Universal Host Materials Research Articles

Improving efficiency in RGB phosphorescent and white organic light emitting diodes via donor-spiro-boron acceptor host materials

The universal host material with boron acceptor core structure is exceptionally sparse to be designed for red, green, and blue in efficient phosphorescent organic light emitting diodes (PhOLEDs) accompanying white OLEDs, possessing indistinguishable device feature. Herein, two boron acceptors spiro-host materials namely 1'-(4-(dimesitylboranyl)phenyl)-10-phenyl-10H-spiro[acridine-9,9′-fluorene] (TPA-PBM) and 1-(4-(dimesitylboranyl)phenyl)-3′,6′-dimethylspiro[fluorene-9,8′-indolo[3,2,1-de]acridine] (2MeCz-PBM) were designed and synthesized. The designed rigid donor strategy exhibited good device performance among the reported boron donor-spiro-acceptor (D-spiro-A) skeleton for organic light emitting diodes (OLEDs). In particular, we fabricate RGB three-color phosphorescent OLEDs based on TPA-PBM and 2MeCz-PBM. The red devices exhibit a maximum external quantum efficiency (EQE) of nearly 28 % (26.8 % for TPA-PBM and 27.5 % for 2MeCz-PBM), with an electroluminescence spectrum at 608 nm. The green/blue devices obtain maximum EQEs of 21.2 %/15.3 % and 21.8 % 17.3 %, for TPA-PBM and 2MeCz-PBM, respectively. Furthermore, green devices display an extremely low efficiency roll-off with decreasing 1 % and 3 % at luminance of 5000 cd m−2. Additionally, the two-color white OLED using 2MeCz-PBM exhibits EQEmax and PEmax are 24.5 % and 62.9 lm W−1 respectively, the EQE remain 23.8 % at commercial lighting brightness of 1000 cd m−2. The WOLED also shows a stable white spectrum with CIE varying range of (0.41, 0.47) to (0.39, 0.46) at 100 cd m−2 to 15000 cd m−2. All obtained results confirmed that our targeted molecules have advantage of carrier balance and efficient charge recombination, which might replace many other commercial host materials.

Advances in 2D Ultrathin Nanosheets of a2FeSiO4 (A= Li, Na, K) for Next-Generation Alkali-Ion Batteries

Although lithium-ion batteries (LIBs) have become ubiquitous in various applications, the limited resource of Li and increasing cost hinder their scalability for grid-scale energy storage applications. Thus, alternative rechargeable batteries that use more earth-abundant elements such as sodium and potassium have received significant attention. However, the distinct different properties of Li+, Na+, and K+ ions hinder the extensive development of universal host materials for alkali metal ions. Polyanion-type electrode materials, which contain polyanionic groups (XO4)n −, (XO4F)n −, and (XmO2m+1)n − (X = P, Si, S, etc.), have garnered significant interest owing to their remarkable structural stability and adjustable electrode potential; this properties make them excellent candidates as cathode materials in sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs). Among these, A2FeSiO4 (AFS) (A= Li, Na, K) is highlighted as a promising cathode material due to its abundant Fe resources, low cost, and non-toxicity. The crystal structure, transport pathways, and synthesis techniques of Li2FeSiO4 (LFS) have been extensively explored whereas research on Na2FeSiO4 (NFS) as a cathode material is still in the process of maturing and research on K2FeSiO4 (KFS) is very rare. The large ionic radius of sodium and potassium presents a significant challenge to effective ion-intercalation reactions for energy storage in electrodes, destabilizing the crystal structures of host materials. In addition, orthosilicates exhibit poor electronic conductivity, greatly limiting its practical application. To address this issue, various techniques have been explored, which include nanostructuring, coating with conductive polymers, and metal doping. In this study, a facile solvothermal technique for the fabrication of ultrathin 2D AFS nanosheets by hydrothermal transformation and propose the reaction mechanism for this chemical transformation. The as-obtained AFS nanosheets are highly exfoliated using oxalic acid as the exfoliating agent to improve their electrochemical performance. These nanosheets are designed to mitigate the diffusion limitations encountered by alkali ions when traversing the densely packed structure of AFS. The highly exfoliated AFS 2D nanosheets greatly improve the electrochemical performance and cyclability of the cathode. The nanosheets exhibit a specific discharge capacity of 149 mAh g-1 for Li+, 99 mAh g-1 for Na+, and 88 mAh g-1 for K+ when operated at a current density of 100 mA g-1. Moreover, the nanosheets display exceptional capacity retention, with a retention rate of 75% for Li+, 99% for Na+, and 65% for K+ after 100 charge/discharge cycles for each of the cathodes. This study demonstrates the potential of 2D nanosheets for stabilizing the silicate-based cathode materials against structural and volume changes, and for improving their electrochemical performance in SIBs and KIBs. Figure 1

