α-NPD Layer Research Articles

Effect of hole-transport layer thickness on the performance of organic light-emitting diodes

In this study, we investigated the influence of hole-transport layer thickness (0, 5, 10, 15, 20, 25, 30, 35 and 40 nm), N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine (α-NPD) on the performance of thermally evaporated multilayer organic light-emitting diodes (OLEDs) with tris(8-hydroxy-quinolinato)aluminium (Alq3) as electroluminescent material. A device with 5 nm α-NPD thickness showed a current density ∼30 times higher than without any α-NPD layer. However, the current density decreased as the α-NPD thickness increased beyond 5 nm. In contrast to the current density, the device efficiency improved as α-NPD layer thickness increased to 30 nm, after which a drop in efficiency was observed. The current efficiency of the device with 30 nm α-NPD thickness was 10 times higher than the device with 5 nm α-NPD thickness. The results indicated that hole-injection in OLEDs was more efficient with thin 5 nm α-NPD, while optimal efficiency was at 30 nm of α-NPD thickness.

Spin transport properties in a naphthyl diamine derivative film investigated by the spin pumping

We report the spin transport properties in a thin film of a naphthyl diamine derivative: N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)-2,2′-dimethylbenzidine (α-NPD). In a palladium (Pd)/α-NPD/Ni80Fe20 tri-layer structure sample, a pure spin current is generated in the α-NPD layer with the spin pumping driven by ferromagnetic resonance (FMR). The generated spin current is absorbed into the Pd layer, and converted into a charge current with the inverse spin-Hall effect (ISHE) in Pd. An electromotive force due to the ISHE in the Pd layer is observed under the FMR of the Ni80Fe20 layer, which is clear evidence for the spin transport in an α-NPD film. The spin diffusion length in an α-NPD film is estimated to be about 62 nm at room temperature, which is long enough as a spin transport material for spintronic devices.

Electrical conductivity of 1,3-bis(N-carbazolyl)benzene (mCP) on a hole-transport bilayer

We studied the electrical conductivity of a bilayer specimen of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (α-NPD) and 1,3-bis(N-carbazolyl) benzene (mCP). The conductivity of indium tin oxide ITO/(heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane (FSAM)/α-NPD(50 nm)/Al can be explained by the space-charge-limited current (SCLC) model. In an α-NPD(50 nm)/mCP(10 nm) specimen, the current is similar to that in an α-NPD(50 nm) specimen despite the existence of a hole barrier between the α-NPD and mCP. The current in an α-NPD/mCP specimen is also proportional to the square of the voltage. We propose a conduction model that combines SCLC in the α-NPD layer and tunneling in the mCP layer.

Direct visualization of electric potential distribution in organic light emitting diode by phase-shifting electron holography

We utilized phase-shifting electron holography on organic light emitting diodes consisting of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminum (Alq3) layers to visualize their built-in potential distribution. The bilayer showed three different electric fields, namely, the fields in the α-NPD layer, near the α-NPD/Alq3 interface, and in the Alq3 layer measured as −1.8 ± 0.4 MV m−1, −10.0 ± 2 MV m−1, 3.1 ± 0.6 MV m−1, respectively. We show that they are related to hole accumulation in the α-NPD layer, the charge carrier accumulation junction around the α-NPD/Alq3 interface, and the giant surface potential spontaneously polarized in the Alq3 layer.

The origin of changes in electrical properties of organic films fabricated at various vacuum-deposition rates

We investigated the dependence of electrical properties on vacuum-deposition rate for films of N,N՛-diphenyl-N,N՛-bis(1-naphthyl)-1,1՛-biphenyl-4,4՛-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminium (Alq3), hole- and electron-transport materials widely used in organic light-emitting diodes (OLEDs), respectively. Hole-only devices (HODs) of α-NPD showed an increase of hole current when α-NPD layers were fabricated at high deposition rates, while the tendency was opposite for electron-only devices (EODs) of Alq3. We found that the increased hole current at high deposition rates for HODs was caused by a horizontal orientation of α-NPD molecules relative to a substrate plane which facilitates hole transport through the films. On the other hand, the decreased electron current at high deposition rates for EODs could be ascribed to the increased surface roughness of Alq3 films which decreases electron injection. Additionally, we demonstrated that long-term operational stability was enhanced for HODs fabricated at high deposition rates and operated at a constant current. Use of a high deposition rate means a reduction of deposition time which is helpful in OLED manufacturing. The improved device performance when high deposition rates were used and the detailed understanding of its origins as we demonstrated here would lead to fabrication of high-performance OLEDs at a low cost in the future.

