Recent progress in semiconductor synthesis and photophysics has revealed a host of new materials with exciting properties for applications in optoelectronic devices such as sensors, photovoltaics, solid state lighting, and more. One of the most significant recent additions to the field is the class of hybrid and inorganic materials that take on perovskite and perovskite-like structures. These are of acute interest due to their high defect tolerance, which allows them to be synthesized using relatively inexpensive wet-processing techniques. However, there are ambiguities in overall structure and bonding such that the materials’ static properties are frequently difficult to determine. It is even more difficult under the non-equilibrium conditions accessed in optoelectronic devices that are frequently the intended final form of these materials. This class utilizes polarizable ions and organic molecules, introducing inherent crystallographic disorder and stimuli-dependent phase changes. As such, although highly promising, this is a fundamentally confounding material class in need of specific and thorough structural and electronic characterization.This dissertation attempts to answer some of these questions by examining the non-equilibrium properties of perovskite materials. Time-resolved, ultrafast techniques are utilized to examine their behavior under photoexcitation; additionally, phase transitions are studied and a method for eliminating them in device materials is proposed. Using transient X-ray diffraction, we observe photoexcitation-dependent changes in the crystal structures of two types of metal halide perovskites, firstly the two-dimensional hybrid variant, then a three-dimensional, fully inorganic variant (Chapters 2 and 3). We find that 2D variants undergo lattice ordering over the course of a nanosecond post-photoexcitation such that the Pb-I-Pb bonds straighten. The inorganic 3D variant undergoes phase transitions from the static, orthorhombic structure to two high-symmetry structures – tetragonal and cubic. In both cases, some thermal effects are observed, but these are determined to be minor compared with the effects of laser-induced photoexcitation and bond-breaking. In the fourth chapter, six different 2D lead iodide perovskites are examined with respect to their temperature-dependent behavior. Phase transitions are suppressed using cyclic organic cations in place of linear organic cations, and these are additionally found to be largely responsible for effects observed in transient photoluminescence.  

Creator

• Cuthriell, Shelby A

DOI

• https://doi.org/10.21985/n2-j84w-qp64

Subject

• Physical chemistry
• Chemistry
• Materials Science

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:16650
• etdadmin_upload_1000360

Date created

• 2023-01-01

Resource type

• Dissertation

Rights statement

• In Copyright