Efficiency of radiative emission from thin films of a light-emitting conjugated polymer

We examine the efficiency of radiative emission from thin layers of light-emitting conjugated polymers. We compare our experimental results for photoluminescence of the conjugated polymer poly(2-methoxy, 5-(2'-ethyl-hexyloxy) 1,4 phenylenevinylene) (MEH-PPV) with those of a theoretical model, finding good agreement between the two. The specially developed model takes into account several factors including absorption in the emissive layer, a spread of emitter sites within the layer, and the broad emission spectrum of the polymer. We find that the photoluminescence quantum efficiency for radiative emission of a bare MEH-PPV film on a glass substrate is similar to 25%. We then apply our model to study electroluminescent devices. We show that for these structures the efficiency of radiative emission is similar to 10%. There is thus potential for considerable improvement in efficiency for both systems through recovery of some of the wasted waveguided light. Finally we use our model to reexamine some controversial results that indicate the probability of singlet exciton formation to be 0.4 +/-0.05, and thus greater than the 0.25 expected from spin statistics. Our reanalysis supports a probability >0.25. We conclude by discussing the limitations of present models, including our own, in predicting the performance of realistic light-emitting diodes.