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Abstract Thermally activated delayed fluorescence (TADF) is a promising mechanism for harvesting triplet excitons in organic light-emitting diodes (OLEDs). The donor–acceptor (D–A) design is the most conventional strategy for developing efficient TADF emitters. A subsequently emerged approach, known as the multiple resonance (MR) effect, also employs electron-donating and electron-withdrawing functional groups. Thus, developing TADF materials has traditionally relied on ingenuity in selecting and combining two functional units. Here, we have realized a TADF molecule by utilizing only a carbazole donor moiety. This molecule is an unusual example in the family of TADF materials and offers better insight into the electronic structures in the excited states for luminescent materials.

Abstract We propose an organic thermoelectric device having a new power generation mechanism that extracts small-scale thermal energy, i.e., a few tens of millielectronvolts, at room temperature without a temperature gradient. We demonstrate a new operating mechanism based on an organic thermoelectric power generation architecture that uses the charge separation capabilities of organic charge transfer (CT) interfaces composed of copper (II) phthalocyanine and copper (II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine as the donor and acceptor, respectively. With the optimized device architecture, values of open-circuit voltage V OC of 384 mV, short-circuit current density J SC of 1.1 μA/cm2, and maximum output P max of 94 nW/cm2 are obtained. The temperature characteristics of the thermoelectric properties yield activation energy values of approximately 20–60 meV, confirming the low-level thermal energy’s contribution to the power generation mechanism. Furthermore, from surface potential analysis using a Kelvin probe, we confirm that charges are generated at the CT interface, and the electrons and holes are diffused to the counter-electrodes with the aid of Fermi-level alignment between adjacent layers.

Abstract The orientation of a permanent dipole moment during vacuum deposition results in the occurrence of spontaneous orientation polarization (SOP). Previous studies reported that the presence of SOP in organic light-emitting diodes (OLEDs) lowers electroluminescence efficiency because electrically generated excitons are seriously quenched by SOP-induced accumulated charges. Thus, the SOP in a host:guest-based emission layer (EML) should be finely controlled. In this study, we demonstrate the positive effect of dipole-dipole interactions between polar host and polar emitter molecules on the OLED performance. We found that a small-sized polar host molecule that possesses both high molecular diffusivities and moderate permanent dipole moment, well cancels out the polarization formed by the SOP of the emitter molecules in the EML without a disturbance of the emitter molecules’ intrinsic orientation, leading to high-performance of OLEDs. Our molecular design strategy will allow emitter molecules to pull out the full potential of the EMLs in OLEDs.

Abstract Manipulating dynamic behaviours of charge carriers and excitons in organic light-emitting diodes (OLEDs) is essential to simultaneously achieve high colour purity and superior operational lifetime. In this work, a comprehensive transient electroluminescence investigation reveals that incorporating a thermally activated delayed fluorescence assistant molecule with a deep lowest unoccupied molecular orbital into a bipolar host matrix effectively traps the injected electrons. Meanwhile, the behaviours of hole injection and transport are still dominantly governed by host molecules. Thus, the recombination zone notably shifts toward the interface between the emissive layer (EML) and the electron-transporting layer (ETL). To mitigate the interfacial carrier accumulation and exciton quenching, this bipolar host matrix could serve as a non-barrier functional spacer between EML/ETL, enabling the distribution of recombination zone away from this interface. Consequently, the optimized OLED exhibits a low driving voltage, promising device stability (95% of the initial luminance of 1000 cd m−2, LT95 > 430 h), and a high Commission Internationale de L’Éclairage y coordinate of 0.69. This indicates that managing the excitons through rational energy level alignment holds the potential for simultaneously satisfying Rec.2020 standard and achieving commercial-level stability.

