Solution-processed Thermally Activated Delayed Fluorescence Research Articles

Rational molecular design of efficient yellow-red dendrimer TADF for solution-processed OLEDs: a combined effect of substitution position and strength of the donors

AbstractThe development of high-performance solution-processed red organic light-emitting diodes (OLEDs) remains a challenge, particularly in terms of maintaining efficiency at high luminance. Here, we designed and synthesized four novel orange-red thermally activated delayed fluorescence (TADF) dendrimers that are solution-processable: 2GCzBP, 2DPACzBP, 2FBP2GCz and 2FBP2DPACz. We systematically investigated the effect of substitution position and strength of donors on the optoelectronic properties. The reverse intersystem crossing rate constant (kRISC) of the emitters having donors substituted at positions 11 and 12 of the dibenzo[a,c]phenazine (BP) is more than 10-times faster than that of compounds substituted having donors substituted at positions 3 and 6. Compound 2DPACzBP, containing stronger donors than 2GCzBP, exhibits a red-shifted emission and smaller singlet-triplet energy splitting, ΔEST, of 0.01 eV. The solution-processed OLED with 10 wt% 2DPACzBP doped in mCP emitted at 640 nm and showed a maximum external quantum efficiency (EQEmax) of 7.8%, which was effectively maintained out to a luminance of 1,000 cd m−2. Such a device∙s performance at relevant display luminance is among the highest for solution-processed red TADF OLEDs. The efficiency of the devices was improved significantly by using 4CzIPN as an assistant dopant in a hyperfluorescence (HF) configuration, where the 2DPACzBP HF device shows an EQEmax of 20.0% at λEL of 605 nm and remains high at 11.8% at a luminance of 1,000 cd m−2, which makes this device one of the highest efficiency orange-to-red HF SP-OLEDs to date.

Large area inkjet-printed OLED fabrication with solution-processed TADF ink

This work demonstrates successful large area inkjet printing of a thermally activated delayed fluorescence (TADF) material as the emitting layer of organic light-emitting diodes (OLEDs). TADF materials enable efficient light emission without relying on heavy metals such as platinum or iridium. However, low-cost manufacturing of large-scale TADF OLEDs has been restricted due to their incompatibility with solution processing techniques. In this study, we develop ink formulation for a TADF material and show successful ink jet printing of intricate patterns over a large area (6400 mm2) without the use of any lithography. The stable ink is successfully achieved using a non-chlorinated binary solvent mixture for a solution processable TADF material, 3‐(9,9‐dimethylacridin‐10(9H)‐yl)‐9H‐xanthen‐9‐one dispersed in 4,4’-bis-(N-carbazolyl)-1,1’-biphenyl host. Using this ink, large area ink jet printed OLEDs with performance comparable to the control spin coated OLEDs are successfully achieved. In this work, we also show the impact of ink viscosity, density, and surface tension on the droplet formation and film quality as well as its potential for large-area roll-to-roll printing on a flexible substrate. The results represent a major step towards the use of TADF materials for large-area OLEDs without employing any lithography.

Origins of the improved device performance in solution-processed TADF organic light-emitting devices with a polymer underneath layer

In solution-processed organic light-emitting devices (OLEDs), a polymer interface layer underneath the emission layer is essential to improving the device's performance. The typical polymer is Poly (9-vinylcarbazole) (PVK), which can dramatically increase the device efficiency whereas the mechanisms are not well understood. In this work, we explored the related mechanisms by smart experimental design and measurements. The results reveal that in addition to reducing the hole current density and balancing the carrier distribution, the PVK underneath layer also improves the film morphology and enhances the horizontal orientation of the transition dipole moment of the thermally activated delayed fluorescence (TADF) emitters. Consequently, the device's efficiency and stability are notably increased by about three times. This work provides insights into the origins of improved device performance in solution-processed TADF OLEDs with a polymer underneath layer and also highlights the effect of the underneath layer on the film formation of the solution-processed emission layer.

