Materials For Organic Light-emitting Diodes Research Articles

Synthesis and characterization of a novel thermally stable silane-based host material for blue phosphorescent organic light-emitting diodes

We report a novel thermally stable host material, namely, bis[3-(2-indolo[3,2,1-jk]carbazolyl)phenyl)diphenylsilane (Si-IDCz), for use in blue phosphorescent organic light-emitting diodes (PhOLEDs). Here, Si-IDCz exhibits a high glass transition temperature (Tg) of 151 °C as it contains a thermally stable silane moiety. Additionally, the rigid silicon core within Si-IDCz effectively suppresses molecular interactions, resulting in a wide bandgap and high triplet energy. Here, we used Si-IDCz as the single host for the blue PhOLEDs. The blue PhOLED with the single-host Si-IDCz exhibited an external quantum efficiency (EQE) of over 15%; an EQE of over 11% was maintained at a high luminance of 5000 cd/m2 owing to the high thermal stability of the Si-IDCz. Furthermore, an improved EQE of more than 20% was achieved by introducing a mixed-host system to enhance the photoluminescence quantum yield; the efficiency roll-off was reduced as well.

Hot-Exciton Mechanism and AIE Effect Boost the Performance of Deep-Red Emitters in Non-Doped OLEDs.

Luminescent materials possessing a "hot-exciton" mechanism and aggregation-induced emission (AIE) qualities are well-suited for use as emitting materials in nondoped organic light-emitting diodes (OLEDs), particularly in deep-red regions where their ground state and singlet excited state surfaces are in proximity, leading to the formation of multiple nonradiative channels. However, designing molecules that artificially combine the hot-exciton mechanism and AIE attributes remains a formidable task. In this study, a versatile strategy is presented to achieve hot-exciton fluorescence with AIE property by increasing the first singlet excited (S1 ) state through modulation of the conjugation length of the newly created acceptor unit, matching the energy level of high-lying triplet (Tn ) states, and enhancing exciton utilization efficiency by employing suitable donor moieties. This approach reduces the aggregation-caused quenching (ACQ) in the aggregate state, resulting in the proof-of-concept emitter DT-IPD, which produces an unprecedented external quantum efficiency (EQE) of 12.2% and Commission Internationale de I'Eclairage(CIE)coordinates of (0.69, 0.30) in a deep-red non-doped OLED at 685nm, representing the highest performance among all deep-red OLEDs based on materials with hot-exciton mechanisms. This work provides novel insights into the design of more efficient hot-exciton emitters with AIE properties.

Simple-structured nicotinonitrile based material for sky-blue organic light-emitting diodes

Organic light-emitting diodes (OLEDs) perform a vital function in flat-panel displays and solid-state lighting. Herein, a simple structured donor-acceptor based organic light-emitting material (OLEM), 6-(4-(di ([1,1′-biphenyl]-4-yl)amino)phenyl)nicotinonitrile (PTPA-NN), has been prepared and applied as emissive layer in OLEDs. This OLEM has a slightly twisted structure, which enhances its thermal stability. Interestingly, a device consisting of PTPA-NN OLEM demonstrated a promising performance showing a turn-on voltage of 3.8 V with a current efficiency of 9.7 cd/A, external-quantum-efficiency of 4.17% and power efficiency of 8.0 lm/W. Small molecular simple-structured nicotinonitrile based material provide an effective strategy to discover suitable OLEMs for sky-blue OLEDs.

A Comparison between Triphenylmethyl and Triphenylsilyl Spirobifluorenyl Hosts: Synthesis, Photophysics and Performance in Phosphorescent Organic Light-Emitting Diodes.

This study presents the synthesis and characterization of two spirobifluorenyl derivatives substituted with either triphenylmethyl (SB-C) or triphenylsilyl (SB-Si) moieties for use as host materials in phosphorescent organic light-emitting diodes (PHOLED). Both molecules have similar high triplet energies and large energy gaps. Blue Ir(tpz)3 and green Ir(ppy)3 phosphorescent devices were fabricated using these materials as hosts. Surprisingly, SB-Si demonstrated superior charge-transporting ability compared to SB-C, despite having similar energies for their valence orbitals. In particular, SB-Si proved to be a highly effective host for both blue and green devices, resulting in maximum efficiencies of 12.6% for the Ir(tpz)3 device and 9.6% for the Ir(ppy)3 device. These results highlight the benefits of appending the triphenylsilyl moiety onto host materials and underscore the importance of considering the morphology of hosts in the design of efficient PHOLEDs.

