Microcavity Organic Light-emitting Diodes Research Articles

Design and optimization of a blue fluorescent microcavity-organic light-emitting diode (MOLED) on AZO anode for an algae excitation light source application

In this work, we present simultaneous organic light-emitting diode (OLED) stack optimization and optical modelling for a blue microcavity-OLED (MOLED) based on AZO anode to be used as algae excitation light in optical biosensor. Fluorescent materials (MADN and DPAVBi) were chosen as host:guest for the doped emissive layer due their known stability compared to phosphorescent and thermally activated delayed fluorescence (TADF) materials. The MOLED modelling was performed by targeting 470 nm as the maximal excitation wavelength and suppressing the emission in the algae fluorescence bandwidth (600–800 nm) in order to fulfil the sensor requirements. By using a bilayer hole transport layer/electron blocking layer (HTL/EBL) instead of a single HTL, the total thickness was adjusted to meet the resonance wavelength condition without loss of efficiency, while at the same time preserving a maximum electric field intensity in the emissive layer (antinode position). MOLED devices were fabricated by organic semiconductor evaporation on three dielectric distributed Bragg reflectors (DBRs) with 3 different numbers of TiO2/SiO2 (high-index/low-index) pairs and aluminum-doped zinc oxide (AZO) transparent electrode. Devices with 1.5 pairs as DBR showed not only an improved external quantum efficiency (+33%) compared to a standard OLED but also an increase of about 3 times the intensity of the peak at 470 nm combined to a lower emission in the 600–800 nm bandwidth, as aimed. This increase in the peak intensity should lead to a longer device lifetime, as the current density necessary to excite algae at 470 nm is significantly lower owing to the microcavity effect.

Organic Light‐Emitting Diode Based Fluorescence Sensing System for DNA Detection

AbstractConventional fluorescence sensing equipment for disease detection is expensive and bulky, restricting access of patients to accurate diagnosis. Organic light‐emitting diodes (OLEDs) have the potential to enable compact fluorescence sensing compatible with point‐of‐care (POC) testing. However, the limited brightness and broad emission spectra of OLEDs can be a challenge for achieving good sensitivity. Here, co‐host microcavity OLEDs with narrowed spectra, high conductivity, and high brightness are developed for fluorescence sensing. The OLEDs are driven in pulsed mode for achieving higher brightness and stable light output. To sense the presence of single‐stranded DNA (ssDNA), two complementary ssDNA labelled with Cyanine dyes – Cy3 (ssDNA‐Cy3) or Cy5 (ssDNA‐Cy5) are used to form a Förster resonance energy transfer (FRET) pair when the DNA hybridized. The dye‐labelled DNA is then excited by the OLED at the Cy3 excitation wavelength, and the fluorescence is detected at the emission wavelength of Cy5. As a result, the device shows a very high sensitivity that can detect as low as 1 × 10−9 m of ssDNA‐Cy5 in fetal bovine serum (FBS). This work shows a simple approach to highly sensitive fluorescence sensing with OLED light sources that is promising for use in POC diagnostics.

Suppressing surface plasmon losses to improve the efficiency of blue organic light-emitting diodes using the plasmonic quasi-bandgap phenomenon

It is a persistent problem in organic light-emitting diode (OLED) display devices that the efficiency of blue-light source materials is lower than that of green- or red-light source materials. To address this problem, numerous studies have investigated blue-light-emitting materials. However, ensuring the reliability of the blue-light-emitting materials has been difficult in most studies. In this study, electrodes using asymmetric dielectric/metal/dielectric structures with plasmonic quasi-bandgap characteristics were developed to achieve a highly efficient blue fluorescent OLED. The electrodes were applied in a microcavity OLED and a transparent OLED. Using the developed electrode in fabricated OLED devices not only maximized the cavity resonance effect and transparency, but also preserved the advantage of the work function of the metal in terms of electrical properties, with high device stability. The approach also minimized losses caused by surface plasmon polaritons, which is a blind spot in the optical aspect of metal electrodes, resulting in improved light extraction efficiency.

