Fluorescent Organic Light-emitting Diodes Research Articles

Rational Molecular Design Strategy for Host Materials in Thermally Activated Delayed Fluorescence-OLEDs Suitable for Solution Processing.

Herein, a novel core molecule for V-shaped host molecules was synthesized, wherein two carbazoles were directly linked to cyclohexane. Cy-mCP and Cy-mCBP hosts were also successfully prepared for solution-processable thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs). The Cy-mCP and Cy-mCBP molecules contained a cyclohexane linker directly linked to two small molecular hosts (mCP and mCBP), exhibiting twice the molecular weight while maintaining the basic properties of a single host molecule with improved film-forming ability and solubility in organic solvents. These host materials showed superior thermal stability and high glass transition temperatures compared to lower molecular weight hosts. Green TADF-OLEDs were prepared using the two host materials and 2,4,5,6-tetra(3,6-di-tert-butylcarbazol-9-yl)-1,3-dicyanobenzene (t4CzIPN) emitter, achieving device efficiencies similar to that of a low-molecular-weight host. However, after the incorporation of a V-shaped host, superior characteristics were observed in terms of the thermal stability and operational stability of the device. The synthesis of V-shaped molecules by directly linking two carbazoles to a cyclohexane linker is promising for the development of different hosts for solution-processable OLEDs.

Unveiling the In Situ and Solvent Polymerization Engineering for Highly Efficient and Flexible Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes.

Thermally activated delayed fluorescence (TADF) polymer has great potential for the construction of flexible solution-processed organic light-emitting diodes (OLEDs). However, the relationship between polymerization engineering and device functions has rarely been reported. Here, two novel TADF polymers, P-Ph4CzCN and P-Ph5CzCN, with a small energy gap between the first excited singlet and triplet states (ΔEST; <0.16 eV) were newly developed by both solvent and in situ polymerization of a styrene component. Detailed device performance testing indicates that both polymerization strategies ensure that the TADF polymer achieves comparable high efficiencies in commonly rigid devices, and the maximum external quantum efficiencies (EQEmax) were 11.9%, 14.1%, and 16.2% for blue, green, and white OLEDs, respectively. Although in situ polymerization provides a simplified device fabrication process, which avoids the complicated synthesis and purification of the polymer, the inevitable high-temperature annealing makes it fail in a plastic substrate device. In contrast, P-Ph5CzCN achieved by solvent polymerization enables the successful fabrication of a flexible device on a poly(ethylene terephthalate) (PET) substrate, which was the first reported flexible OLED based on a TADF polymer. This work provides a strong guideline for the simple fabrication of TADF polymer devices and the application of TADF polymer materials in OLED flexible panels and flexible lighting.

Enhancing Triplet-Triplet Upconversion Efficiency and Operational Lifetime in Blue Organic Light-Emitting Diodes by Utilizing Thermally Activated Delayed Fluorescence Materials.

In the process of triplet-triplet upconversion (TTU), a bright excited singlet can be generated because of the collision of two dark excited triplets. In particular, the efficiency of TTU is crucial for achieving a high exciton production yield in blue fluorescence organic light-emitting diodes (OLEDs) beyond the theoretical limit. While the theoretical upper limit of TTU contribution yield is expected to be 60%, blue OLEDs with the maximum TTU contribution are still scarce. Herein, we present a proof of concept for realizing the maximum TTU contribution yield in blue OLEDs, achieved through the doping of thermally activated delayed fluorescence (TADF) molecules in the carrier recombination zone. The bipolar carrier transport ability of TADF materials enables direct carrier recombination on the molecules, resulting in the expansion of the recombination zone. Although the external electroluminescence quantum efficiency of OLEDs is slightly lower than that of conventional TTU-OLEDs due to the low photoluminescence quantum yield of the doped layer, the TTU efficiency approaches the upper limit. Furthermore, the operational device lifetime of OLEDs employing TADF molecules increased by five times compared to the conventional ones, highlighting the expansion of the recombination zone as a crucial factor for enhancing overall OLED performance in TTU-OLEDs.

