Cation　substitution　plays　a　crucial　role　in　improving　the　efficiency　of　Cu2ZnSn(S,Se)4　(CZTSSe)　solar　cells.　In　this　work,　we　report　a　significant　efficiency　enhancement　of　flexible　CZTSSe　solar　cells　on　Mo　foils　by　partial　substitution　of　Cu+　with　Ag+.　It　is　found　that　the　band　gap　(Eg)　of　(Cu1-xAgx)2ZnSn(S,Se)4　(CAZTSSe)　thin　films　can　be　adjusted　by　doping　with　Ag　with　x　from　0　to　6%,　and　the　minimum　Eg　is　achieved　with　x　=　5%.　We　also　found　that　Ag　doping　can　obviously　increase　the　average　grain　size　of　the　CAZTSSe　absorber　from　0.4　to　1.1　μm.　Additionally,　the　depletion　width　(Wd)　at　the　heterojunction　interface　of　CAZTSSe/CdS　is　found　to　be　improved.　As　a　result,　the　open-circuit　voltage　deficit　(Voc,def)　is　gradually　decreased,　and　the　band　tailing　is　suppressed.　Benefiting　from　the　enhanced　open-circuit　voltage　(Voc),　the　power　conversion　efficiency　(PCE)　is　successfully　enhanced　from　4.34%　(x　=　0)　to　6.24%　(x　=　4%),　and　the　Voc,def　decreases　from　915　to　848　mV.　©　2018　The　Royal　Society　of　Chemistry.