NOVELTY - A reaction solution (S1) comprises a component (A1) of at least one zinc source; a component (A2) of at least one sulfur source; a component (A3) of at least one citrate compound; a component (A4) of at least one ammonia or ammonium salt; and water. The concentration of the component (A3) is 0.001-0.25 M; the concentration of the component (A4) is 0.41-1.0 M; and the pH before the start of reaction is 9.0-12.0.    USE - As reaction solution for use in manufacturing a Zn compound layer consisting primarily of Zn(S, O) and/or Zn(S, O, OH) included in a buffer layer of a photoelectric conversion device useful for a solar cell (claimed).    ADVANTAGE - The reaction solution prevents from becoming opaque white as long as possible by preventing the generation of colloidal particles, thus allowing a Zn system buffer layer well covering an underlayer to be formed. The reaction solution has a transmission factor of greater than or equal to 15% measured with a wavelength of 550 nm in a quartz cell having a light path length of 10 mm after maintained at 90 plus minus 3 degrees C for 10 minutes or 30 minutes. Use of component of at least one citrate compound allows complexes to be formed easily without use of an excessive amount of ammonia in compassion with reaction solutions that do not use a citrate compound, thus crystal growth by CBD reaction is well controlled and a film well covering an underlayer can be formed stably. The method allows processing of more substrates with the same reaction solution, resulting in increased production efficiency. The pH of the reaction solution is not a strong acid condition. The reaction temperature required is not so high. Thus, the reaction has less environmental burden and the damage to the substrate is reduced to a minimum. The anodized substrates do not have high acid or alkali resistance. The method can perform reaction under a moderate pH condition, so that, even when such a substrate is used, the method does not give damage to the substrate and provides high quality photoelectric conversion device.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a method (M1) of manufacturing a buffer layer of a photoelectric conversion device having a stacked structure in which a lower electrode (20), a photoelectric conversion semiconductor layer (30) that generates a current by absorbing light, the buffer layer (40), a translucent conductive layer (60), and an upper electrode (70) are stacked on a substrate (10) involving i) forming a fine particle layer (41) of several of one kind or at least two kinds of fine particles consisting primarily of at least one selected from ZnS (zinc sulfide), Zn(S, O), and Zn(S, O, OH) on the photoelectric conversion semiconductor layer; ii) preparing reaction solution (S1); and iii) forming a Zn compound layer (42) consisting primarily of Zn(S, O) and/or Zn(S, O, OH) on the fine particle layer by a liquid phase method with a reaction temperature of 70-95 degrees C, using the reaction solution (S1), where in the preparation step, an aqueous solution (I) which includes the component (A1), an aqueous solution (II) which includes the component (A2), an aqueous solution (III) which includes the component (A3), and an aqueous solution (IV) which includes the component (A4) are prepared separately, then the aqueous solutions (I), (II), and (III) are mixed to obtain a mixed solution, and the aqueous solution (IV) is mixed in the mixed solution;    (2) a reaction solution preparation method involving preparing the aqueous solutions (I), (II), (III), and (IV); and mixing the aqueous solutions (I) to (IV), where, in the aqueous solution mixing step, the aqueous solutions (I), (II), and (III) are mixed to obtain a mixed solution and the aqueous solution (IV) is mixed in the mixed solution; and    (3) manufacturing a photoelectric conversion device having a stacked structure in which a lower electrode, a photoelectric conversion semiconductor layer that generates a current by absorbing light, a buffer layer, a window layer, a translucent conductive layer, and an upper electrode are stacked on a substrate, where the buffer layer is manufactured by the method (M1).    DESCRIPTION OF DRAWING(S) - The figure shows a schematic cross-sectional view of the photoelectric conversion device    Substrate (10)    Lower electrode (20)    Photoelectric conversion semiconductor layer (30)    Buffer layer (40)    Fine particle layer (41)    Zinc compound layer (42)    Translucent conductive layer (60)    Upper electrode (70)