Date of Award
Physics and Astronomy
Mark E. Siemens, Ph.D.
Four-wave mixing, Perovskite, Photovolotaic, Spectroscopy, Ultrafast
Metal halide perovskite (MHP) thin films are currently undergoing an intense re- search thrust due to the excellent performance of MHP based photovoltaic (PV) devices, which have the potential to revolutionize the worlds energy production via a unique combination of low-cost fabrication and high power conversion efficiency (PCE). However, the vast majority of research is currently aimed at incremental improvements in device PCE, resulting in a body of work without the foundational understanding of the charge-carrier dynamics of the system upon photoexcitation.
This thesis begins with the development of a phase-modulated multidimensional coherent spectroscopy (PM-MDCS) experiment. PM-MDCS is an ultrafast multidi- mensional coherent spectroscopy (MDCS) technique that can identify photophysical processes unavailable to one-dimensional spectroscopies. The thesis then goes on to describe the development of a novel data acquisition scheme and data process- ing technique, diagonal slice four-wave mixing (DS-FWM). Next, a description of calibrating the absolute phase in PM-MDCS experiments is presented. Finally, the thesis discusses the application of steady-state photoluminescence (PL), MDCS, and DS-FWM to study the charge-carrier dynamics in MHP thin films at 5K. These studies provide crucial information to building a fundamental understanding of the photophysical processes in MHP films under illumination, providing direction for targeted research toward improved MHP PV performance.
A novel technique for collection of PM-MDCS data, and analysis of all MDCS data, DS-FWM succeeds where other MDCS lineshape analyses have failed, analyti- cal separation of broadening mechanisms in MDCS data. This technique significantly shortens data acquisition time for time-domain coherent spectroscopies, such as PM- MDCS, and provides direct access to relevant material paramters, such as the pure dephasing rate in the studied system, without the need for any assumptions.
Phasing PM-MDCS spectra is a central concern because the interpretation of spectra rely critically on the phase. We developed a method of calibrating the absolute phase in PM-MDCS that reconstructs all phase contributions to the signal and removes all but the phase of the material response.
The PM-MDCS data presented on MHP thin films clearly show long-lived exci- tons in the system, the existence of which has long been debated in the literature, with surprisingly long dephasing times up to ~ 1 ps. These excitons show clear in- homogeneous broadening, likely due to the large amount of disorder intrinsic to the MHP system, disproving a widely cited finding in the literature that the emission of the MHP system is homogeneously broadened and the system is well ordered. The data also show multiple isolated states that appear as one peak in steady-state PL data, likely due to defect states in the imperfect MHP lattice. PM-MDCS has the capability to disentangle spectrally broad resonances in ways that steady-state mea- surements cannot, allowing the studies performed to access the individual response of these states directly. Time dependent studies spanning hundreds of femtoseconds to about one nanosecond show multiple relaxation pathways and timescales between these states and some coherent coupling. The interaction of the states and transfer pathways of the charge-carriers are of vital importance, as the coupling of defect- states to current-generating states could lead to marked improvements in MHP PV performance.
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Diederich, Geoffrey Michael, "Multidimensional Spectroscopy of Mixed-Cation Perovskite Thin Films" (2019). Electronic Theses and Dissertations. 1653.
Received from ProQuest
Geoffrey Michael Diederich
Optics, Condensed matter physics