NOVELTY - Purifying a dye (A) for use in dye-sensitized solar cells (DSSC), comprises: (a) providing the dye, which is useful as sensitizer in DSSC; (b) converting the dye into a soluble form, preferably a water-soluble form, by adding an ammonium compound (1a); (c) purifying the soluble form of the dye by reversed-phase chromatography, preferably using HPLC; (d) isolating the dye by acid precipitation; and (e) dissolving the isolated dye in a solvent to provide a dye-solution and adjusting the pH of the dye solution to 4-10.    USE - The dye/(II) is useful as sensitizer for producing the dye-sensitized solar cell (all claimed).    ADVANTAGE - The method provides dye characterized by analytical HPLC showing HPLC-purity of higher than 99%. The dye can be prepared using one-pot method, thus the method involves less synthetic steps. The method is reliable, cost efficient and fast for preparing sensitizer dyes of high quality. The method allows up-scaling and automated purification, since dye shows high solubility even in water. The method: does not use expensive chromatography material; and is ecofriendly, since organic solvents can be partially replaced by water during the purification process.    DETAILED DESCRIPTION - Purifying a dye (A) for use in dye-sensitized solar cells (DSSC), comprises: (a) providing the dye, which is useful as sensitizer in DSSC; (b) converting the dye into a soluble form, preferably a water-soluble form, by adding an ammonium compound of formula (NR4-OH) (1a); (c) purifying the soluble form of the dye by reversed-phase chromatography, preferably using HPLC; (d) isolating the dye by acid precipitation; and (e) dissolving the isolated dye in a solvent to provide a dye-solution and adjusting the pH of the dye solution to 4-10.    R = H or 4-12C alkyl, preferably butyl.    INDEPENDENT CLAIMS are included for:    (1) one-pot method of synthesizing cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium (II) (red dye) comprising (a1) providing, in any order, dimeric (p-cymol)-ruthenium (II) chloride and 2,2'-bipyridine-4,4'-dicarboxylic acid; (b1) allowing the dimeric (p-cymol)-ruthenium (II) chloride and 2,2'-bipyridine-4,4'-dicarboxylic acid to react in a single reaction mixture, and (c1) adding a thiocyanate salt to the reaction mixture and allowing the reaction mixture to react to yield red dye;    (2) the dye (A) purified/prepared by the method and having no impurities, preferably detectable in NMR-spectrum; and    (3) a solution (II) of the purified dye and having a pH of 4-11, preferably 4-10.