Youichi Tsuchiya

To realize electrically pumped organic lasing, single‐layer devices without organic heterointerfaces have been employed to suppress efficiency rolloff. In such architectures, on the other hand, extrinsic trap states result in trap‐assisted nonradiative recombination, thereby decreasing electroluminescence efficiency. Here, we elucidate the dominant factors governing extrinsic trap formation by combining current density ( J )–voltage ( V ) characteristics of single‐layer hole‐only‐devices (HODs) with molecular dynamics (MD) simulations of 6,6′‐bibenzo[ d ]oxazole (BZ) derivatives exhibiting high photostability and low amplified spontaneous emission (ASE) thresholds. Drift‐diffusion analysis of the J – V characteristics reveals that 2,2′‐bis(4‐(9‐phenyl‐9 H ‐carbazol‐2‐yl)phenyl)‐6,6′‐bibenzo[ d ]oxazole (BZCz2) exhibits a higher density of extrinsic trap states than the other BZ derivatives studied. MD simulations further show that amorphous BZCz2 layers contain a high population of large voids arising from bulky substituents, potentially facilitating the incorporation of water clusters during vacuum deposition, thereby contributing to the formation of extrinsic hole traps. Furthermore, dispersing BZCz2 at low concentration in the host material of 4,4′‐bis(carbazol‐9‐yl)biphenyl (CBP), which exhibits dense molecular packing and minimal void formation, effectively suppresses extrinsic hole trapping in host–guest systems. These findings provide a strategy for suppressing extrinsic trap states and pave the way for highly efficient single‐layer EL devices.

We show that integrating acceptor-extended donor dendrons into MR-TADF emitters tunes their electroluminescence spectra while maintaining peak performance across key metrics, including quantum yields and reverse intersystem crossing rates.

TADF‐ or exciplex‐sensitized hyperfluorescence (HF)‐organic light‐emitting diodes (OLEDs) can enhance color purity and accelerate singlet radiative decay from a terminal emitter, offering new insights into future material development, photophysics, and device performance. While the development of wide‐energy‐gap exciplexes for deep‐blue OLEDs remains challenging, this work presents a unique approach to generating a blue exciplex and using it as a sensitizer in HF‐OLEDs. Considering the benefits of efficient triplet confinement in sensitizer‐based OLEDs, we developed a new “ hot‐exciplex ” system that leverages higher‐lying triplet states, confirmed by ultrafast picosecond and nanosecond transient absorption studies. To achieve this system, a simple bispyrimidine motif, BisPMD, and a carbazole derivative, CCP, are used as the acceptor and donor, respectively, owing to their similar high triplet energy levels. This design enables access to the hot excitonic states, reducing the singlet‐triplet energy gap (Δ E _ST ) and enhancing the upper‐level reverse intersystem crossing (RISC) rate ( k _hRISC ). The “ hot‐exciplex ” system proves to be an effective sensitizer for blue TADF and narrowband multi‐resonance (MR) emitters, enabling efficient Förster energy transfer (FRET) and improving rolloff and color purity at high efficiency. This advanced exciplex‐driven HF‐OLED is expected to enhance device performance and open further opportunities.

TADF‐ or exciplex‐sensitized hyperfluorescence (HF)‐organic light‐emitting diodes (OLEDs) can enhance color purity and accelerate singlet radiative decay from a terminal emitter, offering new insights into future material development, photophysics, and device performance. While the development of wide‐energy‐gap exciplexes for deep‐blue OLEDs remains challenging, this work presents a unique approach to generating a blue exciplex and using it as a sensitizer in HF‐OLEDs. Considering the benefits of efficient triplet confinement in sensitizer‐based OLEDs, we developed a new “ hot‐exciplex ” system that leverages higher‐lying triplet states, confirmed by ultrafast picosecond and nanosecond transient absorption studies. To achieve this system, a simple bispyrimidine motif, BisPMD, and a carbazole derivative, CCP, are used as the acceptor and donor, respectively, owing to their similar high triplet energy levels. This design enables access to the hot excitonic states, reducing the singlet‐triplet energy gap (Δ E _ST ) and enhancing the upper‐level reverse intersystem crossing (RISC) rate ( k _hRISC ). The “ hot‐exciplex ” system proves to be an effective sensitizer for blue TADF and narrowband multi‐resonance (MR) emitters, enabling efficient Förster energy transfer (FRET) and improving rolloff and color purity at high efficiency. This advanced exciplex‐driven HF‐OLED is expected to enhance device performance and open further opportunities.

