Emitters In Organic Light-emitting Diodes Research Articles

Novel bis(fluorenyl)benzothiadiazole-cored carbazole dendrimers as highly efficient solution-processed non-doped green emitters for organic light-emitting diodes

Bis(fluorenyl)benzothiadiazole-cored carbazole dendrimers show high thermal and electrochemical stability, and great potential as solution processed hole-transporting non-doped green emitters for OLEDs. A pure green device with CIE coordinates of (0.27, 0.62) and high luminance efficiencies (up to 10.01 cd A(-1)) is achieved, respectively.

Highly efficient green phosphorescent OLEDs based on a novel iridium complex

A new iridium(III) complex Ir(tfmppy)2(tfmtpip) (1, tfmppy = 4-trifluoromethylphenyl-pyridine, tfmtpip = tetra(4-trifluoromethylphenyl)imidodiphosphinate) was synthesized and applied in organic light-emitting diodes (OLEDs). The devices with the structures of ITO/TAPC (1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane, 40 nm)/1 (x wt%): mCP (N,N′-dicarbazolyl- 3,5-benzene, 20 nm)/TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene, 40 nm)/LiF (1 nm)/Al (100 nm) exhibited a maximum power efficiency (ηp,max) of 113.23 lm W−1 and a maximum current efficiency (ηc,max) of 115.39 cd A−1 (0.01342 mA cm2) at the doping level of 5 wt%, which is among the best performances for Ir(III) complex based OLEDs in the green-light-emitting region. Compared with our former work, the excellent device efficiencies are due to the use of TmPyPB as the electron-transporting/hole-blocking layer which has a relatively higher electron mobility than that of TPBi (2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) and the introduction of the –CF3 moiety to the Ir(III) complex, which can increase the electron mobility of the complex. The device performances proved that the complex has potential applications as an efficient green emitter in OLEDs.

Oxadiazole- and triazole-based highly-efficient thermally activated delayed fluorescence emitters for organic light-emitting diodes

We developed highly-efficient thermally activated delayed fluorescence (TADF) emitters containing 2,5-diphenyl-1,3,4-oxadiazole (OXD) or 3,4,5-triphenyl-4H-1,2,4-triazole (TAZ) electron acceptor and phenoxazine (PXZ) electron donor moieties. Oxadiazole-based compounds PXZ-OXD and 2PXZ-OXD showed green emission, while the triazole-based ones PXZ-TAZ and 2PXZ-TAZ exhibited sky-blue emission. In toluene solution, the donor–acceptor–donor-type molecules 2PXZ-OXD and 2PXZ-TAZ showed more efficient TADF and higher photoluminescence quantum yields (PLQYs) than the donor–acceptor-type molecules PXZ-OXD and PXZ-TAZ. When doped into a host material, 2PXZ-OXD displayed a high PLQY of 87%. An organic light-emitting diode using 2PXZ-OXD as an emitter exhibited an external quantum efficiency (EQE) of 14.9%, which exceeds those obtained with conventional fluorescent emitters. This high EQE results from the efficient generation of TADF in 2PXZ-OXD.

Tuning the electronic and photophysical properties of heteroleptic iridium(iii) phosphorescent emitters through ancillary ligand substitution: a theoretical perspective

The development and application of phosphorescent emitters in organic light-emitting diodes (OLEDs) have played a critical role in the push to commercialization of OLED-based display and lighting technologies. Here, we use density functional theory methods to study how modifying the ancillary ligand influences the electronic and photophysical properties of heteroleptic bis(4,6-difluorophenyl) pyridinato-N,C [dfppy] iridium(III) complexes. We examine three families of bidentate ancillary ligands based on acetylacetonate, picolinate, and pyridylpyrazolate. It is found that the frontier molecular orbitals of the heteroleptic complexes can be substantially modulated both as a function of the bidentate ligand family and of the substitution patterns within a family. As a consequence, considerable control over the first absorption and phosphorescence emission transitions, both of which are dominated by one-electron transitions between the HOMO and LUMO, is obtained. Tuning the nature of the ancillary ligand, therefore, can be used to readily modulate the photophysical properties of the emitters, providing a powerful tool in the design of the emitter architecture.

