Warm White Organic Light-emitting Diodes Research Articles

Highly Efficient Hybrid White Organic Light‐Emitting Diodes with External Quantum Efficiency Exceeding 30% by Integrating Electron‐Trapping Sensitized Structure and Gradient‐Doping System

AbstractIn this work, efficient two‐color (blue and yellow) hybrid white organic light‐emitting diodes (WOLEDs) with high efficiencies are constructed by introducing iridium(III) bis(4,6‐(difluorophenyl) pyridinato‐N,C2’)picolinate (FIrpic) as electron sensitizer. Experimental results demonstrate that FIrpic with the deep lowest unoccupied molecular orbital (LUMO) level enables preferential electron trapping to collect excessive electrons, thus realizing carrier's balance and high recombination probability. The obtained cool WOLED exhibits the maximum external quantum efficiency (EQE), current efficiency (ηc), and power efficiency (ηp) of 31.05%, 76.36 cd A−1 and 69.92 lm W−1, respectively. Moreover, by gradient doping FIrpic into yellow EML, the generated forward built‐in electric field originating from gradient electrons distribution reduces carriers transport resistance and improves the transport of holes; the diffusion effect of gradient electrons distribution promotes electrons’ transport, thus enhancing the utilization probabilities of carriers and excitons. Besides, FIrpic acts also as an energy transfer ladder to facilitate the energy transfer from host to yellow emitter. Consequently, warm WOLED with enhanced yellow emission achieves the maximum EQE, ηc, and ηp of 33.27%, 75.66 cd A−1 and 88.04 lm W−1, respectively.

Highly Horizontal Oriented Tricomponent Exciplex Host with Multiple Reverse Intersystem Crossing Channels for High-Performance Narrowband Electroluminescence and Eye-Protection White Organic Light-Emitting Diodes.

Despite multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters with small full-width at half maximum are attractive for wide color-gamut display and eye-protection lighting applications, their inefficient reverse intersystem crossing (RISC) process and long exciton lifetime induce serious efficiency roll-off, which significantly limits their development. Herein, a novel device concept of building highly efficient tricomponent exciplex with multiple RISC channels is proposed to realize reduced exciton quenching and enhanced upconversion of nonradiative triplet excitons, and subsequently used as a host for high-performance MR-TADF organic light-emitting diodes (OLEDs). Compared with traditional binary exciplex, the tricomponent exciplex exhibits obviously improved photoluminescence quantum yield, emitting dipole orientation and RISC rate constant, and a record-breaking external quantum efficiency (EQE) of 30.4% is achieved for tricomponent exciplex p-PhBCzPh: PO-T2T: DspiroAc-TRZ (50: 20: 30) based OLED. Remarkably, maximum EQEs of 36.2% and 40.3% and ultralow efficiency roll-off with EQEs of 26.1% and 30.0% at 1000cd m-2 are respectively achieved for its sky-blue and pure-green MR-TADF doped OLEDs. Additionally, the blue emission unit hosted by tricomponent exciplex is combined with an orange-red TADF emission unit to achieve a double-emission-layer blue-hazard-free warm white OLED with an EQEmax of 30.3% and stable electroluminescence spectra over a wide brightness range.

High-performance full phosphorescent warm white organic light-emitting diodes with external quantum efficiency of 34.5%

In this work, we have reported full phosphorescent warm white organic light-emitting diodes (WOLEDs) consisting of three light-emitting layers not only achieving high external quantum efficiency (EQE) but also demonstrating significant color stability and low efficiency roll-off. The WOLED exhibits slight shift in Commission Internationale de l’Eclairage coordinates (CIEx,y) from (0.409, 0.405) at 1000 cd m−2 to (0.372, 0.398) at 10000 cd m−2. Moreover, the maximum external quantum efficiency is as high as 34.5% and remains >28% over wide brightness range of 100 cd m−2 to 5000 cd m−2. Our work not only provides effective design strategy for physiologically friendly warm WOLEDs but also paves the way for achieving high EQE and low roll-off all phosphorescent WOLEDs.

