Doped Emission Layer 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.

Highly Efficient Deep Blue Cd-Free Quantum Dot Light-Emitting Diodes by a p-Type Doped Emissive Layer.

Environmentally friendly ZnSe/ZnS core/shell quantum dots (QDs) as an alternative blue emission material to Cd-based QDs have shown great potential for use in next-generation displays. However, it remains still challenging to realize a high-efficiency quantum dot light-emitting diode (QLED) based on ZnSe/ZnS QDs due to their insufficient electrical characteristics, such as excessively high electron mobility (compared to the hole mobility) and the deep-lying valence band. In this work, the effects of QDs doped with hole transport materials (hybrid QDs) on the electrical characteristics of a QLED are investigated. These hybrid QDs show a p-type doping effect, which leads to a change in the density of the carriers. Specifically, the hybrid QDs can balance electrons and holes by suppressing the overflow of electrons and improving injection of holes, respectively. These electrical characteristics help to improve device performance. In detail, an external quantum efficiency (EQE) of 6.88% is achieved with the hybrid QDs. This is increased by 180% compared to a device with pure ZnSe/ZnS QDs (EQE of 2.46%). This record is the highest among deep-blue Cd-free QLED devices. These findings provide the importance of p-type doping effect in QD layers and guidance for the study of the electrical properties of QDs.

Systematical Investigation of Ultrathin Doped Emissive Layer Structure: Achieving Highly Efficient and Long‐Lifetime Orange Organic Light‐Emitting Diodes

AbstractPhosphorescent organic light‐emitting diodes (PhOLEDs) with ultrathin emissive layer (UEML) structure have great potential in solid‐state lighting and display applications due to their relatively simple fabrication process. In this paper, the effect of doped UEML architecture on the performance of orange PhOLEDs is systematically investigated and highly efficient and long‐lifetime devices with the doped UEML structures are successfully constructed. Through strategic exciton management, the probability of exciton recombination is significantly enhanced, while simultaneously that of exciton annihilation is greatly suppressed, along with excellent charge balance. The resulting orange PhOLEDs based on tris[2‐(2‐pyridyl) imidazole] gallium(III):bis (2‐phenylbenzothiazolato) (acetylacetonate) iridium as emitter exhibit a maximum current efficiency of 60.8 cd A−1, power efficiency of 65.8 lm W−1, and external quantum efficiency of 23.4%, respectively. In addition, the doped UEML architecture greatly extends the device operational stability with a nearly fourfold lifetime improvement (time to reduce to 50% of the initial luminance at 2300 cd m−2) over the conventional phosphorescent device with doped bulk emitting layer. This should be the best results reported so far for orange PhOLEDs, indicating the use of doped UEML structure displays great potential value in designing highly efficient and long‐lifetime PhOLEDs.

Comparative study of multi-functional luminogens with 1,3,5-triazine as the core and phenothiazine or phenoxy donors as the peripheral moieties for non-doped/doped fluorescent and red phosphorescent OLEDs

A series of three-armed fluorescent emitters based on a 1,3,5-triazine acceptor as the central core and phenothiazine or phenoxy moieties as the different peripheral units, are designed and synthesized. All of them exhibit aggregation-induced emission enhancement and thermally activated delayed fluorescence. Photoluminescence quantum yields exceeded 37 and 67% for non-doped and doped solid-state samples respectively. The compounds were found to be capable of transporting both holes and electrons. The highest mobility values of more than 1 × 10−4 cm2V−1s−1 were obtained for the compound having three phenothiazine moieties at electric field of more than 1 × 106 Vcm−1. Additionally, three types of organic light emitting diodes based on these materials were fabricated. The device containing non-doped emission layer with the compound containing single phenothiazine moiety as the emitter gave maximum external quantum efficiency of 5.4%. The device with doped emission layer containing 5% solid solution of the compound having three phenothiazine moieties in an appropriate host gave maximum external quantum efficiency of 9.9%. The phosphorescent device hosted by the compound with single phenothiazine unit exhibited maximum external quantum efficiency of 10.3% and a low-efficiency roll-offs of 1% at 1000 cd/m2.

Control of Au nanoantenna emission enhancement of magnetic dipolar emitters by means of VO2 phase change layers.

