Efficient Near‐Infrared Electroluminescence at 840 nm with “Metal‐Free” Small‐Molecule:Polymer Blends

Due to the so‐called energy‐gap law and aggregation quenching, the efficiency of organic light‐emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating fluorescent materials free from heavy or toxic metals, however, we are not aware of any report of EQEs over 1% (again for emission peaking at wavelengths > 800 nm), even for devices leveraging thermally activated delayed fluorescence (TADF). Here, the development of polymer light‐emitting diodes (PLEDs) peaking at 840 nm and exhibiting unprecedented EQEs (in excess of 1.15%) and turn‐on voltages as low as 1.7 V is reported. These incorporate a novel triazolobenzothiadiazole‐based emitter and a novel indacenodithiophene‐based transport polymer matrix, affording excellent spectral and transport properties. To the best of knowledge, such values are the best ever reported for electroluminescence at 840 nm with a purely organic and solution‐processed active layer, not leveraging triplet‐assisted emission.     

Narrowband and Pure Violet Organic Emitter with a Full Width at Half Maximum of 14 nm and y Color Coordinate of Below 0.02

Violet organic light‐emitting diodes (OLEDs) with a very narrow emission spectrum with a full width at half maximum of 14 nm and y color coordinate of 0.02 are developed using a indolo[3,2,1‐jk]carbazole‐derived pure violet emitter. The violet emitter, 2,5,13,16‐tetra‐tert‐butylindolo[3,2,1‐jk]‐indolo[1′,2′,3′:1,7]indolo[2,3‐b]carbazole (tDIDCz), is designed to have a very rigid molecular structure driven by the multiresonance‐type core structure through the alternating carbon and nitrogen atoms. The violet emitter is decorated with t‐butyl groups to prevent intermolecular aggregation and packing, which allow pure violet emission without excimer emission. The violet OLEDs derived from the tDIDCz emitter show a violet color coordinate of (0.164, 0.018) with a narrow emission spectrum and a full width at half maximum of 14 nm (105 meV). The external quantum efficiency of the pure violet OLEDs is 3.3%. This is the first work reporting pure violet emission without any ultraviolet emission below 380 nm and blue emission above 450 nm by showing a very narrow emission spectrum.     

Versatile Indolocarbazole‐Isomer Derivatives as Highly Emissive Emitters and Ideal Hosts for Thermally Activated Delayed Fluorescent OLEDs with Alleviated Efficiency Roll‐Off

Maintaining high efficiency at high brightness levels is an exigent challenge for real‐world applications of thermally activated delayed fluorescent organic light‐emitting diodes (TADF‐OLEDs). Here, versatile indolocarbazole‐isomer derivatives are developed as highly emissive emitters and ideal hosts for TADF‐OLEDs to alleviate efficiency roll‐off. It is observed that photophysical and electronic properties of these compounds can be well modulated by varying the indolocarbazole isomers. A photoluminescence quantum yield (η_PL) approaching unity and a maximum external quantum efficiency (EQE_max) of 25.1% are obtained for the emitter with indolo[3,2‐a]carbazolyl subunit. Remarkably, record‐high EQE/power efficiency of 26.2%/69.7 lm W^?1 at the brightness level of 5000 cd m^?2 with a voltage of only 3.74 V are also obtained using the same isomer as the host in a green TADF‐OLED. It is evident that TADF hosts with high η_PL values, fast reverse intersystem crossing processes, and balanced charge transport properties may open the path toward roll‐off‐free TADF‐OLEDs.     

Aggregation‐Induced Dual‐Phosphorescence from Organic Molecules for Nondoped Light‐Emitting Diodes

Aggregation‐induced emission (AIE) is a beneficial strategy for generating highly effective solid‐state molecular luminescence without suffering losses in quantum yield. However, the majority of reported AIE‐active molecules exhibit only strong fluorescence, which is not ideal for electrical excitation in organic light‐emitting diodes (OLEDs). By introducing various substituent groups onto the biscarbazole compound, a series of molecular materials with aggregation‐induced phosphorescence (AIP) is designed, which exhibits two distinctly different phosphorescence bands and an absolute solid‐state room‐temperature phosphorescence quantum yield up to 64%. Taking advantage of the AIE feature, the AIP molecules are fabricated into OLEDs as a homogeneous light‐emitting layer, which allows for relatively small efficiency roll‐off and shows an external electroluminescence quantum yield of up to 5.8%, more than the theoretical limit for purely fluorescent OLED devices. The design showcases a promising strategy for the production of cost‐effective and highly efficient OLED technology.     

