OLED Applications Research Articles

Stereoisomeric Homo‐ and Hetero‐Binuclear Iridium(III) Complexes with 3‐Oxidopicolinate Bridging Ligand and Their Application in OLEDs

Stereoisomeric Homo‐ and Hetero‐Binuclear Iridium(III) Complexes with 3‐Oxidopicolinate Bridging Ligand and Their Application in OLEDs

64‐1: Protrusion‐free LTPS Using the CMP Process and Its OLED Application

Low Temperature Polysilicon (LTPS) is the most popular channel material used in the thin film transistor and it has the protrusions due to the volume expansion of Si and the presence of its native oxide. The protrusions cause the negative effects such as interface trap, hysteresis and so on. In that sense, the heights need to be lowered or eventually flattened.In this study, we used the chemical mechanical planarization (CMP) technique to reduce the surface roughness of LTPS to the 1nm and even lower. One of the most difficult problmes in the large sized CMP process is the removal of slurry residues after CMP and we successfully overcome the issue by using the hydrophilic angent.Finally, we are succesfully able to prepare AMOLED panel with reduced gate insulator thickness by using the CMP process and it shows less hysteresis results.

High-performance solution-processed blue OLEDs based on “hot exciton” materials

“Hot exciton” luminescent materials are being researched as potential candidates for application in OLEDs due to their ability to undergo high-level reverse intersystem crossing (hRISC). While evaporated OLEDs using “hot exciton” luminescent materials have shown impressive efficiency, there is a dearth of information on solution-processed “hot exciton” OLEDs, specifically in the realm of deep blue OLEDs. In this work, we developed novel solution-processed blue OLEDs by employing “hot exciton” material, 2-(4-(10-(3-(9H-carbazol-9-yl)phenyl) anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro [9,10-d] imidazole (PAC), as the luminescent material and host material (the classical fluorescent material BD as a guest). An improvement to the quantum efficiency of the luminescent layer was achieved by incorporating a hole transport layer (PVK) between the hole injecting layer (PEDOT:PSS:PFI) and the luminescent layer. Finally, the resultant non-doped blue OLED achieved EQEmax and EUEmax of 4.5 % and 56.3 %, respectively, while the doped blue OLED showed EQEmax and EUEmax of 5.3 % and 54.8 %, respectively. The efficiency roll-off at a brightness of 1000 cd m−2 for the doped device was 11.5 %, whereas for the non-doped device was only 4.2 %. Analysis of the magneto-electroluminescence and transient EL decay suggests that the high EUE in the solution-processed devices primarily derives from the “hot exciton” process. This study confirms that “hot exciton” materials can serve as a key factor in achieving efficient solution-processed blue OLEDs through a well-designed device architecture.

Color-Tunable Afterglows with Red Colors from Difluoroboron Derivatives and Their Applications in OLEDs

Color-Tunable Afterglows with Red Colors from Difluoroboron Derivatives and Their Applications in OLEDs

Syntheses of multi-resonance frameworks towards narrowband organic light-emitting diodes.

Multi-resonance (MR)-type organic emitters are highly attractive in the field of organic light-emitting diodes because of their narrowband emission and thermally activated delayed fluorescence (TADF) properties. Compared with conventional TADF emitters, MR-featured emitters have more complex chemical structures and building logics. The core structures of MR emitters are MR frameworks, i.e., polyaromatic frameworks, precisely embedded with electron-donating and electron-withdrawing atoms/groups. Generally, electron-donating units can be easily introduced through the dedicated design of the precursors/intermediates, while integrating electron-withdrawing units is the key point and bottleneck for the synthesis of the MR framework. In this review, we briefly summarize the synthetic strategies of MR frameworks, focusing on the means of introducing various electron-withdrawing atoms/groups, which will aid the further exploration of MR-TADF emitters and their applications in OLEDs.

Computational design of boron-free triangular molecules with inverted singlet-triplet energy gap.