Tetradentate Pt(II) Complex as Phosphorescent Sensitizer for Highly Efficient Green Organic Light‐Emitting Diodes with Low Efficiency Roll‐Off

AbstractTetradentate Pt(II) complex (Pt‐Trz)‐sensitized green fluorescent organic light‐emitting‐diodes (OLEDs) are developed to improve the external quantum efficiency (EQE) of a green fluorescent emitter (PhtBuPAD). Pt(II) complex and a green fluorescent emitter with bulky blocking units are chosen to efficiently manage energy transfer in phosphor‐sensitized OLEDs. The Pt‐Trz phosphor‐sensitized fluorescent OLED with 2 wt% PhtBuPAD exhibits high EQE of 24.2% and maintains high EQE of 23.4% at a luminance of 3000 cd m−2, realizing low efficiency roll‐off of 3.8%. Moreover, high EQE is achieved even at a high PhtBuPAD‐doping concentration of 3 wt%. In addition, the 2 wt% PhtBuPAD‐doped phosphor‐sensitized device achieves high maximum power efficiency of 91.1 lm W−1, one of the highest power efficiencies among existing green sensitized devices. This study demonstrates that Pt‐Trz phosphor‐sensitized devices with an excellent triplet exciton utilization ability can achieve high EQE, low efficiency roll‐off, and weak doping concentration quenching simultaneously.

Cyanopyridone-cored fluorophores with triphenylamine peripheries: From molecular design to OLED fabrication studies

• Seven novel cyanopyridone-based D-A-D type molecules with a fixed triphenylamine donor, were synthesized. • Theoretical, optical, solvatochromic, thermal, electrochemical, electrical, and EL studies were performed to evaluate structure-property relationships. • Doped OLEDs using novel fluorophores as dopants outperform non-doped devices. • OLEDs emit yellow light with maximum efficiencies of 10.72 cd/A, 7.87 lm/W, and 5.32%. Amongst many types of fluorescent organic materials, compounds with π-conjugation between an electron donor (D) and electron acceptor (A) have been intensively investigated as effective organic electroluminescence (EL) device components. Herein, we report the synthesis and structural characterization of a new series of D-A-D configured small molecule-based fluorophores ( TPA-CyP1-7 ) having triphenylamine ( TPA ) as a donor unit and 3-cyanopyridine-2-one ( CyP ) as an acceptor core with different architects for use as efficient yellow light emitters in fluorescent organic light-emitting diodes (OLED). The detailed photophysical, solvatochromic, thermal, electrochemical, and EL properties, including quantum chemical calculations, were systematically investigated to study their relation between structure and properties. All the fluorophores show high fluorescent quantum yields in the solid film state and display high thermal stability with decomposition temperatures above 350 o C. The study reveals that they possess appropriate HOMO and LUMO energies level for effective charge injection. Finally, these yellow luminogens were employed to fabricate new OLED devices as sole emitters and dopants with CBP host materials. Interestingly, the host-guest devices doped with CBP host emitters show a remarkable improvement in the overall device performance. Among them, the TPA-CyP3 -based doped device has achieved a maximum current efficiency ( η c ), high power efficiency ( η p ), and good external quantum efficiency ( η EQE ) of 10.72 cd/A, 7.87 lm/W, and 5.32%, respectively.

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.

Molecularly engineered host materials for high performance inkjet-printed thermally activated delayed fluorescence organic light-emitting diodes

Host material with appropriate thermal stability and solubility is essential for inkjet printing (IJP) organic light-emitting diodes (OLEDs).Herein, three new host materials with different molecular symmetries have been designed for IJP OLEDs.

A dual-locked triarylamine donor enables high-performance deep-red/NIR thermally activated delayed fluorescence organic light-emitting diodes.

