Organic light-emitting diodes (OLEDs)1–5, quantum-dot-based LEDs6–10, perovskite-based LEDs11–13 and micro-LEDs14,15 have been championed to fabricate lightweight and flexible units for next-generation displays and active lighting. Although there are already some high-end commercial products based on OLEDs, costs must decrease whilst maintaining high operational efficiencies for the technology to realise wider impact.  Here we demonstrate efficient action of radical-based OLEDs16, whose emission originates from a spin doublet, rather than a singlet or triplet exciton. While the emission process is still spin-allowed in these OLEDs, the efficiency limitations imposed by triplet excitons are circumvented for doublets. Using a luminescent radical emitter, we demonstrate an OLED with maximum external quantum efficiency of 27 per cent at a wavelength of 710 nanometres—the highest reported value for deep-red and infrared LEDs. For a standard closed-shell organic semiconductor, holes and electrons occupy the highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs), respectively, and recombine to form singlet or triplet excitons. Radical emitters have a singly occupied molecular orbital (SOMO) in the ground state, giving an overall spin-1/2 doublet. If—as expected on energetic grounds—both electrons and holes occupy this SOMO level, recombination returns the system to the ground state, giving no light emission. However, in our very efficient OLEDs, we achieve selective hole injection into the HOMO and electron injection to the SOMO to form the fluorescent doublet excited state with near-unity internal quantum efficiency. Organic light-emitting devices containing radical emitters can achieve an efficiency of 27 per cent at deep-red and infrared wavelengths based on the excitation of spin doublets, rather than singlet or triplet states.

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Efficient radical-based light-emitting diodes with doublet emission

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Near-Infrared (NIR) Organic Light-Emitting Diodes (OLEDs): Challenges and Opportunities

Light in the second near-infrared window, especially beyond 1500 nm, shows enhanced tissue transparency for high-resolution in vivo optical bioimaging due to decreased tissue scattering, absorption, and autofluorescence. Despite some inorganic luminescent nanoparticles have been developed to improve the bioimaging around 1500 nm, it is still a great challenge to synthesize organic molecules with the absorption and emission toward this region. Here, we present J-aggregates with 1360 nm absorption and 1370 nm emission formed by self-assembly of amphiphilic cyanine dye FD-1080 and 1,2-dimyristoyl-sn-glycero-3-phosphocholine. Molecular dynamics simulations were further employed to illustrate the self-assembly process. Superior spatial resolution and high signal-to-background ratio of J-aggregates were demonstrated for noninvasive brain and hindlimb vasculature bioimaging beyond 1500 nm. The efficacy evaluation of the clinically used hypotensor is successfully achieved by high-resolution in vivo dynamic vascular imaging with J-aggregates.

J-Aggregates of Cyanine Dye for NIR-II in Vivo Dynamic Vascular Imaging beyond 1500 nm

Traditional photothermal therapy requires high-intensity laser excitation for cancer treatments due to the low photothermal conversion efficiency (PCE) of photothermal agents (PTAs). PTAs with ultra-high PCEs can decrease the required excited light intensity, which allows safe and efficient therapy in deep tissues. In this work, a PTA is synthesized with high PCE of 88.3% based on a BODIPY scaffold, by introducing a CF3 "barrier-free" rotor on the meso-position (tfm-BDP). In both the ground and excited state, the CF3 moiety in tfm-BDP has no energy barrier to rotation, allowing it to efficiently dissipate absorbed (NIR) photons as heat. Importantly, the barrier-free rotation of CF3 can be maintained after encapsulating tfm-BDP into polymeric nanoparticles (NPs). Thus, laser irradiation with safe intensity (0.3 W cm-2 , 808 nm) can lead to complete tumor ablation in tumor-bearing mice after intravenous injection of tfm-BDP NPs. This strategy of "barrier-free rotation" provides a new platform for future design of PTT agents for clinical cancer treatment.

NIR Light-Driving Barrier-Free Group Rotation in Nanoparticles with an 88.3% Photothermal Conversion Efficiency for Photothermal Therapy.

Researchers have spared no effort to design new thermally activated delayed fluorescence (TADF) emitters for high-efficiency organic light-emitting diodes (OLEDs). However, efficient long-wavelength TADF emitters are rarely reported. Herein, a red TADF emitter, TPA-PZCN, is reported, which possesses a high photoluminescence quantum yield (ΦPL ) of 97% and a small singlet-triplet splitting (ΔEST ) of 0.13 eV. Based on the superior properties of TPA-PZCN, red, deep-red, and near-infrared (NIR) OLEDs are fabricated by utilizing different device structure strategies. The red devices obtain a remarkable maximum external quantum efficiency (EQE) of 27.4% and an electroluminescence (EL) peak at 628 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.65, 0.35), which represents the best result with a peak wavelength longer than 600 nm among those of the reported red TADF devices. Furthermore, an exciplex-forming cohost strategy is adopted. The devices achieve a record EQE of 28.1% and a deep-red EL peak at 648 nm with the CIE coordinates of (0.66, 0.34). Last, nondoped devices exhibit 5.3% EQE and an NIR EL peak at 680 nm with the CIE coordinates of (0.69, 0.30).

