Utilization Of Triplet Excitons Research Articles

Ultrafast Spectroscopic Investigation of the Aggregation Induced TADF from High-Level Reversed Intersystem Crossing.

The thermally activated delayed fluorescence (TADF) originating from high-level intersystem crossing (hRISC) presents great potential in realizing a more full utilization of triplet excitons. In this study, DPA-FBP and TPA-FBP were doped in a PMMA film with different weight fractions to study the effect of aggregation on the luminescence properties. As a result, the TADF feature from hRISC was only found in the 50 wt % doped film, whereas the 1 wt % doped film only shows prompt fluorescence. The fs-TA spectroscopy results reveal that the 50 wt % film will generate charge transfer species to lower the energy gap, so that the high-lying triplet exciton can transition back to the singlet state, whereas that of the 1 wt % film will quickly transition to the lowest triplet state due to the unfavorable energy splitting. This study provides a new insight into aggregation effects on the excited-state properties of hot exciton materials and the solid-state photodynamic.

Achieving Efficient X-ray Scintillation of Purely Organic Phosphorescent Materials by Chromophore Confinement.

Scintillators have attracted significant attention due to their wide-ranging applications in both industrial and medical fields. However, one of the ongoing challenges is the efficient utilization of triplet excitons to achieve high radioluminescence efficiency. Here, a series of purely organic phosphors is presented for X-ray scintillation, employing a combined rigid and flexible host-guest doping strategy. The doped crystals exhibit a remarkable maximum phosphorescence efficiency of 99.4% under UV excitation. Furthermore, upon X-ray irradiation, the radioluminescence intensities of the doped phosphors are markedly higher compared to their single-component crystal counterparts. Through systematic investigations, it is demonstrated the crucial role of confining isolated chromophores in enhancing scintillation efficiency. Additionally, a transparent scintillator screen fabricated with the doped phosphor exhibits excellent X-ray imaging performance, achieving a high spatial resolution of 18.0 lpmm-1. This work not only offers valuable insights into suppressing non-radiative transitions of triplet excitons during scintillation but also opens a new avenue for designing highly efficient purely organic phosphorescent scintillators.

Insights into the Structural Modification of Selenium-Doped Derivatives with Narrowband Emissions: A Theory Study.

The research on boron/nitrogen (B/N)-based multiresonance thermally activated delayed fluorescence (MR-TADF) emitters has been a prominent topic due to their narrowband emission and high luminous efficiency. However, devices derived from the common types of narrowband TADF materials often experience an efficiency roll-off, which could be ascribed to their relatively slow triplet-singlet exciton interconversion. Since inserting the heavy Se atom into the B/N scheme has been a proven strategy to address the abovementioned issues, herein, extensive density functional theory (DFT) and time-dependent DFT (TD-DFT) simulations have been employed to explore the effects of the structural modification on a series of structurally modified selenium-doped derivatives. Furthermore, the two-layered ONIOM (QM/MM) model has been employed to study the pressure effects on the crystal structure and photophysical properties of the pristine CzBSe. The theoretical results found that the introduced tert-butyl units in Cz-BSeN could result in a shorter charge transfer distance and smaller reorganization energy than the parent CzBSe. In contrast to directly incorporating the o-carborane (Cb) unit to CzBSe, incorporating the bridged phenyl units is important in order to achieve narrowband emissions and high luminous efficiency. The lowest three triplet excited states of CzBSe, Cz-BSeN and PhCb-BSeN all contribute to their triplet-singlet exciton conversions, resulting in a high utilization of triplet excitons. The pressure has an evident influence on the photophysical properties of the aggregated CzBSe and is favored for obtaining narrowband emissions. Our work is promised to provide a feasible strategy for designing selenium-doped derivatives with narrowband emissions and rapid triplet-singlet exciton interconversions.

Ultralow Roll-off Thermally Activated Delayed Fluorescent Light-Emitting Diodes Based on Furo[2,3-b]quinoxaline Emitters.

