Microcavity Effect Research Articles

Highly conductive and transparent thin metal layer for reducing microcavity effect in top-emitting white organic light-emitting diode

The optical and electrical properties of the top electrode are crucial for the structural design of top-emitting organic light-emitting diodes (OLEDs). A uniform metal layer, if obtainable, can be electrically excellent and optically well-tuned. In this study, Ag deposited with Mg at a concentration of 9:1 to form a metal thin film exhibits higher conductivity and higher transmittance than those of a neat Ag thin film within a thickness of 10 nm or less. In addition, the thin Mg:LiF (1:1) layer renders the Mg:Ag metal film more uniform and enhances its electron injection property. If a Mg:LiF/Mg:Ag thin film is applied to top-emitting white OLEDs, the driving voltage is lowered and the efficiency is improved. In addition, color shifts for viewing angles are reduced, and the emission characteristics are close to Lambertian. • 10-nm-thick Mg:Ag (1:9) metal forms a more uniform film compared with neat Ag thin metal of the same thickness. • Thin Mg:LiF (1:1) layer improves the optical and electrical properties of the Mg:Ag thin film. • Top-emitting white OLED device with thin Mg:Ag shows higher efficiency and superior angular emission characteristics.

Over 32.5% Efficient Top-Emitting Quantum-Dot LEDs with Angular-Independent Emission.

Top-emitting quantum-dot light-emitting diodes (TQLEDs) with light emitted from the top semitransparent electrodes can improve the aperture ratio, efficiency, and color saturation of displays. However, because of the microcavity effect induced by the top semitransparent electrodes, the devices usually exhibit angular-dependent emission, which is unfavorable for display applications. In this study, highly transparent and conductive indium zinc oxide (IZO) is used as the top electrode to reduce the microcavity effect, thereby improving the angular color stability. By further applying a nanosphere scattering layer on the IZO top electrode, the trapped light is greatly extracted, and thus, the external quantum efficiencies (EQEs) are remarkably improved by 35, 50, and 133% for the red, green, and blue devices, respectively. The champion red TQLEDs exhibit a record EQE of 32.5% with angular-independent color emission. The demonstrated TQLEDs with high efficiency, high color stability, and a high aperture ratio could be the ideal candidates for QLED displays.

Broadband spectral photodetector based on all-amorphous ZnO/Si heterostructure incorporating Ag intermediate thin-films

The purpose of the present work is to experimentally investigate a new high-performance broadband photodetector (PD) based on all-amorphous ZnO/Si heterostructure incorporating Ag ultrathin films. The sensor was developed using the RF magnetron sputtering method at room temperature conditions. In this context, a-ZnO/Ag/a-ZnO/a-Si/Ag/a-Si embedded ultrathin-films were sputtered on the glass substrate. The device structural, morphological, optical, and photodetection characteristics were studied by performing XRD, SEM, EDX, UV–Vis–NIR spectroscopy, and photoresponse characterizations. The analysis showed an improved absorbance behavior over a wide spectral range. It was demonstrated that the use of a-ZnO/a-Si heterostructure with Ag intermediate ultrathin-films incorporation leads to achieving multiband photodetection property with low dark noise effects, where the photodetector provides high responsivity values of 208 mA/W, 160 mA/W, and 180 mA/W over UV, Visible and NIR spectral ranges. These improvements are attributed to the combined effects of improved light-scattering due to the effect of agglomeration of silver on the surface, and the absorbance improvement enabled by optical micro-cavity effects generated by the inserted Ag ultrathin-films. Therefore, the present experimental study can offer new strategies for the development of highly-detective broadband multispectral photosensors based on a-Si photonics platform, which are suitable for optoelectronic applications.

9‐4: Fabrication of the Indirect X‐Ray Detector Using Organic Photodiode

In this work, we introduce an indirect X‐ray detector with organic photodiode (OPD). OPD, as a photodetector, is attractive due to its easy fabrication of even large area or flexible form factor. The top electrode of OPD is fabricated with a translucent organic‐metal‐organic (OMO) structure to absorb more light energy by the surface plasmonic effect at the organic‐metal interface. In addition, the new fabricated OPD has a microcavity effect, which is generated between the top and bottom electrodes, makes the broad absorption spectrum of CuPc (active layer) narrower, and tune to the narrow Gd2O2S scintillator emission wavelength. Through X‐ray experiments, the microcavity effect was shown to be useful and controllable by varying the thickness of C60 in the bulk heterojunction (BHJ) structure of the active layers which are composed of C60 and CuPc.

