α-NPD Layer Research Articles

Thermal Diffusivities of Tris(8-hydroxyquinoline)aluminum and N,N'-Di(1-naphthyl)-N,N'-diphenylbenzidine Thin Films with Sub-Hundred Nanometer Thicknesses

The thermal diffusivities of tris(8-hydroxyquinoline)aluminum (Alq3) and N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD) films have been characterized quantitatively using a “rear heating/front detection-type” nanosecond thermoreflectance system. Alq3 or α-NPD films sandwiched by Al films, Al/(Alq3 or α-NPD)/Al three-layered films, were prepared by vacuum evaporation. Al acted as a reflective layer for pulse lasers in the thermoreflectance system. The nominal thicknesses of Alq3 and α-NPD layers varied roughly from 30 to 100 nm. The thermal diffusivities of Alq3 and α-NPD films were found to be (1.4–1.7)×10-7 and 1.4×10-7 m2 s-1, respectively.

Remote doping of a pentacene transistor: Control of charge transfer by molecular-level engineering

We demonstrate that holes from a p-doped N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) layer transfer to an adjacent pentacene film. The spatial separation of carriers from dopants, or remote doping, is demonstrated with a combination of photoemission spectroscopy and current-voltage measurements for a p-doped α-NPD/pentacene heterojunction. Increased conductivity of the pentacene film is observed in both nongated temperature-dependent conductivity and gated thin-film transistor measurements.

Study on MoO3−x films deposited by reactive sputtering for organic light-emitting diodes

The authors investigate the role of reduced molybdenum trioxide [MoO3−x (x≤1)] films in organic light-emitting diodes, particularly from the viewpoint of the oxidation state of Mo. MoO3−x films were deposited by reactive sputtering under a mixture of argon (Ar) and oxygen (O2). The O2 gas-flow ratio (GFR) [O2/(Ar+O2)] was adjusted between 10% and 100%. Mo with six, five, and four valence electrons was detected in MoO3−x film deposited with an O2 GFR of 10% and 12.5%, whereas, under higher O2 GFRs, only six valence electrons for Mo in the MoO3−x film were detected. N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) layer, hole-transport material, were deposited over the MoO3−x layer by subsequent vacuum evaporation. At the α-NPD/MoO3−x interface, it was found that α-NPD cations were generated and that MoO3−x was reduced, which provided evidence of charge transfer across the interface by Raman spectroscopy and x-ray photoelectron spectroscopy.

Photo- and electroluminescent properties of zinc(II) complexes with tetradentate Schiff bases, derivatives of salicylic aldehyde

It is studied how the introduction of various substituents into the composition of organic ligands affects the photoluminescence spectra of new zinc(II) complexes with tetradentate Schiff bases H2L (derivatives of salicylic aldehyde (H2SAL1, H2SAL2) and o-vanillin (H2MO1, H2MO2) with ethylenediamine and o-phenylenediamine) in the form of bulk solids and thin films. It is demonstrated that the emission spectra of bulk solid complexes without o-phenylenediamine bridges (ZnSAL1 and ZnMO1) contain additional long-wavelength bands compared to the spectra of corresponding thin films. In the case of films obtained from [ZnSAL1]2 dimer complexes, the long-wavelength band is dominant. At the same time, the photoluminescence spectra of ZnSAL2 and ZnMO2 complexes with o-phenylenediamine bridges are similar in the case of solid samples and thin films. The electroluminescent properties of organic light-emitting diodes (OLEDs) with the ITO/α-NPD/ZnL/Ca:Al structure are studied. The bathochromic shift of the electroluminescence peaks of OLEDs with respect to the photoluminescence spectra of bulk solid samples and thin films is probably related to the formation of exciplexes at the α-NPD/ZnL interface. The electroluminescence spectra of OLEDs based on [ZnSAL1]2 show a hypsochromic shift of the emission maximum, which can be caused by a shift of the recombination region into the α-NPD layer.

