Tandem White Organic Light-emitting Diodes Research Articles

Tandem white organic light-emitting diodes stacked with two symmetrical emitting units simultaneously achieving superior efficiency/CRI/color stability

Abstract In this work, a series of three and four-color tandem white organic light-emitting diodes (WOLEDs) are developed by using the optimized charge generating unit (CGU) to connect two electroluminescence (EL) units with symmetrical emitting layers, in which symmetrical emitting layers are constructed based on the mixed hosts; sandwiched between hole and electron-transporting hosts and the light emitted from two EL units that are absolutely complementary for forming white emission. All resulting tandem WOLEDs realize good white emission with maximum color rendering index (CRI) beyond 77 and 90 for three and four-color white devices and extremely high EL spectra stability, and also achieve high device efficiency with maximum external quantum efficiency (EQE) exceeding 33.10%. For example, for the optimized four-color tandem WOLED, the maximum CRI and EQE of 91 and 34.78% are demonstrated and only very slight CIE (Δx, Δy) variation of (0.002, −0.010) was observed at a wide luminance range from 170.9 cd/m2 to 13,870 cd/m2. To the best of our knowledge, this is the first tandem WOLED with only two EL units realizing such high device performances. More importantly, the proposed tandem WOLEDs here avoid introducing carrier or exciton blocking layer or using more EL units to realize high color quality white emission that provides a novel approach to develop simple, but high-performance tandem WOLEDs.

Recent Developments in Tandem White Organic Light-Emitting Diodes.

Tandem white organic light-emitting diodes (WOLEDs) are promising for the lighting and displays field since their current efficiency, external quantum efficiency and lifetime can be strikingly enhanced compared with single-unit devices. In this invited review, we have firstly described fundamental concepts of tandem device architectures and their use in WOLEDs. Then, we have summarized the state-of-the-art strategies to achieve high-performance tandem WOLEDs in recent years. Specifically, we have highlighted the developments in the four types of tandem WOLEDs (i.e., tandem fluorescent WOLEDs, tandem phosphorescent WOLEDs, tandem thermally activated delayed fluorescent WOLEDs, and tandem hybrid WOLEDs). Furthermore, we have introduced doping-free tandem WOLEDs. In the end, we have given an outlook for the future development of tandem WOLEDs.

High performance hybrid tandem white organic light-emitting diodes by using a novel intermediate connector

Using Liq/Ca/HAT-CN as an intermediate connector, high-performance hybrid tandem WOLEDs with EQE of 39.57% and CRI of 93 were developed.

Doping-free tandem white organic light-emitting diodes

Tandem white organic light-emitting diodes (WOLEDs) are of great research interest since they can greatly boost the performance compared with the single-unit counterparts. However, their structures are more complicated than those of single-unit OLEDs. Besides, to achieve high performance, the doping technology is required to tandem OLEDs, particularly for tandem WOLEDs, further complicating the structures. Herein, doping-free tandem WOLEDs, for the first time, have been demonstrated. By managing an effective doping-free charge generation layer to interconnect doping-free emitting layers/charge transport layers, high-performance doping-free tandem WOLEDs have been developed. The blue-yellow device accomplishes the simplified structure/short fabrication time/reduced cost/high efficiency/low efficiency roll-off/low voltage/high luminance trade-off, which cannot be achieved by previous tandem WOLEDs. Remarkably, the efficiency (81.2cdA−1) is ∼2-fold higher than the highest efficiency of previous doping-free WOLEDs and even higher than those of some best doping tandem WOLEDs. The maximum luminance is 44,886cdm−2, which is the highest for doping-free WOLEDs. Besides, the blue-red device can exhibit a color rendering index (CRI) of 67, which is even higher than that of some representative three-color tandem WOLEDs. Such findings may not only represent a significant step for doping-free WOLEDs, but unlock a novel avenue that doping-free tandem WOLEDs are promising to achieve the simplicity and high performance trade-off.

