Strong Interactions in Dye-Sensitized Interfaces

Abstract

Phthalocyanines (Pcs) are capable of converting sunlight into electric energy when adsorbed on TiO 2 in a dye-sensitized solar cell. Of special interest in this type of cell is the energy level alignment as well as how molecules adsorb on the surface as it determines the output of the cell. We investigated the FePc-TiO 2 (110) interface using scanning tunneling microscopy, synchrotron-based photoelectron spectroscopy, and X-ray absorption spectroscopy. We found a strong coupling of the first-layer FePc to the substrate resulting in an alteration of the electronic structure and charge transfer from the molecules. The FePc in the second layer is not severely affected by the bonding to the surface and has bulk-like electronic properties. The growth of FePc thin films proceeds in a layer plus island mode, and the molecular plane is parallel to the surface. The energy level alignment at the interface is determined, and the lowest unoccupied molecular orbital is found above the conduction band minimum of the oxide substrate.

Inhomogeneous charge transfer within monolayer zinc phthalocyanine absorbed on TiO2(110)

“…30 Based on our previous study on the 4 tert-butyl pyridine on TiO 2 , oxygen vacancies should still exist according to the similar sample preparation procedure. 31 At low coverage the molecular film does not display any HOMO structure, similar to FePc, 13 indicating that HOMO has a lower electron density for interfacial molecules, as previously suggested by the core level shifts and the absence of shake-up structure in the interfacial component. As the coverage increases, the HOMO gradually emerges at 1.4 eV aligned with the HOMO of the thick film.…”

“…Similar to FePc/TiO 2 , no shake-up structure is observed for the interfacial components. 13 In Figure 2(d), the N1s shift between interfacial molecules and bulk-like molecules is about 1.7 eV.…”

“…13 The shifts between interfacial and bulk-like molecules were: 1.2 eV for C B , 1.1 eV for C P and 1.3 eV for N1s. 13 In the present case, the C B shift is considerably smaller than the N1s splitting. In order to extract the binding energy of each component, curve fitting was done.…”

“…• which may block the direct contact between Zn and substrate; whereas, FePc has a smaller contact angle seen by STM, 13 facilitating the central atom − substrate orbital hybridization. Thirdly, theoretical calculations 25 show that the bond length between the Fe and N 3c (∼1.92 Å) is shorter than that between Zn and N 3c (∼1.98 Å), which may also intrinsically influence the screening.…”

“…TiO 2 is usually used as the working electrode for such devices. Previously, we have found that Pc molecules (FePc, 13,14 TiOPc, 15 and H 2 Pc (Ref. 16)) suffer from electron depletion when absorbed on TiO 2 .…”