Band Gap Of Host Material Research Articles

Synthesis of 5-(Aryl)amino-1,2,3-triazole-containing 2,1,3-Benzothiadiazoles via Azide-Nitrile Cycloaddition Followed by Buchwald-Hartwig Reaction.

An efficient access to the novel 5-(aryl)amino-1,2,3-triazole-containing 2,1,3-benzothiadiazole derivatives has been developed. The method is based on 1,3-dipolar azide-nitrile cycloaddition followed by Buchwald-Hartwig cross-coupling to afford the corresponding N-aryl and N,N-diaryl substituted 5-amino-1,2,3-triazolyl 2,1,3-benzothiadiazoles under NHC-Pd catalysis. The one-pot diarylative Pd-catalyzed heterocyclization opens the straightforward route to triazole-linked carbazole-benzothiadiazole D-A systems. The optical and electrochemical properties of the compound obtained were investigated to estimate their potential application as emissive layers in OLED devises. The quantum yield of photoluminescence (PLQY) of the synthesized D-A derivatives depends to a large extent on electron-donating strengths of donor (D) component, reaching in some cases the values closed to 100%. Based on the most photoactive derivative and wide bandgap host material mCP, a light-emitting layer of OLED was made. The device showed a maximum brightness of 8000 cd/m2 at an applied voltage of 18 V. The maximum current efficiency of the device reaches a value of 3.29 cd/A.

A comparative study on the structural, optical, and magnetic behavior of transition metal (Co and Mn) ions substituted Cr2O3 nanoparticles

A comparative study on structural, optical, and magnetic properties of TMxCr2−xO3 (TM = Co and Mn) nanoparticles was synthesized using a simple co-precipitation method with subsequent heat treatment. The structural characterizations of as-prepared nanoparticles were performed using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. XRD pattern revealed the formation of rhombohedral Cr2O3 phase with space group R3¯c. The FTIR spectra illustrated the shifting of absorption bands of Cr2O3 nanoparticles with Co and Mn substitution due to strain induced within the crystal lattice. X-ray photoelectron spectroscopy (XPS) confirmed the presence of a mixed oxidation state of Cr and oxygen defects in the prepared nanoparticles. Furthermore, XPS spectra also demonstrated the presence of Co and Mn having +2 oxidation state. The reduction in optical band gap of host material was observed with Co and Mn substitution. The magnetic properties of as-synthesized nanoparticles were recorded using a SQUID magnetometer and temperature-dependent magnetic measurements illustrated the decrease of Neel temperature with Co and Mn substitution. It was also demonstrated that magnetization increases below about ∼ 120 K due to spin canting and spin frustration for both TM ions substituted Cr2O3 nanoparticles and is fitted using the 3D spin-wave model with Curie-Weiss law. The magnetic studies revealed the mixed paramagnetic (PM) and antiferromagnetic (AFM) order at room temperature and AFM mix with ferromagnetic (FM) order at low temperature explained using the bound magnetic polarons (BMP) model.

Analysis of efficiency variations in ν-DABNA based thermally activated delayed fluorescence OLED devices

Recently, diboron based blue thermally activated delayed fluorescence (TADF) material, N7,N7,N13,N13,5,9,11,15-octaphenyl-5,9,11,15-tetrahydro-5,9,11,15-tetraaza-19b, 20b-diboradinaphtho[3,2,1-de:1′,2′,3′-jk] pentacene-7,13-diamine (ν-DABNA) has drawn attention from the researchers due to its good device performances as well as narrow full-width at half maximum (FWHM) in OLED devices. In this work, we report a detailed analysis to understand the exact reason behind the efficiency variations in ν-DABNA based TADF devices using different types of host materials. Interestingly, we found that triplet exciton density of ν-DABNA molecule is mainly located at the central region, so the triplet exciton energy transfer from the host material is hindered due to the presence of peripheral diphenylamine extensions. Hence, our analysis suggests that utilizing wide band gap host materials in ν-DABNA based TADF devices exhibit excellent device efficiencies with the support of trap assisted emission mechanism. This study brings an in-depth understanding of efficiency variations in the ν-DABNA based OLED devices.

Pure-Blue Fluorescence Molecule for Nondoped Electroluminescence with External Quantum Efficiency Approaching 13%

A pure-blue light-emitting material is one of the key components in the preparation of organic light-emitting diode (OLED) displays. Although high-efficiency blue OLEDs have been realized in therma...

