Green Organic Light-emitting Devices Research Articles

Secondary donor-acceptor group enable efficient pure green organic light-emitting devices based on multi-resonance TADF emitters

Multi-resonance thermally activated delayed fluorescence (MR-TADF) presents great potential for applications in high-quality organic light emitting diodes (OLEDs) displays due to their superb properties of high color purity and high quantum efficiencies. Nonetheless, most reported MR-TADF emitters show unsatisfactory devices performances with severe efficiency roll-off resulted from their inherent issues of slow reverse intersystem crossing rate (kRISC) and aggregation-caused quenching (ACQ). Herein, we propose a novel strategy by employing quinazoline derivatives as secondary acceptor (PQ) and secondary donor–acceptor (PQCz) to provide intermediate locally excited triplet (3LE) states and then enhance spin-orbital coupling (SOC). In addition, the secondary donor–acceptor group PQCz in tCzBN-PQCz can suppress intermolecular interaction and provide dual long-range charge transfer (LRCT) channels to further enhance kRISC. Therefore, tCzBN-PQ and tCzBN-PQCz exhibit high kRISC of 5.6 × 105 s−1 and 7.3 × 105 s−1 with large photoluminescence quantum yield (PLQY) of 89 % and 95 % respectively. Consequently, OLED based on tCzBN-PQ and tCzBN-PQCz show excellent external quantum efficiency (EQE) of 30.2 % and 35.1 % with small efficiency roll-off and small full width at half maxima (FWHM) of 37 nm and 34 nm respectively. Especially, tCzBN-PQCz exhibits better performances and less-sensitive to the doping concentration which proves the effectiveness of this secondary donor–acceptor group.

High-efficiency green organic light-emitting devices with low efficiency roll-off using π-shape materials as host

High triplet energy level and thermal stability are crucial properties for host materials to fulfill organic light-emitting devices (OLEDs) with great performance. In this work, four π-shape bipolar host materials, GHA1-4 were designed and synthesized using dibenzofuran core. These hosts exhibit high triplet energy levels as well as excellent thermal stability. Green top-emission OLEDs with GHA1-4 as hosts and Ir (ppy)3 as emitter exhibited excellent performance with high external quantum efficiency (EQEmax) values of above 40%. In addition, devices incorporating GHA1 and GHA3 exhibit a low turn on voltage of 2.07 V, EQEmax of 45.0% and long LT95 device lifetime of 184 h, as well as remarkably small roll-off of only 2.5% at the brightness of 10,000 cd/m2.

Recent Advances in Triplet–Triplet Annihilation-Based Materials and Their Applications in Electroluminescence

Efficient and stable blue organic light-emitting devices (OLEDs) represent one of the most important components in future solid-state lighting and full color displays. Although red and green OLEDs have been successfully commercialized, it remains challenging to realize blue OLEDs showing high efficiency and high stability simultaneously, restricting their commercialization. Triplet–triplet annihilation (TTA)-based materials have been emerging as a new category of electroluminescent or host materials with promising potential for highly efficient and stable blue devices due to their appealing photophysical properties (e.g., ability to generate emissive singlet excited states through TTA process, high photoluminescence quantum yield, ease of molecular design, etc.). Recently, research on TTA-based materials toward high-performance blue OLEDs has received an increasing attention. In this review, recent research advances on the molecular design strategies, photophysical properties of TTA-based materials (both blue emitters and host materials), mechanism of TTA-based electroluminescence, as well as their OLED applications are presented comprehensively. Moreover, potential perspectives with future research opportunities in this field are also described. We believe that the emerging TTA-based materials will bring more opportunities for developing a large family of novel photofunctional materials showing appealing and tunable photophysical properties, thus providing new opportunities for high-efficiency and stable blue electroluminescence in the future.

