Semitransparent Cathode Research Articles

ZnO/Ag/ZnO multilayers as an n-type transparent conducting electrode for transparent organic light-emitting diodes

Alternative transparent conducting electrodes (TCEs) have gained significant attention in recent decades in an effort to replace costly indium-tin oxide (ITO) in organic optoelectronic devices. For example, state-of-the-art top-emitting and transparent organic light-emitting diodes (OLEDs) in the display industry require a transparent or semitransparent cathode for electron injection. Current OLED display technology employs ultrathin metallic (typically Mg:Ag) layers as semitransparent cathodes. However, recent studies have shown that metal oxide/metal/metal oxide multilayers can perform better optically and offer improved stability compared to semitransparent metal cathodes. Deposition of these multilayers with minimal damage to the underlying organic emissive layer is imperative. Here, we investigate a ZnO/Ag/ZnO TCE multilayer TCE deposited under mild sputtering conditions that are safe for the emissive layer. We demonstrate ∼ 85 % transparent electrodes with a sheet resistance of 1.47 Ω/sq, which is more conductive than the industry standard, ITO. We also integrate the multilayer TCE as the cathode in a transparent OLED device built on ITO, demonstrating the compatibility of this TCE with the existing OLED technology.

Top-Emitting Active-Matrix Quantum Dot Light-Emitting Diode Array with Optical Microcavity for Micro QLED Display.

An electroluminescent quantum-dot light-emitting diode (QLED) device and a micro QLED device array with a top-emitting structure were demonstrated in this study. The QLED device was fabricated in the normal structure of [ITO/Ag/ITO anode]/PEDOT:PSS/PVK/QDs/[ZnO nanoparticles]/Ag/MoO3, in which the semi-transparent MoO3-capped Ag cathode and the reflective ITO/metal/ITO (IMI) anode were designed to form an optical microcavity. Compared with conventional bottom-emitting QLED, the microcavity-based top-emitting QLED possessed enhanced optical properties, e.g., ~500% luminance, ~300% current efficiency, and a narrower bandwidth. A 1.49 inch micro QLED panel with 86,400 top-emitting QLED devices in two different sizes (17 × 78 μm2 and 74 × 40.5 μm2) on a low-temperature polysilicon (LTPS) backplane was also fabricated, demonstrating the top-emitting QLED with microcavity as a promising structure in future micro display applications.

The Selection of Anode and Cathode Materials for Top Emission Organic Light-Emitting Diodes

In this study top emission organic light-emitting diodes (TE-OLED) were successfully fabricated on flexible PET/multilayer disordered silver nanonetwork (MDSN) substrate with conventional LiF/Al as a semitransparent cathode and Ag as a reflective anode. The effects of the hole injection layer, anode buffer layer, and an electron injection layer on the luminescence characteristics of TE-OLEDs were investigated. At first, the thickness of cathode Al is adjusted based upon the overall transmittance of the top-emission cathode and its conductivity. There is a high energy barrier for the hole between the work function of the anode Ag (4.2 eV) and the NPB highest occupied molecular orbital (HOMO) energy level (5.5 eV), which is not favorable for hole injection. This study tested four types of hole injection layer (HIL) materials. Finally, MoO3 was selected as an optimal HIL material, and the optimum thickness was adjusted, enabling the hole to be injected smoothly from the anode to the NPB and then to an emitting layer. The TE-OLED luminance reached 268 cd/m2. There is a high energy barrier between the work function of Ag and MoO3 HOMO (5.3 eV)—about 1.1 eV—which is still not conducive to the hole injection, so a thin layer of high work function metal Au (work function 5.1 eV) was added to the top of the anode silver, which more matches with the MoO3 energy level. It can make the hole easier to inject from the anode to MoO3 (HIL) and protect the silver from oxidation. At 8 V, the luminance is increased to 413.7 cd/m2, and the current efficiency is 0.81 cd/A. The luminance is significantly improved. An electron transport/hole blocking layer TPBi (10 nm) was added to enhance the electron transport capability and effectively block the holes with higher electron mobility and a higher HOMO energy level of TPBi. So that more holes can remain in the emitting layer and increase the chance of the electron-hole recombination to improve the luminance and current efficiency of TE-OLED. At 8 V, the luminance and current efficiency can reach 611.4 cd/m2 and 0.95 cd/A, respectively.

