Center for Excitonics

Events

Harvesting Solar Energy from Singlet Fission Materials

April 19, 2016 at 4:30pm/ 36-428

Sean Roberts
Department of Chemistry, The University of Texas at Austin

A loss mechanism that strongly impacts the performance of photovoltaic cells is the thermalization of charge carriers produced by high energy photons. One strategy to negate these losses is to combine PV cells with materials that capture energetic photons and use their energy to excite multiple electron-hole pairs. Organic thin films that undergo singlet exciton fission (SF), a process wherein a high energy spin-singlet excitation divides its energy to form a pair of low energy triplet excitations, can serve as the photon downconversion material in this scheme. In particular, perylene diimide (PDI) dyes are robust, photostable materials that possess the correct ordering of singlet and triplet energy states for SF. However, excitonic interactions between these molecules in the solid state can strongly impact the rate and yield of this process. By systematically altering functional groups placed at the PDI core’s imide positions, we can shift the preferred packing structure of these materials and alter their level of excitonic coupling. Using ultrafast transient absorption spectroscopy, we find that subtle shifts in PDI intermolecular structure, particularly along the long axis of the PDI core, have a substantial impact on SF.  In addition to this work, we report electronic sum frequency generation (ESFG) spectra of PDI thin films. While PDI triplet excitons possess sufficient energy to resonantly transfer to inorganic semiconductors such as silicon, changes in the electronic structure of these materials near their junction can strongly impact this process. As an even order technique, ESFG selectively probes regions of a sample that experience a breakage of symmetry such as the junction formed by PDIs and silicon. While ESFG is selective for this buried interface, ESFG signals of thin organic films can be subject to strong intensity modulations due to optical interference. As such, we have built an optical model that accounts for interference between ESFG signals emitted by different regions of a sample. We demonstrate that this model can quantitatively reproduce ESFG spectra of PDI thin films and use it to examine how the electronic structure of PDI films distorts at semiconductor junctions.

Sean T. Roberts is an Assistant Professor in the Department of Chemistry at the University of Texas at Austin. Dr. Roberts received his PhD in Physical Chemistry from the Massachusetts Institute of Technology for his work using multidimensional infrared spectroscopy to study hydrogen bond rearrangements and proton transport in liquid water.  After completing his degree in 2010, Dr. Roberts moved to the University of Southern California where as a researcher within the Center for Energy Nanoscience he investigated singlet exciton fission and its potential application to light harvesting systems. In 2014 he joined the faculty at the University of Texas at Austin where he heads a group that utilizes time-resolved nonlinear spectroscopy to examine energy and charge transport in nanostructured electronics. Dr. Roberts’ awards include a Research Grant from the Robert T. Welch Foundation (2015), Research funding from the Air Force Office of Scientific Research (2015), a Doctoral New Investigator Award from the ACS Petroleum Research Fund (2014), and a College of Natural Sciences Advisory Council Teaching Award (2014).