Analyses of emission efficiencies of white organic light-emitting diodes having multiple emitters in single emitting layer

Efficient white organic light-emitting devices (OLEDs) are highly required in various applications. Yet analyses of light extraction/out-coupling efficiencies of white OLEDs having multiple emitters are much more complicated and difficult than single-emitter monochromatic OLEDs, as ratios of excitons formed on different emitters, different optical (microcavity) effects on emission of different emitters, and effects of emission characteristics (e.g., emitting dipole orientations) of different emitters etc. need to be considered simultaneously. In this work, we report efficient single-emitting-layer white OLEDs adopting blue and orange-red emitters and a systematic and quantitative optical simulation approach taking into account all these factors, for analyzing optical out-coupling efficiencies of single-emitting-layer multiple-emitter white OLEDs. It is believed to be useful for development of efficient white OLEDs. • Efficient white OLEDs with a single emitting layer having blue and orange-red emitters. • Analyses of exciton partition ratios on different emitters of white OLEDs with considering optical microcavity effects. • Quantitative analyses of optical out-coupling efficiencies of white OLEDs having multiple emitters.

Top-Emitting Microcavity Light-Emitting Diodes Based on All-Thermally Evaporated Lead-Free Copper Halide Self-Trapped-Exciton Emitters.

Lead-free metal halide light-emitting diodes (LEDs) based on cesium copper halide (CsCu2I3) self-trapped-exciton (STE) emissions show great potential in lighting and color display applications, especially because of their nontoxicity and earth abundance. However, so far, the efficiency and color purity of CsCu2I3-based LEDs remain low. Here we demonstrate the emission of a CsCu2I3 emitter can be enhanced and narrowed in a top-emitting microcavity device. Consequently, the CsCu2I3-based LED device with the assistance of a top-emitting microcavity has significantly narrowed and enhanced the emission spectrum with a full width at half-maximum of 59 nm and a maximum forward brightness of 14767 cd m-2. To the best of our knowledge, this work achieves the narrowest CsCu2I3 LED spectra and demonstrates the potential of employing the microcavity effect to increase the efficiency and color purity of STE-based light-emitting devices.

Fully Vacuum-Free Fabrication of Bi-Directional Polymer Light-Emitting Diodes Based on a Hybrid Lamination Top Electrode

This study investigates a hybrid lamination electrode based on a silver nanowire and a conductive polymer for the top electrode of bi-directional polymer light-emitting diodes (PLEDs). Through a vacuum-free hot-press lamination step, the proposed hybrid electrode employed a complete and concrete electrical contact with the underlying emissive polymer layer. In addition, by inserting a hole injecting poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) layer to increase the level of work function, the bi-directional PLED device with a laminated top electrode showed a more competitive device performance such as an operating voltage, a current efficiency, and a total external quantum efficiency compared to the counterpart device with a multilayered electrode of MoO3/Ag/MoO3 by the vacuum thermal evaporation method. An analysis carried out on the angular characteristics of bi-directional PLEDs including the variation of color coordinates and change in luminance values according to emission angles confirmed that the hybrid lamination electrode provided a more uniform angular distribution regardless of the direction of emission without any optical micro-cavity effect. Detailed optical and electrical analyses were also performed to evaluate the compatibility of the hybrid lamination electrode for the low-cost fabrication of efficient PLEDs.

Coarse Surface Microcavities Permit Bone Ingrowth and Improve Implant Osseointegration.

