Green Phosphorescent Organic Light-emitting Diodes Research Articles

Toward high efficiency green phosphorescent organic light-emitting diodes by fine tuning the charge transporting properties of 1,2,4-thiadiazole based hosts

Two novel host materials, from bipolar-type BzCzTHZ (5-(2-(9H-carbazol-9-yl)phenyl)-3-(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)-1,2,4-thiadiazole) to electron-type DBzTHZ (3,5-bis(3-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)-1,2,4-thiadiazole), were designed and synthesized utilizing the unsymmetric intermediate of 3-bromo-5-chloro-1,2,4-thiadiazole as the center. Their thermal, photoelectronic, energy levels, charge transport mobility properties can be easily tuned by changing the radio of benzimidazole/carbazole moieties. Using a common device structure ITO/MoO3 (10nm)/TAPC (50nm)/TCTA (5nm)/host: Ir(ppy)3 (3wt%) (20nm)/TmPyPB (45nm)/LiF (1nm)/Al (100nm), highly efficient green electrophosphorescent devices were successfully achieved. Devices hosted by DBzTHZ and BzCzTHZ achieved maximum power efficiencies (ηp, max) of 95.2 and 86.2lm/W, corresponding to maximum quantum efficiency (ηEQE, max) of 23.4 and 22.2%, respectively. Moreover, the electron-type host DBzTHZ based device exhibits an ideal turn-on voltage (2.5V at 1.0cd/m2). These results demonstrate that electron-type hosts are competitive with bipolar hosts.

Efficient solution-processed green and white phosphorescence organic light-emitting diodes based on bipolar host materials

Four carbazole-based bipolar host materials are utilized for solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). These bipolar materials consist of an electron-donor unit (carbazole) linking to a fluorene unit bearing various electron-acceptor units (oxadiazole, cyano, and benzimidazole) via a saturated carbon, giving sufficiently high triplet energies due to the lack of direct electronic coupling between the donor and acceptor(s). The resulting physical properties and bipolar characteristics render the realization of efficient solution-processed green and white OLEDs feasible. The best green light-emitting device based on bipolar host CzFCBI incorporating a stepwise hole-injection/transporting system exhibit a low drive voltage, a maximum external quantum efficiency of 14.0%, a current efficiency of 49.0cd/A, and a power efficacy of 55.0lm/W. Moreover, the CzFOXa-based two-component (blue–orange) white light-emitting device shows a warmish-white emission with a maximum external quantum efficiency of 6.9% and stable chromaticity coordinates at different luminance levels and yield a high color rendering index (CRI) reaching 76 at a luminance of 1000cd/m2.

High triplet energy n-type dopants for high efficiency in phosphorescent organic light-emitting diodes

High triplet energy n-type dopants, lithium 2-(oxazol-2-yl)phenolate (LiOx) and lithium 2-(1-methyl-imidazol-2-yl)phenolate (LiIm), were synthesized as n-type doping materials for phosphorescent organic light-emitting diodes and the effect of the n-type doping materials on the electron mobility and device performances of the phosphorescent organic light-emitting diodes was investigated. The LiOx and LiIm n-type dopants were effective to increase the electron mobility of electron transport materials and improve the quantum efficiency of green and blue phosphorescent organic light-emitting diodes.

Alternative p-doped hole transport material for low operating voltage and high efficiency organic light-emitting diodes

We investigate the properties of N,N′-[(Diphenyl-N,N′-bis)9,9,-dimethyl-fluoren-2-yl]-benzidine (BF-DPB) as hole transport material (HTL) in organic light-emitting diodes (OLEDs) and compare BF-DPB to the commonly used HTLs N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD), 2,2′,7,7′-tetrakis(N,N′-di-p-methylphenylamino)-9,9′-spirobifluorene (Spiro-TTB), and N,N′-di(naphtalene-1-yl)-N,N′-diphenylbenzidine (NPB). The influence of 2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ p-dopant) concentration in BF-DPB on the operation voltage and efficiency of red and green phosphorescent OLEDs is studied; best results are achieved at 4 wt. % doping. Without any light extraction structure, BF-DPB based red (green) OLEDs achieve a luminous efficacy of 35 .1 lm/W (74 .0 lm/W) at 1000 cd/m2 and reach a very high brightness of 10 000 cd/m2 at a very low voltage of 3.2 V (3.1 V). We attribute this exceptionally low driving voltage to the high ionization potential of BF-DPB which enables more efficient hole injection from BF-DPB to the adjacent electron blocking layer. The high efficiency and low driving voltage lead to a significantly lower luminous efficacy roll-off compared to the other compounds and render BF-DPB an excellent HTL material for highly efficient OLEDs.

