Recent Interests

Chemists have focused the majority of their attention on synthesizing and studying compounds on a molecular level. The end result of this work is the ability control both molecular structure and properties very efficiently. The major thrust of my research at USC is aimed at extending the control chemists have developed for molecular species to solid materials. We have focused on molecular polymeric materials for optical studies in recent years and have recently extended out interests to include tailoring the properties of the inorganic/biological interface and nanoscience. In the material given below I will highlight our recent work in the areas of electroluminescence in organic materials as well as the photochemical (energy conversion).

Organic LEDs

Considerable research is currently focused on the development of new light emitting device technologies for flat panel displays. One technology that shows promise involves organic light emitting diodes (OLEDs). These devices are built form a variety of different molecular and polymeric materials, which serve as electron and hole carriers, sites of recombination and luminescent zones. Our research on OLEDs addresses a number of issues, including the mechanism of electroluminescence, the stability of and lifetimes of OLEDs, and the identification of new materials and device architectures for OLEDs. We have spent a great deal of time focusing on the color tuning of these devices, which has led to a deep understanding of the mechanism of electroluminescence as well as a range of interesting photophysical studies of organometallic Ir and Pt complexes. With the use of both fluorescent and phosphorescent dopants we have tuned the OLED color from blue to red with high efficiency. Our best devices emit with nearly 100% efficiency (photons/electrons), exceeding the best efficiencies reported for conventional LEDs. We have recently turned our attention from the emission process in OLEDs to the carrier injection and conduction issues related to these devices. In this research we are trying to determine what parameters are the most important for optimizing these processes in organic devices. The knowledge we gain here will be instrumental in developing better OLEDs as well other devices such as solar cells, transistors, memories, etc.. It is important to stress that while our work often involves the fabrication and testing of devices, our principal interest is in understanding the underlying chemical and photophysical properties of the materials. The devices are typically used to study these properties, but achieving high device efficiency or lifetime is not a goal in itself. The real goal is to understand how the molecular properties affect the bulk properties of the materials.

Photochemical Energy Conversion

We are also interested in the use of organic and organometallic materials for studying photochemical energy conversion. We are taking a lead from natural photosynthesis, which involves systems that undergo photoinduced charge separation. The charge separation typically involves a redox reaction between a photoexcited donor and a suitable acceptor, resulting in the production of radical ion pairs (equation (1)). This can be thought of as a microscopic reverse of the electroluminescence process, but the materials demands for achieving high efficiency solar energy conversion are very different. We are working to develop novel materials sets that are tailored to the photochemical energy conversion process

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In a process to generate chemical energy, D+ and A- are used to drive uphill chemical reactions. In order for this process to be efficient, back electron transfer (equation 2) must be prevented. In order to retard back electron transfer it is important to control both the structural and electronic properties of the system. In natural photosynthetic reaction centers this goal is achieved by fixed geometrical arrangements of electron donors, intermediate carriers and electron acceptors within the membrane. In our research we use self assembled thin films to organize acceptors and donors and predetermined distances in multilayer thin films. These thin films are grown by a sequential wet chemical technique, which leads to excellent control over the film structure. We are also investigating the use of vacuum deposited materials to achieve high efficiency devices. Using a combination of synthetic, physical and theoretical approaches we are currently working to understand the nature of energy and charge propagation in these thin films as well as to extend the active wavelengths for our films into the visible part of the spectrum. There is a great deal of interest in the scientific community in the development of renewable energy sources. Solar energy has the potential to replace some of our dependence on fossil fuels, but only if the solar panels can be made very inexpensively and have reasonable to high efficiencies. Organic solar cells have this potential.

Selected publications

 

1.	Highly Efficient, Near-Infrared Electrophosphorescence from a Pt-Metalloporphyrin Complex. Carsten Borek; Kenneth Hanson; Peter I. Djurovich; Mark E. Thompson; Kristin Aznavour; Robert Bau; Yiru Sun; Stephen R. Forrest; Jason Brooks; Lech Michalski; Julie Brown. Angewante Chemie Int. Ed., 2007, 46, 1109-1112.
2.	Whole-Cell Sensing for a Harmful Bloom-Forming Microscopic Alga by Measuring Antibody-Antigen Forces. Alexander S. Lee; Mrinal Mahapatro; David A. Caron; Aristides A. G. Requicha; Life Fellow, IEEE, Beth A. Stauffer; Mark E. Thompson; Chongwu Zhou. IEEE Transactions on Nanobioscience, 2006, 5, 149-156.
3.	Platinum Binuclear Complexes as Phosphorescent Dopants for Monochromatic and White Organic Light-Emitting Diodes. Biwu Ma; Peter I. Djurovich; Simona Garon; Bert Alleyne; Mark E. Thompson. Advance Functional Materials, 2006, 16, 2438-2446.
4.	Management of Singlet and Triplet Excitions for Efficient White Organic Light-Emitting Devices. Yiru Sun; Noel C. Giebink; Hiroshi Kanno; Biwu Ma; Mark E. Thompson; Stephen R. Forrest. Letters to Nature, 2006, 440, 908-912.
5.	Reduced geminate recombination in iridium-based electrophosphorescent materials. Russel J. Holmes, Stephen R. Forrest, Tissa Sajoto, Arnold Tamayo, Peter I. Djurovich, Mark E. Thompson, Organic Electronics 2006, 7(3), 163-172.
6.	Ultraviolet electroluminescence and blue-green phosphorescence using an organic diphosphine oxide charge transporting layer. Paul E. Burrows, Asunga Padmaperuma, Linda S. Sapochak, Peter I. Djurovich, Mark E. Thompson, Applied Physics Letters 2006, 88(18), 183501-183503.
7.	Cyclometalated iridium(III)-sensitized titanium dioxide solar cells. Elizabeth I. Mayo, Kristine Kilsa, Timothy Tirrell, Peter I. Djurovich, Arnold Tamayo, Mark E. Thompson, Nathan S. Lewis, Harry B. Gray, Photochemical & Photobiological Sciences, 2006, 5(10), 871-873.
8.	Enhanced Open-Circuit Voltage in Subphthalocyanine/C60 Organic Photovoltaic Cells. Kristin L. Mutolo, Elizabeth I. Mayo, Barry P. Rand, Stephen R. Forrest, and Mark E. Thompson, Journal of the American Chemical Society, 2006, 128, 8108-9109.
9.	Living Radical Polymerization of Bipolar Transport Materials for Highly Efficient Light Emitting Diodes. Lan Deng, Paul T. Furuta; Simona Garon; Jian Li; David Kavulak; Mark E. Thompson; Jean M. J. Frechet, Chemistry of Materials, 2006, 18(2), 386-395.
10.	Synthesis and electron properties of individual single-walled carbon nanotube/polypyrrole composite nanocables. Xiaolei Liu; James Ly; Song Han; Daihua Zhang; Ari Requicha; Mark E. Thompson; Chongwu Zhou, Advanced Materials, 2005, 17(22), 2727-2732.