Orange-red Organic Light-emitting Diodes Research Articles

Efficient and ultra-high luminance orange-red organic light emitting diode (OLED) based on triphenylamine-benzothiadiazole-phenoxazine hybrid molecule with hybrid local and charge-transfer (HLCT) characteristic

Due to the energy gap law, red emission needs relatively narrow energy gap, thus the nonradiative transition rate (knr) grows exponentially and eventually impacts the materials fluorescent efficiency. Here, an orange-red emission donor-acceptor-π-donor (D-A-π-D’) material N, N-diphenyl-4-(7-(4-(10-phenyl-10 H-phenoxazin-3-yl) phenyl) benzo [c] (Huang et al., 2019; Park et al., 2008; Caspar et al., 1982) [1,2,5] thiadiazol-4-yl) aniline (TBphPP) was designed and synthesized. Introducing a benzene π-bridge between the strong donor 10-phenyl-10H-phenoxazin (PPOX) and acceptor 2,1,3-benzothiadiazole (BTZ) not only is able to stretch the molecular conjugation so as to increase the proportion of LE state in hybrid local and charge-transfer (HLCT) state but also increase the overlap area of the electron cloud to improve luminous efficiency. As a result, the non-doped organic light-emitting diode (OLED) of TBphPP exhibited high luminance of 17,886 cd m−2 and relatively high exciton utilization efficiency (EUE) of 44 % with a standard red electroluminescent peak at 620 nm. Importantly, the optimized doped device based on TBphPP achieved high performance that the maximum external quantum efficiency (EQEmax) was up to 9.86 % with a higher EUE of 66.2 % and the efficiency roll-off (ηroll-off) at 100,000 cd m−2 was only 3.55 %. Simultaneously, it has reached ultra-high luminescence of 180,583 cd m−2, high current efficiency of 29.02 cd A−1 and power efficiency of 16.47 l m W−1. To our knowledge, the TBphPP device with such ultra-high luminance and high efficiency is one of the best performance orange-red materials based on HLCT characteristic.

Extended Conjugation Strategy Enabling Red-Shifted and Efficient Emission of Orange-Red Thermally Activated Delayed Fluorescence Materials.

In account of the energy gap law, the development of efficient narrow-band gap thermally activated delayed fluorescence (TADF) materials remains a major challenge for the application of organic light-emitting diodes (OLEDs). The orange-red TADF materials are commonly designed with either large π-conjugated systems or strong intramolecular donor-acceptor (D-A) interactions for red-shift emission and small singlet-triplet energy gap (ΔEST). There are rare reports on the simultaneous incorporation of these two strategies on the same material systems. Herein, two orange-red emitters named 1P2D-BP and 2P2D-DQ have been designed by extending the conjugation degree of the center acceptor DQ and increasing the number distribution of the peripheral donor PXZ units, respectively. The emission peak of 1P2D-BP is red-shifted to 615 nm compared to 580 nm for 2P2D-DQ, revealing the pronounced effect of the conjugation extension on the emission band gap. In addition, the distorted molecular structure yields a small ΔEST of 0.02 eV, favoring the acquisition of a high exciton utilization through an efficient reverse intersystem crossing process. As a result, orange-red OLEDs with both 1P2D-BP and 2P2D-DQ have achieved an external quantum efficiency (EQE) of more than 17%. In addition, the efficient white OLED based on 1P2D-BP is realized through precise exciton assignment and energy transport modulation, showing an EQE of 23.6% and a color rendering index of 82. The present work provides an important reference for the design of high-efficiency narrow-band gap materials in the field of solid-state lighting.

All-fluorescent white organic light-emitting diodes employing a deep-blue HLCT material simultaneously as emitter and host achieving excellent electroluminescence performance with efficiency roll-off