Herein, a Y-type compound (67dMeOTPA-FQ) and a T-type compound (58dMeOTPA-FQ) based on furo[2,3-b]quinoxaline were synthesized. The theory calculation shows the S1 and T1 of both compounds own a charge-transfer feature while their T2 states have a local excitation feature. The calculated kRISC(T2-S1) is one to 2 orders of magnitude larger than kRISC(T1-S1). Thus, the nonadiabatic spin-vibronic mechanism involved in the T2 state is suggested to be responsible for the thermally activated delayed fluorescence (TADF) feature. Meanwhile, when 2-methyl-9,10-bis(naphthalen-2-yl)anthracene is selected as host, the maximum luminance of the device based on 67dMeOTPA-FQ is up to 104215 cd·m-2, and the external quantum efficiency (EQE) keeps in the 8.2-8.0% range with the luminance changed from 55.0 cd·m-2 to 90000 cd·m-2, only 2.4% efficiency roll-off. As for 58dMeOTPA-FQ, a slightly lower EQE of 7.1-6.7% with the luminance range of 1-40000 cd·m-2 was achieved for orange-red emission. Both the reverse intersystem crossing (RISC) and triplet-triplet annihilation mechanisms are supposed to concurrently contribute to the utilization of triplet excitons and suppress the notorious efficiency roll-off observed in TADF-based devices.

Helical polyisocyanides with thermally activated delayed fluorescence pendants for efficient circularly polarized light emission and detection

Chiral organic polymeric semiconductors are widely regarded as promising candidates for circularly polarized light (CPL) detection due to their advantages of easy chemical modification, solution processing and low cost. However, traditional organic polymeric materials face low photoresponsivity and photocurrent asymmetry factor when constructing CPL detectors. To address this issue, we develope single-handed helical polyisocyanides with thermally activated delayed fluorescence (TADF) feature to fabricate a donor-acceptor heterojunction photodetector with C60, where efficient triplet exciton utilization enables a high photocurrent response while the static single-handed helical main chains of polyisocyanides ensure a high photocurrent asymmetry factor, simultaneously. Furthermore, the performance of TADF polyisocyanides is conveniencely optimized by copolymerizing the host. Benefiting from the comprehensive functionality of TADF polyisocyanides, the prepared photodetectors exhibit a high responsivity of 0.21 A W−1 and a very high photocurrent asymmetry factor of up to 0.12, which make it superior to the reported CPL photodetectors based on organic polymers. In addition, the detector has excellent reproducibility enabling no photocurrent roll-off after 1000 cycles. The long-term stability in ambient air also manifests its robustness. This work paves a new way for high-efficiency polymers based CPL detectors.

29‐2: Distinguished Paper: High Efficiency and High Color Purity Deep‐Blue Organic Light‐Emitting Diodes with Blue Index >500

Thermally activated delayed fluorescent (TADF) and phosphorescent blue organic light‐emitting diodes (OLEDs) have been developed to overcome the relatively low triplet exciton utilization of traditional fluorescent OLEDs. However, broad emission spectra originating from charge transfer process limits the commercial application of such blue OLEDs. Herein, an effective phosphor‐sensitized fluorescent (PSF) OLED device structure was designed. PSF OLED exhibited a maximum blue index (BI) of 508 cd/A/CIEy with CIEy of 0.060, which has been the highest efficiency result reported for PSF deep‐blue OLEDs. Impressively, the sensitized OLEDs maintained blue index of 407 cd/A/CIEy, narrow emission spectra (FWHM = 17 nm) and high color purity (CIEy = 0.046), revealing the attractive technical advantages and commercial application potential.

