Top-emitting OLEDs are the workhorses of high-resolution active-matrix displays because their light exits away from the substrate, decoupling the emitting aperture from the transistor backplane beneath. They also lend themselves naturally to a Fabry–Perot microcavity, formed between a reflective bottom electrode and a semitransparent top cathode. The cavity narrows the electroluminescence spectrum and concentrates emission toward the forward direction, boosting frontal efficiency and color purity. But the same physics that sharpens the spectrum also makes extraction design unforgiving: much of the generated light ends up confined in waveguide modes traveling laterally through the organic stack and in surface plasmon polaritons hugging the metal electrodes. Fixing the air-side interface alone cannot recover those internal losses.

Conventional internal outcoupling structures, such as wrinkles, corrugations, and strongly curved microlens textures, scatter trapped modes back toward useful directions, but they carry a hidden cost. When the underlying texture is steep, the organic layers deposited on top fail to follow it conformally, so the local optical thickness of the cavity varies across the device and the resonance condition drifts from point to point. Stronger scattering then does not translate into a better device, because the cavity-defined forward emission benefit is simultaneously eroded. The key insight of the new work is that the geometry of an internal extraction layer must be judged not just by how much light it scatters but by how gently it does so.

The researchers call their solution a gentle internal corrugation layer, or GICL. Atomic force microscopy over a 50 by 50 micrometer area revealed a peak-to-valley height of about 1.12 micrometers with root-mean-square roughness of 144.5 nanometers, but the defining feature is the slope: representative surface angles of only 2.1 to 6.9 degrees along one axis and 1.8 to 3.5 degrees along the other, spread over lateral spans of nearly 9 micrometers. Cross-sectional electron microscopy showed the entire multilayer stack following this soft undulation without discontinuities. The contrast with reference structures is instructive. A steep imprinted random microlens array reached surface angles of 25.3 degrees with a peak-to-valley height of 7.87 micrometers, while one wrinkle reference achieved low roughness yet still exhibited angles above 53 degrees, demonstrating that roughness amplitude alone says little about geometric severity.

Finite-difference time-domain simulations confirmed that gentler is genuinely better. Among the compared structures, GICL yielded the highest relative total simulated power, outperforming both wrinkle references and the steep microlens array, even though the latter produced far stronger local field perturbation. Ray tracing sharpened the picture: GICL directed 77.3 percent of input power into the forward collection region, versus 43.5 percent for the microlens array, which instead diverted light into wide-angle, internally reflected, and backward trajectories. Under identical conditions, the calculated light extraction efficiency rose from 37.6 percent for the planar microcavity device to 74.1 percent with GICL, an increase of nearly a factor of two. An intermediate-index clear resin with a refractive index of 1.59 was applied at the outer surface to soften the air interface and let the redistributed light actually escape.

The emissive system matters as much as the geometry. In a microcavity, only the spectral portion of the emitter that overlaps the resonance band contributes efficiently to the enhanced external output, so a broad emitter wastes much of its spectrum on off-resonant wavelengths. The team used a phosphor-assisted thermally activated delayed fluorescence sensitization scheme, in which a TADF host and a phosphorescent assistant funnel excitation energy to a terminal multi-resonance TADF emitter called tCzphB-Fl, producing green emission with a full width at half maximum of only about 19 to 22 nanometers. When the microcavity was formed, the narrowband system’s peak shifted just 2 nanometers and its FWHM narrowed from 22 to 19 nanometers, whereas a conventional phosphorescent comparator shifted 13 nanometers and narrowed from 65 to 25 nanometers, evidence that most of the narrowband emission already sat inside the resonance band.

Device results bore out the combined strategy. All GICL devices shared the same 2.25-volt turn-on voltage and essentially unchanged current density, ruling out electrical effects, and the forward spectra showed only minor changes, ruling out spectral reshaping. Under matched resin conditions, adding GICL raised the external quantum efficiency from 34.8 to 69.2 percent, current efficiency from 302.7 to 378.4 candela per ampere, and power efficiency from 199.3 to 396.7 lumens per watt, with maximum luminance reaching nearly 120,000 candela per square meter. Across five samples the efficiencies averaged 68.2 plus or minus 0.7 percent, indicating good reproducibility. The angular color shift at 60 degrees off-axis also fell from 0.0197 to 0.0086 in CIE 1976 coordinates, and the luminance distribution became more Lambertian, both valuable for display uniformity.