Creation of Dual Thermally Activated Delayed-Fluorescence Exciplexes in a Bulk Emitting Layer and Its Interface with an Electron Transport Layer for Promoting the Performance of Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes Fabricated by a Solution Process.

An exciplex, which is composed of electron donor and acceptor molecules and formed by intermolecular charge transfer, is an excited-state species that is able to emit light or transfer its energy to a lower-energy emitter. In reported exciplex-based organic light-emitting diodes (OLEDs), their working mechanism is to generate exciplexes either in the bulk emitting layer (bulk exciplex) or at its interface with an electron transport layer (interface exciplex); both types give promising device performance. Here, we propose a novel strategy of creating both types of exciplexes simultaneously (dual exciplexes) for the generation of more exciplexes for better device performance as indicated in the improved photoluminescence quantum yield (PLQY). Impressively, the dual exciplex-based device with blue thermally activated delayed fluorescence (TADF) emitter 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) exhibits a record-high maximum external quantum efficiency (EQEmax) of 26.7% among the solution-processed TADF blue OLEDs. By further doping with the red-emitting phosphor emitter into the EML, the white device also gives a record-high EQEmax of 24.1% among the solution-processed TADF-phosphor hybrid white OLEDs (T-P WOLEDs) with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.34, 0.42), color rendering index of 70, and correlated color temperature of 5198 K. Furthermore, both blue and white devices show an ultralow efficiency roll-off with external quantum efficiencies at a practical brightness value of 1000 cd m-2 (EQE1000) of 25.1 and 23.9%, respectively. This is the first report of employing a dual exciplex-based OLED with excellent device performance.

Versatile Host Materials for Both D-A-Type and Multi-Resonance TADF Emitters toward Solution-Processed OLEDs with Nearly 30% EQE.

Fabricating solution-processible host material for thermally activated delayed fluorescence (TADF) emitter remains a formidable challenge for organic light-emitting diodes (OLEDs). In this work, two new host materials, namely 3CzAcPy and 9CzAcPy, are found to exhibit high triplet energy levels, high thermal stability, and excellent film morphology from a solution process. An in-depth analysis on the photophysical data and device performance reveals the isomeric effect of the host materials has a significant impact not only on the host properties, but also on the host-dopant interactions and thus the performance of the resulting solution-processed TADF OLEDs. Impressively, the new hosts are proven to be suitable for both donor-acceptor type and multi-resonance TADF emitters, achieving state-of-the-art device performance. By using the new host 9CzAcPy, solution-processed OLED based on a donor-acceptor TADF emitter of DPAC-PCN, a maximum external quantum efficiency (EQE) of 29.5% is achieved, and solution-processed narrowband OLED based on a multiple-resonance TADF emitter of BN-CP1 acquires a maximum EQE of 26.6%. These efficiencies represent the highest values among the solution-processed TADF OLEDs. This study highlights the significance of host-dopant interactions in modulating the electroluminescence performance of TADF emitters, and provides an effective design principle for solution-processible host materials.

Highly Efficient Solution-Processed Bluish-Green Thermally Activated Delayed Fluorescence Compounds Using Di(pyridin-3-yl)methanone as Acceptor

Solution-processed devices with thermally activated delayed fluorescence (TADF) compounds have gained great attention due to their low cost and high performance. Here, two solution-processable TADF emitters named ACCz-DPyM and POxCz-DPyM were synthesized by coupled 9,10-dihydro-9,9-dimethylacridine or phenoxazine modified carbazole as donor with di(pyridin-3-yl)methanone as acceptor. Both TADF compounds show same small ΔΕST of 0.04 eV and high PLQY of 66.2% and 58.2%. The devices fabricated by ACCz-DPyM and POxCz-DPyM as emitters show excellent performance as solution-processed with low turn-on voltage of 4.0 and 3.4 V, high luminance of 6209 and 3248 cd m−2 at 8 V, the maximum current efficiency of 9.9 and 15.9 cd A−1, the maximum external quantum efficiency of 6.6% and 6.5% and low efficiency roll-off. The solution-processed device based on ACCz-DPyM shows bluish-green emission. These results show that ACCz-DPyM and POxCz-DPyM are suitable for solution processing devices.

