THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING 

GEORGIA INSTITUTE OF TECHNOLOGY   

Under the provisions of the regulations for the degree
 

DOCTOR OF PHILOSOPHY
 

on Monday, June 26, 2023

2:30 PM EST
in person in Kendeda 210

or via Zoom

https://gatech.zoom.us/j/4654817205?pwd=a3FsVnBQckVRaEIrNWVpZWxrMmJxQT09 

Meeting ID: 465 481 7205

Passcode: 093557

will be held the

DISSERTATION DEFENSE

for

Juanita Hidalgo

"Structure-Property Relationships in Lead Halide Perovskites for Solar Cells"

Committee Members:

Prof. Juan-Pablo Correa-Baena, Advisor, MSE

Prof. Hamid Garmestani, MSE

Prof. Faisal Alamgir, MSE

Prof. Nazanin Bassiri-Gharb, ME/MSE

Prof. Angus Wilkinson, CHEM

Abstract:

Lead halide perovskites (LHPs) solar cells, particularly FA-based, have made impressive advancements in solar energy conversion, achieving high power conversion efficiencies exceeding 26% for a single junction device. However, the limited long-term stability of these devices has hindered their commercialization. The instability issues are influenced by both internal and external factors, leading to rapid degradation of the perovskite phase and overall device performance. Various strategies have been used to stabilize the perovskite phase, but it is crucial to investigate the underlying mechanisms that govern the structural characteristics of the polycrystalline thin films. This dissertation tackles the challenges associated with the instability of LHPs by investigating the complex relationship between structure, properties, and performance in perovskite solar cells. Advanced X-ray characterization techniques are employed to examine the structural properties of FA-based compositions. Understanding the mechanisms and establishing correlations between structure and properties is possible to lay the foundation of a more robust, stable, and efficient material. This dissertation presents a first step toward the design and optimization of LHPs.

The first part of this dissertation explores the crystallographic orientation in lead bromide perovskites, demonstrating that the solvent and organic cation used in the precursor solution significantly affect the preferred orientation of the deposited perovskite thin films. The solvent affects the early stages of crystallization and induces preferred orientation by preventing colloidal particle interactions. The choice of organic cation influences the degree of crystallographic orientation, with methylammonium-based perovskites showing a higher degree of orientation than formamidinium-based ones due to a lower surface energy of a specific facet. These findings identify the importance of understanding (1) the precursor solution chemistry, (2) the facet properties and its correlation with the structural properties of the polycrystalline LHP film, and (3) the effect of crystallographic orientation on charge carrier transport in perovskite solar cells.

The second part of this dissertation studies the mechanisms causing FA-based lead iodide perovskites to degrade under water and oxygen exposure. Contrary to common knowledge on humidity-induced degradation, the synergistic role of water and oxygen in accelerating phase instability of LHPs is revealed. The study uncovers a surface reaction pathway involving the dissolution of formamidinium iodide (FAI) by water followed by the oxidation of iodide, playing a crucial role in causing the subsequent and irreversible undesired phase transformations from perovskite into non-perovskite phases. The interplay of in-situ experimental techniques with theoretical calculations provides a detailed understanding of the degradation mechanisms, leaving the foundation to design strategies to design more durable and efficient materials. This dissertation delves into strategies for stabilizing the perovskite phase. A hydrophobic molecule, phenethylammonium iodide (PEAI), is used to stabilize FA-based perovskites. Adding PEAI hinders undesired phase transformations and leads to a more stable material with improved solar cell power conversion efficiency and enhanced charge carrier mobilities and lifetimes. Further, adding Br to mixed cation lead iodide perovskites improves their phase stability at low temperatures.

Overall, understanding structure-property-performance relationships in lead halide perovskites is necessary for resolving the main challenge of instability in perovskite solar cells. This dissertation lays the groundwork for future research efforts to investigate the fundamentals of LHPs and broaden their applications in solar cells and beyond.