High External Electroluminescence Quantum Efficiencies Research Articles

Three-dimensional sensing of surfaces by projection of invisible electroluminescence from organic light-emitting diodes.

Organic light-emitting diodes (OLEDs) that efficiently emit near-infrared (NIR) light and consume little power will create valuable applications for OLEDs beyond just displays. Here, we report such a NIR-OLED with high operational stability that can be used as a light source for three-dimensional sensing of object's surfaces. Using a narrow-energy-gap material as a host for producing NIR hyperfluorescence system, we fabricated a NIR-OLED exhibiting intense emission at 930 nm with a high external electroluminescence quantum efficiency of more than 1% at a current density of 100 milliamperes per square meter without any degradation even after more than 300 hours of operation. The NIR-OLEDs were integrated with dense complementary metal-oxide semiconductor circuits to make a micro-NIR-OLED projector (0.21 inch, 230,400 pixels). By actively driving the projector on a pixel by pixel and projecting their emission onto objects, we successfully scanned and sensed the surfaces in three dimensions with invisible NIR.

Blue Thermally Activated Delayed Fluorescence with Sub‐Microsecond Short Exciton Lifetimes: Acceleration of Triplet–Singlet Spin Interconversion via Quadrupolar Charge‐Transfer States

AbstractExciton lifetime is a critical factor in determining the performance of optoelectronic functional systems and devices. Thermally activated delayed fluorescence (TADF) emitters that can concurrently achieve a high fluorescence quantum yield and short exciton lifetime are desirable for application in organic light‐emitting diodes (OLEDs) with suppressed efficiency roll‐off. Herein, phenoxaborin and xanthone‐cored TADF emitters with quadrupolar electronic structures are reported to exhibit sub‐microsecond TADF lifetimes as short as 650 and 970 ns, respectively, while preserving high fluorescence quantum yields. By extending the El‐Sayed rule to the quadrupolar π‐systems, the contribution of doubly degenerate charge‐transfer excited states induced by dual donor units can enhance the spin–orbit coupling between them, leading to a spin‐flip acceleration between the excited triplet and singlet states. This electronic feature is advantageous for mitigating exciton annihilation processes in the emission layer, thereby reducing the efficiency roll‐offs in OLEDs. Consequently, a high external electroluminescence quantum efficiency over 20% can be retained, even under operating the device at a high luminance of 1000 cd m−2.

2,6‐Dicarbonitrile Diphenyl‐1λ5‐Phosphinine (DCNP)—A Robust Conjugated Building Block for Multi‐Functional Dyes Exhibiting Tunable Amplified Spontaneous Emission

AbstractHighly efficient organic light‐emitting diodes (OLEDs) with the concurrent achievement of high external electroluminescence quantum efficiency (EQE) and low light amplification thresholds under optical excitation have been considered as a crucial evolution towards the development of high‐performance electrically pumped organic semiconductor laser diodes. Herein, a series of 2,6‐dicarbonitrile‐diphenyl‐1λ5‐phosphinine (DCNP) based donor (D)‐acceptor (A) type dyes with different electron‐withdrawing and donating moieties have been designed and characterized. The well‐manipulated D‐A strength with tunable optical properties guaranteed the low amplified spontaneous emission (ASE) thresholds of below 10 µJ cm−2 and furnished a wide‐range color‐tuning capability in the visible region (485–595 nm). Furthermore, employing a thermally‐activated delayed fluorescence (TADF) molecule as a triplet harvester boosted the performance of OLEDs based on mDMCz that exhibits an exceptional EQE value of 18.4% which is an eightfold enhancement as compared with that of standard fluorescence OLEDs. Also, the TADF‐assistant fluorescence (TAF) system enables a reduction of the ASE threshold to 3 µJ cm−2 and excellent ASE stability. These results provide a rational design strategy to construct color‐tunable lasing dyes with reduced ASE thresholds and clarify their potentiality as the fluorescent dopant in the TAF system to utilize up‐converted triplet excitons via efficient energy transfer.