Effects of N-Substitution on Phosphorescence Efficiency and Color Tuning of a Series of Ir(III) Complexes with a Phosphite Tripod Ligand: A DFT/TDDFT Study

A DFT/TDDFT investigation was applied to understand the unusual properties of the recently synthesized blue-emitting Ir(III) complexes [Ir(PMe2Ph)(dppit)(py2pz)] (1) [PMe2Ph = dimethylphenylphosphine; dppit = diphenyl phenylphosphonite; py2pz = 3,5-di(2-pyridyl)pyrazole] and [Ir(PMe2Ph)(dppit)(bptz)] (2) [bptz =3-tert-butyl-5-(2-pyridyl)triazolate], which are successfully used as emitters in organic light-emitting diodes (OLEDs). The influence of N-substitution on optical and electronic properties of Ir(III) complexes was also explored by introducing a N atom on the pyridine moiety of N∧N ligands for 1 and 2. The calculated results reveal that introduction of N substitution leads to a blue shift for 1a, 1b, 1c, and 1d (a, b, c, d indicate different positions for N substitution) and slightly red shift for 2a–2d in absorption spectra compared with that of 1 and 2, respectively. The N substitution at different positions on N∧N ligands may also be an efficient approach of tuning emitting color for 1 and 2. Th...

Achievement of complete electromer emission in organic light-emitting diodes

Electromer is a unique excited-state species which can only be formed under electrical excitation. Currently, a clear understanding of electromer has not been realized yet. A big obstacle is that the electromer emission can hardly be separated from the emission of molecular excitons. Obtaining a complete electromer emission is essential for photophysical investigations of this kind of excited-state species and is useful for its potential application in optoelectronic logic gates. We report that a complete electromer emission can be achieved by introduction of a weak-fluorescence layer in organic light-emitting diodes, which will quench the molecular excitons produced in the adjacent emissive layer but leave the electromers undisturbed. In addition, a possible mechanism for the electromer formation is proposed based on the analysis of the single crystal structure of the compound 9,9-bis[4-(di-p-tolyl)aminophenyl]-2,7-bis(9-carbazolyl)fluorene which can produce electromers.

A carbazole-functionalized Ir complex used in efficient single-layer electrophosphorescent devices

The fabrication of electroluminescent devices that combine high device performance with simple device configuration remains an attractive challenge for their low costs and simple fabrication process. In this paper, a novel red phosphorescent iridium complex containing a carbazole-functionalized β-diketonate ligand, bis(1-phenylisoquinolinato-C2,N) iridium 1-(carbazol-9-yl)-5,5-dimethylhexane-2,4-diketonate (Ir(piq)2(CBDK)), is designed, synthesized, and characterized. The electrophosphorescence of Ir(piq)2(CBDK) as an emitter in organic light-emitting diodes was examined and compared with that of Ir(piq)2(acac) (acac=acetylacetonate) without carbazolyl group. A series of single-layer devices containing different concentration gradients of Ir(piq)2(CBDK) as an emitting and hole-transporting material and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene as an electron-transporting material were constructed. A maximum current efficiency of 9.9cdA−1 and power efficiency of 7.8lmW−1 were obtained, which is one of the best performances obtained for a single-layer device containing one material as a host.

Detection of sub-10 nm emission profile features in organic light-emitting diodes using destructive interference

The position of light-emitting molecules can be identified using interferometric approaches. Standard schemes utilize constructive interference to obtain a sectioned area of interest with high detection efficiency. The examination of organic light-emitting diodes (OLED) removes the common constraint of low light levels and enables a more generalized analysis. The OLED emitters are located in the front of a metal mirror, giving rise to an approximate two-wave fringe pattern in the far field. It is demonstrated theoretically and experimentally that positions around the field nodes enable the extraction of emitter distribution details within an electroluminescent layer of only 10nm thickness.

Pyrrolo[1,2-a]quinoxalines: Novel Synthesis via Annulation of 2-Alkylquinoxalines

In an attempt to synthesize a novel homoleptic complex 3 from 2-methyl-3-phenylquinoxaline 1 and Ir(acac)(3) for application as a triplet emitter in OLEDs (organic light-emitting diodes) no cyclometalation was observed. Instead, an annulation to 1-methyl-4-phenylpyrrolo[1,2-a]quinoxaline 2 was observed. Since pyrroloquinoxalines are potentially bioactive and few paths for their synthesis are known, selected reactions and conditions were investigated, suggesting Ir(acac)(3) as catalyst and proving glycerol to be a reactant.

