During the radiative decay of singlet excitons in polymer light-emitting diodes (PLEDs), the newly developed molecule dynamics not only describes the evolution of the lattice distortion of the polymer chain but also illustrates the time-dependent electron transitions and electron populations in detail. Furthermore, based on this method, it is revealed that the nonemissive triplet excitons in PLEDs become emissive through the fusion of injected positive polarons. The obtained maximum quantum efficiency is consistent with recent experimental results.