New Light-Green Thermally Activated Delayed Fluorescence Polymer Based on Dimethylacridine-Triphenyltriazine Light-Emitting Unit and Tetraphenylsilane Moiety as Non-Conjugated Backbone.

In the search for solution-processable TADF materials as a light emitting layer for OLED devices, polymers have attracted considerable attention due to their better thermal and morphological properties in the film state with respect to small molecules. In this work, a new polymer (p-TPS-DMAC-TRZ) with thermally activated delayed fluorescence (TADF) light-emitting characteristics was prepared from a conjugation-break unit (TPS) and a well-known TADF core (DAMC-TRZ). This material was designed to preserve the photophysical properties of DAMC-TRZ, while improving other properties, such as thermal stability, promoted by its polymerization with a TPS core. Along with excellent solubility in common organic solvents such as toluene, chloroform and THF, the polymer (Mn = 9500; Mw = 15200) showed high thermal stability (TDT5% = 481 °C), and a Tg value of 265 °C, parameters higher than the reference small molecule DMAC-TRZ (TDT5% = 305 °C; Tg = 91 °C). The photoluminescence maximum of the polymer was centered at 508 nm in the solid state, showing a low redshift compared to DMAC-TRZ (500 nm), while also showing a redshift in solution with solvents of increasing polarity. Time-resolved photoluminescence of p-TPS-DMAC-TRZ at 298 K, showed considerable delayed emission in solid state, with two relatively long lifetimes, 0.290 s (0.14) and 2.06 s (0.50), and a short lifetime of 23.6 ns, while at 77 K, the delayed emission was considerably quenched, and two lifetimes in total were observed, 24.6 ns (0.80) and 180 ns (0.20), which was expected from the slower RISC process at lower temperatures, decreasing the efficiency of the delayed emission and demonstrating that p-TPS-DMAC-TRZ has a TADF emission. This is in agreement with room temperature TRPL measurements in solution, where a decrease in both lifetime and delayed contribution to total photoluminescence was observed when oxygen was present. The PLQY of the mCP blend films with 1% p-TPS-DMAC-DMAC-TRZ as a dopant was determined to be equal to 0.62, while in the pure film, it was equal to 0.29, which is lower than that observed for DMAC-TRZ (0.81). Cyclic voltammetry experiments showed similarities between p-TPS-DMAC-TRZ and DAMC-TRZ with HOMO and LUMO energies of -5.14 eV and -2.76 eV, respectively, establishing an electrochemical bandgap value of 2.38 eV. The thin film morphology of p-TPS-DMAC-TRZ and DMAC-TRZ was compared by AFM and FE-SEM, and the results showed that p-TPS-DMAC-TRZ has a smoother surface with fewer defects, such as aggregations. These results show that the design strategy succeeded in improving the thermal and morphological properties in the polymeric material compared to the reference small molecule, while the photophysical properties were mostly maintained, except for the PLQY determined in the pure films. Still, these results show that p-TPS-DMAC-TRZ is a good candidate for use as a light-emitting layer in OLED devices, especially when used as a host-guest mixture in suitable materials such as mCP.

