Alq3 Layer Research Articles

Visualization of the carrier transport dynamics in layered Organic Light Emitting Diodes by Modulus spectroscopy

Abstract Carrier transport behavior in organic light emitting diode (OLED) is visualized by Modulus spectroscopy using typical layered devices ITO/α-NPD/Alq3/LiF/Al. Two techniques of Modulus analysis, Dynamic Modulus Plot (DMP) and Distributed Circuit Element (DCE) analysis are presented. The hole distribution in NPD layer is visualized by DCE model. The DMP and the DCE model show that the hole and electron transport in NPD/Alq3 devices up to 10 mA/cm2 obey not to a space charge limited current but an Ohmic conduction. It is well known that holes accumulate in NPD even below the turn-on voltage of DC current. This hole accumulation depends on both of NPD and Alq3 layer thicknesses. The voltage dependence is scaled by the Alq3 thickness, whereas the carrier density dependence by the NPD thickness. We propose a model that the accumulated holes are supplied by the thermal activation of the finite number of shallow trap sites localizing at ITO/NPD interface.

Optical Properties and Stability of Bilayer Rubrene-Alq3 Films Fabricated by Vacuum Deposition

We report on the optical and structural characterization of the two-component vacuum deposited (VD) rubrene (Rub)-Alq3 films. As is known, Rub-doped OLED active materials demonstrate both promising electroluminescence and transistor characteristics. However, in terms of operational lifetime, the Rub practical application in basic devices has a few draw-backs related to its chemical instability. Our main attention was focused on the role of the Alq3 coverage and the isomeric transformation of a Rub molecule on its chemical stability in these structures. By monitoring the evolution of PL emission in time, we found that the Rub degradation in Rub-Alq3 films is slower than that in vacuum-deposited Rub layers. These results demonstrate that the deposition of an Alq3 layer can be a way to enhance the stability of Rub to the photo-oxidation in optoelectronic devices. The Rub amorphous film crystallization at elevated temperatures in open air was observed for the first time.

TiO2 as diffusion barrier at Co/Alq3 interface studied by x-ray standing wave technique

Nano-scale diffusion at the interfaces in organic spin valve thin films plays a vital role in controlling the performance of magneto-electronic devices. In the present work, it is shown that a thin layer of titanium dioxide at the interface of Co/Alq3 can act as a good diffusion barrier. The buried interfaces of Co/Alq3/Co organic spin valve thin film has been studied using x-ray standing waves technique. A planar waveguide is formed with Alq3 layer forming the cavity and Co layers as the walls of the waveguide. Precise information about diffusion of Co into Alq3 is obtained through excitation of the waveguide modes. It is found that the top Co layer diffuses deep into the Alq3 resulting in incorporation of 3.1% Co in the Alq3 layer. Insertion of a 1.7 nm thick barrier layer of TiO2 at Co/Alq3 interface results in a drastic reduction in the diffusion of Co into Alq3 to a value of only 0.4%. This suggests a better performance of organic spin valve with diffusion barrier of TiO2.

Effect of trapped electrons on the transient current density and luminance of organic light-emitting diode

Transient current density and luminance from an organic light-emitting diode (OLED) driven by voltage pulses were investigated. Waveforms with different repetition rate, duty cycle, off-period, and on-period were used to study the injection and transport characteristics of electron and holes in an OLED under pulse operation. It was found that trapped electrons inside the emitting layer (EML) and the electron transporting layer (ETL) material, tris(8-hydroxyquinolate)aluminum (Alq3) helped for attracting the holes into the EML/ETL and reducing the driving voltage, which was further confirmed from the analysis of capacitance–voltage and displacement current measurement. The relaxation time and trapped filling time of the trapped electrons in Alq3 layer were ~200 µs and ~600 µs with 6 V pulse operation, respectively.

