Top-emitting Organic Light-emitting Devices Research Articles

A flexible organic mechanoluminophore device

A flexible mechanoluminophore device that is capable of converting mechanical energy into visualizable patterns through light-emission holds great promise in many applications, such as human-machine interfaces, Internet of Things, wearables, etc. However, the development has been very nascent, and more importantly, existing mechanoluminophore materials or devices emit light that cannot be discernible under ambient light, in particular with slight applied force or deformation. Here we report the development of a low-cost flexible organic mechanoluminophore device, which is constructed based on the multi-layered integration of a high-efficiency, high-contrast top-emitting organic light-emitting device and a piezoelectric generator on a thin polymer substrate. The device is rationalized based on a high-performance top-emitting organic light-emitting device design and maximized piezoelectric generator output through a bending stress optimization and have demonstrated that it is discernible under an ambient illumination as high as 3000 lux. A flexible multifunctional anti-counterfeiting device is further developed by integrating patterned electro-responsive and photo-responsive organic emitters onto the flexible organic mechanoluminophore device, capable of converting mechanical, electrical, and/or optical inputs into light emission and patterned displays.

Efficient Photon Extraction in Top-Emission Organic Light-Emitting Devices Based on Ampicillin Microstructures.

The desire to enhance the efficiency of organic light-emitting devices (OLEDs) has driven to the investigation of advanced materials with fascinating properties. In this work, the efficiency of top-emission OLEDs (TEOLEDs) is enhanced by introducing ampicillin microstructures (Amp-MSs) with dual phases (α-/β-phase) that induce photoluminescence (PL) and electroluminescence (EL). Moreover, Amp-MSs can adjust the charge balance by Fermi level (EF ) alignment, thereby decreasing the leakage current. The decrease in the wave-guided modes can enhance the light outcoupling through optical scattering. The resulting TEOLED demonstrates a record-high external quantum efficiency (EQE) (maximum: 68.7% and average: 63.4% at spectroradiometer; maximum: 44.8% and average: 42.6% at integrating sphere) with a wider color gamut (118%) owing to the redshift of the spectrum by J-aggregation. Deconvolution of the EL intensities is performed to clarify the contribution of Amp-MSs to the device EQE enhancement (optical scattering by Amp-MSs: 17.0%, PL by radiative energy transfer: 9.1%, and EL by J-aggregated excitons: 4.6%). The proposed TEOLED outperforms the existing frameworks in terms of device efficiency.

High-performance narrow spectrum green phosphorescent top-emitting organic light-emitting devices with external quantum efficiency up to 38%

In this work, we have experimentally demonstrated the efficacy of micro-cavity effect in realizing high-performance top-emitting organic light-emitting diodes (TEOLEDs). By optimizing the thickness of top Yb/Ag electrode and cavity length, highly efficient green TEOLED with external quantum efficiency as high as 38% was achieved. A strong dependence of electroluminescent (EL) performances and spectrum on cavity length was observed, and there was also a significant angle dependence of EL spectrum. Ultimately, ultra-high current efficiency up to 161.17 cd A−1 (3.2 V) was obtained by the device with emission peak at 552 nm, which is 35 nm longer than the intrinsic emission peak (517 nm) of utilized green emitter. Interestingly, this device displayed narrow emission with full-width at half-maximum of less than 20 nm, which was obtained by increasing the Ag layer thickness.

Organic/Inorganic Hybrid Thin-Film Encapsulation Using Inkjet Printing and PEALD for Industrial Large-Area Process Suitability and Flexible OLED Application.