A novel, computationally designed, class of triangular-shape organic molecules with an inverted singlet-triplet (IST) energy gap is investigated with ab initio electronic structure methods. The considered molecular systems are cyclic oligomers and their common feature is electronic conjugation along the molecular rim. Vertical excitation energies from the electronic ground state to the lowest singlet and triplet excited states were computed, as well as vertical emission energies from these states to the ground state. The results underscore the significance of optimizing excited-state geometries to accurately describe the optoelectronic properties of IST molecules, in particular with respect to their application in OLEDs.

WS2 incorporated PANI-rGO nanocomposites tailored for inflated thermal, optical and electrical properties used as ETL for OLEDs

This work reports on the synthesis of polymeric PANI-rGO-WS2 (PGW) nanocomposites were synthesized with varying concentrations of WS2 via chemical oxidative polymerization. FESEM images elucidated that the surface morphology of PGW3 carried a structure comprising of PANI nanofibers embedded on the top of the overlapped rGO and WS2 nanosheets. XRD and FTIR spectroscopy showed the molecular interaction among the constituents and affirmed the formation of PGW nanocomposite. UV–Vis. spectroscopy revealed the absorption of light in visible region with the for PGW3 nanocomposite. Photoluminescence spectroscopy showed light absorption in blue color region with excellent color purity. TGA showed the optimized PGW3 nanocomposite has inflated thermal stability. Current density for PGW3 nanocomposite was examined using J-V characteristic curve and found 96% higher than polyaniline. The increased conductivity and high electron-hole recombination rate attributed to the formation of enhanced charge transport network in PGW3 nanocomposite. These results evinced the optimized PGW3 nanocomposite as an ETL for OLED applications.

2,3-Diphenylquinoxalin-6-yl based dyes: Synthesis and studies of photophysical, aggregation-induced emission enhancement and electrochemical properties

Herein, 2,3-diphenylquinoxalin-6-yl based derivatives were synthesized, characterized by various spectroscopic method and studied for photophysical, aggregation-induced emission enhancement (AIEE) and electrochemical properties. Existence of intramolecular charge transfer transition (ICT) at 372−379 nm in absorption spectra of molecules confer weak push-pull interaction within it. Broad range of emission from blue to orange with λem = 412−592 nm in dyes arose from different energy state emission viz., locally excited (LE), Twisted intramolecular charge transfer (TICT) and excimer emission. Switching in emission state of molecules from excimer to TICT emission were observed by changing the polarity of solvent due to stabilization of TICT in polar media. Dyes have good solid state emission and AIEE activity desirable for its solid/aggregate state application in OLED's. Good Stokes shift (2183−9703 cm−1), thermal stability and comparable HOMO (−5.65 to −6.02 eV) energy levels with reported hole transporting materials confer desirable role of molecules in various optoelectronic applications. Further, DFT calculation of dyes, designate comparable experimental and theoretical photophysical and electrochemical properties with respect to structure of dyes.

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Solution-processable red phosphorescent OLEDs based on Ir(dmpq)2(acac) doped in small molecules as emitting layer

Stable red phosphorescent materials are of high importance in OLED applications and a promising scenario relies on the organometallic emitters, such as phosphorescent dopants, constituting an effective emissive layer of OLEDs, which is usually embedded in an appropriate host matrix. Iridium (III) complexes are the most widely used dopants in Phosphorescent OLEDs because of their high internal quantum efficiency. In this work, Bis(2-(3,5-dimethylphenyl)quinoline-C,N)(acetylacetonato)iridium(III), (Ir(dmpq)2(acac)), was doped in four different small molecule host materials, such as 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP), 1,3-Bis(N-carbazolyl)benzene (mCP), 1,1-Bis[(di-4-tolylamino) phenyl]cyclohexane (TΑPC) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA), with doping concentrations of 4%, 6%, and 8%. Τhe net host materials and the Ir(dmpq)2(acac) were first characterized in respect of their optical and photophysical properties. Following, the doped thin films were implemented as the active layers in OLED devices, formed via the solution deposition method. It was found that OLED devices fabricated for all studied cases emit red light, a characteristic of Ir(dmpq)2(acac), with a maximum wavelength of approximately 620 nm. By exploring the different combinations between the host and the dopant, as well as the different doping concentrations, we aim to provide insights into the selection of the best performing host materials for PhOLEDs.