Donor engineering for diphenylsulfone derivatives with both thermally activated delayed fluorescence and aggregation-induced emission properties

Two new aggregation-induced thermally activated delayed fluorescence (AIE-TADF) luminogens based on diphenylsulfone derivatives with asymmetrical D-A-D′ configuration have been designed and synthesized, namely 10-(4-((9-phenyl-9H-carbazol-3-yl)sulfonyl)phenyl)-10H-phenoxazine (3CP-DPS-PXZ), and 9,9-dimethyl-10-(4-((9-phenyl-9H-carbazol-3-yl)sulfonyl)phenyl)-9,10-dihydroacridine (3CP-DPS-DMAC). The TADF and AIE properties of the new luminogens can be tuned by adjusting donor moiety (D′). Since phenoxazine featured stronger electron-donating capability compared with 9,9-dimethylacridine, 3CP-DPS-PXZ manifested a bathochromic-shift emission, enhanced AIE effect and higher reverse intersystem crossing (kRISC) rate constant in comparision with 3CP-DPS-DMAC. Systematic research on the thermal, electrochemical and photophysical properties for these two new compounds have been carried out. The fabricated nondoped green organic light-emitting device (OLED) utilizing 3CP-DPS-PXZ as emitter achieved a maximum external quantum efficiency (EQE) of 17.9 ± 0.3% and the EQE remained 14.5% at the luminance of 1000 cd/m2. However, the maximum EQE of 3CP-DPS-DMAC based blue nondoped OLED was only 9.1 ± 0.2%. These manifested asymmetrical D-A-D’ structure molecular is an efficient way to optimize the performance of AIE-TADF emitter.

Two N,N-chelated difluoroboron complexes containing phenanthroimidazole moiety: Synthesis and luminescence properties

Two new N,N-chelated difluoroboron complexes, PIBF2-Cz and PI-BODIPY, containing phenanthroimidazole moiety were synthesized and characterized by elemental analysis, NMR spectroscopy and MS. Their photophysical and electrochemical properties and thermal stabilities were investigated systematically. The results showed that these N,N-chelated difluoroboron complexes not only exhibit excellent thermal stabilities but also emit strong green emissions with photoluminescence quantum efficiency of 95.71% and 67.22% in chloroform solutions. Furthermore, based on these N,N-chelated difluoroboron complexes, the green organic light-emitting devices with a configuration of ITO/NPB (20 nm)/TBADN: difluoroboron complex (x wt%, 30 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm) have been successfully fabricated by vacuum-deposition method, in which the devices fabricated from PIBF2-Cz exhibited the best electroluminescence performance with a maximum brightness of 20290 cd/m2 and a maximum luminous efficiencies of 7.97 cd/A and a maximum external quantum efficiency (EQE) of 1.88%.

Highly efficient green organic light-emitting devices based on terbium complex by employing hole block material as host

In this work, efficient green electroluminescent (EL) devices with simplified device structure were prepared by doping trivalent terbium complex Tb(PMIP)3 into hole block material TmPyPB. The high triplet energy of TmPyPB helps to confine excitons within light-emitting layer, while the electron transport characteristic of TmPyPB facilitates the balance of carriers on Tb(PMIP)3 molecules. By optimizing the doping concentration of Tb(PMIP)3 and the thickness of each functional layer, highly efficient green EL device with the structure of ITO/MoO3 (3 nm) /TAPC (50 nm) /Tb(PMIP)3 (30 wt%):TmPyPB (25 nm) /TmPyPB (60 nm) /LiF (1 nm) /Al (100 nm) displayed pure Tb3+ characteristic emission with maximum current efficiency, power efficiency and brightness up to 47.24 cd/A (external quantum efficiency (EQE) of 14.4%), 43.63 lm/W and 1694 cd/m2, respectively. At certain brightness of 100 cd/m2, the device still maintained a current efficiency of 19.96 cd/A (EQE=6.1%). Such a device design strategy helps to improve the EL performances of Tb(PMIP)3 and to simplify device fabrication processes, thus reduce the fabrication cost.

Two novel phenanthrene-based host materials in red and green organic light-emitting devices with low efficiency roll-off

Two novel phenanthrene-based host materials showed significant differences due to the different moieties at the same position of the phenanthrene backbone.

