Field Of Organic Light-emitting Diodes Research Articles

NIR TADF emitters and OLEDs: challenges, progress, and perspectives.

Near-infrared (NIR) light-emitting materials show excellent potential applications in the fields of military technology, bioimaging, optical communication, organic light-emitting diodes (OLEDs), etc. Recently, thermally activated delayed fluorescence (TADF) emitters have made historic developments in the field of OLEDs. These metal-free materials are more attractive because of efficient reverse intersystem crossing processes which result in promising high efficiencies in OLEDs. However, the development of NIR TADF emitters has progressed at a relatively slower pace which could be ascribed to the difficult promotion of external quantum efficiencies. Thus, increasing attention has been paid to NIR TADF emitters. In this review, the recent progress of NIR TADF emitters has been summarized along with their molecular design strategies and photophysical properties, as well as electroluminescence performance data of their OLEDs, respectively.

Efficient Exciplex-Based Deep-Blue Organic Light-Emitting Diodes Employing a Bis(4-fluorophenyl)amine-Substituted Heptazine Acceptor.

The realization of a deep-blue-emitting exciplex system is a herculean task in the field of organic light-emitting diodes (OLEDs) on account of a large red-shifted and broadened exciplex emission spectrum in comparison to those of the corresponding single compounds. Herein, 2,5,8-tris(di(4-fluorophenyl)amine)-1,3,4,6,7,9,9b-heptaazaphenalene (HAP-3FDPA) was designed as an electron acceptor by integrating three bis(4-fluorophenyl)amine groups into a heptazine core, while 1,3-di(9H-carbazol-9-yl)benzene (mCP) possessing two electron-donating carbazole moieties was chosen as the electron donor. Excitingly, the exciplex system of 8 wt% HAP-3FDPA:mCP exhibited deep-blue emission and a high photoluminescence quantum yield of 53.2%. More importantly, an OLED containing this exciplex system as an emitting layer showed deep-blue emission with Commission Internationale de l’Eclairage coordinates of (0.16, 0.12), a peak luminance of 15,148 cd m−2, and a rather high maximum external quantum efficiency of 10.2% along with a low roll-off. This study not only reports an efficient exciplex-based deep-blue emitter but also presents a feasible pathway to construct highly efficient deep-blue OLEDs based on exciplex systems.

Photophysical and DFT investigation of imidazole-based hole transporting materials for phosphorescent OLEDs with high current efficiency

Organic small molecules have played a significant role in the field of organic light-emitting diodes (OLEDs) technology. Improving the stability of OLEDs continues to be a major task in organic electronics. To overcome this issue, it is essential to enhance the carrier injection into the emissive layer for better charge balance. Though OLEDs are highly desirable for the solution process, enabling simple and cost-effectively manufacturing to fabricate large-area devices, the development of hole-transporting materials (HTMs) still a considerable challenge. Herein the IMzN (1-(4-methoxyphenyl)-2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole) and IMzB (4-(1-(4-methoxyphenyl)-4,5-diphenyl-1H-imidazol-2-yl)benzoic acid) with excellent solution processability and stability suitable for efficient OLED devices using a simple and efficient microwave-assisted synthetic procedure. Thermogravimetric analysis of IMzN and IMzB suggested that the material be thermally stable up to 320–345 °C. No glass transition has occurredas confirmed by DSC indicating the amorphous nature of the materials, which is well supported by XRD analysis. Materials acquire the appropriate HOMO levels for favorable hole injection as well as superior hole mobility. As a result, both HTMs show better device performance than the commercially used TAPC. The IMzB as HTL shows remarkable current efficiency of 91.13 cd A-1 and EQE of 21.11%. This is the highest current efficiency reported using solution-processed green phosphorescent OLEDs. Devices with IMzB as HTL also exhibit relatively better stability under continuous stress.

Difluoroboron locking tactic enhances photo- and electroluminescence of TADF emitter

Luminescent organoboron materials have attracted considerable interests in the field of organic light-emitting diodes (OLEDs). However, their applications are limited by their complicated synthetic routes. Herein, a simple and efficient difluoroboron locking tactic was employed to enhance molecular rigidity and electron accepting strength of thermally activated delayed fluorescence emitters. With this tactic, the resulting difluoroboron compound, MBF, exhibited more rigid configuration and higher acceptor strength than the reference emitter of MPA, resulting in nearly three times higher photoluminescence quantum yield in doped film and a bathochromic emission shift of 91 nm in dilute toluene solution. In comparison with the MPA-based reference device, the MBF-based OLED rendered six times higher maximum external quantum efficiency accompanied with an electroluminescence peak at 630 nm.