Simultaneously achieving high efficiency and small efficiency roll-off remains a huge challenge for white organic light-emitting diodes (OLEDs), which greatly restricts their industrial application in the lighting field. Recently developed hybridized local and charge transfer (HLCT) materials have a “hot exciton” channel, which can maximize the utilization rate of excitons and suppress efficiency roll-off via rapid reverse intersystem crossing (RISC) from high-lying triplet state (Tn, n ≥ 2) to singlet state (S1). In this work, employing blue HLCT material pCzAnN as the sensitizing host, by precise concentration management, the traditional fluorescent C545T- and DCJTB-based green and orange-red OLEDs are first fabricated, which achieve the maximum external quantum efficiency (EQE) exceeding the theoretical limit of 5 %, reaching 8.51 % and 6.64 %, respectively. On this basis, by strategic device structure design, all-fluorescent two- and three-color white OLEDs are demonstrated having pCzAnN simultaneously as blue emitter and sensitizing host of C545T and DCJTB. The resulting two- and three-color white devices obtain maximum EQE beyond 5 %. What's more, all three-color white devices exhibit extremely small efficiency roll-off, where the efficiency drops are less than 1 % at a practical luminance of 1000 cd/m2, which is comparable to the best works reported in the literatures. At the same time, the optimized three-color white device realizes ideal white emission, in which the electroluminescence (EL) spectra contain three almost equal intensity emission wavebands accompanied by a high color rendering index of 87. Stable EL spectra under a wide luminance range (3863 ∼ 27710 cd/m2) indicate a huge potential application. We believe this work can provide new idea for developing high-efficiency and low roll-off white OLEDs in the future.

Precisely Regulation of Peripheral Decoration of Multi‐Resonance Molecules and Construction of Highly Efficient Solution‐Processed Orange–Red OLEDs with External Quantum Efficiency Approaching 20%

AbstractThe advent of multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials, heralding cutting‐edge emitters with superior efficiency and color fidelity, represents a momentous stride in the realm of organic light‐emitting diodes (OLEDs). In this particular investigation, substantial advancements have been made by enhancing, synthesizing, and characterizing three distinct MR‐type emitters through meticulous control of peripheral decorations using frontier molecular orbital engineering. Through strategic attachment of various pyrimidine derivatives to the positions of the lowest unoccupied molecular orbitals (LUMO) of the parent molecule, the molecular excited state attributes are meticulously tailored, leading to the development of orange‐red MR‐TADF emitters. These designed molecules have demonstrated an emission spectrum shift from orange‐red to orange, exhibiting peak wavelengths spanning the range of 585–608 nm and full width at half‐maximums (FWHM) between 36 and 43 nm. This signifies a remarkable precision in managing the emission maxima of MR‐TADF emitters. Astonishingly, the solution‐processed OLEDs have showcased vibrant orange‐red electroluminescence, characterized by peak wavelengths ranging from 587 to 611 nm, accompanied by a notable external quantum efficiency nearing 20%.

Rational design of dibenzo[a,c]phenazine-derived isomeric thermally activated delayed fluorescence luminophores for efficient orange-red organic light-emitting diodes

It is an immense challenge to develop efficient long-wavelength (orange-to-red) thermally activated delayed fluorescence (TADF) materials due to the increasing nonradiative decay rates following the energy-gap law. Herein, two pairs of asymmetric isomers; DPyPzTPA and TPAPzDPy, and PyPzDTPA and DTPAPzPy based on electron-deficient moieties dibenzo[a,c]phenazine (Pz) and pyridine (Py) combined with electron-donor units of triphenylamine (TPA) were designed and synthesized. Their photophysical properties could be finely modulated by changing the position and number of Py groups as well as TPA fragments onto Pz cores. DPyPzTPA and DTPAPzPy possess much more rigidity and thus less geometry relaxation and non-radiative decay between ground states and excited states than those of PyPzDTPA and TPAPzDPy. Intriguingly, DPyPzTPA exhibits the highest relative photoluminescence quantum yield (ΦPL) and the fastest reverse intersystem crossing (rISC) rate among them owing to relatively stronger rigidity and spin-orbit coupling (SOC) interactions between the lowest singlet (S1) and energetically close-lying excited triplet state and therefore, the device showed the highest maximum external quantum efficiency (EQEmax) of 16.6% (60.9 lm/W, 53.3 cd/A) with Commission Internationale de I'Eclairage (CIE) coordinates of (0.43, 0.55), peak wavelength 556 nm. In stark contrast, due to its lower rigidity and extremely weak delayed fluorescence (DF) characteristic and thus the much lower ΦPL, TPAPzDPy-based devices are only half as efficient (30.8 lm/W, 27.5 cd/A, 8.3% EQE) despite the isomers possessing equal singlet-triplet energy gaps (ΔEST) of 0.43 eV. On the other hand, the device based on DTPAPzPy also demonstrated a strongly enhanced performance (59.1 lm/W, 52.7 cd/A, 16.1% EQE) than its isomer PyPzDTPA-based device (39.5 lm/W, 35.2 cd/A, 10.3% EQE). This work explicitly implicates that the asymmetric and isomeric molecular design is a potential strategy for promoting the development of highly efficient long-wavelength TADF materials.