To evaluate the effects of coarse microcavities added to micron and submicron rough implant surfaces in the implant-bone anchorage in rabbits. Confocal interferometry was used to quantify roughness. Electron microscopy and energy-dispersive x-rays characterized the surfaces prior to and after implantation. X-ray photoelectron spectroscopy and contact angle determined the surface chemistry and energy, respectively. Fifteen New Zealand White rabbits received, respectively, one cavity-less (C-) and one cavity-rich (C+) implant per femoral condyle and were allowed to integrate for 2 and 8 weeks. The bone-to-implant contact (BIC), bone volume density (BVD), and removal torque (RTQ) were then analyzed. The cavities produced on the surfaces were 48.4 ± 16.8 μm in diameter and 37.8 ± 36.5 μm deep (5.9% ± 1.1% surface coverage). C+ did not alter the surface chemistry or energy. In vivo, C+ implants produced more BIC and RTQ at 8 weeks (P = .002 and P = .059, respectively) and more BVD at 2 and 8 weeks postimplantation (P = .031 and P = .078, respectively). Bone tissue was observed inside the cavities of C+ both histologically and by scanning electron microscopy after implant removal. Unevenly distributed coarse cavities within a micron and submicron rough surface allow bone ingrowth and increase implant stability and bone-surface unions in rabbits. These results encourage the design of implants with multilevel surface topographies to improve implant-based regeneration.

Controlling the Emission Zone by Additives for Improved Light-Emitting Electrochemical Cells.

The position of the emission zone (EZ) in the active material of a light-emitting electrochemical cell (LEC) has a profound influence on its performance because of microcavity effects and doping- and electrode-induced quenching. Previous attempts of EZ control have focused on the two principal constituents in the active material-the organic semiconductor (OSC) and the mobile ions-but this study demonstrates that it is possible to effectively control the EZ position through the inclusion of an appropriate additive into the active material. More specifically, it is shown that a mere modification of the end group on an added neutral compound, which also functions as an ion transporter, results in a shifted EZ from close to the anode to the center of the active material, which translates into a 60% improvement of the power efficiency. This particular finding is rationalized by a lowering of the effective electron mobility of the OSC through specific additive: OSC interactions, but the more important generic conclusion is that it is possible to control the EZ position, and thereby the LEC performance, by the straightforward inclusion of an easily tuned additive in the active material.

Efficient and Stable Quantum‐Dot Light‐Emitting Diodes Enabled by Tin Oxide Multifunctional Electron Transport Layer

AbstractZinc oxide (ZnO) based nanoparticles (NPs) are the most commonly used electron transport layer (ETL) materials in quantum dot light‐emitting diodes (QLEDs). However, numerous defects, severe quenching, and poor stability of ZnO NPs limit the commercialization of QLEDs. Herein, tin oxide (SnO2) NPs are prepared as a substitute to ZnO. The obtained SnO2 NPs exhibit high conductivity, good transparency, and excellent stability, thus enabling them to be applied as multifunctional ETLs for various structured QLEDs, e.g., 1) as a phase tuning layer to tune the microcavity effect in inverted QLEDs, 2) as a thick protection layer to improve the stability of noninverted QLEDs, 3) as a sputtering buffer layer to protect the functional layers from the ion bombardment damage in transparent QLEDs, and 4) as an internal light extraction layer to enhance the light coupling efficiency of top‐emitting QLEDs. With the multifunctional SnO2 ETL, the resultant QLEDs are highly efficient (external quantum efficiency of 27.6%) and stable (half lifetime of 323 h at 4,459 cd m−2), which are comparable and even better than ZnMgO based devices. The results suggest that SnO2 is a promising ETL material for realizing efficient and stable QLEDs.

Improving color rendering index of top-emitting white OLEDs with single emitter by using microcavity effects

We have developed a high color rendering index (CRI) top-emitting white organic light-emitting diode (TWOLEDs) using single emitter by enhancing the yellow-green part between the monomer and excimer emission of platinum [1,3-difluoro-4,6-di(2-pyridyl)benzene] chloride via microcavity effect. Resonant wavelengths can well-tuned by adjusting the thickness of the hole transport layer to modify the optical cavity length. The TWOLEDs exhibits four-peak spectra at 480, 512, 556, and 600 nm with maximum color rendering index of 83 and full half-maximum width of 200 nm. Meanwhile, the chromaticity of TWOLEDs is quite stable, showing slight Commission Internationale deL'Eclairage coordinate shifts of (±0.005, ±0.004) over the driving voltage range of 4–10 V. • High color rendering index and chromaticity stable top-emitting white OLEDs with single emitter. • Four peak emission are achieved by combining monomer, excimer emission and resonant cavity. • Weak microcavity white OLEDs were investigated and modelled. • The device emission shows a Lambertian-like characteristic.

