OLED Devices Research Articles

Synthesis and characterization of a new semiconducting indacenodithiophene-carbazole derivative with interesting optical and electronic properties

A novel indacenedithiophene derivative 9,9'-((4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(4,1-phenylene))bis(9H-carbazole) (IDT-CB) by a Suzuki coupling reaction was synthesized, its chemical structure was confirmed by 1H-NMR and 13C-NMR, as well as FTIR. Opto-electronic properties were measured in solution and thin solid films through UV-Vis absorbance and photoluminescence (PL) in solution was also determined. UV-Vis absorption was observed at λmax=425 nm, and the optical band gap was 2.59 eV. The HOMO-LUMO frontier orbitals were determined by cyclic voltammetry (CV) with HOMO=−5.9 eV and LUMO=−3.89 eV, then, a band gap of 2.01 eV was estimated. Photoluminescence had a maximum emission at 556 nm. OLED devices were constructed from the compound IDT-CB as emitting layer (EL), with the simple structure glass/ITO/PEDOT:PSS(40 nm)/IDT-CB(52 nm)/FM. Previously, the chemical compound was solubilized in THF at 5 mg/mL and corresponding analyses in thin films and OLED fabrication were carried out. Solution processable yellow-greenish OLEDs were assembled by spin coating with CIE coordinates (0.38, 0.51). J-V plots, as well as luminance, were determined, a turn-on-voltage of 5.76 V and maximum luminance of 1000 cd/m2 were reached.

Theoretical and experimental investigation of exciplex-forming and electroluminescent properties of 4-ethyl-3,5-bis[40-(N, N-diphenylamine)biphenyl-4-yl]-4H-1,2,4-triazole

The recently synthesized 4-ethyl-3,5-bis [40-(N, N-diphenylamine)biphenyl-4-yl]-4H-1,2,4-triazole (TPA-TZ) is in this work thoroughly studied as an efficient luminophore with an account of density functional theory. The intense fluorescence is explained as an intramolecular charge-transfer transition from the diphenylamine (donor) moiety to the central part of the dye with a characteristic electron redistribution in the triazole ring (acceptor). The photoluminescence (PL) and phosphorescence (Ph) spectra of TPA-TZ in the film and in THF solution at 77 K were studied together with the well-known 2,4,6-tris [3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) dye. The latter compound was used as an electron-withdrawing component of the investigated TPA-TZ: PO-T2T exciplex, which is characterized by a relatively high energy level of the excited lowest triplet state. The films of a solid-state mixture of TPA-TZ and PO-T2T showed exciplex-type emission being slightly red-shifted compared to the PL of the individual donor and acceptor compounds. Two OLED devices with emitting layers of TPA-TZ and of the exciplex-forming molecular mixture of TPA-TZ and PO-T2T were prepared. The exciplex–based OLED demonstrated 4 103 cd/m2 brightness, a current efficiency of 22 cd/A, a power efficiency of 6.77 lm/W, and an external quantum efficiency (EQE) of 7%. This confirms the possibility of triplet harvesting in the molecular mixture of TPA-TZ and PO-T2T, since the theoretical EQE limit for OLED based on the prompt fluorescence is 5%.

Mononuclear Cu(I) halide complexes containing asymmetric diphosphine and diphenyl pyridyl monophosphine: Structures and photophysical properties

Thermally activated delayed fluorescent (TADF) Cu(I) halide complexes have attracted attention due to their highly emission and short lifetimes (several μs scale). Here, three mononuclear four-coordinate Cu(I) halide complexes containing asymmetric diphosphine and pyridyl phosphine, [CuX(L1)(L2)] (L1 = 2,3-bis(diphenylphoshino)thiophene, L2 = 2-(diphenylphosphino)pyridine, X = I (1), Br(2), Cl (3)), were synthesized and their structures and photophysical properties were characterized. At room temperature, complexes 1–3 in powder exhibit intense green to yellow green delayed fluorescence (λem = 497–523 nm, τ = 1.5–4.3 μs, Φ = 0.19–0.28). The emissions of the complexes 1–3 are originated from (metal + halide)-to-ligand and intra-ligand charge transfer. More contributions from MLCT and XLCT can result in the shorter decay lifetimes (1.6 ∼ 4.3 μs), which will prevent a roll-off of efficiency with increasing current density in OLED devices.

