Efficient Non-doped Organic Light-emitting Diodes Research Articles

Highly efficient non-doped organic light-emitting diodes based on long-range coupling and efficient energy transfer

An effective approach to realize high-efficiency exciplex-based organic light-emitting diodes (OLEDs) is proposed by employing long-range coupling between electron-hole pairs and efficient energy transfer. The space-separated holes and electrons form exciplexes through charge-transfer states, and devices with such structures can reduce exciton quenching in a certain extent compared to those based on interfacial exciplexes. Remarkable external quantum efficiency about 12.6% is achieved by introducing a yellow exciplex system in a donor/spacer/acceptor structure because of the suppression of exciton quenching and the efficient energy transfer. Such a unique strategy is beneficial to better utilizing the triplet excitons and thus achieving high performance exciplex-based OLEDs.

Highly Efficient Non-Doped Organic Light-Emitting Diodes Based on Long-Range Coupling and Efficient Energy Transfer

Highly Efficient Non-Doped Organic Light-Emitting Diodes Based on Long-Range Coupling and Efficient Energy Transfer

Unraveling the Important Role of High-Lying Triplet-Lowest Excited Singlet Transitions in Achieving Highly Efficient Deep-Blue AIE-Based OLEDs.

Aggregation-induced emission (AIE) materials are attractive for achieving highly efficient nondoped organic light-emitting diodes (OLEDs) owing to their strong luminescence in the solid state. However, the electroluminescence efficiency of most AIE-based OLEDs remains low owing to the waste of triplet excitons. Here, using theoretical calculations, photophysical dynamics, and magnetoluminescence measurements, the spin conversion process is demonstrated between the high-lying triplet state (Tn ) and the lowest excited singlet state (S1 ) in AIE materials. Moreover, the relative positions of Tn (n<4) and S1 are shown to have a significant impact on the spin-conversion efficiency, thus influencing the harvesting of triplet excitons and the device efficiency. Finally, by selecting an upconversion material with an appropriate energy level for further utilizing the triplet excitons, a deep-blue fluorescent OLED with CIE coordinates of (0.15, 0.08), amaximum external quantum efficiency of 10.2%, low efficiency roll-off, and a high brightness of 16817cdm-2 is developed. This is one of the most efficient deep-blue OLEDs based on AIE materials reported so far. These findings also provide new insights into the design of more efficient AIE molecules and corresponding OLEDs by managing high-lying triplet excitons.

Quinoline-based TADF emitters exhibiting aggregation-induced emission for efficient non-doped organic light-emitting diodes

Three quinoline-based TADF emitters with aggregation-induced emission were synthesized and fabricated for efficient non-doped OLEDs with EQEmax up to 17.3%.

Quinoline-based aggregation-induced delayed fluorescence materials for highly efficient non-doped organic light-emitting diodes

Three new emitters, namely 10,10'-(quinoline-2,8-diyl)bis(10H-phenoxazine) (Fene), 10,10'-(quinoline-2,8-diyl)bis(10H-phenothiazine) (Fens) and 10,10'-(quinoline-2,8-diyl)bis(9,9-dimethyl-9,10-dihydroacridine) (Yad), featuring quinoline as a new electron acceptor have been designed and conveniently synthesized. These emitters possessed small singlet–triplet splitting energy (ΔEst) and twisted structures, which not only endowed them show thermally activated delayed fluorescence (TADF) properties but also afforded a remarkable aggregation-induced emission (AIE) feature. Moreover, they also showed aggregation-induced delayed fluorescence (AIDF) property and good photoluminescence (PL) property, which are the ideal emitters for non-doped organic light-emitting diodes (OLEDs). Furthermore, high-performance non-doped OLEDs based on Fene, Fens and Yad were achieved, and excellent maximum external quantum efficiencies (EQEmax) of 14.9%, 13.1% and 17.4%, respectively, were obtained. It was also found that all devices exhibited relatively low turn-on voltages ranging from 3.0 V to 3.2 V probably due to their twisted conformation and the AIDF properties. These results demonstrated the quinoline-based emitters could have a promising application in non-doped OLEDs.

