Thermally Activated Delayed Fluorescence Host Research Articles

HLCT-Type Acceptor Molecule-Based Exciplex System for Highly Efficient Solution-Processable OLEDs with Suppressed Efficiency Roll-Offs.

Exciplex systems are promising candidates for thermally activated delayed fluorescence (TADF) molecules because of the small energy difference between the lowest singlet and triplet excited states (ΔEST). However, realizing high-efficiency and low-external-quantum-efficiency (EQE) roll-off in solution-processed organic light-emitting diodes (OLEDs) using an exciplex system remains a formidable challenge. In this study, two (HLCT)-type isomers with a spiro skeleton, 2-tBuspoCz-TRZ and 10-tBuspoCz-TRZ, are designed and synthesized as acceptors of exciplexes, where tert-butylspirofluorene indole is regarded as a donor and the triazine unit as an acceptor. Green exciplex emissions are observed for the 2-tBuspoCz-TRZ:TAPC and 10-tBuspoCz-TRZ:TAPC exciplexes, indicating distinct TADF characteristics with a very small ΔEST of 35 ± 5 meV. By using the TADF exciplex system based on the HLCT acceptor as an emitter, solution-processable OLEDs achieve a maximum external quantum efficiency (EQEmax) of 20.8%. Furthermore, a high EQEmax > 25% with a very low-efficiency roll-off (≈3.5% at 1000cd m-2) is obtained for solution-processable phosphorescent devices using HLCT-based exciplexes as the host matrix of phosphors. This study paves the way for a novel strategy for designing acceptor exciplex molecules for effective TADF molecules and host matrices in solution-processable OLEDs.

Low-trigger voltage and efficient phosphorescent organic light-emitting diodes with bipolar TADF host

Thermally activated delayed fluorescence (TADF) materials have continued to attract interest due to their rapid improvements in efficiency, non-toxic and potential to be manufactured at energy-saving organic light-emitting diode for full-color display and solid-state lighting. Here, The bipolar and electroluminescence (EL) characteristics of TADF host, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), were investigated. By incorporating the phosphorescent tris[2-(p-tolyl)pyridine]iridium(III) [Ir(mppy)3] dopant into the matrix, DPDDC showed more excellent EL performance compared with the typical 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host. The turn-on voltage for phosphorescent organic light-emitting diodes (PhOLEDs) was decreased to 2.2 V from 2.5 V by exploiting the TADF host instead of CBP. Crucially, we found that the ratio of Ir(mppy)3 in TADF host plays a significant role in EL process. Remarkable low efficiency roll-off was achieved with above 90% of the current efficiency maintained even at a brightness of 3000 cd m−2, along with high power efficiency of 84.8 lm·W−1 and the external quantum efficiency of 20.6% for DPDDC: 5 wt% Ir(mppy)3-based device.

Cluster Light‐Emitting Diodes Containing Copper Iodine Cube with 100 % Exciton Utilization Using Host‐Cluster Synergy

AbstractClusters combine the advantages of organic molecules and inorganic nanomaterials, which are promising alternatives for optoelectronic applications. Nonetheless, recently emerged cluster light‐emitting diodes require further excited state optimization of cluster emitters, especially to reduce population of the cluster‐centered triplet quenching state (3CC). Here we report that redox‐active ligands enhance reverse intersystem crossing (RISC) of Cu4I4 cluster for triplet‐to‐singlet conversion, and thermally activated delayed fluorescence (TADF) host can provide an external RISC channel. It indicates that the complementarity between TADF host and cluster in RISC transitions gives rise to 100 % triplet conversion efficiency and complete singlet exciton convergence, rendering 100‐fold increased singlet radiation rate constant and tenfold decreased triplet non‐radiation rate constant. We achieve a photoluminescence quantum yield of 99 % and a record external quantum efficiency of 29.4 %.

Cluster Light-Emitting Diodes Containing Copper Iodine Cube with 100% Exciton Utilization Using Host-Cluster Synergy.

Clusters combine the advantages of organic molecules and inorganic nanomaterials, which are promising alternatives for optoelectronic applications. Nonetheless, recently emerged cluster light-emitting diodes require further excited state optimization of cluster emitters, especially to reduce population of the cluster-centered triplet quenching state (3CC). Here we report that redox-active ligands enhance reverse intersystem crossing (RISC) of Cu4I4 cluster for triplet-to-singlet conversion, and thermally activated delayed fluorescence (TADF) host can provide an external RISC channel. It indicates that the complementarity between TADF host and cluster in RISC transitions gives rise to 100% triplet conversion efficiency and complete singlet exciton convergence, rendering 100-fold increased singlet radiation rate constant and tenfold decreased triplet non-radiation rate constant. We achieve a photoluminescence quantum yield of 99% and a record external quantum efficiency of 29.4%.

