Structural heterogeneity and dynamics of dyes on TiO₂: implications for charge transfer across organic–inorganic interfaces

Abstract

Charge transfer across organic-inorganic interfaces plays a vital role in many important applications. Dye-semiconductor systems are the prototypical such interface and provide an excellent platform for exploring the underlying molecular-level factors that affect charge transfer dynamics and efficiency. Experiments often show multi-exponential electron injection kinetics from adsorbed dyes to a semiconductor substrate, suggesting the presence of interfacial heterogeneity. Nonetheless, both the diversity of interfacial structures and the associated implications for electronic dynamics are poorly understood. In the present work, we examine the effect of structural heterogeneity and dynamics on charge injection (as measured by dye-semiconductor electronic coupling) from plane wave density functional theory and ab initio molecular dynamics calculations on model dye-semiconductor systems. We demonstrate that dye binding motif, conformation, solvation, and corresponding thermal fluctuations significantly affect charge injection kinetics. We suggest that the experimentally observed multi-exponential kinetics likely result not only from an intrinsic heterogeneous distribution of electronic coupling strengths, but also from the conformational or solvent dynamics that in turn modulate the coupling strength and/or band alignment.

Optically Reconfigurable Monolayer of Azobenzene Donor Molecules on Oxide Surfaces

“…Molecular donors deposited on semiconducting oxides such as TiO 2 , − ZnO, and SnO 2 can inject electrons into the oxide from their metal-to-ligand charge transfer excited state on the picosecond timescale. − Molecular monolayers and thin films exhibit variation in molecular orientation, conformation, and binding motif as well as in macroscopic parameters including thickness, roughness, packing density, and aggregation, all of which can affect the charge transfer rates. Likewise, the rate of charge transfer, either through conjugated links or through space tunneling, depends on factors such as the composition and structure of the chromophore and the anchoring group, the extent of conjugation of the molecular bridge group, and macroscopic structural factors such as packing density and aggregation state. ,,, In this context, rhenium (Re)–bipyridine complexes have been extensively studied as photoabsorbers and electron donors. ,, …”

“…Donor molecule/semiconductor interfaces are electronically and structurally intriguing because of the strong connection between charge transfer rates and the spatial separation of the donating group and the substrate. This donor–surface distance can be manipulated through the incorporation of a reconfigurable group that changes the distance between the donor and the substrate in response to an external stimulus. ,, The challenge has been to fabricate reconfigurable interfaces with a specific structure and to correlate the corresponding electronic kinetics systematically. Additionally, it is challenging to directly evaluate the structure and structural changes quantitatively during and after the external stimulus in these reconfigurable systems.…”

“…Likewise, the rate of charge transfer, either through conjugated links or through space tunneling, depends on factors such as the composition and structure of the chromophore and the anchoring group, the extent of conjugation of the molecular bridge group, and macroscopic structural factors such as packing density and aggregation state. 16,18,21,22 In this context, rhenium (Re)−bipyridine complexes have been extensively studied as photoabsorbers and electron donors. 18,23,24 Donor molecule/semiconductor interfaces are electronically and structurally intriguing because of the strong connection between charge transfer rates and the spatial separation of the donating group and the substrate.…”

Geometry of Molecular Motions in Dye Monolayers at Various Coverages

“…Characterizing such motions is highly nontrivial, partly because of the organic/inorganic nature of the interface where the dye monolayer resides, which is difficult to probe both experimentally and theoretically. , Nevertheless, serious efforts have been made to gain insight into the structural and vibrational dynamics of sensitized metal oxide surfaces. Scanning tunneling microscopy (STM); X-ray, nuclear magnetic resonance (NMR), infrared (IR), and photoelectron spectroscopies; and molecular dynamics (MD) simulations allow for an atomistic description of the dye molecule/metal oxide interface with estimations of the surface area per dye molecule and the binding modes and molecular orientations of individual dye molecules relative to the surface. ,,− Nonlinear optical methods such as sum frequency generation spectroscopy (SFG) are especially relevant to dye-sensitized systems because of their applicability to liquid/solid interfaces and monolayer sensitivity. − Adsorption structures and orientation distributions of sensitized surfaces are keys to building an accurate picture of DSSCs. − Others have investigated the effects of thermal fluctuations on these structural features, as well as on the electronic properties of dyes. ,− Nevertheless, to date, issues remain unresolved, including how the presence of other dye molecules affects these orientation distributions and whether this effect changes over time and, if so, on what time scale. As a result, not many models explicitly account for the effect of molecular motion on charge injection, diffusion, and/or recombination in DSSCs. ,, …”

“…[50][51][52][53][54][55] Others have investigated the effect of thermal fluctuations on these structural features as well as on the electronic properties of the dyes. 33,[56][57][58][59][60] Nevertheless, up to this date, we do not know how the presence of other dyes affects these orientation distributions, nor whether it changes over time, and if so on what timescale. As a result not many models explicitly account for the effect of molecular motion on charge injection, diffusion and/or recombination in DSSCs.…”