Probing interactions of molecules and pigments on titanium dioxide surfaces for photovoltaic cells, using QCM-D

Photovoltaic cells offer a sustainable way of generating electricity from sunlight, and they will be of growing economical importance for decades to come. Hybrid organic-inorganic solar cells, such as dye sensitized solar cells or perovskite solar cells, are an area of active research, and fundamental scientific advances can lead to impressive improvements of device efficiency. In this thesis, I present experimental studies on some of the fundamental properties of molecules and pigments on titanium dioxide surfaces for solar energy conversion. My main instrument is a quartz crystal microbalance with dissipation technique (QCM-D), and I develop special methods of measurement, which enable the characterization of mass uptake in mesoporous systems. In dye sensitized solar cells (DSC), dye molecules are adsorbed onto a mesoporous titanium dioxide (TiO2) photoanode. The dye molecule absorbs light, subsequently injects an electron into the conducting TiO2, and is regenerated from a hole transport material. Furthermore, the molecular film of dye molecules blocks recombination between electrons in the TiO2 conduction band and the oxidized form of the redox mediator in the hole transport material. In the first chapter, I study the adsorption of the ruthenium-based dye Z907 by QCM-D on flat TiO2 as a model system with sub-monolayer resolution. An adsorption isotherm is measured for Z907, which leads to a formal adsorption equilibrium constant of 5.1e6 M-1 and a number density of 0.76 molecules per nm2. The coadsorbate chenodeoxycholic acid (cheno) enhances the recombination-blocking capability of the adsorbed molecular layer and it prevents aggregation of dye molecules. In chapter two, I develop a method to quantify the ratio between cheno and dye molecules when coadsorbed on the TiO2 using a combination of QCM-D and subsequent fluorescence spectroscopy. I apply this method to study coadsorption with triphenylamine-, porphyrin-, ulllazine- and diketopyrrolopyrrole-based dyes. The role of cheno depends on its specific interaction with the dye molecule. In chapter three, I present a microscopic study on the molecular layer of the amphiphilic dye Z907 adsorbed on TiO2. In collaboration with two groups at EPFL, we present in-situ atomic force microscopy images with sub-nanometer resolution in a functionally relevant liquid environment. Our results reveal changes in the conformation and the lateral arrangement of the dye molecules, depending on their average packing density on the surface, and these results are confirmed by molecular dynamics simulations. Chapter four aims to understand the QCM-D signals on mesoporous TiO2 films attached to the QCM-D sensor. Their high accessible surface area makes mesoporous films interesting for sensing applications. For films of up to 200 nm thickness, I find a regular inertial loading, which allows for an archimedic measure of molecules adsorbed inside the mesoporous matrix. For thicker films, I observe a particular behaviour that is explained by the resonant coupling of the quartz crystal driving frequency to an eigenmode of the TiO2 film. In chapter five, I contribute to the fundamental understanding of the perovskite conversion reaction between PbI2 and CH3NH3PbI3 perovskite. This material is central to the latest, record-breaking perovskite solar cells. I provide evidence for that the back-conversion is reversible by release of methylammonium iodide from the perovskite film into selective solvents.

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