First-principles investigation of structural, electronic, optical and thermal properties of Zinc doped SrTiO3

This work reports the first-principles study, based on the density functional theory (DFT), by using generalized gradient approximation (GGA) and ultra-soft pseudo-potential (USP), to explore effects of zinc (Zn) doping on structural, electronic, optical and thermal properties of cuboctahedral SrTiO3. We observe significant reduction in unit cell volume upon doping Zn into SrTiO3. Furthermore, Zn doping introduces new sates at Brillouin zone symmetry points turning the indirect band gap of host material into direct one. Replacement of Zn with Sr in host lattice repositions density of states at lower energies resulting in stronger interactions between Zn-atom and its neighbors as compared to interactions between Sr-atom and its surroundings. This refers to substantial modification in electronic band structure of host material by Zn doping. Physical properties of SrTiO3 also change significantly upon Zn doping in accordance with the electronic band structure. Significant changes in electronic structure and properties of SrTiO3 by Zn doping opens new prospects for potential applications of these materials in optoelectronics.

Capacitance and impedance spectroscopy studies of polymer light emitting diodes based on MEH-PPV:BT blends

Light emitting polymer poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is blended with a wide bandgap electron transport material benzothiadiazole (BT) and its effect on the electronic properties has been studied by capacitance and impedance spectroscopy (IS) in PLEDs. The impedance data is fitted using equivalent circuit models and the minimum parallel resistance (Rp) at zero bias have been obtained for 1:3 ratio of MEH-PPV:BT blended devices. The negative capacitance (NC) shows the occurrence of the trap-assisted non-radiative recombination mechanism at low frequencies in the unblended MEH-PPV PLEDs. Further, this behavior is seen to be reduced in PLEDs with MEH-PPV:BT blends. This clearly suggests that the blending of MEH-PPV and BT at different weight ratios results in the suppression of trap-assisted recombination. This can be attributed to the elimination of trap states due to the dilution of semiconductor material on account of the addition of wide bandgap host material. Moreover, the blended devices have shown a significant improvement in the conductivity at small bias voltages.

Phenanthro[9,10-d]triazole and imidazole derivatives: high triplet energy host materials for blue phosphorescent organic light emitting devices

A class of wide bandgap host materials is introduced as an alternative to carbazole-based hosts to enhance the efficiency and transport properties of organic light emitting diodes (OLEDs).

Molecular understanding of diphenylether-, 9,9-biphenylfluorene- and tetraphenylsilane-centered wide bandgap host materials for highly efficient blue phosphorescent OLEDs

Organic wide bandgap semiconductor is a hot research topic in optoelectronic fields. In this work, a series of diphenylether-, 9,9-biphenylfluorene- and tetraphenylsilane-centered wide bandgap materials are designed and synthesized. Comprehensive experiments and theoretical calculations are conducted to investigate their intrinsic thermal stability, photophysical property, electrochemical behavior and device performance, aiming at exploring the potentials of oxygen, sp3-hybridized carbon and silicon atoms for wide bandgap materials. They all can interrupt molecular conjugation to generate wide bandgaps. By modification of appropriate donor and acceptor, relatively high triplet energy level (ET) can be obtained in the resulting materials. Quantum chemical calculations indicate the sort of triplet energy levels of N-carbazole > fluorene > benzonitrile, which fully supports the experimental results. Tetraphenylsilane-based materials display lower LUMO levels than corresponding diphenylether- and 9,9-biphenylfluorene-centered materials owing to the dπ-pπ conjugation. Among them, DCzSiPy/FIrpic doped device gives the best device performance with maximum current efficiency of 35.6 cd/A and maximum external quantum efficiency of 16.8%, demonstrating the superiority of tetraphenylsilane skeleton. 9,9-Biphenylfluorene-based doped devices have comparable efficiencies with tetraphenylsilane-hosted ones, meaning 9,9-biphenylfluorene unit also can be a good constructing skeleton for wide bandgap host materials.

P‐196: High Efficiency Blue Phosphorescence Organic Light Emitting Device with Novel CbzBZ host

The high efficiency blue phosphorescent organic light‐emitting device with the wide bandgap host material CbzBZ as the host material and bis[2‐(4,6‐difluorophenyl)pyridinato‐C2,N](picolinato)iridium(III) (FIrpic) as blue emitter was demonstrated. After the adjustment of FIrpic doping ratio and thickness of emitting layer, the optimums device showed the current efficiency of 61.05 cd/A, power efficiency of 63.51 lm/W, and EQE over 28%.

Exciton-Induced Degradation of Carbazole-Based Host Materials and Its Role in the Electroluminescence Spectral Changes in Phosphorescent Organic Light Emitting Devices with Electrical Aging.

