. Joint spectral quantities, when computed exactly, require complete knowledge of the spectra of two operators. In practical situations the computational cost of such a calculation is outside the realm of possibility. Instead, we focus on implementing an inexpensive and accurate method to approximate joint spectral quantities which relies on the Lanczos algorithm.
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    Noise detection and transport measurements of spin valve systems.
    (2011-08) Guo, Feng
    Electronic noise not only limits the performance of magnetic devices in practical applications but also provides valuable physical insights into these devices. The first part of this thesis discusses how the low frequency noise in magnetic tunnel junctions and giant magnetoresistance devices can be used to understand the fundamental noise sources. Previously, the low frequency noise in these systems has been reported to have an enormously large magnitude when the magnetization switches. This was attributed to magnetic fluctuations. An alternative mechanism of a slow drift in the device resistance is discussed, and we show how it produces noise spectra that are similar to those in previous reports. We conclude that this resistance drift causes a measurement artifact and the low frequency magnetic noise is not present in the measured samples within measurement error. As a second part of the thesis, we discuss a pronounced voltage dependent conductance feature present at nonzero bias in some magnetic tunnel junctions. The presence of this feature depends upon the oxidation condition for creating the barrier, and this effect is found to be interfacial in nature. We describe how the electronic structures and density of states at the barrier interfaces could be responsible for this effect, and possibility of utilizing the conductance measurement to probe the interfacial states.