Tandem Organic Light-emitting Diodes Research Articles

Light extraction in tandem organic light emitting diodes

Since the invention of organic light emitting diodes (OLEDs), great research efforts have been dedicated to improving their efficiency and lifetime. For high-brightness applications, tandem OLED structures have advantages because of the lower current densities required to achieve high brightness. With the successful development of highly efficient charge generation layers, high brightness tandem OLEDs are used in displays and lighting. However, the major challenge for tandem OLEDs is the low light extraction efficiency, because about 50% of the light is trapped inside the device as waveguide modes. In this Perspective, we first review the recent works done on light extraction, analyze different waveguide mode extraction structures, and then identify the key factors determining the extraction efficiencies in tandem OLEDs.

Highly efficient tandem PHOLEDs with lithium-doped BPhen/NDP-9-doped TAPC as a charge generation layer

The development of large-area organic light-emitting diode (OLED) displays requires a highly efficient tandem device architecture and an easily processable charge generation layer (CGL) with a low voltage drop and high optical transparency. In this study, we investigated and applied a doped organic n-CGL/p-CGL using thermal vacuum deposition in tandem OLED devices. A doping concentration of 1.0 wt.% for Li in 4, 7-Diphenyl-1, 10-phenanthroline (BPhen) was optimal for the n-CGL with 8 wt.% for 2-(7-dicyanomethylene-1,3,4,5,6,8,9,10-octafluoro-7H-pyrene-2-ylidene)-malononitrile (NDP-9)-doped N,N-bis(4-methylphenyl)benzenamine (TAPC) as a p-CGL. Maximum luminous efficiencies of 42.5 and 63.4 cd/A and a 4,000 cd/m2 current density for the target luminance values of 11.2 and 6.5 mA/cm2 were demonstrated for double-stack and triple-stack tandem blue phosphorescent OLED devices, respectively. Implementing these highly efficient tandem device structures will improve the overall lifetime of OLED displays by lowering their operating current density at the target luminance.

Accurate optical simulation method of tandem organic light-emitting diode with consideration of Purcell effect

In this paper, we report an accurate optical calculation method of tandem organic light-emitting diode (OLED) with consideration of Purcell effect. Using the normal reported method, there are some discrepancies between calculation and experimental external quantum efficiencies. To improve such discrepancies, we considered Purcell effect in optical calculation method. This improved calculation with Purcell effect were well matched with experimental results. Additionally, we measured exciton lifetime to identify the validity of Purcell factor which was applied to calculation. Through this measurement, we identified theoretical calculation is well matched with experimental tendency. Thus, this calculation can be more useful in tandem OLED applications.

Investigation and optimization of the charge generation layer (CGL) in tandem OLEDs using Taguchi’s orthogonal arrays and nondestructive capacitance-voltage (C-V) measurements

In this study, we utilize a simple, reliable Taguchi’s orthogonal array to optimize a tandem organic light-emitting diode (OLED). Furthermore, we use a capacitance-voltage (C-V) measurement to verify the charge generation layer (CGL) performance and make tandem OLED performance predictions. We demonstrate that Taguchi’s orthogonal array works correctly. In addition, it was determined that C-V measurement is a reliable way to resolve the CGL mechanisms and predict OLED efficiency directly. Based on the experimental results, it was proven that using the Taguchi’s orthogonal array improved the efficiency of the tandem OLED device. The current efficiency increased from 3.56 cd/A to 6.13 cd/A. The results of the C-V measurement were compared with the Taguchi’s orthogonal array, and the performance of the device was analyzed based on changes in the trend of the capacitance value. These two methods can be used together to achieve reliable optimization results with a limited number of experiments.

Highly efficient ultra-flexible tandem organic light-emitting diodes adopting a non-doped charge generation unit

An efficient CGU is applied to the high efficiency super-flexible white TOLEDs, which show a higher PE than the single OLEDs. After being bent for 3000 cycles, the devices retain ∼90% of the original luminance.

Efficient tandem organic light-emitting diodes with non-doped structures.

