NOVELTY - Purifying (M1) a dye for use in dye-sensitized solar cells (DSSC), involves: (A) providing a dye which is useful as sensitizer in DSSC; (B) converting the dye into a soluble form, preferably a water-soluble form, of the dye by adding ammonium hydroxide; (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 dye resulting from step (D) in a solvent to provide a dye-solution and adjusting the pH of the dye solution to 4-10.    USE - For purifying a dye useful in dye-sensitized solar cells (DSSC) (claimed).    ADVANTAGE - The dye purified by the method or prepared by the method having no impurities, preferably no impurities detectable in an NMR-spectrum. The method for preparation is applicable on large scale of sensitizer dyes conventionally used in dye-sensitized solar cells (DSSC). The method is reliable, cost efficient and fast method for preparing sensitizer dyes of high purity and quality for use in DSSCs; is automatable in order to ensure the reliable processing of DSSCs in production lines with high efficiencies; allows up-scaling and automated purification because it circumvents (e.g. for the red dye) the insolubility of the dye in methanol by purifying the dye in its soluble form 4tetrabutylammonium (TBA) red dye, which shows high solubility even in water. The synthesis of red dye is performed using a one-pot method, and thus involves less synthetic steps (only one). The method does not use expensive chromatography material, such as Sephadex LH-20 and is also environmentally friendly, since organic solvents can be partially replaced by water during the purification process.    DETAILED DESCRIPTION - Purifying (M1) a dye for use in dye-sensitized solar cells (DSSC), involves:    (A) providing a dye which is useful as sensitizer in DSSC;    (B) converting the dye into a soluble form, preferably a water-soluble form, of the dye by adding ammonium hydroxide compound of formula N(R)4-OH;    (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 dye resulting from step (D) in a solvent to provide a dye-solution and adjusting the pH of the dye solution to 4-10.    R=H or alkyl (preferably 4-12C alkyl, especially butyl).    INDEPENDENT CLAIMS are included for the following:    (1) one-pot method of synthesizing cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II) (red dye) involves: a1) providing, in any order, dimeric (para-cymol)-ruthenium(II)chloride and 2,2'-bipyridine-4,4'-dicarboxylic acid; b1) allowing the dimeric (para-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 mixture to react to yield red dye;    (2) a solution (S1) of a dye purified by the method (M1) and having a pH of 4-11 (preferably 4-10);    (3) a dye obtained by evaporating (M2) the solvent from the solution (S1); and    (4) a dye-sensitized solar cell produced using the dye obtained by method (M2) or produced using directly the solution (S1).