Efficient Phosphorescent Organic Light Research Articles

Efficient Green Phosphorescent Organic Light Emitting Diode using Iridium Complex

We report here the fabrication and characterization of an efficient green phosphorescent organic light emitting diodes (PhOLED) using phosphorescent iridium complex, tris(2-phenylpyridine) iridium (III) (Ir(ppy)3). Ir(ppy)3 is an organic phosphorescent material, which emits green light via both the singlet as well as triplet exciton relaxation. To avoid triplettriplet annihilation and for efficient light emission, the phosphorescent emitters should be doped in a suitable host material which has band-gap larger than the doped guest material and the band gap of the guest should lie within the band-gap of the host. We optimally doped phosphorescent Ir(ppy)3 into a fluorescent 4,4'-bis(9-carbazolyl) biphenyl (CBP) host and used it as the emitter to prepare PhOLEDs. The PhOLED exhibited green electroluminescence (EL) in the range ~ 480-650 nm with EL peak at 550 nm along with a shoulder at 510 nm. The Commission Internationale de-I’eclairage (CIE) colour coordinate of the device was measured to be (0.25, 0.60). The current density vs voltage vs luminescence (J-V-L) characteristics of the device led us to calculate its current efficiency, which was found to be 28.9 cd/A at 7V. The efficiency of PhOLED decreased with increment in the applied voltage beyond 7 V and has been attributed to the triplet-triplet annihilation due to high injected current densities.

Novel benzothiadiazine 1,1-dioxide based bipolar host materials for efficient red phosphorescent organic light emitting diodes

Abstract In this work, three novel bipolar host materials TPA-SA, 3CBZ-SA and 4CBZ-SA have been designed and synthesized by incorporating triphenyl amine and carbazole as donor and benzothiadiazine 1,1-dioxide as an acceptor. These molecules exhibit moderately high triplet energies and bipolar carrier transport characteristics (ambipolarity) which is useful for the exothermic energy transfer to the dopants and also for the balanced carrier injection/transport in the emissive layers. These materials exhibited good performances in PhOLEDs and furnished external quantum efficiency in the range of 10.0–15.0%. Notably, a red phosphorescent device using TPA-SA as the host doped with Ir(pq)2(acac) exhibited a maximum EQE, power efficiency and current efficiency of 15.0%, 16.0 lm/W, and 25.3 cd A−1, respectively.

Solution-Processable Csp3-Annulated Hosts for High-Efficiency Deep Red Phosphorescent OLEDs.

In this report, a solution-processable cohost system incorporating N,N'-di(naphtalene-1-yl)-N,N'-diphenylbenzidine (NPB) and Csp3-annulated phenylquinoline derivatives, including spiro[indeno[1,2-b]quinoline-11,8'-indolo[3,2,1-de]acridine] (IAIQ), 10-phenyl-10H-spiro[acridine-9,11'-indeno[1,2-b]quinoline] (PAIQ) and 3,3'-(11H-indeno[1,2-b]quinoline-11,11-diyl)bis(N-phenyl-N-(m-tolyl)aniline) (m-TPA-DPIQ), is developed for highly efficient saturated red phosphorescent organic light emitting diodes (OLEDs). IAIQ, PAIQ, and m-TPA-DPIQ, designed with the increase of molecular flexibility, are systematically investigated. Solution-processable devices based on the efficient phosphorescent emitter bis[2-(3,5-dimethylphenyl)isoquinolinato](2,8-dimethyl-4,6-nonanedionato)Iridium [Ir(mpiq)2divm] are successfully fabricated, and give electroluminescent peaks at 634-636 nm with Commission Internationale de L'Eclairage coordinates of (0.70, 0.30). Under optimized conditions, the devices incorporating IAIQ, PAIQ, and m-TPA-DPIQ exhibit high external quantum efficiency with the maximum value at 25.1%, 23.4%, and 23.3%, respectively, and all exceeding 18% at the luminance of 1000 cd/m2. In application, the supersaturated red devices with excellent performance could facilitate the development of wet-made displays. The newly developed Csp3-annulated host materials with their excitonic properties also showoff the tactic to construct cohost system for high-quality phosphorescent OLEDs.

