Electrophosphorescent Devices Research Articles

Simple Bipolar Hosts with High Glass Transition Temperatures Based on 1,8‐Disubstituted Carbazole for Efficient Blue and Green Electrophosphorescent Devices with “Ideal” Turn‐on Voltage

Two hybrids based on 1,8-disubstituted carbazole, 1,8-OXDCz and 1,8-mBICz, have been designed and synthesized through a facile process. The incorporation of oxadiazole or N-phenylbenzimidazole moieties at the 1,8-positions of carbazole greatly improves its morphological stability, giving glass transition temperatures (T(g)) as high as 138 and 154 °C, respectively. Blue phosphorescent organic light-emitting devices (PhOLEDs) with 1,8-mBICz exhibit almost the same performance as a similarly structured device based on the mCP host, and green PhOLEDs employing the new host material 1,8-OXDCz exhibit an ideal turn-on voltage (2.5 V at 1.58 cd m(-2)), a maximum current efficiency (η(c,max)) of 73.9 cd A(-1), and a power efficiency (η(p,max)) of 89.7 lm W(-1). These results are among the best performances of [Ir(ppy)(3)]-based devices with simple device configurations.

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.

Highly Efficient Electron‐Transporting/Injecting and Thermally Stable Naphthyridines for Organic Electrophosphorescent Devices

AbstractA series of 1,8‐naphthyridine derivatives is synthesized and their electron‐transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light‐emitting device (OLED) using fac‐tris(2‐phenylpyridine)iridium [Ir(ppy)3] as a green phosphorescent emitter doped into a 4,4′‐N,N′‐dicarbazolebiphenyl (CBP) host with 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (a‐NPD) as the hole‐transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4‐bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole‐injecting layer. The turn‐on voltage of the device is 2.5 V using 2,7‐bis[3‐(2‐phenyl)‐1,8‐naphthyridinyl]‐9,9‐dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole‐blocking (HB) and exciton‐blocking layer of bathocuproine (BCP), 13.2–13.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.2–54.5 lm W−1 are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass‐transition temperatures much higher than that of aluminum(III) bis(2‐methyl‐8‐quinolinato)‐4‐phenylphenolate (BAlq), and decomposition temperatures of 510–518 °C, comparable to or even higher than those of Alq3.

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.

2-Phenyl-1,2,3-benzotriazole Ir(III) complexes with additional donor fragment for single-layer PhOLED devices

Abstract Phosphorescent bis-cyclometallated iridium (III) complexes based on 2-phenyl-1,2,3-benzotriazole with additional donoric diphenylamine or carbazole moieties were synthesized, studied and tested as phosphorescent materials in the single-layer phosphorescent organic light emitting diodes. These single-layer devices were fabricated employing a simple technological approach, based upon the simultaneous vacuum deposition from one crucible of host material and the corresponding phosphorescent 1,2,3-triazole-based iridium complexes. Red and orange electrophosphorescent single-layer devices with current efficiencies of 5.3 cd/A and 6.8 cd/A respectively are reported. Results of the impedance spectroscopy suggest that the proposed method of formation of the working layer provides homogeneous distribution of molecular guest in the matrix of host.

Influence of molecular configuration and functional substituents on excited state energy levels in two naphthyl-based phosphine oxide hosts

Two phosphine oxide hosts, 1,8-bis(diphenylphosphoryl)naphthalene (NAPO) and 2,2′-bis(diphenylphosphoryl)-1,1′-binaphthyl (BiNAPO), were designed and synthesized to investigate the influence of molecular configuration and the density of functional substituents on the excited energy levels. With the much bigger conjugated area, BiNAPO has the T1 of 2.46eV, which is only 0.17eV less than that of naphthalene. This should be owing to the orthogonal configuration of BiNAPO. Meanwhile, NAPO with the higher density of functional substituents has the much lower T1 of 2.26eV. It is showed that shortening the conjugated length by tuning molecular configuration and reducing the density of functional substituents can effectively remain T1 of the chromophore. Simultaneously, Gaussian simulation showed that the larger conjugated area and the rational distribution of functional substituents endow BiNAPO with more balanced carrier injecting/transporting ability, which was further proved by nominal singly-carrier-only devices. The low-voltage driving and efficient green-emitting electrophosphorescent devices based on BiNAPO were demonstrated.