Thermally activated delayed fluorescent small molecule sensitized fluorescent polymers with reduced concentration-quenching for efficient electroluminescence

Thermally activated delayed fluorescence (TADF) small molecule bis-[3-(9,9-dimethyl-9,10-dihydroacridine)-phenyl]-sulfone (m-ACSO2) was used as a universal host to sensitize three conventional fluorescent polymers for maximizing the electroluminescent performance. The excitons were utilized via inter-molecular energy transfer and the non-radiative decays were successfully refrained in the condensed states. Therefore, the significant enhancement of the electroluminescent efficiencies was demonstrated. For instance, after doping poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) into m-ACSO2, the external quantum efficiency (EQE) was improved by a factor of 17.0 in the solution-processed organic light-emitting device (OLED), as compared with the device with neat F8BT. In terms of the other well-known fluorescent polymers, i.e., poly (para-phenylene vinylene) copolymer (Super Yellow, SY) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), their EQEs in the devices were respectively enhanced by 70% and 270%, compared with the reference devices based on the conventional host 1,3-di(9H-carbazol-9-yl) benzene (mCP). Besides the improved charge balance in the bipolar TADF host, these were partially ascribed to reduced fluorescence quenching in the mixed films.Graphical abstract

Thermally activated delayed fluorescence dendrimers with AIE property and functional dendrons for high-efficiency solution-processed white OLEDs

Thermally activated delayed fluorescence (TADF) materials with encapsulation dendrons have exhibited superior advantages in the solution-processed OLEDs. Herein, we utilized electron-donating carbazole and electron-withdraw phenylbenzimidazole as the peripheral wrapping unit to design and synthesize two TADF materials, TRZCz-Cz and TRZCz-Bz, to discuss the distinction about different encapsulations for one identical emissive core. The photophysical investigation demonstrate that both compounds have small ΔE ST and clearly aggregation-induced enhancement (AIE) properties, which are benefit for enhancing the exciton unitization of electroluminescence devices. Despite similar characters in photophysical property, the blue phosphorescence OLEDs host with TRZCz-Cz obtain the maximum current efficiency and external quantum efficiency of 38.6 cd A −1 and 18.4%, respectively, which far exceed TRZCz-Bz based one. Thanks to the high performance of blue device, the fully phosphorescent white OLEDs with Ir(MDQ) 2 (acac) as red phosphorescent dopant achieves the maximum luminescence, current efficiency, external quantum efficiency as high as 12874 cd m −2 , 33.8 cd A −1 and 16.5%, respectively, which are among the highest efficiency of solution-processed white OLEDs. Moreover, the hybrid white OLEDs with TADF as blue host and red phosphor as dopant was also achieved with a low turn-on voltage of 3.2 V, and more standard white emission with CIE coordinate of (0.34,0.33). Thus, this work significantly extends the range of AIE TADF materials and paves the useful way for the fabrication of high efficiency white OLEDs through solution-process. • The optimized wrapping unit enhance the device performance almost two times higher. • The TADF host ensure the solution-processed blue phosphorescence OLEDs with the EQE max of 18.4%. • Both the fully phosphorescent and hybrid white OLEDs achieves high luminescence and stable color purity.

Molecular glasses based on 1,8-naphthalimide and triphenylamine moieties as bipolar red fluorescent OLED emitters with conventional versus TADF hosting

Three new donor–acceptor molecular glasses were designed and synthesized linking 1,8-naphthalimide and triphenylamino groups though the different bridges. The comprehensive characterization of the compounds was carried out using theoretical and experimental approaches. The compounds showed efficient orange-red emission in solid state with photoluminescence intensity maxima in the range of 584–654 nm. The compounds showed extremely high thermal stability with 5 % weight loss temperatures up to 477 °C. They formed molecular glasses with glass-transition temperatures in the range of 161–186 °C. The fabricated organic light-emitting diodes (OLEDs) based on the developed emitters and conventional host showed maximum external quantum efficiency of 2.5 % in the best case. This value was increased up to 4.7 % by the usage of the host exhibiting thermally activated delayed fluorescence (TADF). OLED containing the TADF host displayed orange emission peaking at 589 nm with colour coordinates x of 0.53 and y of 0.45 combined with power efficiency of 6.7 lm·W−1 and current efficiency of 11.8 cd·A-1. Time-resolved electroluminescence technique was used to study the effect of the different guest–host systems on exciton utilization efficiency in devices based on the same emitter exhibiting prompt fluorescence and on the conventional or TADF hosts.