High efficiency and high color purity deep‐blue organic light‐emitting diodes with blue index >500

AbstractThermally activated delayed fluorescent (TADF) and phosphorescent blue organic light‐emitting diodes (OLEDs) have been developed to overcome the relatively low triplet exciton utilization of traditional fluorescent OLEDs. However, broad emission spectra originating from charge transfer process limit the commercial application of such blue OLEDs. Herein, an effective phosphor‐sensitized fluorescent (PSF) OLED device structure was designed. PSF OLED exhibited a maximum blue index (BI) of 508 cd/A/CIEy with CIEy of 0.060, which has been the highest efficiency result reported for PSF deep‐blue OLEDs. A nearly 100% improvement in operational lifetime LT95 (L/L0 = 95%) was achieved successfully by using lower triplet energy (T1) and deuterated host materials. Impressively, the sensitized OLEDs maintained BI of 407 cd/A/CIEy, narrow emission spectra (FWHM = 17 nm), and high color purity (CIEy = 0.046), revealing the attractive technical advantages and commercial application potential.

Efficient OLEDs with Alleviated Efficiency Roll‐Off Based on MR‐TADF Materials Containing Indolo[3,2,1‐jk]carbazole

AbstractThe multiple resonance thermally activated delayed fluorescence (MR‐TADF) materials can meet the requirement of a high color gamut of displays due to their narrowband emission. However, most reported organic light‐emitting diodes (OLEDs) based on MR‐TADF materials suffer from severe efficiency roll‐off. Herein, three green MR‐TADF emitters, p‐ICz‐BNCz, m‐ICz‐BNCz, and dm‐ICz‐BNCz, are obtained by introducing bulky indolo[3,2,1‐jk]carbazole (ICz) units into the classical DtBuCzB skeleton at the para or meta positions relative to the boron‐substituted phenyl ring. Compared with the para‐substitution of ICz, the meta‐substitution not only increases the reverse intersystem crossing rate constants of m‐ICz‐BNCz and dm‐ICz‐BNCz by nearly three times, but also makes their configurations more twisted. The two factors work together to improve the utilization of triplet excitons of the emitters, showing photoluminescence quantum yields exceeding 90%. As a result, the two corresponding OLEDs exhibit maximum external quantum efficiency (EQEmax) values of 28% and 32.9%, respectively, with low‐efficiency roll‐off. At the high brightness of 1000 cd m−2, the EQE values of the two devices still maintain at 19.3% and 25.0%, respectively. In addition, green emissions with Commission Internationale de l'Eclairage coordinates of (0.20, 0.70) and (0.30, 0.67) are also observed.

Through‐Space Charge‐Transfer Organogold(III) Complexes Enable High‐Performance X‐ray Scintillation and Imaging

AbstractOrganic scintillators have recently attracted growing attention for X‐ray detection in industrial and medical applications. However, these materials still face critical obstacles of low attenuation efficiency and/or inefficient triplet exciton utilization. Here we developed a new category of organogold(III) complexes, Tp‐Au‐1 and Tp‐Au‐2, through adopting a through‐space interaction motif to realize high X‐ray attenuation efficiency and efficient harvesting of triplet excitons for emission. Thanks to the efficient through‐space charge transfer process, this panel of complexes achieved higher photoluminescence quantum yield and shorter radiative lifetimes compared with the through‐bond reference complexes. Inspiringly, these organogold(III) complexes exhibited polarity‐dependent emission origins: thermally activated delayed fluorescence and/or phosphorescence. Under X‐ray irradiation, Tp‐Au‐2 manifested intense radioluminescence together with a record‐high scintillation light yield of 77,600 photons MeV−1 for organic scintillators. The resulting scintillator screens demonstrated high‐quality X‐ray imaging with >16.0 line pairs mm−1 spatial resolution, outstripping most organic and inorganic scintillators. This finding provides a feasible strategy for the design of superior organic X‐ray scintillators.

Through-Space Charge-Transfer Organogold(III) Complexes Enable High-Performance X-ray Scintillation and Imaging.