Active, ultra-fast external control of the emission properties at the nanoscale is of great interest for chip-scale, tunable and efficient nanophotonics. Here we investigated the emission control of dipolar emitters coupled to a nanostructure made of an Au nanoantenna, and a thin vanadium dioxide (VO2) layer that changes from semiconductor to metallic state. If the emitters are sandwiched between the nanoantenna and the VO2 layer, the enhancement and/or suppression of the nanostructure's magnetic dipole resonance enabled by the phase change behavior of the VO2 layer can provide a high contrast ratio of the emission efficiency. We show that a single nanoantenna can provide high magnetic field in the emission layer when VO2 is metallic, leading to high emission of the magnetic dipoles; this emission is then lowered when VO2 switches back to semiconductor. We finally optimized the contrast ratio by considering different orientation, distribution and nature of the dipoles, as well as the influence of a periodic Au nanoantenna pattern. As an example of a possible application, the design is optimized for the active control of an Er3+ doped SiO2 emission layer. The combination of the emission efficiency increase due to the plasmonic nanoantenna resonances and the ultra-fast contrast control due to the phase-changing medium can have important applications in tunable efficient light sources and their nanoscale integration.

High CRI and stable spectra white organic light-emitting diodes with double doped blue emission layers and multiple ultrathin phosphorescent emission layers by adjusting the thickness of spacer layer

The color rendering index (CRI) and Commission Internationale de L'Eclairage coordinate (CIE x,y ) are significant parameters for the white organic light-emitting diodes (WOLEDs). We fabricate a series of the phosphorescent WOLED devices, in which there are two doped phosphorescent layers (for blue emission) on both sides of the emitting layer (EML) and three ultrathin phosphorescent layers (for red, orange and green emission) separated by the spacer layers (mCP) in EML. One doped blue emission layer near the hole transport layer contributes to CRI enhancement by increasing the doping concentration (x %) of the blue phosphorescent dye because Dexter energy transfer increases from the blue emission to the red emission. When x is 20, CRI reaches to 90 at 5 V in device A3. Based on device A3, CRI can be further improved to 94 in device B1 with a thinner thickness of the spacer layer (y = 1 nm) between the green emission layer and orange emission layer in EML because Dexter energy transfer from the green emission layer to the orange and red emission layer becomes easier. Although CRI declines, the spectra become more stable by increasing the thickness of the spacer layer (y = 3 nm) in device B2 or decreasing the thickness of the green emission layer (z = 0.05 nm) in device C, in which the CIE x,y coordinate shift is only (0.0214, 0.0102) from 5 V to 8 V. • The double doped blue phosphorescent layers (EML1 and EML5) on both sides of EML can adjust CRI and spectra stability. • The EML1 near HTL is beneficial for CRI and the red emission by increasing the doping concentration of FIrpic. • The EML5 near ETL can improve spectra stability by increasing the thickness of the spacer layer (Spacer2). • The EML5 near ETL can improve CRI by decreasing the thickness of green ultrathin phosphorescence layer (EML4).

A new series of short axially symmetrically and asymmetrically 1,3,6,8-tetrasubstituted pyrenes with two types of substituents: Syntheses, structures, photophysical properties and electroluminescence

A new series of short axially symmetrically (4a and 4b) and asymmetrically (4c and 4d) 1,3,6,8-tetrasubstituted pyrene-based compounds with two phenyl moieties and two diphenylamine units on the pyrene core were designed and synthesized based on stepwise synthetic strategy. These compounds were structurally characterized and their photoelectric properties were investigated by spectroscopy, electrochemical and theoretical studies. The structures of 4a and 4b were determined by single-crystal X-ray diffraction analysis, indicating that the compounds are twisted by the peripheral substituents and the intermolecular π-π interactions have been efficiently interrupted. The four compounds exhibit high absolute fluorescence quantum yields (ФF) in dichloromethane (83.31–88.45%) and moderate ФFs in film states (20.78–38.68%). In addition, compounds 4a and 4b display relatively higher absolute ФFs than those of 4c and 4d in film states. All the compounds exhibit high thermal stability with decomposition temperatures above 358 °C and the values of 4c and 4d are higher than 4a and 4b. Compounds 4a and 4b can form morphologically stable amorphous thin films with Tg values of 146 °C and 149 °C, respectively. However, there are no obvious Tg observed in compounds 4c and 4d. Electroluminescent devices using 4a and 4b as doped emission layer show promising device performance with low turn-on voltage (3.0 V), maximum brightness around 15100 cd/m2 and 16100 cd/m2, maximum luminance efficiency of 12.4 cd/A and 13.6 cd/A and maximum external quantum efficiency of 5.34% and 5.63%, respectively.