New bipolar host materials for high power efficiency green thermally activated delayed fluorescence OLEDs

Four bipolar molecules, namely m-CzPym, p-CzPym, m-CzTrz, and p-CzTrz, with carbazole (Cz) donor and a benzonitrile-substituted pyrimidine (Pym) or triazine (Trz) acceptor core were synthesized and characterized. The electron deficiency of heteroaryl cores together with the substitution pattern of benzonitrile were employed to tune the energy levels as well as the thermally activated delayed fluorescence (TADF) characteristics. The four molecules exhibited TADF behavior with inferior photoluminescent quantum yields (PLQYs) that limit their applications as emitters. These bipolar molecules were employed as TADF host materials for the benchmark TADF emitter 4CzIPN to achieve high-performing green TADF organic light-emitting diodes (OLEDs). Among the molecules, m-CzPym-hosted TADF-OLEDs achieved a maximum external quantum efficiency (EQEmax) of 31.5%, maximum power efficiency (PEmax) of 95.6 lm/W, and maximum current efficiency (CEmax) of 100.2 cd/A. Notably, p-CzPym-hosted TADF-OLEDs also achieved a PEmax of 116.5 lm/W, turn-on voltage of 2.5 V, and impressive low efficiency roll-off performance (>89% of EQEmax at 5000 cd/m2), representing one of the highest efficiencies ever reported in 4CzIPN-doped devices. The high device efficiency can be ascribed to the balanced ambipolar carrier-transporting character of the host materials and high PLQY as well as the outstanding light outcoupling efficiency of the emitting layer.

Achieving High Efficiency and Pure Blue Color in Hyperfluorescence Organic Light Emitting Diodes using Organo‐Boron Based Emitters

AbstractIn the field of organic light‐emitting diodes, organo‐boron based thermally activated delayed fluorescence (TADF) emitters have witnessed outstanding achievements. However, it is still challenging to achieve pure blue color (CIE y < 0.20) along with high efficiencies. To overcome these hurdles, the hyperfluorescence (HF) system suggests a key strategy for future display applications. Here, two TADF host materials, pMDBA‐DI and mMDBA‐DI, and a pure blue multi‐resonance type tert‐butyl substituted TADF fluorescence emitter t‐Bu‐ν‐DABNA are reported, for efficient HF devices. Among the synthesized TADF sensitized host materials, the mMDBA‐DI HF device exhibits a high external quantum efficiency of 39.1% along with narrow emission with full width at half maximum of 19 nm (CIE y = 0.15). The high device efficiency is mainly attributed to the high molecular orientation factor, enhanced photoluminescence quantum yield, and a good TADF characteristic of t‐Bu‐ν‐DABNA with efficient Förster energy transfer.

Thermally Activated Delayed Fluorescence Warm White Organic Light Emitting Devices with External Quantum Efficiencies Over 30%

AbstractWhile monochrome organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have achieved over 30% external quantum efficiencies (EQEs), all‐TADF white OLEDs (WOLEDs) are still lagging behind. Herein, a simple system based on two color‐complementary TADF emitters is exploited to realize high‐performance WOLEDs. By doping a high‐performance orange–red TADF fluorophor (BPPZ‐DPXZ) into a blue TADF host (DBFCz‐Trz), energy transfer, and triplet‐to‐singlet conversion in the host‐dopant system can be optimized to simultaneously achieve full exciton utilization and color balance. With this design, all‐TADF single‐emitting‐layer WOLEDs with a maximum EQE up to 32.8% are demonstrated. This high efficiency surpasses EQEs of reported WOLEDs based on both TADF as well as phosphorescence. It is expected that this finding can provide new insight for designing highly efficient all‐TADF WOLEDs.

Noble Dibenzothiophene-Based Bipolar Hosts for Blue Organic Light-Emitting Diodes Using Thermally Activated Delayed Fluorescence.

