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

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 (Φ_PL) of 97% and a small singlet–triplet splitting (ΔE_ST) 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).     

Adamantyl Substitution Strategy for Realizing Solution‐Processable Thermally Stable Deep‐Blue Thermally Activated Delayed Fluorescence Materials

Highly efficient solution‐processable emitters, especially deep‐blue emitters, are greatly desired to develop low‐cost and low‐energy‐consumption organic light‐emitting diodes (OLEDs). A recently developed class of potentially metal‐free emitters, thermally activated delayed fluorescence (TADF) materials, are promising candidates, but solution‐processable TADF materials with efficient blue emissions are not well investigated. In this study, first the requirements for the design of efficient deep‐blue TADF materials are clarified, on the basis of which, adamantyl‐substituted TADF molecules are developed. The substitution not only endows high solubility and excellent thermal stability but also has a critical impact on the molecular orbitals, by pushing up the lowest unoccupied molecular orbital energy and triplet energy of the molecules. In the application to OLEDs, an external quantum efficiency (EQE) of 22.1% with blue emission having Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.19) is realized. A much deeper blue emission with CIE (0.15, 0.13) is also achieved, with an EQE of 11.2%. These efficiencies are the best yet among solution‐processed TADF OLEDs of CIE y < 0.20 and y < 0.15, as far as known. This work demonstrates the validity of adamantyl substitution and paves a pathway for straightforward realization of solution‐processable efficient deep‐blue TADF emitters.     

Bis‐Tridentate Ir(III) Metal Phosphors for Efficient Deep‐Blue Organic Light‐Emitting Diodes

Emissive Ir(III) metal complexes possessing two tridentate chelates (bis‐tridentate) are known to be more robust compared to those with three bidentate chelates (tris‐bidentate). Here, the deep‐blue‐emitting, bis‐tridentate Ir(III) metal phosphors bearing both the dicarbene pincer ancillary such as 2,6‐diimidazolylidene benzene and the 6‐pyrazolyl‐2‐phenoxylpyridine chromophoric chelate are synthesized. A deep‐blue organic light‐emitting diode from one phosphor exhibits Commission Internationale de l'Eclairage (CIE_{(x ,y )}) coordinates of (0.15, 0.17) with maximum external quantum efficiency (max. EQE) of 20.7% and EQE = 14.6% at the practical brightness of 100 cd m^?2.     

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.     

Efficient orange-red organic light-emitting diodes using 9,10-bis[4-(di-4-tert-butylphenylamino)styryl] anthracene as a fluorescent orange-red emitter

The authors have demonstrated efficient orange-red organic lighting diodes (OLEDs) using a new fluorescent orange-red material, 9,10-bis[4-(di-4-tert-buthylphenylamino)styryl]anthracene (ATBTPA). The optimized orange-red OLED using ATBTPA achieved a maximum external quantum efficiency (EQE) of 3.78%, a current efficiency (CE) of 9.47 cd/A, and Commision Internationale de L'Eclairage (CIE_x,y) coordinates of (0.51, and 0.48) at 1.61 mA/cm² in comparison with orange-red OLED using (5,6,11,12)-tetraphenyl-naphthacene (rubrene) which showed a maximum EQE of 1.65%, CE of 4.94 cd/A, and CIE_x,y coordinates of (0.50, and 0.49) at 0.61 mA/cm², respectively. The optimized orange-red device using ATBTPA showed higher efficiency of two times the orange-red device using rubrene due to the efficient Förster singlet energy transfer from MADN to ATBTPA in comparison with that from MADN to rubrene. This study clearly suggests that ATBTPA is an excellent fluorescent orange-red material for efficient WOLEDs.     