New-Fashioned Universal and Functional Host-Material from a Near-Ultraviolet Organic Emitter for High-Efficiency Organic Light-Emitting Diodes with Low Efficiency Roll-Offs.

In this work, a near-ultraviolet (NUV) emitter, 2MCz-CNMCz, with hot-exciton property is designed based on a "long-short axis" strategy, which exhibits good thermal stability, bipolar carrier transport ability, and high T1 energy level. Its nondoped NUV organic light-emitting diode (OLED) achieves a record maximum external quantum efficiency (ηext ) of 7.76%, with a peak at 404nm and CIE coordinates of (0.158, 0.039). The corresponding high exciton utilization efficiency (ηr ) in the electroluminescence process reveals its potential as a functional sensitizing host. As expected, the TBPe-based blue fluorescent OLED with 2MCz-CNMCz as the host material shows better efficiency and lower efficiency roll-off than that with traditional host material mCP. Meanwhile, the Ir complexes-based green/yellow/red phosphorescent OLEDs with 2MCz-CNMCz host are also fabricated, reaching high ηext values of 26.1%, 30.4%, and 20.4%, respectively, and displaying negligible efficiency roll-offs at 1000cdm-2 , which are among the best OLED performances based on the same emitters. To the authors' best knowledge, this is the first report on the design of high-quality universal and functional host material, and may bring new inspiration to the preparation of high-efficiency, low roll-off, full-color OLEDs.

A universal thermally activated delayed fluorescent host with short triplet lifetime for highly efficient phosphorescent OLEDs with extremely low efficiency roll-off

A universal TADF host material with a small singlet–triplet energy gap was developed for phosphorescent OLEDs.

TADF material with non-conjugated rigid donor for high-performance full-color phosphorescent OLEDs: Effects of triplet harvest and charge transport on efficiency

Host materials play a crucial role in developing highly efficient OLEDs, of which universal host materials for full-color OLEDs are in great demand. Herein, two wide band-gap thermally activated delayed fluorescence (TADF) host materials based on diphenylsulfone derivatives were developed by rationally tuning the TADF characteristic and charge transport ability through electron-donor engineering. Owning to the non-conjugated rigid donor 9,9-dimethylacridine, mSOAD showed obvious superiority in pronounced TADF effect with an extra small singlet-triplet energy split of 0.02 eV and balanced carrier transport properties. Full-color phosphorescent OLEDs based on mSOAD were achieved with impressive efficiencies of 65.52 cd/A, 66.51 lm/W and 19.00% for green OLED, 36.19 cd/A, 32.46 lm/W and 16.7% for blue OLED and 32.65 cd/A, 30.99 lm/W, and 20.31% for red OLED, respectively. Our results revealed the key effect of efficient triplet harvest and balanced charge transport ability of TADF host materials in high efficiency and low efficiency roll-off in OLEDs.

Spiro-type host materials with rigidified skeletons for RGB phosphorescent OLEDs

A universal host material for red, green and blue phosphors raises challenges for the fabrication of efficient phosphorescent organic light-emitting diodes (PHOLEDs).

