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

Abstract

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.