Electroluminescence emission pattern of organic light-emitting diodes: Implications for device efficiency calculations

The electroluminescence (EL) pattern emitted through the surface and edge of the glass substrate of two efficient polymer light-emitting diodes (LEDs) has been characterized. The surface emission is nearly Lambertian, while the edge emission comprises discrete substrate reflection and leaky waveguide modes. A simple “half-space” optical model that accounts for optical interference effects of the metal cathode–reflector is developed to extract the location and orientation of the emitting dipoles from these patterns. Numerical simulations for a range of polymer and metal refractive indices show that the surface out-coupling efficiency ξ of the internally generated photons can be greater than the 0.5 n−2 relation (where n is the refractive index of the emitter layer) valid for isotropic emitters that are not subjected to optical interference effects. When the emitting dipoles are optimally located for maximum rate of surface emission, the model predicts ξ to vary as 0.75 n−2 for the isotropic case, and as 1.2 n−2 for the in-plane case. For our LEDs, we found that the EL arises from in-plane dipoles that are on average almost optimally located away from the cathode. Using this result, the internal EL quantum yield is estimated to be close to 50% of the free-space photoluminescence yield of the emitter for the devices. This indicates excellent injection balance and recombination efficiency of the charge carriers. By also taking into account of optical interference effects on the radiative rate, we deduced that the lower limit for the probability of forming an emissive singlet exciton from electrical injection is 35%–45% in these conjugated polymers. This greatly exceeds the 25% probability from spin-degeneracy statistics.