UC Berkeley

This dissertation describes the use of two-dimensional electronic spectroscopy to explore the impact of phonons and vibrations on the ultrafast electronic processes in photosynthetic light harvesting, hot carrier persistence in perovskite photovoltaics, and efficient photoluminescence in a 2D semiconductor. Each example represents a case where a unified picture of nuclear and electronic response is needed to understand the mechanism. In the introduction, we summarize the electron-phonon interactions that affect optoelectronic processes, and we introduce ultrafast two-dimensional spectroscopy (2D-ES) and the physical information accessed through this experiment. Coherences and wavepackets are introduced, and a time-dependent picture of quantum mechanics is emphasized throughout. The concept of an open system and the impact of environmental fluctuations on dynamics and measurements is equally important. In Chapter 2 these are explored using numerically exact open-system simulations to show the effect of vibrations in the chlorophyll molecule on coherent energy transfer in a photosynthetic complex and on our ability to measure it using two-dimensional spectroscopy. We show that even weak coupling to vibrations generates vibrational wavepacket signatures, and that resonances between vibrations and electronic energy gaps produce vibronic energy transfer effects that are sensitive to environmental fluctuations.

The experimental portion follows with a discussion of strategies for applying two-dimensional spectroscopy techniques to samples with weak signals and/or strong scatter. These include signal processing methods and a new scheme we have devised for rapid scatter subtraction using a cheap, easily programmed microprocessor. We also discuss extensive modifications that allow fluorescence-detected 2D-ES. In Chapter 4, 2D-ES is used to study methylammonium lead iodide perovskite, a hybrid organic-inorganic photovoltaic. We find a vibrational wavepacket that is a good candidate mode for a phonon bottleneck effect thought to slow hot carrier cooling in this material. Chapter 5 presents preliminary results from a 2D-ES study of another semiconductor, single-layer MoS2. We identify from the temperature-dependent 2D lineshape an optical phonon mode principally responsible for early-time charge carrier scattering. We also observe a coupling between two opposite-spin excitons, in agreement with calculations that predict significant exchange interactions.