Efficient Near-Infrared-Emitting Cationic Iridium Complexes as Dopants for OLEDs with Small Efficiency Roll-off

Two near-infrared- (NIR-) emitting cationic iridium(III) complexes, [Ir(pbq-g)2(Bphen)]+PF6– (1) and [Ir(mpbqx-g)2(Bphen)]+PF6– (2), were synthesized and characterized, where pbq-g, mpbqx-g, and Bphen represent phenylbenzo[g]quinoline, 2-methyl-3-phenylbenzo[g]quinoxaline, and 4,7-diphenyl-1,10-phenanthroline, respectively. By employing sp2-hybridized N opposite the chelating N atom in the cyclometalated ligand, we succeeded in expanding the emission of iridium complexes with simple structures to the truly NIR region of the spectrum. This subtle structural adjustment significantly lowered the LUMOs of the iridium complexes, leading to a 60–80-nm emission red shift of complex 2 relative to complex 1. Based on these solution-processable phosphors, NIR organic light-emitting devices (OLEDs) were fabricated with the emission covering the range 690–850 nm. Compared with common OLEDs, these NIR-emitting electrophosphorescent devices demonstrate exclusive small efficiency roll-off with increasing current density...

Efficient deep blue electrophosphorescent devices based on platinum(II) bis(n-methyl-imidazolyl)benzene chloride

Highly efficient deep blue phosphorescent light emitting diodes were developed using a newly synthesized series of blue emitting tridentate platinum emitters. Devices employing a cohost of hole and electron transport materials yielded high external quantum efficiencies with low turn on voltage and low efficiency roll off. A maximum EQE of 15.7% and CIE coordinates of (0.16,0.13) was achieved in a device based on platinum(II) bis(N-methyl-imidazolyl)benzene chloride (Pt-16).

The effect of carrier injection and transport of hosts on organic electrophosphorescent device

Recently the organic electrophosphorescent host materials have attracted increasing attention. The selection principle of suited host materials is important to obtain high efficient electrophosphorescence. In this paper, we report the effect of carrier injection and transport properties of hosts to organic electrophosphorescent devices. A system, consisting of a red phosphorescent [Ru(4,7-Ph2-phen)3]2+ dopant and two blue-emitting hosts THPH and TCPC introducing carbazole substituent groups, was studied. Both the hosts showed similar electronic properties, forming film ability and energy transfer degree to dopant, however, EL efficiency with TCPC host significantly exceeds that with THPH host. The Cyclic Voltammogram curves and single carrier devices of hosts TCPC and THPH show that TCPC with carbazole groups possesses stronger hole injection and transport ability, and this induced TCPC is more suited host than THPH. This result demonstrates our previous study that the electrochemical properties of dopant [Ru(4,7-Ph2-phen)3]2+ molecule, which reflects the carrier trapping ability of guest, may be a basic design criterion for the selection of host.

Synthesis of Dibenzothiophene-Containing Ladder Polysilsesquioxane as a Blue Phosphorescent Host Material