Inverted organic light-emitting devices using a charge-generation unit as an electron injector

Abstract Efficient green fluorescent and phosphorescent inverted organic light emitting devices (IOLEDs) with a non-doped charge generation unit (CGU) of NPB/MoO3/Al/LiF as an electron injector have been developed. It is found that the electrons are generated effectively at the NPB/MoO3 interface and injected from MoO3 to the electron transporting layer via assistant electron injection layers of Al/LiF. More importantly, the performance of the devices shows insensitivity to the work function of the used cathode. Under the optimal structure of NPB(10 nm)/MoO3(5 nm)/Al(3 nm)/LiF(1 nm), the green fluorescent IOLEDs present a maximum current efficiency (CEmax) of 3.5 cd/A. Furthermore, the green phosphorescent IOLEDs based on this CGU show a turn-on voltage of 3.8 V, a CEmax of 54.6 cd/A and a maximum external quantum efficiency of 15.8%. The results suggest that the introduction of non-doped CGU in IOLEDs provides a promising strategy to integrate IOLEDs with the n-channel thin film transistors for applications in active-matrix OLEDs.

Optimization of Transparent Organic Light-Emitting Diodes by Simulation-Based Design of Organic Capping Layers.

In this work, we use the transfer matrix method to optimize TPBi capping layers deposited on organic light emitting diodes with respect to light extraction and transmittance. The green transparent organic light emitting diodes comprise three organic semiconductors (CBP, Ir(ppy)₃ and TPBi) forming an efficient simplified phosphorescent organic light emitting diode stack. A transparent cathode of 2 nm Cs₂CO₃, 2 nm Al and 16 nm Au is deposited by thermal evaporation. The diode stack as well as the capping layer are deposited by organic vapor phase deposition. The refractive indices and extinction coefficients of all materials in the transparent organic light emitting diodes (glass, indium tin oxide, organic semiconductors and cathode) are determined using spectroscopic ellipsometry combined with optical transmittance and reflectance measurements. With these spectrally resolved data, we calculate the transmittance of transparent organic light emitting diodes with TPBi capping layers of different thicknesses. The results were validated with high accuracy in the visible spectral range and beyond (360 nm-1000 nm) by a series of experiments. By choosing a TPBi capping layer of optimized thickness (here 50 nm), we fabricated transparent organic light emitting diodes with an optical transmittance which was strongly enhanced from 47% (reference without capping layer) to 65%, measured at 555 nm.

Efficient red phosphorescent organic light emitting diodes based on solution processed all-inorganic cesium lead halide perovskite as hole transporting layer

Abstract An efficient red phosphorescent organic light emitting diode (PhOLED) has been realized by utilizing a composite hole transporting layer comprised of all-inorganic cesium lead halide perovskite CsPbBr 3 via spin-coating and 1,3-bis(9-carbazolyl) benzene (mCP) by vacuum depositing, in which CsPbBr 3 film is used as a hole transporting layer and mCP plays a dominant role in electron and exciton blocking. And this PhOLED shows a saturated red emission coordinated at CIE (0.65, 0.33) driven at 7.5 V, a maximum brightness of 20,750 cd/m 2 , and a maximum current efficiency of 10.64 cd/A, which is as 1.87 times as that 5.68 cd/A of the reference PhOLEDs based on traditional small organic molecular hole transporting material N , N ′-bis(naphthalen-1-yl)- N , N ′-bis(phenyl)-benzi (NPB). The electroluminescent (EL) spectra and the energy level alignment of different PhOLEDs are investigated. The enhanced EL performances are ascribed to improved hole injecting and transporting behaviors, and better electron and exciton confinements by introducing the composite hole transporting layer CsPbBr 3 /mCP.