Ab initio modeling of steady-state and time-dependent charge transport in hole-only α-NPD devices

We present an ab initio modeling study of steady-state and time-dependent charge transport in hole-only devices of the amorphous molecular semiconductor α–NPD [N,N′-Di(1–naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine]. The study is based on the microscopic information obtained from atomistic simulations of the morphology and density functional theory calculations of the molecular hole energies, reorganization energies, and transfer integrals. Using stochastic approaches, the microscopic information obtained in simulation boxes at a length scale of ∼10 nm is expanded and employed in one-dimensional (1D) and three-dimensional (3D) master-equation modeling of the charge transport at the device scale of ∼100 nm. Without any fit parameter, predicted current density-voltage and impedance spectroscopy data obtained with the 3D modeling are in very good agreement with measured data on devices with different α-NPD layer thicknesses in a wide range of temperatures, bias voltages, and frequencies. Similarly good results are obtained with the computationally much more efficient 1D modeling after optimizing a hopping prefactor.

Study of Electrical Conduction Mechanism of Organic Double-Layer Diode Using Electric Field Induced Optical Second Harmonic Generation Measurement.

By using electric field induced optical second harmonic generation (EFISHG) and current voltage (I-V) measurements, we studied the electrical transport mechanism of organic double-layer diodes with a structure of Au/N, N'-di-[(1-naphthyl)-N, N'-diphenyl]-(1,1'-biphenyl)-4,4'-diamine (a-NPD)/poly(methyl methacrylate) (PMMA)/indium zinc oxide (IZO). Here the α-NPD is a carrier transport layer and the PMMA is an electrical insulating layer. The current level was very low, but the I-V characteristics showed a rectifying behavior. The EFISHG measurement selectively and directly probed the electric field across the α-NPD layer, and showed that the electric field across the a-NPD layer is completely relaxed owing to the charge accumulation at the a-NPD/PMMA interface in the region V > 0, whereas the carrier accumulation was not significant in the region V < 0. On the basis of these experimental results, we proposed a model of the rectification. Further, by coupling the I-V characteristics with the EFISHG measurement, the I-V characteristics of the diodes were well converted into the current-electric field (I-E) characteristics of the α-NPD layer and the PMMA layer. The I-E characteristics suggested the Schottky-type conduction governs the carrier transport. We conclude that the I-V measurement coupled with the EFISHG measurement is very useful to study carrier transport mechanism of the organic double-layer diodes.

Lifetime improvement mechanism in organic light-emitting diodes with mixed materials at a heterojunction interface

To investigate the lifetime improvement mechanism caused by mixing at the heterojunction interface, organic light-emitting diodes (OLEDs) with stacked and mixed 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (α-NPD)/tris(8-hydroxyquinoline)aluminum (Alq3) interfaces were fabricated, and changes in their displacement current due to continuous operation were measured. A decrease in accumulated holes at the α-NPD/Alq3 interface was observed in the stacked configuration devices over longer operations. These results indicate that the injected hole density was reduced during continuous operation, implying that the carrier balance became uneven in the emission region. However, few accumulated holes and changes in the displacement current due to continuous operation were observed in the devices having the mixed layer. Therefore, it was deduced that the number of holes concentrated between the α-NPD and Alq3 layers was decreased by mixing at the heterojunction interface, and that the change in the number of holes was smaller during continuous operation, resulting in less degradation.