Universal host materials for red, green and blue high-efficiency single-layer phosphorescent organic light-emitting diodes

As simplifying the device structure is a key step for the future of organic electronic, we report herein high efficiency universal host materials for red, green and blue Single-Layer Phosphorescent OLEDs.

Universal host materials based on carbazole-formate derivatives for blue, green and red phosphorescent organic light-emitting diodes

The design of universal hosts suitable for different color dopants is still desired for efficient organic light-emitting diodes (OLEDs). In this work, two novel carbazole-formate based isomers (2CzMC and 4CzMC) with planar donor‒π‒acceptor (D‒π‒A) structures were designed and synthesized as the hosts for phosphorescent OLEDs (PhOLEDs). Due to the weak intramolecular charge transfer character and limited conjugation extensions, both 2CzMC and 4CzMC showed wide-bandgap emissions around 380 nm and high triplet energy levels (2.83 eV for 2CzMC, and 2.90 eV for 4CzMC). The photoluminescent spectra of 2CzMC and 4CzMC doped films with blue, green and red phosphors exhibited completely energy transfer from the hosts to the dopants. PhOLEDs devices with similar structures employing these doped film as emitter layers achieved excellent performance. Especially for 4CzMC-FIrpic based blue device, the external quantum efficiency (EQE) reached as high as 18.9%. This study showed a molecular design strategy to develop novel carbazole-based hosts for different color PhOLEDs.

Phthalonitrile-based bipolar host for efficient green to red phosphorescent and TADF OLEDs

A bipolar host material named 4-(9H-carbazol-9-yl)phthalonitrile (CzPN) was developed by combining carbazole and phthalonitrile as donor and acceptor units, respectively. The introduction of the phthalonitrile unit realized a relatively small energy gap of 3.00 eV as well as a deep lowest unoccupied molecular orbital level of −2.72 eV. The characterization of carrier-only devices demonstrated the bipolar charge transporting property of CzPN. With a triplet energy of 2.70 eV, CzPN was investigated as host in green, yellow, and red phosphorescent organic light-emitting diodes (PhOLEDs) and green thermally activated delayed fluorescence (TADF) devices. A low turn-on voltage of 2.8 V was achieved by all the four diodes. Both PhOLEDs and TADF devices exhibited high external quantum efficiency values over 20% with relatively small efficiency roll-offs. These findings demonstrate that CzPN is an excellent host material for both phosphorescent and TADF emitters, and phthalonitrile is a promising unit to develop universal host materials.

Methoxycarbazolyl-disubstituted dibenzofuranes as holes- and electrons-transporting hosts for phosphorescent and TADF-based OLEDs

In the search of universal host materials for organic light emitting diodes a new series of bipolar host materials containing methoxy-substituted carbazoles as the electron-donating and dibenzofuran as an electron-accepting units were designed and synthesized. Different linking topologies and number of methoxy groups attached to carbazolyl moiety were used to understand the impact of the strength of the donor moiety on the thermal, optical, photophysical, electrochemical and electroluminescent properties. The synthesized compounds exhibited relatively high thermal stability with 5% weight loss temperatures exceeding 378 °C and formed molecular glasses with high glass-transition temperatures ranging from 120 to 148 °C. High triplet energy values of 2.86–2.96 eV were estimated for dilute THF solutions at 77K. Hole and electron drift mobilities estimated using time-of-flight technique in solid layers approached 10−4 cm2V−1s−1 at high electric fields exceeding 3.6 × 105 V cm−1. The synthesized methoxy-carbazole based compounds were tested as hosts in electrophosphorescent and TADF organic light-emitting diodes reaching luminance of 53000 cd m−2 and external quantum efficiency of 12.5%, in the best case.