Abstract To fulfill ultra-high-definition display, efficient and bright green organic light-emitting diodes with Commission Internationale de l’Éclairage y-coordinate ≥ 0.7 are required. Although there are some preceding reports of highly efficient devices based on pure-green multi-resonance emitters, the efficiency rolloff and device stabilities for those pure-green devices are still unsatisfactory. Herein, we report the rational design of two pure-green multi-resonance emitters for achieving highly stable and efficient pure-green devices with CIEx,ys that are close to the NTSC and BT. 2020 standards. In this study, our thermally activated delayed fluorescence OLEDs based on two pure-green multi-resonance emitters result in CIEy up to 0.74. In hyperfluorescent device architecture, the CIExs further meet the x-coordinate requirements, i.e., NTSC (0.21) and BT. 2020 (0.17), while keeping their CIEys ~ 0.7. Most importantly, hyperfluorescent devices display the high maximum external quantum efficiencies of over 25% and maximum luminance of over 105 cd m−2 with suppressed rolloffs (external quantum efficiency of ~20% at 104 cd m−2) and long device stabilities with LT95s of ~ 600 h.

Abstract Despite significant progress made over the past decade in thermally activated delayed fluorescence (TADF) molecules as a material paradigm for enhancing the performance of organic light-emitting diodes, the underlying spin-flip mechanism in these charge-transfer (CT)-type molecular systems remains an enigma, even since its initial report in 2012. While the initial and final electronic states involved in spin-flip between the lowest singlet and lowest triplet excited states are well understood, the exact dynamic processes and the role of intermediate high-lying triplet (T) states are still not fully comprehended. In this context, we propose a comprehensive model to describe the spin-flip processes applicable for a typical CT-type molecule, revealing the origin of the high-lying T state in a partial molecular framework in CT-type molecules. This work provides experimental and theoretical insights into the understanding of intersystem crossing for CT-type molecules, facilitating more precise control over spin-flip rates and thus advancing toward developing the next-generation platform for purely organic luminescent candidates.

Abstract Organic light-emitting diodes (OLEDs) exploiting simple binary emissive layers (EMLs) blending only emitters and hosts have natural advantages in low-cost commercialization. However, previously reported OLEDs based on binary EMLs hardly simultaneously achieved desired comprehensive performances, e.g., high efficiency, low efficiency roll-off, narrow emission bands, and high operation stability. Here, we report a molecular-design strategy. Such a strategy leads to a fast reverse intersystem crossing rate in our designed emitter h-BNCO-1 of 1.79×105 s−1. An OLED exploiting a binary EML with h-BNCO-1 achieves ultrapure emission, a maximum external quantum efficiency of over 40% and a mild roll-off of 14% at 1000 cd·m−2. Moreover, h-BNCO-1 also exhibits promising operational stability in an alternative OLED exploiting a compact binary EML (the lifetime reaching 95% of the initial luminance at 1000 cd m−2 is ~ 137 h). Here, our work has thus provided a molecular-design strategy for OLEDs with promising comprehensive performance.

Abstract A considerable variety of donor–acceptor (D–A) combinations offers the potential for realizing highly efficient thermally activated delayed fluorescence (TADF) materials. Multiple D–A type compounds are one of the promising families of TADF materials in terms of stability as well as efficiencies. However, those emitters are always composed of carbazole-based donors despite a wide choice of moieties used in linearly linked single D–A molecules. Herein, we developed a multiple D–A type TADF compound with two distinct donor units of 9,10-dihydro-9,9-dimethylacridine (DMAC) and carbazole as the hetero-donor design. The new emitter exhibits high photoluminescence quantum yield (PLQY) in various conditions including polar media blend and high concentrations. Organic light-emitting diodes (OLEDs) showed a reasonably high external quantum efficiency (EQE). In addition, we revealed that the multiple-D–A type molecules showed better photostability than the single D–A type molecules, while the operational stability in OLEDs involves dominant other factors.

Efficiency roll-off in organic light-emitting diodes under elevated current densities not only results in high power consumption but also device degradation. Here, the impact of polaroninduced quenching in efficiency roll-off was investigated and strategies to reduce efficiency roll-off were proposed.

