Interfacial structure of dye solar cells under redox electrolyte. J. McCree-Grey, J.M. Cole, S.A. Holt, P.J. Evans and Y. Gong

Dye Sensitised Solar Cells Metal-centred complexes face issues Environmental regulations Scarcity / cost of metal centres Organic Dyes Cheaper and more flexible synthesis Low cost efficient environmentally friendly power generation Transparent, good in diffuse light conditions  Smart window applications Dye interactions at molecular level ? Niche prospects - electricity generating windows Greater molecular design flexibility

Background Highlight XRR and NR results. Supported by UV/Vis and DFT calculations. Full details can be found at McCree-Grey et al. Nanoscale, 2017, 9, 11793 DOI: 10.1039/c7nr03936k

Preferred TiO2 binding modes Molecular Structures MK-2 MK-44 0.3 mM solution of Dye in 1:1:1 acetonitrile : tert-butanol : toluene Preferred TiO2 binding modes

Reflectivity Structure perpendicular to interface X-rays Sensitised TiO2 Silicon Structure perpendicular to interface Neutrons travel ‘through’ the silicon substrate Buried interface X-rays through air

MK-44 on TiO2 MK-2 Thickness 23.3 Mass density 1.11 XRR data at four different locations on substrate 9.6 Å thick. Mass density 1.09 g/cm3

Surface attachment by XRR MK-2 MK-44 Models were created using ChemBio3D (Perkin Elmer)

Lithium (yellow) Iodide (pink) Schematic illustration of the dye sensitised TiO2 sample within the solid-liquid environment iodide:tri-iodide neat d3-MeCN iodide:tri-iodide is the (pink, tri-atomic structure) in d3-MeCN formed upon addition of I2 to the previously stated LiI solution Lithium (yellow) Iodide (pink) in d3-MeCN

Schematic illustration of the dye sensitised TiO2 sample within the solid-liquid environment iodide:tri-iodide is the (pink, tri-atomic structure) in d3-MeCN formed upon addition of I2 to the previously stated LiI solution

MK-2 MK44 Figure 5 Reflectivity profiles for (a) MK-2 and (b) MK-44 dyes sensitised on an amorphous TiO2 thin-film and submerged within solution 1 (d3-MeCN, red), 2 (d3-MeCN + LiI, orange), or 3 (d3-MeCN + LiI + I2, green). The thin overlaid lines represent the co-refined models fitted to their corresponding datasets. Corresponding SLD profiles are presented as Figure insets

MK-44 SLD profile

  Dye Layer TiO2 Layer Solution Dye t /Å SLD (x10-6) /Å-2 R / Å R /Å MK-2 1 23.6±1.9 1.9±0.1 6.0 108.6±2.1 2.1 3.5 4.60 2 23.8±1.9 2.5±0.1 4.74 3 22.2±1.5 2.4±0.1 4.86 MK-44 9.2 ±0.7 2.9 ±0.6 4.0 107.6 ±2.3 4.51 15.9 ±1.0 3.6 ±0.4 4.67 3.8 ±0.4

  Dye Layer Dye Solution t /Å SLD (x10-6) /Å-2 MK-2 1 23.6±1.9 1.9±0.1 2 23.8±1.9 2.5±0.1 3 22.2±1.5 2.4±0.1 MK-44 9.2 ±0.7 2.9 ±0.6 15.9 ±1.0 3.6 ±0.4 3.8 ±0.4

Change in MK-44 surface attachment Figure 6 Molecular structure of MK-44 with bidentate bridging geometry adopted in the presence of Li+ ions, indicating the dmax (15.74 Å) and molecular width (7.74 Å). The red shaded boxes indicate sections of the TiO2 surface which are potentially exposed to solvent/electrolyte upon the change in dye geometry.

Conclusions Suggest that initial surface arrangement is crucial Possible mechanism contributing to lower efficiency of MK-44 First application of NR to DSCs. In situ experiments in simple model of DSC cell.