Tandem Organic Light-emitting Diodes Research Articles

Realization of Optimal Interconnector for Tandem Organic Light‐Emitting Diodes with Record Efficiency

An optimal interconnector is obtained using organic heterojunction with n/bulk heterojunction/p structure. It is proven that the interconnector can provide sufficient charge generation and ohmic charge injection. Record power efficiencies of 57.5, 126.8, and 52.7 lm W−1 are achieved for red, green, and blue color tandem organic light-emitting diodes, respectively, which are enhanced from the corresponding single unit ones. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

High-resolution OLED display with remarkably low power consumption using blue/yellow tandem structure and RGBY subpixels

A top-emission organic light-emitting diode (OLED) with a microcavity structure combined with a blue/yellow tandem structure was developed. A high-resolution active-matrix OLED display with the world's lowest level of power consumption using the tandem OLED with red, green, blue, and yellow subpixels was fabricated.

Highly efficient tandem organic light-emitting diodes based on SubPc:C60 bulk heterojunction as charge generation layer

High efficiency tandem organic light-emitting diodes (OLEDs) were realized using an organic bulk heterojunction (BHJ) as charge generation layer (CGL) consisted of boron subphthalocyanine chloride (SubPc) and fullerene (C60). The results showed that the SubPc:C60 based CGL is a promising connecting unit for the fabrication of high efficiency tandem OLEDs with 2.64 folds enhancement in a current efficiency of 63.6cd/A, compare to the corresponding single-unit OLEDs. The efficiency enhancement of tandem OLEDs is attributed to better charge carrier balance derived from the optimized charge transport pathways and reduced interface energy barrier. It showed that the charge generation ability and charge transport characteristics of CGL were strongly dependent on SubPc:C60 mixing ratio and corresponding film morphology, which was verified by the charge extracting analysis. Additionally, the highest occupied molecular orbital energy level of SubPc is compatible for most of hole transporting layer to minimize the hole injection barrier and suppress the hole accumulation and charge recombination in tandem OLEDs.

Investigations of solution-processed charge generation unit with low concentration of small molecule doped in p-type/HAT-CN6 for tandem OLED

We investigated the charge generation and injection mechanism in solution processed charge generation unit (CGU) used in our high performance tandem organic light emitting diode (OLED) via capacitance–voltage (C–V) and current density–voltage (J–V) measurements. By doping 2wt% of small molecule 1,1-bis-(4-bis(4-tolyl)-aminophenyl) cyclohexene (TAPC) into Poly (N-vinylcarbazole) (PVK) as p-type layer of the CGU, we obtained more than two folds improvement in the tandem device efficiency compared to single device. The performance improvement of the TAPC doped CGU could be attributed to low built-in potential, large vacuum level shift as well as high charge density for efficient charge generation.

적색과 청색 물질을 사용한 2파장 방식 백색 적층 OLED의 광학 및 전기적 특성

We studied optical and electrical properties of two-wavelength white tandem organic light-emitting diodes using red and blue materials. White fluorescent OLEDs were fabricated using Alq3 : Rubrene (3 vol.% 5 nm) / SH-1 : BD-2 (3 vol.% 25 nm) as emitting layer (EML). White single fluorescent OLED showed maximum current efficiency of 9.7 cd/A, and tandem fluorescent OLED showed 18.2 cd/A. Commission Internationale de l`Eclairage (CIE) coordinates of single and tandem fluorescent OLEDs was (0.385, 0.435), (0.442, 0.473) at , respectively. White hybrid OLEDs were fabricated using SH-1 : BD-2 (3 vol.% 10 nm) / CBP : (2 vol.% 20 nm) as EML. White single hybrid OLED showed maximum current efficiency of 7.8 cd/A, and tandem hybrid OLED showed 26.4 cd/A. Maximum current efficiency of tandem hybrid OLED was more twice as high as single OLED. CIE coordinates of single hybrid OLED was (0.315, 0.333), and tandem hybrid OLED was (0.448, 0.363) at . CIE coordinates in white tandem OLEDs compared to those for single OLEDs observed red shift. This work reveals that stacked white OLED showed current efficiency improvement and red shifted emission than single OLED.

