TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The method further comprises washing the nanoparticles in a washing reagent prior to the step (a). The nanoparticles are prepared by reacting in an aqueous solution, a water-soluble salt of Cu and a water soluble compound containing an anion of X. The reaction is carried out in the presence of a capping ligand for the nanoparticles. The preparation of the nanoparticles is carried out at 25 degrees C. The dispersion is: essentially homogeneous; deposited on the substrate by spray coating (preferred), inkjet printing, drop-casting, dip-coating, sputtering, painting or spin coating; and deposited in a layer by layer fashion on the substrate to form a graded precursor thin film. The thickness of the precursor thin film and the Cu-Zn-Sn-X thin film is 100 nm to 10 mu m. The Cu-Zn-Sn-X thin film has an average grain size of 20-200 nm. The Cu-Zn-Sn-X thin film comprises essentially of copper-zinc-tin-zinc-sulfide. The film is a graded Cu-Zn-Sn-X thin film. Preferred Components: The molar ratio of: Cu and (Zn+Sn) is 0.75:1-1.25:1; Zn and Sn is 1.5:1-0.75:1; Cu, Zn and Sn is 2.5:1.5:1-1.9:0.9:1; and Cu, Zn and Sn is 2.025:1.25:1. The water-soluble salt of Cu is a member comprising copper chloride, copper sulfate, copper nitrate and/or copper acetate. The ZnX nanoparticles are prepared by reacting in an aqueous solution, a water soluble salt of Zn and a water soluble compound containing an anion of X. The SnXq nanoparticles are prepared by reacting in an aqueous solution, a water-soluble salt of Sn and a water-soluble compound containing an anion of X. The water-soluble salt of Zn is a member comprising zinc chloride, zinc sulfate, zinc nitrate and/or zinc acetate. The water-soluble salt of Sn is a member comprising tin chloride, tin sulfate, tin nitrate and/or tin acetate. The capping ligand comprises a water soluble surfactant. The water-soluble compound containing an anion of S comprises a compound comprising sodium sulfide nonahydrate or potassium sulfide (both preferred), lithium sulfide, sodium sulfide, rubidium sulfide, cesium sulfide, ammonium sulfide, lithium sulfide monohydrate, potassium sulfide nonahydrate, rubidium sulfide tetrahydrate and/or cesium sulfide tetrahydrate. The substrate comprises a material comprising glass or silicon (both preferred), metal foil or ceramic.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The water-soluble surfactant comprises a compound comprising cetyltrimethylammonium bromide (preferred), dodecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride, hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, octadecyltrimethylammonium bromide, octadecyltrimethylammonium chloride, cetyltrimethylammonium chloride, cetylbenzyldimethylammonium chloride and/or cetyltriethylammmonium bromide. The aqueous solution consists essentially of deionized water or distilled water. The washing reagent comprises an alcohol. The dispersing agent comprises water, an alcohol or its mixture. The alcohol comprises ethanol or isopropyl alcohol (both preferred), methanol, propanol, butanol, amyl alcohol, hexanol, 2-ethoxyethanol, benzyl alcohol, optionally substituted cyclopentanol, cyclohexanol and cycloheptanol and/or glycerol.