Blocking Energy‐Loss Pathways for Ideal Fluorescent Organic Light‐Emitting Diodes with Thermally Activated Delayed Fluorescent Sensitizers

Organic light‐emitting diodes (OLEDs) based on thermally activated delayed fluorescence‐sensitized fluorescence (TSF) offer the possibility of attaining an ultimate high efficiency with low roll‐off utilizing noble‐metal free, easy‐to‐synthesize, pure organic fluorescent emitters. However, the performances of TSF‐OLEDs are still unsatisfactory. Here, TSF‐OLEDs with breakthrough efficiencies even at high brightnesses by suppressing the competitive deactivation processes, including direct charge recombination on conventional fluorescent dopants (CFDs) and Dexter energy transfer from the host to the CFDs, are demonstrated. On the one hand, electronically inert terminal‐substituents are introduced to protect the electronically active core of the CFDs; on the other hand, delicate device structures are designed to provide multiple energy‐funneling paths. As a result, unprecedentedly high maximum external quantum efficiency/power efficiency of 24%/71.4 lm W^?1 in a green TSF‐OLED are demonstrated, which remain at 22.6%/52.3 lm W^?1 even at a high luminance of 5000 cd m^?2. The work unlocks the potential of TSF‐OLEDs, paving the way toward practical applications.     

Long‐Lived Flexible Displays Employing Efficient and Stable Inverted Organic Light‐Emitting Diodes

Although organic light‐emitting diodes (OLEDs) are promising for use in applications such as in flexible displays, reports of long‐lived flexible OLED‐based devices are limited due to the poor environmental stability of OLEDs. Flexible substrates such as plastic allow ambient oxygen and moisture to permeate into devices, which degrades the alkali metals used for the electron‐injection layer in conventional OLEDs (cOLEDs). Here, the fabrication of a long‐lived flexible display is reported using efficient and stable inverted OLEDs (iOLEDs), in which electrons can be effectively injected without the use of alkali metals. The flexible display employing iOLEDs can emit light for over 1 year with simplified encapsulation, whereas a flexible display employing cOLEDs exhibits almost no luminescence after only 21 d with the same encapsulation. These results demonstrate the great potential of iOLEDs to replace cOLEDs employing alkali metals for use in a wide variety of flexible organic optoelectronic devices.     

Junction‐Free Electrospun Ag Fiber Electrodes for Flexible Organic Light‐Emitting Diodes

Fabrication of junction‐free Ag fiber electrodes for flexible organic light‐emitting diodes (OLEDs) is demonstrated. The junction‐free Ag fiber electrodes are fabricated by electrospun polymer fibers used as an etch mask and wet etching of Ag thin film. This process facilitates surface roughness control, which is important in transparent electrodes based on metal wires to prevent electrical instability of the OLEDs. The transmittance and resistance of Ag fiber electrodes can be independently adjusted by controlling spinning time and Ag deposition thickness. The Ag fiber electrode shows a transmittance of 91.8% (at 550 nm) at a sheet resistance of 22.3 Ω □^?1, leading to the highest OLED efficiency. In addition, Ag fiber electrodes exhibit excellent mechanical durability, as shown by measuring the change in resistance under repeatable mechanical bending and various bending radii. The OLEDs with Ag fiber electrodes on a flexible substrate are successfully fabricated, and the OLEDs show an enhancement of EQE (≈19%) compared to commercial indium tin oxide electrodes.     

