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Date of Award
Doctor of Philosophy (PhD)
ABSTRACT OF THE DISSERTATIONRational design and optimization of perovskite semiconductors by Arashdeep Singh Thind Doctor of Philosophy in Materials Science and Engineering Washington University in St. Louis, 2021 Professor Rohan Mishra, Chair
Lead-halide perovskites have emerged as a promising class of semiconductors for next-generation solar cells and optoelectronic devices. Since the report of the photovoltaic effect in 2009 in CH3NH3PbBr3 and CH3NH3PbI3, the power conversion efficiency of these perovskites has rapidly approached 25.2% from 3.8%. These perovskite semiconductors can be synthesized using low-cost solution-based techniques, unlike traditional semiconductors that require slow and expensive growth techniques to reduce the formation of defects. Furthermore, their electronic properties can be tuned easily by changing the cation or anion composition. However, these lead-halide perovskites suffer from poor thermodynamic and environmental stability that combined with the toxicity of lead make them unattractive for most commercial applications. In this thesis, we have addressed these challenges associated with lead-halide perovskites using a combination of first-principles density-functional theory (DFT) calculations and aberration-corrected scanning transmission electron microscopy (STEM). Firstly, we have used DFT calculations to understand the thermodynamic stability and electronic structure of various polymorphs of the prototypical lead-halide perovskite CH3NH3PbI3. We find that the experimental phases of CH3NH3PbI3 are thermodynamically unstable with positive formation enthalpy. Moreover, we find a novel layered hexagonal phase to be the ground state for CH3NH3PbI3. Secondly, we have used STEM and DFT to characterize extended defects in comparatively more stable CsPbBr3 and their effect on the electronic structure. We reveal the atomic and electronic structure of the two dominant planar defects: low-angle grain boundaries; and Ruddlesden-Popper planar faults. We find that the grain boundaries repel electrons and attract holes, similar to an n-p-n junction, and could help separate the electron-hole pairs in solar cells. Whereas, Ruddlesden-Popper planar faults serve as an effective electron and hole barrier and naturally binds two CsPbBr3 regions with a sharp interface, which could be utilized to impose strong quantum confinement in large nanocrystals. Unlike in conventional semiconductors, we show that neither of the planar faults observed in CsPbBr3 induce deep defect levels, but their Br-deficient counterparts do. This leads to a direct conclusion that Br or halide-rich synthesis is desirable for suppressing any deep defect levels in CsPbBr3 or lead-halide perovskites in general. Finally, we use a combination of high-throughput DFT calculations and materials informatics to search for novel and environmentally benign bismuth-based oxide perovskites as alternatives to lead-halide perovskites. We have predicted and successfully synthesized a novel bismuth-based perovskite semiconductor, KBaTeBiO6, which is a promising semiconductor for photovoltaic applications. KBaTeBiO6 has an experimental indirect band gap of 1.88 eV and shows excellent stability for over a year under ambient conditions. The calculated effective mass of the charge carriers for KBaTeBiO6 is comparable to the best-performing Bi-halide double perovskites. We further explore a diverse composition space of bismuth-based oxide perovskites to design stable and non-toxic semiconductors, with electronic properties optimized for various applications such as in solar cells, light-emitting diodes, and photocatalysts. In this thesis, we have shown that the discovery and optimization of inorganic bismuth-based perovskite oxide semiconductors can be accelerated by using a combination of high-throughput DFT calculations, materials informatics and characterization.
Katharine M. Flores, Bryce Sadtler, Pratim Biswas, Philip Skemer,
Available for download on Thursday, May 21, 2026