Browsing by Subject "Organic semiconductors"

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    Luminescent Probes of Emergent Physics from Organic Semiconductor Interfaces
    (2022-12) Concannon, Nolan
    To prevent the most harmful effects of the present climate crisis, development ofhigher performance energy conversion devices is needed to accelerate the adoption of renewable energy and energy efficiency technologies. Organic semiconductor materials have demonstrated exciting efficiency gains in a variety of emerging and in-production devices. These materials exhibit a variety of emergent material and device physics, requiring additional research to understand and design next-generation energy conversion technologies. Thin films of organic semiconductors, common in large-area optoelectronics such as consumer displays, present rich photophysics due to forming room-temperature stable excitons, unlike silicon or III-V semiconductors. A plethora of emergent phenomena of excitons at organic semiconductor interfaces requires a detailed understanding of such processes to optimally design devices such as energy-efficient lighting, flexible or transparent solar cells, photodetectors and displays. This dissertation focuses on investigating the novel optical physics of excited states at organic donor-acceptor interfaces through emission spectroscopy of organic mixtures and bilayer devices. In one study, exciplex diffusion is investigated in several donor-acceptor pairings toward an improved understanding of the mechanism of nanoscale energy transport in organic semiconductor mixtures. Additionally, the effect of electric field on exciplex emission spectra is studied to detail the effect of field on exciplex energy and electron-hole separation. Finally, preliminary data displaying the effect of binary dilution on exciplex energy in a two-component mixture is presented. All together, these findings present new insights into the behavior of key device properties such as exciton diffusion length and excited state energies to aid further study of device performance.
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    Nanoscale engineering of thin film morphology for efficient organic photovoltaic cells
    (2012-07) Pandey, Richa
    Organic photovoltaic cells (OPVs) have received significant industrial and academic interest in the last decade as a promising source of inexpensive renewable energy. However, further improvements in device performance and improved lifetimes are required for the commercialization of OPVs. This work is primarily focused on developing a novel device architecture to improve device performance and characterizing structure-property-performance relationships for OPVs. The excitonic nature of organic semiconductors necessitates the use of an electron donor-acceptor (D-A) heterojunction for efficient exciton dissociation and the generation of photocurrent. In many organic semiconductors, the optical absorption length is much larger than the exciton diffusion length. This trade-off between absorption and exciton diffusion is often overcome by increasing the area of the dissociating D-A interface using engineered film morphologies. This thesis presents an approach to maximize cell efficiency using a continuously graded D-A heterojunction. The graded heterojunction allows for an increase in the D-A interface area for an enhanced exciton diffusion efficiency, while also preserving the charge collection efficiency, leading to a significant improvement in device performance relative to that of optimized planar and uniformly mixed OPVs. In addition, this work correlates the optimized D-A composition gradient to the underlying film morphology and charge transport properties of uniform D-A mixtures. Subsequently, a new characterization technique to calculate the charge collection efficiency of OPVs is discussed. This technique is used to demonstrate the enhanced charge collection efficiency in graded heterojunctions relative to uniformly mixed heterojunctions. Afterwards, the properties of a new material and its potential as an electron donor material in OPVs are examined. Finally, an overview of the results and the ideas for future work are presented.
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    Spin polarized charge carrier injection, transport, and detection in organic semiconductors.
    (2011-05) Yunus, Mohammad
    In this thesis we explore spin polarized charge carrier injection, transport, and detection in organic semiconductors. Device structures considered have one or more ferromagnetic contacts to the organic semiconductor, and the condition for which charge carrier injection from ferromagnetic contacts is strongly spin polarized is discussed. Spin injection into semiconductors can be greatly enhanced if the injection mechanism is spin selective, such as is the case for tunnelling from ferromagnetic contacts. By contrast, if the carrier injection is by thermionic emission or another process that does not depend on spin, the injection is only weakly spin polarized. To discuss spin transport and spin detection, we consider a unipolar organic spin valve consisting of an organic semiconductor layer sandwiched between two ferromagnetic contacts. The polarizations of the magnetic contacts can be parallel or anti-parallel. Spin and charge carrier transport in the organic semiconductor is described by spin dependent transport equations in drift-diffusion approximation and the spin detection process is through magneto-resistance. We discuss the impact of various degrees of spin relaxation in organic semiconductors on the spatial variation of the spin current and its effect on magneto-resistance. The spatial profile of the spin current inside the organic semiconductor depends not only on the spin diffusion length but also on the alignment of the contact polarizations. However, the magneto-resistance decreases strongly with decreasing spin diffusion length. Electron tunnelling from a ferromagnetic contact can have significant spin dependence because the spatial part of the electron wave function is different for the majority and minority spin states of the ferromagnetic contacts. The tunnelling process occurs from the ferromagnetic contact through an insulating layer into the organic semiconductor. The insulating layer is modeled first as an ohmic layer with spin dependent contact resistances. The effectiveness of spin dependent contact resistances on spin polarized injection and magneto-resistance is examined on the basis of a simple analytical model. We then model the insulating layer as a tunnel barrier with spin dependent rate equations. Both majority and minority spin electrons of the ferromagnetic contact tunnel through the insulating layer into the localized molecular states of the organic semiconductor at the semiconductor/insulator interface. Tunnelling matrix elements and transition rates of the two spin types are calculated using a Transfer Hamiltonian approach. The transition rates are thus spin dependent and used in rate equations to calculate the injected (extracted) current for carriers of either spin direction. We explore the various aspects of the ferromagnetic contacts, the thickness and barrier height of the insulating layer, and the energy of the localized molecular states on spin injection and magneto-resistance. Consistent with the experimental data, the spin injection from ferromagnetic contacts can be either positive or negative, and the magneto-resistance decreases strongly with the applied bias across the device.