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Organic light-emitting diodes (OLEDs) 1 – 5 , quantum-dot-based LEDs 6 – 10 , perovskite-based LEDs 11 – 13 and micro-LEDs 14 , 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 OLEDs 16 , 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.

At present, electric lighting accounts for ~15% of global power consumption and thus the adoption of efficient, low-cost lighting technologies is important. Halide perovskites have been shown to be good emitters of pure red, green and blue light, but an efficient source of broadband white electroluminescence suitable for lighting applications is desirable. Here, we report a white light-emitting diode (LED) strategy based on solution-processed heterophase halide perovskites that, unlike GaN white LEDs, feature only one broadband emissive layer and no phosphor. Our LEDs operate with a peak luminance of 12,200 cd m−2 at a bias of 6.6 V and a maximum external quantum efficiency of 6.5% at a current density of 8.3 mA cm−2. Systematic in situ and ex situ characterizations reveal that the mechanism of efficient electroluminescence is charge injection into the α phase of CsPbI3, α to δ charge transfer and α–δ balanced radiative recombination. Future advances in fabrication technology and mechanistic understanding should lead to further improvements in device efficiency and luminance.Heterophase CsPbI3 perovskite gives rise to bright white phosphor-free LEDs.

Spin-flip in purely organic molecular systems is often described as a forbidden process; however, it is commonly observed and utilized to harvest triplet excitons in a wide variety of organic material-based applications. Although the initial and final electronic states of spin-flip between the lowest singlet and lowest triplet excited state are self-evident, the exact process and the role of intermediate states through which spin-flip occurs are still far from being comprehensively determined. Here, via experimental photo-physical investigations in solution combined with first-principles quantum-mechanical calculations, we show that efficient spin-flip in multiple donor–acceptor charge-transfer-type organic molecular systems involves the critical role of an intermediate triplet excited state that corresponds to a partial molecular structure of the system. Our proposed mechanism unifies the understanding of the intersystem crossing mechanism in a wide variety of charge-transfer-type molecular systems, opening the way to greater control over spin-flip rates.

Stretchable light-emitting materials are the key components for realizing skin-like displays and optical biostimulation. All the stretchable emitters reported to date, to the best of our knowledge, have been based on electroluminescent polymers that only harness singlet excitons, limiting their theoretical quantum yield to 25%. Here we present a design concept for imparting stretchability onto electroluminescent polymers that can harness all the excitons through thermally activated delayed fluorescence, thereby reaching a near-unity theoretical quantum yield. We show that our design strategy of inserting flexible, linear units into a polymer backbone can substantially increase the mechanical stretchability without affecting the underlying electroluminescent processes. As a result, our synthesized polymer achieves a stretchability of 125%, with an external quantum efficiency of 10%. Furthermore, we demonstrate a fully stretchable organic light-emitting diode, confirming that the proposed stretchable thermally activated delayed fluorescence polymers provide a path towards simultaneously achieving desirable electroluminescent and mechanical characteristics, including high efficiency, brightness, switching speed and stretchability as well as low driving voltage.We synthesized stretchable electroluminescent polymers capable of reaching a near-unity theoretical quantum yield through thermally activated delayed fluorescence. Their polymers show 125% stretchability with 10% external quantum efficiency and demonstrate a fully stretchable organic light-emitting diode.

Organic light-emitting diode (OLED) technology is promising for applications in next-generation displays and lighting. However, it is difficult—especially in large-area mass production—to cover a large substrate uniformly with organic layers, and variations in thickness cause the formation of shunting paths between electrodes 1 , 2 , thereby lowering device production yield. To overcome this issue, thicker organic transport layers are desirable because they can cover particles and residue on substrates, but increasing their thickness increases the driving voltage because of the intrinsically low charge-carrier mobilities of organics. Chemical doping of organic layers increases their electrical conductivity and enables fabrication of thicker OLEDs 3 , 4 , but additional absorption bands originating from charge transfer appear 5 , reducing electroluminescence efficiency because of light absorption. Thick OLEDs made with organic single crystals have been demonstrated 6 , but are not practical for mass production. Therefore, an alternative method of fabricating thicker OLEDs is needed. Here we show that extraordinarily thick OLEDs can be fabricated by using the organic–inorganic perovskite methylammonium lead chloride, CH 3 NH 3 PbCl 3 (MAPbCl 3 ), instead of organics as the transport layers. Because MAPbCl 3 films have high carrier mobilities and are transparent to visible light, we were able to increase the total thickness of MAPbCl 3 transport layers to 2,000 nanometres—more than ten times the thickness of standard OLEDs—without requiring high voltage or reducing either internal electroluminescence quantum efficiency or operational durability. These findings will contribute towards a higher production yield of high-quality OLEDs, which may be used for other organic devices, such as lasers, solar cells, memory devices and sensors. Extraordinarily thick organic light-emitting diodes can be fabricated using hybrid organic–inorganic perovskites as the transport layers, thus relaxing fabrication constraints without affecting their efficiency, voltage requirement or durability.