Twisted-terphenyl conjugated carbazole derivatives with aggregation-induced emission enhancement for deep-blue fluorescence in the solid state

Scarcity of highly efficient, stable and pure deep-blue chromophores is a key impact on application of OLEDs. In this study, four bipolar molecules were designed and synthesized using carbazolyl as donor and oxadiazole/triazole as acceptors via a twisted terphenyl bridge substituted with trimethylsilyl group. These materials show high thermal/morphological stabilities and deep-blue emission benefited by the twisted structures with high fluorescent efficiency in the solid states. The dual emission properties are well tuned via different acceptors and conjugation sites with aggregation-induced emission enhancement. Electroluminescent device using p-CSiTO as a dopant was fabricated and this device emits deep-blue light at 437 nm with Commission Internationale de l’Eclairage (CIEx',y) of (0.159, 0.110) and good colour purity with full width half maximum (FWHM) of 66 nm.

Mechano-Stimulus and Environment-Dependent Circularly Polarized TADF in Chiral Copper(I) Complexes and Their Application in OLEDs

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exciton decay are highly attractive for electroluminescent devices (OLEDs) or next-generation photonic applications, such as spintronics, quantum computing, cryptography, or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure {Cu(CbzR)[(S/R)-BINAP]} [R = H (1), 3,6-tBu (2)] are efficient thermally activated delayed fluorescence (TADF) emitters with high radiative rate constants of kTADF up to 3.1 × 105 s-1 from 1/3LLCT states according to our temperature-dependent time-resolved luminescence studies. The efficiency of the TADF process and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupted by grinding of the crystalline materials. The origin of this pronounced mechano-stimulus photophysical behavior is a thermal equilibrium between the 1/3LLCT states and a 3LC state of the BINAP ligand, which depends on the relative energetic order of the excited states and is prone to inter-ligand C-H···π interactions. The copper(I) complexes are also efficient CPL emitters displaying exceptional dissymmetry values glum of up to ±0.6 × 10-2 in THF solution and ±2.1 × 10-2 in the solid state. Importantly for application in electroluminescence devices, the C-H···π interactions can also be disrupted by employing sterically bulky matrices. Accordingly, we have investigated various matrix materials for successful implementation of the chiral copper(I) TADF emitters in proof-of-concept CP-OLEDs.

De Novo Synthesis of α-Oligo(arylfuran)s and Its Application in OLED as Hole-Transporting Material.

Tuning the photophysical properties of π-conjugated oligomers by functionalization of skeleton, to achieve an optically and electronically advantageous building block for organic semiconductor materials is a vital yet challenging task. In this work, a series of structurally well-defined polyaryl-functionalized α-oligofurans, in which aryl groups are introduced precisely into each of the furan units, are rapidly and efficiently synthesized by de novo metal-free synthesis of α-bi(arylfuran) monomers for the first time. This new synthetic strategy nicely circumvents the cumbersome substituent introduction process in the later stage by the preinstallation of the desired aryl groups in the starting material. The characterization of α-oligo(arylfuran)s demonstrates that photoelectric properties of coplanar α-oligo(arylfuran)s can be tuned through varying aryl groups with different electrical properties. These novel α-oligo(arylfuran)s have good hole transport capacity and can function as hole-transporting layers in organic light-emitting diodes, which is indicative of significant breakthrough in the application of α-oligofurans materials in OLEDs. And our findings offer an avenue for the ingenious use of α-oligo(arylfuran)s as p-type organic semiconductors for OLEDs.

Emerging hyperfluorescent emitters for solid-state lighting

This paper highlights the molecular design principles of hyperfluorescent emitters, their fundamental photophysics, and the advances of their applications in OLEDs.

Synthesis of fluorescent aromatic species via ring transformation of α-pyranones and their application in OLEDs, chemosensors, and cell imaging

Ring transformation of pyran-2-ones with ketones provides fluorescent aromatic compounds bearing push–pull electronic system. Fluorescent aromatic species exhibited various optical applications such as OLED materials, chemosensors and cell imaging.

Effect of thermal annealing on an emissive layer containing a blend of a small molecule and polymer as host for application in OLEDs.