Suppressing triplet exciton quenching by regulating the triplet energy of crosslinkable hole transport materials for efficient solution-processed TADF OLEDs

Crosslinkable hole transport materials (HTMs) with high triplet energies would have a balance of carrier injection into the emitting material layer, suppressing the triplet exciton quenching and resulting in high-performance solution-processed organic light-emitting diode (OLED) devices. Two novel crosslinkable HTMs with different central units, N2, N8-di-p-tolyl-N2,N8-bis(4-vinylphenyl)dibenzo[b,d]thiophene-2,8-diamine (V-p-DBT) and N2,N8-di-p-tolyl-N2,N8-bis(4-vinylphenyl)dibenzo[b,d]furan-2,8-diamine (V-p-DBF), were designed and synthesized. The use of dibenzothiophene and dibenzofuran units increases the torsion angle compared with the commonly used N, N′-di-p-tolyl-N, N′-bis(4-vinylphenyl)-[1,1′-biphenyl]-4,4′-diamine (V-p-TPD), leading to high triplet energies of 2.57 and 2.64 eV, respectively. The triplet energies of V-p-DBT and V-p-DBF effectively suppress triplet exciton quenching. Furthermore, the crosslinked HTM layer showed excellent solvent-resistant abilities and high thermal stability. An outstanding maximum current efficiency (CEmax) of 79.94 cd A−1 and maximum external quantum efficiency (EQEmax) of 24.35% were obtained by V-p-DBF-based green thermally activated delayed fluorescent (TADF) OLEDs. This work provides a new molecular design strategy for achieving efficient solution-processed TADF OLEDs.

Intramolecular-locked triazatruxene-based thermally activated delayed fluorescence emitter for efficient solution-processed deep-blue organic light emitting diodes

Developing efficient deep blue thermally activated delayed fluorescence (TADF) emitters for solution-processed organic light emitting diodes (OLEDs) remains a formidable challenge. Herein, three new TADF emitters, namely FTAT-MBO, FTAT-HBO and FTAT-FBO, are designed and synthesized by incorporating intramolecular-locked triazatruxene with deep highest occupied molecular orbital and high triplet level as the electron donor and the boron-fused units with different substituents of methyl, hydrogen, and fluorine as the electron acceptors. Three compounds show blue emission (445–473 nm) with photoluminescence quantum yields up to 90% and bandwidths down to 54 nm. They also exhibit distinct TADF effect with small singlet − triplet energy splitting (ΔEST) below 0.2 eV and short delay fluorescence lifetimes (down to 1.8 μs). Solution-processed TADF OLEDs based on FTAT-MBO and FTAT-HBO as emitters displayed deep blue emission with the maximum external quantum efficiency (EQE) and Commission Internationale de l’éclairage (CIE) coordinates of 10.2% and (0.15, 0.08); 11.4% and (0.15, 0.12); respectively, while the solution-processed sky-blue OLEDs based on FTAT-FBO achieved the maximum EQE of 17.5% and CIE coordinates of (0.15, 0.25). This study demonstrates the potential of the intramolecular-locked triazatruxene donor unit to construct efficient TADF materials for the fabrication of solution-processed deep-blue to sky-blue OLEDs.

Novel D-D′-A structure thermally activated delayed fluorescence emitters realizing over 20% external quantum efficiencies in both evaporation- and solution-processed organic light-emitting diodes

Thermally activated delayed fluorescence (TADF) emitters have only realized limited performance in solution-processed organic light-emitting diodes (OLEDs) comparing to in evaporation-processed OLEDs. To address this issue, a novel D-D′-A structure, where A is the electron-accepting group, D is the primary electron-donating group and D′ is the secondary electron-donating group, was proposed to develop efficient solution-processable TADF emitters in this work. As the intermediate D′ spacer weakens the direct intramolecular interaction between D and A groups, D-D′-A structure molecules simultaneously possess intramolecular and intermolecular charge-transfer transition channels, suppressing the aggregation-caused quenching induced by solution process. Accordingly, a novel TADF emitter 2-(3,6-bis(9,9-dimethylacridin-10(9H)-yl)-9H-carbazol-9-yl)thianthrene 5,5,10,10-tetraoxide (DMAC-Cz-TTR) was designed and synthesized. In the optimized evaporation- and solution-processed OLEDs, DMAC-Cz-TTR successfully realized similar maximum external quantum efficiencies (EQEs) of 21.1% and 20.6%, respectively. To the best of our knowledge, this is the first TADF emitter realizing nearly equal performance in both evaporation- and solution-processed OLEDs with over 20% EQEs. The outstanding performance of DMAC-Cz-TTR successfully demonstrates the feasibility of the D-D′-A structure to develop efficient solution-processable TADF emitters. • A novel D-D′-A structure was proposed to develop efficient solution-processed TADF emitters. • A new TADF emitter DMAC-Cz-TTR was developed accordingly with intra- and inter-CT transition channels simultaneously. • In the evaporation- and solution-processed OLEDs, DMAC-Cz-TTR realized maximum EQEs of 21.1% and 20.6%, respectively.