발광층 내의 스페이서가 인광 OLED의 효율 및 발광 특성에 미치는 영향

We have investigated the effects of spacer layer inserted between blue and red doped emission layers on the emission and efficiency characteristics of phosphorescent OLEDs. N,N'-di-carbazolyl-3,5-benzene (mCP) was used as a host layer. Iridium(III)bis[(4,6-di-fluorophenyl)pyridinato-N,C]picolinate (FIrpic) and tris(1-phenyl-isoquinolinato-C ,N)iridium(III) [Ir(piq)3] were used as blue and red dopants, respectively. The emission layer structure was mCP (1-x) nm/mCP:Ir(piq)3 (5 nm, 10%)/mCP (x nm)/mCP:FIrpic (5 nm, 10%). The thickness of mCP spacer layer was varied from 0 to 15 nm. The emission from Ir(piq)3 and the efficiency of the device were dominated by energy transfer from mCP host and FIrpic molecules, and by diffusion of mCP host triplet excitons.

Highly efficient white organic light-emitting device using a single emitter

White organic light-emitting devices (WOLEDs) can be fabricated using a simple, low-cost device structure with a single uniformly doped emissive layer. The Pt-17 emitter used in these devices obtains excellent color rendering (CRI = 80) as well as bright white electrophosphoresence (CIE x = 0.37, y = 0.40) by combining efficient monomer and efficient excimer emission as demonstrated by excellent external quantum efficiency (ηext = 15.9%). The Pt-17 based WOLED is also compatible with state-of-the-art charge injection and blocking materials as well as high out-coupling device structures. Application of these existing technologies is expected to extend luminance efficiencies of Pt-17 devices to world-class values (46 lm/?startWend? and 100 lm/?startWend? respectively). In addition to avoiding the difficulty and cost of fabricating more complex device structures, the color of a single-doped device also is uniquely independent of voltage, current density, and age. Molecules like fluorine-free Pt-17 are uniquely positioned to utilize excimer emissions in order to reduce manufacturing costs and provide solutions to satisfy many of the requirements for the next generation of organic solid-state lighting.

Tailoring balance of carrier mobilities in solid-state light-emitting electrochemical cells by doping a carrier trapper to enhance device efficiencies

We demonstrate the improving balance of carrier mobilities in neat-film light-emitting electrochemical cells (LECs) utilizing a cationic transition metal complex (CTMC) as the emissive material and a cationic near-infrared laser dye as the carrier trapper. This low-gap carrier trapper is judiciously chosen such that a significant energy offset in the highest occupied molecular orbital (HOMO) levels between the CTMC and the carrier trapper impedes hole transport in the emissive layers while similar lowest unoccupied molecular orbital (LUMO) levels of these two materials result in relatively unaffected electron transport. Since the CTMC neat films would intrinsically exhibit characteristics of preferred transport of holes, the balance of carrier mobilities would be improved by doping such carrier trapper. Electroluminescent measurements show that the peak external quantum efficiency (EQE) and the peak power efficiency of the neat-film LECs doped with the carrier trapper reach 12.75% and 28.70 lm W−1, respectively. These device efficiencies represent a 1.4 times enhancement as compared to those of the undoped neat-film LECs and approach the upper limit of EQE (∼15%) that one would expect from the photoluminescence quantum yield of the emissive layer (∼0.75) and an optical out-coupling efficiency of ∼20% from a typical layered device structure, consequently indicating superior balance of carrier mobilities in such a doped emissive layer. These results confirm that the balance of carrier mobilities in the CTMC neat films would be improved by doping a proper carrier trapper and this technique offers a general approach for optimizing device efficiencies of CTMC-based neat-film LECs.

Singlet–triplet quenching in high intensity fluorescent organic light emitting diodes

Electroluminescence (EL) transient turn-on peaks with more than twice the brightness of the steady-state EL are observed in fluorescent organic light emitting diodes with a doped emission layer. By modeling both the singlet and triplet population dynamics, we identify the EL drop after the peak as due to singlet–triplet (ST) quenching. The peak recovery following a double voltage pulse sequence is explained in terms of triplet population relaxation through triplet–triplet quenching. A mechanism of using phosphors in the emission region is proposed for reducing ST quenching.

Improvement in the Stabilities of White Organic Light Emitting Diodes Using a Partially Doped Emission Layer

White organic light emitting devices were fabricated to improve the stability through a structural change using the two peak emission method. The fabricated devices were composed of indium tin oxide (100 nm)/ <TEX>$\alpha$</TEX>-NPD (30 nm)/4,40-bis(2,20-diphenylvinyl)-1,10-biphenyl (DPVBi, d: variable)/DPVBi: Rubrene (40 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline(5 nm)/ <TEX>$Alq_3$</TEX>(5 nm)/ Al (100 nm). A DPVBi for blue emissions was used as the host material in the emitters. The doping concentration of the Rubrene was fixed at 2.0% (by weight). The white emission with Commission Internationale De L'Eclairage coordinates of (0.3342, 0.3439) occurred at 14 V with a thickness d of 1 nm. It was insensitive to the drive voltage, and the devices had a maximum luminance of <TEX>$211\;cd/cm^2$</TEX>. At 19 V, the current density and maximum external quantum efficiency were <TEX>$173\;mAcm^2$</TEX> and 0.478%, respectively.