In order to improve the performance of OLEDs, a host-guest mixture was used as an emissive layer. To have better host properties, a mixture of different materials with suitable properties can also be used as a host. In this study, we used a mixture of a polymer and a small molecule as the host and studied the effect of thermal annealing on the emissive layer properties by using Ir(ppy)3 as the emitter. UV-visible absorption, steady-state and time-resolved photoluminescence, scanning electron microscopy, atomic force microscopy, and optical microscopic studies were performed to study the film properties. Devices were fabricated and their current-voltage and luminance-voltage characteristics were studied. Charge-carrier mobility in the devices was studied by dark CELIV and transient electroluminescence methods. We show that, below the glass transition temperature of the polymer, the small molecules formed aggregates due to thermal annealing, which was beneficial for the device performance in the lower-temperature range, mainly due to the improved electron mobility. However, this aggregate formation was detrimental in the higher-temperature range, as it led to inefficient energy transfer due to the increased pure phase formation. At temperatures above the glass transition temperature of the polymer, the small molecules were seen to be distributed more uniformly into the polymer matrix. However, as a result of the degradation of the polymer property due to degradation of the primary chain of the phenyl ring of the polymer, this uniform distribution was not of any use and the device performance deteriorated.

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 narrowband red-emitting asymmetric iridium(iii) complex featuring B- and N-embedded π-conjugation units: structure, photophysics and OLED application

A new B- and N-embedded asymmetric iridium complex IrBNacac has been prepared. It exhibited an effective red emission in solid films with a very narrow full width at half maximum (FWHM = 50 nm) and finally achieved a good OLED device performance.

A theoretical study on the effects of intramolecular and intermolecular interactions on excited state properties of two NIR-TADF combined with AIE molecules

TADF emitters with both high exciton utilization and luminescence efficiency show promising applications in OLEDs, especially it with AIE characteristics. However, theoretical studies on emitters with amorphous structure with the consideration of the solid-state effect are limited. In this work, a multi-scale simulation is performed to study the reported compounds TBSMCN and O-TBSMCN, which both possess distinct features of NIR-AIE-TADF. The packing modes of the molecules in film and crystal are obtained by molecular dynamics simulations (MD) and experiments, and then the photo physical properties are studied by using the QM/MM method. The results show that geometrical changes between the ground state (S0) and the first singlet excited state (S1) are restricted in aggregate state. Thus, the non-radiative energy consumption process is hindered, this indicates the AIE mechanisms. Second, the molecules exhibit a smaller △ES1-T1 and a more significant SOC in aggregate state, which is more favorable for the RISC. Finally, by analyzing the NTO of the dimers in the film, the O-TBSMCN molecule undergoes intermolecular charge transfer, which may be one of the reasons for the significant reduction in its film efficiency. Our calculations could help one to better understand the NIR-AIE-TADF mechanism from a theoretical perspective.

Recent Progress in Blue Thermally Activated Delayed Fluorescence Emitters and Their Applications in OLEDs: Beyond Pure Organic Molecules with Twist D-π-A Structures.

Thermally activated delayed fluorescence (TADF) materials, which can harvest all excitons and emit light without the use of noble metals, are an appealing class of functional materials emerging as next-generation organic electroluminescent materials. Triplet excitons can be upconverted to the singlet state with the aid of ambient thermal energy under the reverse inter-system crossing owing to the small singlet-triplet splitting energy (ΔEST). This results from a specific molecular design consisting of minimal overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, due to the spatial separation of the electron-donating and electron-releasing part. When a well-designed device structure is applied, high-performance blue-emitting TADF organic light-emitting diodes can be realized with an appropriate molecular design. Unlike the previous literature that has reviewed general blue-emitting TADF materials, in this paper, we focus on materials other than pure organic molecules with twist D-π-A structures, including multi-resonance TADF, through-space charge transfer TADF, and metal-TADF materials. Cutting-edge molecules with extremely small and even negative ΔEST values are also introduced as candidates for next-generation TADF materials. In addition, OLED structures used to exploit the merits of the abovementioned TADF emitters are also described in this review.