Tuning of external magnetic field-driven circularly polarized electroluminescence in OLED devices with a single achiral Pt(II) complex

Tuning of external magnetic field-driven circularly polarized electroluminescence in OLED devices with a single achiral Pt(II) complex

Carbazole‐Decorated Organoboron Emitters with Low‐Lying HOMO Levels for Solution‐Processed Narrowband Blue Hyperfluorescence OLED Devices

AbstractThe hyperfluorescence has drawn great attention in achieving efficient narrowband emitting devices based on multiple resonance thermally activated delayed fluorescence (MR‐TADF) emitters. However, achieving efficient solution‐processed pure blue hyperfluorescence devices is still a challenge, due to the unbalanced charge transport and serious exciton quenching caused by that the holes are easily trapped on the high‐lying HOMO (the highest occupied molecular orbital) level of traditional diphenylamine‐decorated emitters. Here, we developed two narrowband blue organoboron emitters with low‐lying HOMO levels by decorating the MR‐TADF core with weakly electron‐donating carbazoles, which could suppress the hole trapping effect by reducing the hole traps between host and MR‐TADF emitter from deep (0.40 eV) to shallow (0.14/0.20 eV) ones for facilitating hole transport and exciton formation, as well as avoiding exciton quenching. And the large dihedral angle between the carbazole and MR‐TADF core makes the carbazole act as a steric hindrance to inhibit molecular aggregation. Accordingly, the optimized solution‐processed pure blue hyperfluorescence devices simultaneously realize record external quantum efficiency of 29.2 %, narrowband emission with a full‐width at half‐maximum of 16.6 nm, and pure blue color with CIE coordinates of (0.139, 0.189), which is the best result for the solution‐processed organic light‐emitting diodes based on MR‐TADF emitters.

Unraveling the injection barrier between electrode and organic layer in OLED device: A theoretical prediction model

This study developed a new prediction model for hole injection barriers. Based on the electronegativity equalized model (EEM), the vacuum level shift in the adjacent layers was further considered in this model. In addition, the variation in ΦITO, which is dependent on the doping concentration of Sn, was considered as one of the determinants for predicting the hole injection barrier (HIB) between ITO and the organic layer. Further, the effect of p-dopants on the HTL energy level was also introduced in this model. The predicted HIBs of various anode/organic junctions were quantitatively and qualitatively consistent with UPS experiments. We believe that the developed EEM model can aid in determining the appropriate HTL/HIL organic materials without tremendous effort and time costs.

Design and Synthesis of Asymmetric Au(III) Complexes Exhibiting Bright Anisotropic Emission for High‐Performance Organic Light‐Emitting Diodes

AbstractOrganic light‐emitting diodes (OLEDs) are emerging as one of the most promising candidates for next‐generation optoelectronics. However, most commercial OLED displays are fabricated using organometallic phosphors containing rare transition metals, such as Ir(III) and Pt(II). Recently, the development of efficient phosphorescent emitters based on abundant Au(III) is drawing considerable attention. Here, rational molecular design of a series of Au(III) organometallic complexes consisting of asymmetric C^C^N ligands and carbazole moieties functionalized by phenyl or mesityl groups is presented. The synthesized complexes exhibit shortened radiative lifetimes, and reach improved photoluminescence quantum yield, ηPL, of greater than 93% in thin films. Moreover, the asymmetric molecular design induces anisotropic emission with a high ratio of horizontally oriented transition dipole moment of up to 82% in host‐guest films. Accordingly, high‐performance OLED devices with record‐high external quantum efficiencies, ηext, and current efficiencies, ηCE, of up to 27% and 89 cd A−1, respectively, are demonstrated. It is believed that the molecular design of anisotropic Au(III) emitters will be greatly facilitated by the fundamental principles and theoretical analysis presented here.