This paper presents an analysis on the performance of an optical wireless power transfer system in which optical transceivers made from a perovskite are used. Experiments are performed under different settings, whose resulted data reveal interesting findings. First, system performance is only matched with the existing theory when no collimating lens is employed. Hence, a data-driven mathematical model is proposed and verified when a collimating lens is used. Second, a collimating lens helps increase the amount of wirelessly transferred power and the transfer distance if perfect alignment between optical transceivers is guaranteed, but substantially limits the sliding distance between them otherwise.

Molecular designs improving the performance of thermally activated delayed fluorescence (TADF) emitters are greatly desired for optoelectronic applications. The authors propose a molecular geometry with donor and acceptor moieties facing each other separated by an anthracene bridge, giving rise to hot exciplex states producing color pure and fast TADF emission.

Metal halide perovskite light‐emitting diodes (LEDs) are promising for future display applications because of their excellent advantages, such as high external quantum efficiency, emission color tunability, and high emission color purity. Although solution processing widely used for perovskite film fabrication is an additional advantage, there are cases in which fabricating a perovskite‐emitting layer with spin‐coating results in the dissolution of an underlying organic hole transport layer (HTL). As a result, since a perovskite layer comes into partial contact with an indium tin oxide (ITO) anode layer, excited states formed in a perovskite‐emitting layer are quenched by charge transfer to ITO. In this study, it is shown that adding tetraethyl orthosilicate (TEOS) into a HTL material of poly(N‐vinylcarbazole) (PVCz) is an effective method used to overcome this issue. Upon heating, the hydrolysis reaction of TEOS molecules takes place in PVCz films to form a siloxane network, which makes PVCz films insoluble and, therefore, alleviates the excited‐state quenching. It is demonstrated that using the siloxane‐blended PVCz HTL increases external quantum efficiencies of perovskite LEDs to 15.4 ± 0.3% from the original 10.4 ± 0.3% by about 1.5 times.

We studied the photophysical and electroluminescent (EL) characteristics of a series of azaborine derivatives having a pair of boron and nitrogen aimed at the multi-resonance (MR) effect. The computational study with the STEOM-DLPNO-CCSD method clarified that the combination of a BN ring-fusion and a terminal carbazole enhanced the MR effect and spin-orbit coupling matrix element (SOCME), simultaneously. Also, we clarified that the second triplet excited state (T2) plays an important role in efficient MR-based thermally activated delayed fluorescence (TADF). Furthermore, we obtained a blue–violet OLED with an external EL quantum efficiency (EQE) of 9.1%, implying the presence of a pronounced nonradiative decay path from the lowest triplet excited state (T1).

Materials that display photostimulated luminescence are attractive for multiple applications. Here, photostimulated luminescence is demonstrated in an all-organic blend film, in which a dopant molecule functions as both an electron trap and a light emitter.

Though reducing non-emissive triplet excited-states using quenchers effectively improves organic semiconductor laser diode (OSLD) performance, existing quenchers are not suitable for devices. Here, the authors designed a solid-state triplet quencher for OSLD under optical and electrical excitation.

Though organic light-emitting diodes are highly desirable for display and lighting applications, efficiency roll-off at high current densities limits its applicability. Here, the authors report a triplet scavenging strategy for improved triplet exciton management and reduced efficiency roll-off.

Common issues facing perovskite solar cells are current-voltage hysteresis and degradation during illumination. Here, a self-assembled monolayer is applied to an SnO2 electron transport layer, helping to achieve hysteresis-less behavior and limited degradation after 1,000 hours of illumination.

Deep-blue emitting organic materials with low exciton lifetime are required to realize efficient organic light-emitting diodes (OLEDs) at high brightness. Here, the authors report deep-blue OLEDs featuring thermally activated delayed fluorescence molecules with subnano-second exciton lifetime.

Improving the lifetime of organic light-emitting diodes requires that degradation processes are understood. Here it is demonstrated that magnetic field effects can be used as a non-destructive indicator of device degradation.

