Spin-dependent recombination and electroluminescence quantum yield in conjugated polymers - art. no. 075206

We study the mixing of singlet and triplet excitons due to spin-orbital cou pling in conjugated polymers with ring twist angle theta. The mixing, propo rtional to sin(2)theta, determines the singlet-triplet transition rates and the spin-dependent recombination of injected electrons and holes in the el ectroluminescence (EL) process. It is found that the EL quantum yield can i n principle be higher than 1/4 if the exchange energy is either (1) so smal l that the back transition from triplet to singlet is faster than the tripl er decay, or (2) so large that the lowest triplet exciton formation is slow ed down by phonon bottleneck. In addition to large theta and small effectiv e exciton-phonon coupling, heavy atom impurities can also increase the yiel d by enhancing the mixing.