Synthesis and electroluminescent properties of blue-emitting t-butylated bis(diarylaminoaryl)anthracenes for OLEDs

A new series of blue fluorescent emitters based on t-butylated bis(diarylaminoaryl) anthracenes were synthesized and their electroluminescent properties investigated. Into these blue materials, t-butyl groups were introduced to both prevent molecular aggregation between the blue emitters through steric hindrance and reduce self-quenching. As such, this would contribute to overall improvement in OLED efficiency. To explore the electroluminescent properties of these materials, multilayered OLEDs were fabricated into a device structure of: ITO/NPB(50 nm)/blue emitters doped in ADN(30 nm)/Alq₃(20 nm)/Liq(2 nm)/Al(100 nm). All devices showed efficient blue emissions. In particular, one device exhibited highly efficient sky blue emissions with a maximum luminance of 11,060 cd/m² at 12.0 V and respective luminous and power efficiencies of 6.59 cd/A and 2.58 lm/W at 20 mA/cm². The peak wavelength of the electroluminescence was 468 nm with CIEx,y coordinates of (0.159, 0.198) at 12.0 V. In addition, a deep blue device with CIEx,y coordinates of (0.159, 0.151) at 12.0 V showed a luminous efficiency of 4.2 cd/A and power efficiency of 1.66 lm/W at 20 mA/cm².     

Effective lattice stabilization of gadolinium aluminate garnet (GdAG) via Lu3+ doping and development of highly efficient (Gd,Lu)AG:Eu3+ red phosphors

Abstract

The metastable garnet lattice of Gd₃Al₅O₁₂ is stabilized by doping with smaller Lu³⁺, which then allows an effective incorporation of larger Eu³⁺ activators. The [(Gd_1?xLu_x)_1?yEu_y]₃Al₅O₁₂ (x = 0.1–0.5, y = 0.01–0.09) garnet solid solutions, calcined from their precursors synthesized via carbonate coprecipitation, exhibit strong luminescence at 591 nm (the ⁵D₀ → ⁷F₁ magnetic dipole transition of Eu³⁺) upon UV excitation into the charge transfer band (CTB) at ～239 nm, with CIE chromaticity coordinates of x = 0.620 and y = 0.380 (orange-red). The quenching concentration of Eu³⁺ was estimated at ～5 at.% (y = 0.05), and the quenching was attributed to exchange interactions. Partial replacement of Gd³⁺ with Lu³⁺ up to 50 at.% (x = 0.5) while keeping Eu³⁺ at the optimal content of 5 at.% does not significantly alter the peak positions of the CTB and ⁵D₀ → ⁷F₁ emission bands but slightly weakens both bands owing to the higher electronegativity of Lu³⁺. The effects of processing temperature (1000–1500 °C) and Lu/Eu contents on the intensity, quantum efficiency, lifetime and asymmetry factor of luminescence were thoroughly investigated. The [(Gd_0.7Lu_0.3)_0.95Eu_0.05]₃Al₅O₁₂ phosphor processed at 1500 °C exhibits a high internal quantum efficiency of ～83.2% under 239 nm excitation, which, in combination with the high theoretical density, favors its use as a new type of photoluminescent and scintillation material.     

Blue fluorescent OLED materials and their application for high-performance devices

The authors applied two technologies to improve the efficiency of fluorescent blue organic light-emitting diodes (OLEDs). First, an efficiency-enhancement layer (EEL) was introduced to boost triplet–triplet fusion (TTF). Second, new blue dopants with a higher orientation factor in the emitting layer were developed. Consequently, the external quantum efficiency (EQE) was increased up to 11.5% with Commission Internationale de l’Eclairage (CIE) 1931 color coordinates of (0.138, 0.092). The reported results may lead to EQEs that exceed 14% with fluorescent blue emitters.     