Biowaste-Derived,Self-Organized Arrays of High-Performance 2D Carbon Emitters for Organic Light-Emitting Diodes

Low-cost flexible organic light-emitting diodes (OLEDs) with nanoemitter material from waste open up new opportunities for sustainable technology. The common emitter materials generated from waste are carbon dots (CDs). However, these have poor luminescent properties. Further solid-state emission quenching makes application in display devices challenging. Here, flexible and rigid OLED devices are demonstrated using self-assembled 2D arrays of CDs derived from waste material, viz., human hair. High-performance CDs with a quantum yield (QY) of 87%, self-assembled into 2D arrays, are achieved by improving the crystallinity and decreasing the CDs' size distribution. The CD island array exhibits ultrahigh hole mobility (≈10⁻¹ cm² V⁻¹ s⁻¹) and significant reduction in solid-state emission quenching compared to pristine CDs; hence, it is used here as an emitting layer in both indium tin oxide (ITO)-coated glass and ITO-coated flexible poly(ethylene terephthalate) (PET) substrate OLED devices, without any hole-injection layer. The flexible OLED device exhibits a stable, voltage-independent blue/cyan emission with a record maximum luminescence of 350 cd m⁻², whereas the OLED device based on the rigid glass substrate shows a maximum luminescence of 700 cd m⁻². This work sets up a platform to develop next-generation OLED displays using CD emitters derived from the biowaste material.     

“Quasi‐freestanding” Graphene‐on‐Single Walled Carbon Nanotube Electrode for Applications in Organic Light‐emitting Diode

An air‐stable transparent conductive film with “quasi‐freestanding” graphene supported on horizontal single walled carbon nanotubes (SWCNTs) arrays is fabricated. The sheet resistance of graphene films stacked via layer‐by‐layer transfer (LBL) on quartz, and modified by 1‐Pyrenebutyric acid N‐hydroxysuccinimide ester (PBASE), is reduced from 273 Ω/sq to about 76 Ω/sq. The electrical properties are stable to heat treatment (up to 200 ºC) and ambient exposure. Organic light‐emitting diodes (OLEDs) constructed of this carbon anode (T ≈ 89.13% at 550 nm) exhibit ≈88% power efficiency of OLEDs fabricated on an ITO anode (low turn on voltage ≈3.1 eV, high luminance up to ≈29 490 cd/m², current efficiency ≈14.7 cd/A). Most importantly, the entire graphene‐on‐SWCNT hybrid electrodes can be transferred onto plastic (PET) forming a highly‐flexible OLED device, which continues to function without degradation in performance at bending angles >60°.     

Controlling Horizontal Dipole Orientation and Emission Spectrum of Ir Complexes by Chemical Design of Ancillary Ligands for Efficient Deep‐Blue Organic Light‐Emitting Diodes

Deep‐blue emitting Iridium (Ir) complexes with horizontally oriented emitting dipoles are newly designed and synthesized through engineering of the ancillary ligand, where 2′,6′‐difluoro‐4‐(trimethylsilyl)‐2,3′‐bipyridine (dfpysipy) is used as the main ligand. Introduction of a trimethylsilyl group at the pyridine and a nitrogen at the difluoropyrido group increases the bandgap of the emitter, resulting in deep‐blue emission. Addition of a methyl group (mpic) to a picolinate (pic) ancillary ligand or replacement of an acetate structure of pic with a perfluoromethyl‐triazole structure (fptz) increases the horizontal component of the emitting dipoles in sequence of mpic (86%) > fptz (77%) > pic (74%). The organic light‐emitting diode (OLED) using the Ir complex with the mpic ancillary ligand shows the highest external quantum efficiency (31.9%) among the reported blue OLEDs with a y‐coordinate value lower than 0.2 in the 1931 Commission Internationale de L'Eclairage (CIE) chromaticity diagram.     

Inside Front Cover: Towards See‐Through Displays: Fully Transparent Thin‐Film Transistors Driving Transparent Organic Light‐Emitting Diodes (Adv. Mater. 6/2006)

The first fully transparent organic light‐emitting diode (OLED) pixels, fabricated by integrating transparent OLEDs on top of transparent thin‐film transistors (TTFTs), are demonstrated on p. 738 by Riedl and co‐workers, and shown schematically on the inside cover. With an average transmittance of more than 70 % in the visible part of the spectrum (400–750 nm), the presented active pixels pave the way to the realization of fully transparent active matrix displays.     

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low‐cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a focus on linking their optoelectronic behavior with the performance of the organic light‐emitting diode (OLED) and related EL devices. TADF emitters are cross‐compared within specific color ranges, with a focus on blue, green–yellow, orange–red, and white OLEDs. Organic small‐molecule, dendrimer, polymer, and exciplex emitters are all discussed within this review, as is their use as host materials. Correlations are provided between the structure of the TADF materials and their optoelectronic properties. The success of TADF materials has ushered in the next generation of OLEDs.     

