Cation substitution is a promising approach to improve the efficiency of kesterite solar cells. Here, silver has been introduced into Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film to replace Cu partially and form (Ag_xCu_1-x)₂ZnSn(S,Se)₄ (ACZTSSe) through a cost-effective dimethyl sulfoxide-based solution method. Fundamental properties of the mixed Ag-Cu kesterite compound as a function of the Ag/(Ag + Cu) ratio were investigated. It is demonstrated that the varying Ag content significantly impacts the band gap and crystal structure of ACZTSSe thin films. Furthermore, a remarkable decreasing trend in the intrinsic hole concentration accompanies the increasing Ag incorporation, which eventually leads to a doping-type conversion in high Ag containing layers. The changing bulk properties of absorber layers due to the Ag alloying have strong influences on device performance. Benefiting from the enhanced V OC, the power conversion efficiency is improved from 4.77% for the baseline CZTSSe cell to 7.12% for an ACZTSSe device containing 16% Ag, and about a 30 mV smaller VOC deficit is achieved. The VOC enhancement evidences that the solution method is facile and viable to achieve proper cation substitution toward higher efficiency kesterite solar cells.