Efficient donor-acceptor host materials for green organic light-emitting devices: non-doped blue-emissive materials with dual charge transport properties.

Comparative optical, electroluminescence and theoretical studies were performed for (E)-4′-(1-(4-(2-(1-(4-morpholinophenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,1′-biphenyl]-4-amine (SMPI-TPA) and (E)-4-(4-(2-(4-(2-(4-(9H-carbazol-9-yl)phenyl)-1H-phenanthro[9,10-d]imidazol-1-yl)styryl)-1H-phenanthro[9,10-d]imidazol-1-yl)phenyl)morpholine (SMPI-Cz). These compounds show excellent thermal properties, dual charge transport properties and form thin films under thermal evaporation. Blue OLEDs (CIE: 0.16, 0.08) based on SMPI-TPA show efficient device performance (ηex 6.1%; ηc 5.3 cd A−1; ηp 5.2 lm W−1) at low turn-on voltages. Both SMPI-TPA and SMPI-Cz were utilised as hosts for green OLEDs. The devices with SMPI-Cz (30 nm):5 wt% Ir(ppy)3 exhibit maximum luminance of 20 725 cd m−2, and ηc and ηp values of 61.4 cd A−1 and 63.8 lm W−1, respectively. In comparison, devices with SMPI-TPA (30 nm):5 wt% Ir(ppy)3 exhibit high ηc and ηp values of 65.2 cd A−1 and 67.1 lm W−1, respectively. Maximum ηex values of 19.6% and 23.4% were obtained from SMPI-TPA:Ir(ppy)3 and SMPI-Cz:Ir(ppy)3, respectively. These device performances indicate that the phenanthroimidazole unit is a tunable building unit for efficient carrier injection and it may also be employed as a host for green OLEDs.

Small molecular hole-transporting and emitting materials for hole-only green organic light-emitting devices

Three small molecular hole-transporting and emitting materials with naphthalene bridged triphenylamine or carbazole were synthesized. These compounds exhibited high absolute fluorescence quantum yields (70–88%) at the green light region (540–555 nm), suitable highest occupied molecular orbital levels (−5.26 ∼ −5.34 eV) and good hole mobilities (∼10−4 cm2 V−1 s−1). The hole-only organic light emitting diodes with as-synthesized hole-transporting and emitting materials showed a tune-on voltage and a maximum brightness of 2.6 V and 2010 cd m−2, respectively. Three stages of voltage-dependent current were observed, which coincide with the characteristic of Ohmic current, Fowler-Nordheim current and space charge limited current. With analysis of above models, overlarge Fowler-Nordheim current and space charge limited current contribute to the low efficiency of the device. All these results indicated that the high hole mobility showed the negative effect on the device efficiency, especially at high operation voltage.

High-Performance Green OLEDs Using Thermally Activated Delayed Fluorescence with a Power Efficiency of over 100 lm W(-1).

A green organic light-emitting device (OLED) with an extremely high power efficiency of over 100 lm W(-1) is realized through energy transfer from an exciplex. An optimized OLED showed a maximum external efficiency of 25.7%, and a power efficiency of 79.4 lm W(-1) at 1000 cd m(-2) , which is 1.6-times higher than that of state-of-the-art green thermally activated delayed fluorescence (TADF) OLEDs.

Green organic light-emitting devices with external quantum efficiency up to nearly 30% based on an iridium complex with a tetraphenylimidodiphosphinate ligand

High performance green electroluminescent device with maximum brightness, current efficiency, power efficiency and external quantum efficiency (EQE) up to 113 610 cd m−2, 112.30 cd A−1, 97.95 lm W−1 and 29.4%, was realized.