Recent Progress of Singlet‐Exciton‐Harvesting Fluorescent Organic Light‐Emitting Diodes by Energy Transfer Processes

The external quantum efficiency (EQE) of organic light‐emitting diodes (OLEDs) has been dramatically improved by developing highly efficient organic emitters such as phosphorescent emitters and thermally activated delayed fluorescent (TADF) emitters. However, high‐EQE OLED technologies suffer from relatively poor device lifetimes in spite of their high EQEs. In particular, the short lifetimes of blue phosphorescent and TADF OLEDs remain a big hurdle to overcome. Therefore, the high‐EQE approach harvesting singlet excitons of fluorescent emitters by energy transfer processes from the host or sensitizer has been explored as an alternative for high‐EQE OLED strategies. Recently, there has been a big jump in the EQE and device lifetime of singlet‐exciton‐harvesting fluorescent OLEDs. Recent progress on the materials and device structure is discussed herein.     

Highly Efficient Deep-Blue OLEDs using a TADF Emitter with a Narrow Emission Spectrum and High Horizontal Emitting Dipole Ratio

New blue (DBA-SAB) and deep-blue (TDBA-SAF) thermally activated delayed fluorescence (TADF) emitters are synthesized for blue-emitting organic-light emitting diodes (OLEDs) by incorporating spiro-biacridine and spiro-acridine fluorene donor units with an oxygen-bridged boron acceptor unit, respectively. The molecules show blue and deep-blue emission because of the deep highest occupied molecular energy levels of the donor units. Besides, both emitters exhibit narrow emission spectra with the full-width at half maximum (FWHM) of less than 65 nm due to the rigid donor and acceptor units. In addition, the long molecular structure along the transition dipole moment direction results in a high horizontal emitting dipole ratio over 80%. By combining the effects, the OLED utilizing DBA-SAB as the emitter exhibits a maximum external quantum efficiency (EQE) of 25.7% and 1931 Commission Internationale de l'éclairage (CIE) coordinates of (0.144, 0.212). Even a higher efficiency deep blue TADF OLED with a maximum EQE of 28.2% and CIE coordinates of (0.142, 0.090) is realized using TDBA-SAF as the emitter.     

Polarized Light‐Emitting Diodes Based on Patterned MoS2 Nanosheet Hole Transport Layer

Here, this study successfully fabricates few‐layer MoS₂ nanosheets from (NH₄)₂MoS₄ and applies them as the hole transport layer as well as the template for highly polarized organic light‐emitting diodes (OLEDs). The obtained material consists of polycrystalline MoS₂ nanosheets with thicknesses of 2 nm. The MoS₂ nanosheets are patterned by rubbing/ion‐beam treatment. The Raman spectra shows that {poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole), poly[(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,8‐diyl)]} (F8BT) on patterned MoS₂ exhibits distinctive polarization behavior. It is discovered that patterned MoS₂ not only improves the device efficiency but also changes the polarization behavior of the devices owing to the alignment of F8BT. This work demonstrates a highly efficient polarized OLED with a polarization ratio of 62.5:1 in the emission spectrum (166.7:1 at the peak intensity of 540 nm), which meets the manufacturing requirement. In addition, the use of patterned MoS₂ nanosheets not only tunes the polarization of the OLEDs but also dramatically improves the device performance as compared with that of devices using untreated MoS₂.     

Realizing 22.5% External Quantum Efficiency for Solution‐Processed Thermally Activated Delayed‐Fluorescence OLEDs with Red Emission at 622 nm via a Synergistic Strategy of Molecular Engineering and Host Selection

Developing high‐efficiency solution‐processable thermally activated delayed‐fluorescence (TADF) emitters, especially in longer wavelength regions, is a formidable challenge. Three red TADF emitters, namely NAI_R1, NAI_R2, and NAI_R3, are developed by phenyl encapsulation and tert‐butyl substitution on a prototypical 1,8‐naphthalimide‐acridine hybrid. This design strategy not only grants these molecules high solubility, excellent thermal stability, and good film‐forming ability, but also pulls down their charge‐transfer (CT) energy levels excited states. Furthermore, dispersing these emitters into two different host materials of mCP and mCPCN finely tailors their CT‐state energy levels. More importantly, a synergistic combination of molecular engineering and host selection can effectively manipulate the competition between the radiative and nonradiative decay rates of the CT singlet states of these emitters and the reverse intersystem crossing from their triplet to singlet states. Consequently, the optimal combination of NAI_R3 emitter and mCP host successfully results in a state‐of‐the‐art external quantum efficiency (EQE) of 22.5% for solution‐processed red TADF organic light‐emitting diodes (OLEDs) with an emission peak above 620 nm. This finding demonstrates that a synergistic strategy of molecular engineering and host selection with TADF emitters could provide a new pathway for developing efficient solution‐processable TADF systems.     

