Partial　cation　substitution　is　an　effective　way　to　inhibit　defects　and　carrier　recombination,　which　can　improve　the　efficiency　of　Cu2ZnSn(S,Se)4　(CZTSSe)　solar　cells.　Herein,　flexible　Cu2Zn1−xCdxSn(S,Se)4　(x　=　0–15%)　solar　cells　are　fabricated　on　Mo　foils　with　partial　Cd　substitution　for　Zn　via　a　green　solution-process.　The　best　device　performance　can　be　achieved　when　Cd/(Zn　+　Cd)　=　8%,　with　an　efficiency　up　to　6.49%　and　a　significantly　improved　device　repeatability.　The　EU　decreases　from　24　to　15　meV,　indicating　that　antisite　defects　and　band　tailings　are　effectively　suppressed.　C–V　data　reveal　that　Wd　and　Vbi　are　enhanced　after　doping　Cd,　resulting　in　a　stronger　built-in　electric　field　which　facilitates　Fermi-level　splitting　and　hence　increases　band　bending　of　the　absorber　toward　the　junction　interface.　Furthermore,　the　mechanism　of　current　shunting　is　studied　using　an　equivalent　circuit　model　with　three　parallel　current　pathways　to　fit　J–V　curves.　The　key　parameters　for　the　solar　cell　diode　such　as　A,　J0,　and　Rsh　are　significantly　improved　by　partially　substituting　Zn　with　Cd,　demonstrating　that　current　shunting　loss　is　suppressed　and　the　junction　quality　is　improved,　resulting　in　a　significant　improvement　in　device　repeatability.　©　2019　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim