Reverse Intersystem Crossing Process Research Articles

Fine tuning of excited states by trifluoromethylphenyl for blue-shifted color and enhanced EQEs in thermally activated delayed fluorescence emitters

3,6-di-tert-butyl-9H-carbazole and 3,6-diphenyl-9H-carbazole have been widely used to construct blue thermally activated delayed fluorescence (TADF) emitters. However, it is difficult for related emitters to achieve maximum external quantum efficiency (EQEmax) above 20 % with the Commission Internationale de l'Eclairage (CIE) y ≤ 0.12. A new type of donor was developed to construct blue TADF molecules, namely CF3P-BuCz-TRZ and CF3P-PhCz-TRZ. The weak electron-withdrawing trifluoromethylphenyl (CF3P) with moderate steric hindrance was introduced into the 1-site of carbazole donor for improving the color purity for blue TADF emitters. More importantly, the introduction of CF3P tightly compacts the energy levels for the significant excited states in RISC process. The energy gap between the lowest singlet excited (S1) state and the lowest triplet (T1) state (Δ E(S1−T1)), as well as between T1 and the higher triplet (Tn, n ≥ 2) states (Δ E(Tn−T1)) were reduced, so that Tn (n ≥ 2) could be effectively harvested to further promote reverse intersystem crossing (RISC). As a result, the OLEDs based on CF3P-BuCz-TRZ and CF3P-PhCz-TRZ displayed blue electroluminescence with CIE coordinates of (0.156, 0.107) and (0.155, 0.115). In particular, the maximum EQEs of the CF3P-PhCz-TRZ device reached 20.43 % without employing any light out-coupling enhancement or TADF-sensitized structure. This strategy should be valuable for designing more pure-blue and deep-blue TADF emitters.

Modulating the Locally Excited States with a Regulating Substituent for Highly Efficient Red/Near‐Infrared Thermally Activated Delayed Fluorescence Emitters

AbstractDeveloping highly efficient red/near‐infrared (NIR) thermally activated delayed fluorescence (TADF) materials is important for organic light‐emitting diodes (OLEDs). Here, two TADF emitters, APTT and APTI, which have the same D/A backbone but different attaching groups at acenaphtho‐[1,2‐b]pyrazine‐8,9‐dicarbonitrile (APDC) core, are reported. The appended regulating groups can not only suppress the D/A rotation due to the space confinement effects but also modulate the locally excited triplet state (3LE). The improved molecular rigidity suppresses the non‐radiative process, accounting for the improved photoluminescence quantum yields (PLQYs), while the modulated 3LE promotes the reverse intersystem crossing (RISC) process due to the high utilization efficiency of triplets. Consequently, both APTT and APTI demonstrate high PLQY and fast RISC process, thereby enhancing TADF efficiency. The doped devices based on APTT and APTI achieve maximum external quantum efficiency (EQEmax) values of 20.5% and 25.4% with emission peaks at 664 and 670 nm, respectively. The non‐doped devices of APTT and APTI achieve the EQEmax of 2.8% and 2.9% with emission peaks at 788 and 794 nm, respectively. Encouragingly, the non‐doped devices of APTI have set new records for near‐infrared TADF OLEDs based on the APDC core. This study provides an efficient approach to modulating the optoelectronic properties of highly efficient NIR TADF OLEDs.

Advancing Triplet Exciton Harvesting Through Heavy Atom Selenium Manipulation in Multiple Resonance Thermally Activated Delayed Fluorescent Emitters

AbstractIn the development of organic light‐emitting diodes (OLEDs) with high efficiency and minimal efficiency roll‐off, fast reverse intersystem crossing (RISC) in multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials is critical. The RISC process is typically hindered by insufficient spin‐orbital coupling (SOC). Incorporating heavy atom selenium into the MR‐TADF structure has the potential to enhance SOC through the heavy atom effect. However, the specific placement of selenium within the molecule results in different enhancements of SOC, with the detailed interplay between these factors yet to be elucidated. The introduction of a selenium‐containing moiety, phenoxaselenine, into the MR‐TADF structure at different substituted positions is undertaken, revealing that the molecule with 3‐substituted phenoxaselenine exhibits faster RISC transition and a significant increase in SOC between higher triplet excited states and S1 state, compared to the molecule with 2‐substituted phenoxaselenine. Significantly reduced efficiency roll‐off is achieved for the narrow‐band emission OLEDs based on the molecule with 3‐substituted phenoxaselenine owing to the enhanced heavy atom effect, giving an impressive external quantum efficiency above 20% even under 10 000 cd m−2 in the corresponding OLED device. These results underscore the potential of strategic heavy atom effect manipulation in MR‐TADF materials for efficient spin‐flipping.

Self-doping mechanism for thermally activated delayed fluorescence molecules: a theoretical perspective

Interesting properties of dual conformations of thermally activated delayed fluorescence molecules show various novel applications in self-doping systems. However, the species and quantities of self-doping systems are not enough to satisfy the requirements and a theoretical investigation to reveal the self-doping mechanism is highly desired. By analysing the potential energy surface, two stable conformations of TP2P-PXZ-qa (QA) and TP2P-PXZ-qe (QE) are determined. The basic geometric and electronic structures are optimised, frontier orbital distributions and energies are obtained, transition properties are analysed and the energy consumption processes of excited states are analysed. Results indicate that two conformations (QA and QE) coexist in the solid phase, the QA conformation acts as host and the QE conformation acts as guest. Efficient intersystem crossing (ISC) and reverse ISC processes are determined for QE conformation. In addition, a substantial overlap exists between the emission spectrum of the QA conformation and the absorption spectrum of the QE conformation, indicating an effective energy transfer process from host to guest. Thus, relationships between molecular structures and photophysical properties are illustrated and a self-doping mechanism is theoretically revealed. Our work rationalises the experimental observations and offers a theoretical perspective for the self-doping mechanism, contributing to the development of efficient non-doped emitters.

Triplet Excited-State Dynamics in Benzothiadiazole-Based Thermally Activated Delayed Fluorescence Compound.

The "hot exciton" thermally activated delayed fluorescence (TADF) materials have attracted considerable research interest for their utilization of high-lying triplet excitons. In this work, we reported the mechanism of photoluminescence by revealing the spectral evolution from singlet to triplet states in "hot exciton" TADF molecules by transient absorption (TA) spectra and triplet sensitization experiments. The internal conversion and intersystem crossing are much faster than reverse intersystem crossing (RISC), so that high-lying triplet states (Tn) are difficult to accumulate to be observed in the transient absorption spectra. In contrast, the emergence of delayed fluorescence in time-resolved emission spectra demonstrates the existence of a high-lying RISC process (hRISC) from Tn to S1. Triplet sensitization experiments successfully identified the spectral features of the T1 state in the TA spectra. This work sheds light on critical factors for the systematic design of these materials to achieve a high emission quantum yield.

Peripheral Selenium Modification of Multi‐Resonance Thermally Activated Delayed Fluorescence Molecules for High‐Performance Blue Organic Light‐Emitting Diodes

AbstractMulti‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules have attracted much attention in the academia owing to their unique photoelectrical properties. However, MR‐TADF emitters usually show slow reverse intersystem crossing (RISC) rate, resulting in high efficiency roll‐off of organic light‐emitting diodes (OLEDs) and seriously limiting their further development. Here, a peripheral selenium (Se) modification is presented for MR‐TADF molecules to promote the RISC process while keeping the narrowband emission for high‐performance blue OLEDs. Compared to the parent molecules (NBN and tBuNBN), SeNBN and SetBuNBN exhibited narrower full‐width at half maximum (FWHM) value of 23 nm and more obvious delayed fluorescence properties with a high efficiency of delayed fluorescence up to 86%, shorter delayed lifetime of 2.4 µs as well as a faster RISC rate of 3.34×105 s−1. Therefore, high‐performance OLEDs based on these two Se modified MR‐TADF emitters are achieved with a high maximum external quantum efficiency (EQE) up to 25.5% and extremely suppressed efficiency roll‐offs of 3.9% at 100 cd m−2 and 24.4% at 1000 cd m−2. This work demonstrated that the introduction of peripheral Se atom can achieve high‐performance organic semiconductors with both narrowband emission and fast RISC rate constant for high‐performance organic optoelectronic devices.

