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

Abstract

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.

Spectrally narrowed edge emission from organic light-emitting diodes

“…4 To explore the nature of the SNEE, we begin by noting that the optical propagation loss in a metal-clad waveguide is primarily caused by the strong absorption, estimated at ϳ10 3 cm −1 of the evanescent tail of the optical field that penetrates the metal electrode. 4,9 The ITO is also lossy, 10 particularly at Ͻ450 nm. Hence, the waveguided light would be completely absorbed before it contributes to any far-field edge emission ͑see below͒.…”

“…Losses occurred at each reflection; they were larger for TM modes, and upon propagation in the ITO when a complex refractive index n = 1.90− 0.04i was introduced. 10 However, these losses did not preclude propagation distances d ϳ 100 m without optical gain. Yet for d ജ 1 mm, some gain was required.…”

Device optimization of tris-aluminum (Alq3) based bilayer organic light emitting diode structures

“…The simulations of the charge transport and charge distribution in organic light emitting devices have been reported [3][4]. There have been several works on calculating the emission pattern [5] and external coupling in OLEDs [6]. It has been shown that the emission from an OLED can be varied through interference effects by changing the thickness of the organic layer in an OLED with a dielectric layer located below the ITO anode [7] or by changing the thickness of a ZnSe layer on top of the metal cathode [8][9].…”

Phosphorescent dye-based supramolecules for high-efficiency organic light-emitting diodes

“…External quantum efficiencies (EQEs) of 30% have been reported recently [1][2][3][4][5][6] ; this is considered to be the light-outcoupling efficiency limit of phosphorescent OLEDs under isotropically oriented transition dipole moments 4 . It has long been recognized that orienting the transition dipole moment of an emitter along the horizontal direction (parallel to the substrate) can enhance the outcoupling efficiency beyond that achieved under isotropic orientation, as demonstrated in polymer-based and vacuumevaporated fluorescent molecule-based OLEDs [7][8][9][10][11][12][13] . Nonetheless, the orientation of the transition dipole moments of iridium complexes used as phosphorescent emitters in efficient OLEDs is typically considered to be isotropic because they are near-globular and small enough to have configurational diversity in their orientational states.…”