Manipulating Complementarity of Binary White Thermally Activated Delayed Fluorescence Systems for 100% Exciton Harvesting in OLEDs

AbstractDifferent to fluorescent and phosphorescent counterparts, white thermally activated delayed fluorescence (TADF) involves in multiple reverse intersystem crossing (RISC), leading to the correlation but competition between blue and other color components in both singlet and triplet allocations. Herein, three blue TADF emitters SSFAPO, DSFAPO, and TSFAPO, collectively named xSFAPO are developed, featuring a moderately electron‐withdrawing phosphine oxide (PO) acceptor respectively linked 1‐3 donors. Despite nearly identical blue emissions, photoluminescence quantum yields of xSFAPO are proportional to donor number. But, their RISC efficiencies are below 70%, markedly less than 85% of a conventional yellow TADF emitter 2,3,5,6‐tetrakis(3,6‐di‐(tert‐butyl)carbazol‐9‐yl)‐1,4‐dicyanobenzene (4CzTPNBu). Furthermore, sp3 hybrid configuration of PO enlarges steric hindrance of peripheral donor groups. So, Dexter energy transfer is impeded by increasing donor numbers. Among xSFAPO and 4CzTPNBu dually doped white‐emitting films, yellow emission from SSFAPO‐based film is the strongest, reflecting the predominance of fast Dexter energy transfer in triplet allocation. Therefore, SSFAPO endowed its warm‐white organic light‐emitting diodes (WOLEDs) with an external quantum efficiency of 25.1%, corresponding to 100% internal quantum efficiency, which are 1.25 and 1.60 folds of those of DSFAPO and TSFAPO‐based WOLEDs. These results suggest advantage complementarity of different components is crucial for developing white‐emitting systems with 100% exciton utilization.

High performance TADF-phosphorescence hybrid warm-white organic light-emitting diodes with a simple fully doping-free device structure

White organic light-emitting diodes (WOLEDs) with doping-free emissive layer (EML) structures have been increasingly attracting attention due to their excellent advantages, such as easier manufacturing process and more flexible structure design. In this paper, a series of highly efficient hybrid WOLEDs is manufactured using a simple doping-free EML structure, which is composed of two blue thermally activated delayed fluorescence (TADF) emitters and an ultrathin (<0.1 nm) orange phosphorescent emitter. Through the thickness optimization of the blue TADF emitters, the optimal device shows a maximum external quantum efficiency, power efficiency, current efficiency, and luminance of 9.1%, 22.7 lm/W, 29.0 cd/A, and 14 530 cd/m2, respectively. When the brightness is increased from 1000 cd/m2 to 10 000 cd/m2, the chromaticity coordinates migrate from (0.44, 0.48) to (0.41, 0.47), and the correlated color temperature changes from 3454 to 3881 K, which belongs to the warm-white light emission zone. The result indicates that the use of a doping-free blue–orange–blue structure provides a common path for fabricating efficient, simple, and low cost WOLEDs.

Efficient multi-light-emitting layers warm and pure white phosphorescent organic light-emitting diodes with excellent color stability

The warm white organic light-emitting diodes (WOLEDs) consisting of four emissive layers (EMLs) as well as pure WOLEDs consisting of three EMLs have been fabricated by using three primary color phosphors as emitters. Experimental results demonstrated that energy transfer between dopants, direct carriers' trapping and main recombination zone are all important factors in determining the performances of obtained devices. By precisely managing dopants' concentrations and emitting layers' thicknesses, we realized the optimized warm WOLED with a Commission Internationale de l’Eclairage coordinates (CIEx,y) of (0.388, 0.383), giving the highest current efficiency (ηc) of 69.00 cd/A, power efficiency (ηp) of 60.19 lm/W and external quantum efficiency (EQE) of 20.63%. Additionally, by increasing the proportion of blue emission, we also realized the pure WOLED (CIEx,y value is (0.331, 0.385)) giving 60.48 cd/A (EQE = 16.90%, CRI = 72.03) and 51.32 lm/W in highest ηc and ηp, respectively. Especially, the optimal warm and pure WOLEDs exhibited the superior color stability with small CIE discrepancies of (0.020, 0.003) and (0.022, 0.004) in the range of 1–10 mA/cm2, respectively.

