proposal

The proposed center has two thrusts, exciton transport and exciton dynamics

Exciton Transport

Photosynthesis provides a model for excitonic devices that has been optimized for two billion years. Indeed, plants and algae obtain a power efficiency approaching that of silicon solar cells with a far shorter energy payback time: two days versus two years. The success of photosynthesis is built on its control of excitons. Unlike conventional solar cells, photosynthesis relies on antennas—excitonic circuits that absorb, direct and concentrate energy.

Unfortunately, there are no synthetic equivalents to the antennas of photosynthesis. In organic photovoltaic cells, for example, few excitons stochastically diffuse more than ~10 nm to a charge separation site. This forces device designers to build blends from which the charge carriers can be difficult to extract. Indeed, state of the art organic solar cells have around 6% energy conversion efficiency, about four times smaller than crystalline silicon based solar cells.

The grand challenge in exciton transport is to understand and exploit energy transport in synthetic light harvesting systems. The teams within the center will pursue "horizontally integrated" tasks that expand our core disciplines, and there will be two "vertically integrated" tasks that span the disciplines.

Exciton Dynamics

Excitons provide a means to transport energy and convert between photons and electrons. The grand challenge in exciton dynamics is to understand and exploit the energy conversion processes. Consequently, the work is directly relevant to the important energy conversion devices: LEDs (electrons to photons) and solar cells (photons to electrons).

The teams within the center will pursue "horizontally integrated" tasks that expand our core capabilities in excitonic materials and two characterization techniques: Cathodoluminescence Scanning Transmission Electron Microscopy (CL-STEM) and Superconducting Nanowire Single Photon Detectors (SNSPDs). There will also be four "vertically integrated" tasks that span the disciplines and address the four key exciton processes: formation, dissociation, fission, and annihilation.

We will examine the fundamentals of the exciton-mediated conversion of electrons into photons. We will focus on the spin dependence of exciton formation and show that selective application of spin orbit coupling during exciton formation promises to enhance the efficiency of materials currently used in fluorescent white OLEDs—potentially by as much as a factor of four. We will also study the opposite process: that of the excitonmediated dissociation of photons into electrons. We will perform terahertz studies that promise to uniquely characterize this key solar cell pathway. We also propose a method to eliminate recombination losses, a major cause of the relatively low open circuit voltage in many excitonicsolar cells. Next, we propose to develop novel excitonic materials, for application in LEDs, solar cells and antennas. Finally, we address exciton annihilation by other species such as charges.

In total, the tasks within the grand challenge of "Exciton Dynamics" represent a comprehensive examination of crucial energy conversion processes in excitons.