Various methods are used to provide a desired doping metal concentration in a CIGS-containing ink when the CIGS layer is deposited on a photovoltaic device. When the doping metal is sodium, it may be incorporated by: adding a sodium salt, for example sodium acetate, together with the copper-, indium- and/or gallium-containing reagents at the beginning of the synthesis reaction of Cu(In,Ga)(S,Se)2 nanoparticles; synthesizing Cu(In,Ga)(S,Se)2 nanoparticles and adding a sodium salt to the reaction solution followed by mild heating before isolating the nanoparticles to aid sodium diffusion; and/or, using a ligand that is capable of capping the Cu(In,Ga)(S,Se)2 nanoparticles with one end of its molecular chain and binding to sodium atoms with the other end of its chain.