Materials For Organic Light-emitting Diodes Research Articles

Novel deep-blue hot exciton material for high-efficiency nondoped organic light-emitting diodes

A deep-blue OLED based on a novel hot exciton material exhibited an EQEmaxof 7.51% with CIE coordinates of (0.154, 0.075).

Are pure hydrocarbons the future of host materials for blue phosphorescent organic light-emitting diodes?

This Chemistry Frontiers deals with a new generation of host materials for phosphorescent OLEDs only constituted with carbon and hydrogen atoms, which can be beneficial for the future development of the OLED industry.

Understanding excited state properties of host materials in OLEDs: simulation of absorption spectrum of amorphous 4,4-bis(carbazol-9-yl)-2,2-biphenyl (CBP).

4,4-Bis(carbazol-9-yl)-2,2-biphenyl (CBP) is widely used as a host material in phosphorescent organic light-emitting diodes (PhOLEDs). In the present study, we simulate the absorption spectra of CBP in gas and condensed phases, respectively, using the efficient time-dependent long-range corrected tight-binding density functional theory (TD-LC-DFTB). The accuracy of the condensed-phase absorption spectra computed using the structures obtained from classical molecular dynamics (MD) and quantum mechanical/molecular mechanical (QM/MM) simulations is examined by comparison with the experimental absorption spectrum. It is found that the TD-LC-DFTB gas-phase spectrum is in good agreement with the GW-BSE spectrum, indicating TD-LC-DFTB is an accurate and robust method in calculating the excitation energies of CBP. For the condensed-phase spectrum, we find that the electrostatic embedding has a minor influence on the excitation energy. Computing accurate absorption spectra is a particular challenge since static and dynamic disorders have to be taken into account. The static disorder results from the molecular packing in the material, which leads to molecule deformations. Since these structural changes sensitively impact the excitation energies of the individual molecules, a proper representation of this static disorder indicates that a reasonable structural model of the material has been generated. The good agreement between computed and experimental absorption spectra is therefore an indicator for the structural model developed. Concerning dynamic disorder, we find that molecular changes occur on long timescales in the ns-regime, which requires the use of fast computation approaches to reach convergence. The structural models derived in this work are the basis for future studies of charge and exciton transfer in CBP and related materials, also concerning the degradation mechanisms of CBP-based PhOLEDs.

QM/MM Studies on Thermally Activated Delayed Fluorescence of a Dicopper Complex in the Solid State

Dicopper complexes with thermally activated delayed fluorescence (TADF) phenomena are important in enriching the arsenal of organic light-emitting diodes materials. However, the TADF mechanism is still elusive, especially in the solid state. Herein, we chose a TADF dicopper complex and investigated its geometric and electronic structures and absorption and emission spectra using DFT, TD-DFT, and QM/MM methods. On the basis of these results, we further estimate the fluorescence emission rate from the S1 state, phosphorescence emission rate from the T1 state, and forward and reverse intersystem crossing (ISC and rISC) rates between S1 and T1. The present work shows that both the S1 and the T1 states have mixed metal-to-ligand and interligand charge-transfer character. Good spatial separation between the HOMO and the LUMO makes the S1–T1 energy gap small, ca. 2.8 kcal/mol. This small energy gap leads to efficient ISC and rISC processes, whose rates are much larger than the fluorescence and phosphorescence emission rates at 300 K, therefore enabling TADF. In contrast, at 77 K, the rISC process is blocked because its rate is much smaller than the phosphorescence emission. Thus, TADF disappears at 77 K. Further analysis shows that high-frequency deformation and low-frequency torsional vibrational modes make a large contribution to the Huang–Rhys factors, but Duschinsky rotation effects are essentially negligible for both the ISC and the rISC processes.

