Blue Fluorescent Organic Light-emitting Diodes Research Articles

Synthesis and Electrical Properties of a New Bipolar Material Using Spacer Moiety

To develop a deep-blue emitter, a molecule with bipolar characteristics was designed as a donor-spacer-acceptor type, in which 9-(4-(4,6-diphenyl-1),3,5-triazin-2-yl)-2,5-dimethylphenyl)-9H-carbazole (DTPCZ)—with carbazole as an electron donating group and a diphenyl triazine moiety as an electron accepting group—was successfully synthesized. The photoluminescence (PL) maxima of DTPCZ were 421 nm in the solution state and 425 nm in the film state, indicating emission in the deep-blue region. DTPCZ also exhibited high thermal stability, with a degradation temperature of 349 °C. To confirm the electroluminescence (EL) characteristics, DTPCZ was applied as a dopant at 10, 20, and 30 wt% in a blue-fluorescent organic light-emitting diode (OLED) device. The highest efficiency was achieved using the 20 wt% doped device, with a current efficiency of 1.2 cd/A, an external quantum efficiency of 2.3%, and a Commission Internationale de l’Eclairage proceedings y-value of 0.06. Thus, deep-blue emission could be realized in the film state. These molecular design strategies can be applied to various fields, such as organic semiconductors.

Abrupt exciton quenching in blue fluorescent organic light-emitting diodes around turn-on voltage region

The quenching of electrically generated excitons in organic light-emitting diodes (OLEDs) serves as a limiting factor in fully exploiting the potential of organic emissive materials, ultimately resulting in a reduction of electroluminescence (EL) efficiency. As the quenching yield is contingent upon the density of species such as excitons and polarons, extensive efforts have been made to comprehend the mechanism under high excitation conditions. Herein, we present a sudden exciton quenching at the turn-on voltage region in a blue triplet–triplet upconversion (TTU)-based blue fluorescent OLED that limits overall EL efficiency. We confirmed that the photoluminescence intensity was quenched by ∼5% before injecting a current density of 0.1 mA cm−2. The quenching behavior strongly depended on the hole-transporting materials (HTMs), indicating that a specific radical cation of the HTMs triggers exciton-polaron annihilation events at the interface between the hole-transporting layer (HTL) and the emitting layer (EML). Furthermore, the voltage dependence of exciton quenching yield and EL spectra suggests a shift in the emission zone from the interface to the bulk of the EML with increasing voltage. These findings are critical for designing an optimized HTL/EML interface for highly efficient blue TTU-OLEDs.

Spirally Configured cis-Stilbene/fluorene (STIF), 10,11-Benzo-, and Imidazole-fused STIF Hybrids as Electron-Transporting Materials for Organic Light-Emitting Diode Applications.

A new class of cis-stilbene/fluorene (STIF) and 10,11-benzo- and imidazole-fused dibenzosuberane/fluorene (Bz-STIF and Imd-STIF) spiro hybrid systems with paired cyanophenyl, pyridyl, and/or pyrimidyl units was synthesized as electron transporting materials (ETMs) for blue fluorescent OLEDs (FOLEDs) and phosphorescent organic light emitting diodes (PhOLEDs). Their photophysical (UV-Vis absorption and emission), electrochemical (Eox /Ered ), morphological (glassy transition temperature, Tg ), and thermal stability (decomposition temperature, Td ) properties are systematical compared. Their correlations regarding the effects of the fused unit/substituent(s) in the STIF core on the morphological/thermal stability, HOMO/LUMO energy level, π-electron distribution profiles by DFT calculations, and electron mobility are established. Blue FOLEDs are fabricated by using them and TmPyPB (a control) as ETMs. The effects of their LUMO energy levels and electron mobilities on the device turn-on voltage, performance efficiencies including external quantum efficiency/current efficiency (EQE/CE), power efficiency (PE), and device lifetime at 5% luminescence decay (T95 ) are correlated. Among them, three best ETMs and TmPyPB (a control) are selected for further green and blue PhOLED fabrications. The effects of their LUMO energy levels and electron mobilities on the device turn-on voltage and performance efficiencies are confirmed, allowing for potential commercial applications.