Novel 4-(2,2-diphenyl-vinyl)-phenyl substituted pyrene derivatives as efficient emitters for organic light-emitting diodes

Two pyrene derivatives (PVPP and TPVPP) bearing 4-(2,2-diphenyl-vinyl)-phenyl side unit were developed for organic light-emitting diodes. Their photophysical, thermal, electrochemical, and electroluminescent properties as well as the film morphologies have been investigated in detail. Both of them exhibit high solid-state quantum yields, good thermal stability, and high glass-transition temperatures in the range of 127–150°C. In particular, it is found that multiple side units can significantly affect the electrochemical properties to improve the electron injection. The LUMO energy level of TPVPP is −2.76eV, which is very close to that of commonly used electron transport materials. The maximum luminance of OLED devices using TPVPP as an emitter layer is 103835cd/m2 with a maximum current efficiency of 5.19cd/A and a maximum power efficiency of 3.38lm/W.

Synthesis and characterization of fluorene and carbazole dithienosilole derivatives for potential applications in organic light-emitting diodes

Several fluorene or carbazole-based dithienosiloles (DTSs) have been synthesized and their thermal, photophysical, and electrochemical properties have been systematically investigated. These compounds show high thermal stability with glass transition temperature above 110 °C as well as decomposition temperatures at ∼400 °C. Intense green emission is observed in the spectral region of 500–510 nm for all compounds (ΦPL=0.31–0.80), that is, attributed to both the 5,5′-substituents of the DTS ring and DTS-based π–π∗ transition. Based on the emission spectra at 77 K, the triplet energy for these compounds was calculated to be within 2.1–2.2 eV, indicating that they may be used as host materials for red emitters in organic light-emitting diodes (OLEDs). All compounds exhibit reversible oxidation and possess low-lying LUMO energies, owing to the conjugated fluorene/carbazole substituents on the DTS. This along with the high thermal/electrochemical stabilities and high fluorescent quantum efficiencies makes the new DTSs compounds promising candidates for use in OLEDs as emitters, host and electron-transporting materials.

Theoretical Insight into the Origin of Large Stokes Shift and Photophysical Properties of Anilido‐Pyridine Boron Difluoride Dyes

The geometric and electronic structures and photophysical properties of anilido-pyridine boron difluoride dyes 1-4, a series of scarce 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives with large Stokes shift, are investigated by employing density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations to shed light on the origin of their large Stokes shifts. To this end, a suitable functional is first determined based on functional tests and a recently proposed index-the charge-transfer distance. It is found that PBE0 provides satisfactory overall results. An in-depth insight into Huang-Rhys (HR) factors, Wiberg bond indices, and transition density matrices is provided to scrutinize the geometric distortions and the character of excited states pertaining to absorption and emission. The results show that the pronounced geometric distortion due to the rotation of unlocked phenyl groups and intramolecular charge transfer are responsible for the large Stokes shift of 1 and 2, while 3 shows a relatively blue-shifted emission wavelength due to its mild geometric distortion upon photoemission, although it has a comparable energy gap to 1. Finally, compound 4, which is designed to realize the rare red emission in BODIPY derivatives, shows desirable and expected properties, such as high Stokes shift (4847 cm(-1)), red emission at 660 nm, and reasonable fluorescence efficiency. These properties give it great potential as an ideal emitter in organic light-emitting diodes. The theoretical results could complement and assist in the development of BODIPY-based dyes with both large Stokes shift and high quantum efficiency.

Design Rules for Charge-Transport Efficient Host Materials for Phosphorescent Organic Light-Emitting Diodes

The use of blue phosphorescent emitters in organic light-emitting diodes (OLEDs) imposes demanding requirements on a host material. Among these are large triplet energies, the alignment of levels with respect to the emitter, the ability to form and sustain amorphous order, material processability, and an adequate charge carrier mobility. A possible design strategy is to choose a π-conjugated core with a high triplet level and to fulfill the other requirements by using suitable substituents. Bulky substituents, however, induce large spatial separations between conjugated cores, can substantially reduce intermolecular electronic couplings, and decrease the charge mobility of the host. In this work we analyze charge transport in amorphous 2,8-bis(triphenylsilyl)dibenzofuran, an electron-transporting material synthesized to serve as a host in deep-blue OLEDs. We show that mesomeric effects delocalize the frontier orbitals over the substituents recovering strong electronic couplings and lowering reorganization energies, especially for electrons, while keeping energetic disorder small. Admittance spectroscopy measurements reveal that the material has indeed a high electron mobility and a small Poole-Frenkel slope, supporting our conclusions. By linking electronic structure, molecular packing, and mobility, we provide a pathway to the rational design of hosts with high charge mobilities.