Development and characterization of an efficient bio-white polymer light-emitting diode with red and green phosphorescent dyes as dopants

An efficient white polymer light-emitting diode (WPLED) with stable Commission Internationale de l’éclairage (CIE) coordinates is fabricated. A blue electroluminescence (EL)-emitting conducting polymer [poly(9,9-di-n-hexyl-fluorenyl-2,7-diyl)] is used as a host for red [Bis(1-phenyl-isoquinoline)(acetylacetonate)iridium(III)] and green [iridium(III)tris(2-(4-tolyl)pyridinato-N,C²)] phosphorescent dyes. Although efficient triplet energy transfer is not possible in the green phosphorescent dye, the self-trapping mechanism is utilized for the emission of EL in the green region while an efficient triplet exciton energy transfer from the host to the red dye is utilized for EL in the red wavelength region. Concentrations of the three constituents are optimized to obtain pure white light of appropriate CIE coordinates. An efficient electron-blocking layer based on a biomaterial (salmon-DNA) is also incorporated in the WPLED to improve the device performance. The WPLED shows three distinguished peaks for the primary colors and achieved a maximum luminance and luminous efficiency of 350 cd/m² and 0.86 cd/A, respectively.     

Symmetrization of the Crystal Lattice of MAPbI3 Boosts the Performance and Stability of Metal–Perovskite Photodiodes

Semiconducting lead triiodide perovskites (A PbI₃) have shown remarkable performance in applications including photovoltaics and electroluminescence. Despite many theoretical possibilities for A ⁺ in A PbI₃, the current experimental knowledge is largely limited to two of these materials: methylammonium (MA⁺) and formamidinium (FA⁺) lead triiodides, neither of which adopts the ideal, cubic perovskite structure at room temperature. Here, a volume‐based criterion is proposed for cubic A PbI₃ to be stable, and two perovskite materials MA_1?x EA_x PbI₃ (MEPI, EA⁺ = ethylammonium) and MA_1?y DMA_y PbI₃ (MDPI, DMA⁺ = dimethylammonium) are introduced. Powder and single‐crystal X‐ray diffraction (XRD) results reveal that MEPI and MDPI are solid solutions possessing the cubic perovskite structure, and the EA⁺ and DMA⁺ cations play similar roles in the symmetrization of the crystal lattice of MAPbI₃. Single crystals of MEPI and MDPI are grown and made into plates of a range of thicknesses, and then into metal–perovskite photodiodes. These devices exhibit tripled diffusion lengths and about tenfold enhancement in stability against moisture, both relative to the current benchmark MAPbI₃. In this study, the systematic approach to materials design and device fabrication greatly expands the candidate pool of perovskite semiconductors, and paves the way for high‐performance, single‐crystal perovskite devices including solar cells and light emitters.     

Highly Efficient Blue Fluorescent OLEDs Based on Upper Level Triplet–Singlet Intersystem Crossing

Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet–triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T₁) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high‐lying excited state (T₂), referred to here as a “hot exciton” path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S₁ and T₂ and rather large T₂–T₁ energy gap, which are expected to impede the internal conversion (IC) from T₂ to T₁ and facilitate the reverse intersystem crossing from the high‐lying triplet state (hRISC). Through sensitizing the T₂ state with ketones, the existence of the hRISC process with an ns‐scale delayed lifetime is confirmed. Benefiting from this fast triplet–singlet conversion, the nondoped device based on this “hot exciton” material reaches a maximum external quantum efficiency exceeding 10%, with a small efficiency roll‐off and CIE coordinates of (0.15, 0.13). These results reveal that the “hot exciton” path is a promising way to exploit high efficient, stable fluorescent emitters, especially for the pure‐blue and deep‐blue fluorescent organic light‐emitting devices.     