Molecular engineering by σ-linkers enables delayed fluorescence emitters for high-efficiency sky-blue solution-processed OLEDs

Two novel thermally activated delayed fluorescence (TADF) emitters, namely Me2AcBO and F2AcBO, based on the 9,9-dimethrylacridine donor and a rigid oxygen-bridged cyclized boron acceptor, were designed and synthesized for highly efficient blue solution-processed organic light-emitting diodes (OLEDs). Me2AcBO and F2AcBO feature a twin structure derived by coupling two individual TADF monomers through non-conjugated linkers. The two new emitters exhibit 3D molecular structures, excellent thermal/morphological properties, high photoluminescence quantum yields over 97%, and high reverse intersystem crossing rates on the order of 106 s−1, which greatly benefit the high device efficiency. Solution-processed sky-blue TADF OLEDs based on Me2AcBO and F2AcBO achieved the maximum external quantum efficiency of 11.0% and 10.9%, respectively, with Commission Internationale de L’Eclairage (CIE) coordinates of (0.14, 0.25). Interestingly, relatively low efficiency roll-offs of 33% and 34% at the luminance of 1000 cd/m2 were demonstrated for Me2AcBO and F2AcBO based devices, respectively. This study presents a novel strategy to realize highly efficient sky-blue TADF emitters for solution-processed OLEDs.

Solution-processed multiple exciplexes via spirofluorene and S-triazine moieties for red thermally activated delayed fluorescence emissive layer OLEDs

A series of novel binary and ternary components of the exciplexes as the cohosts for a red thermally activated delayed fluorescence (TADF) dopant were investigated in the solution-processed OLEDs, where 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl] phenylene (OXD-7) as a conventional acceptor, and 1,3-bis(carbazol-9-yl)benzene (mCP) as a conventional donor were respectively mixed with two molecules containing spirofluorene and s-triazine moieties (TDP-TRZ or DTDP-TRZ) with excellent thermal stability and high electron mobility as the second acceptors. Particularly, the power efficiencies of the devices with the exciplexes are generally enhanced via this strategy of host engineering. The designed devices could achieve a percentage increase of 179% in the power efficiency, compared with the reference device with single-component host, mainly owing to the synergistic effects of electron block, balanced injection of charge carriers and efficient exciton harvesting. The working mechanism of energy transfer in binary and ternary components of the exciplexes hosted red TADF OLEDs is studied. This work provides a novel device design philosophy with the multiple exciplexes cohosts for solution-processed TADF OLEDs, which would help to simplify the fabrication processing, lower the cost, and popularize OLED technology. • A novel efficient binary components exciplexes co-host system applied in TADF emitter s-OLEDs. • A novel solution-processed ternary components exciplexes co-host with red TADF dopant luminescent system is successfully fabricated. • The power efficiency of TADF s-OLED binary exciplexes co-host exhibited a percentage increase of 179% compared with the reference device. • The synergistic effects of electron block, balanced injection of charge carriers and efficient exciton harvesting occurred.

Managing Locally Excited and Charge-Transfer Triplet States to Facilitate Up-Conversion in Red TADF Emitters That Are Available for Both Vacuum- and Solution-Processes.

