Light-emitting　electrochemical　cells　(LECs)　are　appealing　for　cost-effective,　large-area　emission　applications;　however,　their　luminescence　efficiency　is　significantly　limited　by　exciton　annihilation　caused　by　high　concentration　polarons.　Here,　we　present　thermally　activated　delayed　fluorescence　(TADF)　sensitized　fluorescence　LECs　(TSF-LECs)　that　achieve　a　record　9%　EQE.　The　TADF　sensitizers　with　rapid　reverse　intersystem　crossing　(RISC)　rates　can　effectively　convert　triplet　excitons　to　singlet　excitons　in　LECs,　thereby　establishing　a　more　efficient　overall　energy　transfer　pathway.　Importantly,　magneto-electroluminescence　measurements　indicate　that　the　additional　RISC　route　in　TSF-LECs　significantly　suppresses　the　annihilation　of　triplet　excitons　and　thus　reduces　exciton　loss　under　high　concentration　polaron　conditions.　Compared　to　LECs　without　a　sensitizer,　TSF-LECs　exhibit　improved　EQE　and　luminance,　extended　operational　lifetimes,　and　suppressed　efficiency　roll-off.　A　flexible　display　prototype　based　on　TSF-LECs　was　further　fabricated,　capable　of　stably　displaying　high-brightness　preset　patterns　for　extended　periods.　The　exploration　of　the　exciton　dynamics　in　high　concentration　polaron　environments　offers　valuable　insights　for　future　developments　in　high-efficiency　LEC　technology.　©　2024　The　Royal　Society　of　Chemistry.