High efficiency doping-free warm-white organic light-emitting diodes with strategic-tuning of radiative excitons by combining interfacial exciplex with multi-ultrathin emissive layers

Abstract White organic light-emitting diodes (WOLEDs) have widely drawn attention for applications in flat panel display and solid-state lighting. High efficiency and simple device structure are two important factors for large-scale application for WOLEDs. Here, we have presented a relatively reliable method to develop high-performance and doping-free WOLEDs by integrating interfacial exciplex with phosphorescent multi-ultrathin emissive layers. By reasonable design of device architecture, the resulting WOLED with a warm-white light not only realizes a low turn-on voltage of 2.25 V, but also exhibits the high forward-viewing power efficiency and external quantum efficiency of 91.5 lm W−1 and 21.3%, respectively, without light out-coupling enhancement techniques. More importantly, the power efficiency still remain as high as 49.5 lm W−1 at the high luminance of 1000 cd m−2, which should be one of the best values in reported doping-free WOLEDs. It suggests that multi-ultrathin emissive layers combining with interfacial exciplex can realize the efficient utilization of excitons and improvement of efficiency roll-off.

Color-stable white phosphorescent organic light-emitting diodes based on double bipolar mixed-host layer

In this study, we demonstrated high-efficiency white phosphorescent organic light emitting diodes with stable spectra based on the mixed-host emission layer. By employing a mixed-host system consisting of the hole transport material (4, 4'-N, N' -dicarbazole) biphenyl and electron transport material 1, 3, 5-tris (2-N-pheylbenzimidazolyl) benzene in double emission layers, realizing warm white organic light-emitting diodes characterized by peak efficiencies of 47.24 cd A−1 and 39.12 lm W−1 with the Commission International de L’Eclairage coordinates variations from (0.428, 0.470) to (0.427, 0.471) when the luminance varies from 1000 to 10 000 cd m−2.

High-quality warm white organic electroluminescence from efficient phosphor-only emitting systems based on bipolar iridium(iii) complexes

Warm white organic light-emitting diodes (WOLEDs) based on efficient phosphor-only emitting systems exhibit very high peak electroluminescence (EL) efficiencies of 25.0/22.5% and 71.3/59.3 lm W−1 and desirable color rendering index (CRI) of 81/90.

High efficiency blue and color-stable hybrid warm white organic light-emitting diodes based on a thermally activated delayed fluorescent material as an assistant host

High efficiency blue and color-stable hybrid white OLEDs have been realized by simply incorporating a blue TADF assistant host in a conventional blue fluorescent dye-doped emitting layer.

High efficiency warm white organic light-emitting diodes with precise confinement of charge carriers and excitons in the exciplex host system

A high-performance WOLED with stable spectrum emission was achieved by the ratio regulation between the HTM and ETM in the exciplex host system.

Highly efficient and spectra stable warm white organic light-emitting diodes by the application of exciplex as the excitons adjustment layer

Abstract We aim to simplify the device structure due to the low concentration precision and repeatability of white organic light-emitting diodes (WOLEDs) based on exciplex host with phosphorescent dopant. Two layers of exciplex used here play the role of blue emitter and excitons adjustment layer, respectively, to manage the excitons. Green and red phosphorescent ultrathin layer are inserted into exciplex layer to form white light-emitting. The maximum efficiency values of current efficiency, power efficiency and EQE reach to 38.8 cd/A, 34.9 lm/W and 15.3%, and remain 29.7 cd/A, 20.4 lm/W and 11.6% at 1000 cd/m2, respectively. Besides, the change of Commission Internationale de I'Eclairage coordinates is only (0.024, 0.013) with the enhanced driving voltages from 4 V to 6 V. Such a high performance of warm WOLEDs with simple structure demonstrates the reasonable excitons management and efficient energy transfer from exciplex and excitons adjustment layer to ultrathin phosphorescent emitters.