Enhancing Reverse Intersystem Crossing in Triptycene-TADF Emitters: Theoretical Insights into Reorganization Energy and Heavy Atom Effects.

Thermally activated delayed fluorescence (TADF) emitters based on the triptycene skeleton demonstrate exceptional performance, superior stability, and low efficiency roll-off. Understanding the interplay between the luminescent properties of triptycene-TADF molecules and their assembly environments, along with their excited-state characteristics, necessitates a comprehensive theoretical exploration. Herein, we predict the photophysical properties of triptycene-TADF molecules in a thin film environment using the quantum mechanics/molecular mechanics method and quantify their substantial dependency on the heavy atom effects and reorganization energies using the Marcus-Levich theory. Our calculated photophysical properties for two recently reported molecules closely align with experimental values. We design three novel triptycene-TADF molecules by incorporating chalcogen elements (O, S, and Se) to modify the acceptor units. These newly designed molecules exhibit reduced reorganization energies and enhanced reverse intersystem crossing (RISC) rates. The heavy atom effect amplifies spin-orbit coupling, thereby facilitating the RISC process, particularly at a remarkably high rate of ∼109 s-1.

Highly efficient ionic thermally activated delayed fluorescence emitters with short exciton lifetimes towards High-Performance Solution-Processed OLEDs

Ionic thermally activated delayed fluorescence (iTADF) materials are newly emerging as promising candidates for constructing solution-processed electroluminescence devices. Developing iTADF materials with high emission efficiency, short exciton lifetime, and fast reverse intersystem crossing (RISC) is crucial for achieving stable and efficient organic light-emitting diodes (OLEDs), but it remains challenging. Here we report two high-performance iTADF emitters, namely DMAC-TPPO[BF4] and DMAC-TPPS[BF4], which consist of a conventional acridine as the electron-donor and newly developed oxygen/sulfur-fused arylphosphonium derivatives as the electron-acceptor. DMAC-TPPO[BF4] and DMAC-TPPS[BF4] exhibit high photoluminescence quantum yields of 77 % and 83 %, coupled with short exciton lifetimes of 1.2 and 1.1 μs, and RISC rate constants reaching up to 2.60 × 106 and 2.73 × 106 s−1, respectively. Theoretical and experimental investigations demonstrate that the imbedding of bridging heteroatoms into the phosphonium cation is pivotal for enhancing emission efficiency through molecular skeleton rigidification, as well as for accelerating the RISC process by capitalizing on the non-metal heavy-atom effect. Eventually, solution-processed OLEDs utilizing DMAC-TPPS[BF4] realize a peak external quantum efficiency (EQE) of 17.8 % and show a minor efficiency roll-off of 7.3 % (EQE = 16.5 %) even at a practical high luminance of 1000 cd/m2.

Fully space-confined donor-acceptor interaction for highly efficient thermally activated delayed fluorescence emitters

The emerging through-space charge transfers (TSCT) emitters with thermally activated delayed fluorescence (TADF) have drawn increasing interest. However, the low luminescence efficiency and slow reverse intersystem crossing (RISC) always limit their device performances. A new molecular design strategy for TSCT-TADF emitters with a fully space-confined donor/acceptor conformation that integrates high luminescence efficiency and fast RISC is proposed herein. Two new TSCT-TADF emitters, TRZ-STFMe and TRZ-STFPh, are developed by introducing the methyl and phenyl substituents at the C2 of the rigid fluorene scaffold, respectively. The repulsive interaction between methyl and the acceptor induces a close and face-to-face donor–acceptor stacking to generate highly efficient TADF and suppress structural changes during the RISC process in TRZ-STFMe. In contrast, TRZ-STFPh shows relatively low luminescence efficiency and slow RISC because of its conformationally unstable phenyl substituent. With a high luminescence efficiency of 97 % and a significant RISC rate of 5.23 × 105 s−1, TRZ-STFMe demonstrates a high electroluminescence performance with a maximum external quantum efficiency (EQE) of 29.6 % and an EQE of 26.5 % at 1000 cd m−2.

