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Organic light-emitting diodes have become a mainstream display technology because of their desirable features. Third-generation electroluminescent devices that emit light through a mechanism called thermally activated delayed fluorescence are currently garnering much attention. However, unsatisfactory device stability is still an unresolved issue in this field. Here we demonstrate that electron-transporting n-type hosts, which typically include an acceptor moiety in their chemical structure, have the intrinsic ability to balance the charge fluxes and broaden the recombination zone in delayed fluorescence organic electroluminescent devices, while at the same time preventing the formation of high-energy excitons. The n-type hosts lengthen the lifetimes of green and blue delayed fluorescence devices by > 30 and 1000 times, respectively. Our results indicate that n-type hosts are suitable to realize stable delayed fluorescence organic electroluminescent devices. 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.

Charge-transfer (CT) complexes, formed by electron transfer from a donor to an acceptor, play a crucial role in organic semiconductors. Excited-state CT complexes, termed exciplexes, harness both singlet and triplet excitons for light emission, and are thus useful for organic light-emitting diodes (OLEDs). However, present exciplex emitters often suffer from low photoluminescence quantum efficiencies (PLQEs), due to limited control over the relative orientation, electronic coupling and non-radiative recombination channels of the donor and acceptor subunits. Here, we use a rigid linker to control the spacing and relative orientation of the donor and acceptor subunits, as demonstrated with a series of intramolecular exciplex emitters based on 10-phenyl-9,10-dihydroacridine and 2,4,6-triphenyl-1,3,5-triazine. Sky-blue OLEDs employing one of these emitters achieve an external quantum efficiency (EQE) of 27.4% at 67 cd m −2 with only minor efficiency roll-off (EQE = 24.4%) at a higher luminous intensity of 1,000 cd m −2 . As a control experiment, devices using chemically and structurally related but less rigid emitters reach substantially lower EQEs. These design rules are transferrable to other donor/acceptor combinations, which will allow further tuning of emission colour and other key optoelectronic properties. The use of rigid linkers to control the relative position and interaction of donor and acceptor units in exciplex emitters leads to the realization of organic light-emitting devices with enhanced external quantum efficiency.

A spin-flip from a triplet to a singlet excited state, that is, reverse intersystem crossing (RISC), is an attractive route for improving light emission in organic light-emitting diodes, as shown by devices using thermally activated delayed fluorescence (TADF). However, device stability and efficiency roll-off remain challenging issues that originate from a slow RISC rate (kRISC). Here, we report a TADF molecule with multiple donor units that form charge-resonance-type hybrid triplet states leading to a small singlet–triplet energy splitting, large spin–orbit couplings, and a dense manifold of triplet states energetically close to the singlets. The kRISC in our TADF molecule is as fast as 1.5 × 107 s−1, a value some two orders of magnitude higher than typical TADF emitters. Organic light-emitting diodes based on this molecule exhibit good stability (estimated T90 about 600 h for 1,000 cd m−2), high maximum external quantum efficiency (>29.3%) and low efficiency roll-off (<2.3% at 1,000 cd m−2).An organic molecule, 5Cz-TRZ, with multiple donor units supports fast reverse intersystem crossing, allowing fabrication of high-performance organic light-emitting diodes.

Thermally activated delayed fluorescence enables organic semiconductors with charge transfer-type excitons to convert dark triplet states into bright singlets via reverse intersystem crossing. However, thus far, the contribution from the dielectric environment has received insufficient attention. Here we study the role of the dielectric environment in a range of thermally activated delayed fluorescence materials with varying changes in dipole moment upon optical excitation. In dipolar emitters, we observe how environmental reorganization after excitation triggers the full charge transfer exciton formation, minimizing the singlet–triplet energy gap, with the emergence of two (reactant-inactive) modes acting as a vibrational fingerprint of the charge transfer product. In contrast, the dielectric environment plays a smaller role in less dipolar materials. The analysis of energy–time trajectories and their free-energy functions reveals that the dielectric environment substantially reduces the activation energy for reverse intersystem crossing in dipolar thermally activated delayed fluorescence emitters, increasing the reverse intersystem crossing rate by three orders of magnitude versus the isolated molecule. The role of the dielectric environment in thermally activated delayed fluorescence (TADF) is not yet fully understood. Here the authors reveal the relevance of environment–emitter interactions in gating the reverse intersystem crossing and its particular relevance in dipolar TADF emitters.

Engineering a low singlet-triplet energy gap ( ΔE ST ) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 10 5  cm −1 ) and a relatively large ΔE ST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔE ST in organic DF emitters. A low singlet-triplet energy gap, necessary for delayed fluorescence organic semiconductors, results in a small radiative rate that limits performance in OLEDs. Here, the authors show that it is possible to reconcile these conflicting requirements in materials that can access both high oscillator strength intramolecular excitations and intermolecular charge transfer states.

Understanding the host‐guest interactions for thermally activated delayed fluorescence (TADF) emitters is critical because the interactions between the host matrices and TADF emitters enable precise control on the optoelectronic performance, whereas technologically manipulating the singlet and triplet excitons by using different kinds of host‐guest interactions remains elusive. Here, we report a comprehensive picture that rationalizes host‐guest interaction‐modulated exciton recombination by using time‐resolved spectroscopy. We found that the early‐time relaxation is accelerated in polar polymer because dipole‐dipole interaction facilitates the stabilization of the 1CT state. However, an opposite trend is observed in longer delay time, and faster decay in the less polar polymer is ascribed to the π‐π interaction that plays the dominant role in the later stage of the excited state. Our findings highlight the technological engineering singlet and triplet excitons using different kinds of host‐guest interactions based on their electronic characteristics. The previous work about host‐guest interactions of TADF materials mainly focus on the origin of solid‐state solvation and the temporal behavior of TADF emitters in various hosts, which are all based on the singlet state with CT character. Within this context, technologically manipulating the singlet and triplet excitons by using different kinds of host‐guest interactions remains elusive. Here, we report a comprehensive picture that rationalizes host‐guest interaction‐modulated exciton recombination by using time‐resolved spectroscopy. The present investigation provides valuable insights for manipulating the TADF device in glassy matrices by introducing codopants with polar side chains or conjugate π‐planar structures.

Perovskite light-emitting diodes (LEDs) have attracted broad attention due to their rapidly increasing external quantum efficiencies (EQEs) 1 – 15 . However, most high EQEs of perovskite LEDs are reported at low current densities (<1 mA cm −2 ) and low brightness. Decrease in efficiency and rapid degradation at high brightness inhibit their practical applications. Here, we demonstrate perovskite LEDs with exceptional performance at high brightness, achieved by the introduction of a multifunctional molecule that simultaneously removes non-radiative regions in the perovskite films and suppresses luminescence quenching of perovskites at the interface with charge-transport layers. The resulting LEDs emit near-infrared light at 800 nm, show a peak EQE of 23.8% at 33 mA cm −2 and retain EQEs more than 10% at high current densities of up to 1,000 mA cm −2 . In pulsed operation, they retain EQE of 16% at an ultrahigh current density of 4,000 mA cm −2 , along with a high radiance of more than 3,200 W s −1  m −2 . Notably, an operational half-lifetime of 32 h at an initial radiance of 107 W s −1  m −2 has been achieved, representing the best stability for perovskite LEDs having EQEs exceeding 20% at high brightness levels. The demonstration of efficient and stable perovskite LEDs at high brightness is an important step towards commercialization and opens up new opportunities beyond conventional LED technologies, such as perovskite electrically pumped lasers. Perovskite LEDs with exceptional performance at high brightness are demonstrated achieving an operational half-lifetime of 32 hours, an important step towards commercialization opening up new opportunities beyond conventional LED technologies, such as perovskite electrically pumped lasers.