70.1: High‐Resolution OLED Display with the World's Lowest Level of Power Consumption Using Blue/Yellow Tandem Structure and RGBY Subpixels

A top‐emission organic light‐emitting diode (OLED) with a a microcavity structure combined with a blue/yellow tandem structure was developed. A high‐resolution active‐matrix OLED display with the world's lowest level of power consumption using the tandem OLED with red, green, blue, and yellow subpixels was fabricated.

Highly stable and high power efficiency tandem organic light-emitting diodes with transition metal oxide-based charge generation layers

Tandem organic light-emitting diodes (OLEDs) have been studied to improve the long-term stability of OLEDs for 10years. The key element in a tandem OLEDs is the charge generation layer (CGL), which provides electrons and holes to the adjacent sub-OLED units. Among different types of CGLs, n-doped electron transporting layer (ETL)/transition metal oxide (TMO)/hole transporting layer (HTL) has been intensively studied. Past studies indicate that this kind of CGL can achieve the desired efficiency enhancement, however, its long-term stability was reported not good and sometime even poor than a single OLED. This issue was not well addressed over the past 10years. Here, for the first time, we found that this is caused by the unwanted diffusion of TMO into the underlying n-doped ETL layer and can be well resolved by introducing an additional diffusion suppressing layer (DSL) between them. Our finding will fully release the potential of TMO-based CGL in tandem OLEDs.

Charge generation layers for solution processed tandem organic light emitting diodes with regular device architecture.

Tandem organic light emitting diodes (OLEDs) utilizing fluorescent polymers in both sub-OLEDs and a regular device architecture were fabricated from solution, and their structure and performance characterized. The charge carrier generation layer comprised a zinc oxide layer, modified by a polyethylenimine interface dipole, for electron injection and either MoO3, WO3, or VOx for hole injection into the adjacent sub-OLEDs. ToF-SIMS investigations and STEM-EDX mapping verified the distinct functional layers throughout the layer stack. At a given device current density, the current efficiencies of both sub-OLEDs add up to a maximum of 25 cd/A, indicating a properly working tandem OLED.

Inverted Tandem Phosphorescence Organic Light-Emitting Diodes Based on <formula formulatype="inline"><tex Notation="TeX">$\hbox{MoO}_{3}/\hbox{Al/Cs}_{2}\hbox{CO}_{3}$</tex> </formula> Charge Generation Unit

A multilayer charge generation unit (CGU) $\hbox{MoO}_{3}/\hbox{Al/Cs}_{2}\hbox{CO}_{3}$ has been developed for inverted tandem organic light-emitting diodes (OLEDs). The current efficiency of inverted tandem phosphorescence OLED with $\hbox{MoO}_{3}$ (8 nm)/Al (1 nm)/ $\hbox{Cs}_{2}\hbox{CO}_{3}$ (0.5 nm) CGU is almost twice as high as that of individual electroluminescent (EL) device. Charges generate at hole-transporting host material/ $\hbox{MoO}_{3}$ interface and $\hbox{MoO}_{3}$ bulk layer, and electrons are extracted by $\hbox{Cs}_{2}\hbox{CO}_{3}$ /electon-transporting layer (ETL) interface. The significantly enhanced performance of the tandem device with $\hbox{MoO}_{3}/\hbox{Al/Cs}_{2}\hbox{CO}_{3}$ CGU compared with a $\hbox{MoO}_{3}/\hbox{Cs}_{2}\hbox{CO}_{3}$ -based one may be attributed to preventing chemical reaction of $\hbox{MoO}_{3}$ and $\hbox{Cs}_{2}\hbox{CO}_{3}$ by the inserted ultrathin Al layer. Moreover, the current efficiency and EL spectrum of another inverted tandem OLED combining red and green stack further prove the effective charge generation and separation with the multilayer $\hbox{MoO}_{3}/\hbox{Al/Cs}_{2}\hbox{CO}_{3}$ CGU. It is expected that such a CGU is a promising candidate for inverted tandem white OLED for active matrix organic light-emitting diode display driven by an n-type thin-film transistor backplane.

