Structural characterization of the interfacial structure of the working electrode in dye-sensitized solar cells using X-ray and neutron techniques

The transparency and low cost of dye-sensitized solar cells (DSSCs) renders this technology promising for the development of electricity-generating windows in future energy-sustainable buildings. Typically, DSSCs contain a working electrode (photoanode) that consists of a dye adsorbed onto the surface of a semiconductor (often TiO2), where photo-induced electrons are injected into the conduction band (CB) of the semiconductor; the electrical current passes to the counter-electrode, which is often coated with platinum in order to catalyze the electrolytic process. The electrolyte is a redox couple (e.g. I-/I3-) that fills the void between the two electrodes. This thesis examines the interfacial structure of the working electrode in DSSCs using X-ray and neutron techniques. Chapters 1 and 2 provide an introduction to X-ray reflectometry (XRR) and neutron reflectometry (NR) techniques, i.e., the principal analytic tools used for research carried out in the context of this thesis. Chapters 3, 4, and 5 contain the results obtained from applying ex-situ air-solid X-ray techniques to investigate the structure of the dye···TiO2 interface for three high-performance DSSC dyes: the organometallic dyes N3 and N749, as well as the metal-free organic dye MK-2. Chapter 3 discusses how XRR and grazing-incidence small angle X-ray scattering (GISAXS) can be used to explore the molecular packing behavior of these three dyes on TiO2 as a function of the dye-sensitization concentration. Chapters 4 and 5 examine the relationships between the structure of the dye···TiO2 interface and the DSSC fabrication parameters based on several case studies using ex-situ XRR, and a more systematic study using high-throughput ex-situ XRR. Chapters 6 and 7 discuss the structural aspects of the dye···TiO2 interface in the presence of solvent and electrolyte based on the results of NR techniques. Chapter 6 examines the structure of the dye···TiO2 interface and the effect of solvent ingression on the dye···TiO2 interface in an emulated DSSC, using in-situ NR combined with ex-situ XRR. Given that the electrolyte solution plays an important role in the photon-to- electron conversion efficiency, Chapter 7 examines the modulation of the dye···TiO2 interfacial structure from solvent ingression and in the presence of different levels of electrolyte composition, using high-flux in-situ NR with contrast-matching. These aspects are of paramount importance for the determination of various properties of the DSSC working electrode and ultimately of the DSSC device. Chapter 8 concludes the thesis and explores improvements and other potentially feasible techniques to study the structure of the dye···TiO2 interface based on current research findings. The importance of understanding our sample systems, based on various supporting techniques such as IR spectroscopy, XPS as well as imaging techniques is discussed; the results provide additional structural information for the X-ray and neutron reflectivity analyses (fittings).