Effect of dendrimer surface groups on the properties of phosphorescent emissive films

Fac -tris[2-phenylpyridinato- C 2 , N ]iridium(III) [Ir(ppy) 3 ] is a well-known phosphorescent material for organic light-emitting diodes (OLEDs) and while uniform thin films can be formed using evaporation under high vacuum, it is not sufficiently soluble to enable it to be processed from solution. Ir(ppy) 3 -cored dendrimers with solubilizing surface groups can be processed from solution to form good quality neat or guest:host blends, even when the host has relatively poor solubility. In this manuscript, we report the effect of adding different solubilizing surface groups, namely 2-ethylhexyloxy, n -propyl or t -butyl to first generation dendrons attached to the Ir(ppy) 3 core on the optoelectronic properties of neat and blend films with tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The different dendrons were found to have minimal effect on the photoluminescence spectra and efficiency of energy transfer from the host to the guest in the blend films. The hole mobility of neat films of the solution-processed Ir(ppy) 3 -cored dendrimer films was around 10 −6 cm 2 V −1 s −1 , which was an order of magnitude less than Ir(ppy) 3 films formed by evaporation. Blending the dendrimers with TCTA at a concentration of 11 mol% (the concentration of the highest film photoluminescence quantum yield) led to an order of magnitude decrease in the hole mobility compared to the neat films. However, despite the relatively low mobilities, simple two-layer OLEDs composed of a blend light-emitting layer and an electron transporting layer were found to reach a maximum EQE of 11.1% at a luminance of 1000 cd m −2 for a film with a PLQY of 60%. • Ir(ppy)3-cored dendrimers with solubilising surface groups are solution processed. • Good quality guest:host blend films formed with poorly soluble TCTA host. • Dendron type was found to have minimal effect on the host to guest energy transfer. • Charge mobility in dendrimer:TCTA blends less than in neat film. • Simple two-layer OLEDs had maximum EQEs of 11.1% at a luminance of 1000 cd m-2.

Spiro Compounds for Organic Light-Emitting Diodes

ConspectusThe development of cutting-edge optoelectronic devices is rapidly changing science and technology. The organic light-emitting diodes (OLEDs), as one of the most applicable techniques of organic electronics, are experiencing the process of materials’ upgrade. Among all the organic units to construct OLED materials, the spiro structure and its derived compounds have been widely studied due to the good thermal stability and unique orthogonally rigid structure. It had shown great potential in first generation fluorescent OLEDs (FOLEDs) and second generation phosphorescent OLEDs (PhOLEDs). Recently, it has achieved outstanding performance in third generation thermally activated delayed fluorescence (TADF) OLEDs, which is attributed to its orthogonal structure caused by the spiro atom; thus the spiro compound can innately separate the hole and electron which is beneficial for minimizing the singlet–triplet splitting energy (ΔEST). The most classic aromatic spiro unit is 9,9′-spriobifluorene (SBF), which was first reported in 1930. The study of aromatic spiro compounds is closely related to the development of organic semiconductors; now, the structure–property relationship of spiro compounds is getting clearer and attracting wide attention from the material community.In this Account, we briefly summarize the efforts on the functionalization and application of spiro compounds in OLEDs. The functionalization of spiro compounds in our group mainly focuses on the development of electron-rich spiro backbone and the adjustment of different substitution positions. In this regard, we have systematically studied the electron-rich spiro backbones which could greatly facilitate the carrier injection/transport and the new derivation positions (C1, C3, and C4 at fluorene moiety) can markedly affect the energy levels of triplet excitons, which is crucial for host materials in OLEDs, especially for the blue ones. The two main methods, i.e., backbone modification and position engineering, can cooperate to construct a rigid scaffold to realize spatial donor/acceptor intramolecular interaction, which confines the donor/acceptor distance. Afterward, the application of spiro compounds in OLEDs is elucidated from FOLEDs to PhOLEDs, then to TADF OLEDs. The spiro[acridine-9,9′-fluorene] (SAF) unit is our first example to enhance the performance of blue and violet fluorescent emitters, which have reached the theoretical upper limit of FOLEDs. For PhOLEDs, the spiro-type compounds are mainly used as host materials. The substitution positions of the SBF unit are thus thoroughly investigated to afford host materials with high triplet energy. And the SAF unit is modified to form ambipolar host materials to balance hole/electron injection for better white PhOLEDs. During this research, we understood the importance of the orthogonal arrangement in the spiro structure for ambipolar material design. Finally, TADF materials based on the spiro compounds are also presented. The spiro materials used in TADF are usually performed as an individual donor or acceptor before our contribution. Our tendency is to utilize the through-space charge transfer, i.e., charge transfer occurring between the spatial-close donor and acceptor, to construct efficient TADF materials, which paves a new avenue for the future study of spiro compounds.