High‐Efficiency Top‐Emitting Green Perovskite Light Emitting Diode with Quasi Lambertian Emission

AbstractMetal halide perovskite light‐emitting diodes (PeLEDs) are regarded as alternative candidates for next‐generation display technologies due to their high efficiency, superior color purity, tunable bandgap. However, the research on transparent and top‐emitting PeLED with Lambertian emission profile significantly lags behind due to optical microcavity effect, which has become one of the main obstacles for potential practical display applications. Here, strategies are developed to suppress the microcavity effect by enhancing the transmission of the semitransparent electrode and extending the optical cavity length. Besides, a nanopatterned structure is incorporated to suppress the surface plasma and waveguide mode effect. Based on the above combination strategies, a transparent PeLED yields a record total external quantum efficiency (EQE) of 16.1% with Lambertian emission by depositing high refractive index molybdenum oxide on the semitransparent cathode. Besides, a high EQE of 13.6% of a top‐emitting device with quasi‐Lambertian emission is achieved by stretching the optical distance between two reflective layers and enhancing the transmission of the semitransparent electrode. The nanopatterned structure enables suppressed waveguide mode and surface plasmon polariton dilapidation of top‐emitting PeLEDs. This work paves a path to design transparent and top‐emitting PeLEDs for potential applications in both traditional and novel transparent displays.

An Improved Compact and High-Efficiency Relativistic Magnetron With TE11 Mode Radiation

An improved compact and high-efficiency A6 relativistic magnetron (RM) with axial extraction is proposed and studied by numerical simulations. Compared with the conventional design, two pairs of diametrically opposite cavities (four cavities) of the anode block are tapered onto the horn antenna for TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode radiation with high output efficiency. Due to the cutoff radius of the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode is the smallest in the circular waveguide, the diameter of the output waveguide could be reduced obviously. And the total axial length of the horn antenna could also be shortened accordingly. At the same time, a semitransparent cathode is used for quick startup and efficient beam-wave interaction. Results of the 3-D particle-in-cell (PIC) simulations show that over 1.25 GW output power with TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode is obtained at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}$ </tex-math></inline-formula> -band, when the electron beam voltage is 600 kV and the applied axial guiding magnetic field is 0.54 T. And the efficiency exceeds 66%. Simulation results also show that the output efficiency of the proposed RM keeps almost unchanged between 550 and 620 kV with fixed guiding magnetic field.

A highly transparent laminated composite cathode for organic light-emitting diodes

Transparent conductive electrodes are one of the important components of organic optoelectronic devices. Mg:Ag alloys have been widely used as a semi-transparent cathode for top-emitting organic light-emitting diodes (OLEDs) or transparent OLEDs (TrOLEDs) due to their high conductivity and relatively low work function. However, their low transmittance makes the light-extraction efficiency to be comparatively low. In this study, a highly transparent (84.6% in the visible light range), conductive, laminated cathode with the structure of ZnO/Mg:Ag/ZnO was fabricated via atomic layer deposition (ALD). ALD-deposited ZnO is used to protect the vulnerable ultrathin Mg:Ag-alloy layer, while preserving its low work function for the entire structure. In particular, the bottom ZnO layer is used as an ETL, which can provide reasonable energy alignment between the cathode layer and the emission layer and prevent metal particles from penetrating into the underlying organic layer. The top-emission OLED, which is prepared in this study by using the laminated cathode, showed an 85.8% increase in brightness, with a current density of 11.2 mA/cm3 at 4.0 V, which is an ultra-low voltage compared with devices made with Mg:Ag cathodes. In addition, the total luminance of TrOLEDs increases 34.4% compared with devices using Mg:Ag cathodes. Furthermore, the average transmittance of the device was still as high as 77.19% for visible light.