Photoemission spectroscopy study on interfacial energy level alignments in tandem organic light-emitting diodes

Organic light-emitting diodes (OLEDs) using a tandem structure offer a highly attractive option for the applications of next-generation flat panel displays and solid-state lighting due to the extremely high brightness and efficiency along with the long operational lifetime. In general, reliable information about interface energetics of the charge generation layers (CGLs), which plays the central role in charge generation and carrier injection into the stacked emission units, is highly desirable and advantageous for interface engineering and the performance optimization of tandem OLEDs. In this review, our recent studies on tandem OLEDs are overviewed, especially from interface energetics perspective via photoemission spectroscopy. The electronic structures of various transition metal oxide (TMO)-based CGLs and their role in charge generation process are reviewed, addressing the n-type doping impact of organic layers in CGLs, thermal annealing-induced oxygen vacancy in TMOs, and the interfacial stability of CGLs on the device operational lifetime. The resulting energy level alignments are summarized in correspondence with tandem OLED performance.

High Efficiency Tandem Organic Light Emitting Diode Using an Organic Heterojunction as the Charge Generation Layer: An Investigation into the Charge Generation Model and Device Performance

Organic heterojunctions (OHJs) are frequently used as charge generation layers (CGLs) in the construction of high efficiency tandem organic light emitting diodes (OLEDs). However, the charge generation mechanism still remains unclear. In this article, the working principle of a typical OHJ CGL composed of 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile (HAT-CN)/ 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) is studied. We found that the charge generation process in this OHJ results from electron tunneling, which is shown through temperature-dependent J–V characteristics and model fitting. Additionally, we fabricated an ultrahigh efficiency green tandem OLED using only commercially available organic semiconductor materials for OLED. The maximum power efficiency reaches 120 lm/W, yet remains at 110.3 lm/W at 1000 cd/m2, which is greatly enhanced from when compared to the corresponding single-unit device. The maximum current efficiency and external quantum efficiency reach as high as 201 cd/A and 54.5%, respectively.

High performance organic-inorganic perovskite-optocoupler based on low-voltage and fast response perovskite compound photodetector.

Organic-inorganic hybrid photodetectors attract considerable attention because they can combine the advantages of both organic and inorganic systems. Here, a perovskite compound with a broad absorption spectrum and high power conversion efficiency is used as a photosensitive layer in an organic/inorganic hybrid heterojunction photodetector with a high and fast response. The high sensitivity exceeding 104 is obtained at bias of 0–4 V. Using a tandem organic light-emitting diode (OLED) as the light source, we fabricated an optocoupler device. The optocoupler achieved a maximum photoresponsivity of 1.0 A W−1 at 341.3 μWcm−2 at an input voltage of 6 V. The device also exhibits rapid response times of τrise ~ 20 μs and τfall ~ 17 μs; as well as a high current transfer ratio (CTR) of 28.2%. After applying an amplification circuit, the CTR of the optocoupler increases to 263.3%, which is comparable with that of commercial inorganic optocouplers. The developed hybrid optocoupler thus shows great promise for use in photonics.

Highly efficient and stable tandem organic light-emitting devices based on HAT-CN/HAT-CN:TAPC/TAPC as a charge generation layer

We demonstrated an effective charge-generation layer (CGL) composition consisting of three layers: an organic acceptor material HAT-CN, HAT-CN blended with an organic donor material TAPC and TAPC, to fabricate a high efficiency tandem organic light-emitting diode (OLED).

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.

Lithium hydride doped intermediate connector for high-efficiency and long-term stable tandem organic light-emitting diodes.