Carbon dots (CDs) represent a rapidly advancing class of fluorescent nanomaterials. Due to the dominant nonradiative decay channels of the triplet excited state of CDs, these materials generally exhibit no observable phosphorescence (PH) or delayed fluorescence. Here, we report the development of a sustainable and efficient room-temperature phosphorescence (RTP) material by embedding CDs into a covalently bonded silica matrix, with both components derived from a previously unreported biomass precursor. Our effort has focused on stabilizing triplet excited states of CDs by constructing hybrid systems of CDs dispersed in SiO2 matrices, aiming to achieve heavy-metal-free RTP. In this work, we used natural coconut shells to synthesize CDs through the sol–gel method. Furthermore, we leveraged the intimate association between carbon and silica to develop CDs@SiO2. We found that calcination temperature critically influences the exciton confinement efficiency of CDs@SiO2. High-temperature calcination at 600 °C forms a highly rigid, cross-linked SiO2 network with strong Si–O–C and Si–C covalent bonds that connect to the CDs, effectively stabilizing triplet states and achieving an instrumentally fitted RTP lifetime of 285 ms and a visible afterglow duration >10 s (naked-eye detection after 365 nm excitation), and a total photoluminescence quantum yield of ΦPL = 18.5%. We note that the emission from CDs@SiO2 originates from the intrinsic luminescent centers of CDs immobilized within the rigid Si–O network, and the emission intensity scales with CD concentration.

Two new multiresonant thermally activated delayed fluorescence (MR‐TADF) emitters, BINAP‐BN1 and BINAP‐BN2 , containing a 1,1'‐binaphthalene (BINAP) unit and a varying number of tCzBN units (one in BINAP‐BN1 and two in BINAP‐BN2 ), were synthesized. In toluene, both showed emission maxima at ca. 490 nm, photoluminescence quantum yields (Φ _PL ) up to 82%, and Δ E _ST of ∼0.15 eV. Significant differences in their optoelectronic properties emerged as their 1 wt.% doped films in mCBP. BINAP‐BN1 has a Φ _PL of 82%, while that of BINAP‐BN2 is 64%. Aggregation‐induced emission broadening at higher doping concentrations is suppressed for BINAP‐BN1 but not for BINAP‐BN2 , attributed to the less sterically hindered BINAP moiety in the former. These photophysical differences translated in device performance. The organic light‐emitting diodes (OLEDs) with BINAP‐BN1 and BINAP‐BN2 achieved maximum external quantum efficiencies (EQE _max ) of 20.1 and 14.1%, respectively, at corresponding CIE coordinates of (0.08, 0.42) and (0.10, 0.50). A hyperfluorescence (HF) device employing HDT‐1 as sensitizer and BINAP‐BN1 as terminal emitter improved EQE _max to 22.0% and reduced efficiency roll‐off to 32% at 1000 cd/m ² , compared to 52% in the binary device. In comparison, a device with 1 wt.% tCzBN in mCBP showed an EQE _max of 15.6%, rising to 21.6% in a HF configuration.