Manipulation of blue TADF top-emission OLEDs by the first-order optical cavity design: toward a highly pure blue emission and balanced charge transport

The broad luminescence spectrum of a thermally activated delayed fluorescence (TADF) organic light-emitting diode (OLED) is a critical issue to overcome for its application in high-color-purity displays. Herein, a novel device structure that utilizes the first-order microcavity optical mode with a high radiance intensity is demonstrated to solve this problem by considering the charge transport properties through the analysis of hole-only and electron-only devices. In addition, by tuning the optical interference near the semitransparent top cathode layers consisting of thin silver and organic capping layers, light extraction is increased by nearly 2 times compared to the device without a capping layer. Consequently, the optimized blue TADF top-emission OLED exhibits much lower full width at half-maximum, higher maximum current efficiency, and external quantum efficiency compared to the device before optimization. This approach is expected to provide a simple but effective way to further enhance the spectral purity of the conventional TADF-based OLEDs.

Semitransparent Circularly Polarized Phosphorescent Organic Light‐Emitting Diodes with External Quantum Efficiency over 30% and Dissymmetry Factor Close to 10−2

AbstractTo concurrently realize large electroluminescence dissymmetry factor and high device efficiency remains a formidable challenge in the development of circularly polarized organic light‐emitting diode (CP‐OLED). In this work, by introducing a famous chiral resource of R‐camphor, two green chiral iridium(III) isomers of Λ/Δ‐Ir‐(R‐camphor) containing dual stereogenic centers at iridium and ancillary ligand, are efficiently synthesized. Benefiting from their high phosphorescence quantum yields (≈93%) and obvious circularly polarized photoluminescence property with dissymmetry factors (|gPL|) in the 10−3 order of magnitude, the circularly polarized phosphorescent organic light‐emitting diodes using these chiral emitters show excellent performances with the maximum external quantum efficiency (EQEmax) of 30.6%, low efficiency roll‐off, and intense circularly polarized electroluminescence signal with dissymmetry factor (gEL) in the 10−4 order of magnitude. By the novel device engineering with semitransparent cathode, the resulting gEL values are significantly boosted by one order of magnitude, up to 7.70 × 10−3, which is the highest among the CP‐OLEDs with chiral iridium complexes. Noteworthy, the semitransparent devices deliver a record‐high EQEmax of 18.8% among the semitransparent OLEDs. The combination of novel chiral Ir(III) complexes and the semitransparent devices sheds light on the development of high performance CP‐OLEDs with simultaneously high efficiency and large dissymmetry factor.

A Deep Blue Strong Microcavity Organic Light‐Emitting Diode Optimized by a Low Absorption Semitransparent Cathode and a Narrow Bandwidth Emitter

Herein, deep blue strong microcavity top‐emitting organic light‐emitting diodes (TEOLEDs) optimized by a low absorbed silver cathode and a narrow bandwidth emitter are reported for realizing high‐efficiency and narrow spectral characteristics. Usually, the Mg:Ag cathode is widely used as a semitransparent cathode in TEOLED devices due to its favorable reflectivity and proper electron injection ability. However, the high extinction coefficient of Mg causes a decrease in the microcavity effect, causing a low optical efficiency of devices. A Ag cathode is applied as a semitransparent cathode to modify the high absorption property for a strong microcavity blue OLED. At optimal conditions, the optically calculated device efficiency of TEOLEDs using a BPPyA emitter is 70.2 and 91.1 cd (A * y)−1 for Mg:Ag and Ag TEOLEDs, respectively. To further improve the device efficiency, an alternative emitter (DABNA‐NP‐TB) is applied which has a narrow bandwidth of 25 nm. The optically calculated efficiency enhancement in the TEOLED is about 23% compared with a control emitter, which is well correlated with the result of fabricated TEOLEDs.