Front Cover: Highly Luminescent Blue Emitter with Balanced Hybridized Locally and Charge‐Transfer Excited‐States Emission (ChemPhotoChem 3/2023)

The Front Cover illustrates the structure of a highly blue luminescent emitter lighting up dark space. The electronic structure of this compound reveals emission from a balanced hybridized local and charge-transfer (HLCT) excited state as shown by multi-reference quantum chemical calculations. The emitter was implemented by vacuum deposition in an OLED device that exhibits an EQE (external quantum efficiency) of 2%. More information can be found in the Research Article by Thomas J. J. Müller and co-workers.

Highly Luminescent Blue Emitter with Balanced Hybridized Locally and Charge‐Transfer Excited‐States Emission

AbstractThe Front Cover image is provided by Julia Wiefermann, Institute of Organic Chemistry and Macromolecular Chemistry (HHU, Düsseldorf, Germany). The image shows the highly blue luminescent emitter 4’’‐(diphenylamino)‐2’’‐methyl‐[1,1’:4’,1’’‐terphenyl]‐4‐carbonitrile which exhibits balanced hybridized local and charge‐transfer (HLCT) excited state emission. This compound was successfully employed in an OLED device. Read the full text of the Research Article at 10.1002/cptc.202200265.

Two new red-emitting ternary europium(III) complexes with high photoluminescence quantum yields and exceptional performance in OLED devices.

Two new organo-europium complexes (OEuCs) [Eu(btfa)3(Bathphen)] (OEuC-1) and [Eu(tta)3(Bathphen)] (OEuC-2) where btfa and tta are the anions of 4,4,4-trifluoro-1-phenyl-1,3-butanedione and 2-thenoyltrifluoroacetone while Bathphen = Bathophenanthroline have been synthesized and characterized. Both complexes in the solid state exhibit strong red emissions with photoluminescence quantum yields (PLQYs) of 80% ± 10%. These complexes were tested as dopants in the emitting layer (EML) to fabricate red organic light emitting diodes (R-OLEDs). Through device engineering involving the amalgamation of appropriate host materials and complexes, we have achieved exceptional overall R-OLED performance for an OEuC-2 based device, with maximum current efficiency (CEmax) = 9.91 cd A-1, maximum power efficiency (PEmax) = 9.15 lm W-1, maximum external quantum efficiency (EQEmax.) = 6.24%, brightness (B) of 545 cd m-2, and (CIE)x,y = 0.620, 0.323 at J = 10 mA cm-2.

Investigation on the Effect of Applying Different Structures on OLED Devices to Its Performance and Quality

OLED is a kind of device that contains multiple organic layers to produce electroluminescence. OLED has the benefit such as the easy process of manufacturing and only needs a low driving voltage which is environmentally friendly. OLED has low thickness and it doesn’t cost a lot of power compared with LED. By making it as a screen, it can be flexible and have a high capability to emit high light intensity. In this case, more and more experts and enterprises are developing and investigating more in this field. Nowadays, OLED is widely used in the manufacture of foldable screen on mobile phones. Besides, OLED still has a huge room to improve. The external quantum efficiency (EQE), which is the ratio of electrons collected to the incident photons, was well acknowledged to be a good indicator for researchers to evaluate the performance in OLED devices. Due to the difficulties on significant improvement on materials used to make OLED, experts put their effort on designing better structures for OLED to increase its percentage EQE. Therefore, in this work, the performance of OLED devices with different structures were compared.

