Abstract
We study the mixing of singlet and triplet excitons due to spin-orbital coupling in conjugated polymers with ring twist angle θ. The mixing, proportional to sin 2 θ, determines the singlet-triplet transition rates and the spin-dependent recombination of injected electrons and holes in the electroluminescence (EL) process. It is found that the EL quantum yield can in principle be higher than 1/4 if the exchange energy is either (1) so small that the back transition from triplet to singlet is faster than the triplet decay, or (2) so large that the lowest triplet exciton formation is slowed down by phonon bottleneck. In addition to large θ and small effective exciton-phonon coupling, heavy atom impurities can also increase the yield by enhancing the mixing.