Modulation of Exciton Generation in Organic Active Planar pn Heterojunction: Toward Low Driving Voltage and High-Efficiency OLEDs Employing Conventional and Thermally Activated Delayed Fluorescent Emitters

Tuning Charge-Transfer States by Interface Electric Fields

Intermolecular charge-transfer (CT) states are extended excitons with a charge separation on the nanometer scale. Through absorption and emission processes, they couple to the ground state. This property is employed both in light-emitting and light-absorbing devices. Their conception often relies on donor-acceptor (D-A) interfaces, so-called type-II heterojunctions, which usually generate significant electric fields. Several recent studies claim that these fields alter the energetic configuration of the CT states at the interface, an idea holding prospects like multicolor emission from a single emissive interface or shifting the absorption characteristics of a photodetector. Here, we test this hypothesis and contribute to the discussion by presenting a new model system. Through the fabrication of planar organic p-(i-)n junctions, we generate an ensemble of oriented CT states that allows the systematic assessment of electric field impacts. By increasing the thickness of the intrinsic layer at the D-A interface from 0 to 20 nm and by applying external voltages up to 6 V, we realize two different scenarios that controllably tune the intrinsic and extrinsic electric interface fields. By this, we obtain significant shifts of the CT-state peak emission of about 0.5 eV (170 nm from red to green color) from the same D-A material combination. This effect can be explained in a classical electrostatic picture, as the interface electric field alters the potential energy of the electric CT-state dipole. This study illustrates that CT-state energies can be tuned significantly if their electric dipoles are aligned to the interface electric field.

A simple molecular design towards the conversion of a MCL backbone to a multifunctional emitter exhibiting polymorphism, AIE, TADF and MCL

Compared with the large number of single-function materials such as aggregation-induced emission (AIE), mechanochromic luminescence (MCL), or thermally activated delayed fluorescence (TADF) emitters, multifunctional emitting materials offer more opportunities in practical applications. In this report, we provide a simple molecular design strategy towards the conversion of a MCL building block to a multifunctional emitter. Through altering the substituent sites and increasing the number of electron donors and steric hindrance on a normal MCL backbone benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one, a novel multifunctional material 10,11-bis-(4-diphenylamino-phenyl)-benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (10,11-2TPA-BBI) is designed and synthesized. 10,11-2TPA-BBI exhibits simultaneous polymorphism, AIE, MCL and TADF properties. It can form four different aggregate species: yellow solid (YS) and orange solid (OS), orange flake-shaped crystal (OC), and red prism-like crystal (RC). Among them, because of the small energy gaps (ΔE_STs < 0.3 eV) between the singlet and triplet excited states, OS, OC and RC exhibit TADF properties, while YS show normal fluorescence characteristics with a large ΔE_ST of 0.33 eV. OS can be reversibly transformed into YS upon external stimuli, which can be attributed to the emission switch between local excited state and charge transfer state. Crystallographic study indicates that the bulky structure and weak intermolecular interactions account for polymorphism and AIE properties. This work will provide a simple molecular design strategy for multifunctional materials.

Recent progress on organic light-emitting diodes with phosphorescent ultrathin (<1nm) light-emitting layers

In recent years, phosphorescent dyes forming ultrathin light-emitting layers (<1 nm, UEMLs) have been widely applied to fabricate monochromatic and white organic light-emitting diodes (OLEDs) owing to its merits of simplified device structure and preparation process, more flexible design, lower material consumption, and complete exciton utilization. In addition, it was demonstrated that the OLEDs with UEMLs achieved high electroluminescence performance comparable to the conventional doping-based devices. Structurally, OLEDs were structured with phosphorescent UEMLs inserted into nonluminous materials, heterojunction interface as well as into luminescent materials including phosphorescent, conventional fluorescent, thermally activated delayed fluorescence, and exciplex emitters. We carefully reviewed the successful applications of UEMLs in OLEDs and underlying working mechanism of corresponding devices, and also emphasized the representative achievements about OLEDs with UEMLs, aimed at forming a comprehensive summary of the present research for UEMLs-based OLEDs. In the end, we also gave an outlook for the future development of UEMLs-based OLEDs

