Electrophosphorescent Organic Light-emitting Diodes Research Articles

High efficiency electroluminescence devices using a series of Ir(III)-tetrazolate phosphors: Mechanisms for the drive current evolution of quantum yield

We demonstrate high-brightness and high-efficiency blue-green to yellow-green electrophosphorescent organic light-emitting diodes employing a series of organic Ir complexes [Ir-(C∧N)2(N∧N)]. Three different complexes have been synthesized showing high photoluminescence solid blend efficiencies up to 44%. A low current density increase of the electroluminescence (EL) external quantum efficiency (φEL(ext)) is observed and a maximum of φEL(ext)=10.6%±0.8% photon/e and power efficiency η=27±2 lm/W are achieved at a current density of j=0.01 mA/cm2. We examine various electronic processes that underlie a nonmonotonous current density dependence of the EL quantum efficiency of electrophosphorescent light-emitting diodes. The shape of φEL(ext) versus j is shown to reflect a trade off between electron-hole encounter and charge carrier transit times, electric field effect on electron-hole pair dissociation time, and current driven triplet molecular exciton lifetime.

Phosphine-Oxide-Containing Bipolar Host Material for Blue Electrophosphorescent Devices

We report highly efficient blue electrophosphorescent organic light-emitting diodes (OLEDs) incorporating a bipolar host, 2,7-bis(diphenylphosphine oxide)-9-(9-phenylcarbazol-3-yl)-9-phenylfluorene (PCF), doped with iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2′]picolinate (FIrpic). PCF, which contains diphenylphosphine oxide groups appended onto a carbazole/fluorene hybrid, displays both electron- and hole-transporting characteristics, resulting in a low turn-on voltage (2.6 V) and greatly improved power efficiencies. In addition, the sterically hindered structure of PCF provides a compatible environment for the FIrpic dopant, alleviating concentration quenching of the phosphor at high doping levels. The device doped with 28 wt % FIrpic exhibited maximum EL efficiencies of 30.8 cd/A and 26.2 lm/W (at 121 cd/m2). Even at a high brightness of 1000 cd/m2, the efficiencies remained high (26.9 cd/A and 19.6 lm/W).

Efficient red electrophosphorescent organic light-emitting diodes based on the new sensitized heteroleptic tris-cyclometalated Ir(III) complexes

Organic light-emitting device (OLED) was fabricated using the novel red phosphorescent heteroleptic tris-cyclometalated iridium complex, bis(2-phenylpyridine)iridium(III)[2(5′-methylphenyl)-4-diphenylquinoline] [Ir(ppy) 2(dpq–5CH 3)], based on 2-phenylpyridine (ppy) and 2(5′-methylphenyl)-4-diphenylquinoline (dpq–5CH 3) ligand. Generally, the ppy ligand in heteroleptic iridium complexes plays an important role as “sensitizer” in the efficient energy transfer from the host (CBP; 4,4, N, N′-dicarbazolebiphenyl) to the luminescent ligand (dpq–5CH 3). We demonstrated that high efficiency through the “sensitizer” can be obtained, when the T 1 of the emitting ligand is close to T 1 of the sensitizing ligand. The device containing Ir(ppy) 2(dpq–5CH 3) produced red light emission of 614 nm with maximum luminescence efficiency and power efficiency of 8.29 cd/A (at 0.09 mA/cm 2) and 5.79 lm/W (at 0.09 mA/cm 2), respectively.

Phosphorescence Color Tuning by Ligand, and Substituent Effects of Multifunctional Iridium(III) Cyclometalates with 9‐Arylcarbazole Moieties

The synthesis, isomeric studies, and photophysical characterization of a series of multifunctional cyclometalated iridium(III) complexes containing a fluoro- or methyl-substituted 2-[3-(N-phenylcarbazolyl)]pyridine molecular framework are presented. All of the complexes are thermally stable solids and highly efficient electrophosphors. The optical, electrochemical, photo-, and electrophosphorescence traits of these iridium phosphors have been studied in terms of the electronic nature and coordinating site of the aryl or pyridyl ring substituents. The correlation between the functional properties of these phosphors and the results of density functional theory calculations was made. Arising from the propensity of the electron-rich carbazolyl group to facilitate hole injection/transport, the presence of such a moiety can increase the highest-occupied molecular orbital levels and improve the charge balance in the resulting complexes relative to the parent phosphor with 2-phenylpyridine ligands. Remarkably, the excited-state properties can be manipulated through ligand and substituent effects that allow the tuning of phosphorescence energies from bluish green to deep red. Electrophosphorescent organic light-emitting diodes (OLEDs) with outstanding device performance can be fabricated based on these materials, which show a maximum current efficiency of approximately 43.4 cd A(-1), corresponding to an external quantum efficiency of approximately 12.9 % ph/el (photons per electron) and a power efficiency of approximately 33.4 Lm W(-1) for the best device. The present work provides a new avenue for the rational design of multifunctional iridium-carbazolyl electrophosphors, by synthetically tailoring the carbazolyl pyridine ring that can reveal a superior device performance coupled with good color-tuning versatility, suitable for multicolor-display technology.

