Wide Bandgap Host Materials 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.

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...

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.

Adamantane-based wide-bandgap host material: blue electrophosphorescence with high efficiency and very high brightness.

An adamantane-based host material, namely, 4-{3-[4-(9H-carbazol-9-yl)phenyl]adamantan-1-yl}benzonitrile (CzCN-Ad), was prepared by linking an electron-donating carbazole unit and an electron-accepting benzonitrile moiety through an adamantane bridge. In this approach, two functional groups were attached to tetrahedral points of adamantane to construct an "sp(3)" topological configuration. This design strategy endows the host material with a high triplet energy of 3.03 eV due to the disruption of intramolecular charge transfer. Although CzCN-Ad has a low molecular weight, the rigid nonconjugated adamantane bridge results in a glass transition temperature of 89 °C. These features make CzCN-Ad suitable for fabricating blue phosphorescent organic light-emitting diodes (PhOLEDs). The devices based on sky-blue phosphor bis[(4,6-difluorophenyl)pyridinato-N,C(2')](picolinato)iridium(III) (FIrpic) achieved a high maximum external quantum efficiency (EQE) of 24.1%, which is among the best results for blue PhOLEDs ever reported. Furthermore, blue PhOLEDs with bis(2,4-difluorophenylpyridinato)-tetrakis(1-pyrazolyl)borate iridium(III) (FIr6) as dopant exhibited a maximum EQE of 14.2% and a maximum luminance of 34 262 cd m(-2). To the best of our knowledge, this is the highest luminance ever reported for FIr6-based PhOLEDs.

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.

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.

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.

Blue organic light-emitting diodes with low driving voltage and maximum enhanced power efficiency based on buffer layer MoO 3

Blue organic light-emitting devices based on wide bandgap host material, 2-( t-butyl)-9, 10-di-(2-naphthyl) anthracene (TBADN), blue fluorescent styrylamine dopant, p-bis(p- N, N-diphenyl-amino-styryl)benzene (DSA-Ph) have been realized by using molybdenum oxide (MoO 3) as a buffer layer and 4,7-diphenyl-1,10-phenanthroline (BPhen) as the ETL. The typical device structure used was glass substrate/ITO/MoO 3 (5 nm)/NPB (30 nm)/[TBADN: DSA-Ph (3 wt%)](35 nm)/BPhen (12 nm)/LiF (0.8 nm)/Al (100 nm). It was found that the MoO 3∥BPhen-based device shows the lowest driving voltage and highest power efficiency among the referenced devices. At the current density of 20 mA/cm 2, its driving voltage and power efficiency are 5.4 V and 4.7 Lm/W, respectively, which is independently reduced 46%, and improved 74% compared with those the m-MTDATA∥Alq 3 is based on, respectively. The J– V curves of ‘hole-only’ devices reveal that a small hole injection barrier between MoO 3∥NPB leads to a strong hole injection, resulting low driving voltage and high power efficiency. The results strongly indicate that carrier injection ability and balance shows a key significance in OLED performance.

A phosphine oxide derivative as a universal electron transport material for organic light-emitting diodes

A phosphine oxide based organic material, 2-diphenylphosphine oxide-spiro[fluorene-7,11′-benzofluorene] (SPPO2), was developed as a universal electron transport layer with a thermal stability and stable morphology for organic light-emitting diodes (OLEDs). The SPPO2 was synthesized by the reaction of 2-bromo-spiro[fluorene-7,11′-benzofluorene] with diphenyl phosphine chloride. SPPO2 showed a high glass transition temperature (Tg) of 118 °C and good film-forming properties. The SPPO2 reduced the driving voltage of the OLEDs irrespective of the energy level of the host materials due to efficient electron injection from the SPPO2 to host material. In particular, the SPPO2 reduced the driving voltage of the blue device of a wide bandgap host material by more than 3 V. Therefore, the SPPO2 can be used as a universal electron transport layer for OLEDs.

