NOVELTY - An aqueous solution composition comprises thiol-containing compound (I), and metal ion capable of forming poorly soluble sulfide.    USE - Aqueous solution composition is used for preparing metal sulfide particles and metal sulfide thin film for photoelectric conversion element (all claimed) e.g. solar cell.    ADVANTAGE - The aqueous solution composition provides metal sulfide having reduced amount of impurities with quick reaction rate by solution precipitation method such as chemical bath deposition process even at low reaction temperature. The photoelectric conversion element obtained using aqueous solution composition has reduced characteristic falls and time-independent deterioration.    DETAILED DESCRIPTION - An aqueous solution composition comprises thiol-containing compound of formula: R1,R2-N-C(=S)-N-R3,R4 (I), and metal ion capable of forming poorly soluble sulfide.    R1-R4=H or hydrocarbyl having hetero atom, in which at least one of R1-R4 is group other than H or thiourea derivative containing N.    INDEPENDENT CLAIMS are included for the following:    (1) metal sulfide thin film, which has maximal absorption intensity (IR-2) of 2000-2100 cm-1, maximal absorption intensity (IR-1) of 1000-1200 cm-1, maximal absorption intensity (IR-3) of 1500-1700 cm-1, the ratio of maximal absorption intensity (IR-1) and maximal absorption intensity (IR-2) of less than 0.5, and the ratio of maximal absorption intensity (IR-1) and maximal absorption intensity (IR-3) of less than 0.5, in infrared absorption spectrum;    (2) metal sulfide microparticles, which has maximal absorption intensity (IR-2) of 2000-2100 cm-1, maximal absorption intensity (IR-1) of 1000-1200 cm-1, maximal absorption intensity (IR-3) of 1500-1700 cm-1, the ratio of maximal absorption intensity (IR-1) and maximal absorption intensity (IR-2) of less than 0.5, and the ratio of maximal absorption intensity (IR-1) and maximal absorption intensity (IR-3) of less than 0.5, in infrared absorption spectrum; and    (3) photoelectric conversion element (10), which has metal sulfide thin film as a buffer layer (18).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of photoelectric conversion element.    Photoelectric conversion element (10)    Substrate (12)    Lower electrode (14)    Photoelectric converting layer (16)    Buffer layer (18)