Enhanced light extraction efficiency and viewing angle characteristics of microcavity OLEDs by using a diffusion layer

The viewing angle characteristics and light extraction efficiency of organic light-emitting diodes (OLEDs) with a micro-cavity structure were enhanced. This was accomplished by inserting a diffusion layer composed of nano-sized structures of a transparent polymer poly(methyl methacrylate) (PMMA) combined with a zinc oxide (ZnO) semi-planarization layer with a high refractive index (n = 2.1) into the devices. The PMMA nanostructures were fabricated by employing a reactive ion etching (RIE) process. The height and density of the PMMA nanostructures were controlled by varying the speed at which the PMMA was spin-coated onto the substrate. The insertion of the diffusion layer into the micro-cavity OLEDs (MC-OLEDs) improved the external quantum efficiency (EQE) by as much as 17% when compared to that of a MC-OLED without a diffusion layer. Furthermore, adjustment of the viewing angle from 0° to 60° halved the peak shift distance of the electroluminescence (EL) spectra from 42 to 20 nm. Additionally, changing the viewing angle from 0° to 60° changed the color coordinate movement distance of the MC-OLED with the diffusion layer to 0.078, less than half of the distance of the MC-OLED without the diffusion layer (0.165).

Fabrication and characterization of microcavity organic light-emitting diode with CaF2/ZnS distributed Bragg reflector

We fabricated alternately multilayered films using CaF2 and ZnS with a different number of pairs by thermal evaporation method and evaluated their reflectance as a distributed Bragg reflector (DBR). A 7-pair CaF2/ZnS multilayered film had an enough high reflectance for use as DBR in the wavelength region of 400–600 nm, which is good agreement with the theoretically calculated reflectance. The fabricated CaF2/ZnS multilayered film was incorporated to the organic light-emitting diode (OLED) devices. We found that the inversed-type OLED (iOLED) structure is suitable for achieving the working device. From the result of electroluminescence (EL) measurement, the spectral linewidth of EL of inversed OLED with DBR (μ-cavity iOLED) was significantly narrowed compared to that of conventional OLED (cOLED). The EL peak wavelength of μ-cavity iOLED was centered around 520 nm, which was well corresponded to the designed cavity length. The observed narrowing of EL spectra for μ-cavity iOLED was originated from the microcavity effect. Color coordinate of the emission were changed from cOLED: (0.31, 0.54) to μ-cavity iOLED (0.20, 0.72), which indicates a significant improvement of color purity of green emission.

Microcavity‐Enhanced Blue Organic Light‐Emitting Diode for High‐Quality Monochromatic Light Source with Nonquarterwave Structural Design

AbstractDespite their rapid development, organic light‐emitting diodes (OLEDs) are limited to display and lighting applications, and exploiting the application and development of OLEDs has become one of the critical issues. In this study, a new type of distributed Bragg reflector (DBR) with low absorption spacer as a bottom mirror is developed through nonquarterwave structural design. Different factors affecting the electroluminescence (EL) properties of microcavity OLEDs (MOLEDs) are investigated by using the optical microcavities with different structures to regulate the EL properties of 4,4′‐bis[(N‐carbazole)styryl] biphenyl (BSB‐Cz). The optimized MOLEDs with quarterwave DBRs exhibited a maximum external quantum efficiency of 8.86 ± 0.06%, demonstrating an enhancement of 79% compared to a reference device. Under the premise of almost not changing the electrical injection of the functional layer materials in the cavity, the spectral full‐width at half maximum of MOLEDs with a nonquarterwave structural design is as narrow as 2 ± 0.05 nm. These improvements are not only suitable for small organic molecules or polymeric materials, but also for novel nanoluminescent materials, such as quantum dots, perovskites, etc. Overall, the current study suggests the general applicability of this novel concept to obtain high‐quality monochromatic light, irrespective of the type of materials used.

