High　performances　of　Cu2ZnSn(S,Se)4　(CZTSSe)　solar　cells　are　heavily　influenced　by　the　quality　of　heterojunctions.　Herein,　an　oxygen　(O)　doping　of　CZTSSe/CdS　heterojunction　is　performed　to　suppress　the　formation　of　the　defects　by　an　ultraviolet　ozone　(UV-O3)　treatment　for　the　efficient　flexible　CZTSSe　solar　cells.　The　introduction　of　O　reduces　the　non-radiative　recombination　and　increases　the　carrier　concentration　of　the　CdS　films.　Furthermore,　the　defect　density　of　the　CdS　film　has　been　reduced　from　8.24　x　1016　to　2.91　x　1016　cm-3　by　the　O-doping.　The　results　indicate　that　the　electron　transport　is　effectively　promoted　due　to　the　decreased　conduction　band　offset　(CBO)　at　the　heterojunction　interface.　As　a　result,　the　champion　flexible　CZTSSe　solar　cell　achieves　a　power　conversion　efficiency　(PCE)　of　11.21%,　with　a　significantly　improved　short　circuit　current　density.　The　study　for　improving　the　CZTSSe/CdS　heterojunction　through　O-doping　treatment　provides　a　new　insight　for　enhancing　the　PCE　of　the　flexible　CZTSSe　solar　cells.　(c)　2025　Science　Press　and　Dalian　Institute　of　Chemical　Physics,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.　and　Science　Press.　All　rights　are　reserved,　including　those　for　text　and　data　mining,　AI　training,　and　similar　technologies.