Revealing Topological Influence of Phenylenediamine Unit on Physicochemical Properties of Donor-Acceptor-Donor-Acceptor Thermally Activated Delayed Fluorescent Macrocycles

A new thermally activated delayed fluorescence (TADF)-displaying macrocyclic compound m-1 comprising of two electron-donors (N,N'-diphenyl-m-phenylenediamine) and two electron-acceptors (dibenzo[a,j]phenazine) has been synthesized. The macrocycle developed herein is regarded as a regioisomer of the previously reported TADF macrocycle p-1, which has two N,N'-diphenyl-p-phenylenediamines as the donors. To understand the influence of the topology of the phenylenediamine donors on physicochemical properties of TADF-active macrocycles, herein the molecular structure in the single crystals, photophysical properties, electrochemical behavior, and TADF properties of m-1 have been investigated compared with those of p-1. The substitution of p-phenylene donor with m-phenylene donor led to distinct positive solvatoluminochromism over the full visible-color range, unique oxidative electropolymerization, and slightly lower contribution of TADF, due to the lower CT character in the excited states.

OBO-Fused Benzo[fg]tetracene as Acceptor With Potential for Thermally Activated Delayed Fluorescence Emitters

Six luminophores bearing an OBO-fused benzo[fg]tetracene core as an electron acceptor were designed and synthesized. The molecular structures of three molecules (PXZ-OBO, 5PXZ-OBO, 5DMAC-OBO) were determined by single crystal X-ray diffraction studies and revealed significant torsion between the donor moieties and the OBO acceptor with dihedral angles between 75.5 and 86.2°. Photophysical studies demonstrate that blue and deep blue emission can be realized with photoluminescence maxima (λ_PL) ranging from 415 to 480 nm in mCP films. The emission energy is modulated by simply varying the strength of the donor heterocycle, the number of donors, and their position relative to the acceptor. Although the DMAC derivatives show negligible delayed emission because of their large singlet-triplet excited state energy difference, ΔE_ST, PXZ-based molecules, especially PXZ-OBO with an experimental ΔE_ST of 0.25 eV, demonstrate delayed emission in blend mCP films at room temperature, which suggests triplet exciton harvesting occurs in these samples, potentially by thermally activated delayed fluorescence.

Achieving High-Performance Pure-Red Electrophosphorescent Iridium(III) Complexes Based on Optimizing Ancillary Ligands

Two new iridium(III) complexes were synthesized by introducing two trifluoromethyl groups into an ancillary ligand to develop pure-red emitters for organic light-emitting diodes (OLEDs). The electron-donating ability of the ancillary ligands is suppressed, owing to the electron-withdrawing nature of trifluoromethyl groups, which can reduce the HOMO energy levels compared with those of compounds without trifluoromethyl groups. However, the introduction of trifluoromethyl groups into the ancillary ligand has little impact on the LUMO energy levels. Therefore, a well-tuned, pure-red, excited-state energy was achieved by regulating the relative energy level between the HOMO and LUMO. OLEDs with these complexes as emitters showed high external quantum efficiencies (EQEs) of 26 % and realized high EQEs of about 25 % and fairly low driving voltages of 3.3-3.6 V for practical luminance of 1000 cd m^-2 , as well as excellent Commission Internationale de L'Eclairage (CIE) coordinates of (0.66, 0.33) and (0.67, 0.33); thus, this demonstrates the successful molecular design strategy by modifying the electron-donating ability of ancillary ligand.

Recent Progress of Singlet-Exciton-Harvesting Fluorescent Organic Light-Emitting Diodes by Energy Transfer Processes

The external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs) has been dramatically improved by developing highly efficient organic emitters such as phosphorescent emitters and thermally activated delayed fluorescent (TADF) emitters. However, high-EQE OLED technologies suffer from relatively poor device lifetimes in spite of their high EQEs. In particular, the short lifetimes of blue phosphorescent and TADF OLEDs remain a big hurdle to overcome. Therefore, the high-EQE approach harvesting singlet excitons of fluorescent emitters by energy transfer processes from the host or sensitizer has been explored as an alternative for high-EQE OLED strategies. Recently, there has been a big jump in the EQE and device lifetime of singlet-exciton-harvesting fluorescent OLEDs. Recent progress on the materials and device structure is discussed herein.