High‐Performance Fluorescent Organic Light‐Emitting Diodes Utilizing an Asymmetric Anthracene Derivative as an Electron‐Transporting Material

Fluorescent organic light‐emitting diodes with thermally activated delayed fluorescent sensitizers (TSF‐OLEDs) have aroused wide attention, the power efficiencies of which, however, are limited by the mutual exclusion of high electron‐transport mobility and large triplet energy of electron‐transporting materials (ETMs). Here, an asymmetric anthracene derivative with electronic properties manipulated by different side groups is developed as an ETM to promote TSF‐OLED performances. Multiple intermolecular interactions are observed, leading to a kind of “cable‐like packing” in the crystal and favoring the simultaneous realization of high electron‐transporting mobility and good exciton‐confinement ability, albeit the low triplet energy of the ETM. The optimized TSF‐OLEDs exhibit a record‐high maximum external quantum efficiency/power efficiency of 24.6%/76.0 lm W^?1, which remain 23.8%/69.0 lm W^?1 at a high luminance of even 5000 cd m^?2 with an extremely low operation voltage of 3.14 V. This work opens a new paradigm for designing ETMs and also paves the way toward practical application of TSF‐OLEDs.     

Inside Front Cover: Excimer‐Based White Phosphorescent Organic Light‐Emitting Diodes with Nearly 100 % Internal Quantum Efficiency (Adv. Mater. 2/2007)

By combining the monomer and excimer/aggregate emission of FPt, a white OLED can be obtained. Incorporating the novel host material 26mCPy and engineering the charge balance properties, Jabbour and co‐workers used FPt to demonstrate, for the first time, nearly 100% internal quantum efficiency in white OLEDs (as shown in the image). The work is reported on p. 197.     

Wide‐Range Tuning and Enhancement of Organic Long‐Persistent Luminescence Using Emitter Dopants

Most long‐persistent luminescent (LPL) materials, which slowly release energy absorbed from ambient light, are based on inorganic compounds. Organic long‐persistent luminescent (OLPL) systems have advantages over inorganic LPL materials in terms of solubility, transparency, and flexibility. Here, the characteristics of OLPL emission are improved by doping emitter molecules into an OLPL matrix. Greenish‐blue to red and even warm white emission are achieved by energy transfer from exciplex in the OLPL matrix to the emitter dopants. The dopants also improve brightness and emission duration through efficient radiative decay and the trapping of electrons, respectively. This technique will enable the development of a wide range of organic glow‐in‐the‐dark paints.     

Highly efficient and stable white organic light emitting diode base on double recombination zones of phosphorescent blue/orange emitters

We demonstrated efficient and stable white phosphorescent organic light-emitting diodes (OLEDs) with double-emitting layers (D-EMLs), which were comprised of two emissive layers with a hole transport-type host of N,N'-dicarbazolyl-3,5-benzene (mCP) and a electron transport-type host of 2,2',2"-(1,3,5-benzenetryl)tris(1-phenyl)-1H-benzimidazol (TPBi) with blue/orange emitters, respectively. We fabricated two type white devices with single emitting layer (S-EML) and D-EML of orange emitter, maintaining double recombination zone of blue emitter. In addition, the device architecture was developed to confine excitons inside the D-EMLs and to manage triplet excitons by controlling the charge injection. As a result, light-emitting performances of white OLED with D-EMLs were improved and showed the steady CIE coordinates compared to that with S-EML of orange emitter, which demonstrated the maximum luminous efficiency and external quantum efficiency were 21.38 cd/A and 11.09%. It also showed the stable white emission with CIE(x,y) coordinates from (x = 0.36, y = 0.37) at 6 V to (x = 0.33, y = 0.38) at 12 V.