On the electronic and vibronic behaviour of organic and perovskite photovoltaics
File(s)
Author(s)
Gallop, Nathaniel
Type
Thesis
Abstract
Emerging so-called third-generation photovoltaic technologies, in contrast to their predecessors, encompass a broad catalogue of different material systems and present distinct challenges-- such as material softness and energetic disorder-- to their use in photovoltaics. Herein, we attempt to deepen our understanding of two such challenges: the effect of low donor:acceptor energetic offset on the charge dynamics of organic photovoltaic materials incorporating nonfullerene acceptors, and the interaction between the organic and inorganic sublattices of organohalide perovskites.
Initially, we address the problem of charge dynamics in low-offset polymer:NFA blends using multiple ultrafast spectroscopic techniques. Here we find that the unique properties of nonfullerene acceptors necessitate modifications to the standard experimental approach in order to yield useful information. By modifying our spectroscopic technique we find that, despite the formation of long-lived so-called 'Charge Transfer states' within the low-offset blend, the net efficiency of charge separation becomes compromised as the donor:acceptor energetic offset approaches zero. This results in an overall decrease in the net yield of free charge for low-offset blends versus high offset blends which ultimately obviates any enhancement of open-circuit voltage brought about through low-offset blends.
We then shift our attention to perovskite photovoltaics. Here, we study the interplay between the organic and inorganic sublattices of organohalide perovskites. Using two-dimensional IR spectroscopy, we find that the substitution of as little as 10% of the organic A-site ions with the inorganic cation caesium strongly inhibits the facile rotation of organic cation, likely as a result of both rigidisation and a change in inorganic lattice geometry. This further supports the view that the dynamics of the inorganic sublattice drive the dynamics of the organic sublattice, as well as the interplay between static and dynamic disorder within organohalide perovskites. Additionally, we study the degree to which coupling between the organic and inorganic sublattices of the perovskite influence the electronic properties of the perovskite. To achieve this, we develop and subsequently employ a novel spectroscopic technique, which we term PC/VIPER to observe how the band edge of the perovskite FAPbBr3 change with stimulation of the vibrational modes of the formamidinium cation. Our results point towards a weak coupling of the C=N stretching mode of the cation to the electronic properties of the perovskite and moreover point to hydrogen bonding as the underlying cause of this coupling. We ultimatley find, contrary to popular belief, that organic cations do play a role (albeit a small one) in determining the electronic properties of the FAPbBr3.
Initially, we address the problem of charge dynamics in low-offset polymer:NFA blends using multiple ultrafast spectroscopic techniques. Here we find that the unique properties of nonfullerene acceptors necessitate modifications to the standard experimental approach in order to yield useful information. By modifying our spectroscopic technique we find that, despite the formation of long-lived so-called 'Charge Transfer states' within the low-offset blend, the net efficiency of charge separation becomes compromised as the donor:acceptor energetic offset approaches zero. This results in an overall decrease in the net yield of free charge for low-offset blends versus high offset blends which ultimately obviates any enhancement of open-circuit voltage brought about through low-offset blends.
We then shift our attention to perovskite photovoltaics. Here, we study the interplay between the organic and inorganic sublattices of organohalide perovskites. Using two-dimensional IR spectroscopy, we find that the substitution of as little as 10% of the organic A-site ions with the inorganic cation caesium strongly inhibits the facile rotation of organic cation, likely as a result of both rigidisation and a change in inorganic lattice geometry. This further supports the view that the dynamics of the inorganic sublattice drive the dynamics of the organic sublattice, as well as the interplay between static and dynamic disorder within organohalide perovskites. Additionally, we study the degree to which coupling between the organic and inorganic sublattices of the perovskite influence the electronic properties of the perovskite. To achieve this, we develop and subsequently employ a novel spectroscopic technique, which we term PC/VIPER to observe how the band edge of the perovskite FAPbBr3 change with stimulation of the vibrational modes of the formamidinium cation. Our results point towards a weak coupling of the C=N stretching mode of the cation to the electronic properties of the perovskite and moreover point to hydrogen bonding as the underlying cause of this coupling. We ultimatley find, contrary to popular belief, that organic cations do play a role (albeit a small one) in determining the electronic properties of the FAPbBr3.
Version
Open Access
Date Issued
2021-01
Date Awarded
2021-08
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Bakulin, Artem
Nelson, Jenny
Sponsor
European Research Council
Publisher Department
Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)