We present herein the first report of organic/inorganic hybrid thin-film encapsulation (TFE) developed as an encapsulation process for mass production in the display industry. The proposed method was applied to fabricate a top-emitting organic light-emitting device (TEOLED). The organic/inorganic hybrid TFE has a 1.5 dyad structure and was fabricated using plasma-enhanced atomic layer deposition (PEALD) and inkjet printing (IJP) processes that can be applied to mass production operations in the industry. Currently, industries use inorganic thin films such as SiNx and SiOxNy fabricated through plasma-enhanced chemical vapor deposition (PECVD), which results in film thickness >1 μm; however, in the present work, an Al2O3 inorganic thin film with a thickness of 30 nm was successfully fabricated using ALD. Furthermore, to decouple the crack propagation between the adjacent Al2O3 thin films, an acrylate-based polymer layer was printed between these layers using IJP to finally obtain the 1.5 dyad hybrid TFE. The proposed method can be applied to optoelectronic devices with various form factors such as rollables and stretchable displays. The hybrid TFE developed in this study has a transmittance of 95% or more in the entire visible light region and a very low surface roughness of less than 1 nm. In addition, the measurement of water vapor transmission rate (WVTR) using commercial MOCON equipment yielded a value of 5 × 10-5 gm-2 day-1 (37.8 °C and 100% RH) or less, approaching the limit of the measuring equipment. The TFE was applied to TEOLEDs and the improvement in optical properties of the device was demonstrated. The OLED panel was manufactured and operated stably, showing excellent consistency even in the actual display manufacturing process. The panel operated normally even after 363 days in air. The proposed organic/inorganic hybrid encapsulant manufacturing process is applicable to the display industry and this study provides basic guidelines that can serve as a foothold for the development of various technologies in academia and industry alike.

Top-emitting thermally activated delayed fluorescence organic light-emitting devices with weak light-matter coupling

Resonance interaction between a molecular transition and a confined electromagnetic field can lead to weak or strong light-matter coupling. Considering the substantial exciton–phonon coupling in thermally activated delayed fluorescence (TADF) materials, it is thus interesting to explore whether weak light-matter coupling can be used to redistribute optical density of states and to change the rate of radiative decay. Here, we demonstrate that the emission distribution of TADF emitters can be reshaped and narrowed in a top-emitting organic light-emitting device (OLED) with a weakly coupled microcavity. The Purcell effect of weak microcavity is found to be different for TADF emitters with different molecular orientations. We demonstrate that radiative rates of the TADF emitters with vertical orientation can be substantial increased in weakly coupled organic microcavity. These observations can enhance external quantum efficiencies, reduce efficiency roll-off, and improve color-purities of TADF OLEDs, especially for emitters without highly horizontal orientation.

Extracting internal modes of top emission organic light emitting diodes by using internal random mesoscopic wrinkles

Angular electroluminescence spectra stabilized and efficiency enhanced top-emitting organic light-emitting devices (TOLEDs) were demonstrated using randomly distributed mesoscopic internal wrinkles. Compared to planar TOLEDs, wrinkled TOLEDs showed a faster exciton decay rate in the emissive layer and high rate of radiative recombination, which leads to enhanced efficiency. Our wrinkled TOLEDs showed angular spectral dependency (u′v′(θ)) of 0.0032, which is low enough to be imperceptible to the human eyes, and 38.7 % enhanced external quantum efficiency relative to the planar counterpart. The approach described here provides an effective scheme for improving TOLEDs and obviating the necessity of modifying the stack structure of the device.

Easily reproducible top-emitting organic light-emitting devices for microdisplays adapted to aluminum contact from the standard CMOS processes

Organic light-emitting devices (OLEDs) on the silicon backplanes processed via the standard foundry complementary metal–oxide-semiconductor (CMOS) processes were developed with the state-of-the-art electrical doping technology to meet the optical and electrical requirements, resulting in high luminance (over 5000 cd/m2) for the green emissive OLED microdisplay. The unavoidable oxidized aluminium contact after the CMOS processes on the top layer of the pixel was found to significantly increase the driving voltage of the device (up to 1 V at 100 cd/m2 luminance). To aid in the extraction of the accurate device parameters for setting up an equivalent circuit, the reference top-emitting OLEDs without bottom metal contacts were deposited directly on interconnection-metal-only silicon substrates from the CMOS foundry. The distribution of the AlOx of the top-layer metal contact on silicon (the bottom anode of OLEDs) was confirmed by the X-ray photoelectron spectroscopy (XPS) depth profiles.

High-refractive-index capping layer improves top-light-emitting device performance.

The performance of top-emitting organic light-emitting devices (TEOLEDs) was improved with capping layers having high refractive indices. The TEOLED electroluminescence was examined with different capping materials on the same device structure. The luminance and power efficiency in the forward direction was improved significantly by using a high-refractive-index capping layer.

