Phosphorescent Organic Light-emitting Diodes Devices Research Articles

A Comparative Study of Ir(dmpq)2(acac) Doped CBP, mCP, TAPC and TCTA for Phosphorescent OLEDs

In this work, we present the fabrication and characterization of solution-processable red Phosphorescent Organic Light-Emitting Diodes (PhOLEDs). The proposed approach is based on Ir(III) complex, namely Bis(2-(3,5-dimethylphenyl)quinoline-C,N)(acetylacetonato)Iridium(III), also known as Ir(dmpq)2(acac), which was doped in four different host materials: (a) 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP), (b) 1,3-Bis(N-carbazolyl)benzene (mCP), (c) 1,1-Bis[(di-4-tolylamino) phenyl]cyclohexane (TAPC), and (d) tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The metal–organic complex offers unique optical and electronic properties arising from the interplay between the inorganic metal and the organic material. The optical and photophysical properties of the produced thin films are investigated in detail using spectroscopic ellipsometry and photoluminescence, whereas the structural characteristics are examined by atomic force microscopy. This comparative study of the four different Host:Ir-complex systems provides valuable information to evaluate the emission characteristics in order to achieve pure red light. Finally, these materials were applied as a single-emissive layer in PhOLED devices, and the electroluminescence characteristics were studied.

New boomerang-shaped host materials for phosphorescent organic light-emitting diodes

Inspite of being a good triplet sensitizer, benzophenones are not useful as host materials for phosphorescent organic light-emitting diodes (PhOLEDs), primarily due to their low thermal stability. In this work, we have addressed this shortcoming by judicial tethering of benzoyl units onto different positions of a rigid Tröger’s base (TB) scaffold to develop two new benzophenone tethered TBs, namely TB-Bz1, and TB-Bz2 as thermally stable host materials for applications in PhOLEDs. The TB-based host molecules were readily synthesized in good yields from the corresponding amines, and were fully characterized using various spectroscopic techniques. These TBs displayed high thermal decomposition temperatures above 320 °C, imparted by the rigid, non-planar, boomerang-shaped scaffold of the Tröger’s base. It was further observed that the tethering position of the benzoyl units can significantly affect the photophysical properties of the TBs due to the twisting of the benzoyl unit with respect to the TB-core. This assumption was further confirmed by DFT simulations. Taking advantage of the high triplet energy of TBs (>2.8 eV), PhOLED devices were fabricated exploring TBs as the host material and Ir(ppy)3 as the dopant. The maximum external quantum efficiency of 6.1 %, and 6.9 % and maximum luminance of greater than 5000 cd/m2 were obtained using TBs as the host for the proof-of-concept PhOLED devices, where Ir(ppy)3 was employed as the dopant.

Efficient Blue Electrophosphorescence and Hyperphosphorescence Generated by Bis-tridentate Iridium(III) Complexes.

Four blue-emissive iridium(III) complexes bearing a 3,3'-(1,3-phenylene)bis[1-isopropyl-6-(trifluoromethyl)-3H-imidazo[4,5-b]pyridin-2-ylidene]-based pincer chelate, which are derived from PXn·H3(PF6)2, where n = 1-4, and a cyclometalating chelate given from 9-[6-[5-(trifluoromethyl)-2λ2-pyrazol-3-yl]pyridin-2-yl]-9H-carbazole [(PzpyCz)H2], were successfully synthesized and employed as both an emissive dopant and a sensitizer in the fabrication of organic light-emitting diode (OLED) devices. These functional chelates around a IrIII atom occupied two mutually orthogonal coordination arrangements and adopted the so-called bis-tridentate architectures. Theoretical studies confirmed the dominance of the electronic transition by the pincer chelates, while the dianionic PzpyCz chelate was only acting as a spectator group. Phosphorescent OLED devices with [Ir(PX3)(PzpyCz)] (B3) as the dopant gave a maximum external quantum efficiency (EQE) of 21.93% and CIExy of (0.144, 0.157) and was subjected to only ∼10% of roll-off in efficiency at a high current density of 1000 cd m-2. Blue-emissive narrow-band hyperphosphorescence was also obtained using B3 as an assistant sensitizer and ν-DABNA as a terminal emitter, giving both an improved EQE of 26.17% and CIExy of (0.116, 0.144), confirming efficient Förster resonance energy transfer in this hyperdevice.