Deep-red/near-infrared (DR/NIR) organic light-emitting diodes (OLEDs) have attracted a great deal of attention due to their widespread application fields, such as night-vision devices, optical communication, and information-secured displays. However, most DR/NIR OLEDs show low electroluminescence efficiencies, hampering their applications. Herein, we constructed a high-performance DR/NIR thermally activated delayed fluorescence (TADF) emitter based on an advanced dual-locked triarylamine donor (D) unit. Promisingly, such a novel D segment brings numerous advantages: a larger stereoscopic architecture, an enhanced electron-donating ability, and a stiffer molecular structure. In view of these features, the newly developed emitter DCN-DSP shows redshifted emission, a narrowed ΔEST, an enhanced PLQY value and aggregation-induced emission (AIE) properties, which allows for effectively alleviating concentration quenching compared to the control compound using a conventional triarylamine derivative as D units. The DCN-DSP-based OLEDs with modulated doping concentrations exhibit champion EQEs of 36.2% at 660 nm, 26.1% at 676 nm and 21.3% at 716 nm, which are record-high efficiencies among all TADF OLEDs in the similar emission ranges. This work realizes the efficiency breakthrough of DR/NIR TADF OLEDs, and such a promising molecular design approach may inspire even better DR/NIR TADF emitters in the future.

Highly efficient near-infrared thermally activated delayed fluorescence organic light-emitting diodes with emission beyond 800 nm

Near-infrared emission was achievedviathe intermolecular through-space electronic coupling and intramolecular charge transfer.

Recent endeavors and perspectives in developing solution-processable host materials for thermally activated delayed fluorescence organic light-emitting diodes

Indeed, the development of solution-processable host materials for TADF OLEDs is less explored. This review aims to provide a comprehensive overview of the progress made in the exploration of host materials for solution-processable TADF OLEDs.

Carbon Nanodots as Electron Transport Materials in Organic Light Emitting Diodes and Solar Cells.

Charge injection and transport interlayers play a crucial role in many classes of optoelectronics, including organic and perovskite ones. Here, we demonstrate the beneficial role of carbon nanodots, both pristine and nitrogen-functionalized, as electron transport materials in organic light emitting diodes (OLEDs) and organic solar cells (OSCs). Pristine (referred to as C-dots) and nitrogen-functionalized (referred to as NC-dots) carbon dots are systematically studied regarding their properties by using cyclic voltammetry, Fourier-transform infrared (FTIR) and UV-Vis absorption spectroscopy in order to reveal their energetic alignment and possible interaction with the organic semiconductor's emissive layer. Atomic force microscopy unravels the ultra-thin nature of the interlayers. They are next applied as interlayers between an Al metal cathode and a conventional green-yellow copolymer-in particular, (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(1,4-benzo-{2,1',3}-thiadiazole)], F8BT)-used as an emissive layer in fluorescent OLEDs. Electrical measurements indicate that both the C-dot- and NC-dot-based OLED devices present significant improvements in their current and luminescent characteristics, mainly due to a decrease in electron injection barrier. Both C-dots and NC-dots are also used as cathode interfacial layers in OSCs with an inverted architecture. An increase of nearly 10% in power conversion efficiency (PCE) for the devices using the C-dots and NC-dots compared to the reference one is achieved. The application of low-cost solution-processed materials in OLEDs and OSCs may contribute to their wide implementation in large-area applications.

Operational Stability Analysis of Blue Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes Using the Capacitance-Voltage Method

We analyzed the degradation features by measuring the capacitance–voltage characteristics after electrically aging blue thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). The measurement was investigated in terms of the hole transfer layer (HTL) and electron transfer layer (ETL) structures. For the HTL, three different materials—N,N′–bis(naphthalen–1–yl)–N,N′–bis(phenyl)–benzidine (NPB), 4,4′,4″-tris(carbazol–9–yl)triphenylamine (TCTA), and 1,3–bis(carbazol–9–yl)benzene (mCP)—were used at the HTL/emission layer (EML) interface; the TCTA/EML interface had the highest stability among the interfaces. For the ETL, bis [2–(diphenylphosphino)phenyl] ether oxide (DPEPO) without further dopants was used as an exciton blocking layer (ExBL) to effectively confine the excitons at the EML. However, DPEPO has low stability and carrier mobility. Therefore, 0, 10, and 40 nm-thick ExBL devices were investigated; it was found that the 0 nm-thick ExBL device was the most stable. However, the 10 nm-thick ExBL is essential to confine the excitons at the EML, which ensures a high EL performance.