Performance evaluation and analysis of two-stack warm white organic light emitting diodes with three spectral peaks

Abstract In this paper, we report a highly efficient two-stack warm white organic light emitting diodes (WOLEDs) with three spectral peaks using all phosphorescent organic emitters. A multi-stack OLED architecture enhances the color stability along electric field variation by separating emissive layer despite its complex structure. Performances of two- and three-stack WOLEDs having three spectral peaks are theoretically evaluated using our optical structure design method. The theoretically designed two- and three-stack structures show power efficiency of 51.5 and 33.7 lm/W at a constant current density of 1 mA/cm2 with color rendering index (CRI) of 84 and 88, respectively. Two-stack WOLED is expected to have higher power efficiency due to its lower driving voltage and smaller blue portion than that of three-stack WOLED. An optically optimized and fabricated two-stack WOLED with red and co-doped green-blue emissive layers exhibits a power efficiency as high as 58.1 lm/W and CRI of 85. The driving voltage of fabricated WOLED is (5.9 V) well correlated with the expected value of connected unit devices. In addition, our two-stack WOLED with diffusive film and hemispherical structure shows an enhanced power efficiency of 71.0 and 101.3 lm/W, respectively.

High-efficiency and color-stable warm white organic light-emitting diodes utilizing energy transfer from interface exciplex

Abstract We demonstrate high-efficiency warm white organic light-emitting diodes (WOLEDs) with an interface-exciplex-forming structure of hole-transporting 1,3-Bis(carbazol-9-yl)benzene (mCP) as electron-donor and electron-transporting 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-(pyridin-3-yl)pyrimidine (B3PyMPM) as electron-acceptor. The exciplex can be formed effectively at the interface of mCP/B3PyMPM due to the excellent charge-transporting properties of mCP and B3PyMPM and the large energy-levels offset of mCP/B3PyMPM interface, and then its energy transfers to the low-lying triplet states of the blue TADF fluorophors or yellow phosphors. As a result, a maximum power efficiency (PE) of 79.2 lm/W and maximum external quantum efficiency (EQE) of 22.3% were achieved from the warm WOLED. The key factor for such high efficiency WOLED is believed to be from the effective harvest of triplet excitons of the interface-exciplex via reverse intersystem crossing process (RISC).

Exciton‐Adjustable Interlayers for High Efficiency, Low Efficiency Roll‐Off, and Lifetime Improved Warm White Organic Light‐Emitting Diodes (WOLEDs) Based on a Delayed Fluorescence Assistant Host

AbstractRecently, a new route to achieve 100% internal quantum efficiency white organic light‐emitting diodes (WOLEDs) is proposed by utilizing noble‐metal‐free thermally activated delayed fluorescence (TADF) emitters due to the radiative contributions of triplet excitons by effective reverse intersystem crossing. However, a systematic understanding of their reliability and internal degradation mechanisms is still deficient. Here, it demonstrates high performance and operational stable purely organic fluorescent WOLEDs consisting of a TADF assistant host via a strategic exciton management by multi‐interlayers. By introducing such interlayers, carrier recombination zone could be controlled to suppress the generally unavoidable quenching of long‐range triplet excitons, successfully achieving remarkable external quantum efficiency of 15.1%, maximum power efficiency of 48.9 lm W−1, and extended LT50 lifetime (time to 50% of initial luminance of 1000 cd m−2) exceeding 2000 h. To this knowledge, this is the first pioneering work for realizing high efficiency, low efficiency roll‐off, and operational stable WOLEDs based on a TADF assistant host. The current findings also indicate that broadening the carrier recombination region in both interlayers and yellow emitting layer as well as restraining exciplex quenching at carrier blocking interface make significant roles on reduced efficiency roll‐off and enhanced operational lifetime.

Efficient triplet harvesting in fluorescence–TADF hybrid warm-white organic light-emitting diodes with a fully non-doped device configuration

An efficient triplet harvesting strategy provided for fluorescence–TADF hybrid white OLEDs with a fully non-doped device configuration.