Developing red thermally activated delayed fluorescence (TADF) emitters for high-performance OLEDs is still facing great challenge. Herein, three red TADF emitters, pDBBPZ-DPXZ, pDTBPZ-DPXZ, and oDTBPZ-DPXZ, are designed and synthesized with same donor-acceptor (D-A) backbone with different peripheral groups attaching on the A moieties. Their lowest triplet states change from locally excited to charge transfer character leading to significantly enhance reverse intersystem crossing process. In particular, oDTBPZ-DPXZ exhibits efficient TADF feature and exciton utilization. It not only achieves an external quantum efficiency (EQE) of 20.1 % in red vacuum-processed OLED, but also realize a high EQE of 18.5 % in a solution-processed OLED, which is among the best results in solution-processed red TADF OLEDs. This work provides an effective strategy for designing red TADF molecules by managing energy level alignments to facilitate the up-conversion process and thus enhance exciton harvesting.

High-Performance, Solution-Processable Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes Realized via the Adjustment of the Composition of the Organoboron Acceptor Monomer in Copolymer Host Materials

Organic polymers that exhibit features pertinent to functioning as host materials for thermally activated delayed fluorescence (TADF) emitters have considerable potential in solution-processable organic light-emitting diodes (OLEDs), allowing simple, low-cost, and large-area applications. In particular, polymer hosts have superior characteristics, including facile functionality to introduce various electron donor and acceptor entities, the ability to uniformly disperse and contain small molecular dopants, and the ability to produce more smooth and homogeneous films, compared to those of their small-molecule counterparts. This manuscript describes the design and development of three new styrene-based copolymers (ABP91, ABP73, and ABP55) bearing diphenylacridine as the electron donor and 2,12-di-tert-butyl-7-phenyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene as the electron acceptor. In particular, ABP91, ABP73, and ABP55 were synthesized via variations in the ratio of donor to acceptor monomers to substantiate their influence in OLED applications. With the ability of the styrene backbone of interrupting the direct electronic coupling between the adjacent electron donor and acceptor entities through non-conjugated linkages, high triplet energy can be inherited by the resulting polymers (>2.70 eV). Furthermore, these materials manifest thermal robustness through high decomposition temperatures (between 348 and 366 °C) and high glass transition temperatures (between 234 and 277 °C). Consequently, solution-processable OLEDs fabricated using the newly synthesized copolymers as host materials and the familiar t4CzIPN as a green-emissive TADF dopant deliver state-of-the-art performance with maximum external quantum efficiencies of 21.8, 22.2, and 19.7% for ABP91, ABP73, and ABP55, respectively. To our knowledge, this is, to date, the best performance reported when organic polymers are used as host materials in solution-processable TADF OLEDs. The pragmatic outcomes obtained in this study can provide useful insights into the structure-property relationship to the OLED community for the further development of efficient polymer hosts for use in solution-processable TADF OLEDs.

Alkoxy encapsulation of carbazole-based thermally activated delayed fluorescent dendrimers for highly efficient solution-processed organic light-emitting diodes

Two n-butoxy-encapsulated dendritic thermally activated delayed fluorescent (TADF) emitters (namely O-D1 and O-D2) with the first-/second-generation carbazoledendrons are designed and synthesized via CN coupling between carbazoledendrons and 2,4,6-tris(4-bromophenyl)-1,3,5-triazine core. It is found that, compared with the commonly-used tert-butyl groups, the use of n-butoxy encapsulation groups can lead to smallersinglet-triplet energy gap for the dendrimers, producing stronger TADF effect together with faster reverse intersystem crossing process. Solution-processed TADF organic light-emitting diodes (OLEDs) utilizingalkoxy-encapsulated dendrimers O-D1 and O-D2 as emitters exhibitstate-of-the-art device efficiency withthe maximum external quantum efficiency up to 16.8% and 20.6%, respectively, which are ∼1.6 and ∼2.0 times that of the tert-butyl-encapsulated counterparts. These results suggest that alkoxy encapsulation of the carbazole-based TADF dendrimers can be a promising approach for developing highly efficient emitters for solution-processed OLEDs.