Efficient single layer solution-processed blue-emitting electrophosphorescent devices based on a small-molecule host

We report highly efficient single layer solution-processed blue small-molecule electrophosphorescent organic light-emitting diodes with iridium (III) bis[2-(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) doped into a wide-gap 9,9-bis[4-(3 c6-di-tert-butylcarbazol-9-yl)phenyl]fluorene (TBCPF) as the emission layer. An optimized device with 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxidiazolyl]phenylene as the electron-transporting component doped into the emission layer, with a blue electrophosphorescence peaked at 474nm from FIrpic, shows a maximum luminance efficiency of 12.7cd∕A at 190A∕m2 and brightness of 19900cd∕m2 at 20V. We attribute the high performance to the excellent film forming ability and high triplet energy of TBCPF.

Electroluminescence of an iridium complex phosphorescent material doped polymeric system

With a novel iridium complex (pbi)2Ir(acac) doped into carbazole copolymer, polymer doped electrophosphorescent organic light-emitting diodes with the structure of indium-tin oxide(ITO)/poly(N-vinylcarbazole) (PVK):(pbi)2Ir(acac) (x)/2,9-dimethyl-4,7-diphenyl-1,10-phenan throline (BCP) (20nm)/8-Hydroxyquinoline aluminum(Alq3) (10nm)/Mg:Ag were fabricated. The photoluminescent (PL) and electroluminescent (EL) characteristics of the polymer doped system were investigated at the low doping concentrations of 0.1% and 0.5%, respectively. The results demonstrate that the luminescent spectra with different intensities of PVK and (pbi)2Ir (acac) co-existed in the PL and EL spectra of blend system, which was ascribed to an incomplete energy transfer process and direct charge trapping in the electroluminescence process. The 0.1% (pbi)2Ir(acac) doped device achieved a white light emission with the commissions internationale de 1'eclairage (CIE) coordinates of (0.32, 0.38) when the bias voltage was 19V; the 0.5% (pbi)2Ir(acac) doped device had a maximum luminance of 11827 cd·m-2 and a maximum luminance efficiency of 4.13 cd·A-1 corresponding to a bias of 20.6V and 13.4V, respectively.

Iridium(III) Complexes Bearing Quinoxaline Ligands with Efficient Red Luminescence Properties

Abstract Newly designed cyclometalated iridium(III) complexes bearing 2,3-diphenylquinoxalines, which were shown to be highly efficient and pure-red emitting materials for electrophosphorescent organic light-emitting diodes (OLEDs), were synthesized and characterized. Excellent quantum efficiencies for photoluminescence within the range of 50–79% were attained in dichloromethane solution at room temperature. Luminescence peak wavelengths of the complexes were within the preferred range of 653–671 nm in thin films. The most vivid red electrophosphorescence with CIE chromaticity coordinates x = 0.70 and y = 0.28, which are the closest to the ultimate limit of pure red among reported OLEDs, was achieved with the acetylacetonatoiridium complex bearing 2,3-diphenylquinoxaline.

Optimization of the Organic Lightemitting Diodes with a Red Phosphor

We demonstrate very high efficiency electro phosphorescence in organic light-emitting diodes (OLEDs) employing a phosphorescent molecule doped into a conductive host material. Electrophosphorescent OLEDs were fabricated with bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetylacetonate) [btp2Ir(acac)] as a pure red phosphor which has photoluminescence spectrum centered at 615 nm. The device structure was as follows; ITO/2-TNATA/NPB/btp2Ir(acac) doped in host/BCP/Alq3/Liq/Al. CBP, BCP and Alq3 were used as a host, and the concentration of btp2Ir(acac) was varied from 8 to 11%. The device with 11% btp2Ir(acac) doped in Alq3 showed saturated electroluminescence (EL) peak at 615 nm and an efficiency of 4.76 cd/A with an initial brightness of 1100 cd/m2. The CIE coordinates of the device showed x = 0.67, y = 0.32 at a current density of 1 mA/cm2. The photoluminescence spectrum of Alq3 centered at 510 nm is sufficiently overlap with absorption of dopant, therefore, energy transfer from host to guest is efficient.