Novel benzonitrile-based AIE host with high triplet energy for highly efficient solution-processed blue TADF OLEDs

Developing high-performance host materials for blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) is an effective way to improve emission efficiency through overcoming intermolecular interactions. In this work, three novel AIE-active materials with twisted donor-acceptor structures (3phCN-Cz, 3phCN-ICz and 3phCN-tCzICz) are designed and synthesized by modifying the donor unit. Encouragingly, all the molecules exhibited high triplet energies (>2.80 eV) to prevent exciton returning and aggregation‐induced emission (AIE) property for suppressing exciton quenching. Furthermore, systematic studies demonstrated that 3phCN-ICz and 3phCN-tCzICz with larger steric resistance units can not only possess good thermal and electrochemical stability but also exhibit excellent inhibition ability of intermolecular interactions, which are suitable for constructing high-efficiency solution-processed blue TADF devices. Ultimately, the 4TCzBN was employed as the blue TADF emitter, and the solution-processed OLEDs based on 3phCN-ICz achieved the maximum external quantum efficiency (EQEmax), the maximum current efficiency (CEmax), and the lowest turn-on voltage of 22.04%, 41.88 cd A−1, and 3.5 V, respectively. This work demonstrates the potential application of AIE materials with high triplet energy for solution-processed blue TADF OLEDs.

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

Study on charge dynamics and recombination process via transient electroluminescence analysis in blue fluorescence organic light-emitting diodes

Fluorescent organic light-emitting diodes (FOLEDs) exhibit substantially different efficiency-lifetime characteristics depending on dopant materials even in the same host material. Thus, obviously charge and exciton dynamics and recombination process may vary depending on the dopants. However, an accurate understanding of charge and exciton dynamics with dopants in FOLEDS is lacking so far. Herin, triplet–triplet-fusion-based FOLEDs with two blue dopants, 2,5,8,11-tetra-tert-butylperylene (TBPe) and 2,12-di-tert-butyl-5,9-bis(4-(tert-butyl)phenyl)-5,9-dihydro-5,9-diaza-13b-boranaphtho[3,2,1-de] anthracene (t-DABNA) were investigated via transient electroluminescence (TrEL). Interestingly, the TrEL of the blue FOLEDs elucidated different charge profiles and recombination processes within the emission layers. The TrEL spikes after pulse-off and the characteristic times of EL intensity after pulse-on revealed the accumulated charge profile concerning the charge balance. In temperature-dependent TrEL, the delayed fluorescence, exciton decay time, and EL spikes after pulse-on reflected the emission mechanisms as well. The results showed that the electrical properties and the device performances of blue FOLEDs can be significantly impacted by the alignment of the energy levels of hosts and dopants, even with a low doping concentration. Furthermore, depending on the charge trap conditions, it was found that efficiency roll-off can be attributed to Dexter energy transfer from host to dopant and singlet–triplet annihilation by direct recombination on dopant molecules.

Synthesis and Late-Stage Diversification of BN-Embedded Dibenzocorannulenes as Efficient Fluorescence Organic Light-Emitting Diode Emitters.

We report the synthesis and late-stage diversification of a new class of hetero-buckybowl, BN-embedded dibenzocorannulenes (B2 N2 -DBCs). The synthesis is achieved via one-shot halogenative borylation, comprising the nitrogen-directed haloboration of alkyne and an intramolecular bora-Friedel-Crafts reaction, which provides BN-embedded dibenzocorannulene possessing two bromo substituents (B2 N2 -DBC-Br). B2 N2 -DBC-Br undergoes diversification via coupling reactions to provide a variety of arylated derivatives (B2 N2 -DBC-R), exhibiting strong blue fluorescence. An organic light-emitting diode (OLED) employing one of the derivatives as an emitter exhibited a high external quantum efficiency of 6.6 % and long operational lifetime of 907 h at an initial luminance of 1000 cd m-2 , indicating the significant potential for the development of efficient and stable hetero-buckybowl-based OLED materials.

