Dual-color emissive OLED with orthogonal polarization modes

The study addresses the challenge of realizing high-performance linearly polarized OLEDs (LP-OLEDs), which are attractive for displays, communications, and encryption but typically suffer from low polarization extinction ratio (ER), light losses, limited color control, and costly fabrication. Conventional approaches using aligned organic crystals or post-integrated polarizers either lack scalability or incur high optical losses and broad emission angles. The authors propose embedding a dielectric/metal nanograting into the OLED stack to engineer optical modes so that a strong TE-polarized waveguide mode can be selectively outcoupled while suppressing TM/SPP, substrate, and air modes. The goal is to achieve a dual-color device with orthogonal polarization, high ER, narrow bandwidth, controlled angular distribution, and cost-efficient, large-area manufacturability.

Early LP-OLEDs using aligned conjugated polymers achieved polarization but with very low EQE (e.g., anisotropy 3.1 and EQE ~0.1%). Subsequent molecular alignment techniques (friction-transfer, tensile alignment, Langmuir-Blodgett, pre-aligned substrates) often damaged active layers and yielded low ER with broad spectra. Externally integrated birefringent photonic polarizers can enforce linear polarization but at the expense of ~50% light loss and large emission angles, offering only passive modulation. Embedding nanogratings into the OLED stack emerged as a promising route; however, earlier realizations often relied on costly, time-consuming processes (e-beam, ion etching, nanoimprinting). A 2021 demonstration achieved ER ~11.1 dB at ~520 nm with EQE ~7% and a small divergence angle, illustrating feasibility of internal optical cavities. Remaining challenges include simultaneously boosting EQE, increasing ER, narrowing divergence angles, lowering fabrication costs, and enabling multiwavelength modulation with different polarization modes within a single device.

Design and simulation: The authors designed a bottom-emitting inverted OLED comprising glass substrate/cathode (Ag or dielectric/metal nanograting)/electron injection layer/electron transport layer (ETL)/emitting layer (EML)/hole transport layer (HTL)/hole injection layer (HIL)/Al anode (120 nm). For corrugated devices, the cathode is a photoresist/MgF2 (100 nm)/Ag (25 nm) dielectric/metal (D/M) nanograting; for controls, a planar 25 nm Ag cathode is used. The EML uses blue phosphorescent FIrpic doped in TCTA and mCP (each 8 wt%, 15 nm layers). Finite-difference time-domain (FDTD) simulations modeled optical mode distributions, near-field and far-field patterns, and polarization-resolved angular emission versus ETL thickness and grating period. Reflectivity of the 25 nm Ag cathode (~70%) and 120 nm Al anode (~97%) defines a metal-reflection waveguide Fabry-Pérot cavity. By tuning ETL thickness (TmPyPb, n higher than HTL), the TE waveguide mode at 470 nm is maximally confined while air, substrate, TM/SPP modes are suppressed. Momentum matching for outcoupling is achieved with a Bragg grating satisfying k0 = kwg ± m·kG, with grating period Λ setting the coupling condition. Simulations identified ETL ~107 nm and grating Λ ~300 nm (groove depth ~80 nm, ridge width ~150 nm) as optimal for TE 470 nm extraction and minimal TM diffraction. Fabrication: Large-area (up to 3 × 3 cm2) one-dimensional nanogratings were produced by laser interference lithography (343 nm) on spin-coated photoresist (AR-P3170), followed by development, O2 plasma cleaning, deposition of 100 nm MgF2 and 25 nm Ag via vacuum thermal evaporation to form Ag/MgF2/photoresist nanograting. OLED stacks were thermally evaporated: Ag (25 nm)/LiF (0.8 nm)/TmPyPb (ETL, typically 107 nm)/mCP:FIrpic (8%, 15 nm)/TCTA:FIrpic (8%, 15 nm)/TCTA (5 nm)/TAPC (40 nm)/HAT-CN (10 nm)/Al (120 nm). Planar control and a reference device with 60 nm ETL were also fabricated. Characterization: EL spectra, EQE-luminance-voltage were measured with integrating sphere to capture non-Lambertian emission. Angle-resolved polarized EL used a linear polarizer, goniometer, and fiber spectrometer to obtain TE and TM mode dispersion and divergence. Cross-sectional SEM and AFM verified corrugation transfer. Material optical constants were measured by ellipsometry and used in FDTD. Simulations used periodic boundary conditions along grating and PML orthogonal; far-field air-mode projections were analyzed; parameter scans varied ETL thickness and Λ with 1 nm resolution.

