NOVELTY - Preparing (P1) a sandwich layer containing copper, zinc, tin and optionally additionally at least one chalcogen involves sequentially depositing copper monolayers, zinc monolayers and tin monolayers, and optionally at least one chalcogen to at least one of the monolayer by using wet-chemical metal plating compositions to form sandwich layer.    USE - For preparing sandwich layer containing copper, zinc and tin useful in thin film solar cell (claimed).    ADVANTAGE - The process is safe and easy; avoids the use of toxic substances such as cyanide compounds; and provides protection against corrosion. The process effectively produces the thin film solar cell having ubiquitous material.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a metal plating composition (C1) for the deposition of a copper-zinc-tin alloy, optionally containing at least one chalcogen comprising at least one copper plating species, at least one zinc plating species, at least one tin plating species and at least one complexing agent. The metal plating composition (C1) additionally comprising at least one additive, selected from disubstituted benzene compound of formula (I);    (2) depositing (P2) of a copper-zinc-tin alloy optionally containing at least one chalcogen involving contacting a substrate and an anode with the metal plating composition (C1) and flowing an electric current between substrate and the anode;    (3) a thin film solar cell comprising a substrate film, optionally a barrier layer deposited on the substrate film, which serves as an electrical isolator or as a diffusion barrier to prevent any constituent of the substrate to diffuse there through, an electrically conductive back contact layer, a p-type absorber layer and copper-zinc-tin alloy further comprising at least one chalcogen and having chemical formula CuxZnySnzSa or CuxZnySnzSaSeb, at least one n-type buffer layer and at least one window layer;    (4) a copper-zinc-tin alloy of CuxZnySnzSaSeb.    R1 and R2=OH, SH, NR3R4, CO-R5, COOR5, CONR3R4, COSR5, SO2OR5, SO2R5, SO2NR3R4;    R1+R2=condensation chain;    R3 and R4=H or alkyl;    R5=H, alkyl or hydroxyalkyl;    X=1.5-2.5;    y=0.9-1.5;    z=0.5-1.1;    a=3.8-4.2;    b=0.1-4.2.