Optical simulation and optimization of weak-microcavity tandem white organic light-emitting diodes

We systematically studied the influence of weak microcavity effects on the optical properties of tandem white organic light-emitting diodes (WOLEDs) using optical simulation. Based on the simulation results, an image processing method is developed and verified for rational design of high-performance tandem WOLEDs. The results indicate that low operating voltage, good color rendering index, and angular emission properties can be simultaneously obtained in tandem WOLEDs by engineering the device structure. This study provides promising direction for the development of high-performance tandem WOLEDs.

All-phosphorescent three-color two-stack tandem white organic light emitting diodes with high-color-rendering index values

Reported herein are high-color-rendering index two-stack phosphorescent tandem white organic light-emitting diodes using three-color phosphorescent dopants: iridium(III) bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) (472 nm) as a blue emitter for the first stack layer, and iridium(III) bis(4-methyl-2-(3,5-dimethylphenyl)quinolinato-N,C2′) tetramethylheptadionate (612 nm) and iridium(III) bis(4-(4-tbutylphenyl)thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (562 nm) as red and yellow triplet emitters for the second stack layer, respectively. By locating the blue and red emissive layers slightly far from their antinode position in terms of photonic mode density, the electroluminescent spectra could be changed, and the CRI values could be increased from 74 to over 85. There is some efficiency loss, however, from 38.6 to 29.4 lm/W due to the inevitable electrical and optical losses.

Fabrication of transparent Al:LiF composite/MoO3 interconnecting layers for tandem white organic light emitting devices.

We report transparent Al:LiF composite/molybdenum oxides (MoO3) as interconnecting layers for tandem white organic light emitting diodes (WOLEDs) consisting of blue and red phosphorescent unit devices. The Al:LiF (3 nm)/MoO3 (10 nm) interconnecting layers show a high transmittance, good carrier generation and injection capability for tandem WOLEDs. The performance of tandem WOLEDs is sensitive to the LiF doping concentration, which is mainly attributed to the difference in efficiency of carrier injection into the adjoining electroluminescent units. For 10~20% LiF concentration, the external quantum efficiency of tandem device is almost equal to the sum of the efficiencies of blue and red OLEDs at high current density; furthermore, a small variation of Commission Internationale de l'Eclairage (CIE) coordinates with the current density is obtained.

High-efficiency orange and tandem white organic light-emitting diodes using phosphorescent dyes with horizontally oriented emitting dipoles.

Tandem white organic light-emitting diodes (WOLEDs) using horizontally oriented phosphorescent dyes in an exciplex-forming co-host are presented, along with an orange OLED. A high external quantum efficiency of 32% is achieved for the orange OLED at 1000 cd m(-2) and the tandem WOLEDs exhibit a high maximum EQE of 54.3% (PE of 63 lm W(-1)).

A novel intermediate connector with improved charge generation and separation for large-area tandem white organic lighting devices

An intermediate connector is developed for fabricating tandem white organic light-emitting diodes.

Tandem white organic light-emitting diodes adopting a C60:rubrene charge generation layer

Organic bulk heterojunction fullerence (C60) doped 5, 6, 11, 12-tetraphenylnaphthacene (rubrene) as the high quality charge generation layer (CGL) with high transparency and superior charge generating capability for tandem organic light emitting diodes (OLEDs) is developed. This CGL shows excellent optical transparency about 90%, which can reduce the optical interference effect formed in tandem OLEDs. There is a stable white light emission including 468 nm and 500 nm peaks from the blue emitting layer and 620 nm peak from the red emitting layer in tandem white OLEDs. A high efficiency of about 17.4 cd/A and CIE coordinates of (0.40, 0.35) at 100 cd/m2 and (0.36, 0.34) at 1000 cd/m2 have been demonstrated by employing the developed CGL, respectively.

Aligned energy-level design for decreasing operation voltage of tandem white organic light-emitting diodes

In general, organic light-emitting devices (OLEDs) need to operate at higher current density levels to ensure an ample light flux. However, stressed operation will result in poor performance and limited device lifetime. Recently, a tandem structure has been proposed as a pivotal technique to meet the stringent lighting requirements for OLED commercialization, with a research focus on decreasing the concomitant higher operation voltage. Driving two connected emission units (EMUs) in a tandem structure often requires more than twice the driving voltage for a single EMU. This study investigates bipolar host materials and their effective employment in fabricating tandem white phosphorescent OLEDs (PhOLEDs). In addition, the design of a mechanism to align the energy level between the hole transport layer/emitting layer is shown to effectively mitigate operational voltages. In sharp contrast to devices using a unipolar host material, we demonstrate that the turn-on voltage of blue PhOLEDs could be decreased from 3.8V to 2.7V through utilizing a bipolar host. Furthermore, applying the proposed techniques to tandem white PhOLEDs produces a luminance of 103cd/m2 by a 10.1V driving voltage.

