Ab Initio Calculations of the Structural and Electronic Properties of Tin Based Perovskite Solar Absorber Materials
Creators: name: UNDERWOOD, CAMERON CHRISTOPHER
Publisher: name: University of Surrey
Year: 2021

description: Perovskite solar cells have made unprecedented advancements since their rapid emergence in
description: 2009, increasing in efficiency from 3.8% to 25.5% in 2020; giving efficiencies comparable to commercial
description: photovoltaics, such as crystalline silicon, at a fraction of the cost. Perovskites have the
description: remarkable ability of being solution processable, which can lead to extremely low cost solar cells.
description: The development of this material system is still relatively young: there are still a wide selection
description: of problems which need to be overcome before these are readily available worldwide, and there
description: are still many gaps in the understanding behind this new class of solar absorber.
description: In this thesis, ab initio quantum mechanical computational techniques such as density functional
description: theory (DFT) and density functional perturbation theory (DFPT), are used to study perovskite
description: solar absorbers, with an emphasis on tin based materials, to replace toxic lead as well as to tune
description: the band gap of the material. Firstly, the behaviour of neutral and charged unit cells in tin based
description: 3D bulk cubic perovskites are studied, with the structural, electronic and phononic properties
description: investigated. This study shows a transition from semiconducting to metallic behaviour under
description: charge, which was due to the loss of occupancy of the states highest in the valence band maximum,
description: causing the Fermi energy to drop to these partially filled states. Secondly, a parametric
description: study is undertaken to understand the effect that GGA-level input parameters have on phonon
description: band structure calculations for CsSnI3, and the effect this has on inferring stability in the material.
description: These results show that PBEsol largely calculates some imaginary modes at 0 K, but not at
description: 500 K, which is in line with the phase stability. Whereas, PBE largely calculates only positive
description: frequencies at both 0 K and 500 K. Finally, a study is undertaken in mixed Pb-Sn 2D Ruddlesden-
description: Popper perovskites, with an investigation on the nonlinear band gap dependence, as seen in 3D
description: bulk perovskites. Nonlinearity was calculated for both MA2Pb1??xSnxI4 and PEA2Pb1??xSnxI4 for
description: x = 0, 0.25, 0.5, 0.75 and 1, with a small difference between the two, due to different resulting
description: structural parameters. The effect of spin-orbit coupling (SoC) and permutations of Pb and Sn
description: was also investigated, with results showing that SoC mostly affects the linear term, and did not
description: affect the nonlinear term. The short range ordering of Pb and Sn is shown to be crucial for x =
description: 0.5, as there are two classes of permutations which cause a significant difference to the band gap.
description: This research highlights the behaviour of tin-based perovskites, in both 3D and 2D perovskite
description: configurations. The effect of partial or complete replacement of tin in both cases are investigated
description: in depth.