Highly efficient tandem organic light-emitting diodes (TOLEDs) were achieved based on a non-doped charge generation unit (CGU) consisting of LiF/Al/C60/4,4',4"-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA) and ultrathin emitting layers. The current-voltage characteristics of the CGU devices and electron-only devices and the capacitance-voltage characteristics of the CGU-based capacitance devices were characterized to explore the charge generation and injection mechanisms. The charge generation process occurs at the interface of C60/m-MTDATA through electron transferring from the highest occupied molecular orbital of m-MTDATA to the lowest unoccupied molecular orbital of C60. It is found that the thinner C60 layer contributes to efficient electron injection. Under the optimal structure, the blue TOLEDs exhibit a maximum current efficiency (CEmax) of 43.3 cd/A. The CEmax and maximum external quantum efficiency (EQEmax) of the white TOLEDs reach 84.6 cd/A and 26.7%, respectively.

The study of charge injection mechanism of C60/CuPc organic heterojunction connector layer

The charge injection mechanism of a connector layer is one of the critical issues influencing the performance of tandem organic light-emitting diodes (OLEDs). In this paper, to explore the charge injection mechanism of an organic heterojunction connector (OHJC) layer, we studied the device current density (J) characteristics generated by the C60/CuPc OHJC layer under different applied voltages (V). By analyzing the log(J)–V1/2 and ln(J/E)–1/E2 characteristics of devices, we found that the charge injection mechanism of the C60/CuPc OHJC layer is in accordance with the Richardson–Schottky model. This study can be a theoretical basis to design high-efficiency OLEDs.

Optical design of connecting electrodes for tandem organic light-emitting diodes.

Connecting electrodes play a crucial role to assist charge injection into the adjacent electroluminescent units in tandem organic light-emitting diodes (OLEDs). In this study, we demonstrate that Mg:Ag alloy is an effective connecting electrode for bottom- and top-emitting tandem OLEDs. Optical cavity design and simulation are also conducted to predict the luminance of tandem OLEDs. It is found that the theoretical luminance of tandem OLEDs is close to but not higher than twofold enhancement over the luminance of a single OLED optimized to the first resonance mode, which is theoretically higher than high-order resonance modes. It is also found that the optical properties of Mg:Ag connecting electrodes, while having relatively small influence on weak microcavity bottom-emitting tandem OLEDs, have large influence on strong microcavity top-emitting tandem OLEDs.

Highly efficient charge generation and injection in HAT-CN/TAPC heterojunction for high efficiency tandem organic light-emitting diodes

An efficient planar organic heterojunction (OHJ) consisted of 1,4,5,8,9,11-hexaazatriphenylene- hexacarbonitrile (HAT-CN)/1,1-bis [(di-4-tolyamino)phenyl)]cyclohexane (TAPC) is employed simultaneously as both charge generation layer (CGL) and charge injectors to fabricate high efficiency and long lifetime tandem organic light-emitting diodes (OLEDs). It can be seen that compared with conventional tandem OLEDs, the tandem OLEDs utilizing OHJ as both CGL and charge injectors significantly improved not only the current and quantum efficiencies, more importantly, but also the power efficiency and stability. The maximum efficiencies were as high as 42.0%, 159.5 cd A-1 and 92.7 lm W-1, and the lifetime reached 1350 h, which are more higher than the efficiencies of 34.3% 130.2 cd A-1 and 75.0 lm W-1 and the lifetime of 859 h of conventional tandem OLEDs. The more balanced charge generation and injection due to the simultaneous utilization of OHJ as CGL and charge injectors contributed to the significant improvement in the performance of resulting tandem OLEDs. These results fully indicate the validity of OHJ as CGL and charge injectors, and also provide a new route for the development of high performance OLEDs.

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.