Efficient Red Thermally Activated Delayed Fluorescence Emitters Based on a Dibenzonitrile-Substituted Dipyrido[3,2-a:2',3'-c]phenazine Acceptor.

How to construct efficient red-emitting thermally activated delayed fluorescence (TADF) materials is a challenging task in the field of organic light-emitting diodes (OLEDs). Herein, an electron acceptor moiety, 3,6-DCNB-DPPZ, with high rigidity and strong acceptor strength was designed by introducing two cyanobenzene groups into the 3,6-positions of a dipyrido[3,2-a:2′,3′-c]phenazine unit. A red-emitting compound, 3,6_R, has been designed and synthesized by combining the rigid acceptor unit with two triphenylamine donors. Due to high molecular rigidity and strong intramolecular charge transfer characteristic in donor–acceptor–donor skeleton, 3,6_R exhibited a red emission with a high photoluminescence quantum yield of 86% and distinct TADF nature with short delayed fluorescence lifetime of about 1 microsecond. Accordingly, the OLED using 3,6_R as the guest emitter gained a high external quantum efficiency of 12.0% in the red region with an electroluminescence peak of 619 nm and favorable Commission Internationale de l’Eclairage coordinates of (0.62, 0.38).

High-Efficiency Red Electroluminescence Based on a Carbene-Cu(I)-Acridine Complex.

How to develop efficient red-emitting organometallics of earth-abundant copper(I) is a formidable challenge in the field of organic light-emitting diodes (OLEDs) because Cu(I) complexes have weak spin-orbit coupling and a serious excited-state reorganization effect. Here, a red Cu(I) complex, MAC*-Cu-DPAC, was developed using a rigid 9,9-diphenyl-9,10-dihydroacridine donor ligand in a carbene-metal-amide motif. The Cu(I) complex achieved satisfactory red emission, a high photoluminescence quantum yield of up to 70%, and a sub-microsecond lifetime. Thanks to a linear geometry and the acceptor and donor ligands in a coplanar conformation, the complex exhibited a high horizontal dipole ratio of 77% in the host matrix, first demonstrated for coinage metal(I) complexes. The resulting OLEDs delivered high external quantum efficiencies of 21.1% at a maximum and 20.1% at 1000 nits, together with a red emission peak at ∼630 nm. These values represent the state-of-the-art performance for red-emitting OLEDs based on coinage metal complexes.

Rational design of perfectly oriented thermally activated delayed fluorescence emitter for efficient red electroluminescence

How to control the dipole orientation of organic emitters is a challenge in the field of organic light-emitting diodes (OLEDs). Herein, a linear thermally activated delayed fluorescence (TADF) molecule, PhNAI-PMSBA, bearing a 1,8-naphthalimide-acridine framework was designed by a double-site long-axis extension strategy to actively control the dipole orientation. The horizontal ratio of emitting dipole orientation of PhNAI-PMSBA reaches 95%, substantially higher than that of isotropic emitters (67%). This unique feature is associated with the intrinsically horizontal molecular orientation of PhNAI-PMSBA and the good agreement between its transition dipole moment direction and molecular long axis. The PhNAI-PMSBA-based OLED achieves an ultrahigh optical outcoupling efficiency of 43.2% and thus affords one of the highest red electroluminescence with an external quantum efficiency of 22.3% and the Commission International de l’Eclairage 1931 coordinates at around (0.60, 0.40).