Highly efficient tandem PHOLEDs with lithium-doped BPhen/NDP-9-doped TAPC as a charge generation layer

The development of large-area organic light-emitting diode (OLED) displays requires a highly efficient tandem device architecture and an easily processable charge generation layer (CGL) with a low voltage drop and high optical transparency. In this study, we investigated and applied a doped organic n-CGL/p-CGL using thermal vacuum deposition in tandem OLED devices. A doping concentration of 1.0 wt.% for Li in 4, 7-Diphenyl-1, 10-phenanthroline (BPhen) was optimal for the n-CGL with 8 wt.% for 2-(7-dicyanomethylene-1,3,4,5,6,8,9,10-octafluoro-7H-pyrene-2-ylidene)-malononitrile (NDP-9)-doped N,N-bis(4-methylphenyl)benzenamine (TAPC) as a p-CGL. Maximum luminous efficiencies of 42.5 and 63.4 cd/A and a 4,000 cd/m2 current density for the target luminance values of 11.2 and 6.5 mA/cm2 were demonstrated for double-stack and triple-stack tandem blue phosphorescent OLED devices, respectively. Implementing these highly efficient tandem device structures will improve the overall lifetime of OLED displays by lowering their operating current density at the target luminance.

A theoretical design of bipolar host materials for blue phosphorescent OLED

A series of host molecules have been designed and characterized for use in phosphorescent organic light-emitting diode devices. The parent host molecule was modified by adding nitrogen-containing group at different positions. The first triplet excited (T1) and first singlet excited (S1) states energies, frontier orbital energy levels, reorganization energies and injection barriers of the molecules designed were calculated in comparison to those of the reference host, emitter, hole and electron transport materials. Interestingly, our results suggest that addition of nitrogen group into the dibenzothiophene by a N–C linkage increases the triplet energy separation and decreases the injection barriers making them suitable for use as blue phosphorescent materials.

New structure host material for phosphorescent OLED devices

New carbazole and quinoxaline based low molar mass bipolar derivative was synthesized, characterized, and tested as host derivative in phosphorescent organic light emitting diodes (OLEDs). The material could form homogeneous amorphous materials (layers) with very high glass transition temperature of 136 °C and also with very high initial thermal decomposition temperature of about 387 °C. The electron photoemission spectra of the layers of the synthesized compound showed ionization potentials of about 5.85 eV. The compound was tested as host material for green Ir(ppy)3 , blue FIrpic, and yellow PO-01 triplet phosphorescent guests. The device with the yellow PO-01 triplet emitter exhibited the best overall performance. The most efficient yellow OLED demonstrated turn-on voltage of 4.9 V, a maximum brightness exceeding 4050 cd/m2 and current efficiency of 10.8 cd/A with power efficiency of 6.9 lm/W at 100 cd/m2.

Bis-tridentate IrIII Phosphors Bearing Two Fused Five-Six-Membered Metallacycles: A Strategy to Improved Photostability of Blue Emitters.

Iridium complexes bearing chelating cyclometalates are popular choices as dopant emitters in the fabrication of organic light-emitting diodes (OLEDs). In this contribution, we report a series of blue-emitting, bis-tridentate IrIII complexes bearing chelates with two fused five-six-membered metallacycles, which are in sharp contrast to the traditional designs of tridentate chelates that form the alternative, fused five-five metallacycles. Five IrIII complexes, Px-21-23, Cz-4, and Cz-5, have been synthesized that contain a coordinated dicarbene pincer chelate incorporating a methylene spacer and a dianionic chromophoric chelate possessing either a phenoxy or carbazolyl appendage to tune the coordination arrangement. All these tridentate chelates afford peripheral ligand-metal-ligand bite angles of 166-170°, which are larger than the typical bite angle of 153-155° observed for their five-five-coordinated tridentate counterparts, thereby leading to reduced geometrical distortion in the octahedral frameworks. Photophysical measurements and TD-DFT studies verified the inherent transition characteristics that give rise to high emission efficiency, and photodegradation experiments confirmed the improved stability in comparison with the benchmark fac-[Ir(ppy)3 ] in degassed toluene at room temperature. Phosphorescent OLED devices were also fabricated, among which the carbazolyl-functionalized emitter Cz-5 exhibited the best performance among all the studied bis-tridentate phosphors, showing a maximum external quantum efficiency (EQEmax ) of 18.7 % and CIEx,y coordinates of (0.145, 0.218), with a slightly reduced EQE of 13.7 % at 100 cd m-2 due to efficiency roll-off.

Benzophenone-imbedded benzoyltriptycene with high triplet energy for application as a universal host material in phosphorescent organic light-emitting diodes (PhOLEDs)

Benzoyltriptycene functions as a very good host material for blue, green, yellow and red dopants in PhOLED devices.