Organic non-volatile memory based on pentacene/tris(8-hydroxy quinoline) aluminum heterojunction transistor

Non-volatile heterojunction transistor memory was fabricated by successively depositing pentacene/tris(8-hydroxy quinoline) aluminum(Alq3) layers. In this specified configuration, pentacene functioned as the active layer between the source and drain of field-effect transistor memory, and Alq3 was adopted as the charge-trapping layer. Operation of non-volatile memory was implemented by an electrical writing and light illumination erasing procedure. Band bending between pentacene and Alq3 due to the formation of a type II heterojunction prohibited back-injection of trapped charges, which demonstrated hysteresis for the transfer characteristics. The specified heterojunction structure (without a tunneling layer) enabled carriers to facilely transport between the active layer and charge trapping layer. Therefore, a large memory window (40.3 V, for a programming voltage of −80 V) and fast writing speed of less than 1 μs were demonstrated; in addition, the 1-bit non-volatile memory device showed a moderate retention time of more than 2 years. The large memory window, fast writing speed, and long retention time can potentially enable practical applications of non-volatile memory.

Conformable n -channel Organic Phototransistors with Enhanced Photosensitivity and Broadened Response Range via Insertion of an Alq3 Layer

Organic phototransistors (OPTs) have attrac- ted increasing research interest due to their outstanding advantages in optoelectronic integration and the development of wearable and portable electronics. However, research on ${n}$ -channel OPTs, and particularly, ${n}$ -channel OPTs based on commercialized materials, is limited. In this letter, conformable ${n}$ -channel OPTs are fabricated, which show good adherence onto a curved human eye model. By inserting a tris(8-hydroxyquinoline)aluminum (Alq3) layer, the photosensitivity, ${P}$ , i.e., ( ${I}_{\textsf {Light}}-{I}_{\textsf {Dark}})/{I}_{\textsf {Dark}}$ , can be improved by over two orders of magnitude. The changed thickness of Alq3 layer introduces the competition between enhanced light absorption and increased carrier trapping/scattering processes, resulting in the optimized Alq3 layer at 24 nm for the highest ${P}$ value. Under illumination at a wavelength of 489 nm with a light intensity of 243 $\mu \text{W}$ cm−2, the average ${P}$ value of N,N1-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13)/Alq3 OPTs is over 105. This letter provides a method for fabricating conformal high-performance ${n}$ -channel OPTs via the insertion of an Alq3 layer.

Color switching behaviors of organic bistable light-emitting devices fabricated utilizing an aluminum-nanoparticle-embedded tris(8-hydroxyquinoline)aluminum layer and double emitting layers

Abstract Organic bistable light-emitting devices (OBLEDs) with an aluminum (Al)-nanoparticle-embedded tris(8-hydroxyquinoline)aluminum (Alq3) layer and double emitting layers (EMLs) were fabricated to investigate their color switching behaviors. Scanning electron microscopy images showed that Al nanoparticles were formed on the Alq3 layer. The Al nanoparticles in the Alq3 layer improved the storage margin of the organic bistable devices (OBDs), and the double EMLs changed the emission color of the organic light-emitting devices (OLEDs) according to the variations of the ON and the OFF states of the OBDs. The variations of the ON and the OFF states of the OBDs could be clearly distinguished by the color switching of the OLED. The luminances of the OBLEDs with double EMLs in the ON and the OFF states were 641.80 and 22.25 cd/m2, respectively, and their CIE coordinates at 20 V were (0.42, 0.46) and (0.51, 0.47), respectively, which corresponded to the ON and the OFF states of the OBLEDs.

Organic bistable memory devices based on MoO3 nanoparticle embedded Alq3 structures

Organic bistable memory devices were fabricated by embedding a thin layer of molybdenum trioxide (MoO3) between two tris-(8-hydroxyquinoline)aluminum (Alq3) layers. The device exhibited excellent switching characteristics with an ON/OFF current ratio of 1.15 × 103 at a read voltage of 1 V. The device showed repeatable write–erase capability and good stability in both the conductance states. These conductance states are non-volatile in nature and can be obtained by applying appropriate voltage pulses. The effect of MoO3 layer thickness and its location in the Alq3 matrix on characteristics of the memory device was investigated. The field emission scanning electron microscopy (FE-SEM) images of the MoO3 layer revealed the presence of isolated nanoparticles. Based on the experimental results, a mechanism has been proposed for explaining the conductance switching of fabricated devices.

