TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The precursor of at least one element of groups 12-14 elements is chloride, nitrate, sulfate, acetate, phosphate, silicate, and/or hydrochloride. The precursor of at least one element of groups 12-14 elements is a third chalcogenide of one or at least two elements of groups 12-14 elements. The composite particle contains 10-900 pts. wt. of the first chalcogenide based on 100 pts. wt. of the second chalcogenide. A molar ratio of at least one element of groups 12-14 elements and contained in the ink to the group 11 metal contained in the first and second chalcogenides is 1:0-1.2. The first chalcogenide includes cuprous selenide and the second chalcogenide includes copper selenide and/or copper diselenide. The third chalcogenide includes one or at least two particles comprising indium gallium sulfur selenide of formulae ((InxGa1-x)4(SySe1-y)3) (a1), ((InxGa1-x)(SySe1-y)) (a2), ((InxGa1-x)6(SySe1-y)3) (a3) or ((InxGa1-x)5(SySe1-y)7) (a4), or tin-sulfur-selenide of formulae (Sn(SySe1-y)) (b1), (Sn2(SySe1-y)3) (b2) or (Sn(SySe1-y)2) (b3), or zinc-sulfur-selenide of formula (Zn(SySe1-y)) (c1).x, y = 0-1.The group 12 element includes zinc, the group 13 element is at least one of indium or gallium, the group 14 element includes tin, and a chalcogen element is at least one of sulfur or selenium. A molar ratio of one or at least two elements of groups 12-14 elements and contained in the ink to copper is 1:0.7-1.2. Preferred Method: The first chalcogenide maintains a solid phase at the time of the heat treatment, and the second chalcogenide maintains a molten phase at the time of the heat treatment. The second chalcogenide has a melting point of 220-550 degrees C. The heat treatment is performed at 400-550 degrees C under chalcogen atmosphere. The composite particle including the first and second chalcogenides is prepared by preparing a first solution containing a precursor of a group 11 metal and a second solution containing a surfactant, which is trioctylphosphine, trioctylphosphine oxide and/or their acids, and a chalcogenide precursor and then injecting the first solution into the second solution having a temperature of 160-240 degrees C. The copper nanoparticles are prepared by heating and stirring a copper precursor solution containing a copper precursor, acid, and amine and then injecting a reducing agent into it to be capped with acid and amine. A single phase multinary chalcogenide is formed by the heat treatment. EXAMPLE - 50 ml Oleylamine was put into two flasks, respectively, and a temperature was fixed to 60 degrees C. Then, 10 mmol copper(II) chloride was added to a first flask, and 3 g of 77 mmol trioctylphosphine oxide and 10 mmol selenium powder were added to a second flask. Next, after the first flask was heated to 130 degrees C and the second flask was heated to 200 degrees C, cuprous selenide nanoparticles containing copper selenide second phase were synthesized by instantaneously injecting a solution in the first flask into the second flask and maintaining the temperature to 200 degrees C for 1 hour. Then, the synthesized particles were separated using a centrifugation method and then washed with toluene. The cuprous selenide nanoparticles containing the copper selenide second phase were added to tetrahydrofuran, so that a dispersion solution containing 20 wt.% of copper chalcogenide particles (nanoparticles in which copper selenide and cuprous selenide were mixed with each other) was prepared. Indium(III) chloride was added to the prepared dispersion solution so that a molar ratio of copper in the copper chalcogenide particles (copper selenide and cuprous selenide nanoparticles) to indium was 1:1, to give ink. At this time, for homogeneous mixing of the ink, ball milling was performed at 20 Hz for 60 minutes.