Abstract
Perovskite solar cells have made unprecedented advancements since their rapid emergence in
2009, increasing in efficiency from 3.8% to 25.5% in 2020; giving efficiencies comparable to commercial
photovoltaics, such as crystalline silicon, at a fraction of the cost. Perovskites have the
remarkable ability of being solution processable, which can lead to extremely low cost solar cells.
The development of this material system is still relatively young: there are still a wide selection
of problems which need to be overcome before these are readily available worldwide, and there
are still many gaps in the understanding behind this new class of solar absorber.
In this thesis, ab initio quantum mechanical computational techniques such as density functional
theory (DFT) and density functional perturbation theory (DFPT), are used to study perovskite
solar absorbers, with an emphasis on tin based materials, to replace toxic lead as well as to tune
the band gap of the material. Firstly, the behaviour of neutral and charged unit cells in tin based
3D bulk cubic perovskites are studied, with the structural, electronic and phononic properties
investigated. This study shows a transition from semiconducting to metallic behaviour under
charge, which was due to the loss of occupancy of the states highest in the valence band maximum,
causing the Fermi energy to drop to these partially filled states. Secondly, a parametric
study is undertaken to understand the effect that GGA-level input parameters have on phonon
band structure calculations for CsSnI3, and the effect this has on inferring stability in the material.
These results show that PBEsol largely calculates some imaginary modes at 0 K, but not at
500 K, which is in line with the phase stability. Whereas, PBE largely calculates only positive
frequencies at both 0 K and 500 K. Finally, a study is undertaken in mixed Pb-Sn 2D Ruddlesden-
Popper perovskites, with an investigation on the nonlinear band gap dependence, as seen in 3D
bulk perovskites. Nonlinearity was calculated for both MA2Pb1xSnxI4 and PEA2Pb1xSnxI4 for
x = 0, 0.25, 0.5, 0.75 and 1, with a small difference between the two, due to different resulting
structural parameters. The effect of spin-orbit coupling (SoC) and permutations of Pb and Sn
was also investigated, with results showing that SoC mostly affects the linear term, and did not
affect the nonlinear term. The short range ordering of Pb and Sn is shown to be crucial for x =
0.5, as there are two classes of permutations which cause a significant difference to the band gap.
This research highlights the behaviour of tin-based perovskites, in both 3D and 2D perovskite
configurations. The effect of partial or complete replacement of tin in both cases are investigated
in depth.