Effect of lithium and silver diffusion in single-stack and tandem OLED devices

A study of metal (Li, Ag) diffusion has been carried out in an archetypal OLED device based on N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine|tris(8-hydroxyquinolinato) aluminum|4,7-diphenyl-1,10-phenanthroline (NPB|Alq3|Bphen). Using single-stack and two-stack tandem OLED structures with variations of layer thicknesses and metal layer placements, we have found that Ag vapor-deposited on Alq3 layer can diffuse or penetrate deep into Alq3, up to ∼2,000 Å, causing luminescence quenching. This diffusion can be substantially prevented by a thin layer of Li or Bphen deposited on Alq3 prior to the deposition of Ag. In contrast, Li diffusion in either Alq3 or Bphen is limited to about 50–100 Å. Li appears to be able to diffuse into Bphen irrespective of the order of Li and Bphen depositions.

Fabrication of NPB/Alq3 small‐molecule multilayer structures with suppressed interface mixing by multi‐jet mode electrospray deposition

In this study, we investigated the multi‐jet mode modified electrospray deposition (ESD) production of NPB/Alq3 small‐molecule multilayer structures with suppressed interface mixing, using common solvents. Observations of the initial deposition stage and analysis of the photoluminescence spectra of the multilayer structure were conducted. It was demonstrated that, by optimizing the deposition conditions, surface erosion of the NPB layer and the initiation of interface mixing between the NPB and Alq3 layers could be suppressed. A 55‐nm NPB/60‐nm Alq3 multilayer structure with suppressed intermixing and smooth surface morphology (RMS roughness of 3.9 nm) was obtained at the lowest levels of under‐layer erosion.Schematic of a modified electrospray (nanomist deposition: NMD) system and an NPB/Alq3 multilayer structure deposited by NMD.

Influence of doped Alq3 layer on performance of MEH-PPV, MDMO-PPV, and P3HT polymer light-emitting diodes

The present study examined the influence of a doped Tris(8-hydroxyquinolinato) aluminium (Alq3). Layer on the performance of polymer-based organic light-emitting diodes (PLEDs) using Silvaco Atlas software. The luminance and recombination rate of a device depends on the physical properties of the active layer and of the effective layers on the active layer. Insertion of a doped Alq3 layer between the active layer and cathode increased the recombination rate of the middle layer that had been limited by the anode and cathode of the device. Better understanding the effect of the doped Alq3 layer on the parameters of mobility, lifetime, and band gap of the active layer and the performance of the device is necessary to achieve further improvement in these applications. This work presents a theoretical study of the influence of the Alq3 layer on PLEDs having different active layers. Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), poly(3-hexylthiophene) (P3HT), and poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) organic layers were deposited onto an ITO substrate that was completed using calcium as a cathode with a work function of 2.9 eV. The Langiven model was used to analyze the recombination rate and luminance and indicated that the doped Alq3 layer between the active layer and cathode increased performance of the middle layer and improved the P3HT, MEH-PPV, and MDMO-PPV PLEDs. The MEH-PPV device showed the most improvement in comparison with the MDMO-PPV and P3HT devices. This originated from appropriate overlapping of the doped Alq3 layer and the MEH-PPV active layer.

Highly efficient inverted organic light-emitting diodes based on thermally activated delayed fluorescence

Green inverted organic light-emitting diodes (OLEDs) based on a thermally activated delayed fluorescent (TADF) emitter, 2-pheyl-4-carbazole-9-H-thioxanthen-9-one-10,10-dioxide, with a cathode of indium tin oxide (ITO)/ZnO injecting electrons efficiently into the electron transporting layer of Alq3, are demonstrated for the first time. The insertion of 7-nm Cs2CO3 can further enhance the electron injection. The optimized device with 25-nm ZnO and 7-nm Cs2CO3 affords the highest performance of the inverted devices with a current efficiency of 28.1 cd A−1, a power efficiency of 16.1 lm W−1, and an external quantum efficiency of 9.8%. The inverted OLEDs based on TADF emitters are competitive compared with conventional OLEDs because of their air-stable electrodes and TADF emitters, which enable simpler encapsulation.