A bipolar thiophene substituted 1,3,5-triazine host material for green phosphorescent OLEDs with low onset voltage and current efficiency roll-off

A novel bipolar host material containing 1,3,5-triazine, thiophene and triphenylamine moieties, 4,4′-(6-(thiophen-2-yl)-1,3,5-triazine-2,4-diyl)bis(N,N-diphenylaniline) (TTT), was used as a host for three IrIII complexes in fabricating high efficiency phosphorescent organic light emitting diodes (OLEDs) with low driving voltage. The onset voltages of these three OLEDs having the configuration ITO (indium tin oxides)/TAPC (1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane, 60 nm)/TTT: IrIII complex (92:8, 30 nm)/TPBi (2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole, 90 nm)/LiF (1 nm)/Al (100 nm), are between 2.9 V and 3.0 V, which is about 0.5 V less than the onset voltages of similar structural OLEDs with mCP as the host material. Maximum current and power efficiencies are more than 30.0 cd A−1 and 15.0 lm W−1, respectively, with no significant current efficiency roll-off. All the devices exhibit high efficiency at the high brightness of 10,000 cd m−2. The small roll-off is due to the improved charge balance and the wide charge recombination zone in the emissive layer of these devices.

Water‐Soluble Lacunary Polyoxometalates with Excellent Electron Mobilities and Hole Blocking Capabilities for High Efficiency Fluorescent and Phosphorescent Organic Light Emitting Diodes

High performance solution‐processed fluorescent and phosphorescent organic light emitting diodes (OLEDs) are achieved by water solution processing of lacunary polyoxometalates used as novel electron injection/transport materials with excellent electron mobilities and hole blocking capabilities. Green fluorescent OLEDs using poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐(1,4‐benzo‐{2,1′,3}‐thiadiazole)] (F8BT) as the emissive layer and our polyoxometalates as electron transport/hole blocking layers give a luminous efficiency up to 6.7 lm W−1 and a current efficiency up to 14.0 cd A−1 which remained nearly stable for about 500 h of operation. In addition, blue phosphorescent OLEDs (PHOLEDs) using poly(9‐vinylcarbazole) (PVK):1,3‐bis[2‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazo‐5‐yl]benzene (OXD‐7) as a host and 10.0 wt% FIrpic as the blue dopant in the emissive layer and a polyoxometalate as electron transport material give 12.5 lm W−1 and 30.0 cd A−1 power and luminous efficiency, respectively, which are among the best performance values observed to date for all‐solution processed blue PHOLEDs. The lacunary polyoxometalates exhibit unique properties such as low electron affinity and high ionization energy (of about 3.0 and 7.5 eV, respectively) which render them as efficient electron injection/hole blocking layers and, most importantly, exceptionally high electron mobility of up to 10−2 cm2 V−1 s−1.

Indenocarbazole based bipolar host materials for highly efficient yellow phosphorescent organic light emitting diodes

We report bipolar host materials with robust indenocarbazole and biphenyl moiety as hole-electron-transporting unit for phosphorescent yellow organic light-emitting diodes (OLEDs). New host materials demonstrated an excellent morphological stability with high glass transition temperature of 207 °C. Simultaneously, it also revealed appropriate triplet energy of about 2.6 eV for ideal triplet energy transfer to yellow phosphorescent dopant. A phosphorescent yellow OLED with new host ICBP1 (and ICBP2) and conventional yellow dopant iridium(III)bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (Ir(tptpy)2acac) shows a low driving voltage of 3.4 (and 3.6 V) at 1000 cd/m2, and maximum external quantum efficiency as high as 26.4%. Such efficient performance of phosphorescent yellow OLEDs is attributed to a good charge balance and high electron transport properties of host materials.

An Exciplex Forming Host for Highly Efficient Blue Organic Light Emitting Diodes with Low Driving Voltage

The exciplex forming co‐host with phosphorescent dopant system has potential to realize highly efficient phosphorescent organic light emitting didoes (PhOLEDs). However, the exciplex forming co‐host for blue phosphorescent OLEDs has been rarely introduced because of higher triplet level of the blue dopant than green and red dopants. In this work, a novel exciplex forming co‐host with high triplet energy level is developed by mixing a phosphine oxide based electron transporting material, PO‐T2T, and a hole transporting material, N,N′‐dicarbazolyl‐3,5‐benzene (mCP). Photo‐physical analysis shows that the exciplexes are formed efficiently in the host and the energy transfer from the exciplex to blue phosphorescent dopant (iridium(III)bis[(4,6‐difluorophenyl)‐pyridinato‐N,C2′]picolinate; FIrpic) is also efficient, enabling the triplet harvest without energy loss. As a result, an unprecedented high performance blue PhOLED with the exciplex forming co‐host is demonstrated, showing a maximum external quantum efficiency (EQE) of 30.3%, a maximum power efficiency of 66 lm W−1, and low driving voltage of 2.75 at 100 cd m−2, 3.29 V at 1000 cd m−2, and 4.65 V at 10 000 cd m−2, respectively. The importance of the exciton confinement in the exciplex forming co‐host is further investigated which is directly related to the performance of PhOLEDs.

