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Title: Materials modeling of halide perovskites and related compounds
Author: Yang, Ruoxi
ISNI:       0000 0004 7967 762X
Awarding Body: University of Bath
Current Institution: University of Bath
Date of Award: 2018
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Halide perovskites have emerged from a decade ago with soaring efficiencies in optoelectronic applications, competing with many traditional inorganic semiconductors. Despite their excellent performance in laboratories, issues such as toxicity, stability, defects properties have to be addressed in order for perovskites to be employed in real world applications. First-principles calculation is a powerful yet relatively inexpensive way to investigate their physical properties. This thesis focuses on modeling the materials chemistry and physics of a class of inorganic halide perovskites and related compounds. The results consist of three chapters. The first results chapter aims to design new materials beyond halide perovskites with earth abundant, non-toxic, high efficiency materials. Antimony sulfides, cesium antimonysulfides and methylammonium antimony sulfides are reported, which exhibit suitable band gaps, dispersive band structure and comparable work function to hybrid perovskites. The second results chapter focuses on the phonon properties and soft modes of 24 different inorganic halide perovskites. It is shown, through lattice dynamics calculation, that all these inorganic perovskites are unstable at their cubic phase, a confusion often arise in conflicting experimental measurements. The anharmonic energy surface is computed to quantify the strength of the instabilities and correlate with the chemical composition and crystal structure. Different types of instabilities are also categorised according to the irreducible representations, the occurrence of which depends on the chemical identity. The last results chapter investigates the vacancy defects of 9 commonly synthesised inorganic halide perovskites. We have calculated the formation diagram, neutral vacancy formation energy at different synthesis conditions, and charged defect formation energy as a function of the Fermi level. The results show that these perovskites are defect tolerant, i.e. they do not tend to form deep level trap states, which are detrimental for optoelectronic devices. To link this chapter with the previous chapter, we calculate the relaxation energy and correlate it to the formation energy of the defects, and found that there is a negative relationship between the two. This sheds light on the effect of "giant" relaxation energy on the formation of defects. All the work has been compared with experiments, if available, and shown good agreements. The understanding developing from our first-principles simulations have important implications for optoelectronic applications for these materials.
Supervisor: Walsh, Aron Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available