Carrier Transport Materials Research Articles

Improvement of Photo-Carrier Transport Efficiency From the Semiconductor Particles to Electrode By Using Organic Carrier Transport Materials

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High Power Efficiency Yellow Phosphorescent OLEDs by Using New Iridium Complexes with Halogen-Substituted 2-Phenylbenzo[d]thiazole Ligands

On the basis of the yellow iridium phosphor, bis(2-phenylbenzothiozolato-N,C2′)iridium(acetylacetonate) [(bt)2Ir(acac)], the three halogen-substituted analogues were designed and synthesized by introducing the F, Cl, and Br atoms to the 4-position of phenyl ring in the ligand of 2-phenylbenzo[d]thiazole. The optoelectronic properties of all the four iridium complexes were fully investigated. Compared to the 559 nm peak emission of (bt)2Ir(acac) in CH2Cl2 solution, adding F atom caused the peak emission of (4-F-bt)2Ir(acac) blue shift to 540 nm, while adding Cl and Br atoms made the peak emissions of (4-Cl-bt)2Ir(acac) and (4-Br-bt)2Ir(acac) slightly blue shift to 554 and 555 nm, respectively. The PhOLEDs using the four iridium complexes as dopants were initially fabricated in the conventional device structure (device I): ITO/MoO3/NPB/CBP/CBP:dopants/TPBi/LiF/Al. The three halogen-substituted analogues exhibited turn-on voltages of 3.5–3.9 V, maximum current efficiencies of 35.5–52.4 cd A–1, maximum power efficiencies of 18.3–29.4 lm W–1 and maximum external quantum efficiencies (EQE) of 12.1–17.3%, which were superior than the (bt)2Ir(acac)-based device (28.4 cd A–1, 19.9 lm W–1, 9.8%). After reducing the hole-injecting barrier and using better carrier-transporting materials in the optimized device II, ITO/MoO3/TAPC/TCTA/CBP:dopants/TmPyPB/LiF/Al, all the four devices exhibited lower turn-on voltages of 2.9–3.1 V and excellent performance with maximum EQE over 20%. As a result, they showed high power efficiencies in the range of 55.9–83.2 lm W–1. Among the four optimized devices, the (4-F-bt)2Ir(acac)-based device achieved the highest power efficiency of 83.2 lm W–1. Remarkably, the (bt)2Ir(acac)-based device still possessed high current efficiency of 53.5 cd A–1, power efficiency of 23 lm W–1, and EQE of 19.6% at extremely high luminance of 10 000 cd m–2.

Theoretical study on the electronic structures and photophysical properties of carbon nanorings and their analogues

Density functional theory (DFT) is used in a series of hexacene nanoring ([6]CA), its boron nitride analogue ([6]CA-BN) and lithium ion doping derivatives to obtain an insight into electronic structure, aromaticity property, energy gap, ionization potential, electron affinity and reorganization energy. DFT calculations of these nanorings indicate that the energy gaps of the carbon nanorings are smaller than those of the boron nitride nanorings. The lithium ion doping will remarkably reduce the HOMO and LUMO energy. The aromaticities of the rings are investigated though nucleus-independent chemical shift (NICS) values. The NICS scan suggests that the aromaticities of carbon nanoring systems are more than those of boron nitride analogues, the aromaticities of boron nitride compounds are very weak due to orbital localization. We also calculate the reorganization energy to investigate the charge transport properties. The results show that the carbon nanoring and their analogues could serve as bipolar carrier transport materials in photoelectric functional materials, and the lithium ion doping significantly improves the charge transport properties. The [6]CA-BN nanorings serve as better electron-transport materials. Furthermore, the lithium ion doping significantly affects the charge transfer property of [6]CA-BN nanoring, making it used as bipolar carrier transport materials. The time dependant DFT investigations show that the boron nitride substitution leads to an important change in absorption spectrum with blue-shift. And lithium ion doping has no obvious influence on absorption spectrum.

Synthesis of Disilanyl Double-Pillared Bisdibenzofuran with a High Triplet Energy

A double-pillaring strategy for the synthesis of silacyclophanes has been applied to dibenzofuran to create a new cyclophane molecule that links two dibenzofuran molecules through σ(SiSi)-π conjugation. The performance of disilanyl double-pillared dibenzofuran [(Si)DPBD(O)] in green phosphorescent organic light-emitting diode (OLED) devices was evaluated as both a carrier-transport material and a host material for an Ir-based phosphorescent emitter.

