Alq3 Layer Research Articles

Room-temperature organic magnetoresistance based on interfacial ground charge transfer state

We propose an organic magnetoresistance (OMR) based on the weak interfacial ground charge transfer (IGCT) state at m-MTDATA/Alq3 heterojunction. The maximum OMR reaches 13.2% at room temperature under an external magnetic field of 300 mT, which is well interpreted by the different roles of singlet and triplet IGCT states in hole transport. As demonstrated, the hole transport is blocked by singlet IGCT and passed through by triplet IGCT. By manipulating the Larmor frequencies of mixing singlet and triplet IGCT states through Δg mechanism under an external magnetic field, the hole current is modulated, thus generating OMR. Furthermore, OMR can be further enhanced to 15.4% at 300 mT and 27.9% at 1000 mT by inserting a thin BCP interlayer between m-MTDATA and Alq3 layers. This picture will help to implement a giant OMR with interfacial CT states in the future.

Bands alignment between organic layers of Alq3, Gaq3, Erq3 and graphene on 6H-SiC(0 0 0 1)

This paper concerns physicochemical properties of three interfaces composed of organic layers of Alq3, Gaq3 or Erq3 vapor deposited on graphene/6H-SiC(0 0 0 1) surfaces. A graphene termination of 6H-SiC(0 0 0 1), about one monolayer thick, was produced in situ by thermal annealing of the substrate. The formation of the final interfaces and their characterization were also performed in situ under ultrahigh vacuum conditions. X-ray/UV photoelectron spectroscopies (XPS/UPS) were employed in this study. XPS results showed that the organic layers did not react chemically with the graphene/6H-SiC(0 0 0 1) surfaces at room temperature. In turn, UPS measurements showed that the adsorption of organic layers decreased the position of the vacuum level of the initial surface and one covered with the organic layers which initiated the surface dipoles. The vacuum level was lowered by 0.15 eV, 0.4 eV, and 0.9 eV for Alq3, Gaq3, Erq3 layers, respectively. The positions of highest occupied molecular orbitals (HOMOs) were also found. They were located at 1.55 eV, 1.75 eV, and 2.0 eV below the Fermi level, respectively, for Alq3, Gaq3, and Erq3 layers. The band energy diagrams for the three interfaces were constructed.

Luminescence of tris (8-hydroxyquinoline) aluminium thin films under synchrotron radiation excitation

The work is dedicated to the investigation of the luminescent properties of Tris (8-hydroxyquinoline) aluminium (Alq3) thin layers using the conventional spectral methods (cathodoluminescence and photoluminescence), as well as the advanced time-resolved luminescent spectroscopy under excitation by the synchrotron radiation in the fundamental absorption edge of this material. The Alq3 films with a thickness of 100–300 nm were deposited by the PVD (Physical Vapor Deposition) method on silicon substrates. The structural properties of Alq3 layers were studied using scanning electronic microscopy and X ray diffraction. The structure of the luminescence spectra of the Alq3 layer in the green-yellow range at 10 K and 300 K confirms the complex structure of the emission centers of this material, associated with the internal fast radiation transitions in the Al3+ molecule surrounded by three 8-hydroxynoline ligands. It has been determined the energy of creation of excitons associated with Alq3 green emission centers, which has been equal to 6.78 eV and 6.86 eV at 10 K and 300 K, respectively.

Low-dose measurement of electric potential distribution in organic light-emitting diode by phase-shifting electron holography with 3D tensor decomposition.

To improve the performance of organic light-emitting diodes (OLEDs), it is essential to understand and control the electric potential in the organic semiconductor layers. Electron holography (EH) is a powerful technique for visualizing the potential distribution with a transmission electron microscope. However, it has a serious issue that high-energy electrons may damage the organic layers, meaning that a low-dose EH is required. Here, we used a machine learning technique, three-dimensional (3D) tensor decomposition, to denoise electron interference patterns (holograms) of bilayer OLEDs composed of N,N'-di-[(1-naphthyl)-N,N'-diphenyl]-(1,1'-biphenyl)-4,4'-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminum (Alq3), acquired under a low-dose rate of 130 e- nm-2 s-1. The effect of denoising on the phase images reconstructed from the holograms was evaluated in terms of both the phase measurement error and the peak signal-to-noise ratio. We achieved a precision equivalent to that of a conventional measurement that had an exposure time 60 times longer. The electric field within the Alq3 layer decreased as the cumulative dose increased, which indicates that the Alq3 layer was degraded by the electron irradiation. On the basis of the degradation of the electric field, we concluded that the tolerance dose without damaging the OLED sample is about 1.7 × 105 e- nm-2, which is about 0.6 times that of the conventional EH. The combination of EH and 3D tensor decomposition denoising is capable of making a time series measurement of an OLED sample without any effect from the electron irradiation.