Efficiency Improvement of Top-Emission Organic Light Emitting Diode by Using UV-Ozone, Doped Emission Layer, and Hole-Blocking Layer

This work investigates systematic approaches to improve the current efficiency of the devised top-emission organic light emitting diode (TOLED) with a UV-ozone-treated reflective Ag anode, doped emission layer, and hole-blocking layer. The work function of the Ag anode can be increased under the UV-ozone exposure to improve the current density–voltage property of TOLED. Yet, degraded sheet resistance and reflectance characteristics of the Ag anode are observed. Therefore, a trade-off to obtain optimum performance of the studied TOLED is investigated. Besides, structural design by doping 2,3,6,7-tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino-[9,9a,1gh]coumarin into the emission layer to improve the carrier recombination efficiency to obtain enhanced current efficiency is also studied.

An organic light-emitting devices of highly efficient white phosphor using an electron/exciton blocker

Highly efficient white phosphorescent organic light-emitting devices (WOLEDs) was fabricated using an electron/exciton blocker. The device structure is ITO/2T-NATA(25 nm)/ NPBX(25-dnm)/CBP:5%Ir(ppy)3:0.5%Rubrene(8 nm)/NPBX(dnm)/DPVBi(30 nm)/TPBi(20 nm)/Alq(10nm)/LiF(1nm)/Al, in which N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1, 1′-biphenyl-4,4′-diamine (NPBX) functions as a hole transport layer and electron/exciton blocker, 4,4,N,N′-dicarbazolebiphenyl (CBP) is host, 4,4′-bis(2,2-diphenyl vinyl)-1,1′-biphenyl (DPVBi) is blue fluorescent dye, 5,6,11,12,-tetraphenylnaphthacene (rubrene) is fluorescent dye, factris (2-phenylpyridine) iridium (Ir(ppy)3) is phosphorescent sensitizer and tris(8-hydroxyquinoline) aluminum (Alq3) is an electron transport layer. The WOLEDs have obtained white light emission by adjusting the thickness of NPBX, when the concentration of Ir(ppy)3 is 5-wt% and rubrene is 0.5-wt%, respectively, the thickness of the doped emissive layer is 8 nm, the WOLEDs show a maximum luminous efficiency is 11.2 cd/A with d of 10 nm at 7 V and a maximum luminance of 28170 cd/m2 at 17 V, the CIE coordinates is (0.37.0.42), which is in white region.

White top-emitting organic light-emitting diodes using one-emissive layer of the DCJTB doped DPVBi layer

White top-emitting organic light-emitting diodes (TEOLEDs) composed of one doped emissive layer which emits two-wavelength light though the radiative recombination were fabricated. As the emissive layer, 4,4-bis(2,2-diphenylethen-1-yl)biphenyl (DPVBi) was used as the host material and 4-(dicyanomethylene)-2- tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) was added as the dopant material. By optimizing the DCJTB concentration (1.2%) and the thickness of the DPVBi layer (30 nm), the intensity ratio of the two wavelengths could be adjusted for balanced white light emission. By using the device composed of glass/Ag (100 nm)/ITO (90 nm)/2-TNATA (60 nm)/NPB (15 nm)/DPVBi:DCJTB (1.2%, 30 nm)/Alq 3 (20 nm)/Li (1.0 nm)/Al (2.0 nm)/Ag (20 nm)/ITO (63 nm)/SiO 2 (42 nm), the Commission Internationale d'Eclairage (CIE) chromaticity coordinate of (0.32, 0.34) close to the ideal white color CIE coordinate could be obtained at 100 cd/m 2.

Continuous, Atmospheric Process to Create Organic Clusters and Nanostructured, Functional Films

An atmospheric process based on compressed CO2 is used to create stable clusters of small organic molecules. These clusters, 1–10 nm in size, are used as building blocks to assemble thin films on various substrates. Cluster assembly of these films is verified by using low-angle X-ray diffraction. The surface quality of these cluster-assembled films is similar to that of films usually prepared via the vacuum process. Several functional organic light-emitting diode devices have been prepared, in which only the doped emissive layer has been deposited by our process. The radiometric features and efficiencies of these devices match those of vacuum-built devices. Atomic force microscopy of these molecular clusters reveals that they are liquid-like at standard atmospheric conditions. Coatings of these clusters on cloth and stainless steel have been found to be superhydrophobic in nature.