Long-persistent luminescence can be realized in all-organic systems (OLPL) but still with unsatisfactory performance. Here the authors investigate the relationships between energy levels and emission properties in a series of OLPL materials and propose a design strategy for efficient OLPL performance.

Though highly emissive charge-transfer type molecules in a host matrix is an attractive material for organic opto-electronics, concentration quenching limits photoluminescence quantum yield. Here, the authors report concentration quenching in fluorophores based on spontaneous exciton dissociation.

In this study, we developed two thermally activated delayed fluorescence (TADF) emitters, ICzCN and ICzCYP, to apply to organic light-emitting diodes (OLEDs). These emitters involve indolocarbazole (ICz) donor units and nicotinonitrile acceptor units with a twisted donor-acceptor-donor (D-A-D) structure for small singlet (S1) and triplet (T1) state energy gap (ΔEST) to enable efficient exciton transfer from the T1 to the S1 state. Depending on the position of the cyano-substituent, ICzCN has a symmetric structure by introducing donor units at the 3,5-position of isonicotinonitrile, and ICzCYP has an asymmetric structure by introducing donor units at the 2,6-position of nicotinonitrile. These emitters have different properties, such as the maximum luminance (Lmax) value. The Lmax of ICzCN reached over 10000 cd m−2. The external quantum efficiency (ηext) was 14.8% for ICzCN and 14.9% for ICzCYP, and both achieved a low turn-on voltage (Von) of less than 3.4 eV.

This paper proposes a novel conceptual system of optical wireless power transfer (OWPT) between objects, which is different from the existing OWPT systems such that a single device—an optical transceiver—is employed. This optical transceiver, which is capable of both absorbing and emitting light, is fabricated from a metal halide perovskite known for its superior features that can help significantly reduce the whole system size and cost. The proposed system contributes to realizing a thing-to-thing OWPT network, in which surfaces of objects/things are covered by perovskite transceivers (fully or partially), enabling them to wirelessly charge or discharge from the others.

Though triplet-triplet upconversion is a promising strategy for designing new deep blue-emitting organic materials, maximizing the efficiency of this process remains difficult. Here, the authors report the upconversion efficiency in anthracene derivatives based on a spin-orbit coupling mechanism.

Thermally activated delayed fluorescence (TADF) emitters deliver impressive performance in sophisticated multilayer devices. Here, the authors report efficient TADF emission from light-emitting electrochemical cells that feature a solution-processed active layer in between two air-stable electrodes.

Organic light‐emitting diodes (OLEDs) exhibiting thermally activated delayed fluorescence (TADF) have attracted great interest because of their excellent exciton harvesting ability in electroluminescence (EL). While TADF‐OLEDs show a high EL efficiency, the device operational stability is not necessarily satisfactory for commercial applications. Herein, the isotope effect of the host material on the device operational stability in TADF‐OLEDs is investigated. It is unveiled that the deuterated host, i.e., PYD2Cz‐d16, forms a denser film than that of the nondeuterated host, PYD2Cz, demonstrating enhanced stable amorphous nature and balanced carrier transport properties. In green TADF‐OLEDs, the PYD2Cz‐d16‐based OLED considerably lengthens the device operational stability of LT95 (95% of the initial luminance) to ≈140 h at an initial luminance of 1000 cd m−2, which is 1.7 times longer than that of PYD2Cz. Device stabilities of blue TADF‐OLEDs with PYD2Cz‐d16 as a host are also demonstrated with an enhancement of 2 times in LT50 at an initial luminance of 1000 cd m−2. It is suggested that the deuterated materials have a positive effect on the device stability in not only TADF‐OLEDs, but also all other OLEDs having fluorescence and phosphorescence emitters.