Phenoxazine and phenothiazine embedded Multi-Resonance emitters for highly efficient Pure-Red OLEDs with improved color purity

Multi-resonance (MR) emitters based on boron-nitrogen doped polycyclic aromatic hydrocarbons are the cutting-edge light-emitting materials for organic light-emitting diodes (OLEDs) due to their high photoluminescence quantum yield and narrowband emission spectra. Among all the widely-reported MR molecules, emitters showing pure-red emission are the least developed. The difficulty in designing pure-red MR emitters lies in the limit of the weak charge transfer nature of the MR framework, as well as the large molecular weight which may bring difficulty in vacuum sublimation process. Herein, through introducing the strong electron donating phenoxazine and phenothiazine moiety into the double-boron-based MR molecular skeleton, two novel MR emitters namely PXZBNO and PTZBNO were designed and synthesized. These two emitters exhibited pure-red emission with emission maxima up to 627 nm, small full-width at half-maximum (FWHM) of 45 nm. Red OLEDs based on PTZBNO demonstrated very high maximum external quantum efficiency (EQEmax) of 34.5%, together with very small efficiency roll-off. Meanwhile, PXZBNO-based purity pure-red OLEDs exhibited high color purity with color coordinates of (0.69, 0.31), which is approaching BT. 2020 standard.

Forthcoming hyperfluorescence display technology: relevant factors to achieve high-performance stable organic light emitting diodes.

Over the decade, there have been developments in purely organic thermally activated delayed fluorescent (TADF) materials for organic light-emitting diodes (OLEDs). However, achieving narrow full width at half maximum (FWHM) and high external quantum efficiency (EQE) is crucial for real display industries. To overcome these hurdles, hyperfluorescence (HF) technology was proposed for next-generation OLEDs. In this technology, the TADF material was considered a sensitizing host, the so-called TADF sensitized host (TSH), for use of triplet excitons via the reverse intersystem crossing (RISC) pathway. Since most of the TADF materials show bipolar characteristics, electrically generated singlet and triplet exciton energies can be transported to the final fluorescent emitter (FE) through Förster resonance energy transfer (FRET) rather than Dexter energy transfer (DET). This mechanism is possible from the S1 state of the TSH to the S1 state of the final fluorescent dopant (FD) as a long-range energy transfer. Considering this, some reports are available based on hyperfluorescence OLEDs, but the detailed analysis for highly efficient and stable devices for commercialization was unclear. So herein, we reviewed the relevant factors based on recent advancements to build a highly efficient and stable hyperfluorescence system. The factors include an energy transfer mechanism based on spectral overlapping, TSH requirements, electroluminescence study based on exciplex and polarity system, shielding effect, DET suppression, and FD orientation. Furthermore, the outlook and future positives with new directions were discussed to build high-performance OLEDs.

A Compact History of the OLEDs

People's demand for lighting and display is gradually increasing as society and technology progress. Therefore, the scientific community has been eager to find a new kind of luminescent material suitable for industrial society. The effort paid off, as scientists discovered light conversion and its efficient Organic Light Emitting Diode materials through experiments. These materials can be divided into three distinct generations as a result of continuous updating and iteration: the first-gen (first generation) of fluorescent materials, the second- gen of phosphorescent materials, and the third-gen of TADF. In order to let the public better understand the development history of OLED materials, this paper will focus on the mechanism of different materials to discuss the schemes and introduce some of the latest applications of these materials. At the end of this review, it will illustrate why TADF the third generation of OLED light emitting material is the most promising in terms of practical applications compared with other materials and the vision of the future of OLED.