Nonlinear optoelectronic processes in organic optoelectronic devices: Triplet-triplet annihilation and singlet fission

Organic semiconductors with optoelectronic properties have attracted intensive interests in the fields of organic light-emitting diodes, organic photovoltaics and organic photodetectors. In these functional devices, exciton evolution is the key process. Generally, linear process is dominant that one photon is created from each injected electron-hole pair, vice versa. This linear process restricts the internal quantum efficiency to be 100% in optoelectronic devices. Alternatively, nonlinear optoelectronic processes, like triplet-triplet annihilation (TTA) and singlet fission (SF), can theoretically break the limitation stated in linear optoelectronic processes. TTA and SF are two dynamic reversible processes, which link one high-lying energy singlet with two low-lying energy triplet excitons on two neighboring chromophores. These nonlinear processes bring the possibility of novel functions and application scenarios, and hence have attracted more and more attentions recently.This review will systematically and comparatively summarize the basic properties of TTA and SF processes, and their applications in optoelectronic devices in the view of device physics. It starts with a concise introduction of basic knowledge of TTA and SF. Then, the characteristics used to identify these processes are described. Subsequently, implementing them in OLEDs are summarized, followed by the application examples in OPVs. Emphasis is laid on the triplet quenching mechanisms in working devices, since both TTA and SF processes highly rely on triplet excitons. These discussions on the recent important progresses aim to gain some insight into the device physics for device engineering. Finally, a perspective is presented.

Efficient Non-doped Blue Fluorescent Organic Light-Emitting Diodes Based on Anthracene-Triphenylethylene Derivatives.

The development of efficient blue materials has been a continuous research topic in the field of organic light-emitting diodes (OLEDs). In this paper, three aggregation-induced emission enhancement active blue emitters, PIAnTPE, TPAAnTPE and CzAnTPE, are successfully synthesized by attaching a triphenylethylene unit and phenanthroimidazole/triphenylamine/carbazole moieties to the 9,10-positions of anthracene, respectively. The three compounds exhibit good thermal stabilities, appropriate for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels and display high photoluminescence quantum yields (PLQYs) of 65, 70 and 46 % in the solid state. Non-doped blue devices using PIAnTPE, TPAAnTPE and CzAnTPE as the emitting layers show good electroluminescent performances, with the maximum external quantum efficiencies (EQEs) of 4.46, 4.13 and 4.04 %, respectively. More importantly, EQEs of all the three devices can be still retained when the luminescence reaches 1000 cd m-2 , exhibiting quite small efficiency roll-offs in the non-doped OLEDs.

A Novel trans-1-(9-Anthryl)-2-phenylethene Derivative Containing a Phenanthroimidazole Unit for Application in Organic Light-Emitting Diodes.

Aryl-substituted phenanthroimidazoles (PIs) have attracted tremendous attention in the field of organic light-emitting diodes (OLEDs), because they are simple to synthesize and have excellent thermal properties, high photoluminescence quantum yields (PLQYs), and bipolar properties. Herein, a novel blue-green emitting material, (E)-2-{4'-[2-(anthracen-9-yl)vinyl]-[1,1'-biphenyl]-4-yl}-1-phenyl-1H-phenanthro[9,10-d]imidazole (APE-PPI), containing a t-APE [1-(9-anthryl)-2-phenylethene] core and a PI moiety was designed and synthesized. Owing to the PI skeleton, APE-PPI possesses high thermal stability and a high PLQY, and the compound exhibits bipolar transporting characteristics, which were identified by single-carrier devices. Nondoped blue-green OLEDs with APE-PPI as the emitting layer show emission at λ=508 nm, a full width at half maximum of 82 nm, a maximum brightness of 9042 cd m-2 , a maximum current efficiency of 2.14 cd A-1 , and Commission Internationale de L'Eclairage (CIE) coordinates of (0.26, 0.55). Furthermore, a white OLED (WOLED) was fabricated by employing APE-PPI as the blue-green emitting layer and 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) doped in tris-(8-hydroxyquinolinato)aluminum (Alq3 ) as the red-green emitting layer. This WOLED exhibited a maximum brightness of 10029 cd m-2 , a maximum current efficiency of 16.05 cd A-1 , CIE coordinates of (0.47, 0.47), and a color rendering index (CRI) of 85. The high performance of APE-PPI-based devices suggests that the t-APE and PI combination can potentially be used to synthesize efficient electroluminescent materials for WOLEDs.

A Novel Spiro[acridine-9,9'-fluorene] Derivatives Containing Phenanthroimidazole Moiety for Deep-Blue OLED Application.