Optical and morphological properties of ZnO and Alq3 incorporated polymeric thin layers fabricated by the dip-coating method

In this report, we present the influence of polymer matrix on morphological and optical properties of thin films containing zinc oxide (ZnO), tris(8-hydroxyquinolinato)aluminium (Alq3) and ZnO:Alq3. Polyvinylpyrrolidone (PVP) and poly(methyl methacrylate) (PMMA) dissolved in isopropanol and tetrahydrofuran, respectively, were used as polymeric matrices of fabricated composites. The analysed thin layers were deposited on Si substrates using a dip-coating method and characterized by Fourier-Transform Infrared Spectroscopy (FTIR), Energy-Dispersive X-ray spectroscopy (EDX), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Spectroscopic Ellipsometry (SE) and photoluminescence (PL). It was found that adding the polymer to Alq3 causes a blueshift in absorption compared to pure Alq3 layers. We also observed photoluminescence in the region of 2.2–2.8 eV for ZnO:Alq3:PMMA and ZnO:Alq3:PVP, as well as for Alq3:PMMA at room temperature. PL measurements showed that adding ZnO to Alq3:polymer matrix did not result in any shift in PL spectra compared to the results of Alq3:polymer layer. AFM and SEM measurements show that relatively smooth films were obtained in the case of composites based on PVP and PMMA. Moreover, a change in the size of ZnO agglomerates depending on polymer used is observed for the three-component layers. We also noticed that the values of the refractive index are higher for the samples in the PVP matrix. However, the opposite behaviour was observed in the case of the extinction coefficient.

Interfacial Polarization of Thin Alq3, Gaq3, and Erq3 Films on GaN(0001).

This report presents results of research on electronic structure of three interfaces composed of organic layers of Alq3, Gaq3, or Erq3 deposited on GaN semiconductor. The formation of the interfaces and their characterization have been performed in situ under ultrahigh vacuum conditions. Thin layers have been vapor-deposited onto p-type GaN(0001) surfaces. Ultraviolet photoelectron spectroscopy (UPS) assisted by X-ray photoelectron spectroscopy (XPS) has been employed to construct the band energy diagrams of the substrate and interfaces. The highest occupied molecular orbitals (HOMOs) are found to be at 1.2, 1.7, and 2.2 eV for Alq3, Gaq3, and Erq3 layers, respectively. Alq3 layer does not change the position of the vacuum level of the substrate, in contrast to the other layers, which lower it by 0.8 eV (Gaq3) and 1.3 eV (Erq3). Interface dipoles at the phase boundaries are found to be −0.2, −0.9, −1.2 eV, respectively, for Alq3, Gaq3, Erq3 layers on GaN(0001) surfaces.

Ultraviolet Light-Activated Charge Modulation Heterojunction for Versatile Organic Thin Film Transistors.

Organic thin film transistors (OTFTs) are a promising technology for the application of photosensors in smart wearable devices. Light-induced electrical behavior of OTFTs is explored to achieve diverse functional requirements. In most studies, OTFTs show an increased drain current (ID) under light irradiation. Here, we use an ultraviolet (UV) light absorption top layer, tris(8-hydroxyquinoline) aluminum (Alq3), to improve the UV light response of poly(3-hexylthiophene-2,5-diyl) (P3HT)-based OTFTs. Unexpectedly, the Alq3-covered device operated at the accumulation mode demonstrates a decreased ID during the UV light irradiation. N,N'-Ditridecyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI, electron acceptor), pentacene (electron donor), and lithium fluoride (LiF, insulator) as an interlayer were inserted between the P3HT and the Alq3 layers. The PTCDI/Alq3-covered device also shows an unusual decrease in ID under the UV light but an increase in ID under the green light. The pentacene/Alq3-covered device shows an increased ID during the UV light irradiation and, unexpectedly, a memory effect in ID after removing the UV light. The LiF/Alq3-covered device exhibits an electrical behavior similar to the bare P3HT-based device under the UV light. Results of spectroscopic analyses and theoretical calculations have shown that the occurrence of charge transfer at heterojunctions during the UV light irradiation causes charge modulation in the multilayered P3HT-based OTFTs and then results in an unusual decrease or memory effect in ID. In addition, the unexpected ID reduction can be observed in the Alq3-covered poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene]-based OTFTs under UV light. The features, including opposite electrical responses to different wavelengths of light and optical memory effect, provide the multilayered P3HT-based OTFTs with potential for various optical applications, such as image recognition devices, optical logic gates, light dosimeters, and optical synapses.