A ladder polysilsesquioxanes with side chain of dibenzothiophene groups (BS-LPSQ) was successfully synthesized. The ladder structure of BS-LPSQ was characterized by MALDI-TOF MS, XRD, and (1)H NMR spectroscopy. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), atomic force microscopy (AFM), and spectroscopic analyses revealed that the BS-LPSQ has good film-forming ability, high thermal and morphological stability, and good miscibility to the dopant iridium bis(4,6-difluorophenyl)pyridinato-N,C(2)-picolinate (FIrpic), high triplet energy, and a wide bandgap. In addition, compared with the ringed polysiloxane BS-PSQ phosphorescent host material reported previously, the ladder structure of BS-LPSQ has not only a higher thermal resistance, but also could prevent molecular aggregation and effectively avoid quenching of fluorescence. Thus, the BS-LPSQ may be used as a better host for the blue-light-emitting iridium complex FIrpic. The performance of the electrophosphorescent device, based on the ladder BS-LPSQ as the active layer, is superior to that of ringed BS-PSQ and any other polyhedral oligomeric silsesquioxane (POSS)-based or polymer host materials.

New dibenzofuran/spirobifluorene hybrids as thermally stable host materials for efficient phosphorescent organic light-emitting diodes with low efficiency roll-off

A new class of host materials DBFSF (DBFSF2 and DBFSF4) is facilely synthesized through a Suzuki coupling reaction between dibenzofuran and spirobifluorene. Their thermal, electrochemical, electronic absorption and photoluminescent properties are fully investigated. High glass transition temperatures (T(g)) of 115 °C and 124 °C are observed for DBFSF2 and DBFSF4, respectively, due to the introduction of bulky spirobifluorene groups. As expected, the DBFSF4 with a twisted-linkage exhibits higher triplet energy than DBFSF2 and can be used in blue and green phosphorescent OLEDs. Electrophosphorescent devices with DBFSF2 and DBFSF4 as hosts were fabricated. Besides the good current efficiencies of 22.2 cd A(-1) for blue and 64.4 cd A(-1) for green, low efficiency roll-off has also been achieved for both devices.

Peripheral modification of 1,3,5-triazine based electron-transporting host materials for sky blue, green, yellow, red, and white electrophosphorescent devices

A systematic comparison of the physical properties and the dual-role (host and electron transport) applications of star-shaped 1,3,5-triazine-based ET-type hosts (T2T, 3N-T2T, 3P-T2T, and oCF3-T2T) with differential peripheral groups was reported. The introduction of N-heterocyclic polar peripheries onto a 1,3,5-triazine core gave evident benefits to the electron injection/transport properties, rendering efficient PhOLEDs with a simpler device configuration feasible. Among these hosts, 3P-T2T can serve both as a promising host and electron-transport material for various Ir-based electrophosphorescence devices. These PhOLEDs configured with the same device structure exhibited low operation voltages with a maximum ηext of 8%, 15.7%, 16.9%, 16.4% and 10.8% for sky blue (FIrpic), green [(PPy)2Ir(acac)], yellow [(Bt)2Ir(acac)], red [(Mpq)2Ir(acac)] and white [FIrpic + 0.5 wt%(Mpq)2Ir(acac)], respectively.

New rhenium(i) complexes with substituted diimine ligands for highly efficient phosphorescent devices fabricated by a solution process

A series of new carbonyl rhenium(I) complexes chelated by a substituted 1,10-phenanthroline ligand with the general formula Re(CO)3LCl, where L = 2-(2′-methoxyphenyl)-1,10-phenanthroline (L11), 2-(4′-methoxyphenyl)-1,10-phenanthroline (L22) and 2-(4′-diphenylaminophenyl)-1,10-phenanthroline (L33), have been systemically synthesized. The molecular structure of complex 1 was determined by single crystal X-ray diffraction studies, showing that the complex adopts a distorted octahedral geometry. The electrochemical, photophysical, and thermal properties, as well as the electroluminescent behaviors of three rhenium(I) complexes, were investigated. The solution processable complex 1, 2 or 3 was used as a yellow emitting dopant to fabricate electrophosphorescent devices with a polymer host. The device based on complex 3 exhibits a maximum current efficiency of 12.2 cd A−1 and a peak brightness in excess of 7300 cd m−2, respectively. Even at a high luminance of 5000 cd cm−2 with current density of 81 mA cm−2, the current efficiency of this device remains as high as 6.4 cd A−1. These results represent the best values reported for electophosphorescent devices based on solution processable rhenium(I) complexes.