Semi-transparent organic-inorganic hybrid perovskite light-emitting diodes fabricated under high relative humidity

In this study, we fabricated semi-transparent organic-inorganic hybrid perovskite light-emitting diodes (st-PeLEDs) using the solution processing of all its functional layers including the silver nanowire network (AgNW) cathode under ambient air conditions with relative humidity (RH) of over 40%. Aerogel nanoparticle film was used to passivate the film defects in the ambient air processed methylammonium lead bromide (MAPbBr3) perovskite nanocrystal. The semi-transparent aerogel nanoparticle film and AgNW cathode also allowed the emitted light of the st-PeLED to be transmitted through them in addition to the transmission of emitted light through the indium tin oxide (ITO) transparent anode. Under ambient air fabrication conditions with RH of over 40%, the st-PeLEDs fabricated using the optimized nitrogen (N2) gas blowing method provided high maximum luminance (L) of approximately 657 cd·m−2 emission through the bottom ITO anode and 320 cd·m−2 emission through the top AgNW cathode. Furthermore, excellent operational stability was confirmed during bending and twisting of the st-PeLEDs with aerogel passivation layer fabricated on flexible polymer substrate.

Electro-optically Efficient and Thermally Stable Multilayer Semitransparent Pristine Ag Cathode Structure for Top-Emission Organic Light-Emitting Diodes

To attain high efficiency and stability in a top-emission organic light-emitting diode (TEOLED), the semitransparent metal cathode must have high transmittance, low absorbance, and uniform surface ...

Highly efficient and wide-color-gamut organic light-emitting devices using external microcavity

An external microcavity structure was introduced to the stacked cathode in organic light-emitting devices (OLEDs), and its influence on the luminescent characteristics was investigated. From the detailed optical analysis that was conducted, it was found that the surface plasmon loss in a metal cathode can be reduced to about one-fifth by using a semi-transparent thin-film cathode, and can be extracted outside through the microcavity effect after being converted to the thin-film waveguide mode. No less than 50% of the optical power in dipole emission was successfully utilized as the external and substrate modes. The luminous efficiency increased about 1.4-fold, and the color purity of the luminance also improved. As a trial preparation of red, green, and blue OLEDs with the external microcavity structure, the color gamut improved greatly and approached the BT.2020 national standard. The effect of the external microcavity will be discussed from the viewpoint of multi-scale optical analysis.

Theoretical Modeling of the Internal Power Flow and Absorption Loss of the Air Mode Based on the Proposed Poynting Vector Analysis in Top-emitting Organic Light-emitting Diodes

We propose the Poynting vector analysis of the air mode in a top-emitting organic light-emitting diode (OLED) by combining the transfer matrix method and dipole source term. The spatial profiles of the time-averaged optical power flow of the air mode are calculated inside and outside the multilayer structure of the OLED with respect to the thickness of the semi-transparent top cathode and capping layer (CPL). We elucidate how the micro-cavity effect controlled by the thickness variation of the semi-transparent top cathode or CPL affects the internal optical power and absorption loss inside the OLED multilayer and the external optical power coupled into the air. When the calculated absorption loss and external power obtained by the proposed Poynting vector and currently-used point dipole models are compared, two calculation results are identical, which demonstrates the validity of the two models.

Dual mode OPV-OLED device with photovoltaic and light-emitting functionalities

The rapid development of organic optoelectronic devices such as organic photovoltaics (OPVs) and organic light-emitting devices (OLEDs) is largely attributable to their advantageous properties of their large area, ultrathin thickness, flexiblility, transparency, and solution processability. Herein, we fabricate and characterize a dual mode OPV-OLED device with three-terminal structure comprising a polymer-based bulk-heterojunction inverted OPV unit and a top-emission white phosphorescent OLED unit back-to-back connected via intermediate metal alloy electrode. Sputter-deposited indium tin oxide was used as a transparent cathode of the inverted OPV unit, whereas Ag-doped Al served as a common OPV/OLED anode, allowing the decoupling of electricity generation and light mission functions. Notably, the doping of Al by Ag facilitated the reduction of surface roughness, allowing the above electrode to be used as a common anode and dramatically reducing the leakage current. Finally, the top-emission OLED unit featured an ultrathin layer of Ag-doped Mg as a semitransparent cathode. Thus, successful integration of the OPV-OLED elements results in the decoupling of electricity generation and light emission functionalities, achieving a power conversion efficiency of 3.4% and an external quantum efficiency of 9.9%.