Analysis of Organic Light-Emitting Diode As a Maxwell−Wagner Effect Element by Time-Resolved Optical Second Harmonic Generation Measurement

An organic light-emitting diode (OLED) was analyzed as a system of the Maxwell−Wagner effect element, by using time-resolved optical second harmonic generation (SHG) measurement. A transient SHG signal generated from N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) layer in an indium zinc oxide (IZO)/α-NPD/tris(8-hydroxy-quinolinato) aluminum(III) (Alq3)/ LiF/Al OLED device was selectively probed with applying alternating current (AC) square wave voltage, and the electric field distribution change in the device was examined. Results showed that charge Qs at the α-NPD/Alq3 interface changed in accordance with charging and discharging processes accompanied by electroluminescence (EL). We found that observed SHG response reflected well charging and discharging of carriers on electrodes and carrier transit across α-NPD and Alq3 layers, and accounted for nonreversal charging and discharging processes.

Probing of carrier behavior in organic electroluminescent diode using electric field induced optical second-harmonic generation measurement

By using the electric field induced optical second-harmonic generation (EFISHG) measurements, we probed the transient electric field in a double-layer indium zinc oxide (IZO)/N, N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4, 4′-diamine(α-NPD)/tris(8-hydroxy-quinolinato)aluminum(III) (Alq3)/Al electroluminescent (EL) diode. Results evidently showed that EL was initiated by the injected hole transport across α-NPD layer, and holes accumulated at the α-NPD/Alq3 interface while EL was enhanced. Analysis based on the Maxwell–Wagner effect model well accounted for the hole accumulation. EFISHG measurement is useful as a tool for probing carrier behavior in organic EL devices.

Structural and electronic implications for carrier injection into organic semiconductors

We report on the structural and electronic interface formation between ITO (indium-tin-oxide) and prototypical organic small molecular semiconductors, i.e., CuPc (copper phthalocyanine) and α-NPD (N,N′-di(naphtalen-1-yl)-N,N′-diphenyl-benzidine). In particular, the effects of in situ oxygen plasma pretreatment of the ITO surface on interface properties are examined in detail: Organic layer-thickness dependent Kelvin probe measurements revealed a good alignment of the ITO work function and the highest occupied electronic level of the organic material in all samples. In contrast, the electrical properties of hole-only and bipolar organic diodes depend strongly on the treatment of ITO prior to organic deposition. This dependence is more pronounced for diodes made of polycrystalline CuPc than for those of amorphous α-NPD layers. X-ray diffraction and atomic force microscopic (AFM) investigations of CuPc nucleation and growth evidenced a more pronounced texture of the polycrystalline film structure on the ITO substrate that was oxygen plasma treated prior to organic layer deposition. These findings suggest that the anisotropic electrical properties of CuPc crystallites, and their orientation with respect to the substrate, strongly affect the charge carrier injection and transport properties at the anode interface.

Dual luminescence from organic/inorganic hybrid p-n junction light-emitting diodes

We fabricated a hybrid p-n junction structure using n-type InGaN/GaN multiple quantum wells (MQWs) and p-type N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD). The hybrid structure shows a good current rectifying characteristic similar with conventional p-n junction diodes. Electroluminescence (EL) of the hybrid device exhibits two emission bands originated from InGaN/GaN MQWs, as well as α-NPD layer. The EL properties can be explained by either (or both) electron-hole charge transport between the components or (and) efficient energy transfer via Föster mechanism. The device characteristics could be applicable to various multicolor light-emitting diodes by constructing other organic/inorganic hybrid junctions.

EL Behavior for Blue-Emitting Aluminum Quinoline-Based Organic Light-Emitting Diodes

We fabrucated bilayer organic light-emitting diodes (PLEDs) consisting of a diamine derivative (α-NPD) and aluminum(III) bis(2-methyl-8-quinolinate)4-phenylphenolate (BAlq) and investigated their EL properties depending on BAlq's thickness and a kind of cathode metal. Since the OLEDs showed EL spectra including both α-NPD and Balq3 emissions, we discussed the recombunation mechanism of OLEDs from the ratio of α-NPD and Balq3 emissions. The α-NPD/Balq3 OLEDs have two recombination zones in α-NPD and BAlq layer, so that electrons are injected into α-NPD layer. To our surprise, when Al with a poor electron injection is used as the cathode, α-NPD content was found to increase as compared with the use of LiF/Al in spite of decreasing electron current. Electron injection into α-NPD was thought to be one of degradation factors.