Excited-state engineering of universal ambipolar hosts for highly efficient blue phosphorescence and thermally activated delayed fluorescence organic light-emitting diodes

The development of high-performance universal host materials for both blue electrophosphorescence and thermally activated delayed fluorescence (TADF) is one of the main challenges for organic light-emitting diode (OLED) technology. In this work, based on four quaternary ambipolar hosts with insulated donor-acceptor (D-A) structures, the influence of the excited-state characteristics for these hosts on their device performance was systematically investigated. It was showed that on the premise of the similar electrical properties and excited-state energy levels for positive host-dopant energy transfer (ET), the weak exciton capture ability, embodied as low exciton binding energy (Eb), for the hosts could facilitate the Dexter ET to the dopant through charge migration. Since the charge-transfer -featured excited states and small Eb of blue TADF emitters, in contrast to the blue phosphorescence (PH) counterparts, the influence of the host Eb on the device efficiencies of blue TADF dyes was dramatically enlarged, reflecting the higher sensitivity of the latter to host characteristics. In consequence, 9CzFDBFDPO with the smallest Eb endowed its blue TADF and PH devices with the state-of-the-art performance, e.g. EQE beyond 20% and EQE roll-off within 20% at 500 cd m−2. This work showed that based on the systematically optimized excited-state characteristics, the ambipolar hosts can be competent in both high-efficiency blue PH and TADF OLEDs, which is crucial for their commercial applications.

Universal Bipolar Host Materials for Blue, Green, and Red Phosphorescent OLEDs with Excellent Efficiencies and Small-Efficiency Roll-Off.

Host materials are indispensable for the fabrication of organic light-emitting diodes (OLEDs) with phosphorescent emitters, but high-quality host materials that can efficiently and simultaneously function in blue, green, and red phosphorescent OLEDs (PHOLEDs) are much rare. In this work, four bipolar materials are developed using carbazole and 9,9-dimethyl-9,10-dihydroacridine as hole-transporting groups, pyridine as electron-transporting groups, and biphenyl and m-methylbiphenyl as π-spacers. The crystal and electronic structures indicate that these materials have highly twisted conformations, which endow them with aggregation-induced emission features, intramolecular charge transfer processes, wide energy band gaps, and high triplet energies. The carrier transport ability and energy transfer property analyses show that these materials are able to achieve balanced hole and electron transports and can serve as bipolar host materials for PHOLEDs. A series of monochromatic PHOLEDs with different phosphorescent dopants, including blue-emissive FIrpic, green-emissive Ir(ppy)2(acac), and red-emissive Ir(piq)2(acac), are fabricated by employing these four host materials. The green PHOLEDs can provide an impressive luminance of up to 230 200 cd m-2. Based on an identical host material, excellent external quantum efficiencies as high as 25.12, 24.73, and 19.71%, as well as minor efficiency roll-off, are attained for blue, green, and red PHOLEDs, respectively, clearly demonstrating the promising applications as universal bipolar host materials in PHOLEDs with monochromatic light and white light.

Recent Progress in Multivalent Metal (Mg, Zn, Ca, and Al) and Metal-Ion Rechargeable Batteries with Organic Materials as Promising Electrodes.

The emerging demand for electronic and transportation technologies has driven the development of rechargeable batteries with enhanced capacity storage. Especially, multivalent metal (Mg, Zn, Ca, and Al) and metal-ion batteries have recently attracted considerable interests as promising substitutes for future large-scale energy storage devices, due to their natural abundance and multielectron redox capability. These metals are compatible with nonflammable aqueous electrolytes and are less reactive when exposed in ambient atmosphere as compared with Li metals, hence enabling potential safer battery systems. Luckily, green and sustainable organic compounds could be designed and tailored as universal host materials to accommodate multivalent metal ions. Considering these advantages, effective approaches toward achieving organic multivalent metal and metal-ion rechargeable batteries are highlighted in this Review. Moreover, organic structures, cell configurations, and key relevant electrochemical parameters are presented. Hopefully, this Review will provide a fundamental guidance for future development of organic-based multivalent metal and metal-ion rechargeable batteries.

