Geometry of Molecular Motions in Dye Monolayers at Various Coverages

Abstract

Molecular motion in monolayers is thought to influence the kinetics of charge transport and recombination in systems such as dye sensitized solar cells (DSSCs). In this work, we use ab-initio molecular dynamics to evaluate the geometry and timescale of such molecular motion in a D102 monolayer. D102 is a dye routinely used in DSSCs, which contains two chemical groups, indoline and triphenylethylene, that are also present in many other dyes. We find that, at low surface coverage, the dye exhibits two main tilting axes around which it heavily distorts within 10 ps. Further, the two benzene rings in the triphenylethylene group rotate with a 3-4 ps period. We observe that these large amplitude movements are suppressed at full coverage, meaning that dyes in a monolayer are locked into place and only undergo minor conformational changes. Our observations indicate that, counter-intuitively, charge diffusion across dye monolayers may be faster in the parts of the system that are characterized by a lower surface coverage. Since charge transport in dye monolayers has been proven to accelerate recombination kinetics in DSSCs, these results provide the basis for a new understanding of the electronic properties of sensitized systems and device efficiency.

Probing energy losses from dye desorption in cobalt complex-based dye-sensitized solar cells

“…A possible explanation is that the sensitizers anchored at the TiO 2 surface tend to form a molecular layer by aggregation. 65 As a consequence, the oxyalkyl chains and the phenothiazine groups in the sensitizer M26 are expected to overlap. This overlapping allows some of the sensitizers to be removed without leaving sufficient space for the approach of bulky cobalt complexes.…”