Alternating Donor–Acceptor π‐Conjugated Macrocycle Exhibiting Efficient Thermally Activated Delayed Fluorescence and Spontaneous Horizontal Molecular Orientation

A versatile design of thermally activated delayed fluorescence (TADF) π‐conjugated macrocycles incorporating electron‐donor (D) and acceptor (A) units into a cyclo‐meta‐phenylene motif with an alternating pattern is presented. The new π‐conjugated macrocycle, which combines three 5‐(N‐carbazolyl)‐phenylen‐1,3‐diyl as D units and three 6‐phenyl‐1,3,5‐triazin‐2,4‐diyl as A units, possesses a small singlet–triplet energy gap and hence can emit efficient green TADF both in solution and doped thin films. Comparative experimental and computational investigations of the electronic and photophysical properties of the macrocycle with its analogous noncyclic compound reveal key advantages of the cyclic molecular configuration for actual emitters. Organic light‐emitting diodes incorporating the TADF π‐conjugated macrocycle as an emitter demonstrate high external electroluminescence quantum efficiencies of up to 15.7%, outperforming the devices based on the noncyclic emitter. Herein, the importance of geometric design for producing novel organic emitters with fascinating optoelectronic and morphological characteristics is highlighted.

Cis-Quinacridone-Based Delayed Fluorescence Emitters: Seemingly Old but Renewed Functional Luminogens.

Herein, we report a material design of linear cis-quinacridone (cis-QA) derivatives as delayed fluorescence luminogens. In contrast to the widely studied traditional trans-isomers, the functionality of cis-QA and its derivatives remains unexplored and unclarified. Through combined computational and experimental investigations, we revealed that cis-QA derivatives can function as fascinating narrowband deep-blue delayed fluorescence emitters for organic light-emitting diodes (OLEDs). The best-performing deep-blue OLEDs incorporating these cis-QA luminogens achieved high external electroluminescence quantum efficiencies of up to 19.0 % and high color purity with chromaticity coordinates of (0.13, 0.14).

Cis‐Quinacridone‐Based Delayed Fluorescence Emitters: Seemingly Old but Renewed Functional Luminogens

AbstractHerein, we report a material design of linear cis‐quinacridone (cis‐QA) derivatives as delayed fluorescence luminogens. In contrast to the widely studied traditional trans‐isomers, the functionality of cis‐QA and its derivatives remains unexplored and unclarified. Through combined computational and experimental investigations, we revealed that cis‐QA derivatives can function as fascinating narrowband deep‐blue delayed fluorescence emitters for organic light‐emitting diodes (OLEDs). The best‐performing deep‐blue OLEDs incorporating these cis‐QA luminogens achieved high external electroluminescence quantum efficiencies of up to 19.0 % and high color purity with chromaticity coordinates of (0.13, 0.14).

Long-Lived, Non-Geminate, Radiative Recombination of Photogenerated Charges in a Polymer/Small-Molecule Acceptor Photovoltaic Blend.

Minimization of open-circuit-voltage ( VOC) loss is required to transcend the efficiency limitations on the performance of organic photovoltaics (OPV). We study charge recombination in an OPV blend comprising a polymer donor with a small molecule nonfullerene acceptor that exhibits both high photovoltaic internal quantum efficiency and relatively high external electroluminescence quantum efficiency. Notably, this donor/acceptor blend, consisting of the donor polymer commonly referred to as PCE10 with a pseudoplanar small molecule acceptor (referred to as FIDTT-2PDI) exhibits relatively bright delayed photoluminescence on the microsecond time scale beyond that observed in the neat material. We study the photoluminescence decay kinetics of the blend in detail and conclude that this long-lived photoluminescence arises from radiative nongeminate recombination of charge carriers, which we propose occurs via a donor/acceptor CT state located close in energy to the singlet state of the polymer donor. Additionally, crystallographic and spectroscopic studies point toward low subgap disorder, which could be beneficial for low radiative and nonradiative losses. These results provide an important demonstration of photoluminescence due to nongeminate charge recombination in an efficient OPV blend, a key step in identifying new OPV materials and materials-screening criteria if OPV is to approach the theoretical limits to efficiency.