The interplay of intermolecular interactions, packing motifs and electron transport properties in perylene diimide related materials: a theoretical perspective

Perylene diimide (PDI) and its derivatives hold great promise, since they are undeniably considered as an important family of organic n-type semiconductors with both high carrier mobilities and air stabilities comparable to p-type ones, although they traditionally stand out as a class of high-performance dyes and pigments. In this feature article, we summarize the influences of substituents on different positions (imide, ortho, bay) of PDI on their electronic and morphological (packing) properties, which are in close connection with the ability for carrier transport. Then representative molecular packing motifs for PDIs are also classified, with an emphasis on the intricate interplay of intermolecular interactions, packing motifs and electron transport properties of perylene imide related carrier transport materials from a theoretical point of view, towards paving the way for boosting and improving the electron transport mobilities and air stabilities of PDIs-based materials.

A theoretical study of bipolar organic transport material: Disilanyl double-pillared bisanthracene (SiDPBA)

A novel anthracene derivative, disilanyl double-pillared bisanthracene (SiDPBA), has been recently synthesized, which effectively functions as a bipolar carrier transport material in OLEDs. Its charge transport properties have been systemically investigated by band model and hopping model. Band structure calculations of SiDPBA show that the dispersions of the larger valence band and conduction band are comparable, which demonstrates that both electron and hole are favoured for transport, and that SiDPBA has the potential to be used as bipolar transport material from the viewpoint of band model. The density of states and transfer integrals in the main pathways show that it is the edge-to-face CH···π interaction that determines the charge transport property, and that the SiMe2 group contibutes little to it. The calculated charge mobilities of holes and electrons are 0.461 and 0.116 cm2·V–1·s–1, respectively. They are the same order of magnitude, and the electron mobility is on the order of magnitude of perylene-3,4:9,10-tetracarboxylic acid bisimide (PBI) derivatives (0.34 cm2·V–1·s–1). Both models demonstrate that ·SiDPBA has the potential to be used as a bipolar transport material.

N]Cyclo‐2,7‐naphthylenes: Synthesis and Isolation of Macrocyclic Aromatic Hydrocarbons having Bipolar Carrier Transport Ability

Mothball macrocycles: Naphthalene has been coupled to form macrocylic oligomers composed of five, six, and seven naphthalene units (see picture). Thermally stable macrocycles bearing 50, 60, or 70 π electrons within the hydrocarbon structure form columnar assemblies in crystals and serve as bipolar carrier transport materials in organic light-emitting diode devices.

Development of efficient solution-processed red phosphorescent organic light-emitting diodes using carrier transport materials

We report on efficient solution-processed red emissive phosphorescent organic light-emitting diodes (PhOLEDs) employing bis[9-ethyl-3-(4-phenylquinolin-2-yl)-9H-carbazolato- N, C 2′]iridium acetylacetonate (Et-Cvz-PhQ) 2Ir(acac) and bis[9-(2-(2-methoxyethoxy)ethyl)-3-(4-phenylquinolin-2-yl)-9H-carbazolato- N, C 2′]iridium acetylacetonate (EO-Cvz-PhQ) 2Ir(acac) doped into polymer host PVK, blended with electron transporting material OXD-7, and optimized concentration of carrier-transporting material TPD. The optimized PhOLEDs doped with 16 wt% carrier-transporting material shows a maximum luminance efficiency and power efficiency of 4.66 cd/A and 1.22 lm/W, which were increased by 40% and 20%, respectively, compared with that of reference device. The electroluminescence (EL) spectra exhibited the characteristic spectrum of (Et-Cvz-PhQ) 2Ir(acac) and (EO-Cvz-PhQ) 2Ir(acac) with its maximum at 630 nm and CIE (Commission International de L’Eclairage) coordinate of (0.66, 0.33).

Development of organic light‐emitting diodes for electro‐optical integrated devices

AbstractOrganic light‐emitting diodes (OLEDs) are discussed for electro‐optical integrated devices that are used for optical signal transmission. Organic optical devices including polymeric optical fibers are used for optical communication applications to realize polymeric electro‐optical integrated devices. The OLEDs were fabricated by vacuum process, i.e. the organic molecular beam deposition (OMBD) technique or a solution process on a polymeric or a glass substrate, for comparison. Optical signals faster than 100 MHz have been created by applying pulsed voltage directly to the OLED utilizing rubrene doped in 8‐hydoxyquinolinum aluminum (Alq3), as an emissive layer. OLEDs fabricated by solution process utilizing rubrene doped in carrier‐transporting materials have also discussed. OLEDs utilizing polymeric materials by solution process are also fabricated and discussed. Moving‐picture signals are transmitted utilizing both vacuum‐ and solution‐processed OLEDs, respectively.