Incorporation of a CN group into mCP: a new bipolar host material for highly efficient blue and white electrophosphorescent devices

A novel bipolar host material mCPCN has been designed and synthesized by incorporating the electron-accepting CN group into the well-known benchmark host material mCP. Compared to mCP, the incorporation of the simple and small CN group significantly improves thermal/morphological stabilities (Tg = 97 °C and Td = 313 °C) and increases the electron affinity, while keeping electronic transition energies unaltered and maintaining a high triplet energy (ET = 3.03 eV). Characteristics of single-carrier devices containing mCPCN indicate its rather balanced hole/electron injection and transport properties. Highly efficient blue phosphorescent organic light emitting devices (PhOLEDs) with maximum external quantum, current and power efficiencies of 26.4%, 58.6 cd A−1, and 57.6 lm W−1, respectively, were achieved using mCPCN as the bipolar host material and bis[(4′,6′-difluorophenyl)pyridinato-N,C2′]iridium(III) picolinate (FIrpic; ET = 2.65 eV) as the triplet emitter. Furthermore, blue PhOLEDs adopting mCPCN exhibit impressively low efficiency roll-offs, retaining high quantum efficiencies of ∼25% at 1000 cd m−2 and ∼20% even at 8000 cd m−2. mCPCN has also been successfully used in implementing highly efficient white PhOLEDs having external quantum efficiencies in excess of 23%.

Bipolar material with spiro-fluorenyl terminals: synthesis, characterization and application for enhancement of electrophosphorescence

We have designed a novel bipolar material (FTzCz) consisting of spiro-fluorenyl terminals and a bipolar core to enhance emission efficiency of phosphorescent light-emitting diodes based on a conventional poly(9-vinylcarbazole) host and Ir(ppy)3 dopant. The core is composed of directly linked hole-transporting carbazolyl and electron-affinitive aromatic 1,2,4-triazolyl groups. The bipolar FTzCz was synthesized by the Suzuki coupling reaction and was well characterized. It exhibited not only good thermal stability due to its rigid and non-planar chemical structure, but also facilitated hole- and electron-affinities simultaneously. Blending the bipolar FTzCz with PVK significantly enhanced the performance of electrophosphorescent devices [ITO|PEDOT:PSS|(PVK + FTzCz):Ir(ppy)3 (4 wt%)|BCP (10 nm)|Ca (50 nm)|Al (100 nm)]. The maximum luminance and maximum luminance efficiency were enhanced from 3550 cd m−2 and 5.6 cd A−1 (neat PVK-based device) to 4510 cd m−2 and 9.9 cd A−1 (blend device with PVK : FTzCz = 8 : 2), respectively. Our results demonstrate the efficacy of the bipolar FTzCz in enhancing performance of electrophosphorescent devices.

Carbazole-based coplanar molecule (CmInF) as a universal host for multi-color electrophosphorescent devices

The synthesis, isomerism, photophysics and electrophosphorescent characterization of some functional cyclometallated iridium(III) complexes containing 2-[2-(N-phenylcarbazolyl)]pyridine and 2-[3-(N-phenylcarbazolyl)]pyridine molecular frameworks are described. A carbazole-based coplanar molecule (CmInF) obtained through the intramolecular ring closure of aryl substitutions at the C3 and C6 positions exhibits a high triplet energy (ET = 2.77 eV), morphological stability (Tg = 195 °C) and hole mobility in the range of up to 5 × 10−3 cm2 V−1 s−1. Highly efficient multi-color electrophosphorescent devices have been successfully achieved employing CmInF as the universal host material doped with phosphorescent dopants of various colors under the same device configuration of ITO/PEDOT:PSS (300 A)/TCTA (250 A)/CmInF: dopant (250 A)/TAZ (500 A)/LiF/Al (PEDOT:PSS = poly(ethylene dioxythiophene):polystyrene sulfonate; TCTA = 4,4′,4′′-tri(N-carbazolyl)triphenylamine; TAZ = 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole). Through the mixing of two phosphorescent dopants of complementary colors, we also fabricated a two-color white organic light-emitting device (WOLED) with the same device structure consisting of 12 wt% FIrpic and 0.3 wt% (Mpg)22Ir(acac) co-doped into CmInF as a single-emitting-layer, which exhibits peak WOLED efficiency of 13.4% (23.4 cd A−1) and 11.2 lm W−1 with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.37). In addition, the use of such device structure in full-color OLEDs has the advantages of simplifying manufacturing process and reducing production cost that are the critical issues of commercialization.