Molecular Engineering Modulating the Singlet-Triplet Energy Splitting of Indolocarbazole-Based TADF Emitters Exhibiting AIE Properties for Nondoped Blue OLEDs with EQE of Nearly 20.

The development of efficient blue thermally activated delayed fluorescence (TADF) emitters with an aggregation-induced emission (AIE) nature, for the construction of organic light-emitting diodes (OLEDs), is still insufficient. This can be attributed to the challenges encountered in molecular design, including the inherent trade-off between radiative decay and reverse intersystem crossing (RISC), as well as small singlet-triplet energy splitting (ΔEST) and the requirement for high photoluminescence quantum yields (ΦPL). Herein, we present the design of three highly efficient blue TADF molecules with AIE characteristics by combining π-extended donors with different acceptors to modulate the differences in the electron-donating and electron-withdrawing abilities. This approach not only ensures high emission efficiency by suppressing close π-π stacking, weakening nonradiative relaxation, and enhancing radiative transition but also maintains the equilibrium ratio between the triplet and singlet excitons by facilitating the process of RISC. These emitters exhibit AIE and TADF properties, featuring quick radiative rates and low nonradiative rates. The ΦPL of these emitters reached an impressive 88%. Based on their excellent comprehensive performance, nondoped PICzPMO and PICzPMO OLEDs achieved excellent electroluminescence performance, exhibiting maximum external quantum efficiency (EQEmax) of up to 19.5%, while the doped device achieved a higher EQEmax of 20.8%. This work demonstrated that by fusing π-extended large rigid donors with different acceptors, it is possible to regulate the difference in electron-donating and electron-withdrawing abilities, resulting in a small ΔEST, high ΦPL, and fast RISC process, which is a highly feasible strategy for designing efficient TADF molecules.

Molecular design for high exciton utilization based donor-π-acceptor type fluorescent emitter for OLEDs application

The “hot exciton” mechanism becomes an effective strategy to enhance singlet exciton utilization through reverse intersystem crossing (RISC) process from high-lying triplet to low-lying singlet excited states, resulting in high electroluminescent (EL) performance of organic light emitting diodes (OLEDs). Herein, six compounds (1–6) based on D-π-A molecular architecture using 5-butyl-5,10-dihydroindolo[3,2-b]indole (IDL) or 10H-benzo [4,5]thieno[3,2-b]indole (BTI) as electron donating units, and 1,3,5-triphenyl- triazine (TAZ) derivative as a core of electron acceptor were demonstrated as an emissive material for OLEDs application. All compounds showed good thermal stability and photophysical properties. Utilizing them as an emitter, doped OLED devices of IDL-TAZ series (1–3) displayed green emission, while BIT-TAZ series (4–6) illuminated sky-blue. Interestingly, all devices showed EQEs higher than that of theoretical EQEs. Moreover, their doped OLED devices exposed the exciton utilization efficiency (ηs) over the spin statistic limitation of traditional fluorescent OLEDs at 25%. Among them, the doped OLEDs device of compound 3 exhibited the most superior EL performance of emission peak at 540 nm, with the maximum luminance of 17960 cd m−2 and the maximum external quantum efficiency (EQE) of 7.50%. It indicated that the efficient RISC process via “hot exciton” channel can be achieved through the combination between an electron donating IDL or BTI with an electron acceptor TAZ.