High-Performance Solution-Processed Red Thermally Activated Delayed Fluorescence OLEDs Employing Aggregation-Induced Emission-Active Triazatruxene-Based Emitters

Two novel red thermally activated delayed fluorescence (TADF) emitters (TAT-DBPZ and TAT-FDBPZ) were developed by incorporating triazatruxene (TAT) as the electron donor (D) and dibenzo[a,c]phenazine (DBPZ) or fluorine-substituted dibenzo[a,c]phenazine (FDBPZ) as the electron acceptor (A). Both compounds showed aggregation-induced emission (AIE) behaviors and bright red emission in neat films. Benefited from the rigid and large planar conjugated structures of TAT and DBPZ, TAT-DBPZ and TAT-FDBPZ realized high photoluminescence quantum yields (PLQYs) in solid states. Meanwhile, the large steric hindrance between TAT and DBPZ segments produced small singlet-triplet energy splitting (EST), leading to short delayed fluorescence lifetimes and high reverse intersystem crossing (RISC) rate (106 s-1) for both compounds. The solution-processable doped organic light-emitting diodes (OLEDs) based on TAT-DBPZ achieved a high external quantum efficiency (EQE) of 15.4% with a red emission peak at 604 nm, which was one of the highly efficient solution-processable red TADF OLEDs. TAT-FDBPZ based doped devices also showed a red emission peak at 611 nm with a maximum EQE of 9.2% and low efficiency roll-off ratios of 1.0% at 100 cd m-2 and 19% at 1000 cd m-2. Furthermore, their solution-processable non-doped devices displayed EQEs of 5.6% and 2.9% with the red-shifted emission peaks at 626 nm and 641 nm, respectively. These results indicate the huge potential of utilization of triazatruxene as donor unit to achieve highly efficient and low efficiency roll-off solution-processable red TADF OLEDs.

Molecular engineering by σ-Bond spacer enables solution-processable host materials for TADF emitter towards high-performance OLEDs

High-performance solution-processable host materials for thermally activated delayed fluorescence (TADF) emitters are of great significance to the development of organic light-emitting diodes (OLEDs). In this study, three solution-processable hosts, namely mCP-Ph, mCP-Py, and mCP-BmPy featuring a basic structure paradigm of donor-σ-acceptor (D-σ-A) are reported for green TADF OLEDs. Attributing to the well-designed structure, the three host materials exhibited high triplet energy levels and excellent solution processability. It was found that the host materials had a significant impact on the excited state characteristics of the TADF emitter, and the mCP-Ph host with the lowest dipole moment showed the highest intersystem crossing rate constants (kISC and kRISC). In consequence, solution-processed TADF OLEDs based on the emitter 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (DMAC-PCN) and the host 9-(3-(9H-carbazol-9-yl)phenyl)-3-(2,7-di-tert-butyl-9-phenyl-9H-fluoren-9-yl)-9H-carbazole (mCP-Ph) secured a maximum external quantum efficiency of 21.6%, demonstrating the effectiveness of the host engineering by σ-spacer. This work reveals the important principle for designing bespoke host for TADF emitters.

Synergetic interface and morphology modification to achieve highly efficient solution-processed sky-blue organic light-emitting diodes

Abstract Solution-processed organic light-emitting diodes (OLEDs), especially blue OLEDs, generally suffer from the low efficiency. Herein, we report an efficient approach to achieve high efficiency by synergetic interface and morphology modification with a polymer, Poly(9-vinylcarbazole) (PVK). Sky-blue thermally activated delayed fluorescent (TADF) material, 10-(4-(4,6-diphenyl-1,3,5- triazin-2-yl)phenyl)-10H- spiro[acridine-9,9′-fluorene] (SpiroAC-TRZ), is used as the emitter. The incorporation of PVK thin layer at the hole injection layer/emission layer (EML) interface and a small amount of PVK inner the EML modify the carrier behaviors at the interface and improve the EML morphology. As a result, balanced carrier distribution and reduced carrier recombination are realized at the interface and inner the EML. Through these strategies, the maximum external quantum efficiency and current efficiency of the optimal OLED achieve 25.1% and 53.5 cd/A. To the best of our knowledge, the efficiencies are the highest values ever achieved by the solution-processed sky-blue TADF OLEDs.