The Role of Balancing Carrier Transport in Realizing an Efficient Orange-Red Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diode.

Simultaneously realizing improved carrier mobility and good photoluminescence (PL) efficiency in red thermally activated delayed-fluorescence (TADF) emitters remains challenging but important. Herein, two isomeric orange-red TADF emitters, oPDM and pPDM, with the same basic donor-acceptor backbone but a pyrimidine (Pm) attachment at different positions are designed and synthesized. The two emitters show similarly good PL properties, including narrow singlet-triplet energy offsets (0.11 and 0.15 eV) and high photoluminescence quantum yields (ca. 100 and 88%) in doped films. An orange-red organic light-emitting diode (OLED) employing oPDM as an emitter achieves an almost twice as high maximum external quantum efficiency (28.2%) compared with that of a pPDM-based OLED. More balanced carrier-transporting properties are responsible for their contrasting device performances, and the position effect of the Pm substituent leads to significantly distinct molecular packing behaviors in the aggregate states and different carrier mobilities.

Solution-processed high-performance orange-red organic light-emitting diode (OLED) based on ionic phosphorescent iridium(III) complex

A new and efficient ionic orange-red Ir(III) complex is rationally designed by utilizing 2-(4-chloro-3-(trifluoromethyl)phenyl)-4-methylquinoline and a large conjugated 2,2′-biquinoline as main ligand and ancillary ligand, respectively. The complex shows bright phosphorescence peaking at 583 and 626 nm (shoulder) with high absolute phosphorescent quantum efficiency of 81.3% and moderate emission lifetime of 1.54 μs in dichloromethane solution, which is suitable for fabricating high-performance solution-processed organic light-emitting diode. The device based on this ionic complex exhibits excellent electroluminescence performance, with the maximum luminance, external quantum efficiency, current efficiency, and power efficiency of up to 1209 cd/m2, 10.20%, 21.9 cd/A and 9.57 lm/W, respectively, thereby demonstrating that this new cationic Ir(III) complex possesses tremendous potentials for the future solution-processed electroluminescence.

Extremely High Power Efficiency Solution‐Processed Orange‐Red TADF OLEDs via a Synergistic Strategy of Molecular and Device Engineering

AbstractThe development of high‐performance, solution‐processed, orange‐red organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters is a challenging endeavor. In this study, two orange‐red TADF emitters, namely 2DMAC‐DBP‐2tBuCz and 2SPAC‐DBP‐2tBuCz, are developed by a novel donor–acceptor–functional‐group (D‐A‐R) molecular design strategy. This design makes the molecules highly soluble and inhibits concentration quenching of excitons, rendering the emitter suitable for use in devices with high concentration to boost their performance. The solution‐processed, orange‐red OLEDs manufactured in this study achieve a state‐of‐the‐art maximum external quantum efficiency (EQEmax) value of 23.7% and an extremely high maximum power efficiency (PEmax) of 48.8 lm W−1, which is nearly twice higher than the previously reported best value (27.1 lm W−1). Therefore, the collaboration of molecular engineering and sophisticated device design provides a novel method for extremely low power consumption solution‐processed OLEDs.

High-Performance Orange-Red Organic Light-Emitting Diodes with External Quantum Efficiencies Reaching 33.5% based on Carbonyl-Containing Delayed Fluorescence Molecules.

Developing orange to red purely organic luminescent materials having external quantum efficiencies (η exts) exceeding 30% is challenging because it generally requires strong intramolecular charge transfer, efficient reverse intersystem crossing (RISC), high photoluminescence quantum yield (Φ PL), and large optical outcoupling efficiency (Φ out) simultaneously. Herein, by introducing benzoyl to dibenzo[a,c]phenazine acceptor, a stronger electron acceptor, dibenzo[a,c]phenazin‐11‐yl(phenyl)methanone, is created and employed for constructing orange–red delayed fluorescence molecules with various acridine‐based electron donors. The incorporation of benzoyl leads to red‐shifted photoluminescence with accelerated RISC, reduced delayed lifetimes, and increased Φ PLs, and the adoption of spiro‐structured acridine donors promotes horizontal dipole orientation and thus renders high Φ outs. Consequently, the state‐of‐the‐art orange–red organic light‐emitting diodes are achieved, providing record‐high electroluminescence (EL) efficiencies of 33.5%, 95.3 cd A−1, and 93.5 lm W‒1. By referring the control molecule without benzoyl, it is demonstrated that the presence of benzoyl can exert significant positive effect over improving delayed fluorescence and enhancing EL efficiencies, which can be a feasible design for robust organic luminescent materials.

