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Center for Organic Photonics and Electronics
Center for Organic Photonics and Electronics
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Item2014 COPE Fellowship Award Winners( 2014-09-25) Bougher, Thomas ; Grand, Caroline ; Kleinhenz, Nabil ; Knauer, KeithThomas Bougher, a graduate student in Mechanical Engineering presents "Thermal Transport in Chain-oriented Amorphous Polymers".
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ItemComplex Morphologies and Molecular Ordering in OPVs: Critical Parameters Determined by Soft X-Ray Scattering(Georgia Institute of Technology, 2013-09-26) Ade, HaraldInterface structure and the quantitative composition of morphology are known to be critical for fullerene based bulk heterojunction solar cells, yet have been very difficult to study previously due to a paucity of characterization methods. Recently developed soft X-ray microscopy and scattering tools provide new avenues and contribute substantially to indentify the number of phases present and to provide a quantitative measurement of their composition fluctuations and size distribution. This led to the realization that mixed domains are prevalent in OPVs and rather than just being detrimental can have important beneficial contributions for charge generation and charge transport. Furthermore, polarized x-ray scattering can reveal preferential orientation of the donor polymer (edge-on or face-on) relative to the fullerene aggregate interface. Such ordering has previously not been observed solar cells and is shown here to be a critical factor for high performance in a number of systems.
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ItemRelation of Structure and Charge Transport in 'Single-Stack' Organic Nanowires(Georgia Institute of Technology, 2013-08-28) Frauenrath, HolgerOrganic nanowires may provide insights into the fundamental processes of charge generation and transport in organic semiconductors under nanoscopic confinement. Here, we demonstrate how a simple molecular design results in nanowires with defined lateral dimensions that comprised a single stack of tightly π–π stacked chromophores at their core. Moreover, we prepared well-defined microfibers that exhibited hierarchical structure formation with a remarkably high degree of internal order and enabled us to obtain detailed structural information on all length scales with molecular level precision. The nanowires and microfibers nanowires showed light-induced formation of radical cations that behaved like positive polaron charge carriers. The nanofibrils were semiconducting, showed space-charge injection-limited conductivity behavior, and exhibited photo-current generation, relating their macroscopic electric properties to the spectroscopically characterized charge carriers. Our results, thus, provide an example of a universal organic nanowire model system that successfully links molecular design, well-defined supramolecular structure formation, charge carrier generation, and finally macroscopic charge transport.
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ItemThin Film Solar Cells. New Organic Materials, Large-scale Printing and Nanoparticles(Georgia Institute of Technology, 2013-08-21) Watkins, ScottIn this seminar I will present three main areas of our work on solution processed solar cells. In part one I will describe the development of new donor and acceptor materials for solution processed organic photovoltaics (OPVs). In particular, I will discuss the issue of molecular association with regards to small molecule donor materials and its effect on the open circuit voltage. I will also introduce a new class of small molecule semiconductors based on solubilised indene derivatives and their use as acceptor materials in solution processed bulk heterojunction OPVs. In part two, I will describe our work on large scale printing of organic solar cells. Finally, in part three, I will present our work on the fabrication of thin film solar cells based on nanocrystal inks of CdTe. We have shown that totally solution processable solar cells can be fabricated in air at temperatures as low as 300 °C. Focusing on a CdTe/ZnO thin-film system, we report solar cells that achieve power conversion efficiencies of upto 10% with greater than 90% internal quantum efficiency.
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ItemThe Role of Non-covalent Interactions in Band-gap Tuning, Conformation and Self-assembly(Georgia Institute of Technology, 2013-08-06) Skabara, Peter J.The extent to which the optoelectronic properties of molecules are controlled by their conformation and packing is now well appreciated. In particular, the planarity (or nonplanarity) of a molecule has a dramatic influence on its behavior, and hence on its potential application. In the context of light emission, disorder can be beneficial in controlling aggregation, but for photovoltaics and transistors where low energy absorption and effective charge transport are key issues, planar molecules are preferable. Of particular relevance to this talk is the development of planarised conjugated architectures that should support much-enhanced charge transport properties as a result of their ability to self-assemble into tightly packed frameworks. For example, compound 1 (m = 10 ⁻² cm² V⁻¹ s ⁻¹) orders into a 3-dimensional network of close contacts both in crystalline form and as a thin film. This high level of self-assembly is achieved through p-p stacking in 2D and non-covalent interactions between the S and N atoms of adjacent molecules in the third. The nature of these interactions is not entirely clear, but one possible source of an attractive potential is a 3c-2e interaction between lone pairs and relatively low-lying antibonding orbitals, although these are likely to be counteracted to some extent by significant lone pair-lone pair repulsions. The 3D nature of the material represents a unique motif for highly efficient charge transport. A second example, polymer 2 (m = 10 ⁻³ cm² V⁻¹ s ⁻¹), has a planar repeat unit in the solid state as a result of close intramolecular sulfur/fluorine and hydrogen/fluorine contacts. Interestingly, variations in the absorption spectrum of these molecules suggest that the band gap is very sensitive to the degree of planarity. Whilst the mobility values for 1 and 2 are modest, the importance of the work is that it establishes a principle for the rational design of second generation materials with enhanced charge-transport properties. This lecture will discuss several further examples that underline the importance and influence of non-covalent interaction in organic semiconductors.
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ItemNew Materials for High-Efficiency Organic Light-Emitting Devices (OLEDs) and Solid-State Lighting (SSL)(Georgia Institute of Technology, 2013-03-12) Bryce, Martin R.Fluorescent and phosphorescent organic/organometallic materials are receiving intense interest as emitters in electroluminescent organic light-emitting diodes (OLEDs) for displays and solid-state lighting (SSL). The background concepts will be briefly described before a presentation of recent work in the Durham labs. Topics will include the design, synthesis, photophysical properties and OLED performance of fluorescent copolymers, and phosphorescent Ir(III) complexes. Color tuning of emission, including efficient white light emission will be reported. Phosphorescent device efficiencies as high as 40 cd/A with external quantum efficiencies of 12% for green emission have been achieved in simple solution-processed device architectures.