NOVELTY - Bath formulation (F1) for electroplating a copper-zinc-tin alloy or a copper/zinc/tin metal stack on conductive substrate comprises: solvent system that contains sodium and/or potassium salts of citrate mono-/di-/tri-basic; at least one ionic compound of copper, zinc, or tin, where the ionic compounds are salts of chloride, nitrate or sulfate; and at least one surfactant and aldehyde, where the surfactant comprises sodium dodecyl sulfate, N,N-dimethyl-N-dodecyl glycine betaine or polyethylene glycol, the aldehyde comprises propenal, aromatic aldehyde, benzaldehyde, or formaldehyde.    USE - The bath formulation is useful for electroplating a copper-zinc-tin alloy or copper/zinc/tin metal stack on a conductive substrate, or electroplating a selenium layer on a metal or semiconductor alloy substrate, and producing a copper-zinc-tin-sulfur-selenium alloy which is used in a p-type light absorbing material for solar cell (claimed).    ADVANTAGE - The bath formulation: reduces the cost and energy requirement for producing the solar cell; and simplifies the fabrication of the light absorption layers in the manufacturing process, and achieves scale up into large substrate with good conversion efficiency due to ease of control of film thickness and uniformity.    DETAILED DESCRIPTION - Bath formulation (F1) for electroplating a copper-zinc-tin alloy or a copper/zinc/tin metal stack on a conductive substrate comprises: a solvent system that contains sodium and/or potassium salts of citrate mono-/di-/tri-basic, where the salts of citrate are at 100-500 mM; at least one ionic compound of copper, zinc, or tin, where the ionic compounds are salts of chloride, nitrate or sulfate and are at 5-300 mM; and at least one surfactant and aldehyde, where the surfactant comprises sodium dodecyl sulfate, N,N-dimethyl-N-dodecyl glycine betaine or polyethylene glycol, the aldehyde comprises propenal, aromatic aldehyde, benzaldehyde, or formaldehyde, and the surfactants and aldehydes are at 0.1-10 mM. INDEPENDENT CLAIMS are also included for:    (1) an electrochemical method (M1) for depositing a copper-zinc-tin alloy or a copper/zinc/tin metal stack on a conductive substrate, comprising (a) providing a single bath or multiple baths of electrochemical cell containing the bath formulation (F1) and the conductive substrate, (b) maintaining an electroplating bath temperature at 20-60 degrees C, (c) maintaining an electroplating bath acidity at a pH of 3-7, and (d) applying an electrical potential to the electrochemical cell of 0.3-1 mA/cm2, where for depositing the copper-zinc-tin alloy, the ionic compounds in the bath formulation is more than one of copper, zinc, and tin compounds, and the deposition is performed by co-plating of the bath formulation containing more than one ionic compounds on the conductive substrate in the single bath, and for depositing the copper/zinc/tin metal stack, the ionic compounds in the bath formulation is one of copper, zinc or tin compounds, and the deposition is performed layer-by-layer by repeating steps (a)-(d) in multiple baths;    (2) a bath formulation (F2) for electroplating a selenium layer on a metal or semiconductor alloy substrate, comprising (a1) a solvent system that contains at least one of tetra-alkyl-quaternary ammonium salt and 20-60% of polar solvent, where the tetra-alkyl-quaternary ammonium salt comprises choline chloride, betaine, acetyl choline chloride, methyl triphenylphosphonium bromide, or tetrabutylammonium chloride, and the polar solvent comprises water, alcohol, carboxylic acid, amine, or amide, (b1) at least one selenium compound, where the selenium compounds comprise selenium tetrachloride, selenium dioxide, selenium sulfide, selenium oxychloride, selenous acid, or selenic acid, and the selenium compounds are at 5-100 mM, and (c1) at least one surfactant and aldehyde, where the surfactants comprise sodium dodecyl sulfate and or polyethylene glycol, the aldehydes comprise benzaldehyde or formaldehyde, and the surfactants and aldehydes are at 0.5-4 mM; and    (3) an electrochemical method for depositing a selenium layer on a metal or semiconductor alloy substrate, comprising providing a single bath of electrochemical cell containing the bath formulation (F2) and the alloy substrate, (i) maintaining an electroplating bath temperature at 60-120 degrees C, (ii) maintaining an electroplating bath acidity at a pH of 6-10, and (iii) applying an electrical potential to the electrochemical cell of 3-10 mA/cm2.