Enhancing hole transports and generating hole traps by doping organic hole-transport layers with p-type molecules of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane

We investigated the relationship between the hole-transport and hole-trap characteristics of N, N′-diphenyl- N, N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) doped with p-type molecules of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4-TCNQ) at various concentrations. The results of our current density–voltage, field-effect transistor, and thermally stimulated current studies revealed that the current densities of hole-only α-NPD devices at a certain driving voltage increased in the F 4-TCNQ concentration region between 0 mol% and 3 mol% due to the generation of free holes while the hole mobilities of the α-NPD layers decreased due to an increase in hole-trap concentration and deepened hole-trap energy levels, which were caused by the F 4-TCNQ doping. The optimized doping concentration of F 4-TCNQ was 3 mol%, which provided the highest current density for the hole-only device. On the other hand, since the increase in free-hole concentration was overcome by the decrease in hole mobility, the current density of the hole-only device decreased at the F 4-TCNQ concentration of 4 mol% when compared with the optimized concentration.

Electrical transport in crystalline perylene derivatives films for electronic devices

This paper presents investigations on the electrical transport phenomena in perylene and PTCDA emphasising the particularities of the charge carrier injection at the contacts with inorganic semiconductors (Si) and metals (Cu, Al) and the effect of the supplementary organic layer (ZnPc, α-NPD) on the electrical conduction. The I–V characteristics for different delay and integration times, at forward and reverse bias, have evidenced a dominant ohmic behaviour of these SIS and MIS structures at low voltage (<0.1 V) and limitations of the current induced by the space charge and trap charge at voltages >1 V. An intermediate organic layer (ZnPc) with higher ionisation potential than PTCDA and lower electron affinity than perylene improves the charge carrier injection and the conduction properties of the Si/PTCDA/Si and forward biased Cu/perylene/Si heterostructures. No increase in the hole injection was obtained in Si(p)/perylene/Si(p) heterostructure introducing an α-NPD layer.

Small Molecular Organic Nonvolatile Memory Cells Fabricated with in Situ O2Plasma Oxidation

We developed small molecular organic nonvolatile 4F² memory cells using metal layer evaporation followed by O₂ plasma oxidation. Our memory cells sandwich an upper α-NPD layer, Al nanocrystals surrounded by Al₂O₃, and a bottom α-NPD layer between top and bottom electrodes. Their nonvolatile memory characteristics are excellent: the V th , V p (program), V e (erase), memory margin (I on /I off ), data retention time, and erase and program endurance were 2.6V, 5.3V; 8.5V, ?1.5×10², 1×10 5 s, and 1×10³ cycles, respectively. They also demonstrated symmetrical current versus voltage characteristics and a reversible erase and program process, indicating potential for terabit-level nonvolatile memory.

Current Conduction Mechanism for Low-molecular Organic Nonvolatile Memory

AbstractOrganic devices fabricated with a top metal layer/conductive organic layer/middle metal layer/conductive organic layer/bottom metal layer structure have been reported to demonstrate nonvolatile memory behavior such as an after writing (Ion)/after erasing (Ioff) performance of &gt; 1 × 101 and a response time of ∼10 ns, when the organic conductive layers were AIDCN (2-amino-4, 5-imidazoledicarbonitrile), Alq3 (Aluminum tris(8-hydroxyquinoline)), or α-NPD. We fabricated an organic nonvolatile memory device with a structure of α-NPD/Al nanocrystals surrounded by Al2O3/α-NPD/Al, where α-NPD was N,N'-bis(1-naphthyl)-1,1'biphenyl4-4''diamine. A layer of Al nanocrystals, confirmed by a 1.25-MV high voltage transmission-electron-microscope, was uniformly produced between the α-NPD layers by Al layer evaporation at 1.0 Å/sec on the α-NPD followed by O2 plasma oxidation. We confirmed a conduction bistability of ∼102 and a threshold voltage for a set state of 3 V. Al nanocrystals surrounded by amorphous Al2O3 were formed in the α-NPD. They presented seven different reversible current paths for an electron charge or discharge on the nanocrystals. The current slightly increased with an applied bias from 0 V to Vth (a high resistance state (Ioff)), abruptly increased with an applied bias from Vth to Vp, decreased with an increasing applied bias from Vp to Ve (a negative differential resistance (NDR) region), and slightly increased with an applied bias above Ve. After sweeping the first applied voltage from 0 to 10 V (erase), a second applied bias was swept from 0 to Vp (program), where the current followed a high resistance state (Ioff). Next, a third applied bias was swept from 0 to Vp again, where the current followed a low resistance state (Ion). Surprisingly, the ratio of Ion to Ioff was ∼1×102, which is enough current difference to be nonvolatile memory behavior. These I-V characteristics under a positive applied bias were symmetrically repeated under a negative applied bias. All the current sweeping paths were reproducible and symmetrical for an applied bias polarity. In particular, our device demonstrated multi-level nonvolatile memory behavior. It also revealed the current conduction mechanism for each of its operation regions. We observed that the high resistance and low resistance regions followed space-charge-limited current conduction, the Vth to Vp and VNDR to Ve regions followed precisely thermionic-field-emission current conduction, and the above Ve regions followed space-charge-limited current conduction.