Exciplex System with Increased Donor-Acceptor Distance as the Sensitizing Host for Conventional Fluorescent OLEDs with High Efficiency and Extremely Low Roll-Off

Exciplex systems with efficient thermally activated delayed fluorescence as the sensitizing hosts for fluorescent organic light-emitting diodes (OLEDs) have been flourished recently, while the device performances are still lagging behind. Here, a donor molecule sterically encapsulated with tert-butyl units is designed and synthesized to increase the donor-acceptor separation in an exciplex system, leading to reduced singlet-triplet energy gap (Δ E_STs) and improved reverse intersystem crossing (RISC) efficiency. OLEDs utilizing exciplexes with increased donor-acceptor distance ( r_DA) as the hosts for conventional fluorescent dopants exhibit a maximum external quantum efficiency (EQE_max) as high as 16.5%, benefiting from the enhanced RISC process and suppressed exciton loss by the Dexter interaction. Furthermore, extremely low efficiency roll-off is obtained with EQEs of 16.2% at 5000 cd/m² and 15.2% at 10 000 cd/m². The results here represent the state-of-the-art performances for devices based on exciplexes as the hosts for conventional fluorescent dopants, manifesting the superiority of exciplexes with increased r_DA as the sensitizing hosts for fluorescent dopants.

Exciplex-Based Electroluminescence: Over 21% External Quantum Efficiency and Approaching 100 lm/W Power Efficiency

Benzimidazole-triazine-based electron acceptor PIM-TRZ with high triplet exited-state energy and strong electron-transport ability was newly developed. A series of highly efficient exciplex emitters have been fabricated. The TAPC:PIM-TRZ (TAPC: di-[4-( N, N-ditoly amino)-phenyl]cyclohexane) film shows a high photoluminescence (PL) quantum yields (PLQY, Φ_f) of 93.4%, and the device based on TAPC:PIM-TRZ exhibits a low turn-on voltage of 2.3 V, high maximum efficiency of 71.2 cd A^-1 (current efficiency, CE), 97.3 lm W^-1 (power efficiency, PE), and 21.7% (external quantum efficiency, EQE), as well as a high EQE of 16.2% at a luminance of 5000 cd m^-2. The device displays the highest efficiency among reported organic light-emitting devices with an exciplex film as the emitting layer. Furthermore, a green device is also fabricated with a TAPC:PIM-TRZ cohost using C545T (C545T: (10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1 H,5 H,11 H-benzopyrano[6,7-8- I, j]quinolizin-11-one)) as the dopant, and the highest CE, PE, and EQE are 68.3 cd A^-1, 86.6 lm W^-1, and 20.2%, respectively.

Pyrazine-Based Blue Thermally Activated Delayed Fluorescence Materials: Combine Small Singlet-Triplet Splitting With Large Fluorescence Rate

Metal-free thermally activated delayed fluorescence (TADF) emitters have emerged as promising candidate materials for highly efficient and low-cost organic light-emitting diodes (OLEDs). Here, a novel acceptor 2-cyanopyrazine is selected for the construction of blue TADF molecules via computer-assisted molecular design. Both theoretical prediction and experimental photophysical data indicate a small S₁-T₁ energy gap (ΔE_ST) and a relative large fluorescence rate (k_F) in an o-phenylene-bridged 2-cyanopyrazine/3,6-di-tert-butylcarbazole compound (TCzPZCN). The k_F value of 3.7 × 10⁷ s⁻¹ observed in a TCzPZCN doped film is among the highest in the TADF emitters with a ΔE_ST smaller than 0.1 eV. Blue TADF emission is observed in a TCzPZCN doped film with a short TADF lifetime of 1.9 μs. The OLEDs using TCzPZCN as emitter exhibit a maximum external quantum efficiency (EQE) of 7.6% with low-efficiency roll-off. A sky-blue device containing a derivative of TCzPZCN achieves an improved EQE maximum of 12.2% by suppressing the non-radiative decay at T₁.