Polarization-resolved measurements identified where the recovered light had been hiding. Because waveguide modes are predominantly transverse-electric like and surface plasmon polaritons are transverse-magnetic like, separating the s- and p-polarized emission provides a mode-sensitive diagnostic. Both polarized components, and the non-polarized total, increased across the viewing-angle range in the corrugated device, confirming genuine extraction rather than mere redistribution between channels. Quantitative modal analysis showed the outcoupled fraction rising from 29.7 to 38.6 percent, with the waveguide fraction dropping from 40.0 to 32.0 percent and the plasmon-related fraction changing only modestly. The roughly 30 percent simulated gain in outcoupled power matched the approximately 25 percent uncorrected device-level increase closely, lending quantitative coherence to the extraction interpretation.

Notably, the same corrugation helped the broad phosphorescent system far less, lifting its current efficiency by only about 15.7 percent compared with 25.0 percent for the narrowband system, a difference the authors attribute to spectral overlap with the cavity resonance rather than to bandwidth alone, since the two emissive chemistries differ in other respects as well. The gains did come with a small price: efficiency roll-off at high brightness was moderately larger in the corrugated devices, possibly reflecting local current crowding on the textured surface, and operational lifetime at 1,000 candela per square meter was slightly shorter, though still comparable at about 101 hours to 95 percent of initial luminance. The broader lesson stands: in microcavity OLEDs, internal texture geometry, outer-interface index matching, and emitter-resonance alignment must be engineered together, and gentleness, it turns out, extracts more light than force.

Subject of Research: Cavity-compatible internal light extraction in top-emitting microcavity OLEDs using a gentle corrugation layer and resonance-matched narrowband emission

Article Title: Cavity‐Compatible Light Extraction in Top‐Emitting Microcavity OLEDs via a Gentle Internal Corrugation Layer and Resonance‐Matched Narrowband Emission

Article References: Kim, Y. R., Nimbalkar, A., & Suh, M. C. (2026). Cavity‐Compatible Light Extraction in Top‐Emitting Microcavity OLEDs via a Gentle Internal Corrugation Layer and Resonance‐Matched Narrowband Emission. Advanced Science, Article e78013. https://doi.org/10.1002/advs.78013

Image Credits: AI Generated

DOI: 10.1002/advs.78013

Keywords: OLED, microcavity, light extraction, external quantum efficiency, corrugation, TADF, narrowband emission, waveguide modes, surface plasmon polariton, display technology, FDTD simulation, phosphorescence

To understand why this matters, it helps to consider what actually happens inside an OLED when a current flows. Electrons and holes recombine in an emissive layer, and in phosphorescent OLEDs those recombining excitations are triplet excitons harvested by organometallic emitter complexes, allowing nearly all electrically generated excitons to be converted into photons in principle. Internal quantum efficiencies approaching unity are therefore achievable. The trouble lies further downstream. In a planar, multilayer device, only a modest fraction of the light created in the emissive layer can escape directly into air. The remainder is trapped in three competing channels: photon modes guided laterally through the high-index organic and substrate layers, cavity modes in top-emitting structures, and surface plasmon polaritons, which are coupled oscillations of electromagnetic fields and conduction electrons that propagate along metal-organic interfaces. Each trapped channel represents energy that has already been generated but will never reach the viewer’s eye.

Surface plasmon polaritons are particularly punishing in modern device geometries. Inverted OLED architectures, in which the cathode sits beneath the emissive stack and the anode is deposited on top, are attractive for display manufacturing because they allow robust electrical interconnection, oxide-based thin-film transistors and seamless tiling. Yet the metallic electrodes and thin functional layers characteristic of these designs place the emissive layer within the evanescent near field of metal surfaces, exactly the regime in which coupling to surface plasmon polaritons is strongest. The energy funneled into these modes is dissipated as heat within tens of femtoseconds to picoseconds, contributing nothing to brightness while accelerating the degradation of organic molecules around it. Efficiency and operational stability are therefore fundamentally limited by these photonic losses, and researchers have spent two decades searching for ways to either suppress the coupling or recover the energy after it has been captured.

The new work introduces an unexpectedly versatile medium for doing precisely that: a colloidal quantum dot interlayer positioned within the inverted phosphorescent OLED stack. Quantum dots are nanoscale semiconductor crystals whose optical properties can be tuned simply by changing their size, and they have become workhorses of modern optoelectronics in displays and color-conversion films. Here, the team exploited a different facet of their behavior. Because the emission spectrum of the OLED’s phosphorescent emissive layer overlaps spectrally with the absorption and plasmonic response of the quantum dot layer, excitons in the organic emitter can couple efficiently to the dots through the near field. Rather than acting as a passive spacer, the interlayer functions as a plasmonic coupling medium that intercepts energy flowing into surface plasmon polariton modes before it is irreversibly lost, and makes that energy available for re-emission and extraction.

