In this work, the complex excited state dynamics of a sky-blue emitting co-polymer {poly-[9,9 '-dioctylfluorene-co-bis-N,N '-(4-butylphenyl)-bis-N,N '-phenyl-1,4-phenylenediamine]} is determined, using a time-resolved gated spectroscopy method, which helps to investigate the origin of delayed fluorescence in thin film. It is clearly shown that delayed fluorescence arises from triplet-triplet annihilation mechanism, which opens an indirect pathway to generate singlet excitons. Experimentally, the delayed fluorescence yield varies from 20% at room temperature to 46 +/- 2% at 20 K, suggesting that an alternative triplet harvesting mechanism is operative to convert the triplet into emissive singlet states.