Towards red thermally activated delayed fluorescence emitter: Molecular design and property prediction of phenothiazine-triphenyltriazine derivative

A series of thermally activated delayed fluorescence (TADF) molecules with the linkage of the electron-donating group (phenothiazine, PTZ) and electron-withdrawing groups (2,4,6-triphenyl-1,3,5-triazine type, TRZ) have been designed and investigated for efficient red TADF emitters by means of quantum chemical calculations. The geometrical and electronic structures, absorption and emission properties, reverse intersystem crossing (RISC), radiative and nonradiative decay processes of the PTZ-TRZ-based molecules were analyzed. The calculated results indicate that the designed molecules possess the relatively large RISC rates (>105 s−1) and exhibit the red emission, indicating that they have the potential to be an efficient red emitting TADF material. Furthermore, the position of N atom in the TRZ acceptor group have significant effects on RISC process and emission color. This work provides in-depth understanding of the structure–property relationship of PTZ-TRZ-based TADF molecules, and offers the guideline for the screening of the efficient pure red TADF emitters.

High‐Performance Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters for Narrowband Organic Light‐Emitting Diodes

AbstractMulti‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters have drawn considerable attention because of their remarkable optoelectronic properties of high emission efficiency and narrow emission profile, and represent an active subject of cutting‐edge research in the organic electroluminescence (EL). However, the slow reverse intersystem crossing (RISC) rate of MR‐TADF emitter caused by the large energy gap (ΔEST) and small spin‐orbit coupling (SOC) matrix elements between the singlet and triplet excited states limits their further development in organic EL devices. Currently, innovative molecular design strategies have been developed including heavy atom integration, π‐extended MR framework and metal perturbation, and so on to improve the RISC process of MR‐TADF emitters for high‐performance EL devices. Here, an overview is presented on the recent progress of MR‐TADF emitters with fast RISC rate ( > 10−5 s−1), with particular attention to the molecular design, optoelectronic properties, and device performance of organic light‐emitting diodes (OLEDs), which intends to systematize the knowledge in this subject for the thriving development of highly efficient MR‐TADF emitters. Finally, the challenges and future prospects of MR‐TADF materials are discussed comprehensively.

Charge-Transfer Dynamics in OLEDs with Coexisting Electroplex and Exciton States

The reverse intersystem-crossing (RISC) channels from triplet to singlet charge-transfer states (such as exciplex or electroplex states) frequently occur in donor:acceptor heterojunction organic light-emitting diodes (OLEDs) to achieve high external quantum efficiency. Although exciplexes have been extensively investigated, there are few reports on the physical microscopic processes for electroplex-based devices. Herein, three kinds of donor:acceptor heterojunction OLEDs are fabricated: two coexisting electroplex and exciton devices and one pure exciplex-based device. Amazingly, via the fingerprint magnetoelectroluminescence measurement at room temperature, we observe an abnormal current-dependent RISC, which is enhanced with increasing bias current (I) in the coexisting electroplex and exciton but electroplex-dominated device; the conversion from ISC to RISC in the coexisting electroplex and exciton but exciton-dominated device; and only the normal I-dependent ISC, which weakens with increasing I in the pure exciplex-based device. This is because low-energy-triplet electroplexes are well confined by the high triplet energies of electron donors and acceptors, promoting the occurrence of the RISC process in electroplexes, and Dexter energy transfer from triplet excitons to triplet electroplexes enhances the RISC process with increasing I, causing the abnormal I-dependent RISC and the conversion from ISC to RISC. Moreover, as the operational temperature rises from 10 to 300 K, these two coexisting electroplex and exciton devices present the conversion from ISC to RISC at different temperatures, but the pure exciplex-based device shows only the normal temperature-dependent ISC, which is enhanced with increasing temperature. This is because the RISC channel is an endothermic process and temperature-dependent electroluminescence measurements show that the electroplex emission weakens as the temperature is reduced, causing the weakened RISC process and even the presence of the ISC process at low temperature. These experimental results demonstrate that the electroplex emission plays an important role in the occurrence of strong RISC processes. Our work deepens our microscopic understanding of the charge-transfer states in donor:acceptor heterojunction devices but also paves the way for utilizing electroplex states to design highly efficient OLEDs.

Realizing High-Efficiency Orange-Red Thermally Activated Delayed Fluorescence Materials through the Construction of Intramolecular Noncovalent Interactions.