The experimental evidence for this coupling came from time-resolved photoluminescence measurements, a technique that tracks how quickly excitons in the emissive layer decay after excitation. When excited emitters are placed near a metal surface, coupling to plasmon modes typically accelerates their decay, shortening the measured lifetime while converting much of the energy into heat. By comparing the photoluminescence dynamics of devices in which the quantum dot interlayer was present against configurations with conventional spacer layers, the researchers revealed an efficient plasmonic coupling between the emissive layer and the dots. The modified decay dynamics showed that energy which would otherwise have been dissipated through surface plasmon polaritons was instead being transferred into the quantum dot layer, where it could be recycled into radiative channels. This spectroscopic fingerprint established the physical mechanism before device-level gains were even measured.

Complementing the experiments, the team performed three-dimensional finite-difference time-domain simulations, a computational method that solves Maxwell’s equations on a fine spatial grid and captures how electromagnetic energy flows through complex multilayer structures. The simulations demonstrated that the quantum dot interlayer modifies the local density of optical states, the fundamental quantity that determines how readily an emitter can shed energy into each available optical channel. By reshaping this density of states, the interlayer redistributes trapped optical energy among the device’s modes and enhances outcoupling into air modes, the channels that carry light to the outside world. The modeling quantified where the energy went, confirming that a substantial portion of the plasmonic fraction was being diverted away from lossy surface waves and toward extractable radiation, in quantitative agreement with the device measurements.

The quantitative accounting is striking. According to the analysis, the quantum dot interlayer intercepts approximately 30.7 percent of the energy that would initially have been coupled into surface plasmon polariton modes in the reference device. Recovering nearly a third of this previously doomed energy translated directly into the headline performance figures. The quantum dot-integrated inverted phosphorescent OLED achieved an external quantum efficiency of 20.95 percent, compared with 15.67 percent for the reference device without the interlayer, an improvement factor of 1.3. For an intervention that adds a single solution-processed nanomaterial layer to an existing architecture, and which touches neither the emitter chemistry nor the electrode design, this is a remarkably efficient lever on device performance, and one that should be directly relevant to the display industry, where fractions of a percent in efficiency carry substantial value.

The strategy also stands out against earlier attempts to tame plasmonic losses. Previous approaches have included nanostructured electrodes, microcavity engineering, metal nanoparticles doped into transport layers, plasmonic quasi-bandgap designs and polariton-enhanced Purcell effects. Each has shown promise, but many demand complicated fabrication, introduce new sources of optical absorption or electrical instability, or trade off angular color uniformity for efficiency. Embedding gold or silver nanoparticles, for instance, can enhance emission through localized plasmon resonance but risks quenching excitons and degrading device lifetime. The quantum dot interlayer approach avoids directly doping the charge transport layers and instead exploits a well-controlled, spectrally matched coupling medium, giving designers a knob they can tune through dot size, composition and placement. Because colloidal quantum dots are already manufactured at scale for display applications, the pathway from laboratory demonstration to production integration is unusually short.

The research was carried out by Thi Thuy Truong, Hai Truyen Dang, Nisha Vergineya S and Jang Hyuk Kwon at Kyung Hee University’s Department of Information Display in Seoul, together with Malte C. Gather of the Humboldt Centre for Nano- and Biophotonics at the University of Cologne and the University of St Andrews. It was supported by the Korean Ministry of Trade, Industry and Energy through programs on core AMOLED microdisplay technologies for extended-reality devices and industrial innovation foundations, and by the European Research Council through an Advanced Grant under the Horizon Europe Framework Programme. Beyond the immediate efficiency gains, the findings establish quantum dot interlayers as an effective and versatile platform for suppressing plasmonic losses in high-performance OLEDs, with implications for brighter, longer-lived and more energy-efficient displays, lighting and microdisplays. If roughly a third of plasmonic energy can be intercepted with one inserted layer, the remaining plasmonic losses in OLED technology now look less like a fundamental ceiling and more like a recoverable resource waiting to be harvested.

Subject of Research: Use of colloidal quantum dot interlayers to recover surface plasmon polariton losses and enhance efficiency in inverted phosphorescent OLEDs

Article Title: Inverted phosphorescent OLEDs with plasmon coupling from quantum dot interlayers for enhanced efficiency

Article References: Inverted phosphorescent OLEDs with plasmon coupling from quantum dot interlayers for enhanced efficiency. (n.d.). https://doi.org/10.1038/s41566-026-02001-2

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02001-2

Keywords: OLED, phosphorescent OLEDs, quantum dots, surface plasmon polaritons, light extraction, external quantum efficiency, plasmon coupling, local density of optical states, inverted device architecture, nanophotonics, finite-difference time-domain simulation, display technology