We investigate the origins of the long-wavelength bands that appear in the emission spectra of carbazole-based host materials and play a role in the electroluminescence (EL) spectral changes of phosphorescent organic light emitting devices (PhOLEDs) with electrical aging. 4,4'-Bis(carbazol-9-yl)biphenyl (CBP) is used as a model carbazole host material and is studied using photoluminescence, EL, and atomic force microscopy measurements under various stress scenarios in both single and bilayer devices and in combination with various electron transport layer (ETL) materials. Results show that exciton-induced morphological aggregation of CBP is behind the appearance of those long-wavelength bands and that complexation between the aggregated CBP molecules and ETL molecules plays a role in this phenomenon. Comparisons between the effects of exciton and thermal stress suggest that exciton-induced aggregation may be limited to short-range molecular ordering or pairing (e.g., dimer or trimer species formation) versus longer-range ordering (crystallization) in the case of thermal stress. The findings provide new insights into exciton-induced degradation in wide band gap host materials and its role in limiting the stability of PhOLEDs.

Adjusting Nitrogen Atom Orientations of Pyridine Ring in Tetraphenylsilane-Based Hosts for Highly Efficient Blue Phosphorescent Organic Light-Emitting Devices.

Four wide bandgap host materials, namely, 9-(4-diphenyl(4-(pyridin-3-yl)phenyl)silyl-phenyl)-9H-carbazole (CSmP), 9-(4-diphenyl(4-(pyridin-2-yl)phenyl)silylphenyl)-9H-carbazole (CSoP), 9-(4-diphenyl(4-(pyridin-3-yl)phenyl)silylphenyl)-9H-3,9'-bicarbazole (DCSmP), and 9-(4-(diphenyl(4-(pyridin-2-yl)phenyl)silyl)phenyl)-9H-3,9'-bicarbazole (DCSoP), have developed by incorporation of pyridine with varied N atom orientation and carbazole/dimer carbazole units into the tetraphenylsilane skeleton for blue phosphorescent light-emitting diodes. These host materials all possess wide bandgap (3.54-3.64 eV) and high triplet energies (2.77-2.95 eV). As revealed by the absorption and emission spectra, theoretical calculations, and CV measurements, the N atom orientation exerts a strong influence on the LUMO energy level and electron-transportation behaviors without deterioring the photophysical properties. Among them, DCSmP with 3-pyridyl substituent manifests the best electron-transporting capability. The FIrpic-doped blue phosphorescent device using DCSmP as host material exhibits excellent electroluminescence performance with a maximum current efficiency of 40.1 cd A(-1) and a maximum external quantum efficiency of 20.0%. The current efficiency and external quantum efficiency are improved 3-fold, higher than those fabricated from DCSpP with 4-pyridyl as substituent, demonstrating an effective strategy for large improvement in device performance by a subtle change in molecular structure.

Narrow bandgap host material for high quantum efficiency yellow phosphorescent organic light-emitting diodes doped with iridium(III) bis(4-phenylthieno[3,2-c]pyridine)acetylacetonate

A narrow bandgap host material, 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN), was used as a bipolar host material to improve the device performances of yellow phosphorescent organic light-emitting diodes(PHOLEDs). The device performances of the yellow PHOLEDs were optimized at a low doping concentration of 1%. A low turn-on voltage of 3.0V and high quantum efficiency of 19.3% were achieved using the 2CzPN host material and no efficiency roll-off of the device was observed up to 1000cd/m2 compared with 25% of reference device.

Tetraphenylsilane group containing carbazoles as high triplet energy host materials for solution-processable PhOLEDs

A series of solution processable, wide band-gap host materials composed of carbazole and tetraphenylsilane groups were designed and synthesized. Their thermal, electrochemical, and photophysical properties were fully investigated. The introduction of bulky tetraphenylsilane and \textit{tert}-butyl groups around the carbazole led to high glass transition temperatures (T$_{g})$ between 120 and 204 $^{\circ}$C. The triplet energies (E$_{T})$ of the synthesized materials were examined by low temperature (77 K) photoluminescence studies and determined as E$_{T}>$ 2.6 for all compounds. Phosphorescent organic light-emitting devices with the ITO/PEDOT:PSS/EML/TPBi/Cs$_{2}$CO$_{3}$/Al device structure were fabricated by using synthesized materials as the host and two kinds of phosphorescent emitters, FIrpic and Ir(ppy)$_{3}$, as the guests. The highest luminous and power efficiency values obtained by using FIrpic were 3.6 cd A$^{-1}$ and 1.48 lm W$^{-1}$, respectively, with commission International de I'Eclairage (CIE) coordinates of (0.17, 0.36), whereas these values were 7.8 cd A$^{-1}$ and 2.9 lm W$^{-1}$ for the device structure when Ir(ppy)$_{3}$ was used as the guest [CIE (0.28, 0.62)].

Green phosphorescent organic light-emitting devices based on wide bandgap host materials

Two homoleptic iridium(III) complexes Ir3F and Ir6F were synthesized and tested as wide bandgap organometallic hosts for green phosphorescent OLEDs. While using Ir(ppy)3 as the green triplet emitter, Ir3F-based devices clearly outperformed similar devices fabricated with the traditional CBP host. On the opposite, Ir6F-based devices showed lower performances than the reference device. As appealing feature, Ir6F-based devices showed almost no efficiency roll-off i.e., no reduction of the luminous efficiency with the current density contrarily to that commonly observed for phosphorescent OLEDs.