• Achieved a switchable dual-color emission with orthogonal polarization: sky-blue TE-polarized emission (targeted ~470 nm) and green TM-polarized emission (off-confined, ~500 nm region).
• High polarization extinction ratio: ER_TE/TM = 15.8 dB at 470 nm with a small divergence angle of ±30° and narrow FWHM of 28 nm.
• External quantum efficiency: Corrugated OLED EQE = 2.25% at 8 V; planar control EQE = 1.23% at 8 V. Turn-on voltage reduced to 4.7 V. Luminance exceeded 48 cd m−2 at 8 V (vs 7 cd m−2 for planar), with EL intensity rising from 13 (planar) to 103 (corrugated, arb. units).
• Spectral narrowing and color purity: Corrugated device FWHM reduced to 28 nm from 205 nm (reference with 60 nm ETL). CIE shifted from (0.31, 0.48) to (0.13, 0.35), yielding sky-blue emission.
• Mechanism: ETL thickness tuning (107 nm) maximally confines TE waveguide mode within the metal-reflection waveguide, suppressing air and substrate modes; Bragg grating (Λ ~300 nm) provides momentum matching to outcouple TE into air. TM modes couple to SPP at metal/dielectric interface and are strongly attenuated, leading to orthogonal color/polarization.
• Light-extraction enhancement: TE-polarized light extraction in corrugated vs planar devices shows LEER ~15× at ETL = 107 nm; TM extraction is negligible.
• Scalability and fabrication: Large-area (3 × 3 cm2) D/M nanogratings fabricated by low-cost laser interference lithography and thermal evaporation.
• Application concept: Demonstrated polarization-controlled dual-color imaging and proposed color image encryption using arrays with tailored ETL thickness and grating period; simulations indicate extension to cover visible gamut (e.g., red TE with Λ ~385 nm and ETL ~192 nm).

The work demonstrates that embedding a dielectric/metal nanograting within an OLED microcavity provides strong control over optical modes, enabling selective extraction of a TE waveguide mode with high ER, narrow bandwidth, and controlled angular emission, while TM modes are suppressed via SPP coupling. This directly addresses limitations of prior LP-OLEDs (low ER, broad emission, reliance on lossy external polarizers, and expensive fabrication) by using a scalable, cost-efficient process and intracavity mode engineering. The dual-color, orthogonally polarized emission validates the proposed mechanism: phase-matching in the Fabry-Pérot cavity is broken for the targeted TE mode while grating momentum matching converts it to an air mode; the off-confined TM mode emerges at green wavelengths. The approach is tunable via ETL thickness and grating period, suggesting a path to full-gamut polarized emission suitable for advanced displays, AR/VR, data communications, and optical encryption. While EQE is modest relative to optimized ITO-based planar devices, the demonstrated polarization performance (15.8 dB) meets commercial polarization ratios and reveals a viable route to polarization-encrypted color imaging.

A dual-color emissive LP-OLED with orthogonal polarization was realized using an embedded Ag/MgF2 dielectric/metal nanograting in a hybrid microcavity. Through ETL waveguide thickness tuning (107 nm) and Bragg grating design (Λ ~300 nm), the device achieves sky-blue TE emission with ER 15.8 dB, ±30° divergence, and 28 nm FWHM, alongside green TM emission, with EQE up to 2.25%. The fabrication is scalable (3 × 3 cm2) and cost-efficient (laser interference lithography plus thermal evaporation). The concept enables polarization-controlled dual-color imaging and forms the basis for chromo-encryption, and is extendable to the full visible gamut by co-optimizing ETL thickness and grating period. Future work could focus on improving EQE (e.g., electrode optimization), extending experimental demonstrations across the visible spectrum, and evaluating long-term operational stability for practical deployment.

The optimized corrugated LP-OLED exhibits lower EQE than a conventional planar ITO-based OLED using the same FIrpic-based EML, attributed to the 25 nm Ag cathode and the relatively thick (107 nm) ETL. TM modes experience strong quenching due to SPP coupling at the metal interface, limiting TM emission intensity. The full visible color gamut operation is proposed and simulated but not fully realized experimentally in this work.