Improving the performance of blue phosphorescent organic light-emitting devices using a composite emitter

A composite emitter is constructed by doping a carrier-transporting material into a conventional emitter composing of only host and dopant. The transport of carriers from either hole- or electron-transporting layer into the emitter can be promoted through the carrier-transporting material, in particular, when a wide-band-gap host material is used. A blue phosphorescent OLED based on iridium(III)bis((4,6-difluorophenyl)-pyridinate-N,C 2′)-picolinate (FIrpic) as dopant in the composite emitter achieved a power efficiency of 20 lm/W and a low driving voltage of 4.2 V at 1000 cd/m 2, whose current efficiency at 20 mA/cm 2 was 2.5 times better than that of device using the conventional emitter.

Suppression of efficiency roll off in blue phosphorescent organic light-emitting devices using double emission layers with additional carrier-transporting material

By employing double emission layers (DELs) into blue phosphorescent organic light-emitting device (PHOLED), the device shows improved current efficiency by a factor of 1.8 as compared with that of device using single emission layer. Furthermore, by doping additional carrier-transporting material into DELs, the device shows only a slight efficiency roll off of 24% from low (1 mA/cm2 and 400 cd/m2) to high current density (40 mA/cm2 and 10 000 cd/m2). The dramatic improvement in device performances can be attributed to the formation of a broader carrier recombination zone and the elimination of carrier accumulation at the interface. A blue PHOLED with a current efficiency of 29.5 cd/A, a power efficiency of 21 lm/W, and a low driving voltage of 4.4 V with a Commission Internationale deL’Eclairage (CIEx,y) of (0.16, 0.35) at a practical brightness of 1000 cd/m2 can be achieved.

Theoretical study on the electronic structures and optical properties of oxadisilole-substituted acenes

The electronic structures and optical properties of 12 oxadiasilole-substituted acenes were studied using the density functional theory. The calculated maximal absorption and emission wavelengths of oxadisilole-substituted anthracenes are in good agreement with the experimental results. We further designed oxadiasilole-substituted tetracenes and pentacenes. The HOMO energy level increases with the increasing of benzene ring, however the LUMO energy level decreases with the increasing of oxadisilole ring. The transfer integrals and reorganization energies for hole and electron were similar. As a result, these systems can be used as potentially efficient and balanced carrier transport materials.

Improvement in carrier transport and recombination of white phosphorescent organic light-emitting devices using a composite blue emitter

We demonstrate high-efficiency white phosphorescent organic light-emitting devices (PHOLEDs) based on a yellow and composite blue emitters. The composite blue emitter is constructed from a wide-band-gap host, organometallic iridium dopant, and a carrier-transporting material. Under the same driving current density and emissive color, the current efficiency of the white PHOLEDs can be enhanced by a factor of 1.4 comparing to that of using typical blue emitter composed of host and dopant only. Attaching an outcoupling enhancement film onto glass substrate, the white PHOLEDs with a current efficiency of 47 cd/A, a power efficiency of 32 lm/W, and a CIEx,y (CIE: Commission Internationale d'Eclairage) of (0.40,0.44) at a practical brightness of 1000 cd/m2 can be achieved.

Efficient p-i-n type organic solar cells incorporating 1,4,5,8-naphthalenetetracarboxylic dianhydride as transparent electron transport material

The implementation of proper charge carrier transport materials in p-i-n type organic solar cells strongly influences the device performance. Our investigation focuses on the substitution of the standard layer sequence used at the side of electron transport, usually consisting of either C60/exciton blocking layer/metal or n-C60/metal by a layer sequence including a window layer. Here, we evaluate the transparent electron transport material 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), which guarantees a loss free charge extraction from the active materials due to a good energy level alignment as well as exciton blocking due to its wide bandgap. It is demonstrated that upon the exchange of the electron transport layer n-C60 for n-NTCDA, the solar power conversion efficiency of a p-i-n device can be increased by 10%.

P‐203: The Improvement of White OLED's Performance

AbstractWe have researched further improvement upon three‐component white OLED devices with using fluorescent materials disclosed in SID'07. The long lifetime over 70,000hr@1,000cd/m2 maintaining at a high efficiency of 20cd/A was achieved by optimization of a carrier balance with using new carrier transport materials. Performance of blue and red OLEDs were also developed in order to enhance capability of white OLEDs, respectively. A long lifetime of 50,000hr@1,000cd/m2 was achieved by a new light blue dopant BD‐5. Meanwhile, we studied phosphorescent materials for the red device to progress towards further low power consumption. A low driving voltage of 4.4V at 10mA/cm2 and a long lifetime of 100,000hr@1,000cd/m2 were realized by employing a new host material and new carrier transport materials. These materials will contribute to large‐Size OLED TVs with color filter method or lighting application for practical use.