Solution-processed, top-emitting, microcavity polymer light-emitting diodes for the pure red, green, blue and near white emission

Top-emitting microcavity polymer light-emitting diodes (TMPLEDs) are of great significance for active matrix PLED displays with high color purity. However, the complex device structures of highly efficient microcavity organic light-emitting diodes fabricated by the full vapor deposition technology are not suitable for solution-processed PLEDs. Solution-processed TMPLEDs with simple device structures are promising candidates for large-area, mass production display techniques. In this work, three strategies were used to apply microcavity into PLEDs: (1) double Ag electrodes performed as the mirrors of cavity, instead of a multi-layer Bragg reflector, which simplified the device structure and fabrication process; (2) three solution-processed functional layers were specially designed for avoiding the inter-infiltration between the different solutions and to improve the interface contacts; (3) high order microcavities were utilized according to the optical simulation results, in which thick EMLs benefited from thickness control and reproductivity. As a result, the full-color emission including pure red, green, blue was realized, and quasi-white light was also achieved from a single polymer emitting material. The achievement of color purity always requires the sacrifice of part of the current efficiency due to the spectra narrowing, while the higher current efficiency of green TMPLED (10.08 cd A−1) compared to that of non-cavity PLED (~8.60 cd A−1) cast a light on future improvements.

Optical characteristics of refractive-index-matching diffusion layer in organic light-emitting diodes

We developed a diffusion layer with a refractive index-matching layer added to a transparent nanoscale polymer-based structure to obtain its effective scattering effects. The diffusion layer had higher haze when the refractive index-matching layer to a higher refractive index was used. This diffusion layer was applied to conventional organic light-emitting diodes (OLEDs) and micro-cavity OLEDs (MC-OLEDs) to evaluate the characteristics. When a diffusion layer was applied to conventional OLEDs, the external quantum efficiency (EQE) was 31.1% higher than that of the device without a diffusion layer due to the reduction of the substrate mode, and the viewing angle characteristic was also improved. Then, when the diffusion layer was applied to the MC-OLEDs, all devices showed similar EQE values regardless of the presence or absence of the diffusion layer, and the viewing-angle-dependent characteristics were greatly improved by the diffusion layer. Furthermore, when the diffusion layer was used with polarizer film, the black color implementation was not affected by the polarizer film, proving that it is applicable to actual OLED display products.

Suppression of the color shift of microcavity organic light-emitting diodes through the introduction of a circular polarizer with a nanoporous polymer film

ABSTRACTA circular polarizer (CP) film modified by a nanoporous polymer film (NPF) was applied to improve the viewing angle characteristics of a top-emission organic light-emitting diode (TEOLED) without any serious pixel blur phenomenon. From this approach, the angular color shift (Δu′v′) was significantly improved from 0.096 to 0.056, and the angular luminance distribution was also expanded to the Lambertian distribution direction. The black color had a slightly faded tint, however, like the turn-off condition of the plasma display panel (PDP), regardless of the introduction of NPF below or above the CP. Very interestingly, the surface reflectance of the device with NPF-coated CP was not significantly different from that of the device with non-NPF CP. The pixel blur level according to the distance between the scattering medium and the light source was also investigated. As a result, it was found that the pixel blur phenomenon can be greatly suppressed by applying a thinner encapsulation lid with an NPF film.

Green OLED based on double metallic mirrors in microcavity structures and double light-emitting structure

Green microcavity organic light-emitting diodes (OLEDs) are fabricated by using double metallic mirrors in microcavity structures and double light-emitting structures. The structure of the device consists of Al(15 nm)/MoO3(4 nm)/2T-NATA(10 nm)/NPB(15 nm)/NPB:C545T(x%, 20 nm)/Alq3:C545T(4%, 20 nm)/Bphen(35 nm)/LiF(1 nm)/Al (200 nm), where x is the doping concentration. When the doping concentration is 3%, the device has the best photoelectric properties, which is named as device B1.The reference device B2 is prepared based on ITO, for the analysis of microcavity effect. B1 and B2 CIE are (0.2889, 0.620) and (0.3168, 0.5571), so the microcavity device could emit more green light. At 100 mA/cm2, the brightness of devices B1 and B2 is 5076 cd/m2 and 4818 cd/m2, and the maximum brightness of the both devices are 9277.7 cd/m2 and 10440 cd/m2. At 100 mA/cm2, luminance efficiency of devices B1 and B2 are 6.0 cd/A and 5.61 cd/A, and the luminance efficiency of the both devices are 8.6 cd/A and 7.97 cd/A. Compared with the reference devices, green microcavity device has a higher current efficiency and better color purity, which attributes to the microcavity effect.