Approaching Efficient and Narrow RGB Electroluminescence from D-A-Type TADF Emitters Containing an Identical Multiple Resonance Backbone as the Acceptor.

Highly twisted electron donor (D)-electron acceptor (A)-type thermally activated delayed fluorescence (TADF) emitters can achieve high efficiency while suffering from serious structural relaxations and broad emissions. Multiple resonance (MR)-type TADF emitters can realize narrow emission. However, until now, only a few efficient MR-emitting cores are reported and custom tunning of their emission color remains a major challenge in their wider applications. In this work, by combining the conventional TADF and MR-TADF designs, we demonstrate that color tuning and narrowing the spectral width of conventional TADF emission can be easily achieved simultaneously. We select a prototypical carbonyl (C═O)/N-based MR core as a backbone and attach it with D segments of different electron-donating abilities and numbers to obtain three different TADF emitters with emissions from sky blue to green and orange-red while maintaining the narrow emission of the original MR core. The corresponding sky blue, green, and orange-red organic light-emitting diodes achieve maximum external quantum efficiencies of 20.3, 27.3, and 26.3%, respectively, and narrow full widths at half-maximum all below 0.28 eV. These results provide a new molecular design strategy for developing narrowband TADF emitters with easily tunable emissions covering the full visible range.

Efficient triplet harvest for orange-red and white OLEDs based exciplex host with different donor/acceptor ratios

Abstract Highly efficient orange-red and white organic light-emitting diodes (OLEDs) based exciplex host with different donor/acceptor ratios and doping concentrations are fabricated for efficient triplet excitons harvest. As a result, the maximum efficiency is achieved at 5:5 ratios exciplex host in orange-red OLEDs with current efficiency, power efficiency and external quantum efficiency (EQE) of 37.0 cd/A, 37.3 lm/W and 18.3%, respectively. Furthermore, the white OLEDs under two different doping concentrations give warm white (0.48, 0.41) and cool white (0.28, 0.37) emission with maximum current efficiencies, power efficiencies and EQEs of 38.3/37.6 cd/A, 41.3/43.7 lm/W and 17.8/16.6%, respectively. We found that the triplet exciton harvest on exciplex host through reverse intersystem crossing process and efficient energy transfer are responsible for the high device efficiency.

Highly efficient orange–red electroluminescence enabled by fluorenone-based thermally activated delayed fluorescent emitter

A butterfly shaped orange–red thermally activated delayed fluorescent (TADF) emitter, named ACFO, is developed by integrating electron-donating (D) 9,9-dimethyl-9,10-dihydroacridine units into γ-positions of an electron-accepting (A) fluorenone core to form a D-A-D configuration. ACFO emits intense orange–red light peaking at 600 nm in toluene solution, which is significantly red-shifted compared with its benzophenone-based counterpart. Imparted with a small singlet–triplet splitting energy of 0.13 eV, ACFO exhibits distinct TADF feature with a short delayed fluorescent lifetime of 3.8 μs and displays a moderate photoluminescence quantum yield of 31%. Consequently, employing ACFO as an emitter results in orange–red organic light-emitting diodes that exhibit a maximum external quantum efficiency of nearly 11%, corresponding to a nearly 100% exciton utilization efficiency in electroluminescence process. Our finding demonstrates for the first time that fluorenone derivatives can be employed as TADF emitters for high-efficiency fluorescent organic light-emitting diodes.

Efficient solution-processed orange-red organic light-emitting diodes based on a novel thermally activated delayed fluorescence emitter

A novel thermally activated delayed fluorescence (TADF) emitter was designed and synthesized for solution-processed red organic light-emitting diodes (OLEDs).