Low-voltage, high-efficiency blue phosphorescent organic light-emitting devices

Low-voltage, high-efficiency blue phosphorescent organic light-emitting devices based on a composite emitter, including a wide-band-gap host, a carrier-transporting material, and an organometallic iridium dopant, have been demonstrated. The devices exhibit an external quantum efficiency of 12%, a power efficiency of 17lm∕W and a low voltage of 4.8V at a practical brightness of 1000cd∕m2 with a CIEx,y of (0.16, 0.35), which was twofold higher than that of using the typical emitter composed of host and dopant only. The dramatic enhancement in performance can be attributed to the transport of carriers into the wide-band-gap host, which can be promoted through doping a carrier-transporting material in emitter for increasing carrier recombination.

Synthesis and Electron Transport Layer Properties of Zinc Metallic Complexes Containing Quinoline Moieties in OLED

Metal chelate complexes are often studied as a carrier transport material in OLEDs, because they have a good electron transport property. We have synthesized metal chelate complexes, Bis(8-hydroxyquinolinato)zinc (Znq2) and Bis(10-hydroxybenzo[h]quinolinato)zinc (Zn(bq)2) as well as Bis(10-hydroxybenzo[h]quinolinato)beryllium Be(bq)2 and compared EL performance with commercialized Alq3. We will report the basic EL performance of synthesized complex materials such as EL spectrum, current efficiency and CIE value. When we applied Znq2 and Zn(bq)2 to conventional blue OLED device, the device exhibited better power efficiency of 0.4 lm/w and 0.28 lm/w compared to 0.2 lm/w of Alq3 device. We also introduce overall EL performance of metal chelate device including Be(bq)2.

47.5L: Late‐News Paper: Highly Efficient White OLEDs Using RGB Fluorescent Materials

AbstractA three‐component white OLED device using fluorescent materials was developed for full‐color displays. Very high power‐efficiency of 17 lm/W@10mA/cm2 and long lifetime over 30,000hr@1,000cd/m2 were achieved by using new carrier transport materials with high mobility. Power consumption and half lifetime of a 2.2″ QVGA full‐color display were estimated by simulation to be about 200mW(200cd/m2,100%ON) by combining the device with top‐emission structure. This value is comparable to the state of the art of LCDs.

High-Efficiency Green Phosphorescent Organic Light-Emitting Devices with Chemically Doped Layers

We have developed green phosphorescent organic light-emitting devices (OLEDs) with high quantum and luminous efficiencies. A green phosphorescent metal complex, fac-tris(2-phenylpyridine) iridium [Ir(ppy)3], was used as an emitter material. Wide-energy-gap materials with high triplet excited energy levels were used as host materials for Ir(ppy)3 and as carrier transport materials. Hole injection and electron injection from the electrodes were balanced by placing chemically doped layers at the interface between the electrodes and the organic layers. In addition, a highly reflective Ag cathode was employed as an anode, instead of a conventional Al cathode to enhance the reflectivity of the cathode metal. An optimized device exhibited an external quantum efficiency (EQE) of 27% (95 cd/A) and a high power efficiency of 97 lm/W at 100 cd/m2 at 3.1 V.

Preparation and Characterization of Organic Electroluminescent Devices Using Fluorescent Polyimides as a Light-Emitting Layer

The electroluminescence (EL) properties of the organic electroluminescent devices (OELDs) using three kinds of fluorescent polyimides, ODPA/DCHM (HFPI-1), 10FEDA/DCHM (HFPI2), and P2FDA/DCHM (HFPI-3), for light emitting layers were investigated. The HOMO energy levels of HFPI-1 and HFPI-2 were determined as 6.7 and 6.9 eV, respectively, using cyclic voltammetry. Single-layer EL devices having the ITO/HFPI/Al structure were fabricated using HFPI-1 and HFPI-2, but these devices did not exhibit any current and EL properties. This is due to the quite large energy gaps between ITO (work function: 4.7 eV) and the HOMO levels of polyimides (>2.0 eV). Further, multilayer devices were fabricated using carrier transport materials. For these devices, the carrier injection properties from both electrodes were expected to be improved. However, no current and EL properties were observed for all devices. These experimental facts indicate that fluorescent polyimides have little carrier transportability. The EL properties of the devices using TPD-dispersed fluorescent poly-imides as hole transport layers were also investigated. In case of using HFPI-3 whose excitation spectrum is overlapped with the emission of Alq3, the device emits a distinct EL at 520 nm under the operation at 15 V. Further, the emission was displaced to 560 nm under the operation at 30 V, which can be explained by the energy transfer to HFPI-3 or the displacement of the recombination zone of electrons and holes from Alq3 to HFPI-3+TPD layer. This device mechanism might be promising as a novel wavelength-tunable OELD.