Studies of Dye-Titania Interactions in Dye-sensitised Solar Cells

This work details the synthesis of several bespoke materials to derivatise the surface of titania (TiO2) in order to obtain greater understanding of the sensitisation process in dye-sensitised solar cells and how this can influence device performance. In particular, this work has combined synthetic (dye synthesis), experimental (AR-XPS) and theoretical experiments (computer modelling) in order to investigate the self-assembly of organic dyes onto titania surfaces in dye-sensitised solar cell (DSC) devices. To test the dyes and other materials in this thesis both liquid and solid-state dye-sensitised solar devices were made. The standard liquid DSC device utilised TEC 8 glass (3 cm x 1.5 cm) for both the photo and counter electrode. On the photoelectrode a mesoporous TiO2 layer is deposited and sensitised with N719 dye. The counter electrode is coated with a thin layer of platinum PT-1 paste (GreatCellsolar). The two electrodes are bound together by melting 20 µm surlyn and a standard triiodide electrolyte is injected to the cell and the cell sealed. With this method we have managed to achieve 6.5 % using N719 dye. Solid-state dye-sensitised solar cell (ssDSC) devices we made using TEC 7 glass (2.8 cm2). Onto the glass a compact layer of TiO2 has been deposited via spray coating, thereafter a thin layer of mesoporous TiO2 is spin coated and sensitised overnight in a dye bath. After which, a thin layer of Spiro-OMeTAD is deposited via spin coating, after which Au is deposited under vacuum. With this method 4.0 % has been achieved using LEG4 dye. This thesis reports the first synthesis of ten novel half-squaraine (HfSQ) dyes, five containing a benzothiazole backbone and another five identical dyes with a dimethyl-derivatised carbon. These new dyes have been purified using various preparative (column and flash) chromatography. Once purified the dyes have been analysed using nuclear magnetic resonance (NMR), attenuated total reflectance infra-red (ATR-IR), ultraviolet-visible spectroscopy (UV-Vis) and mass spectrometry. These novel dye materials have then been used to derivatise the titania surface in DSC devices. The dyes have been designed so that a sulphur heteroatom can be used as an atomic probe of the dye-TiO2 interface in order to study surface coverage, dye orientation and dye-electrolyte interactions in dye-sensitized solar cell (DSC) devices. X-ray single crystal structural and opto-electronic data have also been correlated with density functional theory/computer modelling to better understand the widely reported but poorly understood influence of S heteroatoms on dye photochemistry, DSC device performance and lifetime. To the best of our knowledge, for the first time, the S heteroatom in the dyes has been used as a molecular probe along with angle-resolved X-ray photoelectron (ARXPS) data to study how the dye-TiO2 orientation varies with the position of the carboxylate linker on the dye periphery. In further work, a one-step synthesis has been used to link two dye-sensitized solar cell (DSC) dyes together through an anhydride moiety. The dyes we have attempted to combine are triphenylamine dye “Yellow dye” to another Yellow dye and squaraine dye SQ2 to other moieties such as Chenodeoxycholic acid and stearic acid. To the best of our knowledge, this is also the first time this has been attempted in DSC dyes. The resulting anhydride precursor has been used to self-assemble the two dyes onto TiO2 surfaces in a controlled manner. Hence, this represents a brand-new method of sensitising two dyes, in a single sorption process; providing a new method to control dye-sorption kinetics, dye loading and surface organisation. When two dyes are introduced to the titania surface at the same time it is possible to control the positions of the two dyes without the need of bulky co-sorbents (e.g. CDCA) to supress dye aggregation. This one-step synthesis method was applied to combine two different dye materials into one anhydride moiety. Attempts have been made to combine SQ2 dye to co-adsorbent CDCA and SQ2 dye to stearic acid. The photo-physical properties of the anhydride and its precursor dyes have been studied (via UV-Vis, ATR-IR NMR and tested in ssDSC devices) and compared highlighting the potential of this new method for co-sensitisation.