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Grain Boundary Studies and Mechanical Behavior of Halide Perovskites for Solar Cells

Description

Abstract:
Perovskite Solar Cells (PSCs) based on hybrid organic-inorganic halide perovskites (OIHPs) have been under the spotlight due to the unprecedented promise of a low-cost and high-efficiency photovoltaics (PVs). They also offer advantages interms of abundance of raw material and compatibility for solution-processed thin film structures, which facilitate scale up of a light weight, flexible and portable PV technology. However, the poor operational stability of PSCs, primarily stemming from the poor intrinsic stability of the light absorbing layer of OIHPs, is posing a challenge for commercialization and wider adoption of this technology. As is the case with several other functional materials, grain boundaries in OIHPs have a significant impact on the material properties, including stability. This study focusses on evaluating and improving the chemical and mechanical stability of OIHPs for their application in solar cells. The first part of this thesis investigates the critical role played by grain boundaries in phase degradation of the most thermally stable OIHP composition, α-FAPbI3 perovskite, and a new processing protocol is proposed to achieve coarse-grained α-FAPbI3 thin films which are significantly more stable. This provides an alternative to the conventional approach of cation-alloying to stabilize the photoactive α-FAPbI3 phase which results in an undesirable shift in band gap and makes it susceptible to phase segregation. Coarse-grained FAPbI3 films are achieved by controlling nucleation and growth during solid-state δ-to-α FAPbI3 transformation. The mechanisms of grain growth are discussed through a series of in situ microscopy and X-Ray diffraction experiments, which explain the role of different processing parameters used for accelerating the δ-to-α FAPbI3 transformation. The critical role played by grain boundaries and moisture during the δ-to-α and α-to-δ phase transformations is also described. Having studied ways to improve intrinsic stability, thereafter, a pure solvent based surface treatment has been proposed to shield OIHPs from environmental stressors. The antisolvent surface treatment is demonstrated as an effective approach to passivate surface dangling bonds and improve operational stability of PSCs. The second part of the thesis deals with the study of mechanical behavior of OIHPs. Experiments have been designed and performed to evaluate the stability of cracks in these materials. OIHPs are soft, brittle and compliant at the same time, and they readily crack when subjected to bending tension on a flexible substrate. The through-thickness vertical cracks certainly compromise the integrity of the PSCs and, more importantly facilitate the nucleation and propagation of the interface delamination cracks which result in significant loss of active area. In this context, channeling cracks have been characterized using scanning electron microscopy and residual stress analysis. First, the e-beam induced mechanical changes are characterized and understood, and then bending-tension induced cracking experiments were performed to demonstrate facile healing of cracks in two prototypical OIHPS, MAPbI3 and FAPbI3. The evidence of changes in microstructure and variation in residual stress during cracking and healing of OIHP thin films are presented, and the mechanisms are explained by discussing the fundamentals of brittle fracture.
Notes:
Thesis (Ph. D.)--Brown University, 2021

Citation

YADAVALLI, SRINIVAS KARTHIK, "Grain Boundary Studies and Mechanical Behavior of Halide Perovskites for Solar Cells" (2021). Materials Science Engineering Theses and Dissertations. Brown Digital Repository. Brown University Library. https://repository.library.brown.edu/studio/item/bdr:bwm353yb/

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