Origin of improved stability in green phosphorescent organic light-emitting diodes based on a dibenzofuran/spirobifluorene hybrid host

A highly efficient and stable green phosphorescent organic light-emitting diode (PHOLED) was fabricated by using a new dibenzofuran/spirobifluorene hybrid material (named as DBFSF2) as the host material. The DBFSF2-based PHOLEDs exhibited slightly better electroluminescence efficiency than that in the conventional host material 4,4′-bis(carbazol-9-yl)biphenyl (CBP)-based ones. Moreover, further studies demonstrated that the DBFSF2-based PHOLEDs had a better stability with about 1.5 times improvement in half lifetime compared with that of CBP-based ones. To investigate the mechanism behind the enhanced stability in the DBFSF2-based PHOLEDs, the evaluations of dark spots growth, film crystallization properties and carrier recombination zone were carried out.

Engineering of interconnect position of bicarbazole for high external quantum efficiency in green and blue phosphorescent organic light-emitting diodes.

Three bicarbazole based host materials with different interconnect positions between carbazole units, 3,3'-(9,9'-[3,3'-bicarbazole]-9,9'-diyl)dibenzonitrile (3CN33BCz), 3,3'-(9,9'-[3,4'-bicarbazole]-9,9'-diyl)dibenzonitrile (3CN34BCz), and 3,3'-(9,9'-[4,4'-bicarbazole]-9,9'-diyl)dibenzonitrile (3CN44BCz), were developed and their synthesis, material characterization, and device characterization were reported. Two carbazole units were connected via 3,3'-, 3,4'-, and 4,4'-positions to correlate the interconnect positions with photophysical properties and device performances. The linkage via 4,4'-position increased triplet energy and thermal stability of the host materials, while the linkage via 3,3'-position enhanced current density. All bicarbazole host materials showed good device performances and high quantum efficiency above 25% was attained using the biscarbazole derivatives for green and blue phosphorescent organic light-emitting diodes. In particular, the bicarbazole host materials with a linkage via 4-position showed high quantum efficiency above 30% in the green devices.

Surveillance des entérobactéries productrices de bêta-lactamases à spectre élargi à l’hôpital Cheik Zaid de 2009 à 2011

Ultrathin non-doped emissive layer (EML) has been employed in green phosphorescent top-emitting organic light-emitting diodes (TOLEDs) to take full advantages of the cavity standing wave condition in a microcavity structure. Much higher out-coupling efficiency has been observed compared to conventional doped EML with relatively wide emission zone. A further investigation on dual ultrathin non-doped EMLs separated by a special bi-layer structure demonstrates better charge carrier balance and improved efficiency. The resulting device exhibits a high efficiency of 125.0 cd/A at a luminance of 1000 cd/m2 and maintains to 110.9 cd/A at 10,000 cd/m2.

Dimeric SFX host materials for red, green and blue phosphorescent organic light-emitting devices

Two novel spiro[fluorene-9,9′-xanthene]-based derivatives 23′ODSFX and 3′3′ODSFX, without possessing conventional hole- or electron-transporting units, have been synthesized by a simple reaction procedure and been demonstrated to be host materials for efficient and low-voltage blue, green and red phosphorescent organic light-emitting devices (PHOLEDs). They showed good thermal stability with high glass transition temperatures (Tg) at 210°C for 23′ODSFX and 138°C for 3′3′ODSFX. Compared with SFX-based devices, the performance of 23′ODSFX-based devices increased nearly 20 times, although the difference of HOMO/LUMO levels and triplet levels between dimeric SFX and SFX was very small. The blue PHOLEDs based on 23′ODSFX and 3′3′ODSFX exhibited the same low turn-on voltage of ∼2.6V, and maximum current efficiency (CE) of 21.7/12.9cdcdA−1, power efficiency (PE) of 19.5/11.5lmW−1, and external quantum efficiency (EQE) of 11.9%/7.2%, respectively. It was evident that the different position linkage in host materials was closely relative to the device performance.

High efficiency green phosphorescent top-emitting organic light-emitting diode with ultrathin non-doped emissive layer

Ultrathin non-doped emissive layer (EML) has been employed in green phosphorescent top-emitting organic light-emitting diodes (TOLEDs) to take full advantages of the cavity standing wave condition in a microcavity structure. Much higher out-coupling efficiency has been observed compared to conventional doped EML with relatively wide emission zone. A further investigation on dual ultrathin non-doped EMLs separated by a special bi-layer structure demonstrates better charge carrier balance and improved efficiency. The resulting device exhibits a high efficiency of 125.0 cd/A at a luminance of 1000 cd/m2 and maintains to 110.9 cd/A at 10,000 cd/m2.