Typical π-π stacking and aggregation-caused quenching could be suppressed in the film-state by the spiro conformation molecular design in the field of organic light-emitting diodes (OLEDs). Herein, a novel deep-blue fluorescent material with spiro conformation, 1-(4-(tert-butyl)phenyl)-2-(4-(10-phenyl-10H-spiro[acridine-9,9'-fluoren]-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (SAF-BPI), was designed and synthesized. The compound consists of spiro-acridine-fluorene (SAF) as donor part and phenanthroimidazole (BPI) as acceptor part. Owing to the rigid SAF skeleton, this compound exhibits a high thermal stability with a glass transition temperature (Tg ) of 198 °C. The compound exhibits bipolar transporting characteristics demonstrated by the single-carrier devices. The non-doped OLEDs based on the SAF-BPI as the emitting layer shows maximum emission at 448 nm, maximum luminance of 2122 cd m-2 , maximum current efficiency (CE) of 3.97 cd A-1 , and a maximum power efficiency of 2.08 lm W-1 . The chromaticity coordinate is stable at (0.15, 0.10) at the voltage of 7-11 V. The device shows a slow efficiency roll-off with CE of 3.35 and 2.85 cd A-1 at 100 and 1000 cd m-2 , respectively.

Efficient Deep-Blue Electroluminescence Based on Phenanthroimidazole-Dibenzothiophene Derivatives with Different Oxidation States of the Sulfur Atom.

Developing efficient deep-blue materials is a long-term research focus in the field of organic light-emitting diodes (OLEDs). In this paper, we report two deep-blue molecules, PITO and PISF, which share similar chemical structures but exhibit different photophysical and device properties. These two molecules consist of phenanthroimidazole and dibenzothiophene analogs. The distinction of their chemical structures lies in the different oxidation states of the S atom. For PITO, the S atom is oxidized and the resulting structure dibenzothiophene S,S-dioxide becomes electron deficient. Therefore, PITO displays remarkable solvatochromism, implying a charge-transfer (CT) excited state formed between the donor (D) phenanthroimidazole and acceptor (A) dibenzothiophene S,S-dioxide. For PISF, it is constituted of phenanthroimidazole and dibenzothiophene in which the S atom is not oxidized. PISF displays locally excited (LE) emission with little solvatochromism. Compared with PISF, the D-A molecule PITO with an electron-deficient group shows a much lower LUMO energy level, which is in favor of electron injection in device. In addition, PITO exhibits more balanced carrier transport. However, PISF is capable of emitting in the shorter wavelength region, which is beneficial to obtain better color purity. The doped electroluminescence (EL) device of the D-A molecule PITO manifests deep-blue emission with CIE coordinates of (0.15, 0.08) and maximum external quantum efficiency (EQE) of 4.67 %. The doped EL device of the LE molecule PISF, however, reveals an even bluer emission with CIE coordinates of (0.15, 0.06) and a maximum EQE of 4.08 %.

Phosphorescent Iridium(III) Complexes Bearing Fluorinated Aromatic Sulfonyl Group with Nearly Unity Phosphorescent Quantum Yields and Outstanding Electroluminescent Properties.

A series of heteroleptic functional Ir(III) complexes bearing different fluorinated aromatic sulfonyl groups has been synthesized. Their photophysical features, electrochemical behaviors, and electroluminescent (EL) properties have been characterized in detail. These complexes emit intense yellow phosphorescence with exceptionally high quantum yields (ΦP > 0.9) at room temperature, and the emission maxima of these complexes can be finely tuned depending upon the number of the fluorine substituents on the pendant phenyl ring of the sulfonyl group. Furthermore, the electrochemical properties and electron injection/transporting (EI/ET) abilities of these Ir(III) phosphors can also be effectively tuned by the fluorinated aromatic sulfonyl group to furnish some desired characters for enhancing the EL performance. Hence, the maximum luminance efficiency (ηL) of 81.2 cd A(-1), corresponding to power efficiency (ηP) of 64.5 lm W(-1) and external quantum efficiency (ηext) of 19.3%, has been achieved, indicating the great potential of these novel phosphors in the field of organic light-emitting diodes (OLEDs). Furthermore, a clear picture has been drawn for the relationship between their optoelectronic properties and chemical structures. These results should provide important information for developing highly efficient phosphors.

Reliable Prediction with Tuned Range-Separated Functionals of the Singlet-Triplet Gap in Organic Emitters for Thermally Activated Delayed Fluorescence.