Triplet Energy Back Transfer in Conjugated Polymers with Pendant Phosphorescent Iridium Complexes

The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent iridium complex 13 and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescent organic light-emitting diodes. Red-emitting phosphorescent iridium complexes based on the [Ir(btp)(2)(acac)] fragment (where btp is 2-(2'-benzo[b]thienyl)pyridinato and acac is acetylacetonate) have been attached either directly (spacerless) or through a -(CH(2))(8)- chain (octamethylene-tethered) at the 9-position of a 9-octylfluorene host. The resulting dibromo-functionalized spacerless (8) or octamethylene-tethered (12) fluorene monomers were chain extended by Suzuki polycondensations using the bis(boronate)-terminated fluorene macromonomers 16 in the presence of end-capping chlorobenzene solvent to produce the statistical spacerless (17) and octamethylene-tethered (18) copolymers containing an even dispersion of the pendant phosphorescent fragments. The spacerless monomer 12 adopts a face-to-face conformation with a separation of only 3.6 A between the iridium complex and fluorenyl group, as shown by X-ray analysis of a single crystal, and this facilitates intramolecular triplet energy transfer in the spacerless copolymers 17. The photo- and electroluminescence efficiencies of the octamethylene-tethered copolymers 18 are double those of the spacerless copolymers 17, and this is consistent with suppression of the back transfer of triplets from the red phosphorescent iridium complex to the polyfluorene backbone in 18. The incorporation of a -(CH(2))(8)- chain between the polymer host and phosphorescent guest is thus an important design principle for achieving higher efficiencies in those electrophosphorescent organic light-emitting diodes for which the triplet energy levels of the host and guest are similar.

Pure red electrophosphorescent organic light-emitting diodes based on a new iridium complex

A new iridium complex was synthesized and demonstrated a saturated red light emission in organic light-emitting diodes (OLEDs). The maximum brightness of 2800 cd/m 2 and the external quantum efficiency of 5.5% were achieved in multilayer OLEDs. The peak wavelength of the emission was found to be at 677 nm with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.71, 0.27).

Characteristics of emissive bilayers in electrophosphorescent organic light-emitting diodes

New electrophosphorescent organic light-emitting diodes were fabricated by polymer/Ir(ppy)3 bilayers. Already several groups have reported polymer systems that show efficient energy transfer from a polymer host to an organometallic guest. We have fabricated a phosphorescent polymer light-emitting device with fac tris(2-phenylpyridine)iridium [Ir(ppy)3] as a triplet emissive layer on a poly(vinylcarbazole) (PVK) layer. The emission spectrum of the bilayer device exhibited pure Ir(ppy)3 emission, which indicates that the efficient energy transfer from PVK layer to Ir(ppy)3 layer. Our bilayer device system shows a lower driving voltage and a higher current density than those of the general blended device. � 2005 Elsevier B.V. All rights reserved. PACS: 85.60.J

Red Phosphorescent Organic Light-Emitting Diodes Using Mixture System of Small-Molecule and Polymer Host

We study two types of polymer materials, poly(n-vinylcarbazole) (PVK) and starburst small-molecule 1,3,5-tris[4-(diphenylamino)phenyl]benzene (TDAPB), as the host for electrophosphorescent organic light-emitting diodes (PHOLEDs) doped with red-emitting phosphor tris(1-phenylisoquinoline) iridium (III) [Ir(piq)3]. PHOLEDs employed a PVK and TDAPB blend as the host exhibited a maximum red light emission at a wavelength of 630 nm. The external quantum efficiency of 6.3% and power efficiency of 3.0 lm/W have been achieved. The electroluminescent (EL) spectra were not changed at high current and the luminance reached 8,800 cd/m2 at the voltage of 13 V. We found that the roughness of the surface as estimated by means of atomic force microscopy (AFM) smoothened with increasing TDAPB doping concentration, and a slight exciplex emission between TDAPB and an electron transport material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4,-oxadiazole (PBD) was observed at the wavelength of approximately 530 nm.