Formation of Ohmic hole injection by inserting an ultrathin layer of molybdenum trioxide between indium tin oxide and organic hole-transporting layers

Current density–voltage (J-V) characteristics of hole-only devices using indium tin oxide (ITO) anode and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) layers were measured with various thicknesses of a molybdenum trioxide (MoO3) buffer layer inserted between ITO and α-NPD. The device with a 0.75-nm-thick MoO3 layer forms Ohmic hole injection at the ITO∕MoO3∕α-NPD interfaces and J-V characteristics of this device are controlled by a space-charge-limited current. Results of X-ray photoelectron and ultraviolet/visible/near-infrared absorption studies revealed that this Ohmic hole injection is attributable to an electron transfer from ITO and α-NPD to MoO3.

P-186: Enhanced Efficiency and Stability of Organic Light-Emitting Diodes with Copper Hexadecafluorophthalocyanine Doped Hole Transport Layer

We report the enhancement of the electroluminescence (EL) efficiency and the device stability of organic light-emitting diodes (OLEDs) by inserting a copper hexadecafluorophthalocyanine (F16CuPC)-doped N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) layer between an indium-tin oxide and α-NPD. Compared with the reference device consisting of the bilayer structure of α-NPD and tris(8-hydroxyquinoline) aluminum (Alq3), the devices having F16CuPC-doped α-NPD show not only an increased EL efficiency (∼4.5 cd/A compared with ∼3.8 cd/A for the reference device) and lower driving voltage but also a longer lifetime and smaller operating voltage variation over time. It is suggested that the improved device performance can be attributed to the higher hole-injection efficiency and thermal stability of the F16CuPC-doped α-NPD layer.

Effect of Buffer or Barrier Layer on Bistability for Nonvolatile Memory Fabricated with Al Nanocrystals Embedded in α-NPD

ABSTRACTRecently, low molecular organic non-volatile memories have been developed as a next generation of non-volatile memory because of nano-meter device-feature size and nano-second access and store-time. We developed a non-volatile memory fabricated with the device structure of Al/ α-NPD/Al nano-crystals surrounded by Al2O3/α-NPD/Al, where α-NPD is N,N'-bis(1-naphthyl)-1,1'biphenyl4-4”diamine. One layer of Al nano-crystals with ∼20 nm-width ∼20 nm length was uniform produced between α-NPD layers, confirmed by 1.2MV high voltage transmission-electron-microscope. This device showed Vth of 3.0 V, Vprogram of 4.3 V, and Verase of 6.3 V. Particularly, this device exhibited an excellent non-volatile memory behavior performing the bi-stability (Iprogrm/Ierase) of &gt;1×102, program/erase cycles of &gt;1×105 and multi-levels. In addition, previous reports about low molecular organic non-volatile memories have showed a bad reproducible memory characteristic. However, this issue was completely solved via isolating Al nano-crystals embedded in α-NPD by O2 plasma oxidation. The uniformity of Vth, Vp, and Ve were 9.91%, 6.94% and 7.92%, respectively. Furthermore, the effect of buffer or barrier layer on non-volatile memory characteristics was investigate to examine the control ability for Vth, Vp, and Ve. The 0.5-nm LiF showed a barrier layer behavior suppressing the bi-stability of non-volatile memory. Otherwise, 15-nm CuPc exhibited a buffer layer behavior enhancing the bi-stability of nonvolatile memory.