A universal host material with a simple structure for monochrome and white phosphorescent/TADF OLEDs

Highly efficient phosphorescent and thermally activated delayed fluorescence organic light-emitting diodes have been realized by employing a universal host material.

Dual Carbon-Confined SnO2 Hollow Nanospheres Enabling High Performance for the Reversible Storage of Alkali Metal Ions.

To explore a universal electrode material for the high-performance electrochemical storage of Li+, Na+, and K+ ions remains a big challenge. Herein, we propose a "trinity" strategy to coat the SnO2 hollow nanospheres using the dual carbon layer from the polydopamine-derived nitrogen-doped carbon and graphene. Thereinto, hollow structures with sufficient void space could buffer the volume expansion, whereas dual carbon-confined strategy could not only elastically prevent the aggregation of nanoparticle and ensure the structural integrity but also immensely improve the conductivity and endow high rate properties. Benefiting from the effective strategy and specific structure, the dual carbon-confined SnO2 hollow nanosphere (denoted as G@C@SnO2) can serve as the universal host material for alkali metal ions and enable their rapid and reversible storage. As expected, the resulting G@C@SnO2 as a universal anode material shows reversible alkali-metal-ion storage with high performance. We believe this that strategy could pave the way for constructing other metal-oxide-based dual carbon-confined high-performance materials for the future energy storage applications.

A universal solution-processable bipolar host based on triphenylamine and pyridine for efficient phosphorescent and thermally activated delayed fluorescence OLEDs

Host materials play an important role in the development of phosphorescent and thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). Herein, a universal bipolar host material TPA-(PyF)3 based on triphenylamine and pyridine was designed and synthesized through a simple Friedel-Crafts reaction with a high yield of 83.5%. TPA-(PyF)3 possesses a high triplet energy level of 2.83 eV, excellent thermal and morphological stability, and good solution processability. Using TPA-(PyF)3 as a host material, solution-processed OLEDs with a simple single emitting layer were fabricated. Blue phosphorescent OLEDs based on bis((3,5-difluorophenyl)-pyridine) iridium picolinate (FIrpic) show a maximum current efficiency of 18.5 cd A−1 and a maximum brightness of 13,838 cd m−2, with a low current efficiency roll-off of 9.7% at the practical brightness of 100–1000 cd m−2. Green TADF OLEDs using TPA-(PyF)3 as the host and 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyano-benzene (4CzIPN) as the emitter with the same structure of the phosphorescent OLEDs exhibit a maximum current efficiency of 12.7 cd A−1, compared with 10.2 cd A−1 of the control device based on the common host material of 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy). This study demonstrates the effective utilization of TPA-(PyF)3 as a universal bipolar host for both phosphorescent and TADF OLEDs.

1,2,4]Triazolo[1,5-a]pyridine as Building Blocks for Universal Host Materials for High-Performance Red, Green, Blue and White Phosphorescent Organic Light-Emitting Devices.

The electron-accepting [1,2,4]triazolo[1,5-a]pyridine (TP) moiety was introduced to build bipolar host materials for the first time, and two host materials based on this TP acceptor and carbazole donor, namely, 9,9'-(2-([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (o-CzTP) and 9,9'-(5-([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (m-CzTP), were designed and synthesized. These two TP-based host materials possess a high triplet energy (>2.9 eV) and appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital levels as well as the bipolar transporting feature, which permits their applicability as universal host materials in multicolor phosphorescent organic light-emitting devices (PhOLEDs). Blue, green, and red PhOLEDs based on o-CzTP and m-CzTP with the same device configuration all show high efficiencies and low efficiency roll-off. The devices hosted by o-CzTP exhibit maximum external quantum efficiencies (ηext) of 27.1, 25.0, and 15.8% for blue, green, and red light emitting, respectively, which are comparable with the best electroluminescene performance reported for FIrpic-based blue, Ir(ppy)3-based green, and Ir(pq)2(acac)-based red PhOLEDs equipped with a single-component host. The white PhOLEDs based on the o-CzTP host and three lumophors containing red, green, and blue emitting layers were fabricated with the same device structure, which exhibit a maximum current efficiency and ηc of 40.4 cd/A and 17.8%, respectively, with the color rendering index value of 75.

