Individually addressable nanoscale OLEDs

Abstract

When reducing pixel size below the wavelength of light, the conventional stacked geometry of organic light-emitting diodes (OLEDs) is dominated by sharp nanoelectrode contours. This causes spatially imbalanced charge carrier transport and recombination resulting in low external quantum efficiency (EQE) and filament formation accelerating device failure. Here, we circumvent these limitations by selectively passivating nanoelectrode edges with an insulating layer, while simultaneously defining a nanoaperture in flat areas. We thereby ensure controlled charge carrier recombination and suppress filament growth. After demonstrating efficient hole injection by gold nanoelectrodes, we realize individually addressable subwavelength OLED pixels (300 nanometers by 300 nanometers) based on plasmonic gold patch antennas for light extraction. We achieve an EQE of 1%, a maximum luminance of 3000 candela per square meter, and fast response times exceeding video rates. Our results highlight a scalable strategy to overcome key electronic and optical bottlenecks of nanoscale optoelectronic devices and demonstrate the potential of plasmonic patch antennas for high-density, high-performance OLED integration.

Fig. 1.. Conceptual design of nano-OLED pixels with an individually addressable bottom nanoelectrode.

(A) Sketch of the device architecture (cut open explosion view). A subwavelength Au patch electrode supporting plasmonic modes is used as bottom anode. To bypass the detrimental effects of electrode downscaling the edges and corners of the nanoelectrode are insulated, leaving a centered nanoaperture with an area of homogenous electric field distribution (see “insulating nanoaperture”). A standard organic stack consisting of hole injection layer (HIL), hole transport layer (HTL), emissive layer (EML), and electron transport layer (ETL) is applied on top, followed by an extended metallic top cathode. (B) Cut through (A) along α-β. Spatially controlled hole injection (bottom anode) and electron injection (top cathode) lead to exciton formation above the nanoaperture within the emissive layer. Upon recombination, excitons couple to the plasmonic mode(s) of the Au nanopatch, leading to strong emission. (C) Device without nanoaperture: Inhomogeneous charge carrier injection leads to poor emission and promotes filament growth (see inset to the right).

Fig. 2.. Au nanoelectrodes with nanoapertures.

(A) EBL process with HSQ (blue) as negative resist covering the electrode. Cross-linking is controlled by a gradient e-beam dose indicated by the fading red color. Alkalic development with tetramethylammonium hydroxide (TMAH) results in the removal of unexposed HSQ and partial etching of exposed HSQ according to the degree of cross-linking. (B) Tapping-mode AFM images of two adjacent 1 μm–by–1 μm Au electrodes with individual electrical connectors. The electrode edges and the connector are fully covered by HSQ apart from a central 550-nm-diameter nanoaperture. (C) Conductive AFM image of the same area as in (B), revealing current injection only inside the apertures. a.u., arbitrary units. (D) Height profile along the dashed gray line in (B).

Fig. 3.. Electrical properties and device configuration of hole-only macrojunctions (electrode patch, 100 μm by 100 μm; active area, 1.0 × 10⁻⁴ cm²) and nanojunctions (electrode patch, 1 μm by 1 μm; nanoaperture diameter, 550 nm; active area, 2.4 × 10⁻⁹ cm²).

(A) Molecular structures of the interface functionalization material HAT-CN and the hole transport material NPB. (B) Device architecture consisting of an Au bottom anode (50 nm in thickness) functionalized with an ultrathin HAT-CN layer (5 nm), followed by a hole transport layer consisting of NPB (135 nm) and the electron-blocking Au cathode (140 nm). The flat-band energy landscape does not consider Fermi level pinning of HAT-CN and NPB. (C) Principle of operation under an applied voltage V exceeding the built-in voltage V_BI. Hole injection is mediated by the bottom electrode in conjunction with the HAT-CN interface layer, and injected holes are transported via the HOMO of NPB. Electron injection from the top cathode is prevented through the large energy barrier. (D) Semilogarithmic current density-voltage characteristics of representative macrojunction (red lines and inset to the right) and nanojunction (blue lines and inset to the right) diodes. The blocking character of both electrodes under reverse bias is highlighted by the inset to the left. (E) Corresponding double logarithmic presentation of the space charge–limited current and Poole-Frenkel fits in the regime between 3 and 8 V [dashed box in (D)]. From the fits, we deduce a lower absolute number of trap states in the nanojunction (gray arrow).