Exciplex host coupled with a micro-cavity enabling high efficiency OLEDs with narrow emission profile

A combination strategy that unites an exciplex host with the micro-cavity effect is developed to resolve the color purity issue. The proposed strategy enables OLEDs to exhibit 2-fold narrower FWHM (26 nm vs. 71 nm) and a CE of 180.2 cd A−1.

Spectral response tuning of organic photodetectors using strong microcavity effects for medical X-ray detector application

An X-ray detector system was fabricated with an organic small molecule photodetector and a terbium-doped gadolinium oxysulfide (Gd2O2S:Tb) scintillating screen. Though the microsphere cluster structure of the Gd2O2S film had advantage in low material cost, wavelength matching to the proper photodetector is challenging due to its scintillating performance property in the relatively narrow emission spectrum range. To tune the absorption spectrum of the organic photodetector to that of the emission spectrum of the Gd2O2S, a microcavity effect was applied by controlling the thickness of the C60 layer. The peak wavelength of the absorption spectrum was tuned over a broad region of wavelengths, from 461 nm to 776 nm. The fabricated X-ray detector system was demonstrated in a medical X-ray radiography system.

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.

Haze-enhanced ZnO/Ag/ZnO nanomesh electrode for flexible, high-efficiency indoor organic photovoltaics

An oxide/metal/oxide multilayer electrode is employed to improve the mechanical flexibility as well as power conversion efficiency of organic photovoltaics. However, its performance needs to be further improved to provide a higher conversion efficiency and better environmental compatibility with curved indoor electronics. In this study, ZnO/Ag/ZnO nanomesh electrodes are incorporated into the inverted non-fullerene organic photovoltaics to enhance the efficiency under indoor and outdoor lighting illumination in a flexible mode. The opto-electrical properties of the perforated ZnO/Ag/ZnO nanomesh electrode with different hole sizes are compared with those of the planar ZnO/Ag/ZnO and indium tin oxide electrodes. The micro-cavity effect and haze effect, which plays a crucial role in determining the performance of the organic photovoltaics, are directly related to the hole diameter. Despite higher transmittance of indium tin oxide, organic photovoltaics using ZnO/Ag/ZnO nanomesh electrodes with a hole diameter of 350 nm exhibits an average conversion efficiency of 15.7% under a 1000 lux light-emitting diode lamp; this efficiency is 45.3% and 27.6% greater than those of organic photovoltaics using indium tin oxide and planar ZnO/Ag/ZnO electrodes, respectively. Furthermore, all ZnO/Ag/ZnO nanomesh-based organic photovoltaics show much higher mechanical flexible properties than those of the planar ZnO/Ag/ZnO-based organic photovoltaics. • ZnO/Ag/ZnO nanomesh is employed as a cathode for flexible organic photovoltaics. • Opto-electrical properties of the ZnO/Ag/ZnO nanomesh electrode are optimized. • As the hole size decreases, optical haze and micro-cavity effect are found to increase. • Fabricated photovoltaic shows a 15.7% efficiency under 1000 lux LED light. • Fabricated photovoltaic also exhibits an excellent mechanical flexibility.

Electromagnetically induced transparency from first-order dynamical systems

We show how a strongly driven single-mode oscillator coupled to a first-order dynamical system gives rise to induced absorption or gain of a weak probe beam, and associated fast or slow light depending on the detuning conditions. We derive the analytic solutions to the dynamic equations of motion, showing that the electromagnetically induced transparency (EIT) like response is a general phenomenology, potentially occurring in any nonlinear oscillator coupled to first-order dynamical systems. The resulting group delay (or advance) of the probe is fundamentally determined by the system damping rate. To illustrate the practical impact of this general theoretical framework, we quantitatively assess the observable consequences of either thermo-optic or free-carrier dispersion effects in conventional semiconductor microcavities in control/probe experiments, highlighting the generality of this physical mechanism and its potential for the realization of EIT-like phenomena in integrated and cost-effective photonic devices.