Charge Transport Properties of a Series of Metal Quinolates Utilising Dispersion-Corrected Density Functional Theory

The electronic and charge transport properties of Metal-Quinolates (Metal = Li, Na, K, Rb and Cs) compounds are theoretically investigated using AustinFrisch-Petersson functional with dispersion (APFD) corrected density functional theory (DFT). The calculated energy gap between highest occupied molecular orbital and lowest unoccupied molecular orbital ranges from 3.40 eV for LiQ to 0.93 eV for CsQ. The ionisation potential, electron affinity and chemical hardness of the compounds are calculated. We found that the electron hopping rate, kelectron of CsQ is around 150 times greater than LiQ. We suggest that CsQ is the most efficient charge injecting or transport material for organic light-emitting diodes (OLEDs). Dimer formation is desirable with all M-Quinolate with different electronic structures and (CsQ)2 dimer shows the lowest dimerisation energy.

Realizing efficient blue and deep-blue delayed fluorescence materials with record-beating electroluminescence efficiencies of 43.4%

As promising luminescent materials for organic light-emitting diodes (OLEDs), thermally activated delayed fluorescence materials are booming vigorously in recent years, but robust blue ones still remain challenging. Herein, we report three highly efficient blue and deep-blue delayed fluorescence materials comprised of a weak electron acceptor chromeno[3,2-c]carbazol-8(5H)-one with a rigid polycyclic structure and a weak electron donor spiro[acridine-9,9’-xanthene]. They hold distinguished merits of excellent photoluminescence quantum yields (99%), ultrahigh horizontal transition dipole ratios (93.6%), and fast radiative transition and reverse intersystem crossing, which furnish superb blue and deep-blue electroluminescence with Commission Internationale de I’Eclairage coordinates (CIEx,y) of (0.14, 0.18) and (0.14, 0.15) and record-beating external quantum efficiencies (ηexts) of 43.4% and 41.3%, respectively. Their efficiency roll-offs are successfully reduced by suppressing triplet-triplet and singlet-singlet annihilations. Moreover, high-performance deep-blue and green hyperfluorescence OLEDs are achieved by utilizing these materials as sensitizers for multi-resonance delayed fluorescence dopants, providing state-of-the-art ηexts of 32.5% (CIEx,y = 0.14, 0.10) and 37.6% (CIEx,y = 0.32, 0.64), respectively, as well as greatly advanced operational lifetimes. These splendid results can surely inspire the development of blue and deep-blue luminescent materials and devices.

Promising Photoluminescence Enhancement of Tris(8-hydroxyquinoline)aluminum by Simultaneous Localized and Propagating Surface Plasmons of Ag Nanostructures

The continuous performance optimization of tris(8-hydroxyquinoline)aluminum (Alq3) materials is of great significance during the commercialization process of organic light-emitting diodes (OLEDs). In incorporating Ag nanostructures into Alq3, the photophysical properties are greatly improved by the plasmon–exciton coupling effect. Localized surface plasmons (LSPs) in Ag nanoparticles (NPs) efficiently increased the absorption ability. The coexistence of LSPs and propagating surface plasmons (PSPs) in Ag nanowires (NWs) leads to a PL enhancement of 5.3-fold and a full-width at half maximum (FWHM) narrowed by 10 nm. Temperature-dependent PL measurements exhibit that the plasmonic density of states (DOS) increases with decreasing temperature below 40 °C, and the thermal exchange can be accelerated by the introduction of Ag nanostructures. Effective suppression of the thermal accumulation effect is further proved by excitation intensity (EI)-dependent PL measurements. We also found that Ag nanostructures could mainly change the y coordinates in International Commission on Illumination (CIE), leading to a higher brightness. The 5372 K color temperature of an Ag NWs-embedded composite is suitable for daylight-type fluorescent OLEDs. The results would pave an effective way for further optimizing the optical performance of light-emitting materials in OLEDs.