Lifetime Comparisons of Organic Light-Emitting Diodes Fabricated by Solution and Evaporation Processes

In this paper, a blue fluorescent organic light-emitting diode (OLED) with a 1 cm2 emitting area was fabricated by a solution process. The ITO/spin MADN:13% UBD-07/TPBi/Al was used as the basic structure in which to add a hole-injection layer PEDOT:PSS and an electron-injection layer LiF, respectively. The device structure was optimized to obtain a longer lifetime. Firstly, the TPBi, which is an electron transport layer and a hole-blocking layer, was added to the structure to increase the electron transport rate. When the TPBi thickness was increased to 20 nm, the luminance was 221 cd/m2, and the efficiency was 0.52 cd/A at a voltage of 8 V. Since the addition of the hole-injection layer (HIL) increased the hole current but did not increase the electron current, the electron transport layer (ETL) Alq3 with the lowest unoccupied molecular orbital (LUMO) was added as stepped ETL to help the TPBi transport more electron current into the emitting layer. When the thickness of the TPBi/Alq3 was 10 nm/15 nm, the luminance reached 862 cd/m2, the efficiency was 1.29 cd/A, and the lifetime increased to 252 min. Subsequently, a hole-injection layer PEDOT:PSS with a thickness of 55 nm was added to make the ITO surface flatter and to reduce the probability of a short circuit caused by the spike effect. At this time, the luminance of 311 cd/m2, the efficiency of 0.64 cd/A, and the lifetime of 121 min were obtained. Following this, the thickness of the emitting layer was doubled to increase the recombination probability of the electrons and the holes. When the thickness of the emitting layer was 90 nm, and the thermal evaporation method was used, the efficiency was 3.23 cd/A at a voltage of 8V, and the lifetime was improved to 482 min. Furthermore, when the thickness of the hole-injection layer PEDOT:PSS was increased to 220 nm, the efficiency increased to 3.86 cd/A, and the lifetime was increased to 529 min. An infrared thermal image camera was employed to detect the temperature variation of the blue OLEDs. After the current was gradually increased, it was found that the heat accumulation of the device became more and more significant. When the driving current reached 50 mA, the device burnt out. It was found that the maximum temperature that the OLED device could withstand was approximately 58.83 degrees C at a current of 36.36 mA.

Highly Efficient and Pure Blue Fluorescent Organic Light-Emitting Diodes with Excellent Device Stability

Highly Efficient and Pure Blue Fluorescent Organic Light-Emitting Diodes with Excellent Device Stability

Spirobifluorene-based hole-blocking material with enhanced efficiency through hole and exciton confinement in blue fluorescent OLEDs

A new hole-blocking material, 2-(9,9′-spirobi[fluoren]-5-yl)-4-phenylbenzo [4,5]thieno[3,2-d]pyrimidine (SBF-BTPym), was designed and synthesized using spirobifluorene and electron transport moieties. The high triplet energy and high thermal stability of the spirobifluorene moiety were suitable for designing the hole-blocking material for blue fluorescent organic light-emitting diodes (OLEDs). The electron deficiency of benzothienopyrimidine allowed good electron transport properties, and a deep highest occupied molecular orbital energy level of −6.55 eV suppressed leakage of the holes from the emitting layer (EML) to the adjacent layer. Blue fluorescent OLEDs consisting of a triplet-triplet annihilation–induced host were fabricated using SBF-BTPym. The maximum external quantum efficiency of the blue fluorescent device was 7.2%, which was higher than that of the device without a hole-blocking layer by 10%. According to transient electroluminescence analysis, exciton confinement in the EML is mainly responsible for the improved efficiency.

A sensitization strategy for highly efficient blue fluorescent organic light-emitting diodes

Highly efficient blue fluorescent materials have recently attracted great interest for organic light-emitting diode (OLED) application. Here, two new pyrene based organic molecules consisting of a highly rigid skeleton, namely SPy and DPy, are developed. These two blue light emitters exhibit excellent thermal stability. The experiment reveals that the full-width at half-maximum (FWHM) of the emission spectrum can be tuned by introducing different amounts of 9,9-diphenyl-N-phenyl-9H-fluoren-2-amine on pyrene units. The FWHM of the emission spectrum is only 37 nm in diluted toluene solution for DPy. Furthermore, highly efficient blue OLEDs are obtained by thermally activated delayed fluorescence (TADF) sensitization strategy. The blue fluorescent OLEDs utilizing DPy as emitters achieve a maximum external quantum efficiency (EQE) of 10.4% with the electroluminescence (EL) peak/FWHM of 480 nm/49 nm. Particularly, the EQE of DPy-based device is boosted from 2.6% in non-doped device to 10.4% in DMAc-DPS TADF sensitized fluorescence (TSF) device, which is a 400% enhancement. Therefore, this work demonstrates that the TSF strategy is promising for highly efficient fluorescent OLEDs application in wide-color-gamut display field.Graphical