Role of oxygen vacancies in TiOx films in electronic structure at interface with an α-NPD layer

Metal oxide buffer layers are a promising method for improving the efficiency and stability of organic photovoltaic cells and organic light-emitting diodes. To design organic electronic devices intelligently, it is necessary to understand the electronic structure at organic/metal-oxide interfaces. In this study, the electronic structure of films of a well-known hole-transporting material, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) on TiOx underlayers was studied via ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. New electronic states were found near the Fermi level (EF) upon adsorption of α-NPD when the TiOx underlayer was highly oxygen defective. The observed spectra were not fully explained by density functional theory (DFT) calculations for simple ionic α-NPD molecules. In addition, the vacuum level shift at the interfaces was also different from the shift predicted by several theoretical models that do not consider chemical reactions at the interface. These results suggest that the adsorbate–substrate interaction is complicated when a highly defective metal oxide is used. The stoichiometry of the metal oxide buffer layer may have a large effect on carrier injection or extraction properties at the interface.

Efficient organic light-emitting diodes fabricated on cellulose nanocrystal substrates

Organic light-emitting diodes (OLEDs) fabricated on recyclable and biodegradable substrates are a step towards the realization of a sustainable OLED technology. We report on efficient OLEDs with an inverted top-emitting architecture on recyclable cellulose nanocrystal (CNC) substrates. The OLEDs have a bottom cathode of Al/LiF deposited on a 400 nm thick N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) layer and a top anode of Au/MoO3. They achieve a maximum luminance of 74 591 cd/m2 with a current efficacy of 53.7 cd/A at a luminance of 100 cd/m2 and 41.7 cd/A at 1000 cd/m2. It is shown that the α-NPD layer on the CNC substrate is necessary for achieving high performance OLEDs. The electroluminescent spectra of the OLEDs as a function of viewing angle are presented and show that the OLED spectra are subject to microcavity effects.

Reducing the driving voltage of organic light-emitting diodes by inserting a transparent ultrathin layer of oxidized silver as a hole-injecting layer

Organic light-emitting diodes (OLEDs) containing a transparent ultrathin layer of oxidized silver as a hole-injecting layer, placed between a indium–tin-oxide (ITO) electrode and the hole-transporting layer, were fabricated, and their electrical and luminescent properties were investigated. The OLEDs had a structure that consisted of an ITO layer; followed by an ultrathin Ag layer oxidized by a ultraviolet (UV)–ozone surface treatment; a N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD) layer; a 5,6,11,12-tetraphenylnaphthacene (rubrene)-doped 9,10-diphenylanthracene (DPA) layer; a tris-(8-hydroxyquinoline) aluminum (Alq3) layer; a lithium fluoride (LiF) layer; and a Al layer. The operating voltages of the OLEDs with the oxidized Ag (i.e., AgOx) layer were drastically lower than those of the layer-free OLEDs, because the AgOx layer, which had high oxidizability, contributed to hole injection as it oxidized the surface of the α-NPD layer. However, the external quantum efficiency of the OLEDs with the AgOx layer was lower than that of the AgOx layer-free OLEDs, suggesting that the carrier balance (i.e., the balance between the holes and electrons) became uneven in the emission layer, owing to the insertion of the AgOx layer. It was assumed that this imbalance resulted from the number of holes in the emission layer being higher because of the increase in hole injection in the AgOx layer.

Localized surface plasmon resonance effect in organic light-emitting devices with Ag islands

We report on luminescence enhancement of organic light-emitting devices (OLEDs) with silver islands (i-Ag) by a localized surface plasmon resonance (LSPR) effect. The devices were fabricated using tetraphenylporphyrin (TPP) as the red emission material, bis[N-(1-naphthyl)-N-phenyl] benzidine (α-NPD) as the blue emission and hole transport material, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as the electron transport material. To clarify the position of emission enhancement by energy transfer from i-Ag, an ultrathin TPP layer located within the α-NPD layer. In the device with i-Ag and the TPP layer located over 10 nm from i-Ag, TPP emission was enhanced in comparison with the device without i-Ag. The enhancement of TPP emission was suggested to be the effect of the enhanced electric field resulting from LSPR excited by α-NPD emission.