A 3-dimensional spiro-functionalized platinum(ii) complex to suppress intermolecular π–π and Pt⋯Pt supramolecular interactions for a high-performance electrophosphorescent device

A 3-D platinum(II)-based spirometal complex has been designed and synthesized to suppress aggregation and excimer emission. A prototype phosphorescent organic light-emitting device exhibits the high-performance orange emission with an external quantum efficiency of up to 5.2%.

A diarylborane-substituted carbazole as a universal bipolar host material for highly efficient electrophosphorescence devices

Recently bipolar phosphorescent host materials have attracted wide attention since they can achieve better charge balance and hence better device performance. In this work, we report the synthesis and physical properties of a novel bipolar host material containing the dimesityl borane/carbazole hybrid, CMesB. With a high triplet energy, CMesB is considered a promising universal host material and has been applied to phosphorescent OLEDs of various colors. Red/green/blue/white (RGBW) OLEDs based on CMesB all show high external quantum efficiencies (20.7% for red, 20.0% for green, 16.5% for blue, and 15.7% for white) at practical brightnesses. The results indicate that the bipolar host CMesB with high triplet energy has high potential in manufacturing RGBW OLEDs for display or lighting applications.

Solution-processed electrophosphorescent devices with a thin fluoropolymer at the hole transport interfacial layer

Electrophosphorescent devices with ionomer-type hole transport layers were investigated. On top of the 3,4-ethylenedioxy thiophene:poly(4-styrene sulfonate) [PEDOT:PSS] structures, fluoropolymer interfacial layers (FPIs) with different side chain lengths were introduced. Both for the PEDOT:PSS/FPI (layered) and PEDOT:PSS (mixed) structures with soluble phosphorescent emitters, the short-side-chain FPIs showed higher efficiency. The difference in the electrical properties of the two FPIs for bipolar (light-emitting) devices was not clear, but the hole-only device clearly showed the favored hole injection at the PEDOT:PSS/FPI structure with a shorter side chain, a copolymer of tetrafluoroethylene and sulfonyl fluoride vinyl ether.

Solution-Processed Electrophosphorescent Devices Based on Small-Molecule Mixed Hosts

Highly efficient electrophosphorescent organic light-emitting diodes (PHOLEDs) containing mixed hosts of bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2) and 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7) and a soluble derivative of the green emitter fac-tris(2-phenylpyridine)iridium (III) [(Ir(ppy)3 ] have been demonstrated. All organic layers were mixed in a single layer for solution processing during the fabrication of the PHOLEDs. The amorphous mixed host of SimCP2:OXD-7:Ir(ppy)3 exhibited ambipolar charge transport as well as high hole and electron mobilities on the order of 10-7 cm2 /Vs from space-charge-limited current measurement. The values of both the hole and electron mobilities were 100 times greater than that of the widely used poly(9-vinylcarbazole) (PVK):OXD-7:Ir(ppy)3 host, resulting in efficient charge balance in the SimCP2 host. Based on a simple fabrication process of single-layer PHOLED, a green device with the maximum luminous and power efficiencies of 42 cd/A and 20 lm/W, respectively, was obtained at 600 cd/m2.