Efficient Phosphorescent Organic Light Emitting Diodes Using F4TCNQ as the Indium-Tin-Oxide Modification Layer

A phosphorescent organic light emitting diode by using tetrafluorotetracyanoquinodimethane (F4TCNQ) as the indium-tin-oxide modification layer and 4,4′-bis(carbazol-9-yl)biphenyl (CBP) as the hole transporting layer is reported. CBP doped with a green phosphorescent dopant, tris(2-(p-tolyl)pyridine) iridium(III) (Ir(mppy)3) is used as the emission layer in this device, and the maximum current efficiency of 31.3 cd/A is achieved. Furthermore, low efficiency roll-off of 10.4% is observed with device luminance increasing from 100 cd/m2 (29.7 cd/A) to 10000 cd/m2 (26.5 cd/A). It is demonstrated that a charge-generation area is formed at F4TCNQ/CBP interface, which will benefit hole injection into the hole transporting layer. Moreover, use of the CBP hole transporting layer will benefit the low efficiency roll-off by broadening triplet exciton formation, as well as by avoiding accumulation of unbalanced carrier at the hole transporting layer/emission layer interface.

Efficient green phosphorescent tandem organic light emitting diodes with solution processable mixed hosts charge generating layer

A novel solution processable charge generating layer (CGL) that consists of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN6)/Poly(N-vinylcarbazole) (PVK): 1,1-bis-(4-bis(4-tolyl)-aminophenyl) cyclohexene (TAPC) for a tandem green phosphorescent organic light emitting diode (PHOLED) is demonstrated. The use of orthogonal solvent to dissolve HATCN6 and PVK:TAPC is the key to overcome the interface erosion problem for the solution processed CGL. The current efficiency of the 2wt% TAPC mixed with PVK is the highest at 24.2cd/A, which is more than three-folds higher than that of the single device at 1000cd/m2.

Using interlayer step-wise triplet transfer to achieve an efficient white organic light-emitting diode with high color-stability

An efficient phosphorescent white organic light emitting-diode with a red-green-blue tri-emitting-layer structure is reported. The host of the red dopant possesses a lower triplet-energy than the green dye. An interlayer step-wise triplet transfer via blue dye → green dye → red host → red dye is achieved. This mechanism allows an efficient triplet harvesting by the three dopants, thus maintaining a balanced white light and reducing energy loss. Moreover, the color stability of the device is improved significantly. The white device not only achieves a peak external quantum efficiency of 21.1 ± 0.8% and power efficiency of 37.5 ± 1.4 lm/W but shows no color shift over a wide range of voltages.

Efficient and stable red organic light emitting devices from a tetradentate cyclometalated platinum complex

An efficient and stable red phosphorescent organic light emitting diode was developed using a tetradentate cyclometalated platinum complex. Devices employing the phosphorescent molecule, platinum(II)-9-(4-methylpyridin-2-yl)-2-(3-(quinolin-2-yl)phenoxy)-9H-carbazole (PtON11Me), yielded high external quantum efficiencies and high operational lifetimes. A maximum EQE of 12.5% and color coordinates CIE (x=0.61, y=0.36) was achieved in devices employing efficient hole blocking and transporting materials and a high operational lifetime of T0.97∼3200h was achieved in devices utilizing electrochemically stable hole blocking and transporting materials.