High-Efficiency Red Organic Light-Emitting Diodes with External Quantum Efficiency Close to 30% Based on a Novel Thermally Activated Delayed Fluorescence Emitter.

The development of high-efficiency and low-cost organic emissive materials and devices is intrinsically limited by the energy-gap law and spin statistics, especially in the near-infrared (NIR) region. A novel design strategy is reported for realizing highly efficient thermally activated delayed fluorescence (TADF) materials via J-aggregates with strong intermolecular charge transfer (CT). Two organic donor-acceptor molecules with strong and planar acceptor are designed and synthesized, which can readily form J-aggregates with strong intermolecular CT in solid states and exhibit wide-tuning emissions from yellow to NIR. Experimental and theoretical investigations expose that the formation of such J-aggregates mixes Frenkel excitons and CT excitons, which not only contributes to a fast radiative decay rate and a slow nonradiative decay rate for achieving nearly unity photoluminescence efficiency in solid films, but significantly decreases the energy gap between the lowest singlet and triplet excited states (≈0.3 eV) to induce high-efficiency TADF even in the NIR region. These organic light-emitting diodes exhibit external quantum efficiencies of 15.8% for red emission and 14.1% for NIR emission, which represent the best result for NIR organic light-emitting diodes (OLEDs) based on TADF materials. These findings open a new avenue for the development of high-efficiency organic emissive materials and devices based on molecular aggregates.

Highly Efficient Thermally Activated Delayed Fluorescence via J-Aggregates with Strong Intermolecular Charge Transfer.

The simultaneous realization of high quantum yield and exciton utilizing efficiency (ηr) is still a formidable challenge in near-infrared (NIR) fluorescent organic light-emitting diodes (FOLEDs). Here, to achieve a high quantum yield, a novel NIR dye, 4,9-bis(4-(diphenylamino)phenyl)-naphtho[2,3-c][1,2,5]selenadiazole, is designed and synthesized with a large highest occupied molecular orbital/lowest unoccupied molecular orbital overlap and an aggregation-induced emission property, which demonstrates a high photoluminescence quantum yield of 27% at 743 nm in toluene and 29% at 723 nm in a blend film. For a high ηr, an orange-emitting thermally activated delayed fluorescent material, 1,2-bis(9,9-dimethyl-9,10-dihydroacridine)-4,5-dicyanobenzene, is chosen as the sensitizing host to harvest triplet excitons in devices. The optimized devices achieve a good ηr of 45.7% and a high external quantum efficiency up to 2.65% at 730 nm, with a very small efficiency roll-off of 2.41% at 200 mA cm−2, which are among the most efficient values for NIR-FOLEDs over 700 nm. The effective utilization of triplet excitons via the thermally activated delayed fluorescence-sensitizing host will pave a way to realize high-efficiency NIR-FOLEDs with small efficiency roll-off.

High-Efficiency Near-Infrared Fluorescent Organic Light-Emitting Diodes with Small Efficiency Roll-Off: A Combined Design from Emitters to Devices

Near-infrared-II thermally activated delayed fluorescence organic light-emitting diodes.

Pursuing purely organic materials with high-efficiency near-infrared (NIR) emissions is fundamentally limited by the large non-radiative decay rates (knr) governed by the energy gap law. To date, reported endeavors to decelerate knr are mainly focused on reducing the electron-vibration coupling with the electronic nonadiabatic coupling assumed as a constant. Here, we demonstrated a feasible and innovative strategy by employing intermolecular charge-transfer (CT) aggregates (CTA) to realize high-efficiency NIR emissions via nonadiabatic coupling suppression. The formation of CTA engenders intermolecular CT in the excited states; thereby, not only reducing the electronic nonadiabatic coupling and contributing to small knr for high-efficiency NIR photoluminescence, but also stabilizing excited-state energies and achieving thermally activated delayed fluorescence for high-efficiency NIR electroluminescence. This work provides new insights into aggregates and opens a new avenue for organic materials to overcome the energy gap law and achieve high-efficiency NIR emissions.

Qingxin Liang

Papers

Highly Efficient Thermally Activated Delayed Fluorescence via J-Aggregates with Strong Intermolecular Charge Transfer.

High-Efficiency Near-Infrared Fluorescent Organic Light-Emitting Diodes with Small Efficiency Roll-Off: A Combined Design from Emitters to Devices

Near-infrared-II thermally activated delayed fluorescence organic light-emitting diodes.

Intermolecular charge-transfer aggregates enable high-efficiency near-infrared emissions by nonadiabatic coupling suppression

Controlled-fabrication and assembly-induced emission enhancement (AIEE) of near-infrared emitted gold nanoclusters capped by thiolactic acid