High efficiency yellowish green phosphorescent emitter derived from phenylbenzothienopyridine ligand

A yellowish green phosphorescent dopant derived from phenylbenzothienopyridine ligand, iridium (III) [bis(1-phenylbenzo[4,5]thieno[2,3-c]pyridinato-N,C2]picolinate. (Ir(DTNP)2pic) was synthesized and the device performances of the Ir(DTNP)2pic was studied. The Ir(DTNP)2pic dopant exhibited yellowish green emission at 548nm and showed a high quantum efficiency of 22.4% at 1000cd/m2 with a color coordinate of (0.43, 0.57) in yellowish green phosphorescent organic light-emitting diodes.

52.3: Understanding Extrinsic Degradation in Phosphorescent OLEDs

AbstractWe quantified water as an impurity in OLEDs and investigated how the water level impacts initial lifetime decay. We also demonstrated a two to three fold increase in lifetime for blue and green phosphorescent OLEDs by reducing the amount of water in the devices.

Highly efficient bluish green phosphorescent organic light-emitting diodes based on heteroleptic iridium(III) complexes with phenylpyridine main skeleton

A new series of heteroleptic iridium(III) complexes, bis(2-phenylpyridinato-N,C2′)iridium (2-(2′,4′-difluorophenyl)-4-methylpyridine), (ppy)2Ir(dfpmpy) and bis(2-(2′,4′-difluorophenyl)-4-methylpyridinato-N,C2′)iridium (2-phenylpyridine) (dfpmpy)2Ir(ppy), have been synthesized by using phenylpyridine as a main skeleton for bluish green phosphorescent organic light-emitting diodes (PhOLEDs). The Ir(III) complexes showed high thermal stability and high photoluminescent (PL) quantum yields of 95%±4% simultaneously. As a result, the PhOLEDs with the heteroleptic Ir(III) complexes showed excellent performances approaching 100% internal quantum efficiency with a very high external quantum efficiency (EQE) of ∼27%, a low turn-on voltage of 2.4V, high power efficiency of ∼85lm/W, and very low efficiency roll-off up to 20,000cd/m2.

Highly efficient green and red phosphorescent OLEDs using novel bipolar blue fluorescent materials as hosts

We investigated highly efficient green and red phosphorescent organic light-emitting diodes (OLEDs) based on two kinds of iridium complexes as the guests and two novel 1,3,5-triazine derivatives as the host materials, respectively. For comparison, the devices using a common fluorescent host 4,4′-N,N′-dicarbazolebiphenyl (CBP) have also been fabricated. Results show that all devices using 1,3,5-triazine derivatives as host have better performance than that of CBP. This may be attributed to the adoption of the hosts, which have suitable HOMO/LUMO levels enabling excitons to form on host and hence favoring efficient energy-transfer from host to guest. In addition, the high bipolar carrier mobility of the host is found to be critical to this kind of doping system, which would balance the injection of both carriers and improve efficiency.

Numerical analysis of the electrical and the optical properties of green phosphorescent organic light-emitting diodes

In this paper, we report a theoretical study on the electrical-optical properties of phosphorescent organic light-emitting diodes (PHOLEDs). Our simulation reveals that the refractive index of each material plays a crucial role in the emission characteristics and that the barrier height at the interface significantly influences the behavior of charge transport as well as the generation of excitons. The calculated transient profiles indicate that the carrier recombination in the PHOLEDs takes place mainly at the interface between the emitting layer and the hole transport layer after 8 μs. In the case of high index of refraction, the simulation result via modal analysis implies a possibility for improving the light extraction by increasing the substrate mode. As the thickness of each layer has been altered, we observe that the chromaticity of the device changes periodically.

Enhancement of efficiencies for tandem green phosphorescent organic light-emitting devices with a p-type charge generation layer

Tandem green phosphorescent organic light-emitting devices with a 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile or a molybdenum trioxide charge generation layer were fabricated to enhance their efficiency. Current density–voltage curves showed that the operating voltage of the tandem green phosphorescent organic light-emitting device with a 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile layer was improved by 3% over that of the corresponding organic light-emitting device with a molybdenum trioxide layer. The efficiency and the brightness of the tandem green phosphorescent organic light-emitting device were 13.9cd/A and 26,540cd/m2, respectively. The current efficiency of the tandem green phosphorescent organic light-emitting device with a 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile layer was lower by 1.1 times compared to that of the corresponding organic light-emitting device with molybdenum trioxide layer due to the decreased charge generation and transport in the 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile layer resulting from triplet–triplet exciton annihilation.

