Principles

OLED Outcoupling Efficiency

The relationship among material quantum yield, device EQE, and the classical outcoupling limit

An organic emitter may have a quantum yield of 90% while the device EQE remains near 20%. The missing photons are distributed across three stages: charges form emissive excitons, excitons decay radiatively, and the generated light leaves the planar device.

Emission optics primarily addresses the third stage and can also feed back on the second. This article places material metrics, device efficiency, and optical simulation in one loss budget, then uses the classical escape cone to explain the roughly 20% outcoupling scale.

Material Quantum Yield Is Not Device EQE

Photoluminescence quantum yield (PLQY) measures the fraction of absorbed photons that produce emitted photons under optical excitation. OLED external quantum efficiency (EQE) measures how many photons leave the device per injected electron under electrical drive. Exciton formation, nonradiative processes inside the device, and optical trapping still separate these quantities.

Normalizing to one injection event, the device efficiency can be written as

\eta_{\mathrm{EQE}}=C\,q_{\mathrm{eff}}\,\eta_{\mathrm{out}}.

Here, $\eta_{\mathrm{EQE}}$ is the external quantum efficiency; $C$ is the conversion efficiency from charge pairs to the target emissive excitons, including charge balance and the usable-exciton fraction; $q_{\mathrm{eff}}$ is the effective quantum efficiency in the device optical environment; and $\eta_{\mathrm{out}}$ is the light-extraction or outcoupling efficiency (LEE), the fraction of photons entering electromagnetic emission channels that reaches the target external medium. All three factors are dimensionless and lie between 0 and 1.

Material PLQY is most closely related to the intrinsic emitter quantum efficiency $q_0$ , but it cannot replace $C$ , $q_{\mathrm{eff}}$ , or $\eta_{\mathrm{out}}$ . Charge imbalance, exciton quenching, and optical feedback under electroluminescence can all move the device away from the material measurement. Even with $C=0.95$ , $q_{\mathrm{eff}}=0.90$ , and $\eta_{\mathrm{out}}=0.20$ ,

\eta_{\mathrm{EQE}}=0.95\times0.90\times0.20=17.1\%.

This gives the most direct order-of-magnitude explanation for a 90% material yield and a device EQE near 20%.

Classical Escape Cone and Outcoupling Limit

The emissive layer and organic functional layers usually have higher refractive indices than air. When light travels from a high-index to a low-index medium, only directions inside the critical angle can propagate directly across the interface. The critical angle is

\theta_c=\arcsin\left(\frac{n_{\mathrm{out}}}{n_{\mathrm{EML}}}\right),

where $\theta_c$ is measured from the interface normal, $n_{\mathrm{EML}}$ is the emissive-layer index, and $n_{\mathrm{out}}$ is the external-medium index, with $n_{\mathrm{EML}}>n_{\mathrm{out}}$ . Plane waves beyond $\theta_c$ undergo total internal reflection, leaving a finite escape cone.

Bottom-emitting device stack and the classical light-extraction limit versus emissive-layer refractive index — Figure 1 | A high-index emissive layer allows only light inside the escape cone to leave directly; the classical outcoupling limit falls as the emissive-layer index rises.

For an isotropic planar emitter with approximately Lambertian emission into air, classical ray optics gives

\eta_{\mathrm{out}}\approx\frac{1}{2n_{\mathrm{EML}}^2}.

At $n_{\mathrm{EML}}\approx1.7$ , this is about 17%, often summarized as the classical OLED outcoupling scale of roughly 20%. The expression includes only the geometric escape cone and assumes ideal orientation, reflection, and zero absorption. Real OLEDs also exhibit dipole-orientation effects, thin-film interference, metal absorption, waveguiding, and surface-plasmon coupling. The formula is therefore a useful starting intuition, not a final device prediction.

Where the Unextracted Energy Goes

Energy outside the escape cone does not disappear immediately. It enters several channels in the layered structure.

Air, substrate, absorption, waveguide, and surface-plasmon channels in a bottom-emitting device — Figure 2 | Major optical channels in a planar emitter. Only part of the power escapes directly; the remainder enters substrate, absorption, waveguide, and SPP channels.

Energy channel	Origin	Device consequence
External outcoupling	Wavevector lies inside an external-medium light cone	Produces useful light and contributes to EQE
Substrate mode	Light enters a high-index substrate but is totally internally reflected at the substrate–air boundary	Remains trapped in the substrate
Waveguide mode	Total internal reflection and interference in organic layers or transparent electrodes	Propagates laterally and is eventually lost
Absorption	Metals, electrodes, and lossy functional layers absorb electromagnetic energy	Converts optical energy to heat
Evanescent and SPP coupling	Dipole near fields couple to high-wavevector states at a metal–dielectric interface	Often creates strong near-field loss
Exciton nonradiative decay	Excitons do not enter an electromagnetic channel	Represented by $1-q_{\mathrm{eff}}$

Snell's law alone cannot separate these channels. Film thicknesses are comparable to the wavelength, so reflected waves interfere coherently; the energy allocation must be solved with a dipole multilayer model and integrated in in-plane-wavevector space. See Power Dissipation and Optical Modes for the exact definitions.

Introductory Phenomena and Their Physics

Observable phenomenon	Main optical cause	Related article
High material PLQY but device EQE near 20%	Conversion efficiency, effective quantum efficiency, and outcoupling efficiency multiply stage by stage	This article
Changing one layer thickness changes brightness and efficiency with the same emitter	Dipole position and microcavity interference redistribute outcoupling and loss	Microcavity Effect
A display looks greener, bluer, or more saturated off axis	The cavity resonance moves with angle and reweights spectral bands	Microcavity Effect
Some OLEDs are nearly Lambertian while others are strongly forward-directed	A weak cavity can approach constant radiance; a strong cavity selects angles	Microcavity Effect
Horizontally oriented molecules often increase EQE	Horizontal dipoles place more power near the surface normal and inside the escape cone	Dipole Model and Applicability
Moving the EML toward a metal changes both lifetime and efficiency	The optical environment changes the decay rate and high-wavevector loss	Purcell Effect
Increasing electrode reflectance can first raise and then lower efficiency	Constructive interference improves outcoupling while repeated passes and metal absorption grow	Microcavity Effect

Together, these observations show that the material determines what light can be generated, while device optics determines its color, direction, and probability of leaving the device.

Next Step

Continue with Dipole Model and Applicability to see why an emitter is represented by a point dipole and how molecular orientation enters the calculation.

Reference

Tsutsui, T.; Takada, N. Progress in Emission Efficiency of Organic Light-Emitting Diodes: Basic Understanding and Its Technical Application. Jpn. J. Appl. Phys. 2013, 52. https://doi.org/10.7567/JJAP.52.110001
Shim, J. I.; Shin, D. S. Measuring the Internal Quantum Efficiency of Light-Emitting Diodes: Towards Accurate and Reliable Room-Temperature Characterization. Nanophotonics 2018, 7, 1601–1615.
Greenham, N. C.; Friend, R. H.; Bradley, D. D. C. Angular Dependence of the Emission from a Conjugated Polymer Light-Emitting Diode: Implications for Efficiency Calculations. Adv. Mater. 1994, 6, 491–494.
Salehi, A.; Chen, Y.; Fu, X.; Peng, C.; So, F. Manipulating Refractive Index in Organic Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2018, 10, 9595–9601.

Dispersion Theory

Thin-film phase, group delay, GDD, and differential group delay

Dipole Model and Applicability

Point dipoles, molecular orientation, spatial distributions, and model limits in planar emitters