Data-driven machine learning models for quick prediction of thermal stability properties of OLED materials

Organic light-emitting diode (OLED) materials have exhibited a wide range of applications. However, the further development and commercialization of OLEDs requires higher quality OLED materials, including materials with a high thermal stability. Thermal stability is associated with the glass transition temperature (Tg) and decomposition temperature (Td), but experimental determinations of these two important properties generally involve a time-consuming and laborious process. Thus, the development of a quick and accurate prediction tool is highly desirable. Motivated by the challenge, we explored machine learning (ML) by constructing a new dataset with more than 1,000 samples collected from a wide range of literature, through which ensemble learning models were explored. Models trained with the LightGBM algorithm exhibited the best prediction performance, where the values of mean absolute error, root mean squared error, and R2 were 17.15 K, 24.63 K, and 0.77 for Tg prediction and 24.91 K, 33.88 K, and 0.78 for Td prediction. The prediction performance and the generalization of the ML models were further tested by two applications, which also exhibited satisfactory results. Experimental validation further demonstrated the reliability and the practical potential of the ML-based models. In order to extend the practical application of the ML-based models, an online prediction platform was constructed. This platform includes the optimal prediction models and all the thermal stability data under study, and it is freely available at http://www.oledtppxmpugroup.com. We expect that this platform will become a useful tool for experimental investigation of Tg and Td, accelerating the design of OLED materials with desired properties.

Synthesis, crystal structure and photophysical properties of bis-[2,6-di-fluoro-3-(pyridin-2-yl)pyridine-κN](tri-fluoro-methane-sulfonato-κO)silver(I).

In the title compound, [Ag(CF3SO3)(C10H6F2N2)2], the AgI centre adopts a highly distorted trigonal-planar coordination environment resulting from its coordination by one O atom of the tri-fluoro-methane-sulfonate anion and the pyridine N atoms of two crystallographically independent 2',6'-di-fluoro-2,3'-bi-pyridine ligands, which display very similar conformations to one another. Pairwise Ag⋯O-SO2CF3 - [Ag⋯O = 2.8314 (14) Å] inter-actions and inter-molecular C-H⋯O inter-actions between inversion-related units lead to the formation of an eight-membered cyclic dimer in which the silver atoms are separated by 6.2152 (3) Å. In the crystal, the dimers are linked through C-H⋯O hydrogen bonds, halogen⋯π and weak π-π stacking inter-actions, resulting in the formation of a three-dimensional supra-molecular network. The title compound exhibits a strong and broad emission band from 400 nm to 550 nm in solution and its photoluminescence quantum efficiency is estimated to be ca 0.2, indicating that the title compound could have applications as an emitting material in organic light-emitting diodes (OLEDs).

Palladium-Catalyzed C-P Bond-Forming Reactions of Aryl Nonaflates Accelerated by Iodide.

An iodide-accelerated, palladium-catalyzed C–P bond-forming reaction of aryl nonaflates is described. The protocol was optimized for the synthesis of aryl phosphine oxides and was found to be tolerant of a wide range of aryl nonaflates. The general nature of this transformation was established with coupling to other P(O)H compounds for the synthesis of aryl phosphonates and an aryl phosphinate. The straightforward synthesis of stable, isolable aryl nonaflates, in combination with the rapid C–P bond-forming reaction allows facile preparation of aryl phosphorus target compounds from readily available phenol starting materials. The synthetic utility of this general strategy was demonstrated with the efficient preparation of an organic light-emitting diode (OLED) material and a phosphonophenylalanine mimic.

Theoretical insight on electronic structure and photophysical properties of a series of cyclometalated iridium(III) complexes bearing the substituted phenylpyrazole with different electron-donating or electron-accepting groups.