Construction of High Tg Bipolar Host Materials with Balanced Electron–Hole Mobility Based on 1,2,4-Thiadiazole for Phosphorescent Organic Light-Emitting Diodes

A novel electron-transporting moiety, 1,2,4-thiadiazole, was first introduced to construct bipolar host molecules for phosphorescent organic light-emitting diodes (PhOLEDs). By incorporating 1,2,4-thiadiazole with typical hole-transporting carbazole moieties, a series of thiadiazole/carbazole hybrids, o-CzTHZ, m-CzTHZ, and p-CzTHZ, were synthesized. All the hybrids exhibit very high glass transition temperatures (Tg ≥ 167 °C) and show good thermal and morphological stability in films. Moreover, these host materials possess good bipolar charge transporting properties; electron and hole mobilities of these bipolar thiadiazole/carbazole hybrids can be tuned by simply adjusting the linkage modes between thiadiazole and carbazole moieties. The maximum external quantum efficiencies (ηEQE, max) in the green PhOLEDs with o-CzTHZ, m-CzTHZ, and p-CzTHZ as the hosts reached 26.1%, 24.0%, and 22.9%, respectively, and their EQE were still over 20% even at the high luminance of 10,000 cd/m2. This study demonstrates tha...

Improved efficiency and lifetime for green phosphorescent organic light-emitting diodes using charge control layer

We investigated green phosphorescent organic light-emitting diodes (PHOLEDs) with charge control layer (CCL) to produce high efficiency and improve operational lifetime. Three types of devices were fabricated following the number of CCL within emitting layer (EML), maintaining the thickness of whole EML. The CCL and host material, which was 4,4′-bis (carbazol-9-yl)biphenyl (CBP) with bipolar property, can control carrier movement in EML. Therefore, the electroluminescent (EL) performance improvement as efficiency and lifetime was realized with a good charge balance, an effective triplet exciton confinement, and the reduced triplet exciton quenching effect in EML. Device 2 with a CCL as exciton distribution structure exhibits the remarkable EL performances for the maximum luminous and external quantum efficiency of 65.34cd/A and 20.42%, respectively. Moreover, operational lifetime is nearly improved 2.5 times than the conventional device.

Efficient inverted organic light-emitting devices with self or intentionally Ag-doped interlayer modified cathode

Green phosphorescent inverted organic light-emitting devices (IOLEDs) with self or intentionally Ag-doped interlayer modified cathode were demonstrated. The IOLEDs show low driving voltage and high efficiency. For example, the efficiency of inverted bottom-emitting OLED with ITO cathode is comparable with the conventional bottom-emitting OLED with ITO anode. The top-emitting IOLED with Ag cathode shows high current efficiency of 76.4 cd/A which is 2.38 times of that of the conventional bottom-emitting OLED with ITO anode. The results indicate that the electron injection from cathode was observably improved by the Ag-doped interlayer and such interlayer is cathode independent relatively.

High-performance green phosphorescent top-emitting organic light-emitting diodes based on FDTD optical simulation

We have successfully applied finite-difference time-domain (FDTD) method in top-emitting organic light-emitting diodes (TOLEDs) for structure optimization, demonstrating good agreement with experimental data. A mixed host with both hole transport and electron transport materials is employed for the green phosphorescent emitter to avoid charge accumulation and broaden the recombination zone. The resulting TOLEDs exhibit ultra-high efficiencies, low current efficiency roll-off, and a highly saturated color, as well as hardly detectable spectrum shift with viewing angles. In particular, a current efficiency of 127.0cd/A at a luminance of 1000cd/m2 is obtained, and maintains to 116.3cd/A at 10,000cd/m2.

Low driving voltage blue, green, yellow, red and white organic light-emitting diodes with a simply double light-emitting structure

Low driving voltage blue, green, yellow, red and white phosphorescent organic light-emitting diodes (OLEDs) with a common simply double emitting layer (D-EML) structure are investigated. Our OLEDs without any out-coupling schemes as well as n-doping strategies show low driving voltage, e.g. < 2.4 V for onset and < 3 V for 1000 cd/m2, and high efficiency of 32.5 lm/W (13.3%), 58.8 lm/W (14.3%), 55.1 lm/W (14.6%), 24.9 lm/W (13.7%) and 45.1 lm/W (13.5%) for blue, green, yellow, red and white OLED, respectively. This work demonstrates that the low driving voltages and high efficiencies can be simultaneously realized with a common simply D-EML structure.