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

摘要

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)(+) + 4H(2)O + 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-tentbutylpyridine. 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.