Improved Thermal Stability and Operational Lifetime of Blue Fluorescent Organic Light-Emitting Diodes by Using a Mixed-Electron Transporting Layer

Improved Thermal Stability and Operational Lifetime of Blue Fluorescent Organic Light-Emitting Diodes by Using a Mixed-Electron Transporting Layer

P‐124: Student Poster: Multiple‐Resonance‐Type Blue Fluorescent OLEDs with High Efficiency of Over 25% and Long Device Lifetime of Over 500 H

A deep blue boron based multiple resonance (MR) type material was synthesized and evaluated. A di‐tert‐butylphenyl substituent was introduced into the emitter to suppress the intermolecular interactions. A single electroluminescence device and a tandem device were fabricated with a fluorescent OLED using the characteristics of the MR‐type emitter with good color purity and high photoluminescence quantum yield. As a result, the single electroluminescence unit and tandem devices exhibited high maximum external quantum efficiency of 11.3% and 25.1%, and long device lifetime of 208 h and 500 h at 1,000 cd/m2, respectively.

Structurally modified [1,2,4]triazolo[1,5‑a]pyridine derivatives as promising materials for highly efficient blue fluorescent organic light-emitting diodes

Here, we designed and synthesized three donor–acceptor (D-A) emitters, 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-6-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (m-PPI-TP), 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-7-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (p-PPI-TP) and 1-phenyl-2-(4-(2-phenyl-[1,2,4]triazolo[1,5-a]pyridin-8-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (o'-PPI-TP), based on [1,2,4]triazolo[1,5-a]pyridine (TP) as electron-transporting unit and 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (PPI) as hole-transporting moiety serving as emitting block concurrently. All three compounds were demonstrated to be thermally stable with decomposition temperatures (Td) in the range of 454–476 °C and high photoluminescence quantum yields over 86%. Besides, high exciton utilization efficiency (EUE) via “hot exciton” process was proved according to photophysical experiments and theoretical calculations analysis. What’s more, the structure–property relationships between different substituted sites of the TP building block and PPI unit were also unveiled. Finally, the non-doped blue and doped deep-blue OLEDs were fabricated with high external quantum efficiency (EQE) approaching 8%. The yellow and red phosphorescent OLEDs (PHOLEDs) based on m-PPI-TP as the host material achieved the best EL performance with EQE of 26.55% and 20.35%, respectively.

High Efficiency of over 25% and Long Device Lifetime of over 500 h at 1000 nit in Blue Fluorescent Organic Light-Emitting Diodes.

In this study, a multiple resonance (MR) type blue emitter is synthesized, characterized, and evaluated for highly efficient and stable blue fluorescent organic light-emitting diodes (OLEDs). The MR blue fluorescent emitter has a di-tert-butyl benzene substituent in the MR core structure to minimize quenching mechanisms by intermolecular interaction. The emitter shows a high photoluminescence quantum yield and small full width at half maximum of 22 nm, which realize high external quantum efficiency (EQE) of 11.4% in the single unit OLED and device lifetime up to 95% of the initial luminance (LT95 ) of 208 h at 1000 cd m-2 and over 10000 h at 100 cd m-2 . The optimized tandem device of the new blue emitter achieves high EQE over 25% and extremely long LT95 of over 500h at 1000 cd m-2 and 30000 h at 100 cd m-2 . The lifetime of this work is one of the best data of blue OLED lifetime reported in the literature.

Efficient Non‐Doped Blue Electro‐fluorescence with Boosted and Balanced Carrier Mobilities

AbstractOne of the most important issues of the organic light‐emitting diode (OLED) is the highly efficient blue‐emissive material, which demands both excellent photoluminescent quantum yield (PLQY) and balanced carrier mobilities. Herein, a series of blue‐emissive donor–π–acceptor (D–π–A) materials with fluorene π‐bridge and their D–A analogues are synthesized and discovered with a theoretical combined experimental method. Based on the excellent electron mobility of the oxadiazole (OXZ) acceptor, it is further proven that the insertion of the fluorene π‐bridge can not only contribute to the formation of hybrid local and charge‐transfer excited state with high PLQY, but also give rise to the hole mobilities by enhanced intermolecular face‐to‐face stacking. As a result, the non‐doped OLED of TPACFOXZ exhibits a high maximum external quantum efficiency approaching 10% with boosted and balanced hole and electron mobilities of 5.60 × 10−5 and 6.60 × 10−5 cm2 V−1 s−1, respectively, which are among the best results of the non‐doped blue fluorescent OLEDs.

