TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The copper-indium-compound seed particles comprise copper indium sulfide, copper indium selenide, copper indium gallium (di)sulfide, copper indium gallium (di)selenide, and copper indium gallium (di)selenide sulfide. The water soluble copper precursor comprises copper(II) chloride, copper(II) nitrate, copper (II) sulfate, copper acetate, and their hydrates. The water-soluble indium precursor comprises indium chloride, indium nitrate, indium acetate, indium(III) sulfate and their hydrates. The water-soluble gallium precursor comprises gallium trichloride, gallium(III) nitrate, gallium iodide and their hydrates. The water-soluble sulfur precursor is thiourea or its hydrate. The water-soluble selenium precursor comprises selenourea, selenium dioxide, selenious acid, selenium tetrachloride and their hydrates.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The step of forming the thin film: at high temperature is performed using hydrothermal synthesis; and using hydrothermal synthesis is performed at 70-250 degrees C for 1-175 hours. Preferred Components: The solution comprises water or a buffer. In the method, a volume ratio of the water-soluble precursor to the water or buffer of the solution is 1:1-1:10000. The solution further comprises a structure directing agent. EXAMPLE - Copper chloride, indium chloride, gallium chloride, and selenourea were mixed at a molar ratio of 1:1:1:4 in distilled water to form a sample solution. The obtained sample solution was then vigorously stirred for 30 minutes to became homogeneous, and then substrate having the seed particle layer was located at the bottom of a vessel, and then 80% of the vessel was filled with the stirred sample solution. Subsequently, the vessel was placed in an autoclave made of stainless steel, so that hydrothermal synthesis was carried out at 150 degrees C for 18 hours, and fabricating a copper indium gallium selenium, copper indium gallium sulfur or copper indium gallium selenium sulfur thin film.