Perovskite light-emitting diodes (LEDs) show promises for next-generation displays owing to their excellent luminescent properties and low cost. Despite substantial progress with green- and red-emitting devices, the development of efficient blue perovskite LEDs has lagged behind. Here we demonstrate efficient blue perovskite LEDs based on a mixed two-dimensional–three-dimensional perovskite and a multifunctional ionic additive that enables control over the reduced-dimensional phases, non-radiative recombination channels and spectral stability. We report a series of devices that emit efficient electroluminescence from mixed bromide/chloride quasi-three-dimensional regions, with external quantum efficiencies of up to 21.4% (at a luminance of 22 cd m –2 and emission peak at 483 nm), 13.2% (at a luminance of 2.0 cd m – 2 and emission peak at 474 nm) and 7.3% (at a luminance of 6 cd m –2 and emission peak at 464 nm). Devices show a nearly 30-fold increase in operational stability compared with control LEDs, with a half-lifetime of 129 min at an initial luminance of 100 cd m –2 . Our findings demonstrate the performance of blue perovskite LEDs close to that of state-of-the-art blue organic LEDs and inorganic quantum dot LEDs and provide a new approach to design multifunctional molecules to boost the performance of perovskite optoelectronic devices. Addition of a multifunctional ionic additive in mixed two-dimensional–three-dimensional bromide/chloride perovskites enables efficient blue perovskite LEDs with external quantum efficiency of up to 21.4% and half-lifetime of 129 min at an initial luminance of 100 cd m –2 .

Emerging functional materials such as halide perovskites are intrinsically unstable, causing long-term instability in optoelectronic devices made from these materials. This leads to difficulty in capturing useful information on device degradation through time-consuming optical characterization in their operating environments. Despite these challenges, understanding the degradation mechanism is crucial for advancing the technology towards commercialization. Here we present a self-supervised machine learning model that utilizes a multi-channel correlation and blind denoising to recover images without high-quality references, enabling fast and low-dose measurements. We perform operando luminescence mapping of various emerging optoelectronic semiconductors, including organic and halide perovskite photovoltaic and light-emitting devices. By tracking the spatially resolved degradation in electroluminescence of mixed-halide perovskite blue-light-emitting diodes, we discovered that lateral ion migration (perpendicular to the external electric field) during device operation triggers the formation of chloride-rich defective regions that emit poorly—a mechanism that would not be resolvable with conventional imaging approaches. Halide perovskites are promising materials for light-emitting devices, given their narrowband emission and solution processability. However, detailed information on device degradation during operation is required to improve their stability, and this is challenging to obtain. Ji et al. propose a self-supervised deep learning method to capture multi-dimensional images of such devices in their operating regime faster than allowed by conventional imaging techniques.

Thermally-activated delayed fluorescence (TADF) enables organic semiconductors with charge transfer (CT)-type excitons to convert dark triplet states into bright singlets via a reverse intersystem crossing (rISC) process. Here, we consider the role of the dielectric environment in a range of TADF materials with varying changes in dipole moment upon optical excitation. In a dipolar reference emitter, TXO-TPA, environmental reorganisation after excitation in both solution and doped films triggers the formation of the full CT product state. This lowers the singlet excitation energy by 0.3 eV and minimises the singlet-triplet energy gap ({\\Delta}EST). Using impulsive Raman measurements, we observe the emergence of two (reactant-inactive) modes at 412 and 813 cm-1 as a vibrational fingerprint of the CT product. In contrast, the dielectric environment plays a smaller role in the electronic excitations of a less dipolar material, 4CzIPN. Quantum-chemical calculations corroborate the appearance of these new product modes in TXO-TPA and show that the dynamic environment fluctuations are large compared to {\\Delta}EST. The analysis of the energy-time trajectories and the corresponding free energy functions reveals that the dielectric environment significantly reduces the activation energy for rISC, thus increasing the rISC rate by up to three orders of magnitude when compared to a vacuum environment.