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.

A thermally activated delayed fluorescence exciplex to achieve highly efficient and stable blue and green phosphorescent organic light-emitting diodes

The development of a thermally activated delayed fluorescence (TADF) exciplex with high energy is of great significance in achieving highly efficient blue, green, and red organic light-emitting diodes (OLEDs) for use in full-color displays and white lighting. Highly efficient and stable blue and green phosphorescent OLEDs were demonstrated by employing a TADF exciplex (energy: 2.9 eV) based on 4-substituted aza-9,9′-spirobifluorenes (aza-SBFs). Blue PhOLEDs demonstrated a maximum current efficiency (CE) of 47.9 cd A⁻¹ and an external quantum efficiency (EQE) of 22.5% at 1300 cd m⁻² (2.5 times the values of aza-SBF-based systems), with the best blue PhOLED demonstrating a CE, power efficiency (PE) and EQE of 60.3 cd A⁻¹, 52.7 lm W⁻¹, and 26.2%, respectively. Green PhOLEDs exhibited a CE of 78.1 cd A⁻¹ and EQE of 22.5% at 9360 cd m⁻², with the best green PhOLED exhibiting a maximum CE, PE, and EQE of 87.4 cd A⁻¹, 101.6 lm W⁻¹, and 24.5%, respectively. The device operational lifetime was improved over 17-fold compared to reference devices because of the high thermal stability of the materials and full utilization of the TADF exciplex energy, indicating their potential for application in commercial OLEDs.

A high energy TADF exciplex (415 nm) based on aza-spirobifluorene derivatives was demonstrated to achieve efficient and stable PhOLEDs.

Exciplex Cohosts Employing Nonconjugated Linked Dicarbazole Donors for Highly Efficient Thermally Activated Delayed Fluorescence-Based Organic Light-Emitting Diodes

Two new nonconjugated linked dicarbazole materials, dCzPSi and dCzPSO₂, with high triplet energy were synthesized and characterized. dCzPSi and dCzPSO₂ were adopted as unipolar host materials for the green thermally activated delayed fluorescence (TADF) emitter (4CzIPN) to achieve high-efficiency organic light-emitting diodes (OLEDs). The electron-transporting acceptor, PO-T2T, was introduced to mix with dCzPSi and dCzPSO₂ to give two new exciplex-forming systems that can improve the exciton formation propensity in the emitting layer. The relevant properties of these new exciplexes were characterized, and they were suggested as promising cohosts for the green TADF emitter, 4CzIPN. The characteristics of the devices employing single hosts (dCzPSi and dCzPSO₂) and exciplex-forming cohosts (dCzPSi:PO-T2T and dCzPSO₂:PO-T2T) were explored. The obtained results indicate that the Si-bridged dicarbazole compound dCzPSi outperforms its counterpart dCzPSO₂ in which two carbazole groups are linked by an SO₂ group. The device employed with the dCzPSi:PO-T2T cohost with 10 wt % 4CzIPN achieved a low V_on of 2.2 V and maximum efficiencies of 21.1% (external quantum efficiency), 56.4 cd A^-1 (current efficiency), 59.1 lm W^-1 (power efficiency), as compared to those (18.7%, 56.6 cd A^-1, and 68.5 lm W^-1) of the dCzPSO₂:PO-T2T-hosted device. This work verifies the advantages of using a cohost that can form an exciplex for boosting the device efficiency with reduced efficiency roll-off of TADF-based OLEDs.