Instability origin and improvement scheme of facial Alq3 for blue OLED application

Degradation phenomenon and poor stability of tris(8-hydroxyquinoline) aluminum(III)(Alq3)-based organic light-emitting diodes(OLEDs) have attracted much attention. In this paper, we discussed the origin of instability of the facial Alq3-based blue luminescent OLEDs with the help of first-principles calculation. The results show that environmental humidity seriously affects the luminescence stability of Alq3-based OLEDs. H2O molecules in environment can be firmly bound to the oxygen atoms of the facial Alq3, which then act as starting points for further degradation of Alq3. Moreover, the interactions between facial Alq3 and different cathode metal layers were investigated to explain the experiment phenomenon. A design guideline for diminishing the strong attraction from oxygen atoms can be proposed to protect Alq3 and improve the stability of materials applied in OLEDs.

Defect analysis in low temperature atomic layer deposited Al2O3 and physical vapor deposited SiO barrier films and combination of both to achieve high quality moisture barriers

Al2O3 [20 nm, atomic layer deposition (ALD)] and SiO films' [25 nm, physical vacuum deposition (PVD)] single barriers as well as hybrid barriers of the Al2O3/SiO or SiO/Al2O3 have been deposited onto single 100 nm thick tris-(8-hydroxyquinoline) aluminum (AlQ3) organic films made onto silicon wafers. The defects in the different barrier layers could be easily observed as nonfluorescent AlQ3 black spots, under ultraviolet light on the different systems stored into accelerated aging conditions (85 °C/85% RH, ∼2000 h). It has been observed that all devices containing an Al2O3 layer present a lag time τ from which defect densities of the different systems start to increase significantly. This is coherent with the supposed pinhole-free nature of fresh, ALD-deposited, Al2O3 films. For t > τ, the number of defect grows linearly with storage time. For devices with the single Al2O3 barrier layer, τ has been estimated to be 64 h. For t > τ, the defect occurrence rate has been calculated to be 0.268/cm2/h. Then, a total failure of fluorescence of the AlQ3 film appears between 520 and 670 h, indicating that the Al2O3 barrier has been totally degraded by the hot moisture. Interestingly, the device with the hybrid barrier SiO/Al2O3 shows the same characteristics as the device with the single Al2O3 barrier (τ = 59 h; 0.246/cm2/h for t > τ), indicating that Al2O3 ALD is the factor that limits the performance of the barrier system when it is directly exposed to moisture condensation. At the end of the storage period (1410 h), the defect density for the system with the hybrid SiO/Al2O3 barrier is 120/cm2. The best sequence has been obtained when Al2O3 is passivated by the SiO layer (Al2O3/SiO). In that case, a large lag time of 795 h and a very low defect growth rate of 0.032/cm2/h (t > τ) have been measured. At the end of the storage test (2003 h), the defect density remains very low, i.e., only 50/cm2. On the other hand, the device with the single PVD-deposited SiO barrier layer shows no significant lag time (τ ∼ 0), and the number of defects grows linearly from initial time with a high occurrence rate of 0.517/cm2/h. This is coherent with the pinhole-full nature of fresh, PVD-deposited, SiO films. At intermediate times, a second regime shows a lower defect occurrence rate of 0.062/cm2/h. At a longer time span (t > 1200 h), the SiO barrier begins to degrade, and a localized crystallization onto the oxide surface, giving rise to new defects (occurrence rate 0.461/cm2/h), could be observed. At the end of the test (2003 h), single SiO films show a very high defect density of 600/cm2. Interestingly, the SiO surface in the Al2O3/SiO device does not appeared crystallized at a high time span, suggesting that the crystallization observed on the SiO surface in the AlQ3/SiO device rather originates into the AlQ3 layer, due to high humidity ingress on the organic layer through SiO pinholes. This has been confirmed by atomic force microscopy surface imaging of the AlQ3/SiO surface showing a central hole in the crystallization zone with a 60 nm depth, deeper than SiO thickness (25 nm). Using the organic AlQ3 sensor, the different observations made in this work give a quantitative comparison of defects' occurrence and growth in ALD-deposited versus PVD-deposited oxide films, as well as in their combination PVD/ALD and ALD/PVD.

Influence of Pentacene Interface Layer in ITO/α-NPD/Alq3/Al Organic Light Emitting Diodes by Time-Resolved Electric-Field-Induced Optical Second-Harmonic Generation Measurement.