Efficient inverted top-emitting organic light-emitting devices with double electron injection layers

Inverted top-emitting organic light-emitting devices (TEOLEDs) have great potential in flat-panel displays and lighting, yet poor electron injection is a key issue limiting its use in active-matrix OLED. Here, a high-performance green phosphorescent inverted TEOLEDs with ZnO and ZnS double electron injection layers (EILs) were demonstrated. The double EILs which provide a work function gradient can greatly enhance the electron-injection efficiency. For the green phosphorescent TEOLED device based on this double EILs, a high current efficiency of 33.1 cd A−1 is achieved with low turn-on voltage of 4.0 V. Results indicate that a novel pathway of electron transport to accelerate electron injection is created by applying this double EILs.

Efficient and angle-stable white top-emitting organic light emitting devices with patterned quantum dots down-conversion films

Abstract Efficient white top-emitting organic light-emitting devices (TEOLEDs) are fabricated by combining blue TEOLEDs with down-conversion films. Quantum dots (QDs) are integrated with polymethyl methacrylate (PMMA) to obtain freestanding composite films, and the films are used as down-conversion films to produce red emission by converting blue emission from blue TEOLEDs. The blue and white TEOLEDs exhibit the maximum current efficiency of 37.4 and 22.2 cd/A, respectively. Through integrating a microstructure QDs down-conversion film by transfer printing on blue TEOLED, the light extraction performance of white TEOLEDs is improved and the angular distribution of the white emission is almost Lambertian. The CIE color coordinates of white TEOLEDs with patterned QDs down-conversion films shift only (0.025, 0.005) from 0° to 60°.

Flexible Efficient Top-Emitting Organic Light-Emitting Devices on a Silk Substrate

A flexible efficient top-emitting organic light-emitting device (TOLED) on an off-the-shelf silk substrate has been demonstrated by planarizing the silk substrate with photopolymer NOA63. The flexibility of the bare silk substrates was retained in the planarized silk substrates due to ductile characteristics of cured NOA63. The planarized silk substrate has shown superiority on surface morphology, which is beneficial to the performances of OLEDs. Their maximum luminance and current efficiency are 45545 ${\rm{cd/ m}}^{2}$ and 37.7 cd/A, respectively. Moreover, our devices show not only high luminance and efficiency but also high flexibility and mechanical robustness. Emission of operating devices is uniform and free of defects under a very small bending radius and the luminance and efficiency do not deteriorate obviously after repeated bending. TOLEDs on silk substrate are a potential alternative to wearable displays.

Optimization of period and thickness of the corrugated Ag cathode for efficient cross coupling between SPP and microcavity modes in top-emitting OLEDs

The efficient cross coupling between the surface plasmon–polariton (SPP) and microcavity modes was employed in the top-emitting organic light-emitting devices (TOLEDs) to release the trade-off between efficiency and stability by using a periodic corrugated Ag cathode. As the main factors affecting the cross coupling, the thickness and period of the corrugated Ag cathode were optimized to excite the SPP at the two interfaces of the Ag cathode and achieve a matched resonance wavelength between the SPP and the microcavity mode. At the optimal values of the thickness and period, the cross coupling between the SPP and microcavity modes enhanced the light extraction efficiency, and the efficiency of the corrugated TOLEDs was enhanced by 38.6% at the current density of 100 mA cm−2.

Elimination of Plasmon Losses and Enhanced Light Extraction of Top-Emitting Organic Light-Emitting Devices Using a Reflective Subelectrode Grid

We demonstrate high-efficiency light extraction for top-emitting organic light-emitting devices (OLEDs) comprising a transparent conductive oxide on the surface of a nondiffractive, reflecting metal-coated scattering grid located beneath the organic active region. The grid scatters light trapped in waveguide modes without changing the device electrical properties or causing significant plasmonic losses. This results in an increase in external quantum efficiency for green phosphorescent organic light-emitting devices from 20 ± 1% to 30 ± 2%, for structures without and with the reflecting grid. Adding a low refractive index capping layer reduces the spectral angular dependence characteristic of top-emitting OLEDs. The improvement in light extraction by substrate modification allows for optimization of the optical design without necessitating changes in the design or structure of the OLEDs themselves.