Red Organic Light-Emitting Diode with External Quantum Efficiency beyond 20% Based on a Novel Thermally Activated Delayed Fluorescence Emitter

A novel thermally activated delayed fluorescence (TADF) emitter 12,15-di(10H-phenoxazin-10-yl)dibenzo[a,c]dipyrido[3,2-h:2',3'-j]phenazine (DPXZ-BPPZ) is developed for a highly efficient red organic light-emitting diode (OLED). With rigid and planar constituent groups and evident steric hindrance between electron-donor (D) and electron-acceptor (A) segments, DPXZ-BPPZ realizes extremely high rigidity to suppress the internal conversion process. Meanwhile, the highly twisted structure between D and A segments will also lead to an extremely small singlet-triplet energy split to DPXZ-BPPZ. Therefore, DPXZ-BPPZ successfully realizes an efficient fluorescent radiation transition and reverse intersystem crossing process, and possesses an extremely high photoluminescence quantum efficiency of 97.1 ± 1.1% under oxygen-free conditions. The OLED based on DPXZ-BPPZ shows red emission with a peak at 612 nm and a Commission Internationale de L'Eclairage (CIE) coordinate of (0.60, 0.40), and it achieves high maximum forward-viewing efficiencies of 20.1 ± 0.2% (external quantum efficiency), 30.2 ± 0.6 cd A-1 (current efficiency), and 30.9 ± 1.3 lm W-1 (power efficiency). The prepared OLED has the best performance among the reported red TADF OLEDs. These results prove that DPXZ-BPPZ is an ideal candidate for red TADF emitters, and the designing approach is valuable for highly efficient red TADF emitters.

Recent Advances of Exciplex-Based White Organic Light-Emitting Diodes

Recently, exciplexes have been actively investigated in white organic light-emitting diodes (WOLEDs), since they can be effectively functioned as (i) fluorescent or thermally activated delayed fluorescent (TADF) emitters; (ii) the hosts of fluorescent, phosphorescent and TADF dopants. By virtue of the unique advantages of exciplexes, high-performance exciplex-based WOLEDs can be achieved. In this invited review, we have firstly described fundamental concepts of exciplexes and their use in organic light-emitting diodes (OLEDs). Then, we have concluded the primary strategies to develop exciplex-based WOLEDs. Specifically, we have emphasized the representative WOLEDs using exciplex emitters or hosts. In the end, we have given an outlook for the future development of exciplex-based WOLEDs.

Exciplex: An Intermolecular Charge-Transfer Approach for TADF

Organic materials that display thermally activated delayed fluorescence (TADF) are a striking class of functional materials that have witnessed a booming progress in recent years. In addition to pure TADF emitters achieved by the subtle manipulations of intramolecular charge transfer processes with sophisticated molecular structures, a new class of efficient TADF-based OLEDs with emitting layer formed by blending electron donor and acceptor molecules that involve intermolecular charge transfer have also been fabricated. In contrast to pure TADF materials, the exciplex-based systems can realize small Δ E_ST (0-0.05 eV) much more easily since the electron and hole are positioned on two different molecules, thereby giving small exchange energy. Consequently, exciplex-based OLEDs have the prospective to maximize the TADF contribution and achieve theoretical 100% internal quantum efficiency. Therefore, the challenging issue of achieving small Δ E_ST in organic systems could be solved. In this article, we summarize and discuss the latest and most significant developments regarding these rapidly evolving functional materials, wherein the majority of the reported exciplex forming systems are categorized into two subgroups, viz. (a) exciplex as TADF emitters and (b) those as hosts for fluorescent, phosphorescent and TADF dopants according to their structural features and applications. The working mechanisms of the direct electroluminescence from the donor/acceptor interface and the exciplex-forming systems as cohost for the realization of high efficiency OLEDs are reviewed and discussed. This article delivers a summary of the current progresses and achievements of exciplex-based researches and points out the future challenges to trigger more research endeavors to this growing field.

Synthesis by a Cost-Effective Method and Electroluminescence of a Novel Efficient Yellowish-Green Thermally Activated Delayed Fluorescent Molecule

A new thermally activated delayed fluorescent molecule, TRZ 3(Ph-PTZ), containing three phenothiazines as donor units and a 2,4,6-triphenyl-1,3,5-triazine as the acceptor unit was synthesized using a simple cost-effective method based on a cobalt catalyzed cross-coupling. This compound was tested in organic light-emitting diodes and was found to show superior yellowish-green electroluminescence performance with a maximum external quantum efficiency of 17.4% and a maximum luminance value of 7430 cd/m².