The thermally activated delayed fluorescence (TADF) mechanism has recently attracted significant interest in the field of organic light-emitting diodes (OLEDs). TADF relies on the presence of a very small energy gap between the lowest singlet and triplet excited states. Here, we demonstrate that time-dependent density functional theory in the Tamm-Dancoff approximation can be very successful in calculations of the lowest singlet and triplet excitation energies and the corresponding singlet-triplet gap when using nonempirically tuned range-separated functionals. Such functionals provide very good estimates in a series of 17 molecules used in TADF-based OLED devices with mean absolute deviations of 0.15 eV for the vertical singlet excitation energies and 0.09 eV [0.07 eV] for the adiabatic [vertical] singlet-triplet energy gaps as well as low relative errors and high correlation coefficients compared to the corresponding experimental values. They significantly outperform conventional functionals, a feature which is rationalized on the basis of the amount of exact-exchange included and the delocalization error. The present work provides a reliable theoretical tool for the prediction and development of novel TADF-based materials with low singlet-triplet energetic splittings.

Recent Advances in Organic Electroluminescent Materials and Devices

Since the invention of organic light-emitting diodes(OLEDs) by Dr. CW Tang in 1987, researches in related fields have been developed rapidly. Now, small to medium size OLED displays have already been commercialized. Large OLED TVs and OLED lightings have also emerged in real applications. The further development of the OLED technology calls for intensive research on organic light-emitting materials and devices. This report reviewed the most focused issues as well as the main progress in the field of OLEDs in the recent years. It is shown that researchers have paid attentions to both technical innovations and theoretical researches, such as the design of new molecules, developing of new device technology, and understanding of light-emitting and transporting mechanisms of organic semiconductors.The developing trends of organic light-emitting materials and devices are summarized as follows: 1) molecular design for high performance phosphorescent materials; 2) novel fluorescent materials and their light emitting mechanisms; 3) new technologies for high performance white OLEDs; 4) new technologies for solution-processed devices and flexible devices; 5) the influences of molecular packing, disorder and doping on the charge transporting properties of organic semiconductors. Moreover, the current situations and developing trends of international OLED display and lighting industries are discussed.

From Simple Ligands to Complex Structures: Structural Diversity of Silver(I) Complexes Bearing Tetradentate (alkylenebimpy) NHC Ligands

The synthesis of five new silver(I) NHC complexes, bearing alkylenebimpy ligands with bridge lengths ranging from ethylene to hexylene is described. Their solid-state structures exhibit an unforeseen variety, including infinite strands of Ag3 units within a helically wound ligand environment. The very first infinite chain of almost linear Ag3 units within a cage of helically wound ligand molecules is described. All complexes are spectroscopically characterized and emit polychromatic blue light (420 nm ≤ λ ≤ 510 nm) after laser excitation at λ = 337 nm. This latter behavior might lead to applications in the field of organic light-emitting diodes.

ChemInform Abstract: Yellow/Orange Emissive Heavy‐Metal Complexes as Phosphors in Monochromatic and White Organic Light‐Emitting Devices

AbstractReview: 215 refs.

Organic Light - Emitting Diodes and their Applications

Organic light emitting diodes (OLEDs) have been the focus of intense study since the late 1980s, when the low voltage organic electroluminescence in small organic molecules such as Alq3, and large organic molecules such as polymers (PPV), was reported. Since that time, research has continued to demonstrate the potential of OLEDs as viable systems for displays and eco-friendly lighting applications. OLEDs offer full colour display, reduced manufacturing cost, larger viewing angle, more flexible, lower power consumption, better contrast, slimmer, etc. which help in replacing the other technologies such as LCD. The operation of OLEDs involves injection of charge carriers into organic semiconducting layers, recombination of charge carriers, formation of singlet and triplet excitons, and emission of light during decay of excitons. The maximum internal quantum efficiency of fluorescent OLEDs consisting of the emissive layer of fluorescent organic material is 25% because in this case only the 25% singlet excitons can emit light. The maximum internal quantum efficiency of phosphorescent OLEDs consisting of the emissive layer of fluorescent organic material mixed with phosphorescent material of heavy metal complexes such as platinum complexes, iridium complexes, etc. is nearly 100% because in this case both the 25% singlet excitons and 75% triplet excitons emit light. Recently, a new class of OLEDs based on thermally activated delayed fluorescence (TADF) has been reported, in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates of more than 106decays per second. These molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels and provides an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.The OLED technology can be used to make screens large enough for laptop, cell phones, desktop computers, televisions, etc. OLED materials could someday be applied to plastic and other materials to create wall-size video panels, roll-up screens for laptops, automotive displays, and even head wearable displays. Presently, the OLEDs are opening up completely new design possibilities for lighting in the world of tomorrow whereby the offices and living rooms could be illuminated by lighting panels on the ceiling. The present paper describes the salient features of OLEDs and discusses the applications of OLEDs in displays and solid state lighting devices. Finally, the challenges in the field of OLEDs are explored. Contents of Paper