Investigation of 4,4′-bis[(N- carbazole) styryl] biphenyl (BSB4) for a pure blue fluorescent OLED with enhanced efficiency nearing the theoretical limit

4,4′-bis[(N-carbazole) styryl] biphenyl (BSB4 or BSBCz) is one of the widely studied organic fluorescent materials for blue organic electroluminescent devices in the recent times. In this work, BSB4 is used as a guest material to construct the host-guest matrix for the emissive layer (EML) of a pure blue fluorescent organic light-emitting diode (OLED). A pure blue emission suitable for display application with a Commission Internationale de l’Eclairage coordinate of (0.147,0.070) is achieved by the blue-shift of the emission spectrum of the host-guest matrix from that of the pristine guest (BSB4) molecules. The optimization of OLED structures is carried out by considering (a) charge balance in the EML for high exciton density, and (b) optical interference of generated light in the organic layers for increased light outcoupling. A thorough comparative study on the use of different combinations of widely used hole and electron transport layers to obtain charge balance in the EML of the OLED, thereby enhancing the external quantum efficiency (EQE) is shown. Optical interference effects in the fabricated OLEDs are analyzed by optical simulation of each device structure by transfer matrix method. With the optimized device structures, we are able to overcome the 2% EQE limit that has been reported so far for blue fluorescent OLEDs with BSB4 as light emitting material and achieve a maximum EQE of 4.08%, which is near to the theoretical limit of EQE for fluorescent OLEDs.

A systematic investigation to unravel the primary determinant of the operational stability of blue fluorescent organic light-emitting diodes

The primary factor that affects the lifetime of blue FOLEDs is the bond dissociation energy (BDE) in anionic state. The BDE of host molecules has a greater impact on the lifetime than that of electron blocking layer molecules.

Structurally Modified [1,2,4]Triazolo[1,5-A]Pyridine Derivatives as Promising Materials for Highly Efficient Blue Fluorescent Organic Light-Emitting Diodes

Structurally Modified [1,2,4]Triazolo[1,5-A]Pyridine Derivatives as Promising Materials for Highly Efficient Blue Fluorescent Organic Light-Emitting Diodes

Biindenophenanthrene derivatives as non-doped blue fluorescent materials for organic light-emitting diodes

We designed and synthesized biindenophenanthrene derivatives for blue fluorescent organic light-emitting diodes (OLEDs) materials to overcome the disadvantages of blue emitter caused by the wide band gap. To demonstrate the electroluminescence (EL) properties, the OLEDs devices were fabricated in the following sequence: indium-tin-oxide (180 nm)/4,4’,4’’-tris[2-naphthyl(phenyl)amino (30 nm)/4,4’-bis(N-(1-naphthyl)-N-phenylamino) biphenyl (20 nm)/Blue emitting materials (20 nm)/Bathophenanthroline (30 nm)/lithium quinolate (2 nm)/Al (100 nm). Device A that using 13,13,13',13'-tetramethyl-13H,13'H-11,11'-biindeno[1,2-l]phenanthrene in emitting layer showed luminous efficiency, power efficiency, and external quantum efficiency of 1.72 cd/A, 0.58 lm/W, 1.03% at 20 mA/cm2 with Commission Internationale d’Énclairage (CIE) coordinates of (0.15, 0.23) at 8.0 V.

7,7-Dimethyl-9-(10-phenylanthracen-9-yl)-7H-Benzo[b] Fluoreno[3,4-d]Thiophene Derivatives for Blue Fluorescent Organic Light-Emitting Diodes.