The development of highly efficient orange and red thermally activated delayed fluorescence (TADF) materials for constructing full-color and white organic light-emitting diodes (OLEDs) remains insufficient because of the formidable challenges in molecular design, such as the severe radiationless decay and the intrinsic trade-off between the efficiencies of radiative decay and reverse intersystem crossing (RISC). Herein, we design two high-efficiency orange and orange-red TADF molecules by constructing intermolecular noncovalent interactions. This strategy could not only ensure high emission efficiency via suppression of the nonradiative relaxation and enhancement of the radiative transition but also create intermediate triplet excited states to ensure the RISC process. Both emitters exhibit typical TADF characteristics, with a fast radiative rate and a low nonradiative rate. Photoluminescence quantum yields (PLQYs) of the orange (TPA-PT) and orange-red (DMAC-PT) materials reach up to 94 and 87%, respectively. Benefiting from the excellent photophysical properties and stability, OLEDs based on these TADF emitters realize orange to orange-red electroluminescence with high external quantum efficiencies reaching 26.2%. The current study demonstrates that the introduction of intermolecular noncovalent interactions is a feasible strategy for designing highly efficient orange to red TADF materials.

Temperature dependent triplet-exciton relay from thermally activated delayed fluorescence (TADF) to self-sensitized triplet–triplet annihilation (TTA) of asymmetrical diphenylsulfone-based blue emitter

The utilization of triplet excitons is greatly concerned in organic luminophores. For thermally activated delayed fluorescence (TADF) emitters, the reverse intersystem crossing (RISC) which activates the triplet excitons at low temperature remains challenging without thermal stimulation. Herein, a temperature-dependent promotion relay of the triplet excitons from TADF to self-sensitized triplet − triplet annihilation up-conversion (TTA-UC) was demonstrated in an asymmetrical TADF luminophore based on 9,9-dimethyl-9,10-dihydroacridine (DMAC) donor and diphenylfonyl (DPS) acceptor. Single-crystal structure analysis combined with theoretical calculations revealed that the alternate strong-and-weak contact of the molecules reduced the singlet–triplet splitting energy (ΔEST), amplified the singlet–triplet coupling (SOC) strength and exciton coupling (EC), leading to highly efficient RISC process over a wide range of temperatures. Moreover, benefiting from the extreme small ΔEST value and large SOC strength, highly efficient blue OLEDs were achieved with the highest CEmax, PEmax and EQEmax of 32.0 cd A-1, 33.5 lm W−1 and 14.7%, and low efficiency roll-off. This work provides a design guidance for high utilization of the triplet excitons through RISC to improve the luminescence efficiency and reduce the device efficiency roll-off.

Evolution Between Exciton and Exciplex Emission in Planar Heterojunction OLEDs with Different Hole-Injection Characteristics

Planar heterojunction-based organic light-emitting diodes (PHJ OLEDs) usually have simultaneous exciton and exciplex emissions and understanding their evolution processes is crucial for designing high-performance white OLEDs, but relevant evolution mechanisms are still unclear. Here, unreported competitions between exciton and exciplex emissions and an undiscovered energy-transfer channel from triplet exciton to triplet exciplex states in PHJ OLEDs with different hole-injection abilities are investigated by separately measuring their current-dependent electroluminescence (EL) spectra and magneto-EL (MEL) traces at different temperatures. Interestingly, the ratio of emission intensity between exciton and exciplex states ($R={I}_{\mathrm{exciton}}/{I}_{\mathrm{exciplex}}$) from the device with a good hole-injection ability rises with increasing bias current at each temperature, whereas that from the device with a poor hole-injection ability hardly changes. In addition, R from the devices with good and poor hole-injection abilities show nonmonotonic variation and monotonic reduction with decreasing operational temperature at each bias current, respectively. These various current- and temperature-dependent competitions between exciton and exciplex emissions are attributed to distinct current- and temperature-dependent electron-tunneling effects occurring at organic heterojunction interfaces of devices with different hole-injection abilities. More intriguingly, using MEL as a fingerprint detection tool, we find there is a Dexter energy-transfer (DET) channel from the triplet exciton to triplet exciplex (${T}_{1}\ensuremath{\rightarrow}{\mathrm{EX}}_{3}$) state and the DET can enhance the reverse intersystem crossing (RISC) process from triplet to singlet exciplexes (${\mathrm{EX}}_{3}\ensuremath{\rightarrow}{\mathrm{EX}}_{1}$), which cannot be found using the conventional probing tool, EL spectra. Because the numbers of ${T}_{1}$ excitons in these devices have different current and temperature dependencies, various current- and temperature-dependent DET and RISC processes happen, which cause abundant MEL behaviors. Obviously, this work deepens the physical understanding of the competition and DET processes between exciton and exciplex states in PHJ OLEDs.