Highly Efficient Organic Light‐Emitting Diode Based on a Hidden Thermally Activated Delayed Fluorescence Channel in a Heptazine Derivative

An orange-red organic light-emitting diode containing a heptazine derivative exhibits high performance with a maximum external quantum efficiency of 17.5 ± 1.3% and a peak luminance of 17000 ± 1600 cd m⁻² without any light out-coupling enhancement. The high electroluminescence performance can be ascribed to the presence of an efficient up-conversion channel from the lowest triplet state to the lowest singlet state.

Solution-processable iridium complexes for efficient orange-red and white organic light-emitting diodes

Two homoleptic and heteroleptic cyclometalated iridium complexes containing the 2-phenylbenzothiozole derivative as the main ligand, 1 and 2, have been synthesized and characterized as efficient orange-red phosphors. Triarylamine was introduced as an important substituent into the 6-position of the benzothiazole ring to tune the photonic and electronic properties of these complexes. Different from most of small molecular iridium complexes, 1 and 2 are solution-processable and their neat films can be obtained by a spin-coating method. Furthermore, their homogeneously dispersed films in a small molecular matrix, 4,4′-N,N′-dicarbazolebiphenyl (CBP), were successfully prepared by solution method even with low doping levels. Organic light-emitting diodes (OLEDs) were fabricated by solution processing the emitting layer containing 1 and 2 as doped emitters in the CBP host. Efficient orange-red electroluminescence by using 5 wt% 2 as the dopant was realized with a maximum efficiency of 14.49 cd A−1 (7.38 lm W−1 and 8.73%) and Commission Internationale de l'Eclairage (CIE) coordinates of (0.60, 0.40), which are among the highest luminance efficiency ever reported for partially solution-processed red and orange-red OLEDs so far. In addition, two-element white OLEDs were achieved with these orange-red phosphors and the traditional blue emitter by spin coating the emission layer. A maximum luminance efficiency of 8.97 cd A−1 and CIE of (0.33, 0.35) were realized.

Highly efficient orange-red phosphorescent organic light-emitting diode using 2,7-bis(carbazo-9-yl)-9,9-ditolyfluorene as the host

We demonstrate an efficient orange-red organic light-emitting diode using a host, 2,7-bis(carbazo-9-yl)-9,9-ditolyfluorene, doped with tris(2-phenylquinoline) iridium(III). The device exhibits a high current efficiency of 44.8 cd/A at 1000 cd/m2. This may be attributed to the adoption of the host, which favors the injection of holes, as well as the emissive-layer architecture enabling excitons to form on host and hence favoring efficient energy-transfer from host to guest. Moreover, an electron-confining layer is used to modulate excessive holes to be injected into emissive layer and confine the electrons, which would in turn balance the injection of both carriers and improve efficiency.

Highly efficient orange-red organic light-emitting diode using double emissive layers with stepwise energy-level architecture

This study demonstrates a highly efficient orange-red organic light-emitting diode with a double emissive layer architecture that exhibits a record-breaking power efficiency of 47 lm W−1 at 100 cd m−2, or 32 lm W−1 at 1 000 cd m−2. Two factors may contribute to the high efficiency. First, the device architecture has a stepwise energy-level, leading to a significantly reduced energy-barrier for both hole and electron to transport, revealed by the relatively low driving voltage. Second, employing a hole-transporting host and an electron-transporting host leads to a nearly perfect balanced injection of hole and electron to the emissive zones, indicated by the obtained external quantum efficiency that reaches the 20% theoretical limit.

Highly efficient, orange–red organic light-emitting diodes using a series of green-emission iridium complexes as hosts

We investigated highly efficient phosphorescent organic light-emitting diodes (OLEDs) based on an orange–red emission iridium complex as the guest and five green emission iridium complexes as the host material, respectively. For comparison, a device using a common fluorescent host CBP (4,4′-bis( N-carbazolyl)-1,1′-biphenyl) has also been fabricated. Results show that the steric hindrance and exciton transporting property of the iridium complex host are found to be critical to this kind of doping system, a proper steric hindrance and improved exciton transporting ability result in reducing of triplet–triplet annihilation, thus improving of the device performance. In addition, all devices using iridium complexes as host have better performance than that of CBP, which arised from the fact that those green emission iridium complexes have a lower triplet excited energy befitting for energy confinement and a higher highest occupied molecular orbital (HOMO) level for hole injection.