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Center for Organic Photonics and Electronics
Center for Organic Photonics and Electronics
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ItemHow Do We Create and Process Materials for Flexible, Transparent Electronic Circuitry?(Georgia Institute of Technology, 2018-03-29) Marks, TobinThis lecture focuses on the challenging design, realization, understanding, and implementation of new materials families for unconventional electronics. Fabrication methodologies to achieve these goals include high-throughput, large-area, high-resolution printing techniques. Materials design topics will include: 1. Rationally designed high-mobility p- and n-type organic semiconductors for printed organic CMOS, 2. Self-assembled high-k nanodielectrics enabling ultra-large capacitance, low leakage, high breakdown fields, minimal trapped interfacial charge, and device radiation hardness, 3. Polycrystalline and amorphous oxide semiconductors for printable transparent and mechanically flexible electronics, 4. Combining these materials sets to fabricate a thin-film transistor-based circuitries, 5. The relevance of these advances to unconventional photovoltaics.
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ItemOrganic Memories Featuring Multi-Bit Storage( 2018-03-09) Meerholz, KlausPhotochromic molecules provide an intriguing and relatively untapped alternative to traditional materials utilized in organic memory devices. We have recently reported on a new prototype of a nonvolatile light-emitting organic memory (LE-OMEM) that integrates a layer of crosslinkable dithienylethene photochromes (XDTE) into a solution-processed, multilayer OLED. The XDTE molecules undergo a change in both their UV-visible absorption and energy level position due to a photo- and/or electrically-induced ring-opening/-closing reaction. Exploiting the difference in HOMO and LUMO energies of both isomers and the subsequent change in hole-injection barrier we use this XDTE layer to control the hole injection and transport within our OLED layer stack. Optimized devices have displayed ON/OFF ratios in both current and electroluminescence approaching 106. We investigate both, optical and electrical programming of the OMEM devices and show that precise control of the ratio of both isomers in the active layer enables access to a multitude of intermediate states, demonstrating the potential of these devices for future multi-level memory applications. We report a dynamic range of ca. 200.000, thus, storage of up to 16 bits per pixel seem feasible. We also take advantage of these devices with variable energy landscape to study trapping.
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ItemBio-inspired Materials: New Approaches to Molecular Building Blocks( 2018-01-09) Kawker, CraigMarine organisms use organic building blocks in unique ways to achieve materials with exceptional properties. With inspiration from these natural systems, the design of synthetic building blocks to mimic these capabilities and extend them to common polymeric materials will be described.
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ItemDisorder-Order Transitions in p-Conjugated Polymers(Georgia Institute of Technology, 2016-12-02) Köhler, AnnaThe aggregation of p-conjugated materials significantly impacts on the photophysics, and thus on the performance of optoelectronic devices. Nevertheless, we know comparatively little about the laws governing aggregate formation of p-conjugated materials from solution. In this talk, I shall compare, discuss and summarize how aggregates form for three different types of compounds, that is, homopolymers, donor-acceptor type polymers and low molecular weight compounds. To understand how aggregates form, we employ temperature dependent optical spectroscopy, which is a simple yet powerful tool for such investigations. I shall discuss how optical spectra can be analysed to identify distinct conformational states and to obtain quantitative information on changes in the inter-chain coupling, the conjugation length and the oscillator strength upon aggregate formation. We find aggregate formation to proceed alike in all these compounds by a coil-to-globule like first order phase transition. Notably, the chain expands before it collapses into a highly ordered dense state. I will address the role of side chains and the impact of changes in environmental polarization.
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ItemSerendipity in the Quest for New Organic Materials( 2016-09-14) Wudl, FredOn the way to the design of a new molecule for possible ambipolar organic field effect transistors, we discovered a crystal-to-crystal, essentially quantitative, monomer to polymer transformation. The crystalline polymer had the further interesting property of full exfoliation making it possible to isolate and observe a single polymer molecule. We further discovered preparation and X-ray crystallographic characterization of the first crystalline homoatomic polymer chain as part of a semiconducting pyrroloperylene-iodine complex. The crystal structure contains infinite polyiodideI I∞δ-‐ and close similarities in the low-wavenumber Raman spectra of the title compound and starch-iodine complex suggest such infinite chains of polyiodide in the latter as well. The structure of iodine within the insoluble, blue starch-iodine complex has remained elusive since the phenomenon was first observed over two centuries ago, and has since generated much speculation as to how iodine arranges within this complex. This includes suggestions of infinite polymeric iodine chains stabilized by amylose sugars.