High-efficiency/CRI/color stability warm white organic light-emitting diodes by incorporating ultrathin phosphorescence layers in a blue fluorescence layer

Abstract By incorporating ultrathin (<0.1 nm) green, yellow, and red phosphorescence layers with different sequence arrangements in a blue fluorescence layer, four unique and simplified fluorescence/phosphorescence (F/P) hybrid, white organic light-emitting diodes (WOLEDs) were obtained. All four devices realize good warm white light emission, with high color rending index (CRI) of >80, low correlated color temperature of <3600 K, and high color stability at a wide voltage range of 5 V–9 V. These hybrid WOLEDs also reveal high forward-viewing external quantum efficiencies (EQE) of 17.82%–19.34%, which are close to the theoretical value of 20%, indicating an almost complete exciton harvesting. In addition, the electroluminescence spectra of the hybrid WOLEDs can be easily improved by only changing the incorporating sequence of the ultrathin phosphorescence layers without device efficiency loss. For example, the hybrid WOLED with an incorporation sequence of ultrathin red/yellow/green phosphorescence layers exhibits an ultra-high CRI of 96 and a high EQE of 19.34%. To the best of our knowledge, this is the first WOLED with good tradeoff among device efficiency, CRI, and color stability. The introduction of ultrathin (<0.1 nm) phosphorescence layers can also greatly reduce the consumption of phosphorescent emitters as well as simplify device structures and fabrication process, thus leading to low cost. Such a finding is very meaningful for the potential commercialization of hybrid WOLEDs.

Two Host-Dopant Emitting Systems Realizing Four-Color Emission: A Simple and Effective Strategy for Highly Efficient Warm-White Organic Light-Emitting Diodes with High Color-Rendering Index at High Luminance.

A four-color warm-white organic light-emitting diode employing a simple adjacent two-emitting-layer structure as a blue host-orange dopant/green host-red dopant has been fabricated, which exhibited a stable high electroluminescent performance: an external quantum efficiency of 23.3% and a power efficiency of 63.2 lm W(-1) at an illumination-relevant luminance of 1000 cd m(-2) with a high color-rendering index (CRI) of 92 and maintained high levels of 21.6% and 48.8 lm W(-1) with a CRI value of 93 at the extremely high luminance of 5000 cd m(-2). To our knowledge, this should be the best result so far for a white-light organic light-emitting diode with CRI > 90, simultaneously exhibiting very high efficiencies based on a high luminance level for the solid-state lighting.

Simple-Structured Phosphorescent Warm White Organic Light-Emitting Diodes with High Power Efficiency and Low Efficiency Roll-off.

We present phosphorescent WOLEDs fabricated simply by inserting an ultrathin nondoped orange layer within the blue emissive zone, where an efficient exciplex system is applied as the host. The resulting WOLED shows maximum power efficiency of 75.3 and 63.1 lm/W at the luminance of 1000 cd/m(2). The exciton density profile in the emitting layer, the operational mechanism and the quenching process at high luminance are systematically investigated by experimental and theoretical methods, from which it is concluded that the efficient utilization of excitons via exciplex host and the wide recombination zone are the key factors for the prominent achievement of high efficiency and greatly reduced efficiency roll-off.

Highly efficient blue and warm white organic light-emitting diodes with a simplified structure

Two blue fluorescent emitters were utilized to construct simplified organic light-emitting diodes (OLEDs) and the remarkable difference in device performance was carefully illustrated. A maximum current efficiency of 4.84 cd A−1 (corresponding to a quantum efficiency of 4.29%) with a Commission Internationale de l’Eclairage (CIE) coordinate of (0.144, 0.127) was achieved by using N,N-diphenyl-4″-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1, 1′:4′, 1″-terphenyl]-4-amine (BBPI) as a non-doped emission layer of the simplified blue OLEDs without carrier-transport layers. In addition, simplified fluorescent/phosphorescent (F/P) hybrid warm white OLEDs without carrier-transport layers were fabricated by utilizing BBPI as (1) the blue emitter and (2) the host of a complementary yellow phosphorescent emitter (PO-01). A maximum current efficiency of 36.8 cd A−1 and a maximum power efficiency of 38.6 lm W−1 were achieved as a result of efficient energy transfer from the host to the guest and good triplet exciton confinement on the phosphorescent molecules. The blue and white OLEDs are among the most efficient simplified fluorescent blue and F/P hybrid white devices, and their performance is even comparable to that of most previously reported complicated multi-layer devices with carrier-transport layers.