P‐13.9: Simultaneously Determination of Emitting Dipole Orientation and Distribution in OLEDs by a Two‐Step Inversion Method

P‐13.9: Simultaneously Determination of Emitting Dipole Orientation and Distribution in OLEDs by a Two‐Step Inversion Method

Pyrene-based chalcones as functional materials for organic electronics application

Though new generation organic electronic devices have evolved from mere scientific perceptions to real-life marketed applications, considerably less research attention has been focused on n-type or electron transporting small molecule semiconductors. The present study is focused on the exploration of structural, thermal, electrochemical, electrical, and optical properties of two pyrene-based chalcones: PC1 and PC2, synthesized through Claisen Schmidt condensation reaction. The chalcones displayed good thermal stability and wide bandgap n-type semiconducting behaviour with high charge carrier concentration and dielectric constant. The experimental evidences including fluorescence measurements, nanoaggregate size, and morphology analysis, supported by DFT calculations and molecular dynamic simulations advocated the intramolecular charge transfer and aggregation-induced enhanced emission features of the molecules. Successful fabrication of a diode in combination with the current-voltage characteristics established the candidature of PC1 and PC2 for electro-optical devices. The dielectric studies were performed to measure dielectric constant and AC conductivity at different frequency ranges. The cyclic voltammetry and AC impedance response of PC2 differed from PC1 due to the inclusion of a fluorine atom in the molecular scaffold. Further, the functional implication of PC2 as an electrode material was explored by constructing a supercapacitor, which offered a specific capacitance of 220 Fg-1 at a scan rate of 10 mV s−1. Moreover, these chalcone-based organic semiconductors displayed high thermal and charge carrier concentration as well as compatibility with other layers in an OLED device. Hence PC1/PC2 can be further investigated as dopants along with other emissive layers as host materials in OLEDs.

Achieving Solution-Processed Non-Doped Single-Emitting-Layer White Organic Light-Emitting Diodes through Adjusting Pyrene-Based Polyaromatic Hydrocarbon.

Single-emitting-layer white organic light-emitting diodes (SEL-WOLEDs) have developed rapidly in recent years due to the outstanding advantages of high efficiency, simple device structure, low cost, less phase separation, and stable emission color. Nevertheless, the relatively complicated host-dopant system is usually essential for most previous SEL-WOLEDs and the development of simple non-doped SEL-WOLEDs lags behind. Hence the straightforward synthesis of single-white-emitting molecules for non-doped SEL-WOLEDs still remains a great challengeable task. In this article, we designed and synthesized two new pyrene-based polyaromatic hydrocarbons (PAHs) and used them as emitting layer materials in the OLED devices. When the molecules change from the mono-fused one to bis-fused one, the emitting light changes from greenish to white color. Further study indicated that the bis-fused molecule PyD with more twisted and extended backbone packed in neat Cmca space group in single-crystal system compared with P21 /n for PyS, which may be favorable to form excimers in the solid state and broaden the emission spectrum in the OLEDs. As a result, a solution-processed non-doped single-white-emitting-molecule SEL-WOLED with high performance (e. g., a high color rendering index of 66) is reported. The findings will be beneficial not only to further development of simple WOLEDs, but also to other related organic optoelectronic technology.

Numerical investigation of organic light emitting diode OLED with different hole transport materials

In this paper, a comparative study between four OLEDs devices is carried out. The bi- layers device (A) (consists of) Hole Injection Layer (HIL)/Electron Transport Layer (ETL), the multilayer device (B) (consists of) HIL Layer/Hole Transport Layer (HTL)/ETL Layer. The influence of the hole transporting material on the performance of the three layers OLEDs was investigated. Three different HTL materials were used: α- NPD, TAPC and p-TTA with the same electron transporting material as Alq3; (these holes transport material consists the devices (B), (C) and (D) respectively). The carrier injection, Langevin recombination rate, singlet exciton density and the power of luminescent are demonstrated. The simulation results shows that the insertion of a thin HTL layer between HIL and ETL layers increases the characteristics of the device (B)as: 6.19.1025 cm-3s-1 of the Langevin recombination rate, 1.16.1015cm-3 of the singlet exciton density and 0.04232 W/μm2 of the luminescence power. Moreover, the insertion of TAPC as HTL material gives rise to 1.36.1026 cm-3s-1 of the Langevin recombination rate, 2.1015cm-3 of the singlet exciton density and 0.075 w/μm2 of the luminescence power.

