Computationals studies of order, disorder and stability in metal halide perovskite photovoltaic absorbers
File(s)
Author(s)
McMahon, Andrew
Type
Thesis
Abstract
Hybrid organic-inorganic perovskites have attracted much interest for their
potential use as photovoltaic absorbers in highly efficient, cheaply manufactured photovoltaic (PV) devices. This thesis studies several topics of interest to hybrid organic-inorganic perovskite (HOP) photovoltaics, including the
dynamic behaviour of the organic component in methylammonioum lead iodide (MAPbI3), the moisture induced degradation of typical hybrid perovskite
materials and the possibility of engineering ferroelectric photovoltaic devices
using hybrid perovskite-like materials.
The observation of hysteresis in current-voltage curves for most hybrid perovskite devices suggests either dynamic movement of charged ions or a ferroelectric response of the absorber material to applied electric fields. The possibility that alignment or anti-alignment of organic cations, which are molecular
dipoles, could be causing hysteresis in hybrid perovskite devices was examined
using semi-classical Monte-Carlo simulations, informed by density functional
theory calculations and experimental quasi-elastic neutron scattering experiments. I show that the Monte Carlo model does indeed suggest the possibility of either ordered low temperature states, and that the nature of these
states (ferroelectric or anti-ferroelectric) is determined by the magnitude of
a parameter related to the distortion of the inorganic PbI6 framework when
methylammonium cations are aligned or anti-aligned. I also present a basic
model based on the Monte Carlo and quasi-elastic neutron scattering results
in that chapter which gives a characteristic timescale for domain wall motion
within MAPbI3 devices. The timescale for domain wall motion across a typical device is calculated to be on the order of milliseconds, which is deemed to
fast to be consistent with observed current-voltage hysteretic behaviour in hybrid perovskite solar cell devices. These results constitute the first theoretical
results of this thesis.
Despite rapid advances in photovoltaic efficiency of hybrid perovskite PV
devices, their viability as a mainstream commercial technology is impeded
by their susceptibility to a variety of environmental conditions, including the
presence of airborne water vapour. The second part of this thesis studies the
hydration of HOPs using electronic structure methods and ab initio thermodynamics based upon density functional theory calculations. The calculations
presented show that ab initio thermodynamics can capture the key trends observed in experiments, but that it is difficult to reproduce experimental phase
boundaries accurately using this technique (as is explained in that chapter this
is not unexpected). A model of the entropy associated with the presence of
a rotationally mobile organic cation within the perovskite is also introduced
and used to augment the basic ab initio thermodynamic calculations. It is
shown that the magnitude of this entropic contribution could play a key role
in the location of the thermodynamic phase boundary between MAPbI3 and
its hydrated structures. Finally, a mechanism for the hydration of MAPbI3 is
proposed and outlined.
The final part of this thesis discusses the application of the theory of hybrid
improper ferroelectricity, whereby changes in the typical perovskite structure
(for example by creating n = 2 layers of ABX3 perovskite intercalated with
single layers with rock-salt AX stoichiometry) to metal halide perovskite based
materials. It is shown through DFT calculations of polarisation and in depth
analysis of the symmetries present in crystals studied that Ruddlesden-Popper
and superlattice structures based on CsPbI3 metal halide perovskites should
indeed exhibit hybrid improper ferroelectricity. The electronic structures of
the Ruddlesden-Popper Cs3Pb2I7 is compared with that of CsPbI3 and it is
shown that the Ruddlesden-Popper has a larger band gap. It is explained that
this is due to changes in orbital overlap between lead and iodide states within
the structure. A discussion is given of why hybrid improper ferrroelectricity
is a useful property to have for solar cell absorber materials, what this means
for devices based on these materials and key challenges for moving forward
with this line of research is given.
potential use as photovoltaic absorbers in highly efficient, cheaply manufactured photovoltaic (PV) devices. This thesis studies several topics of interest to hybrid organic-inorganic perovskite (HOP) photovoltaics, including the
dynamic behaviour of the organic component in methylammonioum lead iodide (MAPbI3), the moisture induced degradation of typical hybrid perovskite
materials and the possibility of engineering ferroelectric photovoltaic devices
using hybrid perovskite-like materials.
The observation of hysteresis in current-voltage curves for most hybrid perovskite devices suggests either dynamic movement of charged ions or a ferroelectric response of the absorber material to applied electric fields. The possibility that alignment or anti-alignment of organic cations, which are molecular
dipoles, could be causing hysteresis in hybrid perovskite devices was examined
using semi-classical Monte-Carlo simulations, informed by density functional
theory calculations and experimental quasi-elastic neutron scattering experiments. I show that the Monte Carlo model does indeed suggest the possibility of either ordered low temperature states, and that the nature of these
states (ferroelectric or anti-ferroelectric) is determined by the magnitude of
a parameter related to the distortion of the inorganic PbI6 framework when
methylammonium cations are aligned or anti-aligned. I also present a basic
model based on the Monte Carlo and quasi-elastic neutron scattering results
in that chapter which gives a characteristic timescale for domain wall motion
within MAPbI3 devices. The timescale for domain wall motion across a typical device is calculated to be on the order of milliseconds, which is deemed to
fast to be consistent with observed current-voltage hysteretic behaviour in hybrid perovskite solar cell devices. These results constitute the first theoretical
results of this thesis.
Despite rapid advances in photovoltaic efficiency of hybrid perovskite PV
devices, their viability as a mainstream commercial technology is impeded
by their susceptibility to a variety of environmental conditions, including the
presence of airborne water vapour. The second part of this thesis studies the
hydration of HOPs using electronic structure methods and ab initio thermodynamics based upon density functional theory calculations. The calculations
presented show that ab initio thermodynamics can capture the key trends observed in experiments, but that it is difficult to reproduce experimental phase
boundaries accurately using this technique (as is explained in that chapter this
is not unexpected). A model of the entropy associated with the presence of
a rotationally mobile organic cation within the perovskite is also introduced
and used to augment the basic ab initio thermodynamic calculations. It is
shown that the magnitude of this entropic contribution could play a key role
in the location of the thermodynamic phase boundary between MAPbI3 and
its hydrated structures. Finally, a mechanism for the hydration of MAPbI3 is
proposed and outlined.
The final part of this thesis discusses the application of the theory of hybrid
improper ferroelectricity, whereby changes in the typical perovskite structure
(for example by creating n = 2 layers of ABX3 perovskite intercalated with
single layers with rock-salt AX stoichiometry) to metal halide perovskite based
materials. It is shown through DFT calculations of polarisation and in depth
analysis of the symmetries present in crystals studied that Ruddlesden-Popper
and superlattice structures based on CsPbI3 metal halide perovskites should
indeed exhibit hybrid improper ferroelectricity. The electronic structures of
the Ruddlesden-Popper Cs3Pb2I7 is compared with that of CsPbI3 and it is
shown that the Ruddlesden-Popper has a larger band gap. It is explained that
this is due to changes in orbital overlap between lead and iodide states within
the structure. A discussion is given of why hybrid improper ferrroelectricity
is a useful property to have for solar cell absorber materials, what this means
for devices based on these materials and key challenges for moving forward
with this line of research is given.
Version
Open Access
Date Issued
2018-07
Date Awarded
2019-01
Copyright Statement
Creative commons attribution non-commercial No derivatives licence.
Advisor
Harrison, Nicholas
Barnes, Piers
VandeVondele, Joost
Sponsor
Novathera
Society of Chemical Industry (Great Britain)
Engineering and Physical Sciences Research Council
Royal Academy of Engineering (Great Britain)
Royal Society (Great Britain)
Leverhulme Trust
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)