High Performance Yellow Green Organic Light-Emitting Devices Based on Ir(ppy)<SUB>3</SUB>

High Performance Yellow Green Organic Light-Emitting Devices Based on Ir(ppy)<SUB>3</SUB>

Improved light-extraction efficiency from organic light-emitting diodes using hazy SiO2 thin films created by using an aerosol-deposition method

We herein report an investigation of the effect of rough thin films of SiO2 granules deposited on glass substrates of organic light-emitting devices (OLEDs) by using a simple, low-cost and scalable process based on a powder spray of SiO2 granules in vacuum, known as the aerosol-deposition method, with regard to their external light-extraction capabilities. The rough and hazy thin SiO2 films produced by using aerosol-deposition and acting as scattering centers were able to efficiently reduce the light-trapping loss in the glass substrate (glass mode) for internally-generated photons and to enhance the external quantum efficiency (EQE) of the OLEDs. Based on aerosol-deposited silica films with a thickness of 800 nm and a haze of 22% on glass substrates, the EQE of phosphorescent green OLEDs was found to be enhanced by 17%, from an EQE of 7.0% for smooth bare glass substrates to an EQE of 8.2%. Furthermore, the EQEs of fluorescent blue and phosphorescent red OLEDs were shown to be enhanced by 16%, from an EQE of 3.7% to 4.3%, and by 16%, from an EQE of 9.3% to 10.8%, respectively. These improvements in the EQEs without serious changes in the emission spectra or the Lambertian emitter property clearly indicate the high potential of the aerosol-deposition technique for the realization of highly-efficient light extraction in colorful OLED lighting.

Efficient blue green organic light-emitting devices based on a monofluorinated heteroleptic iridium(III) complex

Two heteroleptic iridium complexes were synthesized and their photophysical properties studied. The two complexes vary by their ancillary ligand that is respectively acetylacetone (acac) or dibenzoylmethane (dbm). Interestingly, only the acac-based complex proved to be highly emissive and electroluminescent devices were only fabricated with this complex. In order to determine the emissive properties of this new complex, comparison with complexes previously reported in the literature was established.

Efficient Green Organic Light-Emitting Devices Based on a Solution-Processable Starburst Molecule

We develop high efficiency solution-processed pure green organic light-emitting devices using a starburst molecule 7,7′,7″-(5,5,10,10,15,15-hexahexyl-10, 15-dihydro-5H-diindeno[1, 2-a:1′, 2′-c]fluorene-2,7,12-triyl)tris(4-(4-(9H-carbazol-9-yl)phenyl)benzo[c][1,2,5]thiadiazole) (TRcz) doped 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) as the emitting layers. The electroluminescence properties of the devices with different doping concentrations are investigated. With the increasing doping concentration from 0.5wt% to 5wt%, the maximum efficiency changes from 4.8 cd/A to 8.4 cd/A. Under the optimal concentration of 4wt%, the device shows pure green emission at 516 nm with a chromaticity coordinate of (0.30, 0.59) as well as a high brightness of 19900 cd/m2 and a high efficiency of 10.1 cd/A, which are better than 11490 cd/m2 and 4.2 cd/A obtained in the undoped device.

Phosphorescent Organic Light-emitting Devices to Sense Contact Stresses

ABSTRACTWe studied the electrical and optical responses of organic light-emitting devices (OLEDs) with green and red phosphorescent dyes doped in a polymer matrix to compressive stresses. The green OLED converted stresses as low as 6.8 kPa into measurable and reversible changes in both current density and electroluminescence (EL) intensity. The current showed a nearly linear characteristic response with sensitivity up to 205 μA kPa-1, whereas the EL intensity decreased by over three orders of magnitude at 107 kPa. In contrast, stress-induced modulations in current and light intensity were noticeable in the red OLED only above 160 kPa. The discrepancy has been attributed to different rates of stress-enhanced back exciton energy transfer between guest and host molecules, which quenches the EL of the green OLED, but has a much smaller impact on the performance of the red OLED. It is expected that similar green phosphorescent OLEDs built on large curved surfaces may directly image stress distributions and sense touch on a par with a human finger.

