Energy Frontier Research Center for Excitonics
The Energy Frontier Research Center (EFRC) for Excitonics is a collaboration between Massachusetts Institute of Technology, Harvard and Brookhaven National Laboratory. Our objective is to supersede traditional electronics with devices that use excitons to mediate the flow of energy. Excitons are quasiparticle excitations consisting of a bound electron and hole that mediate the absorption and emission of light, especially in disordered and low-dimensional materials.
The motivation for excitonics is that conventional electronic devices can be difficult to manufacture; their constituent materials require very high levels of order, and achieving such low entropy in a semiconductor requires expensive and energy intensive fabrication. For example, the energy payback time for a crystalline silicon solar cell is on the order of 2 years, and at current manufacturing growth rates, it is expected to take at least 20 years to produce enough silicon-based solar cells to make a significant impact on the world energy supply. Similarly, epitaxial growth constraints are likely to limit solid state lighting sources to a small fraction of the overall demand for lighting.
There is an alternate approach that is more suitable for large scale production. In the Center for Excitonics, we address materials with only short-range order. Such nanostructured materials are compositions of nano-engineered elements such as organic molecules, polymers, or quantum dots and wires, in films bound together by weak van der Waals bonds. These materials are characterized by excitons that are localized within the ordered nanostructures. Excitons provide a unique means to transport energy and convert between photons and electrons. Due to localization of excitons, the optical properties of the films are relatively immune to longer-range structural defects and disorder in the bulk. And in contrast with the painstaking growth requirements of conventional semiconductors, weak van der Waals bonds allow excitonic materials to be readily deposited on a variety of materials at room temperature.
We address two grand challenges in excitonics: (1) to understand, control and exploit exciton transport, and (2) to understand and exploit the energy conversion processes between excitons and electrons, and excitons and photons.
(1) Exciton Transport: We are developing new theory to explain and model the movement of excitons in complex nanostructures. We build artificial excitonic antennas that absorb and guide light in nanofabricated circuits of molecular chromophores, J-aggregates, quantum dots and nanowires. We characterize coherence and energy transfer in our antennas using scanning probe microscopy and our recently developed technique of fully phase coherent 2-d Fourier transform spectroscopy. Finally, our excitonic technologies will be applied to low cost solar cells and luminescent solar concentrators, which promise power efficiencies > 30%.
(2) Exciton Dynamics: We are developing new theory for the dynamics of exciton formation and separation. Applications include increasing the efficiency of organic light emitting devices by up to a factor of four and characterizing the fundamental efficiency limits of excitonic solar cells.
We possess two important tools that are unique to the center: Cathodo-luminescence Scanning Transmission Electron Microscopy (CL-STEM), which is used to characterize the structure-function relationships of excitonic nanomaterials, and Superconducting Nanowire Single-Photon Detectors (SNSPDs). SNSPDs can conclusively determine the efficiency of multiple carrier generation, a process with enormous potential for solar cells. Finally, we aim to characterize the link between exciton annihilation and device degradation.
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