High‐Performance Dibenzoheteraborin‐Based Thermally Activated Delayed Fluorescence Emitters: Molecular Architectonics for Concurrently Achieving Narrowband Emission and Efficient Triplet–Singlet Spin Conversion

AbstractThermally activated delayed fluorescence (TADF) materials, which enable the full harvesting of singlet and triplet excited states for light emission, are expected as the third‐generation emitters for organic light‐emitting diodes (OLEDs), superseding the conventional fluorescence and phosphorescence materials. High photoluminescence quantum yield (ΦPL), narrow‐band emission (or high color purity), and short delayed fluorescence lifetime are all strongly desired for practical applications. However, to date, no rational design strategy of TADF emitters is established to fulfill these requirements. Here, an epoch‐making design strategy is proposed for producing high‐performance TADF emitters that concurrently exhibiting high ΦPL values close to 100%, narrow emission bandwidths, and short emission lifetimes of ≈1 µs, with a fast reverse intersystem crossing rate of over 106 s−1. A new family of TADF emitters based on dibenzoheteraborins is introduced, which enable both doped and non‐doped TADF‐OLEDs to achieve markedly high external electroluminescence quantum efficiencies, exceeding 20%, and negligible efficiency roll‐offs at a practical high luminance. Systematic photophysical and theoretical investigations and device evaluations for these dibenzoheteraborin‐based TADF emitters are reported here.

Highly Efficient Thermally Activated Delayed Fluorescence from an Excited-State Intramolecular Proton Transfer System.

Thermally activated delayed fluorescence (TADF) materials have shown great potential for highly efficient organic light-emitting diodes (OLEDs). While the current molecular design of TADF materials primarily focuses on combining donor and acceptor units, we present a novel system based on the use of excited-state intramolecular proton transfer (ESIPT) to achieve efficient TADF without relying on the well-established donor–acceptor scheme. In an appropriately designed acridone-based compound with intramolecular hydrogen bonding, ESIPT leads to separation of the highest occupied and lowest unoccupied molecular orbitals, resulting in TADF emission with a photoluminescence quantum yield of nearly 60%. High external electroluminescence quantum efficiencies of up to 14% in OLEDs using this emitter prove that efficient triplet harvesting is possible with ESIPT-based TADF materials. This work will expand and accelerate the development of a wide variety of TADF materials for high performance OLEDs.

Cyclohexane-Coupled Bipolar Host Materials with High Triplet Energies for Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence.

Thermally activated delayed fluorescence-based organic light-emitting diodes (TADF-OLEDs) have recently attracted tremendous research interest as next-generation optoelectronic devices. However, there are a limited number of host materials with an appropriately high lowest-excited triplet energy (ET) and bipolar charge transport properties for high-efficiency TADF-OLEDs. Moreover, these host materials should have high thermal and morphological stabilities. In this study, we develop novel bipolar host materials consisting of an electron-donating 9-phenylcarbazole unit and an electron-accepting triphenylphosphine oxide, triphenylphosphine sulfide, or 2,4,6-triphenyl-1,3,5-triazine unit linked by a nonconjugated cyclohexane core. These bipolar host materials possess high glass-transition temperatures of over 100 °C and high ET values of approximately 3.0 eV. TADF-OLEDs employing these bipolar host materials could achieve high external electroluminescence quantum efficiencies of up to 21.7% together with reduced efficiency roll-off characteristics, because of expansion of the charge-recombination zone within the emission layer arising from the bipolar charge transport ability of these host materials.

