Physical & Biological Sciences Division
Associate Professor
Faculty
Physical Sciences Building
146
Physical Sciences Bldg 146 (Office)
Physical Sciences Bldg 186 (Lab)
Chemistry
B.S. Chemistry, University of San Francisco
Ph.D. Physical Chemistry, UCLA
Postdoctoral Scholar, Stanford University / SLAC National Accelerator Laboratory
Physical chemistry of organic semiconductors; energy and electron transfer dynamics; conjugated polymer physics; supramolecular assembly; charge transport in organic thin films
We are broadly interested in understanding the influence of molecular and macromolecular structure of small-molecule and polymeric organic semiconductors on the efficiency and dynamics of light harvesting. Current work in the group aims to address the three fundamental steps involved in photoelectric and photochemical conversion: energy transfer, interfacial electron transfer, and long-range charge transport.
Inspired by the protein-based light-harvesting machinery found in photosynthetic organisms, we are trying to create an artificial photosystem by assembling supramolecular light-harvesting antennae using conjugated polyelectrolytes. The conjugated backbone of these materials renders them semiconducting, and the charged (or ionizable) sidechains lead to complex interactions and hierarchical structure that is sensitive to the local electric field. Using a range of van der Waals, hydrogen-bonding and ionic interactions, we are working on elucidating the physics of conjugated polyelectrolyte assembly. We use this knowledge to organize individual macromolecular pieces into larger structures that absorb a broad region of the electromagnetic spectrum. The ultimate goal is to directionally funnel excitation energy to a "reaction center", where the excited state is trapped and then dissociated via electron transfer to yield a charge-separated state. We probe excited-state dynamics using time-resolved fluorescence spectroscopy via time-correlated single-photon counting. We then connect these time-domain experiments with the equilibrium and non-equilibrium structure of the assemblies using visible light and small-angle X-ray scattering.
For excited states to lead to either photochemistry or a photocurrent, neutral excitations must dissociate to form electron-hole pairs, which must eventually transfer into a charge collector at a hybrid organic/inorganic interface. In thin solid films, the dynamics of electron (or hole) transfer at interfaces often occurs on sub-100 femtosecond timescales. We are working on determining what governs electron transfer from molecular adsorbates to both localized (interface trap) and delocalized states of inorganic semiconductors and insulators. We are also interested in understanding how the interfacial electron-phonon coupling modulates the electron transfer rate from vibrationally-unequilibrated excited states. To probe dynamics at such short times, we use the element-specific "core-hole clock" technique, which utilizes resonantly emitted Auger electrons following absorption of an X-ray photon. The quantity that sets the time resolution of this experiment is the lifetime of the core-to-LUMO(+n) excited state, which is of order 5 femtoseconds for initial states localized on light atoms such as C, N and O. Because excitation energies from the core levels of these elements to unoccupied molecular orbitals are separated by hundreds of electron volts, we can probe electron transfer at hybrid organic/inorganic interfaces in an element-specific manner. Thus, we can ask the question, how does the precise atomic composition of an organic molecule determine the rate of interfacial electron transfer?
Before charges can get collected at electrodes in a photovoltaic device, they must first traverse the photoactive layer. Unlike long-range electron motion in crystalline inorganic solids, charge carriers in organic solar cells composed of donor/acceptor blends navigate through a tortuous landscape filled with energetic and topological trap states. Charges often move within an interpenetrating matrix of the opposite carrier conductor. Since conjugated organic molecules are generally anisotropic, charge transfer rates depend strongly on relative molecular orientation. We are working on understanding how the three-dimensional pi-electron contour of organic semiconductors affects the charge mobility in the thin film. To characterize the charge transport network, we are working on relating the current/voltage characteristics to the microstructure of the phase-segregated network. To obtain the latter, we utilize small-angle and resonant small-angle X-ray scattering at national synchrotron lightsource facilities.
Quantum Mechanics, Thermodynamics, Spectroscopy, Physical Chemistry of Polymers