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.

Quantum mechanical effects for a hydrogen atom confined in a dielectric spherical microcavity

The quantum mechanical effects for a hydrogen atom confined in a dielectric spherical microcavity is discussed for the fist time. Especially we calculate the energy and Shannon entropy of this system. Some unexpected and interesting phenomena appear due to the effect of the dielectric spherical microcavity. Firstly, the energy of this system is not always negative. For smaller spherical microcavity, the energy can be positive. The turning radius for the bound energy changes from positive to negative depends on the dielectric constant of the spherical microcavity sensitively. Second, the dielectric spherical microcavity impacts the rearrangement of the excited state energy, and breaks the energy degeneracy of the excited states. At some given radius, there is energy crossover between different orbital. Third, the dielectric in the spherical microcavity affects the Shannon entropy change for the confined hydrogen atom greatly. When the size of the spherical microcavity is small, the Shannon entropy change is negative, which suggests that the electron density is localized. As we increase the radius of the microcavity, the Shannon entropy change becomes positive, and the confinement of the electron density gets delocalized. Our results show that we can control the quantum mechanical effects of the atom by changing the dielectric constant in the spherical microcavity. This work can guide the future experimental studies for trapping and manipulating of atoms and molecules in the external environment and has some practical applications in metrology and quantum information processing.

Threshold estimation of an organic laser diode using a rate-equation model validated experimentally with a microcavity OLED submitted to nanosecond electrical pulses

We propose a new model for the organic laser diode based on rate equations for polarons, singlet and triplet excitons for both host and guest molecules, and photon densities. The proposed model is validated experimentally by comparing calculated and measured optical responses in the context of pulsed nanosecond electrical excitation of high-speed μ-OLEDs in the limit of weak micro-cavity effects. We predict the laser threshold current density as a function of the micro-cavity quality factor, for two material gain and residual absorption values. We elucidate the crucial role played by the latter in setting the laser threshold and comment on the recently observed threshold value of ∼500 A/cm2 by the group of Adachi [1]. Simulations predict that laser action under short electrical nanosecond pulse single-shot excitation is accompanied by damped relaxation oscillations in the GHz regime. The measured ultra-short experimental optical responses at 45 V are best reproduced numerically when the Langevin recombination rate is larger than usually observed in the literature as a consequence of the field dependence of the Poole-Frenkel law for the mobility.

Improved green thermal activated delayed fluorescence OLEDs based on thermally evaporated distributed Bragg reflector (DBR) of MgF2/ZnS

Unlike the traditional fabrication of distributed Bragg reflector (DBR) structure via atomic layer deposition or spin-coating, here the 1–6 pairs of magnesium fluoride (MgF2)/zinc sulfide (ZnS) alternative dielectric layers were grown via thermal evaporation. The absorption, transmission, reflection, and photoluminescence (PL) spectra were evaluated. 5 pair MgF2/ZnS denotes the largest reflectance (88.5% at 535 nm) together with a stopband at 450–650 nm among the 1– 6 pair dielectric layers, exhibiting the potential for using as DBR. Relative to the bare 4,4’-bis(carbazol-9-yl)biphenyl(CBP):(4s,6s)−2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN) film, the PL intensity of CBP:4CzIPN/5 pair MgF2/ZnS DBR is enhanced and splitted into two peaks. The 5 pair alternative dielectric film presents more uniform aggregation over 4 pair MgF2/ZnS. The cross-sectional scanning electron microscopic image denotes explicit layering for the MgF2 and ZnS. The organic light-emitting diode (OLED) incorporating 5 pair MgF2/ZnS DBR layers illustrates significantly improved electroluminescent (EL) performance due to the photons concentrated in the direction perpendicular to the DBR. The slightly narrowed EL spectrum is originated from the microcavity effect between the two Al electrodes. Here we develop a universal method for the DBR fabrication suitable to most of OLEDs.