Efficient narrowband organic light-emitting devices based on multi-resonance TADF emitters with secondary donor

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters have been considered to be one of the most desirable materials for high-quality organic light emitting diodes (OLEDs) owing to their small full-width at half-maximum (FWHM) and outstanding device performance. However, there are still some inherent issues that restrict their further practical applications, such as slow reverse intersystem crossing (RISC) rate (kRISC), and severe intermolecular aggregation. Herein, three MR-TADF emitters, SPAC-tCzBN, SPBAC-tCzBN and o-SPAC-tCzBN, were designed and synthesized by introducing the secondary donor into tCzBN, which successfully realized the modification of the excited states and therefore achieved the enhanced RISC process. Furthermore, the rigid and steric molecular geometry greatly block the intermolecular forces between adjacent molecule and then reduce the concentration-induced quenching and spectral broadening. Consequently, OLEDs based on these emitters showed state-of-the-art external quantum efficiencies (EQEs) of 34.1%∼36.5% with narrow FWHM of 24 nm ∼ 28 nm. Meanwhile, the emission peaks and FWHM in EL spectra show much-promised less-sensitive to the doping concentration owing to suppressed intermolecular interactions. This work demonstrates a novel and effective MR-TADF molecular design strategy to realize high-quality OLEDs with extreme color purity and remarkable device efficiency.

Peripherally Heavy‐Atom‐Decorated Strategy Towards High‐Performance Pure Green Electroluminescence with External Quantum Efficiency over 40 %

AbstractHeavy‐atom integration into thermally activated delayed fluorescence (TADF) molecule could significantly promote the reverse intersystem crossing (RISC) process. However, simultaneously achieving high efficiency, small roll‐off, narrowband emission and good operational lifetime remains a big challenge for the corresponding organic light‐emitting diodes (OLEDs). Herein, we report a pure green multi‐resonance TADF molecule BN‐STO by introducing a peripheral heavy atom selenium onto the parent BN‐Cz molecule. The organic light‐emitting diode device based on BN‐STO exhibited state‐of‐the‐art performance with a maximum external quantum efficiency (EQE) of 40.1 %, power efficiency (PE) of 176.9 lm W−1, well‐suppressed efficiency roll‐off and pure green gamut. This work reveals a feasible strategy to reach a balance between fast RISC process and narrow full width at half maximum (FWHM) of MR‐TADF by heavy atom effect.

Peripherally Heavy-Atom-Decorated Strategy Towards High-Performance Pure Green Electroluminescence with External Quantum Efficiency over 40 .

Heavy-atom integration into thermally activated delayed fluorescence (TADF) molecule could significantly promote the reverse intersystem crossing (RISC) process. However, simultaneously achieving high efficiency, small roll-off, narrowband emission and good operational lifetime remains a big challenge for the corresponding organic light-emitting diodes (OLEDs). Herein, we report a pure green multi-resonance TADF molecule BN-STO by introducing a peripheral heavy atom selenium onto the parent BN-Cz molecule. The organic light-emitting diode device based on BN-STO exhibited state-of-the-art performance with a maximum external quantum efficiency (EQE) of 40.1 %, power efficiency (PE) of 176.9 lm W-1 , well-suppressed efficiency roll-off and pure green gamut. This work reveals a feasible strategy to reach a balance between fast RISC process and narrow full width at half maximum (FWHM) of MR-TADF by heavy atom effect.