Quantum chemical calculations are necessary to develop advanced emitter materials showing thermally-activated delayed fluorescence (TADF) for organic light-emitting diodes (OLEDs). However, calculation costs become problematic when more accurate functionals were used, therefore it is judicious to use a multimethod approach for efficiency. Here we employed combinatorial chemistry in silico to develop the deep blue TADF materials with a new concept of homo-junction design. The homo-junction materials containing TADF candidates designed by calculation were synthesized and analyzed. We found that these materials showed the emission from charge transfer (CT) state, and the clear delayed emission was provided in solid state. Because the homo-junction TADF materials showed three exponential decayed emission in solid state, we employed novel four-state kinetic analysis.

In our previous paper [T. Matsushima et al., Nature 572, 502 (2019)], current densities of organic light-emitting diodes (OLEDs) did not decrease significantly when the thicknesses of a 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) transport layer were increased from tens of nanometers to 1 μm. To make this mechanism clear, we carried out several experiments in terms of electron transfer with other organic layers and electron mobility of HAT-CN. Finally, we found that the vacuum-evaporated HAT-CN layers have very high electron mobility and, therefore, using a HAT-CN transport layer can suppress the decrease in current density even in thick OLEDs. The electron mobility of vacuum-deposited HAT-CN layers, which was measured using analysis with a space-charge-limited current model, was 0.1–1 cm2 V−1 s−1. This electron mobility is much higher than those of conventional organic transport layers used in OLEDs (

The potential of organic light-emitting diodes (OLEDs) for display and lighting applications is limited by the need for blue emitters that show efficient and stable operation. Here, the authors report stable and efficient thermally activated delayed fluorescence OLEDs based on a sky-blue emitter.

OLEDs based on thermally activated delayed fluorescence have shown high fluorescence efficiency but poor lifetime. Herein, Cui et al. demonstrate that the use of n-type host molecules can increase the device lifetime by 30 times and 1000 times for green and blue OLEDs, respectively.

Abstract The dipole orientation of guest emitters doped into host matrices is usually investigated by angular dependent photoluminescence (PL) measurements, which acquire an out-of-plane PL radiation pattern of the guest-host thin films. The PL radiation patterns generated by these methods are typically analysed by optical simulations, which require expertise to perform and interpret in the simulation. In this paper, we developed a method to calculate an orientational order parameter S without the use of full optical simulations. The PL radiation pattern showed a peak intensity (I sp) in the emission direction tilted by 40°–60° from the normal of the thin film surface plane, indicating an inherent dipole orientation of the emitter. Thus, we directly correlated I sp with S. The S − I sp relation was found to depend on the film thickness (d) and refractive indices of the substrate (n sub) and the organic thin film (n org). Hence, S can be simply calculated with information of I sp, d, n sub, and n org. We applied our method to thermally activated delayed fluorescence materials, which are known to be highly efficient electroluminescence emitters. We evaluated S and found that the error of this method, compared with an optical simulation, was less than 0.05.

Abstract The photoluminescence properties of a thermally activated delayed fluorescence emitter, 1,2-bis(carbazol-9-yl)-4,5-dicyanobenzene (2CzPN), doped in a host matrix consisting of 1,3-bis(9-carbazolyl)benzene and a polar inert molecule, camphoric anhydride (CA), in various concentrations have been investigated. It is found that the addition of CA stabilizes only the lowest singlet excited state (S1) of 2CzPN without changing the energy level of the lowest triplet excited state (T1), leading to a reduction in the energy gap between S1 and T1. The maximum reduction of energy gap achieved in this work has been determined to be around 65 meV from the shift of the fluorescence spectrum and the temperature dependence of the photoluminescence decay rate.

Abstract Improving the performance of blue organic light-emitting diodes (OLEDs) is needed for full-colour flat-panel displays and solid-state lighting sources. The use of thermally activated delayed fluorescence (TADF) is a promising approach to efficient blue electroluminescence. However, the difficulty of developing efficient blue TADF emitters lies in finding a molecular structure that simultaneously incorporates (i) a small energy difference between the lowest excited singlet state (S1) and the lowest triplet state (T1), ΔE ST, (ii) a large oscillator strength, f, between S1 and the ground state (S0), and (iii) S1 energy sufficiently high for blue emission. In this study, we develop TADF emitters named CCX-I and CCX-II satisfying the above requirements. They show blue photoluminescence and high triplet-to-singlet up-conversion yield. In addition, their transition dipole moments are horizontally oriented, resulting in further increase of their electroluminescence efficiency. Using CCX-II as an emitting dopant, we achieve a blue OLED showing a high external quantum efficiency of 25.9%, which is one of the highest EQEs in blue OLEDs reported previously.

