Discovery of Black Dye Crystal Structure Polymorphs: Implications for Dye Conformational Variation in Dye-Sensitized Solar Cells

We present the discovery of a new
crystal structure polymorph (1) and pseudopolymorph (2) of the Black Dye, one of the world’s leading dyes
for dye-sensitized solar cells, DSSCs (10.4% device performance efficiency).
This reveals that Black Dye molecules can adopt multiple low-energy
conformers. This is significant since it challenges existing models
of the Black Dye···TiO2 adsorption process
that renders a DSSC working electrode; these have assumed a single
molecular conformation that refers to the previously reported Black
Dye crystal structure (3). The marked structural differences
observed between 1, 2, and 3 make the need for modeling multiple conformations more acute. Additionally,
the ordered form of the Black Dye (1) provides a more
appropriate depiction of its anionic structure, especially regarding
its anchoring group and NCS bonding descriptions. The tendency toward
NCS ligand isomerism, evidenced via the disordered form 2, has consequences for electron injection and electron recombination
in Black Dye embedded DSSC devices. Dyes 2 and 3 differ primarily by the absence or presence of a solvent
of crystallization, respectively; solvent environment effects on the
dye are thereby elucidated. This discovery of multiple Black Dye conformers
from diffraction, with atomic-level definition, complements recently
reported nanoscopic evidence for multiple dye conformations existing
at a dye···TiO2 interface, for a chemically
similar DSSC dye; those results emanated from imaging and spectroscopy,
but were unresolved at the submolecular level. Taken together, these
findings lead to the general notion that multiple dye conformations
should be explicitly considered when modeling dye···TiO2 interfaces in DSSCs, at least for ruthenium-based dye complexes.