Hierarchical VS2 Nanosheet Assemblies: A Universal Host Material for the Reversible Storage of Alkali Metal Ions.

Reversible electrochemical storage of alkali metal ions is the basis of many secondary batteries. Over years, various electrode materials are developed and optimized for a specific type of alkali metal ions (Li+ , Na+ , or K+ ), yet there are very few (if not none) candidates that can serve as a universal host material for all of them. Herein, a facile solvothermal method is developed to prepare VS2 nanosheet assemblies. Individual nanosheets are featured with a few atomic layer thickness, and they are hierarchically arranged with minimized stacking. Electrochemical measurements show that VS2 nanosheet assemblies enable the rapid and durable storage of Li+ , Na+ , or K+ ions. Most remarkably, the large reversible specific capacity and great cycling stability observed for both Na+ and K+ are extraordinary and superior to most existing electrode materials. The experimental results of this study are further supported by density functional theory calculations showing that the layered structure of VS2 has large adsorption energy and low diffusion barriers for the intercalation of alkali metal ions.

Ultrahigh-efficiency solution-processed simplified small-molecule organic light-emitting diodes using universal host materials.

Although solution processing of small-molecule organic light-emitting diodes (OLEDs) has been considered as a promising alternative to standard vacuum deposition requiring high material and processing cost, the devices have suffered from low luminous efficiency and difficulty of multilayer solution processing. Therefore, high efficiency should be achieved in simple-structured small-molecule OLEDs fabricated using a solution process. We report very efficient solution-processed simple-structured small-molecule OLEDs that use novel universal electron-transporting host materials based on tetraphenylsilane with pyridine moieties. These materials have wide band gaps, high triplet energy levels, and good solution processabilities; they provide balanced charge transport in a mixed-host emitting layer. Orange-red (~97.5 cd/A, ~35.5% photons per electron), green (~101.5 cd/A, ~29.0% photons per electron), and white (~74.2 cd/A, ~28.5% photons per electron) phosphorescent OLEDs exhibited the highest recorded electroluminescent efficiencies of solution-processed OLEDs reported to date. We also demonstrate a solution-processed flexible solid-state lighting device as a potential application of our devices.

New Benzimidazole-Based Bipolar Hosts: Highly Efficient Phosphorescent and Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes Employing the Same Device Structure.

The rapid development of the thermally activated delayed fluorescence (TADF) emitters makes it necessary to produce new versatile host materials for both phosphorescent and TADF emitters. Three new bipolar host materials, 9,9'-(2'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (o-mCPBI), 9,9'-(3'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (m-mCPBI), and 9,9'-(4'-(1H-benzimidazol-1-yl)-[1,1'-biphenyl]-3,5-diyl)bis(9H-carbazole) (p-mCPBI), are designed and synthesized by integrating mCP with benzimidazole moiety via the ortho-, meta-, and para-positions of N-phenyl. The influence of different linking modes on the thermal, photophysical, electrochemical, and charge transport properties of the compounds is studied. By employing the same device structure except the emitting layer, the device performances of the blue, green, yellow, red, white phosphorescent and blue, green TADF organic light-emitting diodes (OLEDs) based on the three hosts are investigated. Among these three hosts, o-mCPBI exhibits the best device performance with external quantum efficiencies of over 20% for phosphorescent OLEDs and enhanced efficiencies for TADF devices. All these devices show relatively low efficiency roll-offs at high brightness. The versatility of the benzimidazole-based bipolar host o-mCPBI, such as an extremely high triplet energy level, suitable molecular orbital energy levels, improved thermal and charge transport properties, and excellent device performances for various colors, makes it a universal host material for highly efficient both phosphorescent and TADF OLEDs.