Chromatographic studies of photodegradation of RuL2(SCN)2 in nanostructured dye-sensitization solar cells

Abstract

The claim that ruthenium complexes attached to nanostructured TiO 2 can sustain 10 8 electron transfer cycles is in conflict with photocurrent imaging studies, which clearly show photoinduced degradation of solar cell efficiency. HPLC chromatographic studies of the cis-RuL 2 (SCN) 2 with L = 2,2-bipyridyl-4,4-dicarboxylatoacid (bpca) in solution and extracted from longterm operating solar cells were undertaken to identify the spectrum of possible photochemical products. While a dramatic increase of photochemical stability of the ruthenium complex when attached to TiO 2 nanoparticles is confirmed with respect to its photochemical reactivity in solution, photoelectrochemical products of the ruthenium complex can clearly be identified after one to a few months of solar cell operation under simulated solar light. The interpretation is that the ruthenium complex when adsorbed to TiO 2 nanoparticles is exposed to different surface states, which warrant different stability conditions to the oxidized ruthenium intermediate. To cite this article: M.

Interface Stability of a TiO2/3‐Methoxypropionitrile‐Based Electrolyte: First Evidence for Solid Electrolyte Interphase Formation and Implications

“…[35] The chemical stability of thiocyanate-based polypyridyl ruthenium complexes N719 and Z907 has been reported in particular by the groups of Tributsch, Hagfeldt, and Lund, and these authors became unanimously alarmed at the vulnerability of the strong electron donor thiocyanate ligand that can exchange at temperatures above 80 8C with many electrolyte components, that is, nitrile-based solvent and/or fragmented parts of the solvent, 4-tert-butylpyridine, water residues, or even iodide. [28,29,[36][37][38][39][40][41][42] Although such ligand-exchange reactions affect the metal-to-ligand charge transfer light absorption, which entails either a slight bathochromic or hypsochromic shift in absorption, [26] the isolated implication of this in situ dye structure modification on the practical cell characteristics remains unclear. Herein, we report on a study with the aim of providing in-depth evidence on the stability of the interface between anatase TiO 2 and the electrolyte; the novel observations highlighted herein call for significant interfacial engineering between TiO 2 , the sensitizer, and the electrolyte to strengthen device stability.…”

Resonant Raman study of dye instability in dye‐sensitized TiO2 system: The effect of surface states

“…The N 3 dye stability depends critically on its chemical state and local environment in a DSSC. Ruthenium complexes may only sustain no more than 100 electron transfer processes in a solution 4, 13, while it can sustain 10 7 –10 8 redox cycles before irreversible degradation occurs when it is properly bonded to the TiO 2 surface 14. In a DSSC device, the rate of dye electron injection into the TiO 2 conduction band is in femtosecond regime due to both the energy‐level matching and the good chemical bonding between the dye and the TiO 2 surface 8.…”

“…The redox couple is reduced at the counter‐electrode. It is therefore supposed that the stability of a DSSC is very sensitive to the TiO 2 surface states and/or defects 4, 13. The most common defects on anatase TiO 2 surface are oxygen vacancy and related Ti 3+ .…”

“…The increase of Fermi‐energy level not only lowers the electron injection rate from the dye, but also enhances electron back injection 28. Carboxylic group bonded to the titanium, which, in its reduced form of Ti 3+ , can engage in transition metal back bonding 13. The surface site Ti 3+ was reported to support transport of electron and hole between dye molecule and photoelectrode 17.…”

A Review on Current Status of Stability and Knowledge on Liquid Electrolyte-Based Dye-Sensitized Solar Cells

“…These authors became consensually alarmed on the vulnerability of the strong electron donor thiocyanate ligand which can easily exchange with electrolyte components as a result of the antibonding character of these orbitals. This ligand exchange reaction is triggered not only by light action (photolysis) but also can be activated by temperatures above 80 ∘ C. The monodentate thiocyanate ligand can then exchange with acetonitrile or 3-methoxypropionitrile when used as a solvent (in integral or fragmented part such as C≡N), with 4-tertbutylpyridine, water residues, or even iodide [165][166][167][168][169][170][171][172][173]. The different exchange reactions reported are summarized in Figure 14.…”