Lithium hydride (LiH) is employed as a novel n-dopant in the intermediate connector for tandem organic light-emitting diodes (OLEDs) because of its easy coevaporation with other electron transporting materials. The tandem OLEDs with two and three electroluminescent (EL) units connected by a combination of LiH doped 8-hydroxyquinoline aluminum (Alq3) and 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) demonstrate approximately 2-fold and 3-fold enhancement in current efficiency, respectively. In addition, no extra voltage drop across the intermediate connector is observed. Particularly, the lifetime (T75%) in the tandem OLED with two and three EL units is substantially improved by 3.8 times and 7.4 times, respectively. The doping effect of LiH into Alq3, the charge injection, and transport characteristics of LiH-doped Alq3 are further investigated by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS).

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.

Field- and thickness-dependent performance of tandem organic light-emitting diodes with buffer-modified fullerene/copper phthalocyanine heterojunction as interconnector

Tandem organic light-emitting diodes (OLEDs) with the buffer-modified fullerene/copper phthalocyanine (LiF/C60/CuPc/MoO3) as interconnector have been investigated. The properties of tandem OLEDs are dependent on electric field, which is induced by electric-field-dependence of charge generation (by electric filed and photons) and separation in such an interconnector structure. At low electric field, because of less generated-charges and light-absorption of C60/CuPc film, current efficiency and brightness of tandem OLEDs with two emissive units exhibit less than twice higher than those of conventional single-unit device. But more than twofold enhancement of current efficiency and brightness is observed at higher electric field due to a large number of charges generated in the interconnector. Meanwhile, the performance of tandem devices is affected by the thickness of C60 and CuPc. The optimal thickness is related to their exciton diffusion length.

The charge generation layer incorporating two p-doped hole transport layers for improving the performance of tandem organic light emitting diodes

We report the charge generation layer (CGL) structure comprising of Li 2 CO 3 doped bathocuproine (BCP:Li 2 CO 3 )/MoO 3 doped 4,4- N , N -bis [ N -1-naphthyl- N -phenyl-amino]biphenyl (NPB:MoO 3 )/MoO 3 doped 4,4′- N , N ′-dicarbazole-biphenyl (CBP:MoO 3 ) for tandem organic light emitting diodes (TOLEDs). Compared to the TOLED using the conventional CGL structure of BCP:Li 2 CO 3 /20 nm CBP:MoO 3 , the one using the CGL structure of BCP:Li 2 CO 3 /5 nm NPB:MoO 3 /15 nm CBP:MoO 3 showed increased electrical and luminous properties, mostly because the introduction of the higher-conductivity NPB:MoO 3 relative to CBP:MoO 3 could improve the current conduction in the CGL structure. Whereas, the performance of the CGL structure of BCP:Li 2 CO 3 / x nm NPB:MoO 3 /20- x nm CBP:MoO 3 decreased with x increasing, mostly due to the fact that the CBP:MoO 3 became depleted of mobile holes upon contacting p-doped NPB: the smaller thickness of CBP:MoO 3 , the worse conductivity for it. We provide some in-depth insights on designing the high-performance CGLs for TOLEDs.

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)).

Effect of NaCl doped into Bphen layer on the performance of tandem organic light-emitting diodes

To improve the performance of tandem organic light-emitting diodes (OLEDs), we study the novel NaCl as n-type dopant in Bphen:NaCl layer. By analyzing their relevant energy levels and carrier transporting characteristics, we discuss the mechanisms of the effective charge generation layer (CGL) of Bphen:NaCl (6 wt%)/MoO3. In addition, we use the Bphen:NaCl (20 wt%) layer as the electron injection layer (EIL) combining the CGL to further improve the performance of tandem device. For this tandem device, the maximal current efficiency of 9.32 cd/A and the maximal power efficiency of 1.93 lm/W are obtained, which are enhanced approximately by 2.1 and 1.1 times compared with those of the single-emissive-unit device respectively. We attribute this improvement to the increase of electron injection ability by introducing of Bphen:NaCl layer. Moreover, the CGL is almost completely transparent in the visible light region, which is also important to achieve an efficient tandem OLEDs.