Perovskite　bandgap　tuning　without　quality　loss　makes　perovskites　unique　among　solar　absorbers,　offering　promising　avenues　for　tandem　solar　cells1,2.　However,　minimizing　the　voltage　loss　when　their　bandgap　is　increased　to　above　1.90　eV　for　triple-junction　tandem　use　is　challenging3-5.　Here　we　present　a　previously　unknown　pseudohalide,　cyanate　(OCN-),　with　a　comparable　effective　ionic　radius　(1.97　angstrom)　to　bromide　(1.95　angstrom)　as　a　bromide　substitute.　Electron　microscopy　and　X-ray　scattering　confirm　OCN　incorporation　into　the　perovskite　lattice.　This　contributes　to　notable　lattice　distortion,　ranging　from　90.5　degrees　to　96.6　degrees,　a　uniform　iodide-bromide　distribution　and　consistent　microstrain.　Owing　to　these　effects,　OCN-based　perovskite　exhibits　enhanced　defect　formation　energy　and　substantially　decreased　non-radiative　recombination.　We　achieved　an　inverted　perovskite　(1.93　eV)　single-junction　device　with　an　open-circuit　voltage　(VOC)　of　1.422　V,　a　VOC　x　FF　(fill　factor)　product　exceeding　80%　of　the　Shockley-Queisser　limit　and　stable　performance　under　maximum　power　point　tracking,　culminating　in　a　27.62%　efficiency　(27.10%　certified　efficiency)　perovskite-perovskite-silicon　triple-junction　solar　cell　with　1　cm2　aperture　area.　Triple-junction　solar　cells　with　cyanate　in　ultrawide-bandgap　perovskites　exhibit　enhanced　defect　formation　energy　and　substantially　decreased　non-radiative　recombination.