Hydroxynaphthyridine-Derived Group III Metal Chelates: Wide Band Gap and Deep Blue Analogues of Green Alq3 (Tris(8-hydroxyquinolate)aluminum) and Their Versatile Applications for Organic Light-Emitting Diodes

A series of group III metal chelates have been synthesized and characterized for the versatile application of organic light-emitting diodes (OLEDs). These metal chelates are based on 4-hydroxy-1,5-naphthyridine derivates as chelating ligands, and they are the blue version analogues of well-known green fluorophore Alq(3) (tris(8-hydroxyquinolinato)aluminum). These chelating ligands and their metal chelates were easily prepared with an improved synthetic method, and they were facially purified by a sublimation process, which enables the materials to be readily available in bulk quantity and facilitates their usage in OLEDs. Unlike most currently known blue analogues of Alq(3) or other deep blue materials, metal chelates of 4-hydroxy-1,5-naphthyridine exhibit very deep blue fluorescence, wide band gap energy, high charge carrier mobility, and superior thermal stability. Using a vacuum-thermal-deposition process in the fabrication of OLEDs, we have successfully demonstrated that the application of these unusual hydroxynaphthyridine metal chelates can be very versatile and effective. First, we have solved or alleviated the problem of exciplex formation that took place between the hole-transporting layer and hydroxynaphthyridine metal chelates, of which OLED application has been prohibited to date. Second, these deep blue materials can play various roles in OLED application. They can be a highly efficient nondopant deep blue emitter: maximum external quantum efficiency eta(ext) of 4.2%; Commision Internationale de L'Eclairage x, y coordinates, CIE(x,y) = 0.15, 0.07. Compared with Alq(3), Bebq(2) (beryllium bis(benzoquinolin-10-olate)), or TPBI (2,2',2''-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole), they are a good electron-transporting material: low HOMO energy level of 6.4-6.5 eV and not so high LUMO energy level of 3.0-3.3 eV. They can be ambipolar and possess a high electron mobility of 10(-4) cm(2)/V s at an electric field of 6.4 x 10(5) V/cm. They are a qualified wide band gap host material for efficient blue perylene (CIE(x,y) = 0.14, 0.17 and maximum eta(ext) 3.8%) or deep blue 9,10-diphenylanthracene (CIE(x,y) = 0.15, 0.06 and maximum eta(ext) 2.8%). For solid state lighting application, they are desirable as a host material for yellow dopant (rubrene) in achieving high efficiency (eta(ext) 4.3% and eta(P) 8.7 lm/W at an electroluminance of 100 cd/m(2) or eta(ext) 3.9% and eta(P) 5.1 lm/W at an electroluminance of 1000 cd/m(2)) white electroluminescence (CIE(x,y) = 0.30, 0.35).

Exciton Confinement and Triplet Harvesting for Efficient White Organic Light Emitting Diodes

A blue light emitting diode based on wide band gap host for a blue fluorescence light emitting layer has been developed to fabricate hybrid white organic light emitting diode comprising of a fluorescence blue and a phosphorescence red light emitting layers. We have tested various wide band gap host materials including CBP, BAlq, and TcTa, and among them the device with TcTa host has shown good performance which could be attributed to tight exciton confinement. A phosphorescence red dopant (RD) has been introduced into BAlq layer to harvest triplet states of the fluorescence blue emission layer and we have succeeded in obtaining the balanced emission of the blue and the red emissions with high efficiency. The device showed the maximum external quantum efficiency of 16% at 0.1 mA/cm2 and 13% of external quantum efficiency with (0.29, 0.23) of CIE coordinates at 10 mA/cm2.

Hole blocking layer free phosphorescent organic light-emitting diodes by using a charge confining structure

Phosphorescent organic light-emitting diodes without hole blocking layer were developed by using a charge confining device structure which can confine excitons within light-emitting layer. Narrow triplet bandgap host material was sandwiched between wide triplet bandgap host materials and excitons could be effectively confined in narrow bandgap host light-emitting layer. The charge confining devices showed excellent device performances even without hole blocking layer because excitons were confined inside light-emitting layer.