The Chemical Structure, Dynamics, and Mechanism program in the Chemistry Division of the National Science Foundation supports Professor Charles Musgrave of the University of Colorado Boulder to lead two closely related research efforts on singlet fission (SF) and spin controlled exciton diffusion (SCED) in organic photovoltaic (PV) materials. This effort focuses on using computational quantum chemistry to explore the fundamental principles involved in these two research thrusts to discover and understand phenomena that might be exploited in future technological applications. The project aims to understand the mechanisms of SF and SCED and the principles that govern these processes. SF involves absorbing a solar photon to create a singlet exciton, or bound electron-hole pair and then splitting this exciton into two coupled triplet excitons of lower energy. The motivation for SF is the ability to absorb high-energy photons and down convert their energy with little loss whereas the excess energy of the high-energy photon would usually be dissipated through conversion into heat. Thus, if successful, SF could lead to more efficient solar cells. A key to this process is the possible role of an optically dark multiexcitonic state in the exciton fission process. A major thrust of this project is to explore the nature of this dark state in various organic PV materials and to determine its role in governing the SF process. SCED is a proposed phenomena that may affect the rate at which excitons, and thus energy is transported in organic PV where the exciton transfer rate from molecule to molecule is affected by a local magnetic field through spin-orbit coupling to create excitons that are no longer Born-Oppenheimer pure spin states, but which can be quickly transported to harvest their energy before they undergo non-radiative decay to dissipate their energy. If the transport rate can be accelerated, it offers the potential to create significantly thicker PV films that could capture a larger fraction of the energy contained in solar radiation.
The goal of this project is to elucidate the governing principals of these processes to predict, discover and understand new SF and SCED materials that can lead to new, more efficient solar cells. These same phenomena might also be exploited to obtain more efficient and environmentally friendly LED lighting. The work also aims to integrate research with education and outreach to disseminate the new knowledge obtained through this research and to educate future technology workers and non-experts about the nature of interconverting solar energy into electrical energy for PVs. Ultimately, this project aims to prepare diverse undergraduate and graduate students for innovative and productive academic, industrial, government laboratory careers.