Blue Thermally Activated Delayed Fluorescence Molecule Having Acridane and Cyanobenzene Units

Highly efficient blue thermally activated delayed fluorescence (TADF) materials consisting of 9,9-diphenylacridan and benzonitrile units were developed. We found that placing 9,9-diphenylacridan in the ortho-position of benzonitrile enhanced the TADF process compared with results from meta- and para-substituted derivatives. We observed blue TADF emission with a high photoluminescence quantum yield (80%) and a high reverse intersystem crossing rate constant (1.2 × 106 s−1) for 2,6-bis(9,9-diphenylacridan-10(9H)-yl)benzonitrile (o-A2CN). An organic light-emitting diode fabricated using o-A2CN as an emitter showed good blue emission (CIEx, CIEy = 0.16, 0.16) with high external electroluminescence quantum efficiency (15.9%).

Thermally Activated Delayed Fluorescence Properties of Regioisomeric Xanthone-Based Twisted Intramolecular Charge-Transfer Luminophores

Abstract Thermally activated delayed fluorescence (TADF) emitters consisting of a xanthone acceptor unit and a para-, meta-linked tercarbazole donor unit were developed. The influence of the regioisomeric donor–acceptor structures on their photophysical and TADF properties was studied. Blue organic light-emitting diodes employing these TADF emitters showed high external electroluminescence quantum efficiencies of up to 14.4% with suppressed roll-off characteristics.

Aggregation-induced delayed fluorescence from phenothiazine-containing donor–acceptor molecules for high-efficiency non-doped organic light-emitting diodes

Highly luminescent donor–acceptor molecules based on a phenothiazine donor unit coupled with a xanthone or benzophenone acceptor unit were developed for use in organic light-emitting diodes (OLEDs). While both molecules are almost non-luminescent in pure tetrahydrofuran (THF) solution, a strong yellow delayed fluorescence was observed upon their aggregation in THF/water mixtures or in neat films. This result demonstrates the unique aggregation-induced delayed fluorescence (AIDF) characteristics of these molecules. OLEDs using these AIDF materials as a non-doped emission layer achieved high external electroluminescence quantum efficiencies of up to 11%, which exceeds the theoretical maximum for conventional fluorescent OLEDs. Highly luminescent donor–acceptor molecules based on a phenothiazine donor unit coupled with a xanthone or benzophenone acceptor unit were developed. While both molecules are almost non-luminescent in pure tetrahydrofuran (THF) solution, a strong yellow delayed fluorescence was observed upon their aggregation in THF/water mixtures and in neat films. This result demonstrates the unique aggregation-induced delayed fluorescence (AIDF) characteristics of these molecules. Organic light-emitting diodes (OLEDs) using these AIDF materials as a non-doped emission layer achieved high external electroluminescence quantum efficiencies of up to 11%, which far exceeds the theoretical maximum for conventional fluorescent OLEDs.

Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages.

Optimization of the energy levels at the donor-acceptor interface of organic solar cells has driven their efficiencies to above 10%. However, further improvements towards efficiencies comparable with inorganic solar cells remain challenging because of high recombination losses, which empirically limit the open-circuit voltage (Voc) to typically less than 1 V. Here we show that this empirical limit can be overcome using non-fullerene acceptors blended with the low band gap polymer PffBT4T-2DT leading to efficiencies approaching 10% (9.95%). We achieve Voc up to 1.12 V, which corresponds to a loss of only Eg/q - Voc = 0.5 ± 0.01 V between the optical bandgap Eg of the polymer and Voc. This high Voc is shown to be associated with the achievement of remarkably low non-geminate and non-radiative recombination losses in these devices. Suppression of non-radiative recombination implies high external electroluminescence quantum efficiencies which are orders of magnitude higher than those of equivalent devices employing fullerene acceptors. Using the balance between reduced recombination losses and good photocurrent generation efficiencies achieved experimentally as a baseline for simulations of the efficiency potential of organic solar cells, we estimate that efficiencies of up to 20% are achievable if band gaps and fill factors are further optimized.