Afterglow from purely organic solids has drawn increasing attention over the past decade. In carbazole-based systems, recent studies revised earlier aggregation-state explanations and implicated trace carbazole analog impurities as the origin. However, the widely assumed charge-trapping mechanism cannot account for exponential decay kinetics, lacks robust experimental validation and leaves some photophysical features unexplained. Here we establish the mechanism by revisiting two representative series of carbazole-analog host-guest systems. Quantitative spectral decomposition and comparative photophysics reveal that the afterglow is dominated by guest phosphorescence, accompanied by an extremely weak power-law long-persistent luminescence. The analog guests are confirmed to act as deep triplet exciton traps and shallow hole traps, the latter of which can enhance hole-transport-type long-persistent luminescence. These insights clarify previously unresolved mechanisms and establish a unified framework for luminescence that integrates energy transfer, exciton diffusion, and three afterglow pathways (delayed fluorescence, room-temperature phosphorescence and long-persistent luminescence), offering design principles for brighter and more persistent purely organic afterglow materials.

Multiresonant thermally activated delayed fluorescence (MR‐TADF) emitters hold significant promise for use in organic light‐emitting diodes (OLEDs) as they deliver bright, narrowband emission. However, their typically moderate reverse intersystem crossing rate constants ( k _RISC ) result in triplet exciton accumulation that feeds undesired annihilation processes, resulting in pronounced efficiency roll‐off, an issue amplified in solution‐processed OLEDs. Here, we report three MR‐TADF emitters, TRZ‐SpAc‐DBN , Ph‐SpAc‐DBN , and DOBNA‐SpAc‐DBN , that efficiently manage exciton kinetics using an intramolecular FRET design that combines fast k _RISC donor‐acceptor (D‐A) TADF cores with highly emissive peripheral MR‐TADF units. The three compounds show narrowband emission (FWHM = 24 nm) and have high photoluminescence quantum yields (up to ∼100%) in both solution and doped films in mCP, with k _RISC values exceeding 1 × 10 ⁵ s ⁻¹ . When employed as terminal emitters in solution‐processed hyperfluorescent OLEDs with 5 t BuCzTRZ as the sensitizer, devices showed high maximum external quantum efficiencies ( EQE _max of up to 20.7%) and remarkably low efficiency roll‐off (below 2.5% at EQE ₁₀₀₀ ). Notably, devices with TRZ‐SpAc‐DBN achieved an extraordinary EQE _10,000 of 13.6%, corresponding to only 34% efficiency roll‐off. These findings highlight the effectiveness of combining a FRET‐enabled MR‐TADF emitter design with hyperfluorescent device architectures to realize high‐efficiency and exceptionally low efficiency roll‐off solution‐processed OLEDs.

Thermally activated delayed fluorescence materials have attracted attention for organic light-emitting diodes owing to their high efficiency and stability. Accurate evaluation of the singlet–triplet energy gap (Δ E _ST ), particularly in near-zero-gap systems, remains a key challenge. Here, we investigate the photophysical properties of HzTFEX ₂ , a proposed inverted singlet–triplet (InvE _ST ) emitter, using temperature- and solvent-dependent analyses based on a dynamic exciton model. The results show that InvE _ST explains the emission decay profiles of HzTFEX ₂ in the Marcus normal region. We demonstrate that conventional kinetic analyses are insufficient in the near-zero Δ E _ST regime, highlighting the necessity of the dynamic exciton model.

Spontaneous orientation polarization (SOP), arising from the partial alignment of permanent dipole moments in organic thin films, is regarded as a crucial factor in enhancing the performance of organic optoelectronic devices. In this study, organic host materials, DPCzBN1, DPCzBN2, and DPCzBN3, were synthesized, and their SOP properties were analyzed. An unusual negative SOP was observed in the DPCzBN3 film, while its counterparts, DPCz, DPCzBN1, and DPCzBN2, showed positive SOP. Further, we found that doping emitter molecules into the DPCz derivatives had an adverse effect on negatively polarized SOP. This finding highlights the significant role of dipole-dipole interactions and glass transition temperature (T_g)-dependent molecular kinetic energy in determining SOP in codeposited films.