Dye-sensitised solar cells (DSCs) have niche prospects for electricity-generating windows that could equip buildings for energy-sustainable future cities. However, this 'smart window' technology is being held back by a lack of understanding in how the dye interacts with its device environment at the molecular level. A better appreciation of the dyeTiO interfacial structure of the DSC working electrodes would be particularly valuable since associated structure-function relationships could be established; these rules would provide a 'toolkit' for the molecular engineering of more suitable DSC dyes via rational design. Previous materials characterisation efforts have been limited to determining this interfacial structure within an environment exposed to air or situated in a solvent medium. This study is the first to reveal the structure of this buried interface within the functional device environment, and represents the first application of in situ neutron reflectometry to DSC research. By incorporating the electrolyte into the structural model of this buried interface, we reveal how lithium cations from the electrolyte constituents influence the dyeTiO binding configuration of an organic sensitiser, MK-44, via Li complexation to the cyanoacrylate group. This dye is the molecular congener of the high-performance MK-2 DSC dye, whose hexa-alkyl chains appear to stabilise it from Li complexation. Our in situ neutron reflectometry findings are built up from auxiliary structural models derived from ex situ X-ray reflectometry and corroborated via density functional theory and UV/vis absorption spectroscopy. Significant differences between the in situ and ex situ dyeTiO interfacial structures are found, highlighting the need to characterise the molecular structure of DSC working electrodes while in a fully assembled device.

The dye…TiO 2 interfacial structure in working electrodes of dye-sensitized solar cells (DSCs) is known to influence its photovoltaic device performance. Despite this, direct and quantitative reports of such structure remain sparse. This case study presents the application of X-ray reflectometry to determine the preferred structural orientation and molecular packing of the organic dye, coumarin 343, adsorbed onto amorphous TiO 2. Results show that the dye molecules are, on average, tilted by 61.1° relative to the TiO 2 surface, and are separated from each other by 8.2 Å. These findings emulate the molecular packing arrangement of a monolayer of coumarin 343 within its crystal structure. This suggests that the dye adsorbs onto TiO 2 in one of its lowest energy configurations, i.e. dye…TiO 2 self assembly is driven more by thermodynamic rather than kinetic means. Complementary DSC device tests illustrate that this interfacial structure compromises photovoltaic performance, unless a suitably sized co-adsorbant is interdispersed between the coumarin 343 chromophores on the TiO 2 surface.

The fabrication process for dye-sensitized solar cells (DSCs) is well-established; yet, reported reliabilities of DSC device photovoltaic properties have been the subject of widespread controversy. The fabrication process will naturally affect these device properties, wherein the most susceptible DSC device component is its working electrode, which comprises a dye···TiO2 interface. While various analytical chemistry methods have probed this fabrication process indirectly, the molecular structure of this interface has not been tracked with varying dye sensitization conditions, via any direct means. Little is thus known about how steric and electronic factors associated with dye counterions, dye deprotonation levels, and the choice of sensitizing solvent impact upon the nature by which the dye···TiO2 interfacial structure forms via self-assembly. This work employs X-ray reflectometry (XRR) in combination with density functional theory calculations to probe the structural nature of dye···TiO2 interfaces featuring the ruthenium-based dye cis-bis­(isothiocyanato)­bis­(2,2′-bipyridyl-4,4′-dicarboxylato)­ruthenium­(II) (N3), and its doubly deprotonated derivative ditetrabutylammonium cis-bis­(isothiocyanato)­bis­(2,2′-bipyridyl-4,4′-dicarboxylato)­ruthenium­(II) (N719). XRR measurements show that N3 and N719 dye···TiO2 interfaces, sensitized using the co-solvent MeCN:t-BuOH (1:1, v/v) exhibit the same preferred bidentate binding mode, although minor structural variations were observed owing to the co-adsorption of tetrabutylammonium counterions in the case of N719. Further XRR studies on N3-adsorbed TiO2 surfaces sensitized using different solvents, MeCN:t-BuOH (1:1, v/v), EtOH:t-BuOH (1:1, v/v), EtOH, and DMSO, showed that the nature of dye···TiO2 adsorption binding modes depends not only on the acidity of the N3-containing solutions, which dictate the dye deprotonation levels, but also on the steric nature of the sensitizing solvent.