By using the density functional theory (DFT) and time-dependent density functional theory (TDDFT), the electronic structure and photophysical properties of a series of cyclometalated iridium(III) complexes bearing the substituted phenylpyrazole have been theoretically investigated. All studied iridium(III) complexes have the distorted octahedral geometry with cis-C,C, cis-O,O, and trans-N,N chelate disposition. The lowest lying singlet → singlet absorptions of all studied iridium(III) complexes are respectively located at 405nm, 387nm, 382nm, 370nm, and 387nm. The calculated emission wavelengths for all studied iridium(III) complexes are 654nm, 513nm, 506nm, 505nm and 499nm, respectively. The calculated emission wavelength for complex 4 at the CAM-B3LYP level is in good agreement with the experimental value. From the theoretical results, it can be seen that the electron-donating substituent groups have the important effect on the electronic structure and photophysical properties of all studied iridium(III) complexes. We hope that this study can provide valuable guidance for the design of new phosphorescent organic light-emitting diodes (OLEDs) materials.

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.

Thermally Activated Delayed Fluorescence Amorphous Molecular Materials for High-Performance Organic Light-Emitting Diodes.

Small-molecule thermally activated delayed fluorescence (TADF) materials have been extensively developed to actualize efficient organic LEDs (OLEDs). However, organic small molecules generally compromise thin film quality and stability due to the tendency of crystallization, aggregation, and phase separation, which hence degrade the efficiency and long-term stability of the OLEDs. Here, for the first time, we exploit the unique molecular configuration of the bimesitylene scaffold to design two highly efficient TADF amorphous molecular materials with excellent thermal and morphological stabilities. The twisted and rigid bimesitylene scaffold thwarts regular molecular packing and crystallization, thereby guaranteeing homogeneous and stable amorphous thin films. Meanwhile, the highly twisted geometry of the bimesitylene scaffold efficiently breaks the molecular conjugation and thus conserves the high energies of the lowest locally excited triplet states (3LE) above the lowest charge transfer states (1CT and 3CT), leading to small singlet-triplet energy splitting and fast reverse intersystem crossing. These TADF emitters exhibit high photoluminescence quantum yields of 0.90 and 0.69 and short TADF lifetimes of 4.94 and 1.44 μs in doped films, based on which the greenish-blue and greenish-yellow OLEDs achieve external quantum efficiencies of 23.2 and 16.2%, respectively, with small efficiency roll-off rates and perfect color stability.

Cyanocarbazole-substituted di(9H-carbazol-9-yl)-1,1'-biphenyl derivatives for efficient blue phosphorescent host materials for organic light emitting diodes

To develop the efficient blue phosphorescent host materials for Organic light-emitting Diodes (OLEDs), cyanocarbazole-substituted 3',5'-di(9H-carbazol-9-yl)-1,1'-biphenyl derivatives are designed and synthesized. Multilayer devices were fabricated in the following sequence; ITO (180 nm)/Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) (75 nm)/Blue phosphorescent host materials (15 nm)/Bis[2-(4,6-difluorophenyl)pyridinato-C 2,N](picolinato)iridium(III) (Firpic) (15 nm)/1,3,5-Tri(m-pyridine-3-ylphenyl)benzene (TmPyPB) (40 nm)/Liq (2 nm)/Al (100 nm). In particularly, blue phosphorescent device using 9-(3',5'-di(9H-carbazol-9-yl)-[1,1'-biphenyl]-4-yl)-9H-carbazole-3-carbonitrile as a host material exhibited efficient emission with a luminous efficiency, a power efficiency, an external quantum efficiency, and CIE coordinates of 11.6 cd/A, 4.10 lm/W, 5.62% at 20 mA/cm2, and (0.15, 0.33) at 8 V, respectively.