Organic scintillators have recently attracted growing attention for X-ray detection in industrial and medical applications. However, these materials still face critical obstacles of low attenuation efficiency and/or inefficient triplet exciton utilization. Here we developed a new category of organogold(III) complexes, Tp-Au-1 and Tp-Au-2, through adopting a through-space interaction motif to realize high X-ray attenuation efficiency and efficient harvesting of triplet excitons for emission. Thanks to the efficient through-space charge transfer process, this panel of complexes achieved higher photoluminescence quantum yield and shorter radiative lifetimes compared with the through-bond reference complexes. Inspiringly, these organogold(III) complexes exhibited polarity-dependent emission origins: thermally activated delayed fluorescence and/or phosphorescence. Under X-ray irradiation, Tp-Au-2 manifested intense radioluminescence together with a record-high scintillation light yield of 77,600 photons MeV-1 for organic scintillators. The resulting scintillator screens demonstrated high-quality X-ray imaging with >16.0 line pairs mm-1 spatial resolution, outstripping most organic and inorganic scintillators. This finding provides a feasible strategy for the design of superior organic X-ray scintillators.

Green solvent-processed organic solar cells based on a small-molecule Ir(iii) complex as electron donor materials

Through the synergistic involvement of an environmentally friendly solvent and high temperature spin-coating process, the power conversion efficiency for small-molecule Ir(iii) complex based organic solar cells could be significantly improved.

Encapsulated TADF macrocycles for high-efficiency solution-processed and flexible organic light-emitting diodes.

Macrocyclic thermally activated delayed fluorescence (TADF) emitters have been demonstrated to realize high efficiency OLEDs, but the design concept was still confined to rigid π-conjugated structures. In this work, two macrocyclic TADF emitters, Cy-BNFu and CyEn-BNFu, with a flexible alkyl chain as a linker and bulky aromatic hydrocarbon wrapping units were designed and synthesized. The detailed photophysical analysis demonstrates that the flexible linker significantly enhances the solution-processibility and flexibility of the parent TADF core without sacrificing the radiative transition and high PLQY. Moreover, benefiting from sufficient encapsulation of both horizontal and vertical space, the macrocyclic CyEn-BNFu further isolated the TADF core and inhibited the aggregation caused quenching, which benefits the utilization of triplet excitons. As a result, the non-doped solution-processed OLEDs based on CyEn-BNFu exhibit high maximum external quantum efficiencies (EQE) up to 32.3%, which were 3 times higher than those of the devices based on the parent molecule. In particular, these macrocyclic TADF emitters ensure the fabrication of flexible OLEDs with higher brightness, color purity and bending resistance. This work opens a way to construct macrocyclic TADF emitters with a flexible alkyl chain linker and highlights the benefits of such encapsulated macrocycles for optimizing the performance of flexible solution-processed devices.

Narrowband TADF emitters with high utilization of triplet excitons: theoretical insights and molecular design.

Narrowband emitters with thermally activated delayed fluorescence (TADF) features, known as multi-resonant TADF (MR-TADF) emitters, are drawing increasing research interest owing to their properties of high efficiency and excellent color purity. However, MR-TADF-based devices often face serious efficiency roll-off at high luminance intensity, which could be attributed to undesired triplet-triplet annihilation (TTA) caused by the structural planarity and relatively small reverse intersystem crossing rate constants (krisc) of MR-TADF emitters. Herein, combining a sp3-C inserted strategy to suppress harmful bimolecular interactions and chalcogens to improve the krisc, a series of asymmetric narrowband emitters, namely, DMAC-O, DMAC-S, and DMAC-Se, have been theoretically designed to break the slow rate-limiting step of krisc of experimental BN-DMAC. For comparison, both O and Se atoms were doped into the MR skeleton to substitute two sp3-inserted units, yielding BN-O-Se. The combination of TD-DFT and the wavefunction-based STEOM-DLPNO-CCSD approach exhibits that those asymmetric molecules are promising for simultaneously exhibiting narrow emission spectral full-width at half-maximums (FWHMs) and high luminous efficiencies. The contributions of chalcogens to hole distributions result in red-shifted fluorescent peaks, and the asymmetric strategy also helps with twisted molecular configuration, which is beneficial for suppressing unfavorable TTA. Furthermore, the incorporation of chalcogens is sufficient to promote the intersystem crossing and reverse intersystem crossing channels of asymmetric emitters. More importantly, the doped heavy Se atom results in a significantly increased krisc of 2.32 × 106 s-1 for DMAC-Se, which is more than 200 times larger than 1.09 × 104 s-1 of pristine BN-DMAC. These results suggest that the combination of the heavy Se atom and an sp3-inserted unit is a feasible strategy for achieving poor planarity and significantly enhancing krisc, which will help in harvesting triplet excitons, thereby inhibiting efficiency roll-off in corresponding narrowband devices.