Enhancement of Efficiency for White-Organic Light-Emitting Diode with a Thin Electron-Blocking Layer

In this study, we have enhanced the emission property of white-organic light-emitting diodes using the method of inserting the ultrathin electron-blocking layer (thin-EBL) between red–green–blue emissive layers. The devices into which three emissive layers are inserted were fabricated with indium tin oxide/CuPc/[-di(naphthalene-1-yl)--diphenyl-benzidine] (α-NPD/DPVBi/thin-EBL/: 4-(dicyano-methylene)-2-methyl-6-(dimethylaminostyryl-4H-pyran (DCM1)/. To enhance the white emission property and improve the efficiency of devices, we suggest the α-NPD as the thin-EBL. According to knowledge of the energy band diagram, α-NPD was inserted between the blue-emitting layer and green-emitting layer. The α-NPD layer showed the effective blocking of electron as a function of thickness. From the Experimental, the effective blocking of electrons through controlling the thin-EBL thickness embodied the balanced white emission and improved the efficiency of the device. In our device with α-NPD, it had a maximum power efficiency of at , a luminance efficiency of at , a luminance of at , and the CIE coordinates were (0.353, 0.357), respectively.

Spectroscopic ellipsometric characterization of organic films obtained via organic vapor phase deposition

Thin films of Tris(8-hydroxyquinoline)-aluminum(III) (Alq3) and N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD ) were deposited on large-area silicon substrates by means of the recently developed organic vapor phase deposition (OVPD) method. Variable-angle spectroscopic ellipsometry was used to measure the optical constants of OVPD Alq3 and α-NPD layers in the 0.8–5 eV energy range. The absorption onset which defines the lower limit of the optical band gap was found to be at ∼2.65 eV and ∼2.9 eV for Alq3 and α-NPD , respectively. Additionally, the thicknesses of the layers as well as the thickness profiles of the organic thin films were determined along the 8′′ diameter of the wafers. The thickness analysis revealed large-area uniform deposition of the films.

Efficient white organic electroluminescent devices consisting of blue- and red-emitting layers

We report the fabrication and the characterization of white-organic-light-emitting devices consisting of a blue-emitting layer of 1,4-bis(2,2-diphenyl vinyl)benzene (DPVBi) and a red-emitting layer of 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-1 H,5 H-benzo[i,j]quinolizin-8-yl)vinyl]-4 H-pyran (DCM2) doped into either 4,4′bis[ N-(1-napthyl)- N-phenyl-amino]-biphenyl (α-NPD) or tris(8-hydroxyquinoline) aluminum (Alq 3). The spectral emission depends on both the doping location of DCM2 and its doping concentration. The electroluminescence (EL) spectra consist of two broad bands around 460 nm (DPVBi) and 560 nm (α-NPD:DCM2) or 590 nm (Alq 3:DCM2). We obtained an efficient white-light emission from the devices with the 0.2% DCM2 doping in α-NPD layer. The device shows the CIE coordinates of (0.33, 0.36), an external quantum efficiency (QE) of about 3.1%, and a luminous efficiency of 3.75 lm/W at luminance 100 cd/m 2. The maximum luminance of about 41,000 cd/m 2 was obtained.

Nondoped-type white organic electroluminescent devices utilizing complementary color and exciton diffusion

We fabricated nondoped white organic electroluminescent devices using vacuum-deposited thin films of blue-emitting 4,4′-bis[N-1-napthyl-N-phenyl-amino]biphenyl (α-NPD) and orange-emitting 4-(dicyanomethylene)-2-metyl-6-(p-dimethyl aminostyryl)-4H-pyran (DCM), a hole-blocking layer of 2-(4-biphenyl)-5-(p-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD) and electron-transporting tris(8-quinolinolato) aluminum (III). Excitons formed at the α-NPD/tBu-PBD interface sequentially transfer their energy to α-NPD via the Förster mechanism. The exciton is captured by an ultrathin DCM layer located within the pure α-NPD layer. The position of the DCM determines the device spectrum, and enables a white emission to be achieved. The spectrum is not sensitive to the voltage applied, and the devices show maximum luminance of about 1000 cd/m2.