Bis(1-phenyl-1H-benzo[d]imidazole)phenylphosphine oxide interlayer for effective hole blocking in efficient phosphorescent organic light emitting diodes based on widely used charge transporting layers

We have developed the efficient phosphorescent organic light emitting diodes (PHOLEDs) based on green-emitting fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] dopant with widely used charge transporting layers. PHOLEDs show the electroluminescence emission from the dopant emitter at 512nm with CIE color coordinates of (0.30, 0.59). We investigated the roll of a 10-nm-thick electron-transporting type bis(1-phenyl-1H-benzo[d]imidazole)-phenylphosphine oxide (BIPO) interlayer in the device efficiencies. BIPO with the deep HOMO energy level and higher electron transporting ability effectively blocks the holes from EML to ETL and improves the charge balance in the EML of the device, resulting in the higher device external quantum efficiency of 14.42%, current efficiency of 44.37cd/A, and power efficiency of 14.67lm/W with a maximum luminance of 75,080cd/m2. The results show that the BIPO interlayer can be very much useful for the efficient low-cost device fabrication.

Wet-process feasible novel carbazole-type molecular host for high efficiency phosphorescent organic light emitting diodes

Wet-process feasible high triplet energy carbazole type host materials are synthesized. On doping a green emitter into a host carbazole, the device shows an efficacy of 51 lm W¬1 and current efficiency of 52 cd A¬1 at 100 cd m−2.

Fluorine-free blue phosphorescent emitters for efficient phosphorescent organic light emitting diodes

Two blue phosphorescent Ir(iii) compounds with a main fluorine-free bipyridine ligand have been synthesized.

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μh(TAPC): 1×10−2cm2/Vs and μe(TpPyPB): 7.9×10−3cm2/Vs] [1]. The optimized device with thin TAPC (5–10nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000cd/m2. The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).

Controllably tunable phenanthroimidazole–carbazole hybrid bipolar host materials for efficient green electrophosphorescent devices

A series of phenanthroimidazole–carbazole (1 : 2) hybrids as bipolar host materials have been designed and synthesized through facile typical Ullmann reactions. These compounds with rigid configurations exhibit excellent thermal and morphological stabilities with high glass transition temperatures (Tg) (143–282 °C). Their photoelectronic properties, energy levels, charge transport mobility, and film morphologies can be controllably tuned through judicious engineering of the linkage modes between the two carbazole groups and the 2,5-diphenyl-1,3,4-phenanthroimidazole (para and meta). The promising physical properties of these new compounds make them suitable for use as hosts doped with Ir-based phosphor for realizing highly efficient phosphorescent organic light emitting diodes (PhOLEDs). A green device hosted by compound PhBIDmpCP shows a maximum current efficiency of 74.3 cd A−1 and a maximum power efficiency of 74.4 lm W−1 (corresponding EQEmax = 20.2%).

A new electron transporting material for effective hole-blocking and improved charge balance in highly efficient phosphorescent organic light emitting diodes

A highly efficient new benzoimidazole phosphine oxide based electron transporting material, bis(1-phenyl-1H-benzo[d]imidazole)phenylphosphine oxide (BIPO), was designed, synthesized by condensation, nucleophilic substitution and oxidation reactions, and confirmed using various spectroscopic studies. It shows a thermal stability (ΔT5%) of 451 °C with a glass transition temperature of 129 °C from the thermogravimetric analysis and differential scanning calorimetry studies. BIPO used as an efficient electron transport layer (ETL) in a green emitting phosphorescent organic light emitting diode, ITO/4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (30 nm)/4,4′,4′′-tris(carbazole-9-yl)triphenylamine 10 nm)/(4,4′-N,N′-dicarbazole)biphenyl (CBP) host doped with Ir(ppy)3 dopant (5%) (30 nm)/ETL (40 nm)/LiF (1 nm)/Al (100 nm), serves as an effective hole-blocking material and improves the charge balance in the device, resulting in higher device efficiencies of 22.19%, 68.3 cd A−1 and 24.4 lm W−1 with a maximum luminance of 72 080 cd m−2 compared to those (17.03%, 52.0 cd A−1 and 20.9 lm W−1 with a maximum luminance of 33 490 cd m−2) of a device using a widely used ETL, 1,3,5-tris(m-pyrid-3-yl-phenyl)benzene. These results show that the new BIPO ETL could be very useful in efficient organic light emitting diodes.