In this study, we designed and synthesized two blue fluorescence materials using 7,7-dimethyl-9-(10-phenylanthracen-9-yl)-7H-benzo[b]fluoreno[3,4-d]thiophene substituted anthracene derivatives. To characterize their electroluminescent properties, we fabricated the OLED devices using these two emitting materials. Particularly, a device using 7,7-dimethyl-9-(10-phenylanthracen-9-yl)-7Hbenzo[ b]fluoreno[3,4-d]thiophene showed maximum values of luminous efficiency, power efficiency, and external quantum efficiency of 2.42 cd/A, 1.48 lm/W, 3.08% at 20 mA/cm2, respectively with CIE (x, y) coordinates of (0.15, 0.09) at 8.0 V.

Stable Blue Fluorescent Organic Light-Emitting Diodes Based on an Inorganically Doped Homojunction

MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Ca were incorporated in ambipolar 4, $4'$ -N, N'-dicarbazolylbiphenyl (CBP) by coevaporation as stable inorganic acceptors and donors, respectively. Both charge transport and injection in CBP single-carrier devices were markedly improved as the doping concentration was raised above 10 wt% for MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and 6 wt% for Ca. P-i-n homojunction blue fluorescent organic light-emitting diodes (OLEDs) based on a single CBP layer with 10 wt% MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -doped and 6 wt% Ca-doped CBP charge transport regions had a luminance of 1680 cd/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the luminous efficiency of 3.4 cd/A, and a lifetime of 3.1 h at 50 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The homojunction blue OLEDs, benefiting from the greatly simplified structure and stable inorganic dopants, exhibited much improved reliability as compared with conventional multilayer heterojunction blue OLEDs.

Bee-shaped host with ideal polarity and energy levels for high-efficiency blue and white fluorescent organic light-emitting diodes

An ideal host for blue emitters with thermally activated delayed fluorescence (TADF) should bear a moderate polarity to relax charge transfer (CT) states of dopant for a reduced singlet-triplet energy gap (ΔEST) while utter-mostly keeping blue emission, a high triplet energy to confine exciton, and a small ΔEST for feasible charge injection. However, it is rather challenging to simultaneously fulfill all those requirements as the strong CT characteristic necessary for a small ΔEST usually enlarge polarity and lower energy levels. Here, a symmetrical ortho-linked diphenylphosphine oxide (DPO)/carbazole derivative is developed, of which the configuration resembles the bee with carbazoles as front wings, phenyl rings on DPO as back wings, hooked together by π-π interaction. This motif features a short distance of charges for a moderate polarity of 3.45 D, a twisted non-conjugation structure for a small ΔEST of 0.24 eV with a high triplet of 2.98 eV and multiple intermolecular interactions for enhanced charge transport. The corresponding blue TADF device realizes a compromise between efficiency and colors with a maximum external quantum efficiency (EQEmax) of 35.7% and a CIEy of 0.20. Furthermore, a full-TADF white device with this host realizes a EQEmax of 26.1%, representing the state-of-the-art performance.

Reliability of organic light-emitting diodes in low-temperature environment* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61775130 and 11974236), the Science and Technology Commission of Shanghai Municipality Program, China (Grant Nos. 19DZ2281000 and 17DZ2281000), and the Research Innovation Program for College Graduates of Jiangsu Province, China (Grant Nos. KYCX20 2545 and KYCX20 2549).

Organic light-emitting diode (OLED) is an electroluminescent technology that relies on charge-carrier dynamics and is a potential light source for variable environmental conditions. Here, by exploiting a self-developed low-temperature testing system, we investigated the characteristics of hole/electron transport, electro-optic conversion efficiency, and operation lifetime of OLEDs at low-temperature ranging from –40 °C to 0 °C and room temperature (25 °C). Compared to devices operating at room temperature, the carrier transport capability is significantly decreased with reducing temperature, and especially the mobility of the hole-transporting material (HTM) and electron-transporting material (ETM) at –40 °C decreases from 1.16 × 10−6 cm2/V⋅s and 2.60 × 10−4 cm2/V⋅s to 6.91 × 10−9 cm2/V⋅s and 1.44 × 10−5 cm2/V⋅s, respectively. Indeed, the temperature affects differently on the mobilities of HTM and ETM, which favors unbalanced charge-carrier transport and recombination in OLEDs, thereby leading to the maximum current efficiency decreased from 6.46 cd⋅A−1 at 25 °C to 2.74 cd⋅A−1 at –40 °C. In addition, blue fluorescent OLED at –20 °C has an above 56% lifetime improvement (time to 80% of the initial luminance) over the reference device at room temperature, which is attributed to efficiently dissipating heat generated inside the device by the low-temperature environment.