Flexible Active-Matrix Organic Light-Emitting Diode Display Using Air-Stable Organic Semiconductor of $\hbox{Dinaphtho}[\hbox{2}, \hbox{3}\hbox{-b}: \hbox{2}^{ \prime}, \hbox{3}^{\prime} \hbox{-f}]\hbox{thieno}[\hbox{3}, \hbox{2} \hbox{-b}] \hbox{-} \hbox{thiophene}$

We developed a flexible active-matrix (AM) organic light-emitting diode (OLED) display driven by an organic thin-film transistor (TFT) (OTFT) using an air-stable organic semiconductor (OSC) of dinaphtho[2, 3 -b:2', 3' -f]thieno[ 3, 2 -b]-thiophene (DNTT) for the first time. We employed a low-temperature cross-linkable olefin-type polymer as the gate insulator (GI) and investigated the properties of the interface between DNTT and polymer GI. The top-contact TFT demonstrated a high mobility of up to 0.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s and a near-zero turn-on voltage. DNTT has a deeper highest occupied molecular orbital level than other OSC materials, and this leads to large contact resistance between the source/drain (S/D) contact and OSC. Surface modifications to the S/D contact were investigated to enable efficient carrier injection to fabricate high-performance bottom-contact TFTs. The short-channel TFT we fabricated exhibited a high hole mobility of 0.5 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V·s, a low subthreshold slope of 0.31, and excellent environmental and operational stability. The DNTT-based TFT also demonstrated good applications to processes with less deterioration. Finally, a 5-in flexible OLED display was successively fabricated by integrating the AM backplane with a phosphorescent OLED device. A high luminescence over 300 cd/ m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was achieved by driving the DNTT-based OTFTs.

Electroluminescent Property of a Beryllium Metal-chelate Complex Used as the Electron-transporting Host Material

A simple and highly efficient single-emitting-layer phosphorescent organic light-emitting diode (PHOLED) that used bis[2-(2-hydroxyphenyl)-4-methyl-pyridine] beryllium(Bempp) as the electron-transporting host material and bis (pyridylphenyl) iridium (III) (N, N'-diisopropyl-benzamidine) (PPP) as the phosphorescent dye to enhance the electro-luminescence (EL) performance was presented. This PHOLED device exhibited substantially lower driving voltage(2.5 V) and higher EL efficiencies than those based on the CBP host. It showed dramatically enhanced peak values of more than 63.1 cd/A for luminous efficiency, 54.0 lm/W for power efficiency. The high EL performance was attributed to the well-matched energy levels of Bempp with the hole/electron-transporting (HT/ET) materials as well as the highly efficient and complete energy transfer between the host and the guest. The results demonstrate that Bempp is a promising host material to replace the commonly used CBP because of its higher EL efficiency and its relative high triplet energy (ca. 2.6 eV).

Novel transport materials for high-performance fluorescent and phosphorescent OLEDs

To improve the performance of blue fluorescent and green phosphorescent organic light-emitting diode devices, Merck developed novel green phosphorescent host and electron-transporting materials. The newly developed electron-transporting material improves the external quantum efficiency of blue fluorescent devices up to 8.7%, with an excellent lifetime. In combination with the newly developed host materials, the efficiency of green phosphorescent devices can be improved by a factor of 1.7, and the lifetime by a factor of 7.

High-efficiency, low-voltage phosphorescent organic light-emitting diode devices with mixed host

We report high-efficiency, low-voltage phosphorescent green and blue organic light-emitting diode (PHOLED) devices using mixed-host materials in the light-emitting layer (LEL) and various combinations of electron-injecting and electron-transporting layers. The low voltage does not rely on doping of the charge-transport layers. The mixed LEL architecture offers significantly improved efficiency and voltage compared to conventional PHOLEDs with neat hosts, in part by loosening the connection between the electrical band gap and the triplet energy. Bulk recombination in the LEL occurs within ∼10 nm of the interface with an electron-blocking layer. A “hole-blocking layer” need not have hole- or triplet-exciton-blocking properties. Optical microcavity effects on the spectrum and efficiency were used to locate the recombination zone. The effect of layer thickness on drive voltage was used to determine the voltage budget of a typical device. The behavior of undoped devices was investigated, and the electroluminescence is attributed to an exciplex of the two host materials. Electrically detected electron paramagnetic resonance was used to study the behavior of doped and undoped PHOLED devices from 20 to 200 K, which largely mimics the behavior of the same devices at room temperature.

P‐209: New Green Phosphorescent Host Materials

AbstractWe report novel bipolar host materials for high efficiency green phosphorescent organic light emitting diode devices (PHOLEDs). Phenyl moieties were inserted in a 4,4′‐N,N′‐dicarbazolebipheyl (CBP) structure to reduce electron injection barrier and enhance electron mobility. The significant improvement of current and power efficiencies and reduction in turn‐on voltage in green light‐emitting PHOLEDs by this modification are observed. At a given constant luminance of 1000 cd/m2, current efficiency 53 cd/A, low driving voltage 6.5 V, and power efficiency 27 lm/W in 4,4′‐N,N′‐dicarbazoleterpheyl (CTP) host with Ir(ppy)3 dopant are reported. Results show better performance in green PHOLEDs with new host materials than CBP.