P‐3.1: 2 Stack‐2&amp;3 Color Top Emission White OLEDs for AR &amp; VR

In this work, we developed top emission white OLED (T/E WOLED) device for an application to high resolution AR and VR. By preparing a good performance of semitransparent cathode and optimizing the critical organic layer to co‐regulate the microcavity structure, we fabricated the 2 stack 3‐color as well as 2 stack 2‐color WOLED devices, and the key impact factors to the performance of T/E WOLED were discussed. Results indicate that the 2 stacked 3‐color WOLED device shows a very wide color gamut, while the microcavity structure leads to a low light‐emitting efficiency. However, the 2 stack 2 color device (blue unit + yellow unit) exhibits a wonderful light‐emitting efficiency and CIE coordinates of (0.37, 0.36). Furthermore, the efficiency of this device can achieve as high as 80% of the bottom emitting device, revealing a superior prospect to the application of AR &amp; VR.

Design and simulation of output mode conversion structure of relativistic magnetron with all cavity output

A relativistic magnetron using all cavity extraction and semi-transparent cathode has the virtues of compactness, high output power and high efficiency. The three-dimensional particle-in-cell simulations show that 1.15 GW output microwave with an efficiency about 50% can be obtained at S-band with pure TE11 mode of the fan waveguide. However, due to the fact that the output structure is composed of three detached fan waveguides, mode conversion structure in the output region is required for the convenience of practical applications. Therefore, two mode conversion structures are studied for the output mode conversion. The first structure is to widen gradually or abruptly the fan waveguide in the azimuthal direction from a given position (starting point) along the microwave transport direction. And then the three fan waveguides are connected into one coaxial waveguide. The effects of the position of the starting point on the beam-wave interaction and microwave extraction are numerically studied. For the convenience of description, we define L as the axial distance between the center of the output coupling hole and starting point. Simulation results show that for the abrupt and gradual variation case, when the length of L changes in a relatively wide region, the output power is larger than 1.0 GW in TEM mode at S-band. It is about 90% of the conventional fan waveguide with 1.15 GW. For the gradual variation case, the optimal value of L equals 10.0 cm, and the corresponding output power is beyond 1.0 GW. For the abrupt variation case, the optimal value of L equals 13.75 cm, the corresponding output power is about 1.15 GW. But in the abrupt variation case, the output power is a little more sensitive to the value of L. The second structure is to convert the fan waveguide into a rectangular waveguide. Ａcompound waveguide composed of a section of fan waveguide and a section of rectangular waveguide is designed for studying its feasibility. In the compound waveguide, the wide edges of the cross section of the rectangular waveguide are tangent to the inner and outer arc of the fan cross section respectively. And the narrow edges cross the end points of the outer arc. Simulation results show that in the compound waveguide the microwave with TE11 mode of the fan waveguide input at the inlet can be changed into the TE10 mode of the rectangular waveguide at the outlet with almost no power loss. In all, the output microwave power larger than 1.0 GW could be obtained after using the two proposed mode conversion structures. In practical applications, one could choose the relevant mode conversion structure according to the requirement.

Efficient micro-cavity top emission OLED with optimized Mg:Ag ratio cathode.

Micro-cavity top-emitting organic light emitting diodes (TEOLEDs) are now receiving prominence as a technology for the active matrix display applications. The semi-transparent metal cathode plays the crucial role in realizing TEOLEDs structure. Here, we report the optimization results on Mg:Ag ratio as the semitransparent cathode deposited by vacuum thermal evaporation. The optimized Mg:Ag cathode with 1:10 ratio (wt %) shows a sheet resistance value as low as 5.2 Ω/□, an average transmittance of 49.7%, reflectance of 41.4%, and absorbance of 8.9% over the visible spectral region (400~700 nm). The fabricated red TEOLEDs device implemented using LiF (1nm)/Mg:Ag (1:10) cathode shows the voltage value of 4.17 V at a current density of 10.00 mA/cm2, and current efficiencies variation from 55.3 to 50.1 cd/A over the brightness range 2,000 - 12,000 cd/m2. The electroluminescence (EL) spectrum displays the light emission at 608 nm wavelength with a half width of 29.5 nm. The narrow half-width of red light emission is attributed to the micro-cavity effects due to the semitransparent cathode.