Intramolecular exciplex-typed molecule based on a stereoscopic V-shaped aromatic macrocycle and investigation of host-dependence of exciton utilization

Through-space charge transfer (TSCT) has been increasingly utilized for the design of efficient thermally activated delayed fluorescence (TADF) emitters. TSCT molecular skeletons with spatially and electronically confined D/A subunits can provide a small ΔEST, and also ease for fine control of CT properties within a single molecule. However so far TSCT-TADF molecules have not been fully investigated. Herein, we report a new ortho-position molecular pattern for TSCT-TADF design, using a stereoscopic V-shaped aromatic macrocycle of electron-deficient oxacalix[2]arene[2]triazine to “clip” the electron-rich triphenylamine unit. The new emitter OC2A2T-TPA exhibits a small ΔEST of 18 meV, and the OLED device that uses high-energy-level host DPEPO realizes the first-time electroluminescence application of OC2A2T-type macrocyclic compounds by achieving an impressive external quantum efficiency (EQE) of 14.6 %. Further host-dependence investigation reveals that OC2A2T-TPA actually behaves like an intramolecular exciplex, exhibiting high exciton utilization once its high-lying locally excited (LE) states on D/A could be sensitized first in the case of DPEPO host, otherwise low exciton utilization in the case of lower-energy-level mCP host. This work opens a way to enrich TSCT-TADF design and clarifies the importance to involve the high-lying LE states in the energy transfer routes for high-efficiency intramolecular-exciplex molecules.

Deep‐blue Fluorophores Based on Phenanthroimidazole Integrated with Benzo[d]thiazole: Experimental and Theoretical Investigation

AbstractThree deep‐blue emissive donor‐acceptor (D−A) fluorophores, 2‐(4′‐(1‐(4‐(tert‐butyl)phenyl)‐1H‐phenanthro[9,10‐d]imidazol‐2‐yl)‐[1,1′‐biphenyl]‐4‐yl)benzo[d]thiazole (PTPISN), 2‐(4′‐(1‐(3‐(trifluoromethyl)phenyl)‐1H‐phenanthro[9,10‐d]imidazol‐2‐yl)‐[1,1′‐biphenyl]‐4‐yl)benzo[d]thiazole (m‐CFPISN) and 3‐(2‐(4′‐(benzo[d]thiazol‐2‐yl)‐[1,1′‐biphenyl]‐4‐yl)‐1H‐phenanthro[9,10‐d]imidazol‐1‐yl)benzonitrile (m‐CNPISN) based on benzothiazole with varying substituents at N1‐position to tune the properties of the materials were designed and synthesized. The synthesized fluorophores were structurally confirmed by spectroscopic techniques. All the fluorophores exhibited good thermal properties with high thermal degradation temperatures. An optical study reveals that the fluorophores are capable of emitting in the blue spectral region in solution and solid. The quantum chemical investigations were also performed with the aim to understand the electronic structures of the fluorophores. Further, solution‐processed OLED device was fabricated by using PTPISN as an emissive dopant and found that bright deep‐blue electroluminescence with CIEy of (∼0.08). These results suggest that the introduction of benzothiazole moiety is a promising design strategy for obtaining new deep‐blue emitters.