Effect of periodically modified n-type electron transport layers on the optoelectrical performance of organic light-emitting diodes

We have investigated the characteristics of Cs2CO3 doped 4, 7-Diphenyl-1, 10-phenanthroline (Bphen) film and the effects of its thickness on the optoelectrical performance of microcavity organic light-emitting diodes. By doping Cs2CO3 into Bphen, higher current density and lower turn-on voltage could be achieved with barely changing the external quantum efficiency. Three types of resonators were fabricated by varying the thickness of every a quarter of electron transport layers, i.e. 152.5nm (0.25 λ), 305nm (0.5 λ), and 610nm (1.0 λ). The 0.25λ-based microcavity organic light-emitting diode was found to exhibit the lowest waveguided loss coefficient of 118.0cm−1, while the relative emission intensity of edge mode to surface reached about 2118.8 times, indicating that this mode underwent a very low attenuation during its propagation.

Sunlight-like white organic light-emitting diodes with inorganic/organic nanolaminate distributed Bragg reflector (DBR) anode microcavity by using atomic layer deposition

White organic light-emitting diodes (WOLED) with inorganic/organic/hybrid nanolaminate distributed Bragg reflector (DBR) prepared by atomic layer deposition (ALD) has been firstly developed, and the improved structural, optical, and electrical properties have been observed. The functional layer consists of periodically alternating layers of ZrO2/Zircone (zirconium alkoxides with carbon-containing backbones) films fabricated using ALD. By well adjusting the thickness and emissive dyes of the emission layer, three white light diodes were realized based on three cavity modes with different resonance wavelength and intensity. The narrowed lectroluminescent (EL) spectra from the microcavity organic light emitting diodes (MOLEDs) are of high-purity, and the off-resonant optical mode is highly suppressed due to the excellent optical properties of the DBR layers with high reflectivity in a wide stop-band range. The CIE chromaticity coordinates of devices are tuned from (0.33, 0.34), (0.43, 0.40) to (0.57, 0.40), which are all on the planckian locus, and the color temperature is adjusted from 5783 K, 3842 K to 2245 K. This work exhibits a simple and novel approach to achieve a sunlight-like WOLED by constructing the inorganic/organic nanolaminate DBR using ALD, which will be very important for healthy display and lighting.

Narrow‐band emitting microcavity OLED with ITO DBR electrode for sensing applications

Microcavity organic light-emitting diodes (OLEDs) with ITO distributed Bragg reflectors (DBRs) were proposed. For the 6.5-period DBR electrode, the Bragg reflectivity and sheet resistance of 92.3% and 30 Ω/ square were obtained. In addition, the full width at half maximum of the electroluminescence spectra of the microcavity OLED was <7 nm and the enhancement of the optical intensity was 4.6 times higher in amplitude than that of the non-cavity OLED.

Effect of hole-transport-layer thickness on deep-blue emission in top-emitting cavity organic light-emitting diodes

The effect of hole-transport-layer thickness on deep-blue emission was experimentally and theoretically elucidated for top-emitting micro-cavity organic light-emitting diodes. As a result of strong optical interference in the micro-cavity, the emission spectrum and intensity from the top-emitting organic light-emitting diodes was strongly dependent on the viewing angle. Optimizing hole-transport-layer thickness lead to deep-blue emissions at a wavelength of about 460 nm with a current efficiency of 5 cd/A at 10,000 cd/m2 luminance. Light out-coupling in the top-emitting organic light-emitting diodes was modeled and calculated to show the power dissipation, and the model explained the experimental results well.

Theoretical and Experimental Studies on Microcavity Organic Light-Emitting Diodes with Different Emitters

We have theoretically and experimentally investigated the microcavity organic light-emitting diodes (MOLEDs) that enhanced the emission intensity and narrowed the spectra simultaneously. In this work, MOLEDs with the reflectivities of 70% and 90% have been successfully fabricated. Comparing to non-cavity OLEDs, the maximum forward electroluminescence intensity and the peak luminescence can be improved by 6.8 times and 2.2 times, respectively. The full width at half maximum could be sharply narrowed to 10 nm. The different configurations of MOLEDs with varied emitting layers have also been evaluated which fitted well with the experimental results.