Substituent effect on anthracene-benzophenone triad system: Photophysical properties and application to OLEDs with “hot-exciton” characteristics

‘‘Hot-exciton’’ materials exhibiting high-lying reverse intersystem crossing (hRISC) are promising candidates for organic light-emitting diode (OLED) materials owing to their improved exciton utilization efficiency and efficiency roll-off. To investigate the excited-state structure–property relationship in anthracene/benzophenone-based materials, we designed and prepared three anthracene-benzophenone derivatives by varying substituents at peripheral benzophenone units, namely, AnBP-F, AnBP-H, and AnBP-OMe. Systematic investigation of the excited-state properties of these compounds and DFT calculations showed that they have hybridized local and charge-transfer (HLCT) characteristics in their emissive transitions. All prepared compounds were adapted in a non-doped OLED device, which exhibited an external quantum efficiency (EQE) of up to 2.88%. To further study the exciton dynamics in the device, magnetoelectroluminescence was measured, which exhibits the combined characteristics of triplet–triplet annihilation (TTA) and HLCT mechanisms.

Spiroborate-Based Host Materials with High Triplet Energies and Ambipolar Charge-Transport Properties.

Organic light-emitting diodes (OLEDs) receive considerable attention because of their commercial use in flat panel displays. Herein, highly efficient spiroborate-based host materials are reported for use in blue OLEDs. Our designed spiroborates (SBOX) were simple to synthesize and exhibited high triplet excitation energies, narrow S-T gaps, and balanced charge carrier mobilities. A blue OLED containing one of the designed spiroborates, SBON, as a host exhibited a high external quantum efficiency (27.6 %) and low turn-on voltage (3.7 V) compared to those observed using 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (17.6 % and 4.5 V, respectively), indicating their potential as host materials in OLEDs.

Blue Organic Light Emitting Diode Materials based on Different Light-emitting Groups

Abstract: Organic light emitting diode (OLED) is a device that uses organic semiconductor materials to emit light under the action of an electric field. Compared with traditional luminescent materials, they have the advantages of good softness, low-temperature resistance, wider field of vision and low energy consumption, and have been widely used in the field of display and lighting in recent years. In addition, compared with red and green light-emitting materials, the maximum external quantum efficiency of blue-light materials-based devices is high, but the CIE coordinate performance is poor and the blue emission is difficult to achieve high efficiency and high color purity at the same time. Researchers continue to design new molecular structures in order to synthesize new high-efficiency blue light materials. It is found that different molecular structures have different effects on the performance of OLED devices. In the design and synthesis of blue-light materials, various light-emitting groups are often used to regulate the stability, singlet-third-line state of the target molecule energy level difference, excited state lifetime, aggregation state structure, electricity luminescent color and its OLED performance, etc. The introduction of different lightemitting groups into the blue light material greatly improves the performance of the material. This paper mainly reviews the research status of blue organic electroluminescent materials in the past five years from different molecular structures, further discusses the photoelectric properties of each compound and the properties of devices based on this material, and briefly analyzes the advantages of molecular design and device production. And finally discusses the improvement methods of blue OLED light-emitting materials in order to provide a reference for future research.

Theoretical Investigation of the Nonlinear Optical and Charge Transport Properties of N-(4-Methoxybenzylidene) Isonicotinohydrazone and Some of Its Derivatives: A DFT and TD-DFT Study

Here, we report the findings from a study on the charge transport and nonlinear optical (NLO) properties of N-(4-methoxybenzylidene) isonicotinohydrazone (INH) and some of its derivatives named INH1-INH15. The density functional theory (DFT) approach was used for ground state computations at the B3LYP-D/6-311G (d,p) level of theory, while the time-dependent density functional theory (TD-DFT) was carried out at the CAM-B3LYP/6-311G (d,p) level. The results show that the energy gaps of all the studied compounds range from 3.933 to 4.645 eV. INH3 and INH4 have the lowest electron and hole reorganization energies (i.e., 0.409 and 0.634 eV, respectively) and can thus be classified as moderate electron and hole-carrying materials for organic light-emitting diode (OLED) applications. TD-DFT computations demonstrate that an extension of the conjugation (in INH2 and INH3) increases the oscillator strength, improving the NLO response. According to the NLO data, INH2 and INH3 have higher static isotropic polarizabilities (38.509 and 37.986 × 10−24 esu, respectively) and second hyperpolarizabilities (54.440 and 57.598 × 10−36 esu, respectively), while INH4 and INH13 have higher first hyperpolarizability values (11.944 and 10.939 × 10−30 esu, respectively). The results reveal that INH derivatives with different groups are viable alternatives for OLED and NLO applications.