Cyclohexane-cored dendritic host materials with high triplet energy for efficient solution-processed blue thermally activated delayed fluorescence OLEDs

Two dendritic host materials (CH-2D1 and CH-2D2) are developed by using non-conjugated cyclohexane core and two first/second generation carbazole dendrons, which show excellent solubility in organic solvents, high thermal stability and high triplet energy (ET) of 2.78–2.84 eV, making them suitable for fabrication of solution-processed blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). Compared to CH-2D1 with the first-generation carbazole dendron which shows low-lying highest occupied molecular orbital (HOMO) energy level of −5.48 eV, CH-2D2 with the second-generation carbazole dendrons exhibits higher HOMO level of −5.37 eV, which is more favourable for hole injection from the anode to the emissive layers. Consequently, solution-processed blue TADF OLEDs utilizing CH-2D2 as host show promising device performance with a low turn-on voltage of 3.4 V, maximum external quantum efficiency of 17.8%, maximum luminous efficiency of 41.3 cd A−1 and power efficiency of 33.9 lm W−1, which can compare with the most efficient solution-processed blue TADF OLEDs based on small-molecule and polymer hosts. These results indicate that dendritic host materials with well-defined chemical structure and good solubility are an attractive approach―beyond the soluble small-molecule hosts and polymer hosts―for the development of efficient solution-processed TADF OLEDs.

Highly Efficient Solution-Processed Thermally Activated Delayed Fluorescence Bluish-Green and Hybrid White Organic Light-Emitting Diodes Using Novel Bipolar Host Materials.

Two pyridine-containing bipolar host materials with high triplet energy, 9,10-dihydro-9,9-dimethyl-10-(3-(6-(3-(9,9-dimethylacridin-10(9H)-yl)phenyl)pyridin-2-yl)phenyl acridin (DDMACPy) and N-(3-(6-(3-(diphenyl amino)phenyl)pyridin-2-yl)phenyl)-N-phenylbenzenamine (DTPAPy), are synthesized from the modification of the commonly adapted host material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (DCzPPy). The highest occupied molecular orbital levels of DDMACPy (5.50 eV) and DTPAPy (5.60 eV) are found to be shallower than that of DCzPPy (5.90 eV) that leads to the improvement in hole injection from the hole transport layer PEDOT:PSS (WF = 5.10 eV). These host materials are used in the emitting layer of bluish-green organic light-emitting diode (OLED) with the thermally activated delayed fluorescence (TADF) emitter, 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine, as the guest. The DDMACPy-based device shows the highest performance among them, with the maximum external quantum efficiency (EQEmax), current efficiency (CEmax), and power efficiency (PEmax) of 21.0%, 53.1 cd A-1, and 44.0 lm W-1 at CIE (0.17, 0.42), respectively. By further doping with the red-emitting phosphor iridium(III) bis(2-phenylquinoline)(2,2,6,6-tetramethylheptane-3,5-ionate) [Ir(dpm)PQ2] and yellow-emitting phosphor iridium(III) bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C20)acetylacetonate (PO-01-TB) emitters into the bluish-green emitting layer, a TADF-phosphor hybrid white OLED (T-P WOLED) is obtained with excellent EQEmax, CEmax, and PEmax of 17.4%, 48.7 cd A-1, and 44.5 lm W-1 at CIE (0.35, 0.44), respectively. Moreover, both the bluish-green and WOLED show a low efficiency roll-off with external quantum efficiencies at the brightness of 1000 cd m-2 (EQE1000) of 18.7 and 16.2%, respectively, which are the highest performance records among the solution-processed TADF bluish-green and T-P WOLEDs.