Small single–triplet energy gap bipolar host materials for phosphorescent blue and white organic light emitting diodes

We report extremely high efficiency tandem white organic light-emitting diodes (OLEDs) with newly synthesized host materials. Our new host materials have well balanced bipolar characteristics and very small singlet to triplet splitting energies (∼0.4 eV) due to almost no orbital overlapping between ground and excited states. The fabricated blue phosphorescent device with one of these host materials shows very high external quantum efficiency of 25.7%, low onset and driving voltages of 2.47 V and 3.49 V at 1 and 1000 cd m−2, respectively. High performance asymmetric tandem OLEDs could be fabricated by adopting these efficient and low driving voltage blue unit and very efficient yellow unit devices. Power efficiency of 63.1 lm W−1 and current efficiency of 128.8 cd A−1 at 1000 cd m−2 were achieved without any light out-coupling technology. We expect that blue phosphorescent OLEDs with our new narrow band-gap bipolar host materials could be a promising solution to make high efficiency white OLEDs for display and lighting applications.

Highly efficient tandem white organic light-emitting diodes based upon C60/NaT4 organic heterojunction as charge generation layer

We present a high-efficiency tandem white organic light-emitting diode based upon a C60/5,5′′′-bis(naphth-2-yl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (NaT4) organic heterojunction as a charge generation layer. A maximum current efficiency, maximum power efficiency, and maximum external quantum efficiency of 111.3 cd A−1, 50.5 lm W−1, and 38.7%, respectively, are obtained. More importantly, the efficiency roll-off is greatly improved with only a slight roll-off to109 cd A−1, 40.3 lm W−1, and 38% at a luminance of 1000 cd m−2. Surprisingly, the electroluminescent spectrum is highly stable at all levels of luminance. Furthermore, our tandem device shows better operational stability than a tandem device employing a conventional charge generation layer. It is concluded that the excellent charge generation, transport, and extraction in the C60/NaT4 organic heterojunction-based CGL is responsible for the high performances.

Tandem white phosphorescent organic light-emitting diodes based on interface-modified C60/pentacene organic heterojunction as charge generation layer

The significant improvement of power efficiency and efficiency roll-off in tandem white organic light-emitting diodes (WOLEDs) was achieved using interface-modified C60/pentacene organic heterojunction as charge generation layer (CGL). The resulting tandem WOLED exhibits a maximum power efficiency of 53.8 lm W−1 and a maximum current efficiency of 101.5 cd A−1 without any out-coupling techniques. More importantly, the efficiency roll-off is greatly improved, yet remaining power efficiency of 53 lm W−1 at 100 cd m−2 and 45 lm W−1 at 1000 cd m−2, which outperforms that of the conventional CGL-based tandem WOLED. This clearly demonstrates that the organic heterojunction is a promising CGL for high-performance tandem WOLEDs.

High-efficiency stacked white organic light-emitting diodes

We report efficient tandem white organic light-emitting diodes (WOLEDs) by using bathocuproine:Cs2CO3∕MoO3 as an effective interconnecting layer. We utilized two primary colors of sky blue and orange fluorescent emitters to obtain efficient white electroluminescence. Although single WOLEDs using two adjacent emitting layers showed a maximum current efficiency of 7.96cd∕A with Commission Internationale d’Eclairage (CIE) coordinates of (0.28, 0.34), the tandem WOLED device made by stacking two single color OLEDs in series demonstrated doubled maximum current efficiency of 17.14cd∕A with CIE coordinates of (0.28, 0.41). The stacking of different single color OLEDs in series instead of double stacking of WOLEDs can be useful to achieve highly efficient WOLEDs because it can reduce the number of layers of the devices.