Improve angular dependence of emission intensity in top-emitting organic light-emitting devices by using curved surfaces as substrates

We have demonstrated an effective way to suppress the viewing angle dependence of the emission intensity of top-emitting organic light-emitting devices (TOLEDs) by fabricating devices on curved surfaces. From this approach, curved light emission surface has been obtained and TOLEDs with curved surface exhibit a relatively dispersed light emission pattern. As a result, the variation of emission intensity with viewing angles can be relieved. In addition, we can almost observe the same luminance and efficiency in OLEDs with and without curved surface light emission.

不同发光染料的顶发射有机电致发光器件的研制

不同发光染料的顶发射有机电致发光器件的研制

Angle-stable inverted top-emitting white organic light-emitting devices based on gradient-doping electron injection interlayer

By adopting an 4,7-diphenyl-1,10-phenanthroline (Bphen):Ag interlayer with the gradient structure between the cathode and electron injection layer, the electron injection of white inverted top-emitting organic light-emitting devices (WITOLED) was enhanced. The structure of Ag–Ge–Ag anode capping with light outcoupling was used to ensure the spectral stability with the angles. The structure of the emission layer was carefully designed to guarantee the spectral stability with the voltages. The highest current efficiency of the WITOLED can reach 21.7cd/A. The CIE 1931 coordinates varies only 0.022 in X and 0.015 in Y (0o–60o).

Hybrid Tamm plasmon-polariton/microcavity modes for white top-emitting organic light-emitting devices

White top-emitting organic light-emitting devices (WTOLEDs), emitting white light through a transparent top metallic electrode, have emerged as promising candidates as energy-efficient solid-state lighting sources and full-color flat-panel displays. The microcavity effect due to usage of metallic electrodes results in emission enhancement solely at a particular color, and therefore sets an obstacle for WTOLEDs, where at least two colors with balanced intensity should be emitted. Current efforts solving the problem basically rely on the relaxation of the microcavity effect, resulting in sacrificed light outcoupling efficiency in the original resonance region. Here, we demonstrate that by integrating a photonic crystal structure upon the top metallic electrode, an additional emission enhancement peak other than the one determined by the microcavity resonance could be provided by the Tamm plasmon-polariton mode. Mode hybridization induced dual hybrid modes with comparable light outcoupling efficiency can then be excited, from which two colors with balanced intensity could be emitted. Both experimental and theoretical results demonstrate that the proposed mode hybridization strategy may pave the way for the realization of WTOELDs towards high white color quality, improved viewing characteristics, and electroluminescence efficiency.

基于Ag/Ge/Ag阳极的蓝光顶发射有机电致发光器件

通过在Ag层中引入一层Ge薄膜,获得了具有低反射率和高反射相移的Ag/Ge/Ag复合阳极,并制备了基于该阳极的蓝光顶发射有机电致发光器件。阳极高的反射相移使得器件在有机层厚度为100 nm时获得了顶发射蓝光发射,且阳极较低的反射率减弱了器件内的微腔效应,使得其电致发光光谱在不同视角下具有良好的稳定性。当Ge的厚度为20 nm时,器件性能表现最为优良,最高亮度和最大电流效率分别可达3 612 cd/m2和5.4 cd/A,且色坐标在视角从0°变化到60°时仅移动了(0.007,0.006)。

高效高亮度硅基顶发射有机电致发光器件的研制

高效高亮度硅基顶发射有机电致发光器件的研制

Angle-stable top-emitting white organic light-emitting devices employing a down-conversion layer

Angle-stable white top-emitting organic light-emitting devices (WTEOLEDs) by the combination of a blue TEOLED with a down-conversion (DC) layer are demonstrated. The DC layer is composed of red fluorescent dye doped in a host of tris (8-hydroxy-quinolinato) aluminum. The TEWOLED shows a CIE coordinates of (0.240, 0.332) and a maximum current efficiency of 3.95 cd/A. Furthermore, the WTEOLED shows excellently angle-stable characteristic, the CIE coordinates at 20 mA/cm2 only shift by (0.023, −0.007) from 0° to 60°. It can be attributed to the angle-stable blue TEOLED based on the Ag/Ge/Ag transparent cathode.