Document Type



Photovoltaic solar cells convert sunlight into electricity by using material heterojunctions to facilitate separation of photo-excited charges in order to generate a current. Understanding and optimizing the efficiency of charge separation at the quantum mechanical level defines the future of the solar energy landscape and its impact on society. In the past decade, new charge separation mechanisms have been researched in heterostructures of layered 2D materials which facilitate ultrafast charge transfer and separation (see Figure 1a).[1, 2] Theoreticians have studied the mechanism of ultrafast transfer in these 2D materials and predict ultrafast transfer occurs by electrons collectively flowing back-and-forth between different layers.[3, 4] The collective/coherent nature of the transfer can lead to a greater efficiency compared to the usual stochastic/incoherent separation accomplished in regular charge separation.[5, 6] These theoretical studies claim that specific lattice vibrations and twist orientations of the heterostructure are important for tuning the efficiency of charge transfer. However, no experimental evidence has been presented for this collective action because conventional, single photon ultrafast probes do not have the quantum mechanical state resolution needed. Specifically, conventional methods lack the ability to determine the affects of lattice modes when coherently coupled to electronic excitations. My dissertation focused on developing new ultrafast probes that are hypothesized to be capable of mapping out ultrafast charge separation at the quantum mechanical level. Past work in the Wright Group has shown that triple sum-frequency, TSF, spectroscopy is a viable method for probing mixed vibrational-electronic coupling in molecular systems.[8{11] In TSF, three different electric fields (laser beams) collectively excite a charge-density polarization which oscillates at the sum of the input frequencies (see Figure 1b for a diagrammatic representation); this polarization emits an electric field (light) at the triple sum- frequency of the input beams which is then measured. The intensity of output radiation is directly related to how closely the input frequencies match (are in resonance with) the energy differences between quantum mechanical states in the system and how strongly those states are coupled together. Thus, TSF spectroscopy can offer insight into the quantum mechanical structure of a system. Because TSF can measure how multiple states are coupled together, it is an attractive option for measuring how collective excitations in a semiconductor manifest and evolve. Before I started my graduate work, TSF had not been accomplished on semiconductor systems. The goals of my graduate work were to demonstrate that TSF is a viable semiconductor measurement tool, demonstrate that TSF can measure ultrafast dynamics, and resolve ultrafast charge transfer in layered semiconductor heterostructures. It is hoped that these new measurement techniques will be important tools for rationally designing the next generation of materials used to build more efficient solar cells.

Publication Date



Link Foundation Fellowship for the years 2018-2020

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Chemistry Commons



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