Molecular Design of Non-doped OLEDs Based on a Twisted Heptagonal Acceptor: A Delicate Balance between Rigidity and Rotatability.

The development of efficient non-doped organic light-emitting diodes (OLEDs) is highly desired but very challenging because of a severe aggregation-caused quenching effect. Herein, we present a heptagonal diimide acceptor (BPI), which can restrict excessive intramolecular rotation and inhibit close intermolecular π-π stacking due to well-balanced rigidity and rotatability of heptagonal structure. The BPI-based luminogen (DMAC-BPI) shows significant aggregation-induced delayed florescence with an extremely high photoluminescence quantum yield (95.8 %) of the neat film, and the corresponding non-doped OLEDs exhibit outstanding electroluminescence performance with maximum external quantum efficiency as high as 24.7 % and remarkably low efficiency roll-off as low as 1.0 % at 1000 cd m-2 , which represents the state-of-the-art performance for non-doped OLEDs. In addition, the synthetic route to DMAC-BPI is greatly streamlined and simplified through oxidative Ar-H/Ar-H homo-coupling reaction.

Creation of Efficient Blue Aggregation-Induced Emission Luminogens for High-Performance Nondoped Blue OLEDs and Hybrid White OLEDs.

Organic blue luminescent materials are essential for organic light-emitting diodes (OLEDs). However, high-quality blue materials that can fulfill the requirements of OLED commercialization are much rare. Herein, two novel blue luminogens, 9-(4-(2,6-di- tert-butyl-10-(4-(1,2,2-triphenylvinyl)phenyl)anthracen-9-yl)phenyl)-9 H-carbazole and 9-(4-(2,6-di- tert-butyl-10-(4-(1,2,2-triphenylvinyl)phenyl)anthracen-9-yl)1,3-di(9 H-carbazol-9-yl)benzene (TPE-TADC), consisting of anthracene, tetraphenylethene, and carbazole groups are successfully prepared, and their thermal, optical, electronic, and electrochemical properties are fully investigated. They exhibit prominent aggregation-induced emission property and strong blue fluorescence at ∼455 nm in neat films. Efficient nondoped OLEDs are fabricated with these blue luminogens, providing blue electroluminescence (EL) at 451 nm (CIE x, y = 0.165, 0.141) and high EL efficiencies of 6.81 cd A-1, 6.57 lm W-1, and 5.71%. By utilizing TPE-TADC as a blue emissive layer, high-performance two-color hybrid white OLEDs are achieved, furnishing modulatable light color from pure white (CIE x, y = 0.33, 0.33) to warm white (CIE x, y = 0.44, 0.46) and excellent EL efficiencies of 56.7 cd A-1, 55.2 lm W-1, and 19.2%. More importantly, these blue and white OLEDs all display ultrahigh color and efficiency stabilities at high luminance, indicating the great potential of these blue luminogens for the application in OLED displays and white illumination.

Novel molecular triad exhibiting aggregation-induced emission and thermally activated fluorescence for efficient non-doped organic light-emitting diodes.

A light-emitting molecular triad (BPCP-2CPC) with dual functionality was successfully synthesized and applied to solution-processed non-doped organic light-emitting diodes. The BPCP-2CPC triad contains 9-phenyl-9H-carbazole units as a host moiety tethered to a green-emitting core (BPCP) through a cyclohexane unit and exhibits thermally activated delayed fluorescence (TADF) and aggregation-induced emission (AIE) characteristics simultaneously. The BPCP-2CPC-based non-doped TADF-OLED devices showed a high external quantum efficiency (EQE) of 13.4%.

A sterically hindered asymmetric D-A-D' thermally activated delayed fluorescence emitter for highly efficient non-doped organic light-emitting diodes.