Low absorption semi-transparent cathode for micro-cavity top-emitting organic light emitting diodes

Abstract We present a low absorption semi-transparent cathode for top-emitting organic lighting diodes (TEOLEDs). To reduce the light loss by electrode absorption, transparent cathode materials with a high refractive index and low extinction coefficient characteristics at the visible light range are investigated for the highly efficient TEOLEDs. Molybdenum trioxide (MoO3) and tungsten trioxide (WO3) are considered as the transparent materials. Due to the low conductivity of MoO3 and WO3, a small amount of silver (Ag) is blended in order to enhance the electrical property as electrodes. Optimized MoO3:Ag mixed film shows the reduced extinction coefficient by half compared to Mg:Ag. With a general organic material (refractive index∼1.8) as a capping layer, the average absorption, transmittance and omnidirectional reflectance of 18.2, 65.9, and 25.6% are observed, respectively, with a sheet resistance of 12.7 Ω/□. The current efficiencies of fabricated red, green and blue TEOLEDs with optimized MoO3:Ag cathode are 32.4, 76.8, and 2.8 cd/A, respectively. The performance enhancement ratio is 9.8, 13.6, and 86.7% for red, green and blue devices compared with Mg:Ag cathode devices.

CuI-based hybrid junction in photovoltaic devices with semitransparent cathode

A hybrid inorganic/organic heterojunction formed by p-type cuprous iodide CuI and n-type fullerene C60 was employed in the archetypal photovoltaic devices. The thickness of the p-CuI layer deposited on a molybdenum oxide (MoOx) anode buffer was optimized to achieve a higher efficiency, while the thickness of the n-C60 photoabsorber layer was fixed. In an attempt to obtain semitransparent devices, a trilayer MoOx/metal/MoOx cathode with a BCP underlayer was applied (metal = Ag or Mg, BCP = bathocuproine). The maximum photovoltage of such devices on a PET/ITO substrate was equal to 0.75 V for an average transparency of about 35% over the range of 380–1000 nm. Similar devices having a non-transparent Al cathode in conjunction with an Alq3 underlayer generate photovoltage of 0.82 V (Alq3 = tris-8-hydroxy­quinolinato-aluminum), which shows the important effect of hybrid junctions in multilayer organic-based devices.

Thin‐film encapsulated white organic light top‐emitting diodes using a WO3/Ag/WO3 cathode to enhance light out‐coupling

An alternative design of a semitransparent cathode for top‐emission white‐fluorescent organic light‐emitting diodes (OLEDs) has been investigated. The scope of this study was to improve the luminance of OLEDs used for displays while keeping the current density versus voltage characteristic unchanged for addressing purposes. The use of an optical simulation tool allowed the optimization of the tri‐layer cathode WO3/Ag/WO3 to increase the light out‐coupling coefficient of the device leading to an increased white emission compared with a reference device with a Ca/Ag cathode. An increase of ~40% in luminance has been calculated by simulation and experimentally confirmed. The p‐i‐n OLED structure underneath the tri‐layer cathode allowed an efficient injection of electrons independently from the work function of WO3. The WO3/Ag/WO3 cathode has been also confirmed to be compatible with the atomic layer deposition technique for thin film encapsulation. Finally, lifetime measurements up to 600 h have been carried out to quantify the enhancements induced by the new cathode compared with the control device. It has been found that lifetimes of both cathode architectures are similar on this time scale, while the WO3/Ag/WO3 cathode shows a lower voltage drift versus aging.