Manipulation of Thermally Activated Delayed Fluorescence of Blue Exciplex Emission: Fully Utilizing Exciton Energy for Highly Efficient Organic Light Emitting Diodes with Low Roll-Off

The application of exciplex energy has become a unique way to achieve organic light-emitting diodes (OLEDs) with high efficiencies, low turn-on voltage, and low roll-off. Novel δ-carboline derivatives with high triplet energy (T₁ ≈ 2.92 eV) and high glass transition temperature (T_g ≈ 153 °C) were employed to manipulate exciplex emissions in this paper. Deep blue (peak at 436 nm) and pure blue (peak at 468 nm) thermally activated delayed fluorescence (TADF) of exciplex OLEDs were demonstrated by utilizing them as emitters with the maximum current efficiency (CE) of 4.64 cd A^-1, power efficiency (PE) of 2.91 lm W^-1, and external quantum efficiency (EQE) of 2.36%. Highly efficient blue phosphorescent OLEDs doped with FIrpic showed a maximum CE of 55.6 cd A^-1, PE of 52.9 lm W^-1, and EQE of 24.6% respectively with very low turn on voltage at 2.7 V. The devices still remain high CE of 46.5 cd A^-1 at 100 cd m^-2, 45.4 cd A^-1 at 1000 cd m^-2 and 42.3 cd A^-1 at 5000 cd m^-2 with EQE close to 20% indicating low roll-off. Manipulating blue exciplex emissions by chemical structure gives an ideal strategy to fully utilize all exciton energies for lighting of OLEDs.

Recent advances in organic thermally activated delayed fluorescence materials

Thermally activated delayed fluorescence: harvesting dark triplet excitons to generate bright emissive singlet excitons.

Synthesis and characterisation of a carbazole-based bipolar exciplex-forming compound for efficient and color-tunable OLEDs

The single-layer and bilayer devices showed blue monomer, electromer or interface exciplex emission.

Polarity-Tunable Host Materials and Their Applications in Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

A series of polarity-tunable host materials were developed based on oligocarbazoles and diphenylphosphine oxide, and their polarities can be tuned through increasing distance of acceptor and donor units. Density functional theory calculations were employed, and photoluminescence spectra in different polar solvents were measured to illustrate different polarities of these host materials. As CZPO has relatively stronger polarity, electroluminescence (EL) spectrum of solution-processed device based on 6 wt % PXZDSO2:CZPO is 7 nm red-shifted relative to that of other host materials based devices. Besides, a comparable impressive external quantum efficiency (EQE) value of 18.7% is achieved for an evaporation-processed yellow device consisting of FCZBn, which is superior to that of the device based on CBP (4,4'-dicarbazolyl-1,1'-biphenyl) (17.0%), and its efficiency roll-off is also obviously reduced, giving an EQE value as high as 16.3% at the luminance of 1000 cd/m². In addition, from CZPO to FCZBn as the polarities of host materials decrease, EL spectra of solution-processed devices based on DMAC-DPS emitter blue-shift constantly from 496 to 470 nm. The current work gives a constructive approach to control EL spectra of organic light-emitting diodes with a fixed thermally activated delayed fluorescence emitter by tuning the polarities of host materials.

High-Performance, Simplified Fluorescence and Phosphorescence Hybrid White Organic Light-Emitting Devices Allowing Complete Triplet Harvesting

Causes of efficiency limitation in common fluorescence and phosphorescence hybrid white organic light-emitting devices (WOLEDs) are discussed, and a new device architecture is proposed to address these issues. This architecture employs a fluorescent emitting layer (EML) of blue exciplex-forming cohost, which shows broad and strong thermally activated delayed fluorescence (TADF). Hybrid WOLEDs based on this architecture not only allow complete triplet harvesting for light generation but also can achieve white light emission with high color rending indexes (CRI) using only two colors. By using 26DCzPPy:PO-T2T as the blue fluorescent EML and 26DCzPPy:Ir complexes as the phosphorescent EML, we prepared a series of two-color WOLEDs with low turn-on voltages of 2.5-3.3 V, high forward-viewing EQEs of 12.7-19.3% and high CRIs of 67-77. These results suggest this new architecture would be an effective way to achieve high performance WOLEDs with simple structures.