Stability of the oxidized form of RuLL′(NCS)2 dyes in acetonitrile in the presence of water and pyridines – Why the dye-sensitized solar cell electrolyte should be dry

The detrimental effect of electrolyte water contamination on the light-soaking lifetime of Dye-sensitized Solar Cells (DSCs) prepared with RuLL′(NCS)₂ dyes and N-additives like 4-tert-butylpyridine (TBP) is not well understood. A new explanation is presented based on investigation of the stability of the ruthenium(III) complexes Ru(bipy)₂(NCS)₂⁺ (1⁺) and RuLL′(NCS)₂⁺ (L = 4,4′-dicarboxy-2,2′-bipyridine, L′= 4,4′-nonyl-2,2′-bipyridine) (Z907⁺) in acetonitrile in the presence of water and pyridines covering a large variation in basicity. 1⁺ reacts with small amounts of water in the acetonitrile containing a pyridine base (X) according to the overall reaction: 6Ru(bipy)₂(NCS)₂⁺ + 4H₂O + 8X → 5Ru(bipy)₂(NCS)₂ + Ru(bipy)₂(NCS)(CN) + SO₄²⁻ + 8XH⁺. The reaction mechanism of 1⁺ (and Z907⁺) is proposed to be initiated by an attack of OH⁻ giving Ru(bipy)₂(NCS)(NCS-OH). The stronger the base the faster the reaction. Extrapolating the life time of Z907⁺ to a typical TBP concentration of 0.5 M in the DSC gives a degradation rate around 7 s⁻¹. Z907⁺ bound to a layer of nano crystalline TiO₂ surface reacted fast too, when inserted in an acetonitrile solution containing 4-tert-butylpyridine. In a "wet" electrolyte, containing more than 500 mM of water the light-soaking lifetime of a DSC prepared with Z907 is predicted to be about 10 days at out-door light soaking conditions, whereas trace amounts of water ( < 25 mM) in a "dry" electrolyte is used up by consumption of only 10% of the Z907 in a typical DSC. Therefore, the DSC is expected to have a long light-soaking lifetime with a "dry" electrolyte.