Fig. 4.. Electrical properties of hole-only devices based on Au patch antennas (1 μm by 1 μm) with (blue lines and inset) and without nanoaperture (red lines and inset).

(A) Current density–voltage characteristics of two representative junctions with and without nanoaperture. The junctions are voltage cycled three times starting from 0 V. An erratic behavior, likely resulting from filament formation and disruption, is observed for a patch antenna without nanoaperture. (B) Constant voltage operation (5 V, dc) of two representative junctions with and without nanoaperture. The device without nanoaperture exhibits breakdown presumably by filament formation after only 3 min of operation, while the device with nanoaperture is stable over the measurement period of 30 min.

Fig. 5.. Device reproducibility.

The blue area indicates the absolute variation in current-voltage (I-V) characteristics across 30 individual nanojunctions. At 20 V, the current variation remains within one order of magnitude. The red curve corresponds to the nanojunction shown in Fig. 3. Inset: White-light reflection micrograph of the electrode layout, featuring 11 pixels per block. The vertical and horizontal pixel pitches are 2 and 10 μm, respectively, demonstrating the scalability of the design for display applications. Three such blocks (33 pixels) were fabricated on a single substrate, with only three pixels exhibiting deviation from the standard J-V behavior.

Fig. 6.. Optoelectronic characterization of a nano-OLED device based on plasmonic Au patch antennas (300 nm by 300 nm) with a 200-nm nanoaperture.

(A) Flat band energy landscape and device architecture. The stack consists of HAT-CN (5 nm; hole injection layer), NPB (30 nm; hole transport layer), mCP doped with TXO-TPA (30 nm; emissive layer), Bphen (75 nm; electron transport and hole blocking layer), Ca (10 nm; electron injection layer), and Al (120 nm; capping electrode). Light emission is mediated by exciton-plasmon coupling via the Au patch antenna bottom electrode. (B) Current density– and electroluminescence (EL)–voltage characteristics recorded between −15 and +15 V. (C) EQE as function of the current density for a representative pixel demonstrating EQE values in the percentage regime. (D) Luminance characteristics of a representative pixel reaching a peak luminance of ~3000 cd m⁻², comparable to that of macroscopic OLEDs using the same emitter material. The luminance is calculated considering the 300 nm–by–300 nm patch antenna area.

Fig. 7.. Nano-OLED light emission mediated by plasmonic modes of the Au patch antenna (300 nm by 300 nm).

(A) Simulated light outcoupling efficiency of the nano-OLED stack, averaged over multiple laterally distributed point dipole positions within the 200-nm nanoaperture. Dipoles are placed 50 nm above the Au patch. The solid curve shows the combined contribution from horizontally and vertically oriented dipoles, while the dashed curve isolates the vertical dipole contribution, illustrating coupling to plasmonic antenna modes. The inset displays the simulated electric field distribution of the dominant quadrupolar-like n₂₂ mode at 650 nm, calculated 15 nm above the Au surface. The n₂₂ resonance (black dotted line) and contributions from lower-order modes (n₂₁, n₁₁, and n₁₀; gray dotted line) are indicated as guides to the eye. (B) Normalized EL spectrum measured at 15 V (gray; antenna-coupled emission). The convolution of a macroscopic standard OLED spectrum (molecular emission) with the simulated outcoupling efficiency spectrum [from (A)] closely resembles the nano-OLED spectrum, confirming predominant coupling to the n₂₂ plasmonic mode. The spectral shaping is highlighted by the normalized difference spectrum of antenna-coupled emission and molecular emission. (C) Spatial emission map from a representative nano-OLED pixel. The pixel area and electrical contact lines are overlaid in yellow. Emission is centered within the nanoaperture-defined pixel. Linecuts along the vertical and horizontal axes reveal a point-like emission pattern with a FWHM below 600 nm, limited by the microscope resolution.