A systematic study on efficiency enhancements in phosphorescent green, red and blue microcavity organic light emitting devices

A systematic study has been conducted on microcavity organic light emitting diodes (OLEDs) based on green, red and blue phosphorescent emitters to elucidate the microcavity effects for different color emitters. We found that the luminance output is determined by the reflectivity of the semitransparent electrode and the photopic response of the green, red and blue emitters. While the luminance enhancements of blue and red phosphorescent microcavity devices are small, a current efficiency as high as 224 cd A−1 is obtained in the green phosphorescent microcavity OLEDs.

Luminous‐efficiency improvement of solar‐cell‐integrated high‐contrast organic light‐emitting diode by applying distributed Bragg reflector

Abstract— Solar‐cell‐integrated organic light‐emitting diodes (OLEDs) were fabricated with both high contrast ratio and energy‐recycling ability. However, the luminous efficiency of the integrated devices is reduced to 50% of that of conventional top‐emitting OLEDs. A novel structure to recover the luminous efficiency from 50% to near 85% by applying a distributed Bragg reflector (DBR) made of 20 layers of GaN/AlN was demonstrated. It saves about 40% of the electric power than that of a device without a DBR. The contrast ratio remains high compared to that of conventional OLEDs. In this paper, simulations were conducted first to prove our models and assumptions. Then, two types of thin‐film solar cells — CdTe and CIGS solar cells — were used. They had different contrast ratios as well as viewing‐angle properties. Finally, the emission spectrum was calculated to be 11 nm FWHM, which is narrower than that for the emission spectrum of a typical microcavity OLED and has the advantage of having saturated colors.

MoO3 as combined hole injection layer and tapered spacer in combinatorial multicolor microcavity organic light emitting diodes

Multicolor microcavity (μC) organic light-emitting diode (OLED) arrays were fabricated simply by controlling the hole injection and spacer MoO3 layer thickness. The normal emission was tunable from ∼490 to 640 nm and can be further expanded. A compact, integrated spectrometer with two-dimensional combinatorial arrays of μC OLEDs was realized. The MoO3 yields more efficient and stable devices, revealing a new breakdown mechanism. The pixel current density reaches ∼4 A/cm2 and a maximal normal brightness ∼140 000 Cd/m2, which improves photoluminescence-based sensing and absorption measurements.

High performance optoelectronic device based on semitransparent organic photovoltaic cell integrated with organic light-emitting diode

A high performance organic optoelectronic device that integrates a front semitransparent organic photovoltaic (OPV) cell and a microcavity organic light-emitting diode (MOLED) connected with a 0.3-nm-thick plasma-polymerized fluorocarbon film (CF X )-modified ultra-thin silver interlayer (Ag/CF X ) has been reported. Through proper selection of the active materials, and optimal design of the device structure, the semitransparent OPV cell in the integrated device achieves power conversion efficiency of 88% of the discrete control OPV device. Here we show that the integrated device possesses a high power conversion efficiency of 3.30% measured under air mass 1.5 global irradiation and a high luminous efficiency of 11.5 cd/A operated at 4.5 V. The performance of devices holds great potential for application in organic sensors possessing both high power conversion efficiency and high luminous efficiency.

分布式布喇格反射镜的穿透深度计算

The analytical expressions for the phase shift on reflection of Distributed Bragg Reflectors(DBRs)with odd and even numbers were derived based on the transfer-matrix method.The expression for the penetration depth of DBR as a function of phase shift was given according to the relationship between the phase shift and the reflection delay.The DBR with the structure of[HL]2H was fabricated by electron beam evaporation and the transmittance spectrum,and phase shift and penetration depth were measured or simulated.The microcavity organic light emitting diode(MOLED)was fabricated using the designed DBR.The measured effective cavity length of the MOLED fit well with the calculated one.