A Phenazaborin-Based High-Efficiency Blue Delayed Fluorescence Material

Abstract A highly efficient blue thermally activated delayed fluorescence (TADF) material based on phenazaborin and spiroacridan units was developed. A blue-emitting organic light-emitting diode containing the phenazaborin derivative as a TADF emitter exhibited a high external electroluminescence quantum efficiency of 18.2%.

Highly Efficient Thermally Activated Delayed Fluorescence Emitters with a Small Singlet–Triplet Energy Gap and Large Oscillator Strength

Abstract A strategy for designing highly efficient thermally activated delayed fluorescence (TADF) emitters was reported. TADF emitters with donor–acceptor–donor (D–A–D)-type structures showed highly efficient TADF because of their small singlet–triplet energy gap and large oscillator strength. An organic light-emitting diode containing a D–A–D-type TADF emitter, cis-BOX2, exhibited a high external electroluminescence quantum efficiency of 17.6%.

Bifunctional Star‐Burst Amorphous Molecular Materials for OLEDs: Achieving Highly Efficient Solid‐State Luminescence and Carrier Transport Induced by Spontaneous Molecular Orientation

Bifunctional star-burst amorphous molecular materials displaying both efficient solid-state luminescence and high hole-transport properties are developed in this study. A high external electroluminescence quantum efficiency up to 5.9% is attained in OLEDs employing the developed amorphous materials. It is revealed that the spontaneous horizontal orientation of these light-emitting molecules in their molecular-condensed states leads to a remarkable enhancement of the electroluminescence efficiencies and carrier-transport properties.

High-efficiency organic light-emitting diodes utilizing thermally activated delayed fluorescence from triazine-based donor–acceptor hybrid molecules

We have designed and synthesized a high-efficiency purely organic luminescent material, 2,4-bis{3-(9 H-carbazol-9-yl)-9 H-carbazol-9-yl}-6-phenyl-1,3,5-triazine (CC2TA) comprising the bicarbazole donor and phenyltriazine acceptor units, which is capable of emitting thermally activated delayed fluorescence. The molecular design of CC2TA allows spatial separation of HOMO and LUMO on the donor and acceptor fragments, respectively, leading to an exceptionally small singlet–triplet exchange energy (ΔEST = 0.06 eV) together with a high triplet energy. Furthermore, a high external electroluminescence quantum efficiency as high as 11% ± 1% has been achieved in the sky-blue organic light-emitting diodes employing CC2TA as an emitter.

Emission Color Tuning in Ambipolar Organic Single‐Crystal Field‐Effect Transistors by Dye‐Doping

AbstractThe effect of dye‐doping in ambipolar light‐emitting organic field‐effect transistors (LE‐OFETs) is investigated from the standpoint of the carrier mobilities and the electroluminescence (EL) characteristics under ambipolar operation. Dye‐doping of organic crystals permits not only tuning of the emission color but also significantly increases the efficiency of ambipolar LE‐OFETs. A rather high external EL quantum efficiency (∼0.64%) of one order of magnitude higher than that of a pure p‐distyrylbenzene (P3V2) single crystal is obtained by tetracene doping. The doping of tetracene molecules into a host P3V2 crystal has almost no effect on the electron mobility and the dominant carrier recombination process in the tetracene‐doped P3V2 crystal involves direct carrier recombination on the tetracene molecules.

New transport layers for highly efficient organic electroluminescence devices

Multiheterostructure electroluminescence (EL) devices with blue emission color based on parahexaphenyl (PHP) as the emitting layer and various new organic materials as hole and/or electron transport layers (HTL, ETL) were fabricated to improve the EL efficiency. We present new ETLs, which were prepared via different metal-catalyzed coupling reactions of brominated educts in high yields. Utilizing these new organic materials enables us to obtain high performance of blue PHP EL devices with low threshold fields (0.3 MV/cm) and high external EL quantum efficiencies up to 3.4%.