The　passivation　of　non-radiative　states　and　inhibition　of　band　tailings　are　desirable　for　improving　the　open-circuit　voltage　(Voc)　of　CZTSSe　thin-film　solar　cells.　Recently,　alkali　metal　doping　has　been　investigated　to　passivate　defects　in　CZTSSe　films.　Herein,　we　investigate　Li　doping　effects　by　applying　LiOH　into　CZTSSe　precursor　solutions,　and　verify　that　carrier　transport　is　enhanced　in　the　CZTSSe　solar　cells.　Systematic　characterizations　demonstrate　that　Li　doping　can　effectively　passivate　non-radiative　recombination　centers　and　reduce　band　tailings　of　the　CZTSSe　films,　leading　to　the　decrease　in　total　defect　density　and　the　increase　in　separation　distance　between　donor　and　acceptor.　Fewer　free　carriers　are　trapped　in　the　band　tail　states,　which　speeds　up　carrier　transport　and　reduces　the　probability　of　deep-level　defects　capturing　carriers.　The　charge　recombination　lifetime　is　about　twice　as　long　as　that　of　the　undoped　CZTSSe　device,　implying　the　heterojunction　interface　recombination　is　also　inhibited.　Besides,　Li　doping　can　increase　carrier　concentration　and　enhance　build-in　voltage,　leading　to　a　better　carrier　collection.　By　adjusting　the　Li/(Li　+　Cu)　ratio　to　18%,　the　solar　cell　efficiency　is　increased　significantly　to　9.68%　with　the　fill　factor　(FF)　of　65.94%,　which　is　the　highest　FF　reported　so　far　for　the　flexible　CZTSSe　solar　cells.　The　increased　efficiency　is　mainly　attributed　to　the　reduction　of　Voc　deficit　and　the　improved　CZTSSe/CdS　junction　quality.　These　results　open　up　a　simple　route　to　passivate　non-radiative　states　and　reduce　the　band　tailings　of　the　CZTSSe　films　and　improve　the　efficiency　of　the　flexible　CZTSSe　solar　cells.　©　2022　Science　Press　and　Dalian　Institute　of　Chemical　Physics,　Chinese　Academy　of　Sciences