Modulation of N3 and N719 dye···TiO₂ Interfacial Structures in Dye-Sensitized Solar Cells As Influenced by Dye Counter Ions, Dye Deprotonation Levels, and Sensitizing Solvent

Abstract

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.

Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces

“…In the photooxidized state of the dye, the hole density distribution is expected to closely resemble the HOMO density distribution. 21 Values for the total height of the dye with respect to the substrate surface lie between 1.0 and 1.3 nm for different anchoring geometries on TiO2, 19,42 which is compatible with the retrieved hole-surface distance based on IPDs.…”

Imaging Dye Aggregation in MK-2, N3, N749, and SQ-2 dye···TiO2 Interfaces That Represent Dye-Sensitized Solar Cell Working Electrodes

“…Once adsorbed onto the TiO 2 surface, an N749 dye molecule can manifest a lateral height ranging from 0.96 to 1.60 nm, given its molecular dimensions and multiple dye···TiO 2 anchoring configuration options (Figure a). , The corresponding lateral height of the N3 dye varies from 1.0 to 1.3 nm, given the four anchoring possibilities shown in Figure b . Both MK-2 and SQ-2 dye molecules exhibit almost perfectly planar structures, , and they each contain only one anchoring group which appears to bind to TiO 2 surfaces via bidentate chelation − (Figure c,d) Accordingly, they present with a more upright anchoring configuration (greater lateral height) on the TiO 2 surface, relative to those of N749 and N3, cf. lateral heights 2.2–2.7 nm for MK-2 , and 1.8 nm for SQ-2, the latter being judged from density functional theory calculations that were carried out for this study (see section Materials and Methods and the Supporting Information for details).…”

Electrolyte/Dye/TiO2 Interfacial Structures of Dye-Sensitized Solar Cells Revealed by In Situ Neutron Reflectometry with Contrast Matching

“…A TBA + cation could occupy a similar area on the TiO 2 surface compared to the Rubased anion in N749, judging from related findings of TBA + influences on the dye self-assembly of N719. 18 Yet, TBA + cations in N749 are heavily disordered, even when contained within a crystal lattice such that they have not to be resolved by X-ray diffraction data. 20 While the motion of TBA + cations would be somewhat restricted within a dye self-assembled structure, this environment is less confined than a crystal lattice; thus, we could reasonably suppose that TBA + cations are also likely to be too dynamic to be resolved by reflectometry.…”

“…A myriad of ex situ materials characterization methods has been employed to determine the interfacial structure of DSC working electrodes outside of the device, including imaging (atomic force microscopy, − scanning tunneling microscopy (STM) , ), X-ray scattering (grazing-incidence X-ray scattering, X-ray reflectometry (XRR) ,− ), X-ray diffraction, − and optical, vibrational (infrared (IR) and Raman), − X-ray absorption, − and photoelectron spectroscopy. , These studies have managed to quantify the essence of dye/TiO 2 interfacial structures that represent an exposed DSC working electrode. However, none of them have been able to directly probe this dye/TiO 2 interfacial structure while it is held within its DSC device assembly.…”