Date of Award

Spring 2023

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Andrew Greytak

Abstract

Organic and inorganic halide perovskite quantum dots (QDs) (in form of ABX3, where A is organic cation e.g CH3NH3+ or inorganic cation e.g Cs+) have emerged as an intriguing alternative to the widely studied chalcogenide and pnictide QDs. Their excellent optoelectronic properties such as large absorption coefficient, high carrier mobility, wide color gamut, long electron-hole diffusion lengths, and tunable bandgap energy are largely responsible for their wide acceptability in various electronic devices. However, despite this enthusiasm, the problem of purification, long-term stability, lead toxicity and inadequate knowledge of their surface chemistry still presents a major obstacle towards their commercialization. The ionic character of these QDs makes them very sensitive to polar solvents that are typically used for purification of conventional QDs. In chapter 2 of this dissertation, we explored the use of gel permeation chromatography (GPC) as an alternative route for effective purification of CsPbBr3 QDs without exposure to polar solvents. Also, the limited stability has driven the search for ligand exchange reactions that could stabilize these particles but sometimes with contradictory reports. For example, dimethyldidodecyl ammonium bromide (DDAB) is one of the most widely studied ligands in efforts to stabilize CsPbBr3 QDs through surface modification. While some researchers have reported improved quantum yield, optoelectronic performance and stability through such ligand exchange, others have reported it to cause a phase transformation to poorly fluorescent 2D CsPb2Br5 nanoplatelets.

In chapter 3 of this dissertation, we report the study of the thermodynamics of DDAB mediated phase transformation of highly luminescent CsPbBr3 to a poorly fluorescence 2D CsPb2Br5 nanoplatelets. Using isothermal titration calorimetry (ITC) we have been able to resolve the different reactions occurring on the surface and throughout the crystal structure of these QDs with their respective enthalpies, entropies, and free energy values. Our surface analysis has offered greater insights into the surface chemistry and the role of DDAB on the surface of perovskite QDs thus resolving conflicts in literature reports about the actual role of this ligand. Furthermore, ion migration has been reported to cause phase separation in mixed halide perovskite-based optoelectronics which ultimately leads to instability and a decrease in photovoltaic performance. Hence, many researchers have devoted attention to understanding the process of this ion exchange/migration in various mixed halide perovskite CsPb(I1-xBrx)3 alloys of different compositions. However, to date, the processes underlying this anion migration/exchange remain under debate and largely unclear. In chapter 4 of this dissertation, we elucidated the processes involved during halide ion exchange in lead halide perovskite nanocrystals. Our fundamental understanding of the process of this anion exchange will help provide better-informed guidelines towards engineering perovskite nanocrystals for different applications such as tandem cells and efficient devices for lighting and display technology

Available for download on Thursday, May 15, 2025

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