Factors that Control the Direction of Excited-State Electron Transfer at Dye-Sensitized Oxide Interfaces

Abstract

Molecular excited states at conductive and semiconductive interfaces were found to transfer an electron to the oxide (injection) or accept an electron from the oxide (hole transfer). The direction of this electron transfer was determined by the energetic overlap of the metal oxide and sensitizer redox-active states and their electronic coupling. Potentiostatically controlled mesoporous thin films based on a nanocrystalline conductive metal oxide [tin-doped indium oxide (ITO)] and semiconducting metal oxides (TiO2 and SnO2) were utilized with the sensitizers (S) [Ru­(bpy)2(P)]­Br2 and [Ru­(bpz)2(P)]­Br2, where bpy is 2,2′-bipyridine, bpz is 2,2′-bipyrazine, and P is 2,2′-bipyridyl-4,4′-diphosphonic acid. For dye-sensitized TiO2, excited-state injection [TiO2|S* → TiO2(e–)|S+] was exclusively observed, and the injection yield decreased at negative applied potentials. In contrast, evidence for both injection [ITO|S* → ITO­(e–)|S+] and hole transfer ([ITO|S* → ITO­(h+)|S–] is reported for ITO and SnO2. Hole transfer became more efficient with negative applied potentials. The direction of electron flow between the metal oxide and excited state sensitizer was correlated with the energetic overlap and the electronic coupling as predicted by Marcus−Gerischer theory. The data reveal that control of the Fermi level enables conductive oxides to function as a photocathode or as a photoanode for solar energy conversion applications.

Dye-sensitized solar cells strike back

“…For fundamental recombination studies, transparent conductive oxide (TCO) materials have some advantages. [119][120][121] They have a metallic character, which permits potentiostatic control of the Fermi level (E F ) and, consequently, of the driving force for charge recombination, ÀDG1 = nF(E1 0 À E F ). Quantifying k cr as a function of ÀDG1 allows analysis through Marcus-Gerischer theory and access to the total reorganization energy (l) and to the electronic coupling.…”

Perspectives on Dye Sensitization of Nanocrystalline Mesoporous Thin Films

“…For fundamental electron transfer, the TCO materials may serve as electron acceptors (n-type behavior) or as electron donors (p-type) . The Fermi level of the TCO is of relevance rather than E CB and/or trap states in semiconducting materials.…”

“…For fundamental electron transfer, the TCO materials may serve as electron acceptors (n-type behavior) or as electron donors (p-type). 125 The Fermi level of the TCO is of relevance rather than E CB and/or trap states in semiconducting materials. Hence a significant advantage of TCOs is that their metallic character allows potentiostatic control of the Fermi level (E F ) and thus the driving force for electron transfer, −ΔG°= nF(E°′ − E F ).…”

Intrinsic and Extrinsic Incorporation of Indium and Single‐Walled Carbon Nanotubes for Improved ZnO‐Based DSSCs

“…[3] Over the years the working principles of DSSCs have been firmly investigated and the above mentioned key parameters have been optimized in DSSCs. [7][8][9][10][11][12] Finding the right semiconductor material is one major research target besides identifying the best photosensitizing dye. [13,14] Two choices of semiconductors stand out with n-type materials, such as titanium dioxide (TiO 2 ) [15,16] or zinc oxide (ZnO), [17,18] and p-type materials, like nickel oxide (NiO) [19] or copper oxide (CuO).…”