Understanding the photophysical process governing the operation of the organic light emitting diodes (OLEDs) and how they are affected by film morphology is crucial to the efficient design of future OLEDs. In particular, delayed fluorescence (DF), is known to contribute a significant fraction of the light emission from polymer-based OLEDs, but its mechanism remains unclear. Here, we investigate the origin of DF in the state of the art OLED polymer Poly (9, 9-dioctylfluorene-alt-benzothiadiazole) (F8BT), under both optical and electrical excitation using time-resolved emission spectroscopy (TRES) as a function of film thickness, excitation fluence, magnetic-field, and temperature. The temperature dependence of the DF for various film thicknesses suggests that thermally activated triplet migration is the dominant process controlling DF at room temperature. We found that thermal activation energy (Eeff) of triplet migration decreases from 179 ± 31 meV to 86 ± 11 meV as film thickness varied from ~110 nm to ~560 nm, respectively. The Eeff of triplet migration is found to be a function of the molecular packing of polymer chains as determined from synchrotron grazing incidence wide angle x-ray scattering (GIWAXS) studies and steady-state photoluminescence studies. Quantum chemical calculations of reorganization energy and singlet–triplet exchange energy gap in F8BT molecule as a function of the dihedral angle between donor & acceptor moiety also confirm the experimental results. Our results show that DF in polymer OLEDs is significantly affected by parameters such as the film thickness and disorder, allowing for a high degree of control over the underlying photophysics to be achieved. © 2018, The Author(s).