Novel thermally activated delayed fluorescence (TADF) host materials for blue electrophosphores-cence were designed by combining the electron acceptor dibenzothiophene (DBT) unit and the electron donor acridine derivatives into a single molecular unit by density functional theory (DFT). Depending on the optimal charge transfer, DFT and time-dependent DFT (TD-DFT) calculations for the ground state were performed to obtain the energy of the singlet (S1) and triplet (T1) excited states of the TADF material for Hartree-Fock percentage of TD-DFT. The sufficiently large separation between the HOMO and LUMO resulted in a small difference in energy (ΔEST) between the S1 and T1 states using DFT and TD-DFT calculations. The host molecules retained high triplet energy and showed great potential for use in blue organic light-emitting diodes (OLED). The results showed that these molecules are a good TADF host materials because they have a low barrier to hole and electron injection with a balanced charge transporting property for both holes and electrons, and a small ΔEST.

P‐168: High Efficiency (EQE>30%) TADF‐OLED with Lightly‐doped Emitter (0.5%) by using TADF‐Host

High efficiency (EQE=30.15%) organic light emitting diode (OLED) was demonstrated by lightly doping the thermally activated delayed fluorescence (TADF) emitter (0.5%) into the TADF host, resulting from: (1) efficient energy transfer from host to dopant, (2) long exciton diffusion length in host, and (3) rare concentration quenching due to lightly‐doped condition.

Highly Charge Transport Thermally Activated Delayed Fluorescence Host Materials Based on Benzimidazole-Acridine Derivatives.

We designed novel thermally activated delayed fluorescence (TADF) host molecules for blue electrophosphorescence by combining the electron acceptor benzimidazole (BI) unit and the electron donor acridine derivatives into a single molecular unit based on density functional theory (DFT). We obtained the energies of the first singlet (S1) and the first triplet (T1) excited states of the TADF materials by performing DFT and time-dependent DFT (TD-DFT) calculations to the ground state using dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. Using DFT and TD-DFT calculations, the large separation between the HOMO and LUMO caused a small difference in energy (ΔEST) between the S1 and T1 states. The host molecules retained high triplet energy and showed great potential for use in blue phosphorescent organic light-emitting diodes. The results showed that these molecules are promising TADF host materials because they have a low barrier to hole and electron injection, balanced charge transport for both holes and electrons, and a small ΔEST.

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.

High-performance sky-blue phosphorescent organic light-emitting diodes employing wide-bandgap bipolar host materials with thermally activated delayed fluorescence characteristics

Abstract Up to date, several kinds of thermally activated delayed fluorescence (TADF) host molecules have been rapidly developed to fulfill the increasing demand of high-efficiency phosphorescent organic light-emitting diodes (PHOLEDs). Herein, three wide-bandgap TADF materials BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz are newly designed and synthesized as host materials for sky-blue PHOLEDs. By modulation of carbazole-units in the molecular skeleton, we find that the optimized molecules can realize both high triplet energies and balanced carrier transport properties with obvious TADF properties. The sky-blue PHOLEDs with ultralow phosphor doping ratio employing BPCN-Cz2Ph, BPCN-2Cz, and BPCN-3Cz as host materials demonstrate really high performance with alleviated roll-offs and achieve impressive current efficiencies (CEs) of 46.5 cd A−1, 50.0 cd A−1, and 34.5 cd A−1, power efficiencies (PEs) of 40.6 lm W−1, 46.8 lm W−1, and 34.8 lm W−1, and external quantum efficiencies (EQEs) of 22.2%, 24%, and 15.8%, respectively. More encouragingly, even at an applied luminance of 1000 cd m−2, the EQEs could maintain high levels of 21.0%, 21.5% and 12.8%, respectively, which represents the potential utility value of the new family of these molecules in blue PHOLEDs. Our molecular design strategy provides a practical way to construct promising wide-bandgap TADF host materials with an excellent combination of TADF property, high triplet energy, and balanced carrier-transporting ability for high-performance devices.

An effective thermally activated delayed fluorescence host material for highly efficient blue phosphorescent organic light-emitting diodes with low doping concentration

A thermally activated delayed fluorescence (TADF) material BCz-2SO has been designed and synthesized as host for blue phosphorescent organic light-emitting diodes (OLEDs). Photophysical studies and theoretical calculations show that the molecule has a small singlet-triplet energy gap (ΔEST) of 0.345 eV, which is beneficial to the reverse energy transferring between the singlet and triplet state. Thanks to the TADF property, the triplet energy can be transmitted to the singlet state through reverse intersystem crossing (RISC), and then transmitted to the guest through the Förster energy transfer (FET) to achieve 100% utilization of energy. Thus, the triplet–triplet annihilation (TTA) of the blue phosphor can be avoided by the extremely low doping concentration of 1%. By using BCz-2SO as the host of FIrpic, the solution-processed blue phosphorescent device achieves the maximum current efficiency (CE), power efficiency (PE), external quantum efficiency (EQE) and highest brightness of 16.38 cd A−1, 9.04 lm W−1, 7.8% and 16,537 cd m-2, respectively. It demonstrates that one can employ the solution-processed method to prepare the high performance phosphorescent OLEDs using the TADF host material we have developed.