Profiling the Mirror Surface from the Fraunhofer Amplitude Distribution that Can be Generated Under Coherent Illumination

Abstract

A procedure is proposed to profile the mirror surface needed to produce under coherent illumination, a pre-specified Fraunhofer amplitude distribution U(ξ, ψ). The unknown optical path function ?(x y) in the diffraction integral is reconstructed from its stationary points {(x _i, y _i)}. The points {(x _i, y _i)} correspond to locations where the first Fresnel zone distribution F(x, y) is an extremum. The reconstruction utilizes the equivalence of the method of stationary points and the method of Fresnel zones. In Fraunhofer diffraction, the F(x, y) has the functional distribution of the inverse Fourier transform of U(ξ, ψ). We consider only Hermitian U(ξ, ψ)'s because they are generated by real F(x, y) distributions. The ?(x, y) is deduced from the behaviour of its first derivatives, which are expressed as product series expansions of {(x _i, y _i)}. The path function ?(x, y) which describes all the possible paths taken by the light waves from the point source to the observation plane via the reflecting surface, is utilized as a constraint in the profiling procedure.     

Synthesis and electroluminescent properties of blue emitting materials based on arylamine-substituted diphenylvinylbiphenyl derivatives for organic light-emitting diodes

This paper reports the synthesis and electroluminescent properties of a series of blue emitting materials with arylamine and diphenylvinylbiphenyl groups for applications to efficient blue organic light-emitting diodes (OLEDs). All devices exhibited blue electroluminescence with electroluminescent properties that were quite sensitive to the structural features of the dopants in the emitting layers. In particular, the device using dopant 4 exhibited sky-blue emission with a maximum luminance, luminance efficiency, power efficiency, external quantum efficiency and CIE coordinates of 39,000 cd/m², 12.3 cd/A, 7.45 lm/W, 7.71% at 20 mA/cm² and (x = 0.17, y = 0.31) at 8 V, respectively. In addition, a blue OLED using dopant 2 with CIE coordinates (x = 0.16, y = 0.18) at 8 V exhibited a luminous efficiency, power efficiency and external quantum efficiency of 4.39 cd/A, 2.46 lm/W and 2.97% at 20 mA/cm², respectively.     

Calibration of a sequentially reflective colorimeter using a tri-color LED

We built a simple colorimeter using a tri-color LED to measure the color of a test object. The color was determined by measuring the reflected intensities of three successively incident light pulses in red, green, and blue colors. The reflected signals reaching the detector at different times were recorded respectively using three sample-and-hold circuits, and then processed by a computer to give the chromaticity coordinates (x, y). In the calibrating procedure, we assumed a simple linear transformation between three CIE color coordinates (X, Y, Z) and the three measured reflectance values. The linear transformation was characterized by a 3?×?3 matrix whose nine elements were determined by four standard samples with their chromaticity coordinates measured by a well-calibrated instrument. Our method works well without any information about the spectrum of illuminant and the color-matching functions. We also demonstrate the applicability of our calibrating method to two conventional illuminants: a bulb and a fluorescent lamp.     

Synthesis and electro-optical properties of carbazole derivatives with high band gap energy

Two materials containing carbazole moieties and exhibiting a high band gap energy, 3,8-di(9H-carbazol-9-yl)-6-phenylphenanthridine (DCzP) and 3,6-di(naphthalene-2-yl)-9-phenyl-9H-carbazole (DNaC), were synthesized via CN coupling and Suzuki coupling reactions, respectively. The compound DCzP exhibited blue emission with the CIE coordinates of x = 0.165 and y = 0.136 from the OLED device, ITO(indium–tin oxide)/NPB(N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine)/DCzP/LiF/Al. The doped device, ITO/2-TNATA(4,4′,4″-Tris(2-naphtylphenyl-phenylamino) triphenyl amine)/NPB/DCzP + Ir(ppy)₃/BCP(2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline)/Alq₃(tris(8-hydroxyquinoline)aluminum/LiF/Al, showed bright yellowish-green emission with a maximum luminance of 23,000 cd/m² when the synthesized DCzP was applied as a host material for the phosphorescent green dopant. From the double layer device, ITO/DNaC/Alq₃/LiF/Al, in which DNaC was used as the hole transporting material, the yellowish-green color arising from the Alq₃ was also observed.     

Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

One-step synthesis and double emission (Ca1 + x − yEuy)Ga2S4 + x phosphor for white LEDs

(Ca_{1 + x − y}Eu_y)Ga₂S_4 + x phosphors have been synthesized one step by solid state reaction. The photoluminescence excitation and emission spectra of phosphors have been studied; the influence of host composition and Eu²⁺ concentration on emission spectra has also been investigated. The emission spectrum consists of yellow emission at 550 nm and red emission at 650 nm. It also indicates that the excitation spectrum is a broadband and can be well matched with the emission of GaN chip. Combined these phosphors with 460 nm-emitting GaN chips, White LEDs have been fabricated. Their electroluminescence spectra have been measured under 20 mA forward-bias current. Their CIE chromaticity coordinates and color temperature indicate that (Ca_{1 + x − y}Eu_y)Ga₂S_4 + x phosphors are promising phosphors for GaN-based white LEDs.     

Influence of color coordinates on the color changes after polymerization and thermocycling of dental resin composites

Color changes after polymerization and aging of dental resin composites are perceptible. If these color changes can be predicted by the shade of material before polymerization or clinical use, color matching of composites with teeth would be improved. The objectives were to measure the correlations among the color coordinates (CIE L^∗, a^∗ and b^∗) of dental resin composites and the changes in color (Δ E^∗_ab) and color parameters (Δ L^∗, Δ C^∗_ab, and Δ H^∗_ab) after polymerization (PO) and thermocycling (TC), and to determine whether the range of color coordinates influenced these correlations. Color of two resin composites (26 shades) was measured before and after PO, and after TC with a reflection spectrophotometer. Regression analyses were performed among the color coordinates and the changes in color and color parameters after PO and TC. After PO, Δ E^∗_ab was correlated with CIE L^∗ value measured after PO, and CIE a^∗ and b^∗ values measured before PO (p < 0.01). After TC, Δ E^∗_ab was correlated with CIE b^∗, a^∗, and L^∗ measured after PO (p < 0.01). Correlations among the color coordinates and the changes in color parameters after PO or TC varied, but were generally significant (p < 0.01). Multiple regression analyses showed that Δ E^∗_ab values after PO and TC were mainly influenced by CIE L^∗ after PO (multiple r = 0.87 after PO and 0.58 after TC). Correlations were also influenced by the range of color coordinates. Within the limit of this study, shade of resin composites significantly influenced the changes in color and color coordinates after PO and TC. Light shades (high CIE L^∗ shades measured after polymerization) showed small color changes after polymerization and thermocycling.     

A new interpretation of Legendre's transformation and consequences

The Legendre transformation has found widespread application in thermodynamics, Hamilton‐Lagrange‐mechanics and optics. It attributes the values of the coordinates (x, y (x)) representing the points of a monotonic piecewise smooth functional curve y (x) the slopes m_x (x) = dy (x)/dx and the intercepts y (m_x) of their tangents on the y‐axis. Thus, the initial curve y (x) is represented by the ordered set of all slopes m_x (x) = dy (x)/dx of its tangents together with their intercepts y (m_x) on the y‐axis. It is shown that the transformed or conjugated function must basically be supplemented by a homogeneous linear function of the relevant variable. This is usually neglected in the literature. In addition, a new interpretation of the Legendre transformation is presented and discussed: For this purpose the derivative m_x (x) is considered as the proper initial function and integrated between x₀ and x. This integral is complemented by the integral of x (m_x) ((the inverse function of m_x (x)) over m_x between m_x0 = m_x (x₀) and m_x (x), if m_x (x) and x (m_x) are strictly monotonic. The sum of both integrals yields the “area” (xm_x – x₀m_x0). Legendre's transformation is obtained by reordering the respective terms. The procedure of transformation corresponds to integration by parts. Some examples and consequences of the properties considered are demonstrated and discussed using the simple model of two‐state systems. The general results of the present work remove possible internal inconsistencies in thermodynamics.     

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.