Effect of desiccant on the performance of green organic light-emitting device

A green organic light-emitting diodes (OLED) with a multilayer structure of indium-tin oxide (ITO)/copper-phthalocyanine (CuPc) (200A)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) (600A)/N′- diphenyl-N,N′-tris(8-hydroxyquinoline) aluminium (Alq3) (400A):10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7- tetrahydro-1H,5H,11H-(l)benzopyropyrano(6,7,8-i, j)quinolizin-11-one (C545T) (2%)/Alq3 (200A)/LiF (10A)/Al (1000A) was prepared via vacuum thermal evaporation. To reduce the impact of water vapor and oxygen on the device, we encapsulated it with a kind of specific and efficient desiccant, called DESIPASTE, under the protection of high-purity nitrogen. By analyzing a series of optical characteristics of OLEDs, the results showed that this desiccant can improve the brightness about 500 and 250 cd/m2 at same driving voltage and current density, respectively. The electroluminescent (EL) spectra were hardly affected except a very weak blue shift of broadband emission peak. It turns out that encapsulation with DESIPASTE is a simple and efficient way to improve the performance of OLED.

Investigation of emission location in top-emitting green organic light-emitting devices by optical analysis

A series of top-emitting organic light-emitting devices with different thicknesses of carrier transporting layers (N,N′-di(1-naphtyl)-N,N′-diphenylbenzidine, tris(8-hyroxyquinloine) aluminum (Alq 3)) and emitting layer (EML, 10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T)-doped Alq 3) were fabricated. C545T-doped Alq 3 was found to bring about double recombination peaks in EML. As the distance between EML and reflective anode was increased, the outcoupling efficiency greatly deviated from optically-simulated values due to charge imbalance in EML and optical loss at the EML/Alq 3 interface. The device with 30 nm of EML exhibited maximized outcoupling efficiency and further increase of EML thickness brought about decrease in efficiency due to decrease in hole–electron recombination at the EML/Alq 3 interface.

Luminance Mechanisms in Green Organic Light-Emitting Devices Fabricated Utilizing Tris(8-hydroxyquinoline)Aluminum/4,7-Diphenyl-1, 10-Phenanthroline Multiple Heterostructures Acting as an Electron Transport Layer

The electrical and the optical properties in green organic light-emitting devices (OLEDs) fabricated utilizing tris(8-hydroxyquinoline)aluminum (Alq3)/4,7-diphenyl-1,10-phenanthroline (BPhen) multiple heterostructures acting as an electron transport layer (ETL) were investigated. The operating voltage of the OLEDs with a multiple heterostructure ETL increased with increasing the number of the Alq3/BPhen heterostructures because more electrons were accumulated at the Alq3/BPhen heterointerfaces. The number of the leakage holes existing in the multiple heterostructure ETL of the OLEDs at a low voltage range slightly increased due to an increase of the internal electric field generated from the accumulated electrons at the Alq3/BPhen heterointerface. The luminance efficiency of the OLEDs with a multiple heterostructure ETL at a high voltage range became stabilized because the increase of the number of the heterointerface decreased the quantity of electrons accumulated at each heterointerface.

Luminance Efficiency Enhancement in Green Organic Light-Emitting Devices Fabricated Utilizing a Cesium Fluoride/Fullerene Heterostructure Electron Injection Layer

Enhancement mechanisms of the luminance efficiency in green organic light-emitting devices (OLEDs) fabricated utilizing a cesium fluoride (CsF)/fullerene (C60) heterostructure acting as an electron injection layer (EIL) were investigated. The luminance efficiencies as functions of the current density showed that the luminance efficiency in the green OLEDs fabricated utilizing a CsF/C60 heterostructure acting as an EIL was higher than that in the green OLEDs fabricated utilizing a CsF, a Liq, or a C60 single EIL. The interfacial dipole existing at the CsF layer decreased the electron injection barrier, and the stability of the OLEDs with a CsF EIL was enhanced due to the lower diffusion rate of Cs atoms in comparison with Li atoms. The enhancement of the luminance efficiency of the OLEDs with a heterostructure EIL was attributed to the increase in the electron injection. These results can help improve understanding of the enhancement mechanisms of the luminance efficiency in green OLEDs utilizing a CsF/C60 heterostructure acting as an EIL.