A cyclometalated platinum(II) complex with the 4-isobutyryl-3-methyl-1-phenylpyrazol-5-onate ligand as a new efficient emitter for organic light-emitting diodes

A cyclometalated platinum(II) complex with the 4-isobutyryl-3-methyl-1-phenylpyrazol-5-onate ligand as a new efficient emitter for organic light-emitting diodes

Escaped and Trapped Emission of Organic Light-Emitting Diodes

By locating the emitters around the first and second antinode of the metal electrode, the escaped and trapped emission of small molecule based bottom emission organic light-emitting diodes is investigated by using an integrating sphere, a fiber spectrometer and a glass hemisphere. It is found that the external coupling ratio by locating the emitters at the second antinode (at a distance of 220 nm from the cathode) is 70%, which is higher than that of an emitter at the first antinode (60 nm from the cathode) in theory and experiment. Extending the “half-space" dipole model by taking the dipole radiation pattern into account, we also calculate the optical coupling efficiency for the emitter at both the first and second antinode. Our experimental and theoretical results will benefit the optimization of device structures for the higher out-coupling efficiency.

Solution processeable organic–inorganic hybrids based on pyrene functionalized mixed cubic silsesquioxanes as emitters in OLEDs

Traditional materials for application in organic light emitting diodes (OLEDs) are primarily based on small molecules and polymers, with much fewer examples of intermediate molecular weight materials. Our interest lies in this intermediate molecular weight range, specifically in hybrids based on 3-dimensional silsesquioxane (SSQ) cores that represents a new class of versatile materials for application in solution processable OLEDs. We report here various SSQ based hybrids that are easily prepared in one high-yield step from the Heck coupling of commercially available 1-bromopyrene, and 1-bromo-4-heptylbenzene with octavinyl-T8-SSQ, and a mixture of octavinyl-T8-, decavinyl-T10- and dodecavinyl-T12-SSQ. The resulting materials offer numerous advantages for OLEDs including amorphous properties, high-glass-transition temperatures (Tg), low polydispersity, solubility in common solvents, and high purity via column chromatography. Solution processed OLEDs prepared from the SSQ hybrids provide sky-blue emission with external quantum efficiencies and current efficiencies of 3.64% and 9.56 cd A−1 respectively.

Investigation on the escaped and trapped emission in organic light-emitting devices

The escaped and trapped emission of small molecule based bottom emission organic light-emitting diodes was investigated by using an integrating sphere and a fiber spectrometer and a glass hemisphere. It was found that the maximum external coupling ratio rext, namely the ratio of escaped emission to the emission of escaped and trapped in the substrate, is 56%. We also extended the “half-space” dipole model by taking dipole radiation pattern into account, which agreed well with our experiments. Our experimental and theoretical results will benefit the optimization of device structures for the higher out-coupling efficiency.

Energy transfer dynamics in organic light emitting diode emission layers doped with triplet emitters

Herein, we report the study of singlet energy transfer dynamics in two host-guest systems: CNPPP (poly [2-(6′-cyano-6′-methylheptyloxy)-1,4-phenylene]) as host in both and as guests btpIr (iridium (III) bis[2-(2′-benzothienyl)pyridinato-N,C3′] (acetylacetonate)) and Irppy (fac tris(2-phenylpyridine) iridium (III), two triplet emitters. In both systems, the energy transfer was evaluated using the Förster transfer model. Utilizing time-resolved and stationazry measurements, it could be concluded that the behavior of both systems was in good agreement with the proposed model. In addition, this analysis provides a clear indication to the homogenous distribution of the dopants.

Two Novel Tetrahedrally Silicon-Based Benzimidazole Derivatives: Potentials as Blue Emitters for OLEDs

Two novel silicon-centered benzimidazole derivatives, Bis(4-(benzimidazol-1-yl)phenyl) dimethylsilane (1) and Bis(3-(benzimidazol-1-yl)phenyl)dimethylsilane (2) have been synthesized and determined by IR, 1H NMR, 13C NMR and mass spectroscopy. These compounds have high thermal stability and are fluorescent with emission in the region of violet to blue, which could be potentially applied as blue emitters in organic light-emitting diodes (OLEDs) display.

Examining micro-cavity parameters of top emission organic light-emitting diode with low-order resonant modes

Emission characteristics of top emitting organic light-emitting devices (TOLEDs) with Ag as reflective anode, Al/Ag as semitransparent cathode and 90–160 nm [N-(1-naphthy1)-N-pheny1-amino] bipheny1/tris-(8-hydroxy quinoline) aluminum (NPB/Alq3) sandwiched in the electrodes are examined. The electroluminescence (EL) spectra of the TOLEDs are simulated based on the Fabry-Perot cavity theory. And the resonant modes in cavity structure of TOLEDs is discussed and clarified which can accurately describe the work principle of the devices. A fairly good match between calculated values and experimental data is achieved at different emission colors from bluish green to orange.