P-141: High-Efficiency and Long Lifetime Electrophosphorescent Organic Light-Emitting Diodes with Improved Hole-Electron Balance by using Alternate Multilayer Structures

Electrophosphorescent organic light-emitting diodes (PHOLEDs) with alternate multilayer structures, which consist of N, N′-bis-(1-naphthyl)-N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) and 5,6,11,12-tetraphenylnaphthacene (rubrene) as hole-transporting layers have been fabricated. The effect of the alternate multilayer structures upon ther performance of PHOLEDs with fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) doped into 4,4′-N, N′-dicarbazole-biphenyl (CBP) as the emitting layer has been investigated. Compared with the conventional diode only with NPB as hole-transporting layer, high efficiency and long lifetime diodes have been obtained with the alternate structures. Such an improvement in the device performance was attributed to the improved hole-electron balance, which can be further attributed to the introduction of the alternate multilayer structure into PHOLEDs.

High-efficiency and low-voltage p-i-n electrophosphorescent organic light-emitting diodes with double-emission layers

We demonstrate high-efficiency organic light-emitting diodes by incorporating a double-emission layer (D-EML) into p-i-n-type cell architecture. The D-EML is comprised of two layers with ambipolar transport characteristics, both doped with the green phosphorescent dye tris(phenylpyridine)iridium. The D-EML system of two bipolar layers leads to an expansion of the exciton generation region. Due to its self-balancing character, accumulation of charge carriers at the outer interfaces is avoided. Thus, a power efficiency of approximately 77 lm∕W and an external quantum efficiency of 19.3% are achieved at 100cd∕m2 with an operating voltage of only 2.65 V. More importantly, the efficiency decays only weakly with increasing brightness, and a power efficiency of 50 lm∕W is still obtained even at 4000cd∕m2.

Ultrahigh Energy Gap Hosts in Deep Blue Organic Electrophosphorescent Devices

Four ultrahigh energy gap organosilicon compounds [diphenyldi(o-tolyl)silane (UGH1), p-bis(triphenylsilyl)benzene (UGH2), m-bis(triphenylsilyl)benzene (UGH3), and 9,9‘-spirobisilaanthracene (UGH4)] were employed as host materials in the emissive layer of electrophosphorescent organic light-emitting diodes (OLEDs). The high singlet (∼4.5 eV) and triplet (∼3.5 eV) energies associated with these materials effectively suppress both the electron and energy transfer quenching pathways between the emissive dopant and the host material, leading to deep blue phosphorescent devices with high (∼10%) external quantum efficiencies. Furthermore, by direct charge injection from the adjacent hole and electron transport layers onto the phosphor doped into the UGH matrix, exciton formation occurs directly on the dopant, thereby eliminating exchange energy losses characteristic of guest−host energy transfer. We discuss the material design, and present device data for OLEDs employing UGHs. Among the four host materials, UGH2 and UGH3 have higher quantum efficiencies than UGH1 when used in OLEDs. Rapid device degradation was observed for the UGH4-based device due to electro- and/or photooxidation of the diphenylmethane moiety in UGH4. In addition to showing that UGH materials can be used to fabricate efficient blue OLEDs, we demonstrate that very high device efficiencies can be achieved in structures where the dopant transports both charge and excitons.

Fabrication and Characteristics of Increased Efficiency of Layered Polymeric Electroluminescent Diodes

We fabricated polymer electrophosphorescent organic light-emitting diodes (PHOLEDs) with a layered structure using water-soluble materials and a poly(n-vinylcarbazole) (PVCz) host doped with fac-tris(2-phenylpyridyl)Ir(III) (Ir(ppy)3). The 3-layered structure of PHOLEDs exhibit emission from Ir(ppy)3 (the emission maximum was observed at 520 nm) and show a significantly improved current efficiency of 21.0 cd/A and a luminous power efficiency of 5.2 lm/W (peak external quantum efficiency of about 6.0% has been realized compared to the conventional PHOLEDs with 2-layered structure. Furthermore, the 4-layered structure of PHOLEDs using tris(8-hydroxyquinoline)aluminum (Alq3) as electron transport layers exhibited a current efficiency of 28.1 cd/A and a luminous power efficiency of 6.0 lm/W (peak external quantum efficiency of about 8.0%). Our results suggest that the layered structure using water-soluble materials improves device performances and is effective for facilitating the fabrication by a wet process.