Electrochemiluminescence of hot exciton nanomaterial with boosted efficiency for visual bioanalysis

The electrochemiluminescence (ECL) efficiency (ΦECL) of organic ECL emitters is greatly influenced by the excited state property and component. Here we design a hot exciton molecule (BCzP-BT) with hybridized local and charge-transfer (HLCT) excited state property to prepare the first hot exciton organic nanorods (BB NRs). The BB NRs exhibit both sensitive annihilation and intense coreactant ECL emissions with a band gap emission model. Due to the reverse intersystem crossing among high-lying excited states (hRISC), the HLCT excited state property leads to highly efficient utilization of triplet excitons and thus high photoluminescence quantum yields (ΦPL) of more than 80% at 554 nm, which is demonstrated by solvatochromic experiments and quantum chemical calculations. The high ΦPL endows BB NRs with superior ΦECL over other nanoemitters, even the thermally activated delayed fluorescence materials, and thus offers highly-efficient nanoemitters for the design of ECL imaging strategy. As a proof of concept, an ECL probe is constructed by assembling BHQ2-ssDNA on BB NRs to integrate CRISPR/Cas12a system for target recognition, which produces a rapid and high-throughput ECL imaging platform. The proposed imaging method for HPV16 DNA detection shows excellent performance with a detection limit of 0.6 fM. This work broadens the application of hot exciton materials in ECL field, and opens a new avenue for developing next-generation ECL devices.

Twisted vibration of Aza-azulene promotes ultra-fast triplet-singlet up-conversion for extremely stable red electroluminescence

The fast triplet-singlet up-conversion based on thermally activated delayed fluorescence (TADF) molecules is a unique effort to improve the utilization of triplet excitons. We construct two TADF molecules using nonbenzenoid aromatic aza-azulene as donor and triazine as acceptor. The distortion structural resonance of aza-azulene enhances the reverse intersystem crossing (RISC) process, resulting in efficient inter/intramolecular charge transfer (CT) emission. The rate constant of RISC (kRISC) is 9.0 × 106 and 8.4 × 106 s−1 in solid states. Efficient Förster Energy Transfer (FRET) and/or Dexter Energy Transfer (DET) between these molecules and dopants make them suitable host materials to improve the performances of red phosphorescent organic light-emitting diodes (OLEDs). Especially, the top-emitting device achieves an external quantum efficiency (EQE) of 51.3% and a current efficiency of 68.2 cd/A at 1000 cd m−2 with a long device lifetime of LT95 = 52,350 and LT50 = 536,200 h.

Performance improved white full-organic light emitting diodes (WOLED) based on tricolor dual-layer of hyperfluorescence (HF)

Efficiency-improved white-light organic light emitting diodes (WOLED) was designed, which based on thermally activated delayed fluorescence (TADF) tricolor-hyperfluorescent dual-layers. Benefited from the efficient utilization of triplet excitons by effective reverse intersystem crossing (RISC), this WOLED features improved efficiency and long lifetime. In this work, blue OLED based on emitting layer of mCP:BD340 was developed and dual-emissive hyperfluorescent OLEDs based on mono-emitting layer of CBP:4CzIPN (green TADF emitter):DBP (red emissive fluorescent molecule) were investigated. Then, the hyperfluorescent WOLED based on the two emitting layers was designed. The good performances were benefited from the highly efficient hyperfluorescent system. This device architecture design demonstrated here represents a kind of full-organic, low-cost white-light OLED for power-efficient light source.