Effect of hybridized local and charge transfer molecules rotation in excited state on exciton utilization

The fluorescent molecules utilizing hybridized local and charge-transfer (HLCT) state as potential organic light-emitting diodes materials attract extensive attention due to their high exciton utilization. In this work, we have performed the density functional theory method on three HLCT-state molecules to investigate their excited-state potential energy surface (PES). The calculated results indicate the T1 and T2 energy gap is quite large, and the T2 is very close to S1 in the energy level. The large gap is beneficial for inhibiting the internal conversion between T1 and T2, and quite closed S1 and T2 energies are favor for activating the T2 → S1 reverse intersystem crossing path. However, considering the singlet excited-state PES by twisting the triphenylamine (TPA) or diphenylamine (PA) group, it can be found that the TPA or PA group almost has no influence on T1 and T2 energy levels. However, the plots of S1 PES display two kinds of results that the S1 emissive state is dominated by charge-transfer (CT) or HLCT state. The CT emission state formation would decrease the S1 energy level, enlarge the S1 and T2 gap, and impair the triplet exciton utilization. Therefore, understanding the relationship between the S1 PES and molecular structures is important for designing high-performance luminescent materials utilizing HLCT state.

Versatile azaryl-ketone-based blue AIEgens for efficient organic light-emitting diodes

We report two donor-acceptor type blue luminophores based on an azaryl-ketone derivative, phenyl(3-phenylimidazo[1,2-a]pyridin-2-yl)methanone (pBZ), as respectively emitter and host materials for organic light-emitting diodes (OLEDs). The two new compounds pBZ-TPA and pBZ-DPA having varying lengths of phenylene linkers are prepared through a facile synthetic method and systematically investigated with thermal, photophysical, electroluminescent, and theoretical studies. They exhibit obvious aggregation-induced emission (AIE) characters. The electron-deficiency pBZ and the electron-rich arylamine (triphenylamine or diphenylamine) respectively serve as electron acceptor and donor, rendering bipolar electronic properties. The two AIE luminophores (AIEgens) exhibit obvious charge transfer character evidenced by positive solvatochromic effect in their photoluminescence. It is also found that the (bi)phenylene π-linker attached in donor would tune the molecular conjugation and degree of charge transfer. The versatility of pBZ-TPA and pBZ-DPA is demonstrated in blue OLEDs as emitters and in red phosphorescent devices as hosts with decent performance and mild efficiency roll-offs.

Organic Light-Emitting Diodes Based on Luminescent Self-Assembled Materials of Copper(I)

Over the past decade, we have seen the rapid advancement in the technology of organic light-emitting diodes (OLEDs) toward practical applications. The high color contrast, wide viewing angles, good flexibility, and low power consumption of OLEDs have rendered them promising candidates for next-generation displays and solid-state lighting applications. Parallel to the optimization in device structures, of equal importance is the search for new classes of emitters. While conventional OLEDs mainly rely on triplet emitters based on precious metal complexes, such as those of platinum(II) and iridium(III), copper(I) self-assembled materials represent attractive alternatives with relatively low cost but comparable emission tunability. In this mini review, the recent progress in the development of copper(I) self-assembled materials, including copper(I) clusters and coordination polymers, as electroluminescent materials in OLEDs will be discussed.

A Novel Deep Blue LE-Dominated HLCT Excited State Design Strategy and Material for OLED

Deep blue luminescent materials play a crucial role in the organic light-emitting diodes (OLEDs). In this work, a novel deep blue molecule based on hybridized local and charge-transfer (HLCT) excited state was reported with the emission wavelength of 423 nm. The OLED based on this material achieved high maximum external quantum efficiency (EQE) of 4% with good color purity. The results revealed that the locally-excited (LE)-dominated HLCT excited state had obvious advantages in short wavelength and narrow spectrum emission. What is more, the experimental and theoretical combination was used to describe the excited state characteristic and to understand photophysical property.