DFT study of electronic and structural properties of Sm:GaN

First principle calculations of Sm:GaN carried out using GGA-PBE, mBJ and GGA+U are presented to demonstrate the structural, electronic and magnetic properties of the system. The effects of Hubbard correction (i.e. U) on band structure and location of Sm 4f levels are discussed in detail. The application of U indicated a considerable splitting of occupied and unoccupied 4f band unlike that of GGA and mBJ. The introduction of Sm related gap states caused narrowing of band gap which is expected to facilitate the tuning of optical transitions. The results also indicated the production of intermediate bands in the band gap of host material which points to possible use of this material in photovoltaic cell generation. The observed spin polarization of 4f and 5d and 6s states of Sm shows possibility of ferromagnetic exchange interactions in the material for spintronic devices.

Electronic structure analysis of rare earth ions Ce and Nd doped gallium nitride

First principle calculations on electronic structure of Rare Earth ions Ce and Nd doped GaN are presented. Band gap narrowing, optical transitions, and hybridization of impurity states with the host are discussed. The dopants introduced 4f states in the band gap of the doped material in such a way that REGa isoelectronic traps facilitate the optical transitions in RE:GaN. The observed formation of intermediate bands and impurity levels in band gap of host material predicted it to be excellent candidate for the future solar cell generation and other optoelectronic devices.

P.121: Good Color Stable Phosphorescent White Organic Light‐Emitting Diodes with Double Emissive Layer Structure

AbstractWe report good color stable phosphorescent white organic light‐emitting diodes (OLEDs) based on double emissive layer (EML) structure composed of blue and red/green phosphorescent units. Deep hole trapping situation of red and green dopants at the red/green EML could realize less voltage dependent white spectral characteristics by restricting the change of exciton generation zone. A wide band‐gap host material of 2,6‐bis(3‐(carbazol‐9‐yl)phenyl)pyridine (26DCzPPy) was used for such deep trapping generation. Fabricated phosphorescent white OLED has very slight color coordinates changes of (‐0.007, +0.006) from 1000 cd/m2 to 10000 cd/m2 with power efficiency of 37.7 lm/W and current efficiency of 48.1 cd/A at 1000 cd/m2.

Color stable phosphorescent white organic light-emitting diodes with double emissive layer structure

In this paper, we report color stable phosphorescent white organic light-emitting diodes (OLEDs) based on a double emissive layer (EML) structure composed of blue and red/green phosphorescent units. Deep hole trapping situation of red and green dopants at the red/green EML could induce less voltage dependent white spectral characteristics by restricting the change of exciton generation zone. A wide band-gap host material, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy), was used for achieving such deep-trap generation. Fabricated phosphorescent white OLED shows a slight color coordinate change of (−0.002, +0.002) from 1000cd/m2 to 5000cd/m2 with power efficiency of 38.7lm/W and current efficiency of 46.4cd/A at 1000cd/m2. In addition, negligible color changes were observed by delaying red dopant saturation time using optimum red dopant concentration.

Low Driving Voltage and High Efficiency Blue Phosphorescent OLEDs with Mixed Host System

We report both low driving voltage and high efficiency characteristics in blue phosphorescent organic light-emitting diodes with a simple device structure. A mixed host system consisting of narrow band-gap host material of 4,4’,4’’-tris(N-carbazolyl)triphenylamine (TCTA) and bipolar carrier transporting material of 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) shows low driving voltage performances with high luminance stability. A very low onset voltage of 3.0 V and a driving voltage of 3.8 V to obtain a brightness of 1000 cd/m2are achieved with a current efficiency of 44.2 cd/A.

Carbazole/oligocarbazoles substituted silanes as wide bandgap host materials for solution-processable electrophosphorescent devices

We have designed and synthesized a series of organic wide bandgap materials, namely DCzSiCz, DDCzSi and DTCzSi, by incorporating carbazole/oligocarbazoles via a silicon-bridged linkage mode. All the materials show good thermal stability and excellent solution-processibility. Their HOMOs and LUMOs could be tuned to facilitate the efficient carriers injection by the incorporated carbazole/oligocarbazoles, while their singlet and triplet energy levels still maintain high levels, all above 3.44eV and 2.87eV, respectively. High efficient blue electrophosphorescent devices with low turn-on voltage are realized using DCzSiCz, DDCzSi and DTCzSi as hosts for FIrpic through solution-processable method. Among them, DCzSiCz-based device demonstrates the best performance, showing a maximum brightness of 6600cdm−2 at 11V and maximum luminous efficiency of 8.40cdA−1 at 5V.