Citation network analysis of organic LEDs

The field of organic light-emitting diodes (OLEDs) is an emergent research domain because of both scientific interest in chemistry and condensed matter physics and industrial importance as flat panel displays. In this paper, we analyzed a citation network of OLED papers and used a topological clustering method to investigate the structure of research and to detect emerging research domains. We found that most papers belong to two main clusters: organics and polymers. These two clusters have distinctive differences in subcluster structures and journals where papers are published. Supposing this discrepancy to indicate research progress, organics are in the applied research stage, while polymers are in the basic research stage.

New Host Materials Containing Carbazole and Naphthyl Moieties for Green Dopant in Phosphorescence Organic Light-Emitting Diodes (OLEDs)

Much effort on the development of new host materials has been devoted to increase the emission efficiency of organic light-emitting diodes (OLEDs). In particular, the use of phosphorescent molecules as guest emitters, which harvests both singlet and triplet excitons, leads to internal quantum efficiency (ηint) approaching 100%. 2 Energy transfer from a host to a dopant in OLEDs can result in high external quantum efficiency (ηext). Therefore, the development of suitable hosts for blue, green and red phosphorescent emitters has been one of the major issues in the field of OLEDs. In general, host materials should have a higher triplet energy than that of the dopant molecules due to the energy transfer from the host to the guest while prohibits the energy transfer of triplet excitons from the guest to the host material. From the point of this consideration, carbazole derivatives are often use as host materials, because they have high triplet energies (2.9 eV or less). Polyaromatic compounds bearing naphthyl groups have formed an important class of highly efficient and stable bluelight emitting materials. It has been suggested that nonplanar derivatives of naphthalene due to steric factors may hinder close packing and improve the device performance; hence, the electroluminescence (EL) polyaromatic compounds bearing naphthyl groups have been designed on the basis of this principle. During our ongoing efforts on the development of emitting materials and host for phosphorescence OLEDs, we have observed that naphthyl derivatives linked by 9-ethylcarbazole unit as a core, such as 3,6-di(1naphthyl)-9-ethylcarbazole (1-DNEC) and 3,6-di(2-naphthyl)-9-ethylcarbazole (2-DNEC), have high thermal stability, reversible electrochemical behaviors and larger band gap compared to those of other carbazole derivatives. These facts have prompted us to investigate electroluminescent (EL) characteristics of both compounds. Herein, we discuss electroluminescent characteristics and device data for OLEDs employing both 1-DNEC and 2-DNEC as host. We fabricated multilayer devices with the configuration of ITO/NPB (30 nm)/(device 1) 1-DNEC and 2-DNEC (device 2): Ir(ppy)3 (45 nm: 3%)/Alq3 (25 nm)/LiF (2 nm)/Al. The layers of the device consist of ITO (indium-tin-oxide) as the anode, NPB (N,N'-di-1-naphythyl-N,N'-diphenyl-byphenyl4,4'-diamine) as the hole transporting layer, 1-DNEC or 2DNEC as host, 3%-Ir(ppy)3 (ppy = 2-phenylpyridine) as the emitter, Alq3 (q = 8-hydroxyquinolate) as the electron transporting layer, LiF as the electron injection layer, and Al as the cathode, respectively (Figure 1). In this study, we chose Ir(ppy)3 as the emissive material, which is a prototypical dopant for green OLEDs. The electroluminescence spectra of all devices consist only of green emission (λmax = 458 nm) from Ir(ppy)3 without any emissions from the host and Alq3,