High-Efficiency Organic Electrophosphorescent Diodes Using 1,3,5-Triazine Electron Transport Materials

This paper reports on novel electron transport materials, 1,3,5-triazine derivatives (TRZ), which have been useful in high-efficiency electrophosphorescent (EP) organic light-emitting diodes (OLEDs). We synthesized four 2,4,6-tris(diarylamino)-1,3,5-triazine derivatives (TRZ1−TRZ4) that had electron-donating substituents and examined their OLED characteristics. Of these, we found that TRZ 2, 3, and 4 function as a host for tris(2-phenylpyridine)iridium (Ir(ppy)3) and, in particular, 2,4,6-tris(carbazolo)-1,3,5-triazine (TRZ2) demonstrated a very high external electroluminescent (EL) quantum efficiency (ηext) of ∼10.2 ± 1.0% and an energy conversion efficiency (ηenergy) of 14.0 ± 2.0 lm/W. Detailed transient photoluminescent (PL) measurement revealed that the triplet energy level of TRZ2 (ET1 = 2.81 eV) is higher than that of conventional 4,4‘-N,N‘-dicarbazol-biphenyl (CBP) (ET1 = 2.56 eV), suggesting that TRZ2 has excellent capabilities in confining Ir(ppy)3 triplet excitons.

High-efficiency low-voltage stable inverted transparent electrophosphorescent organic light-emitting diodes: combining electrically doped carrier transport layers and iridium-complex doped emissive layer

We demonstrated high-efficiency low-voltage stable inverted transparent clectrophosphorescent organic light-emitting diodes employing an indium-tin-oxide coated glass substrate directly as cathode and a semitransparent top Au thin film as anode. The structure contains an iridium-complex doped emissive layer sandwiched in between n- and p-doped charge transport layer with appropriate blocking layers to form a nip structure. The devices are about 50% transparent and emit green light from both sides with peak external quantum efficiency (EQE) of 4.08% (14.3 cd/A). At 100 cd/m 2 , the EQE is 2.8% (13 cd/A) at an operating voltage of 4.3 V. The devices exhibit a lifetime of above 50 hours under continuous constant-current driving for the initial luminance of about 9000 cd/m 2 in vacuum, which project a lifetime of ∼5000 hours for 100 cd/m 2 .

Organic optical bistable switch

We demonstrate an organic optical bistable switch by integrating an efficient organic photodetector on top of a transparent electrophosphorescent organic light-emitting diode (TOLED). The bistability is achieved with an external field-effect transistor providing positive feedback. In the “LOW” state, the TOLED is off and the current in the photodetector is solely its dark current. In the “HIGH” state, the TOLED emits light that is directly coupled into the integrated photodetector through the transparent cathode. The photocurrent then is fed back to the TOLED, maintaining it in the HIGH state. The green electrophosphorescent material, fac tris(2-phenylpyridine) iridium [Ir(ppy)3] doped into a 4,4′-N,N′-dicarbazole-biphenyl host was used as the luminescent material in the TOLED, while alternating thin layers of copper phthalocyanine and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole were used as the active region of the organic photodetector. The circuit has a 3 dB bandwidth of 25 kHz, and can be switched between HIGH and LOW using pulses as narrow as 60 ns. The bistable switch can be both electrically and optically reset, making it a candidate for image-retaining displays (e.g., electronic paper) and other photonic logic applications. The integrated organic device also has broad use as a linear circuit element in applications such as automatic brightness control.

High-efficiency electrophosphorescent organic light-emitting diodes with double light-emitting layers

We demonstrate high-efficiency electrophosphorescent organic light-emitting diodes (PHOLEDs) with double light-emitting layers (D–EMLs) by doping both hole and electron transport hosts with fac tris(2-phenylpyridine)iridium [Ir(ppy)3] simultaneously. The D–EMLs PHOLEDs show significantly improved efficiency (peak external quantum efficiency of about 12.6%, corresponding to a current efficiency of 44.3 cd/A) compared to the conventional PHOLEDs with a single EML and either hole or electron transport host doped with Ir(ppy)3. We attribute this improvement mainly to reduced losses of triplet excitons into regions that are not doped by phosphorescent emitter molecules.