Ultrafast and nonlinear optical studies of chemical vapor deposited two-dimensional hybrid halide perovskite films
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Two-dimensional (2D) Ruddlesden–Popper halide perovskites have emerged as promising materials for next-generation optoelectronic applications due to their structural tunability, high exciton binding energies, and enhanced environmental stability. In this thesis, we investigate the ultrafast excited-state dynamics and carrier relaxation processes in Chemical Vapor Deposition (CVD)-grown 2D Hybrid Halide Perovskite (HHP) thin films, focusing primarily on the prototypical lead based systems butylammonium lead iodide (BA₂PbI₄), phenylethylammonium lead iodide (PEA₂PbI₄), and the tin based perovskite phenylethylammonium tin chloride iodide (PEA₂SnClₓI₄₋ₓ). The first part of this work entailed building a transient absorption spectroscopy setup based on an amplified femtosecond laser system. In addition to a full scale optical setup involving steering optics, delay line, and a spectrometer, we developed custom computer algorithms to interface with the optical table components for automating data acquisition as well as analysis. This system along with optical techniques were used to investigate the steady state, temperature dependent, time resolved, and nonlinear optical properties of HHP thin films. We characterize the interplay between excitons, quasi-free carriers, and defect-mediated recombination pathways. We report a rich evolution of the ground state bleaching and photoinduced absorption features, with multiple decay regimes governed by three rate constants (monomolecular, bimolecular, and Auger recombination). At excited state densities below ∼ 4 × 10¹² cm⁻², the decay of the GSB in PEA₂PbI₄ requires all three rate constants, highlighting the coexistence of free carriers and excitons even in a regime where the Saha equation would predict nearly complete excitonic dominance. These results suggest that even a small fraction of quasi-free carriers can trigger exciton ionization, an insight with significant implications for photovoltaic operation under continuous illumination. We also explore how organic cation chemistry, dimensionality, and defects impact carrier thermalization and recombination. The hot carrier Fermi temperature is higher in PEA₂PbI₄ compared to BA₂PbI₄, attributed to fewer defect states. Despite these differences, both materials exhibit comparable ultrafast cooling times (∼150 fs), consistent with strong Fr¨ohlich coupling. In contrast, the tin based perovskite PEA₂SnClₓI₄₋ₓ displays a slower cooling time of ∼1 ps, indicating its potential as a candidate for hot carrier extraction. This work demonstrates that CVD-grown 2D perovskites offer a powerful platform for tuning excited-state dynamics through chemical and structural control. Our results provide both mechanistic understanding and design strategies for optimizing these materials for use in light-harvesting and light-emitting applications.
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Ph. D.