Insights into OLED functioning through coordinated experimental measurements and numerical model simulations

AbstractApplying this method to the standard OLED device structure that has received broad attention in the literature, we have found a number of surprising results. From our experiments, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. The device simulations fully clarify the situation, giving insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we found that there is a leakage of unrecombined holes towards the cathode at low voltages. We also found a strong variation of the electric field in the Alq3 layer due to space charge effects. By using the laser dye derivatives DCM‐TPA with electron trapping capabilities and DCM‐II with both electron and hole trapping capabilities as dopants in a standard OLED architecture, we could study the effect on transport and emission characteristics. In the case of the exclusively electron trapping dopant, a blue‐shift of the emission color with increasing bias is observed which we find is due to a splitting of the recombination zone.Applying this method to the standard OLED device structure that has received broad attention in the literature, we have found a number of surprising results. From our experiments, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. The device simulations fully clarify the situation, giving insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we found that there is a leakage of unrecombined holes towards the cathode at low voltages. We also found a strong variation of the electric field in the Alq3 layer due to space charge effects. By using the laser dye derivatives DCM‐TPA with electron trapping capabilities and DCM‐II with both electron and hole trapping capabilities as dopants in a standard OLED architecture, we could study the effect on transport and emission characteristics. In the case of the exclusively electron trapping dopant, a blue‐shift of the emission color with increasing bias is observed which we find is due to a splitting of the recombination zone.

Efficient Organic Electrophosphorescent White‐Light‐Emitting Device with a Triple Doped Emissive Layer

An organic electrophosphorescent white‐light‐emitting device (see Figure) containing green, red, and blue emitters doped into a 9 nm thick wide‐bandgap material is shown to attain a maximum forward viewing power efficiency of (26±3) lm W–1. The high power efficiency is attributed to the efficient confinement of excitons and charge within the emissive layer, a thin emissive layer, and highly efficient blue electrophosphorescent dopant.

High-efficiency low-voltage stable inverted transparent electrophosphorescent organic light-emitting diodes: combining electrically doped carrier transport layers and iridium-complex doped emissive layer

We demonstrated high-efficiency low-voltage stable inverted transparent clectrophosphorescent organic light-emitting diodes employing an indium-tin-oxide coated glass substrate directly as cathode and a semitransparent top Au thin film as anode. The structure contains an iridium-complex doped emissive layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers to form a nip structure. The devices are about 50% transparent and emit green light from both sides with peak external quantum efficiency (EQE) of 4.08% (14.3 cd/A). At 100 cd/m 2 , the EQE is 2.8% (13 cd/A) at an operating voltage of 4.3 V. The devices exhibit a lifetime of above 50 hours under continuous constant-current driving for the initial luminance of about 9000 cd/m 2 in vacuum, which project a lifetime of ∼5000 hours for 100 cd/m 2 .

Combinatorial Fabrication and Studies of Small Molecular Organic Light-Emitting

ABSTRACTVarious combinatorial matrix arrays of UV-violet, white, and blue-to-red organic light-emitting devices (OLEDs), fabricated using a sliding shutter technique, are described. In the UV-violet devices, which contain a UV-violet emitting layer of 4,4′-bis(9-carbazolyl) biphenyl (CBP), the optimal radiance R and external quantum efficiency ηext were determined with respect to the thicknesses of the hole transporting layers. In the blue-to-red devices, which contained a blue-emitting layer of 4,4′-bis(2,2′-diphenyl-vinyl) -1,1′-biphenyl (DPVBi) and a red-emitting 5 wt.% dye-doped guest-host layer, the color of the devices evolved continuously from blue to red as the thickness of the doped layer increased from 0 to 35 Å. The (nominal) 2 Å-thick doped layer device exhibited the highest brightness L ∼ 120 Cd/m2 and external quantum efficiency ηext ∼4.4 % at a current density of 1 mA/cm2. In the white OLEDs, which were similar to the blue-to-red devices but with lightly doped emission layer, the highest brightness Lmax was over 74,000 Cd/m2; in all devices Lmax exceeded 50,000 Cd/m2. The maximum efficiencies were 11.0 Cd/A, 5.96 lm/W and 4.6% at 5.8 V, 0.6 mA/cm2, and 68 Cd/m2 in a 0.25 wt.%, 2 nm-thick doped layer device.