Tandem　solar　cells　are　regarded　as　an　effective　way　to　break　through　the　theoretical　efficiency　of　the　Shockley–Queisser　limit,　and　large-size　lead　sulfide　quantum　dots　(PbS　QDs)　are　considered　ideal　infrared　(IR)　photovoltaic　materials　for　absorbing　low-energy　IR　photons　in　the　bottom　subcells　of　tandem　solar　cells　due　to　their　tunable　bandgaps　and　multiexciton　generation　effects.　However,　the　exposed　(100)　surface　of　large-size　PbS　QDs　leads　to　poor　passivation　during　the　ligand-exchange　process,　resulting　in　degraded　device　performance.　Herein,　a　mixed　halide　ions　ligand　passivation　strategy　is　developed　to　synergistically　passivate　the　(111)　and　(100)　facets　of　large-size　QDs.　Particularly,　a　gradual　dropwise　addition　of　lead　chloride　salts　dissolved　in　dimethyl　sulfoxide　for　achieving　precisely　proportional　surface　passivation　of　Cl　ions　in　the　ligand　exchange　is　proposed,　which　enhances　the　quality　of　the　QD　solids　and　obtains　excellent　control　of　the　surface　trap　states.　Additionally,　optical　and　electrical　characterization　results　show　that　the　passivated　IR　PbS　QD　solids　have　higher　carrier　mobility,　longer　carrier　lifetime,　and　lower　defect　density,　which　increases　the　IR　efficiency　of　IR　PbS　QD　solar　cells　to　1.25%.　©　2023　Wiley-VCH　GmbH.