(Invited) Development of Highly Luminescent Radical Core Carbazole Dendrimer

The development of emitting materials for OLEDs has started with fluorescence, moved to phosphorescence, and reached TADF. These materials have singlet ground state, but recently some ground state doublet (radical) emitting materials have been reported.1,2 The excited state of doublet emitters also has a doublet character, and theoretically, can achieve 100% exciton utilization efficiency in an OLED device. Triphenylmethyl radicals such as PTM (perchlorotriphenylmethyl) radical were recognized as emitters from the 1980s. The photostability of these radicals are extremely low, but in 2006 the introduction of donor (carbazole) unit was reported to improve the photostability and opened the door to utilize them as a luminescent materials.3 Carbazole dendrimers are widely used as solution-processable OLED material including TADF emitters.4 The dendron has a highly polarized unique electronic structure due to the summation of the inductive electron-withdrawing effect of carbazole units.5 The attachment of carbazole dendrons to luminescent radical is expected to increase the stability through the steric and electronic effects.A carbazole dendron (G2 to G4) attached TTM (tris(2,4,6-trichlorophenyl)-methyl) radicals were successfully synthesized and photophysical parameters were determined. All dendrimers showed red emission. PLQY (photoluminescence quantum yield) increased when the generation increased and reached 80%. The major reason was the decrease in the internal conversion rate (non-radiative decay). The photostability (PL intensity half-life measured under continuous laser irradiation) in solution has increased 100 to 1000 times compared to simple TTM radicals. These results indicate that the carbazole dendron attached TTM radicals are potential material for efficient OLEDs.

Synthesis, Single-Crystal XRD, Spectral and Computational Analysis of 2-(3,4-Dimethoxyphenyl)-1H-Phenanthro[9,10-d] Imidazole as Electron-Transport and NLO Material

A new Phenanthro-imidazole crystal, 2-(3,4-dimethoxyphenyl)-1H-phenanthro[9,10-d] was synthesized and characterized with spectral studies (IR, 1H-NMR, 13 C-NMR). Single Crystal X-ray and refinement studies reveals that the synthesized crystal system is monoclinic with P21/n space group and crystallographic data has been deposited in the Cambridge crystallographic data center with CCDC number: 2126693. RMSD values (Bond length = 0.035 Å and Bond angle=) and R-squared analysis (Bond length = 0.985 and bond angle = 0.990) reveals very good correlation between the experimental and theoretical structure. Analysis of frontier molecular orbitals and calculation of reorganization energy using DFT methods reveals that the crystals could be used as an electron transport material in OLED devices as the electron reorganization energy ( is comparatively low with a value of 0.373 eV. The computed value for the static first hyperpolarizability of the crystal was found to be 1.40023X10−of esu, which is four times higher than the standard value of urea and hence the crystal can be a potential candidate for future studies in non-linear optics.

Eliminating the Reverse ISC Bottleneck of TADF Through Excited State Engineering and Environment‐Tuning Toward State Resonance Leading to Mono‐Exponential Sub‐µs Decay. High OLED External Quantum Efficiency Confirms Efficient Exciton Harvesting

AbstractThe electronic structure and photophysics of the recently designed organic direct singlet harvesting (DSH) molecule are explored, in which donor (D) and acceptor (A) are held at distance by two bridges. One of the bridges is functionalized with fluorene. This structure leads to an ultrasmall singlet–triplet energy gap of ∆E (S1−T1) ≈ 10 cm−1 (≈1 meV) between the charge transfer states 1,3CT and shows an energetically close‐lying 3ππ* state localized on fluorene. Dielectric constant variation of the environment leads to state crossing of 3ππ* and 1,3CT near ε = 2.38 (toluene), as confirmed through time‐dependent density functional theory (DFT) and state‐specific DFT/polarizable continuum model excited‐state calculations. Transient absorption (TA) and time‐resolved luminescence in the femtosecond to microsecond regimes show rates of intersystem crossing (ISC) and reverse ISC (rISC) of >109 s–1. Thus, a strictly mono‐exponential short‐lived photo‐luminescence decay (431 ns) is observed, revealing that rISC is no longer the bottleneck responsible for long thermally activated delayed fluorescence. Ultrafast TA displays a time constant of ≈700 fs, representing the relaxation time of DSH and its solvent environment to the relaxed 1CT state with a molecular dipole moment of ≈40 D. Importantly, OLED devices, emitting sky‐blue light and showing high external quantum efficiency of 19%, confirm that singlet and triplet excitons are harvested efficiently.