Thermally activated delayed fluorescence (TADF) materials have opened a new chapter for high-efficiency and low-cost organic light-emitting diodes (OLEDs). Herein, we describe a novel and effective design strategy for TADF emitters which includes introducing a carbazole donor unit at the ortho-position, at which the donor and acceptor groups are spatially in close proximity to guarantee the existence of intramolecular electrostatic attraction and through-space charge transfer, leading to reduced structural vibrations, suppressed non-radiative decay and rapid radiative decay to avoid excited state energy loss. As a result, a green TADF emitter (2Cz-DPS) showing high solid-state photoluminescence quantum efficiency (91.9%) and excellent OLED performance was produced. Theoretical simulations reveal that the non-adiabatic coupling accelerates the reverse intersystem crossing of 2Cz-DPS, resulting in a state-of-the-art non-doped OLED with an extremely high external quantum efficiency of 28.7%.

Highly Efficient Nondoped Organic Light Emitting Diodes Based on Thermally Activated Delayed Fluorescence Emitter with Quantum-Well Structure.

Highly efficiency nondoped thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs) with multiquantum wells structure were demonstrated. By using an emitting layer with seven quantum wells, the nondoped TADF OLEDs exhibit high efficiency with EQE of 22.6%, a current efficiency of 69 cd/A, and a power efficiency of 50 lm/W, which are higher than those of the conventional doped OLED and among the best of the TADF OLEDs. The high performance of the devices can be ascribed to effective confinement of the charges and excitons in the emission layer by the quantum well structure. The emission layer with multiquantum well structure is demonstrated to be cost effective for highly efficient nondoped TADF OLEDs and holds great potential for organic electronics.

Tuning the singlet-triplet energy gap of AIE luminogens: crystallization-induced room temperature phosphorescence and delay fluorescence, tunable temperature response, highly efficient non-doped organic light-emitting diodes.

In this contribution, we finely tuned the singlet-triplet energy gap (ΔEST) of AIE-active materials to modulate their fluorescence, phosphorescence and delay fluorescence via rational molecular design and investigated the possible ways to harvest their triplet energy in OLEDs. Noteworthily, two molecules o-TPA-3TPE-o-PhCN and o-TPA-3TPE-p-PhCN with larger ΔEST values (0.59 eV and 0.45 eV, respectively) emitted efficient long-lived low temperature phosphorescence in their glassy solutions and exhibited efficient crystallization-induced room temperature phosphorescence (RTP). Meanwhile, it was the first time to observe a novel crystallization-induced delay fluorescence phenomenon in another AIE-active molecule p-TPA-3TPE-p-PhCN owing to its very small ΔEST value (0.21 eV). It was also found that molecules with various ΔEST values showed significantly different temperature sensitivity. Non-doped electroluminescent (EL) devices using these molecules as light-emitting layers were fabricated, exhibiting external quantum efficiencies (EQE) higher than theoretical values of purely singlet emitter type devices. Particularly, p-TPA-3TPE-p-PhCN showed outstanding device performances with high luminance and efficiencies up to 36,900 cd m(-2), 11.2 lm W(-1), 12.8 cd A(-1) and 4.37%, respectively, considering that its solid-state quantum yield was only 42%. All the above observations suggested that tuning the ΔEST values of AIE materials is a powerful methodology to generate many more interesting and meaningful optoelectronic properties.

Efficient nondoped organic light-emitting diodes with CuI complex emitter

In CuI complex based organic light emitting diodes (OLEDs) a host matrix is traditionally thought to be required to achieve high efficiency. Herein, it is found that the device ITO/MoO3 (1nm)/4,4′-N,N′-dicarbazole-biphenyl (CBP, 35nm)/[Cu(μ-I)dppb]2 (dppb=1,2-bis[diphenylphosphino]benzene, 20nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 65nm)/LiF (1nm)/Al (100nm) with a vacuum thermal evaporated nondoped CuI complex emissive layer (EML) showed external quantum efficiency and current efficiency of 8.0% and 24.3cd/A at a brightness of 100cd/m2, respectively, which are comparable to the maximum efficiencies reported in an optimized doped OLED with the same emitter, higher efficiency than the OLED with a [Cu(μ-I)dppb]2:CBP EML, and much higher efficiencies than the nondoped OLED with a bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)2(acac)] EML. A series of reference films and single carrier devices were fabricated and studied to understand the difference between CuI and IrIII complex based nondoped OLEDs.