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ItemPerovskite Solar Cells; Outshining Silicon( 2015-11-10) Snaith, Henry J.The cost of generating electricity from sun light using photovoltaic devices has continued to drop extensively over the last decade. Now in some locations in the world, PV generated electricity is cheaper than that from any other source. As the costs continue to drop and the efficiency of the PV modules continues to rise, the economic argument for globally widespread deployment of PV will become impossible to ignore. There have already been such advances in manufacturing the PV modules, that now the majority of the cost of a PV installation is the non-module costs, such as physical frames, electrical power handling, land and other soft costs. Therefore, from a PV technology perspective the most straight forward means to ensure the continuing drop in the cost of PV electricity is to enhance the efficiency of the modules. Current deployed PV is predominantly based on single-junction crystalline silicon. Most modules today are around 15 to 17% efficiency, with the more advanced “silicon technologies” promising modules of around 22%, but single junction silicon has a practical efficiency limit of around 25%. To move beyond this will require fundamentally superior technologies. Within the last few years organic-inorganic halide perovskites have risen to become a very promising PV material, captivating the research community, with the lab based cell efficiency rising form 4% to over 20% within a few years. In the most efficient devices, the perovskite semiconductor is present as a solid absorber layer sandwiched between negative (n) and positive (p)-type charge collection contacts. The perovskite itself is crystalized at low temperature by either, mixing precursor salts in a solvent and casting from solution, or via sublimation of the same salts under vacuum. Improving solar cell operation is reliant upon understanding and controlling thin-film crystallisation and controlling the nature of the p and n-type contacts. In addition, understanding and enhancing long term stability of the materials and devices if a key driver. One key advantage perovskites have over silicon is that the band gap (the lowest energy at which the material absorbs light) can be tuned broadly from around 1.2eV to 2.4eV. This enables the possibility of realising multi-junction solar cells, which could deliver much higher efficiency than single junction silicon, by either combining perovskites with silicon, or on their own. Snaith presents his work on developing thin film perovskite solar cells, introducing the technology and putting it into the broader perspective of the global PV industry. He discusses areas in which he has made recent advances and discuss the current status and potential for the “hybrid” perovskite-on-silicon tandem concept. He also presents broader applications where perovskite cells could find markets not currently met with crystalline silicon PV.
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ItemExploring the Synthesis and Applications of Graphene( 2015-05-04) Kaner, RichardGraphene is the ultimate two-dimensional material consisting of a single layer of sp hybridized carbon. Here we explore different approaches to synthesize this carbon allotrope, ranging from chemical conversion to vapor phase deposition. Briefly, graphite can be converted into graphene oxide (GO) sheets, which readily disperse in water, and then can be reduced by various methods. Due to its unique ability to be solution processed and patterned, GO and chemically converted graphene (CCG) hold promise for applications ranging from sensors to transparent conducting electrodes for flexible solar cells, etc. Chemical vapor deposition onto metal substrates enables the growth of continuous, large-area graphene. The challenges of growing graphene, controlling the number of layers, transferring graphene and some exciting uses such as laser scribed graphene for supercapacitors will be discussed.
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ItemProbing the Relationship Between Molecular Packing and Optoelectronic Properties in Molecular Electronic Materials( 2015-04-03) Nelson, JennyThe properties (electronic, optical, mechanical and thermal) of organic semiconductors are strongly dependent on the chemical structure, configuration, conformation and relative position of the component molecules or molecular segments. Predicting the impact of structure on properties requires a detailed understanding of molecular interactions and dynamics at an atomistic level, as well as computationally efficient means of translating structural information into optoelectronic properties at a macroscopic, device relevant level. Although predictive models are not yet available except for simple systems, significant progress has been made toward this goal, making use of multi-scale modeling techniques, coarse graining approaches, and structural measurements of model material systems. In this talk we will review methods used to simulate the structure – property relationship in organic semiconductors, and will present combined experimental and theoretical studies of a number of applications relevant systems including disordered and multicrystalline molecular and conjugated polymer films. We investigate the use of coarse grained methods to rationalize the electronic properties of such systems. We discuss how these methods may contribute to the design of high performance organic semiconductors.
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ItemNew Polymerization Methods for Plastic Electronics( 2015-03-13) Leclerc, MarioConjugated polymers have received a lot of attention since they combine the best features of metals or semiconductors with those of synthetic polymers. For instance, solar cells based on poly(2,7-carbazole) and poly(thienopyrroledione) derivatives have revealed power conversion efficiencies up to 8-9 %. This class of materials could lead to printable and flexible photovoltaic devices as well as other plastic electronic devices. Along these lines, we will describe novel synthetic methodologies for a simple and “green” preparation of such well-defined conjugated polymers. These new synthetic methods are based on direct (herero)arylation reactions that allow the formation of carbon-carbon bonds between heteroarenes and aryl halides. They do not require organometallic intermediates thereby significantly reducing both synthetic steps, metallic by-products, and cost. They even make possible the synthesis of conjugated polymers that were unknown up to now.
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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".