Orange–red organic light emitting diodes with high efficiency and low efficiency roll-off: boosted by a fused acceptor composed of pyrazine and maleimide

The strategy to develop orange–red thermally activated delayed fluorescent (TADF) materials for high-performance organic light-emitting diodes (OLEDs) with both high maximum external quantum efficiencies (EQEsmax) and low efficiency roll-off, is ongoing pursued yet challenging. Herein, two efficient orange–red TADF emitters, T-DMAC-PPyM and P-DMAC-BPyM, with a new acceptor fused by pyrazine and maleimide, were designed and synthesized. Notably, the highly twisted conformation of P-DMAC-BPyM can inhibit the close π-π stacking, so as to reduce exciton quenching, while the “locked” biphenyl bridge can enhance the molecular rigidity, thus reducing the non-radiative decay associated with intramolecular rotation. Consequently, P-DMAC-BPyM exhibits outstanding electroluminescent performance with a remarkably high EQEmax of 26.0% and an impressively low efficiency roll-off (EQE100 is 21.7%; EQE1000 is 18.3%), which are among the best results for orange–red TADF OLEDs.

Deep-blue organic light-emitting diodes based on multi-tert-butyl modified naphthylene

Six tert-butyls modified naphthyl derivative (2,6-di-tert-butyl-1,5-bis-(3,5-di-tert-butyl-phenyl)-naphthalene (N(dtb)4) was synthesized and tested for host and guest materials in organic light-emitting diodes (OLEDs). Blue OLED using N(dtb)4 as a host and an emitter DSA-PH as a guest reaches an efficiency of 11.52 cd·A−1 (maximum external quantum efficiency 7.8 % and maximum power efficiency 10.33 lm·W−1). In addition, for the OLED with N(dtb)4 being a guest, we find that the concentrations of N(dtb)4 will also affect the performance of the OLED, among which ADN: 5 wt.% N(dtb)4 has higher efficiency, and the maximum current efficiency, external quantum efficiency (EQE) and power efficiency of the device reach 3.75 cd·A−1, 3.93 % and 3.36 lm·W−1, respectively. We also investigated the pure N(dtb)4 -based ultraviolet OLED (UV-OLED), and achieved the maximum EQE of the device is 1.12 %. These results indicate that N(dtb)4, a novel synthesized fluorescent organic material, has great potential applications in OLEDs and accordingly advancing lightings and displays.

Appraisal of Structural, Thermal, and Optical Properties of Novel Bluish-Violet Light-Emitting Cyclometallated Iridium (III) (Cl-H-DPQ)2Ir(acac) Complex for OLED Devices

Synthesis and evaluation of structural, thermal, and optical properties of a novel bluish-violet light-emitting cyclometallated Iridium (Ir3+) complex [(Cl-H-DPQ)2Ir(acac)] containing 4-(6-chloro-4-phenylquinolin-2-yl) phenol as main ligand and acetyl acetone (acac) as ancillary ligand by employing Friedlander reaction at 105 °C is reported. X-ray diffraction of the complex reveals its crystalline nature and scanning electron microscopy illustrates polygonal morphology within the nm range. FTIR spectra confirms the structure formation and molecular bonding of the synthesized complex. DTA/TGA curves reveal its thermal stability to about 252 °C demonstrating its potential at elevated temperatures. UV–vis optical absorption and photoluminescence spectra were carried out in solid state form and in various organic solvents ranging from basic to acidic media. This facilitated estimation of the energy band gap and Stokes shift in various organic solvents. When excited at 350 nm light, the complex emits prominent bluish-violet light peaking at 468 nm in the solid-state form. However, it shows a bathochromic shift in acidic media and a hypsochromic shift in basic media. CIE chromaticity coordinates (x, y) of (Cl-H-DPQ)2Ir(acac) complex in solid state form was found to be (0.2631, 0.2358), and in acetic acid and formic acid they were found to be (0.325, 0.159) and (0.402, 0.198), respectively, while in chloroform and dichloromethane they were found to be (0.1616, 0.043) and (0.1756, 0.062), respectively. The Stokes shift was found to be in the range of 6480–15,719 cm−1 in various acidic and basic solvents. The solvent sensitivity to chromophore was probed by a Lippert–Mataga plot. These results reflect the potential of the (Cl-H-DPQ)2Ir(acac) complex as a bluish-violet light-emitting material for organic light-emitting diodes, displays, and solid-state lighting.