High-performance two-photon absorption luminophores: large action cross sections, free from fluorescence quenching and tunable emission of efficient non-doped organic light-emitting diodes

Two novel quadrupolar D–π–D type two-photon absorption (2PA) luminophores PA2F4H8E8 and AnA2F4H8E8 were synthesized by Wittig–Horner reaction in high yields. By using distyrylbenzene as the π-bridge center, PA2F4H8E8 shows two-photon absorption aggregation-induced emission enhancement (2PA-AIEE) and exhibits exceptionally large 2PA and 2PA action cross-sections up to ∼6300 GM and ∼1400 GM at 800 nm in THF, respectively. A non-doped electroluminescent (EL) device of PA2F4H8E8 displays good performance with an extremely low turn-on voltage of 2.6 V and a high brightness of 9837 cd m−2.

Naphthalene-substituted 2,3,4,5-tetraphenylsiloles: synthesis, structure, aggregation-induced emission and efficient electroluminescence

Two thermally stable naphthalene-substituted 2,3,4,5-tetraphenylsiloles, 1,1-dimethyl-2,5-bis[4-(naphthalen-1-yl)phenyl]-3,4-diphenylsilole (D-1-NpTPS) and 1,1-dimethyl-2,5-bis[4-(naphthalen-2-yl)phenyl]-3,4-diphenylsilole (D-2-NpTPS), have been synthesized and fully characterized. D-2-NpTPS shows redder absorption and emission than D-1-NpTPS due to the better conjugation between naphthalen-2-yl groups and phenyl rings at the 2,5-positions of the silole core. While they are weakly fluorescent in solutions, strong luminescence is induced when aggregated in poor solvents or fabricated into solid films, with high fluorescence quantum yields up to 99%, demonstrating their aggregation-induced emission (AIE) feature. Efficient non-doped organic light-emitting diodes utilizing D-1-NpTPS and D-2-NpTPS as light-emitting layers are fabricated. Remarkably high electroluminescence efficiencies of 10.5 cd A−1, 7.3 lm W−1, and 3.2% are acheived by the D-2-NpTPS device.

1,3,6,8-Tetrakis[(triisopropylsilyl)ethynyl]pyrene: A highly efficient solid-state emitter for non-doped yellow electroluminescence devices

Simple molecule possesses high power: 1,3,6,8-Tetrakis[(triisopropylsilyl)ethynyl]pyrene (TTIPSEPy), a simple-structured molecule, shows strong solid-state photoluminescence with unity emission efficiency. Highly efficient non-doped organic light-emitting diodes are fabricated using TTIPSEPy as emitter, exhibiting stable yellow light (CIE = 0.41, 0.56) with high luminance and efficiency up to 26,400 cd/m 2 and 9.2 cd/A, respectively.

Boron-Bridged π-Conjugated Ladders as Efficient Electron-Transporting Emitters

Four diboron-bridged ladder molecules 1-4 have been designed and synthesized. X-ray diffraction analysis revealed that the bulky phenyl substituents on boron centers efficiently prevented π stacking of the luminescent ladder unit. Characterizations of these complexes demonstrated that the construction of diboron-containing ladder-type skeletons endowed these materials with good thermal stability, high fluorescence quantum yields, and strong electron affinity. The highly efficient nondoped organic light-emitting diodes using complexes 1 and 2 as electron-transporting emitters exhibited maximum luminance values of 16,930 and 18,060 cd/m(2) with turn-on voltages of 3.5 and 2.5 V as well as maximum luminous efficiencies of 6.4 and 5.4 cd/A, respectively.