One-pot two-step cross-dehydrogenative-coupling polycondensation for synthesis of tetrafluorobenzene-based conjugated polymer

Polycondensation of 1,2,4,5-tetrafluorobenzene (1) with 3,3′-dihexyl-2,2′-bithiophene via Pd/Ag dual-catalyzed cross-dehydrogenative coupling was examined. In situ deprotonation-metalation of 1 with Ag salt proceeded smoothly at 25 °C, and the subsequent transmetalation with a Pd catalyst provided a cross-coupling product. The sequential one-pot two-step reaction method effectively suppressed undesired homo-coupling defects during polymerization. The obtained highly regioregular polymer had a packing structure and exhibited strong fluorescence in the film state, which was also evaluated for use as an emitting material in organic light-emitting diodes.

Recent Progress of Blue-light Emitting Materials for Organic Light-emitting Diodes

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Recent advances in metal-free phosphorescent materials for organic light-emitting diodes

This review discusses organic room temperature phosphorescent materials for the emitting layer of OLEDs. The existing circumstances, potential and challenges of metal-free phosphorescent OLEDs are summarized.

Benzoate-based thermally activated delayed fluorescence materials.

Compounds PTZ-MBZ (methyl 3-(10H-phenothiazin-10-yl)benzoate) and DMAC-MBZ (methyl 3-(9,9-dimethylacridin-10(9H)-yl)benzoate) were conveniently synthesized, and they exhibited TADF properties with lifetimes of 0.80 and 2.17 μs, respectively. The spatially separated highest occupied molecular orbital and lowest unoccupied molecular orbital resulted in a very small singlet-triplet energy gap of 0.0152 eV and 0.0640 eV, respectively. Thermally activated delayed fluorescence materials with short lifetime could be used as promising luminescent materials for organic light-emitting diodes.

Doublet-emissive materials for organic light-emitting diodes: exciton formation and emission processes.

Doublet-emission is mainly discovered in stable radicals, lanthanide-metal complexes with an f1 electron configuration and transition-metal complexes with a low-spin d5 electron configuration, and has a distinct radiation mechanism from closed-shell luminescent molecules and thus technology opportunities. There exists an unpaired electron in the frontier molecular orbitals which enables efficient nanosecond-scale luminescence in these materials due to the spin-allowed transitions between doublet-spin states. In this review, we summarize recent advances in these materials and their application in organic light emitting diodes (OLEDs). The photoluminescence and electroluminescence mechanisms of different doublet-emissive molecular systems are discussed, in addition to the photophysical phenomena arising from doublet states. We also outline the current challenges faced by each molecular system, and the potential outlook on the future research trends in this field.

A Review on the Milestones of Blue Light-Emitting Materials in India

Since 1987 in the field of optoelectronics, organic light-emitting diodes (OLEDs) have secured their position because of their extreme use in panels of lighting applications such as TV and smartphone displays. At present, OLEDs are at top-notch position in the lighting market for their promising features. The field of OLEDs is rapidly growing day by day in academia and industry due to the success of OLEDs in the form of excellent efficiency, feasible methods, excellent lifetime, color purity, and superb device architecture. As a result, OLEDs are new profitable leading devices of the 21st century. However, the OLED industry has evolved in optoelectronics in the last 30 years and is advancing rapidly just because of the development in OLED materials (fluorescent, phosphorescent, thermally activated delayed fluorescent, and blue light-emitting materials). Blue light-emitting materials have achieved incredible popularity nationally and internationally. At the international level, USA, Japan, Korea, and Germany are at the top of the list in the production of OLEDs. India has also seen rapid progress in OLED development in the last 12 years and details of research in blue OLEDs by key players of India are involved in this report.1 Introduction1.1 OLED Construction1.2 Working of OLED2 OLED Development2.1 Historical Background of OLED2.1.1 International Status2.1.2 National Status3 Progress of Blue Emitters in India4 Present Scenario of Blue OLEDs5 Conclusions and Outlook