Unravelling the mechanism of ultrafast photoinduced charge generation in organic and hybrid photovoltaic systems

At the current time, the world as we know it is at a crossroads; there is an urgent need to lower greenhouse gas emissions to limit the global rise in temperature. However, this is at odds with the increasing energy demands of the world's growing population. As such, to ensure that we can have our cake and eat it too, it is necessary to further develop renewable and low-carbon energy sources to minimise irreversible damage to our planet.

Third generation solar photovoltaics are low-cost materials which can generate electricity whilst having a minimal impact on the environment. To ensure efficiencies of these materials continue to improve, it is necessary to have a good fundamental understanding of how they generate charges, as well as the fate of these charges. This allows us to provide vital information to further material design, and thus push device efficiencies to new highs.

The first chapter of this thesis will provide the scientific background and historical context as well as introducing the material systems that will be studied later. Likewise, the second chapter introduces the laser-based ultrafast spectroscopic techniques that will be used to probe the ultrafast charge generation processes.

The third chapter focuses on pristine pentamethine cyanine dyes; in particular, transient absorption spectroscopy is used to evidence high efficiency intrinsic charge generation occurring via a symmetry-breaking charge separation process. The efficiency of this process was shown to be dependent on the size of the counterion used, allowing us to conclude that the interchromophore distance and degree of H-aggregation are vital parameters in ensuring efficient charge generation.

The first part of the fourth chapter explores whether a similar symmetry-breaking charge separation process could occur in pristine dicyanomethylene-substituted squaraine dyes. Transient absorption spectroscopy, on both pristine squaraine and squaraine/C60 bilayer samples, was used to determine that intrinsic charge generation cannot occur. The second part of this chapter focuses on the photochemistry of spiro-OMeTAD, a material often exploited as a hole transport layer in third generation solar cells. Once again, transient absorption spectroscopy was used to show that oxidised spiro-OMeTAD was generated via a symmetry-breaking charge separation process which could be catalysed by the addition of lanthanum-based TFSI salts.

The final results chapter targets two-dimensional hybrid organic-inorganic lead halide perovskites, where the impact of spacer size and layer orientation on the photogeneration of charge transfer excitons was studied using temperature dependent transient absorption spectroscopy. Furthermore, electroabsorption spectroscopy was used as a novel method to quantify the strength of the photoinduced electric field. Finally, it was possible to demonstrate that charge transfer can occur between the perovskite and electroactive naphthalene diimide-based spacers, a