Georgia Institute of Technology Materials Science and Engineering

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

"Understanding structure-property relationships in metal halide perovskites by advanced synchrotron-based characterization techniques"

Abstract:

Organic-inorganic metal halide perovskites (MHPs) are promising and outstanding energy materials due to their superior and tunable optoelectronic properties. MHPs have been widely studied for solar energy conversion, having achieved to date a maximum power conversion efficiency of 25.5%. Compositional engineering has been one of the most widely used strategies to increase the efficiency of MHP solar cells.  Lead-based MHPs have an APbX3 structure, which has been tailored to include multiple A-site cations and X-site halide anions. However, MHP solar cells’ lifetimes are shorter than those of market-leader silicon solar cells because of their instability under external stimuli such as moisture and temperature. Understanding the effect of external stimuli on structure-property relations of mixed-ion compositions will lead to the design of more robust and efficient materials.

Numerous studies have described in detail the effect of external stimuli in the structure-property relationships of some single-cation and single-halide compositions, but the mixed-ion compositions are still widely unexplored due to their novelty and compositional complexity. By using synchrotron-based X-ray techniques, the structure can be probed from a micro to a molecular level with improved energy and spatial resolution, higher throughput, reduced sample damage, and varied in-situ and in-operando setups. This work proposes to unravel the structure and crystallization processes under external stimuli, through synchrotron-based characterization techniques, in multiple MHP compositions and its correlation with optoelectronic properties for solar cell applications.

In the first chapter, synchrotron grazing incidence wide angle X-ray scattering (GIWAXS) will be used to understand moisture-induced crystallographic reorientations in LHPs thin films and study their effect on charge carrier extraction. In parallel, synchrotron-based X-ray fluorescence with in-operando X-ray beam induced current will be performed to demonstrate the correlation between compositional changes with the electrical current.  Via a moisture treatment, it will be explained the importance of crystal orientation to enhance charge carrier transport and long-term stability in LHP solar cells. Furthermore, a detailed study will be carried out to comprehend the role of the cation and the halide in the orientation changes experienced upon interaction with moisture; an ion’s size-dependent correlation will be seen.

In the second chapter, GIWAXS and synchrotron X-ray diffraction (XRD) will be used to understand the moisture degradation pathways of mixed-ion MHPs thin films and powders. MHPs will be analyzed by in-situ tracking their structural changes under <100% relative humidity. The role of the halide, Br and I, will be unraveled. It will be seen how compositional differences can lead to a reversible or unreversible degradation. The correlation of the structural changes with the optoelectronic properties and solar cell performance will also be explained.

In the third chapter, synchrotron-based XRD and small-angle X-ray scattering studies will explain the effect of temperature in the crystallization of MHP powders.  This study will be carried out to shine insight on the phase behavior under extreme temperature conditions of MHP powders to elaborate a phase diagram for these solid solutions.

Overall, understanding the effect of external stimuli in the structure-property relationships of metal halide perovskites is fundamental to overcome barriers towards commercialization. Studying mixed-ion composition MHPs can lead to an improved performance of these materials in applications such as solar cells and light-emitting devices.