Tailor-made nanostructures bridging chaos and order for highly efficient white organic light-emitting diodes

Organic light-emitting diodes (OLEDs) suffer from notorious light trapping, resulting in only moderate external quantum efficiencies. Here, we report a facile, scalable, lithography-free method to generate controllable nanostructures with directional randomness and dimensional order, significantly boosting the efficiency of white OLEDs. Mechanical deformations form on the surface of poly(dimethylsiloxane) in response to compressive stress release, initialized by reactive ions etching with periodicity and depth distribution ranging from dozens of nanometers to micrometers. We demonstrate the possibility of independently tuning the average depth and the dominant periodicity. Integrating these nanostructures into a two-unit tandem white organic light-emitting diode, a maximum external quantum efficiency of 76.3% and a luminous efficacy of 95.7 lm W−1 are achieved with extracted substrate modes. The enhancement factor of 1.53 ± 0.12 at 10,000 cd m−2 is obtained. An optical model is built by considering the dipole orientation, emitting wavelength, and the dipole position on the sinusoidal nanotexture.

Remarkable enhancement of efficiencies for red, green, and blue tandem phosphorescent organic light-emitting diodes by utilizing a non-doped photovoltaic-type charge generation unit

We have achieved a remarkable enhancement in current efficiencies and power efficiencies for red, green, and blue tandem phosphorescent organic LEDs (T-PhOLEDs) by employing LiF/Al/fullerene/zinc-phthalocyanine/MoO3 as a charge generation unit (CGU). Fullerene/zinc-phthalocyanine with a proper absorption property in the visible region can absorb a part of photons from emission layers within T-PhOLEDs, which facilitates the charge generation and separation within CGU. At the identical luminance of 1,000 cd m−2, the current efficiencies for red, green, and blue T-PhOLEDs are 3.3-, 2.1-, and 2.9-fold those of the corresponding single-unit devices, and power efficiencies are increased by 72.2%, 13.6%, and 37.2%, respectively.

Tandem organic light-emitting diodes with buffer-modified C60/ZnPc as charge generation layer

Tandem organic light-emitting diodes with buffer-modified C60/ZnPc as charge generation layer

Aggregation-induced emission tetraphenylethene type derivatives for blue tandem organic light-emitting diodes

Tetraphenylethene (TPE) has been demonstrated to be an effective core molecule for generating excellent aggregation-induced emissions (AIE). Herein, we propose TPE moieties decorated with different biphenyl or anthracenyl fragments to avoid the crystallization of neat film and simultaneously harvest blue AIE. The two synthesized compounds, 2-(4-biphenyl)-1,1,2-triphenylethylene (2) and 2-(9-antracenyl)-1,1,2-triphenylethylene (3), possess bipolar transport capabilities, high thermal stability, and strong sky-blue AIE. Both compounds 2 and 3 were selected as emitters to investigate their electroluminescence applications. The optimized non-doped sky-blue organic light-emitting diodes (OLEDs) with 2 and 3 exhibited respective maximum efficiencies of 3.8% (8.8 cd/A) and 2.2% (4.0 cd/A). These results illustrate the effective molecular design concepts of the TPE derivatives containing biphenyl or anthracenyl fragments. Furthermore, a tandem device architecture was used to improve device efficiency. Through adopting the specific designs of the intermediate charge generation layer structure and the corresponding device architecture, respective peak efficiencies of 8.2% (19.4 cd/A) and 4.6% (9.5 cd/A) were obtained in tandem devices with 2 and 3 emitters together with slight efficiency roll-off. The superior EL performance demonstrated that the tandem structure possesses high potential for boosting the efficiency of the AIE-based OLEDs.

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.

Design of Tandem Organic Light Emitting Diode using efficient charge generation layer

In this work, highly efficient Tandem Organic Light Emitting Diode (OLED) based on planar organic charge generation layer (CGL) of C70/Pentacene is proposed. It is found that the proposed CGL generate charges as well as exhibits organic photovoltaic type behavior. In addition a very thin layer of Al/LiF and PEDOT:PSS is used as electron and hole injection layer respectively to further increase the transport of electrons and holes from CGL to emissive unit. The current efficiency and luminance of red tandem device is found to be 29 cd/A and 29010 cd/m2 at 20 mA/cm2 and 100 mA/cm2 respectively which is 1.855 times 1.856 times greater than that of single emitter device having current efficiency and luminance of 15.63 cd/A and 15630 cd/cm2 at the same current densities Generally, the turn on voltage of Tandem devices is almost double as compared to single unit device. But with the proposed CGL the turn on voltage of red tandem device is found to be 5.9 V which is less than double the turn on voltage of single unit device (3 V). Therefore, the effective charge generation and transport property of proposed CGL as well as remarkable high injection property of thin Al layer and PEDOT:PSS leads to the enhancement in the efficiency of tandem OLED.