Solution-processable star-shaped and linear π-conjugated oligomers consisting of an electron-donating tetrathienoanthracene (TTA) core and electron-accepting diketopyrrolopyrrole (DPP) arms, namely, TTA-DPP4 and TTA-DPP2, were designed and synthesized. Based on density functional theory calculations, the star-shaped TTA-DPP4 has a larger oscillator strength than the linear TTA-DPP2, and consequently, better photoabsorption property over a wide range of visible wavelengths. The photovoltaic properties of organic solar cells based on TTA-DPP4 and TTA-DPP2 with a fullerene derivative were evaluated by varying the thickness of the bulk heterojunction active layer. As a result of the enhanced visible absorption properties of the star-shaped π-conjugated structure, better photovoltaic performances were obtained with relatively thin active layers (40–60 nm).

The potential of organic molecular devices are limited by our inability to uniformly arrange them as thin films on metals, especially over imperfect surfaces. Here, the authors demonstrate the growth of a self-organized molecular carpet, which can extend over steps and terraces while retaining regularity.

We investigated the influence of vacuum chamber impurities on the lifetime of highly efficient TADF-based OLEDs. Batch-to-batch lifetime variations are clearly correlated with the results of contact angle measurements, which reflect the amount of impurities present in the chamber. Introduction of ozone gas can clean the impurities out of the vacuum chamber, reducing the contact angle to less than 10°. In the vacuum chamber of a new deposition system designed using resin-free vacuum components, various plasticizers and additive agents were initially detected by WTD-GC-MS analysis, but these impurities vanished after ozone gas cleaning. Devices fabricated in the new chamber exhibited lifetimes that are approximately twice those of OLEDs fabricated in a pre-existing chamber. These results suggest that impurities, particularly from plasticizers, in the vacuum chamber greatly influence the OLED lifetime.

Liquid organic light-emitting diodes (liquid OLEDs) are unique devices consisting only of liquid organic semiconductors in the active layer, and the device performances have been investigated recently. However, the device degradation, especially, the origin has been unknown. In this study, we show that material degradation occurs in liquid OLEDs, whose active layer is composed of carbazole with an ethylene glycol chain. Nuclear magnetic resonance (NMR) experiments clearly exhibit that the dimerization reaction of carbazole moiety occurs in the liquid OLEDs during driving the devices. In contrast, cleavages of the ethylene glycol chain are not detected within experimental error. The dimerization reaction is considered to be related to the device degradation.

Two novel deep-blue multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters, 1B-CzCrs and 2B-CzCrs, containing a fused carbazole unit were synthesized. The carbazole contributed to the emergence of TADF in these small molecules. Particularly, organic light-emitting diodes with 1B-CzCrs doped in the mCP host achieve a maximum external quantum efficiency of 12.8% at CIE coordinates of (0.146, 0.062). [ABSTRACT FROM AUTHOR]

In recent decades, various types of aryl-substituted 1,3,5-triazine derivatives have been applied in many research fields, including biomedical chemistry, non-linear optics, and organic electronics. However, the substituent scope for 4,6-diaryl-1,3,5-triazines (DAr-TRZs) remains limited. Here, we present our work on the synthesis of 2-amino- and 2-bromo-DAr-TRZ derivatives bearing electron-withdrawing groups on the aryl rings. Our synthetic methods successfully provided trifluoromethyl-, nitrile-, and nitro-substituted DAr-TRZs. These will expand the structural diversity of conventional triazine-based functional materials. [ABSTRACT FROM AUTHOR]