CAS: Harvesting Electrons From Lower Energy Excited States to Extend Spectral Coverage and Facilitate Multiple Charge Collection Following Singlet Fission on Metal Oxides

With support from the Chemical Structure, Dynamics & Mechanisms-B Program of the Chemistry Division, Professors Blank and Gladfelter from the University of Minnesota-Twin Cities and Professor Pappenfus from the University of Minnesota-Morris seek to build a more thorough understanding of how light can be used to induce separation of charge, a critical first step in the conversion solar energy into electrical or chemical energy. In addition to photovoltaic devices, light-induced charge transfer is part of other devices such as sensors. This project will focus on using well-defined nanocrystals of metal oxides as the scaffold for probing this behavior. A better understanding of how to leverage metal oxide semiconductors is expected to contribute to the development of a range of optoelectronic devices and to improving the efficiency of such devices. This project will directly support the training and education of the next generation of physical/physical organic chemists, including graduate students and undergraduate students, in areas of chemistry that cross the traditional disciplinary boundaries. The University of Minnesota-Morris is a rural, primarily undergraduate institution, and this project will provide undergraduate students at Morris with a collaborative research experience. Nanocrystalline films of semiconducting metal oxides often play a central role in devices that involve the separation of charge. Commonly employed metal oxides have conduction band energies that place thermodynamic and kinetic limitations on the molecules used as sensitizers. Indium oxide has yet to receive much attention, but it has tremendous potential. With a lower energy conduction band, it opens a new window of opportunity to access lower energy sensitizer states. This window would, in principle, allow extension of the spectrum of light that can be harvested and provide access for charge transfer from lower energy triplet states of the sensitizers. The triplet states that are of particular interest in this proposal are those that are generated via singlet fission. This project will use transparent solution phase dispersions of nanocrystals with narrow size distributions as a well-defined experimental platform to investigate the excited state dynamics and charge transfer of the sensitizer. The goal is to realize, characterize and optimize charge multiplication through singlet fission on a transparent metal oxide surface with the potential to increase the efficiency of light harvesting. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.