Triplet–Triplet Annihilation Enhanced Deep‐Blue Organic Light‐Emitting Diodes by Naphtho[1,2‐d]imidazole‐Isomer Derivatives with Spin–Orbit Coupling

AbstractThe utilization of triplet excitons is of great importance for organic light‐emitting diodes (OLEDs). Triplet–triplet annihilation (TTA) is one of the effective tactics to achieve high efficiency deep‐blue organic electroluminescence emitters by converting two triplet excitons into one singlet exciton. Whereas, in addition to the 25% electrogenerated singlet excitons, the proportion of radiative singlet excitons (RSE) produced by the TTA process is usually only 15%; thus the total radiative excitons are 40%. In this study, ≈35% of RSE is achieved by the TTA process (total 60%) with two deep‐blue emitters based on the isomeric naphthoimidazole (NI) unit and anthracene bridge. As a result, non‐doped OLEDs based on the two NI derivatives as emitting layers achieve maximum external quantum efficiencies of 10.9% and 11.2% with an identical deep‐blue emission peak of 452 nm, which are the best TTA OLEDs with a Commission Internationale de l'Eclairage chromaticity Y coordinate below 0.15. Theoretical and experimental results demonstrate that the TTA process can be improved owing to the efficient spin–orbit interactions, even though the energy levels of the triplet pairs are higher than the calculated second triplet excited states.

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.

Tetradentate Pt(II) Complex as Phosphorescent Sensitizer for Highly Efficient Green Organic Light‐Emitting Diodes with Low Efficiency Roll‐Off

AbstractTetradentate Pt(II) complex (Pt‐Trz)‐sensitized green fluorescent organic light‐emitting‐diodes (OLEDs) are developed to improve the external quantum efficiency (EQE) of a green fluorescent emitter (PhtBuPAD). Pt(II) complex and a green fluorescent emitter with bulky blocking units are chosen to efficiently manage energy transfer in phosphor‐sensitized OLEDs. The Pt‐Trz phosphor‐sensitized fluorescent OLED with 2 wt% PhtBuPAD exhibits high EQE of 24.2% and maintains high EQE of 23.4% at a luminance of 3000 cd m−2, realizing low efficiency roll‐off of 3.8%. Moreover, high EQE is achieved even at a high PhtBuPAD‐doping concentration of 3 wt%. In addition, the 2 wt% PhtBuPAD‐doped phosphor‐sensitized device achieves high maximum power efficiency of 91.1 lm W−1, one of the highest power efficiencies among existing green sensitized devices. This study demonstrates that Pt‐Trz phosphor‐sensitized devices with an excellent triplet exciton utilization ability can achieve high EQE, low efficiency roll‐off, and weak doping concentration quenching simultaneously.

Sulfur-Decorated Nonaromatic Amine Emitters Towards Efficient Triplet Exciton Utilization in Organic Light-Emitting Diodes.

Purely organic compounds served as promising materials for organic electronics have intrigued extensive research focuses owing to their unique photophysical and electronic properties. In the field of organic light-emitting diodes (OLEDs), the utilization of triplet excitons is of great significance for realizing high-performance devices. In contrast to the traditional aromatic amine-based counterparts, sulfur atom with a high-level outer orbit simultaneously provides powerful electron-donating ability and striking spin-orbit coupling effect to facilitate the utilization of theoretically spin-forbidden triplet excitons, demonstrating great prospect in constructing highly emissive purely organic emitters for OLED applications. Herein, we summarize the currently developed sulfur-decorated nonaromatic amine-based emitters exhibiting attractive photoelectronic characteristics, and gain insight into the understanding of molecular design and photophysical processes of these emitters, providing new perspectives for enriching the existing luminescent material systems and designing high-performance emitters.