Charge Transfer at Organic/Inorganic Interfaces

Electronic properties of organic/inorganic interfaces are extremely important in emerging technologies ranging from solar cells to molecular electronics. Electron transfer rates at these interfaces are very sensitive to interfacial electronic properties, and in some systems, like dye-sensitized solar cells, the device function depends critically on these electron transfer rates. However, the relationship between interfacial structure and injection rates is not well understood.

In recent years, transient IR spectroscopy has emerged as a way to study electron injection at these interfaces because free electrons injected into the semiconductor give rise to a strong, broad offset which reflects the overall injection kinetics.  Kinetics measurements on a series of dyes synthesized by our collaborators at UW-Madison are shown in Figure 1.  The overall kinetics reflect the dye structures and electronic couplings, but as with other research groups' work on similar systems, we observe multiexponential kinetics. Multiexponential kinetics suggests that the injection reflects contributions from several different pathways, but the identity of these pathways is the subject of ongoing debate.

Charge-transfer kinetics of Re dyes

Figure 1. Electron injection kinetics of three dyes with a series of linkers varying in length and degree of conjugation.

Vibrational spectroscopy is a powerful technique for disentangling the many factors that control the injection kinetics because vibrational modes of the dye molecule are sensitive to both the electronic state and to the local environment of the dye molecule on the surface. In our group, we have used 2D IR and transient 2D IR spectroscopy to study the interfacial structure and relate it to the injection rate, shown in Figure 2.

Transient 2D IR of Re1c dye

Figure 2. (a) 2DIR pulse sequence and corresponding spectrum,(b) t2DIR pulse sequence and corresponding spectrum of Re1C dye attatched a TiO2 nanocrystalline thin film.

Utilizing both 2D IR and density functional theory (DFT), we have explored the relative contributions of binding geometry and intermolecular interactions at the interface. DFT calculations show that binding geometry alone is inadequate to explain the surface coverage-dependent trends in our experimental spectra, as seen in Figure 3.

Figure 3: 2D IR spectra of a single linked dye, ReC (a,b,c) and a double linked dye, ReCC (d,e,f) at full monolayer coverage (a,d) and submonolayer coverages (b,c,e,f).

Instead, they suggest that intermolecular coupling is a more likely explanation for the observed trends, as seen in Figure 4.

Figure 4: Comparison of DFT results and 2D IR results at low surface coverage (top) and high surface coverage (bottom).

Ongoing work seeks to further elucidate the extent of intermolecular copulings and its effect on electron injection processes.