title: First Principles Study of Ferroelectricity in Halide Perovskites for Future Photoferroics
Creators: name: Cowell, Jordan A. R.
Publisher: name: University of Kent
Year: 2023

description: Perovskite and perovskite-like materials have garnered warranted attention over the previous decade due to their potential use in high efficiency photovoltaic devices. Their flexibility both chemically and sterically gives materials' scientists and engi- neers the opportunity to tune the properties of such materials to suit the application that it is needed for. In this thesis we use ab initio methods to describe the funda- mental distortions that appear in a series of structures and their associated optoelec- tronic properties. In the first results chapter we complete a rigorous exploration of a variety of chemistries where we explore the various distortions that can appear in these materials. I show that the system exhibits a very complex interplay between pri- marily the breathing distortion and Jahn-Teller distortion and subsequently all other distortions that appear at lower tolerance factors. We also explore the appearance of a polar instability. In the second results chapter we take the understanding of these distortions and physical aspects of the gold double perovskites to discuss the opto- electronic properties of these materials. We find that the Jahn-Teller distortion opens up the gap in a metallic to insulator phase transition, we examine the effects of each of the distortions on the size of the gap and the effective masses of the structures and we see that the introduction of tilt distortions opens up the gap even more and increases the effective masses of the charge carriers. Examination of pressure presented us with the unusual discovery that hydrostatic pressure suppresses the appearance of the tilt distortions in this perovskite structure due to the cooperative nature between the tilt and Jahn-Teller distortions(something that is suppressed with pressure). With the knowledge of the effects of pressure we predicted a polar phase in the Rb2Au2(Br, I)6 structures with the introduction of a small hydrostatic pressure. We explored the pos- sibility of an enhanced dielectric constant through tuning of lattice parameters with pressure and was able to show an enhanced dielectric constant at a region where the phonon frequency was sufficiently softened. In the final part of the thesis we dis- cussed the application of hybrid improper ferroelectricity. We showed through sym- metry analysis that a polar distortion appears in the low-symmetry A3B2X7(A = Cs, Rb, K; B = Pb, Sn, Ge; X = I, Br, Cl) Ruddlesden-Popper structures via a trilinear coupling term with two non-polar distortions. We explored the effect of chemical substitution on the size of the gap and the effects each change in the chemistry had on the size of the distortions, we suggested that the Cs3Ge2I7 system is a proper ferroelectric (does not follow the improper method) and finally we explored the effects of Spin-Orbit coupling and the type of exchange-correlation functional that was used on the ac- curacy of our results. Overall we believe that the work presented in this review puts forward some exciting and interesting discussions of charge-separation in perovskite photovoltaics. As we look forward in this review we make some suggestions of other structures which would be more suitable for a possible experimental analysis.