Analysis of carrier behavior in α-NPD/P(VDF–TrFE) double-layer capacitor by using electric-field-induced optical second-harmonic generation

The effect of ferroelectric polymer on carrier behaviors in indium–tin oxide (ITO)/poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)]/N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD)/Au structure was investigated. The internal electric field in α-NPD introduced by the dipole moments from ferroelectric P(VDF–TrFE) together with the α-NPD/P(VDF–TrFE) interface accumulated charges plays an important role in the carrier motion in α-NPD. In displacement current measurement (DCM), two asymmetric peaks and reduced polarization resulting from a large injection barrier from Au electrode to α-NPD layer were observed. Coupled with the electric-field-induced optical second-harmonic generation (EFISHG) measurement, the electric field in α-NPD layer was directly probed and gave a support that only part of the dipoles in P(VDF–TrFE) was polarized. We proposed that the formation of an amorphous insulating layer between amorphous α-NPD layer and partially crystallized P(VDF–TrFE) layer markedly decreased the voltage directly applied on the P(VDF–TrFE) film and caused a reduced polarization. Our findings here will be helpful in analyzing the carrier behavior in organic electronic devices using ferroelectric layers.

Study of Hole-Injection Assisted Layer in Double-Layer Organic Light-Emitting Diodes by Electric-Field-Induced Optical Second-Harmonic Generation Measurement

By using electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the physical reason for the decrease in the threshold voltage of indium tin oxide (ITO)/N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD)/tris(8-hydroxy-quinolinato)aluminum(III) (Alq3)/Al diodes with a pentacene layer sandwiched between ITO and α-NPD layers. The amount of charge accumulated at the α-NPD/Alq3 interface was the same as that at ITO/α-NPD/Alq3/Al diodes under an applied DC voltage corresponding to the threshold voltage, although the charge accumulation proceeded faster. Results showed that the pentacene layer assists hole injection and the succeeding hole accumulation at the α-NPD/Alq3 interface, followed by the electroluminescence (EL) emission. The decrease in threshold voltage by inserting a hole-injection assisted layer such as pentacene is one way of improving the EL device performance.

Exciton formation and light emission near the organic–organic interface in small-molecule based double-layer OLEDs

We investigate the efficiency and emission color of small-molecule based double-layer organic light-emitting diodes (OLEDs) based on 4,4′-bis[1-naphthyl (phenyl) amino]-1,1′-biphenyl (α-NPD) and aluminum (III) bis (2-methyl-8-quinolinato)4-phenylphenolato (BAlq) by studying the charge transport and photophysics near the organic–organic interface between the emitting layers. For that purpose, the light-emission profile is reconstructed from full angle, wavelength and polarization dependent electroluminescence spectra. By increasing the thickness of the BAlq layer from 100 to 300nm, at a fixed 160nm α-NPD layer thickness, the emission color is found to vary from deep blue to green, yellow-green, white and back to blue. We demonstrate that this is due to a gradual emission profile shift, in combination with a wavelength and layer thickness dependent light outcoupling efficiency. The emission profile shift, from an approximately 20nm-wide zone on the α-NPD-side of the interface to a very narrow zone on the BAlq-side of the interface, gives rise to a changing balance between the contributions from BAlq excitons, α-NPD excitons and charge-transfer excitons. It also contributes to a pronounced layer thickness dependence of the external quantum efficiency. The shift of the emission profile is explained by a charge transport and recombination model.

Properties of Interface between Organic Hole-Transporting Layer and Indium Tin Oxide Anode Modified by Fluorinated Self-Assembled Monolayer

The electronic structure and chemical properties of the interface between indium tin oxide (ITO) modified by a fluorinated self-assembled monolayer (F-SAM) and a N,N '-bis(1-naphthyl)-N,N '-diphenyl-1,1'-diphenyl-1,4'-diamine (α-NPD) layer were investigated in order to clarify the effects of the F-SAM modification of ITO anodes on the driving voltage and lifetime of organic light-emitting diodes (OLEDs). Ultraviolet and X-ray photoelectron spectroscopy revealed that the F-SAM modification of ITO led to a shallower highest occupied molecular orbital level in the α-NPD layer near the interface than in conventionally treated ITO, a chemical reaction between F-SAM and α-NPD, and the migration of adsorbed fluorine into the α-NPD layer. These results indicate that high conductance, the suppression of crystallization, and the inhibition of oxidation in the hole-transporting layer along with a small hole-injection barrier height at the anode/HTL interface contribute to the excellent properties of OLEDs having ITO anodes modified by F-SAM.