Highly efficient exciplex organic light-emitting diodes by exciplex dispersion in the thermally activated delayed fluorescence host

Abstract In this work, we describe a new approach of improving the external quantum efficiency (EQE) of exciplex organic light-emitting diodes (OLEDs). A new approach of dispersing the exciplex in the thermally activated delayed fluorescence (TADF) host was developed to increase the EQE of the exciplex OLEDs. The new emitting layer structure was made up of a mixture of TADF material and an n-type material which can generate exciplex with the TADF material. The content of the n-type material was maintained below 10 wt % to have a morphology with the exciplex dispersed in the TADF matrix material. The emitting layer structure with the exciplex dispersed in the TADF host suppressed exciton quenching and improved the photoluminescence quantum yield of the exciplex. As a result, the exciplex OLEDs with the n-type material content of 1 wt% showed high EQE of 15.3% compared with EQE of 10.8% of the conventional exciplex OLEDs with the n-type material content of 50%.

Molecular Design of Triazole Based Thermally Activated Delayed Fluorescence Hosts for Blue Electrophosphorescence.

We designed novel thermally activated delayed fluorescence (TADF) host molecules for blue electrophosphorescence by combining electron donor acridine derivatives and the electron acceptor 3,4,5-triphenyl-1,2,4-triazole (TPT) unit into a single molecule based on density functional theory (DFT). We obtain the energies of the first singlet (S1) and first triplet (T1) excited states of TADF materials by performing DFT and time-dependent DFT (TD-DFT) calculations on the ground state using dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. The host molecules retained high triplet energy and showed great potential for use in blue phosphorescent organic light-emitting diodes. The results showed that these molecules are promising TADF host materials because they have a low barrier to hole and electron injection, a balanced charge transport for both holes and electrons, and a small energy difference (ΔEST) between the S1 and T1 states.

Highly Charge Transport Thermally Activated Delayed Fluorescence Host Materials Based on Phenyl-Biscarbazolyl Derivatives.

We designed novel thermally activated delayed fluorescence (TADF) host molecules for blue electrophosphorescence by combining the electron donor 1,3-Bis(N-carbazolyl)benzene (mCP) unit and the electron acceptor diphenylphosphine oxide-4-(triphenylsilyl) phenyl (TSPO1) unit in a single molecule based on density functional theory. We obtained the energies of the first singlet (S1) and the first triplet (T1) excited states of the TADF materials by performing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to the ground state using dependence on charge transfer amounts for the optimal Hartree-Fock percentage in the exchange-correlation of TD-DFT. Using DFT and TD-DFT calculations, the large separation between the HOMO and LUMO caused a small difference in energy (ΔEST) between the S1 and T1 states. The host molecules retained high triplet energy and showed great potential for use in blue phosphorescent organic light-emitting diodes. The results showed that these molecules are promising TADF host materials because they have a low barrier to hole and electron injection, balanced charge transport for both holes and electrons, and small ΔEST.

Understanding and Manipulating the Interplay of Wide‐Energy‐Gap Host and TADF Sensitizer in High‐Performance Fluorescence OLEDs

Comprising an emitting layer (EML) constituting a wide-energy-gap host, a thermally activated delayed fluorescence (TADF) sensitizer and a conventional fluorescent dopant, TADF-sensitizing-fluorescence organic light-emitting diodes (TSF-OLEDs) highly depend on component interplay to maximize their performance, which, however, is still under-researched. Taking the host type (TADF or non-TADF) and the recombination position (on the host or on the TADF sensitizer) into consideration, the interplay of host and TADF sensitizer is comprehensively studied and manipulated. A wide-energy-gap host with TADF and recombination of charges on it are both required to maximize device performances by triggering multiple sensitizing processes to eliminate exciton losses. Based on those findings, a maximum external quantum efficiency (EQE)/power efficiency (PE) of 23.2%/76.9 lm W-1 is realized with a newly developed TADF host, significantly outperforming the reference devices. Further device optimization leads to unprecedently high EQE/PE of 24.2%/89.5 lm W-1 and a half-lifetime of over 400 h at an initial luminance of 2000 cd m-2 , with the peak PE being the highest value among the reported TSF-OLEDs. This work reveals the importance of manipulating the component interplay in EMLs, opening a new avenue toward highly efficient TSF-OLEDs.