By using I-V, EL-V, displacement current measurement (DCM) and time-resolved electric-field-induced optical second-harmonic generation (TR-EFISHG) measurement, we studied the influence of interface pentacene layer inserted between ITO and a-NPD layers in ITO/α-NPD/Alq3/Al OLEDs. All experiments were carried out for the OLEDs with and without a pentacene interface layer. The I-V and EL-V measurements showed the decrease of operating voltage of EL, the DCM showed the lowering of inception voltage of carrier injection by inserting a pentacene interface layer. The TR-EFISHG measurement showed the faster accumulation of holes at the interface between the a-NPD and Alq3 layers, which resulted in the relaxation of electric field of a-NPD layer accomplished by the increase of the conductivity and the increase of the electric field in the Alq3 layer. We conclude that TR-EFISHG measurement is helpful for understanding I-V and EL-V characteristics, and can be combined with other methods to give significant information which are impacted by the interface layer.

Degradation of Bilayer Organic Light-Emitting Diodes Studied by Impedance Spectroscopy

The degradation of bilayer organic light-emitting diodes (OLEDs) with a device structure of N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD) (hole transport layer) and tris-(8-hydroxyquinolate)aluminum (Alq3) (emissive layer and electron transport layer) has been studied by impedance spectroscopy and device simulation. Two modulus peaks are found in the modulus spectra of the OLEDs below the electroluminescence threshold. After aging of the OLEDs, the intensity of electroluminescence is degraded and the modulus peak due to the Alq3 layer is shifted to lower frequency, indicating that the resistance of the Alq3 layer is increased. Device simulation reveals that the increase in the resistance of the Alq3 layer is due to the decrease in the electron mobility in the Alq3 layer.

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.

Significant relaxation of residual negative carrier in polar Alq3 film directly detected by high-sensitivity photoemission

Tris(8-hydroxyquinoline)aluminum (Alq3) has been widely applied as a good electron-injecting layer (EIL) in organic light-emitting diodes. High-sensitivity photoemission measurement revealed a clear photoemission by visible light, although its ionization energy is 5.7 eV. This unusual photoemission is ascribed to Alq3 anions captured by positive polarization charges. The observed electron detachment energy of the anion was about 1 eV larger than the electron affinity reported by inverse photoemission. This difference suggests that the injected electron in the Alq3 layer is energetically relaxed, leading to the reduction in injection barrier. This nature is one of the reasons why Alq3 worked well as the EIL.

Effect of Plasma Induced Surface Damages on the Organic Thin Films by Using Metal Mesh

In this paper we have investigated the effect of plasma induced damages on the organic small molecular electron transport layers in organic light emitting devices with such electroluminescent materials as tris(8-hydroxyquinolinato) aluminum (Alq3) layers was used for studying plasma-induced degradation. We presently use metal mesh that could reduce the influence of plasma exposure of the organic layer films. The photoluminescence, atomic force microscopy, scanning electron microscopy and current-voltage characteristics of electron transport material was measured after exposing to argon plasma. The high-energy species in plasma induces the sputtering of organic layers, quenching of emission sites, and the creation of defect sites in organic layers. In the current-voltage characteristics, negative differential resistance phenomenon was observed corresponding to electron-only device structure of Indium Tin Oxide (ITO)/Alq3/LiF/Al. The carrier transport behavior in the electron-only device was described.

Charge transport in 2,6-bis(5′-hexyl-2,2′-bithiophene-5-yl)naphthalene-based organic devices

Organic semiconductor 2,6-bis(5′-hexyl-2,2′-bithiophene-5-yl)naphthalene (H2T26N) is employed as an active layer in organic field-effect transistors (OFETs) or as a hole-transport layer in organic light-emitting diodes (OLEDs). In OFET device the p-type conductivity is confirmed and the hole mobility as high as 0.34cm2/Vs is achieved. The emission spectrum of ITO/H2T26N/tris(8-quinolinolato) aluminum (Alq3)/Ca OLED shows emission peaks which cannot be ascribed to either the H2T26N or Alq3 layer. Through studies of photoluminescence spectra and electrochemical properties we demonstrate that the additional feature results from an exciplex at the H2T26N/Alq3 bilayer interface.