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 CIE x,y s 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 CIE y up to 0.74. In hyperfluorescent device architecture, the CIE x s further meet the x-coordinate requirements, i.e., NTSC (0.21) and BT. 2020 (0.17), while keeping their CIE y s ~ 0.7. Most importantly, hyperfluorescent devices display the high maximum external quantum efficiencies of over 25% and maximum luminance of over 10 5  cd m −2 with suppressed rolloffs (external quantum efficiency of ~20% at 10 4  cd m −2 ) and long device stabilities with LT 95 s of ~ 600 h. The device performance of pure-green hyperfluorescent organic light-emitting diodes remains unsatisfactory. Here, the authors report multi-resonance emitters with extended π−conjugation or increased donor strength, resulting in bright, efficient and stable pure-green hyperfluorescent devices.

An ultrapure deep-blue multi-resonance-induced thermally activated delayed fluorescence material (DOB2-DABNA-A) is designed and synthesized. Benefiting from a fully resonating extended helical π-conjugated system, this compound has a small ΔE ST value of 3.6 meV and sufficient spin–orbit coupling to exhibit a high-rate constant for reverse intersystem crossing ( k RISC  = 1.1 × 10 6  s –1 ). Furthermore, an organic light-emitting diode employing DOB2-DABNA-A as an emitter is fabricated; it exhibits ultrapure deep-blue emission at 452 nm with a small full width at half maximum of 24 nm, corresponding to Commission Internationale de l’Éclairage (CIE) coordinates of (0.145, 0.049). The high k RISC value reduces the efficiency roll-off, resulting in a high external quantum efficiency (EQE) of 21.6% at 1000 cd m –2 . The realization of a BT.2020 blue emitter is challenging due to the spectral redshift associated with π-extension. Here, the authors report a multi-resonance thermally activated delayed fluorescence emitter, exhibiting ultrapure deep-blue narrow emission and CIEy ≤ 0.05 exceeding 20% efficiency.

The field of plasmonics, which studies the resonant interactions of electromagnetic waves and free electrons in solid-state materials 1 , has yet to be put to large-scale commercial application 2 owing to the large amount of loss that usually occurs in plasmonic materials 3 . Organic light-emitting devices (OLEDs) 4 – 7 have been incorporated into billions of commercial products because of their good colour saturation, versatile form factor 8 and low power consumption 9 , but could still be improved in terms of efficiency and stability. Although OLEDs incorporating organic phosphors achieve an internal charge-to-light conversion of unity 10 , their refractive index contrast reduces the observable fraction of photons outside the device to around 25 per cent 11 – 13 . Further, during OLED operation, a localized buildup of slow-decaying 14 triplet excitons and charges 15 gradually reduces the brightness of the device in a process called ageing 16 , 17 , which can result in ‘burn-in’ effects on the display. Simultaneously improving device efficiency and stability is of paramount importance for OLED technology. Here we demonstrate an OLED that uses the decay rate enhancement 18 of a plasmonic system to increase device stability, while maintaining efficiency by incorporating a nanoparticle-based out-coupling scheme to extract energy from the plasmon mode. Using an archetypal phosphorescent emitter, we achieve a two-fold increase in operational stability at the same brightness as a reference conventional device while simultaneously extracting 16 per cent of the energy from the plasmon mode as light. Our approach to increasing OLED stability avoids material-specific designs 19 – 22 and is applicable to all commercial OLEDs that are currently used for lighting panels, televisions and mobile displays. Plasmonic effects in organic light-emitting devices, which are normally considered a source of energy loss, are harnessed to enhance the stability of these devices while maintaining operational efficiency.

Organic light-emitting diodes (OLEDs) have outperformed conventional display technologies in smartphones, smartwatches, tablets, and televisions while gradually growing to cover a sizable fraction of the solid-state lighting industry. Blue emission is a crucial chromatic component for realizing high-quality red, green, blue, and yellow (RGBY) and RGB white display technologies and solid-state lighting sources. For consumer products with desirable lifetimes and efficiency, deep blue emissions with much higher power efficiency and operation time are necessary prerequisites. This article reviews over 700 papers covering various factors, namely, the crucial role of blue emission for full-color displays and solid-state lighting, the performance status of blue OLEDs, and the systematic development of fluorescent, phosphorescent, and thermally activated delayed fluorescence blue emitters. In addition, various challenges concerning deep blue efficiency, lifetime, and approaches to realizing deeper blue emission and higher efficacy for blue OLED devices are also described.

Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted a rising number of investigations due to their promising applications for high-resolution fullcolor displays and energy-saving solid-state lightings. Near-infrared (NIR) emitting dyes have gained increasing attention for their potential applications in electroluminescence and optical imaging in optical tele-communication platforms, sensing and medical diagnosis in recent decades. And a growing number of people focus on the “heavy metal-free” NIR electroluminescent materials to gain more design freedom with cost advantage. This review presents recent progresses in conjugated polymers and organic molecules for OLEDs/PLEDs according to their different luminous mechanism and constructing systems. The relationships between the organic fluorophores structures and electroluminescence properties are the main focus of this review. Finally, the approaches to enhance the performance of NIR OLEDs/PLEDs are described briefly. We hope that this review could provide a new perspective for NIR materials and inspire breakthroughs in fundamental research and applications.