Probing electric field distribution in organic double-layer diode by electric field induced optical second harmonic generation

Analyzing spectroscopic optical properties of an organic double-layer diode comprised of α-NPD and Alq3 layers, we studied the selectively probing of electric field distribution in one of the two layers by using the microscopic electric field induced optical second harmonic generation (EFISHG) measurement. Spectroscopic SHGs from Indium–Zinc-Oxide/N,N-Di(naphthalene-1-yl)-N,N′-diphenyl-benzidine/tris(8-quinolinolato) aluminium/Al (IZO/α-NPD/Alq3/Al) diodes were measured. Results showed that the SHG peaks were generated at 940 and 1050 nm from the α-NPD and Alq3 layers, respectively, due to the EFISHG process, and the electric field in each layer can be selectively probed. The contribution of the accumulated charge at the double-layer α-NPD and Alq3 interface was also identified by the d.c. voltage dependence on the EFISHG intensity.

Interlayer molecular diffusion and thermodynamic equilibrium in organic heterostructures on a metal electrode

In this paper, we investigate by photoemission the electronic properties of an organic/organic interface consisting in a strong electron acceptor hexaazatriphenylene-hexacarbonitrile (HATCN) deposited on a physisorbed N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) monolayer on Ag(111). At the first HATCN deposition steps (∼1 monolayer), the sample work function increases by 1.05 eV and the hole injection barrier (HIB) in the pre-adsorbed α-NPD monolayer is lowered by 0.65 eV. This results from HATCN diffusion to the silver surface through the α-NPD monolayer. Furthermore, this HATCN monolayer is proposed to form a compact chemisorbed monolayer, with a different structural arrangement than that observed on pristine Ag(111). In a second step, the additional deposited HATCN start growing on top of the α-NPD layer, and Fermi-level pinning, associated with the formation of HATCN negative polarons, is identified at the HATCN/α-NPD interface. Finally, HATCN is deposited on a α-NPD multilayer film. In this case, HATCN is also found to diffuse to the silver surface giving a similar HIB value in the α-NPD multilayer. This work demonstrates the importance of studying in detail the formation of organic heterostructures to understand the interplay between growth mode and electronic properties.

Application of Organic Photodiodes to X-ray Measurements—A Feasibility Study

To elucidate the applicability of organic photodiodes (OPDs) to radiation measurements, three OPDs were fabricated, two with heterojunction structures (OPD-a and OPD-b) and one with a bulk-heterojunction structure (OPD-c). OPD-a and OPD-b have a structure of ITO/α-NPD/td-PTC/Al with a sensing area of 4 × 4mm2, while OPD-c has a 300-nm-thick mixed organic layer between α-NPD and td-PTC layers. All three OPDs showed rectifying properties under forward and reverse voltages. X-ray irradiation experiments have been carried out with these OPD devices. Results revealed that X-rays increased the output current, although some problems persisted, such as low photon-to-current conversion efficiency. When irradiating OPD-a and OPD-b with mono-energy X-rays at 20 keV, the current increased to about 6 pA from about 0.5 pA under dark conditions. The X-ray-induced current increased concomitantly with increasing X-ray intensity with some nonlinearity. Measurements of X-ray-induced current with OPD-c showed that it could yield a higher X-ray-induced current than those observed from OPD-a and OPD-b.

Displacement Current Measurement of MIS Devices with Ionic Liquids to Explore Carrier Behaviors in Model Interfaces of Organic Devices

ABSTRACTCharge injection property of organic thin film devices is a key issue to understand the device operation. Displacement current measurement (DCM) is a powerful technique to probe the charge injection behaviors in terms of a change in the apparent capacitance of test devices. However, it requires to suppress actual current flowing through the device for investigating the details of interface phenomena. We propose here the use of ionic liquids (ILs) as a top contact insulator in organic metal-insulator-semiconductor (MIS) structures. Because of the high stability and dielectric constant of the ILs, the external applied voltage was applied mainly to the organic layer with suppressing the actual current. The DCM curves of Pt wire/IL/α-NPD/ITO structure were measured, and they actually show the signals due to the hole injection from theITO to α-NPD layer and accumulation at the IL/α-NPD.