Optimization for tandem organic light-emitting diodes based on Firpic

A series of single-unit and tandem blue phosphorescent organic light-emitting diodes (OLEDs) were prepared by adjusting the concentration of dopant based on the structure of ITO/NPB/EL unit/Alq3/Cs2CO3/Al. The results show that tandem device with doping concentration of 10 wt% has appropriate energy transfer, which achieves the best performance with a maximum current efficiency of 3.4 cd·A−1. Further study found that current efficiency and power efficiency of the tandem OLED adding BCP as hole blocking layer (HBL) can achieve 7.85 cd·A−1 and 0.72 lm·W−1, respectively. It is 2.88 times and 1.57 times larger than those of sing-unit devices, and green peak is restrained effectively.

Buffer-modified C60/pentacene as organic charge generation layer based on Al and MoO3

We demonstrate tandem organic light-emitting diodes (TOLEDs) with excellent performance using Al and MoO3 buffer-modified C60/pentacene as charge generation layer (CGL). Al and MoO3 were used as the electron and hole injection layers of C60/pentacene CGL, respectively. Green phosphorescence TOLEDs with the structure of ITO/NPB/mCP:Ir(ppy)3/TPBi/Al/C60/pentacene/MoO3/NPB/mCP:Ir(ppy)3/TPBi/Cs2CO3/Al were fabricated. The results show that the inserted Al and MoO3 can effectively increase the charge injection capacity of organic CGL, resulting the improvement of luminance and current efficiency of TOLEDs. The turn-on voltage of TOLEDs is much lower than that of single-unit device, and the current efficiency is more than 2 times larger than that of the single-unit device. TOLEDs can exhibit excellent photoelectric performance when the thicknesses of Al, C60, pentacene and MoO3 are 3 nm, 15 nm, 25 nm and 1 nm, respectively. The maximum luminance and current efficiency are 7 920.0 cd/m2 and 16.4 cd/A, respectively. This work is significant to build new CGL structures for realizing high-performance TOLEDs.

Electrical and optical analyses of tandem organic light-emitting diodes with organic charge-generation layer

The electrical and optical properties of tandem organic light-emitting diodes (OLEDs), in which a fluorescent and phosphorescent emitting units are connected by an organic charge-generation layer (CGL), were experimentally analyzed. To investigate the internal properties of the tandem OLEDs, we fabricated and compared two single, two homo-tandem, and two hetero-tandem OLEDs using the fluorescent and phosphorescent units. From the experimental results of the OLEDs obtained at the same current density, the voltage across the CGL as well as the individual emission spectra and luminance of each unit of tandem OLEDs were obtained and compared with the theoretical simulation results. The analysis method proposed in this study can be utilized as a method to verify the accuracy of optical or electrical computer simulation of tandem OLED and it will be useful to understand the overall electrical and optical characteristics of tandem OLEDs.

Silver-induced activation of 8-hydroxyquinolinato lithium as electron injection material in single-stack and tandem OLED devices

Abstract The 8-hydroxyquinolinato lithium (Liq)|Ag system has been evaluated as the electron-injection layer for the MoOx| N,N′-di (1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) charge-generation layer in tandem organic light-emitting diode (OLED) devices. From current-voltage studies of single-stack devices, it is determined that Liq is a relatively poor electron-transport material. However, Liq is found to be useful as an electron-injection layer with activation by Ag. Efficient tandem devices have been obtained using Liq|Ag as the electron-injection layer adjacent to the MoOx|NPB charge-generation layer, where the optimal Ag thickness is about 20–40 A and the activation of Liq can be seen with as little as 1 A of Ag.