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.

Aluminum doped Zinc oxide anode film for performance enhancement of trilayer fluorescence organic light emitting diode

The industrialization of fluorescence organic light-emitting diodes (OLEDs) has seriously been impacted by high fabrications cost and a short device lifespan. Particularly for transparent conductive anodes (TCA) in an OLED, which benefit manufacturing the simplest configuration with an abundant platform, more flexibility, potentially low cost-effective, and wide-ranging light flux. The goal of this effort has been to look into the possibility of aluminum-doped zinc oxide (AZO) as TCA, typically AZO/SpiroOMeTAD/Alq3: Ir(ppy)3/Bphen/LiF/Al structure is remarkably thought-out by drift–diffusion model. In addition, maximizing the light flux and minimizing the knee voltage necessitates a well-organized trilayer bandgap alignment towards exciton confinement and steadiness charge carrier transport. To enhances the hysteresis of the recombination prefactor through the higher hopping mechanism, the organic layers are staked in between AZO anode and bilayer cathode (LiF/Al) units. Moreover, optimization of the device structure is intended to eliminate the influence on OLED performances under the better thickness adjustment of consecutive layers. The maximum light flux of 1.7095 × 102 Wm−2 has been attained for 70 nm thickness of SpiroOMeTAD as hole conducting layer (HCL), 10 nm thickness of Alq3 as an organic emissive layer (OEL) with Ir(ppy)3 emitter, and 80 nm thickness of Bphen as electron conducting layer (ECL). The exposed different optoelectronic appearance curve signifies the maximum charge carrier density and current density of 1.18 × 1024 m−3, and 1.035 × 106 Am−2 for knee voltages of 2.64 V. The highest recombination prefactor of 1.174 × 10-21 m-6s−1 is achieved at 300 K for different opto-electrical considerations. Hence, future studies may be an alternative strategy for enlightening the efficiency of AZO anode-based OLEDs.

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

Acridone-Based Host Materials for Green Phosphorescent and Thermally Activated Delayed Fluorescent OLEDs with Low-Efficiency Roll-Offs.

By linking the carbazole unit to the nitrogen atom of acridone through phenyl or pyridyl, two compounds, named 10-(4-(9H-carbazol-9-yl)phenyl)acridin-9(10H)-one (AC-Ph-Cz) and 10-(5-(9H-carbazol-9-yl)pyridin-2-yl)acridin-9(10H)-one (AC-Py-Cz) were designed and synthesized. These two materials, characterized with highly twisted and rigid structure, good thermal stability, and balanced carrier-transporting properties, were employed as host materials for green phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes (OLEDs). The carbazole group, despite its small contribution to the highest occupied molecular orbitals (HOMOs) of these two materials, plays an essential role as an intramolecular host in energy delivering and improving the hole transporting ability of these two hosts. The incorporation of the electron-deficient pyridyl group as a linking group slightly improves the electron transporting capability of AC-Py-Cz. The green phosphorescent OLED (PhOLED) based on AC-Py-Cz exhibited excellent device performance with a turn-on voltage of 2.5 V, a maximum power efficiency and an external quantum efficiency (ηext ) of 89.8 lm W-1 and 25.2 %, respectively, benefitting from the better charge-balancing ability of AC-Py-Cz host due to the presence of the pyridyl bridge. More importantly, all the devices based on these two hosts showed low efficiency roll-off at high brightness due to the suppressed non-radiative transition in the emitting layer. In particular, the AC-Py-Cz-hosted green PhOLED exhibited an efficiency roll-off of 1.6 % from the maximum next at a high brightness of 1000 cd m-2 and a roll-off of 15.9 % at an extremely high brightness of 10000 cd m-2 . This study manifests that acridone-based host materials have great potential in fabricating OLEDs with low efficiency roll-off.