TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: In the method (M1), providing the metal cation solution involves dissolving a metal in an acid or dissolving a metal salt in water; and providing hydroxide anions and carbonate anions to the metal cation solution involves: bubbling a gas into the metal cation solution and/or adding a solution of the hydroxide anions and the carbonate anions into the metal cation solution. The nano metal salt has a size of 1-500 nm. In the method (M2), no chemical reduction is performed for the layer of the nano metal salt before the step of selenizing the layer of the nano metal salt. Preferred Components: The metal cation comprises group IB metal ion, group IIIA metal ion, group IIB metal ion and/or group IVA metal ion. The acid comprises acetic acid, oxalic acid, hydrochloric acid, sulfuric acid and/or nitric acid. The gas comprises carbon monoxide, carbon dioxide and/or ammonia. The solution of the hydroxide anions and the carbonate anions comprises ammonium hydrocarbonate solution, lithium hydrocarbonate solution, sodium hydrocarbonate solution and/or potassium hydrocarbonate solution. The absorption layer of the solar cell is copper-indium-gallium-selenide layer or a copper-zinc-tin-selenide layer. The metal salt comprises Cu2(OH)2CO3, Cu3(OH)2(CO3)2, NH4Ga(OH)2CO3, NH4Al(OH)2CO3, In(OH)3asteriskxCO3 (where x is 0-3), Zn7(OH)10(CO3)2, Zn5(OH)6(CO3)2, Zn4CO3(OH)6asteriskH2O, Sn6O4(OH)4asteriskxCO3 (where x is 0-3), Na2Sn2(OH)4, K2Sn(OH)6, Na2Sn(OH)6, (NH4)2Cu2In(2-x)Ga2x(OH)6(CO3)3 (where x is 0-2), or Cu5Zn(5-2.5x)Sn2.5x(OH)9(CO3)3 (where x is 0-2).    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The acid comprises acetic acid and/or oxalic acid. EXAMPLE - 0.5 Mole of copper nitrate and 2 mole of ammonium hydrocarbonate were dissolved in water to form solutions, respectively. The copper nitrate solution and the ammonium hydrocarbonate solution were mixed to precipitate a solid. The solid was collected by centrifugation, then washed by water to remove its excess anions, and then dried to obtain Cu2(OH)2CO3.