Improvement of inverted planar heterojunction solar cells efficiency by using KI/Alq3 hybrid exciton blocking layer

The exciton blocking layer (EBL) as an interfacial layer is extremely critical in determining the organic photovoltaic cell (OPV) performances. Here, we studied inverted planar heterojunction solar cells PHJ-OPVs with the following configuration ITO/EBL/C60/CuPc/MoO3/Al. Upon the EBL functionality which can act as an exciton blocking layer and allows the electron collection at the cathode, we proposed the insertion of hybrid EBL consisted of KI/Alq3 thin layer. The Alq3 is known as an EBL due to its broad bandgap, whereas we found that when a thin layer of 1 nm of KI is introduced in ITO/Alq3 interface, the KI decomposed during the thermal deposition, and only potassium interacts and diffuses in the Alq3 layer, which effectively enhances the electrons collection at the ITO/C60 interfaces leading to the improvement of open-circuit voltage (Voc), and device power conversion efficiency by 36% than the device using Alq3 alone as EBL.

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.

Active Control of Spontaneous Orientation Polarization of Tris(8‐hydroxyquinolinato)aluminum (Alq3) Films and Its Effect on Performance of Organic Light‐Emitting Diodes

AbstractVacuum‐deposited amorphous organic films with an anisotropic amorphous structure are known to exhibit spontaneous orientation polarization (SOP). In this study, amorphous structures of tris(8‐hydroxyquinolinato)aluminum (Alq3) films are controlled by substrate temperature (Tsub) during vacuum deposition. The effect of density, molecular orientation, and SOP of Alq3 films on the performance of organic light‐emitting diodes (OLEDs) is comprehensively investigated. The SOP of Alq3 films is significant at low Tsub and becomes weaker as Tsub increases. The presence of SOP certainly improves electron injection from a metal cathode and, therefore, increases the current in OLEDs. External quantum efficiencies (EQEs) of OLEDs are increased by ≈40% when Tsub for the Alq3 deposition is increased from 260 to 328 K. The increased EQEs are attributed to several factors, which include not only increased photoluminescence quantum yields and light outcoupling efficiencies but also suppressed singlet‐polaron annihilation originating from the reduced SOP. Operational stability of OLEDs is also affected by Tsub and becomes the highest when Alq3 layers are vacuum‐deposited at Tsub = 299 K. These findings indicate the importance of controlling organic amorphous structures and SOP in films to maximize OLED performance and contribute to a better understanding of working mechanisms of OLEDs.

Growth behavior, physical structure, and magnetic properties of iron deposited on Tris(8-hydroxy quinoline)-aluminum

Growth behavior and evolution of magnetism as a function of Fe thin film thickness onto organic semiconductor tris(8-hydroxyquinoline)-aluminum, (Alq3) thin film was investigated in-situ using electrical resistance and magneto-optic Kerr effect (MOKE) measurements, respectively. The variation of electrical resistance with Fe film thickness reveals the Volmer-Weber growth of Fe on Alq3 film. Reflection high energy electron diffraction (RHEED) pattern resembles polycrystalline BCC structure of Fe. Ex-situ x-ray reflectivity (XRR) measurement from the Fe/Alq3/Si sample reveals that Fe diffuses into Alq3 to form an alloy layer of 50 Å thickness with Fe volume fraction of 0.4. In-situ MOKE study affirms anomalous magnetic behavior at the initial stage of Fe growth. Initially hysteresis loop appears and develops in a particular direction, and then vanishes at certain thickness. With further growth of Fe hysteresis loop appears again and grows in the opposite direction. The observed unusual changes in magnetic behavior as a function of Fe thickness may be explained in terms of the contribution from the magnetic Fe-Alq3 alloy present in the intermix layer, the metallic Fe on top of Alq3 layer, and their antiferromagnetic coupling.

Organic small-molecule heterointerface for use in transistor-type non-volatile memory

A chargeable layer is an essential element for charge transfer and trapping in a transistor-based non-volatile memory device. Here we demonstrate that a heterointerface layer comprising of two different small molecules can show electrical memory characteristics. The organic heterointerface layer was fabricated with a pentacene and tris(8-hydroxyquinoline) aluminum (Alq3) layers by sequential vapor deposition without breaking the vacuum state. Pentacene was adopted as the active layer on the top, and Alq3 was used as the bottom layer for charge trapping. The bottom-gate top-contact transistor with an organic heterointerface layer showed distinct non-volatile memory behaviors and showed high air stability and reliability. We investigated the energy structure of the pentacene/Alq3 heterointerface layer to reveal the operation mechanism of the non-volatile memory and suggested that the writing/erasing gate bias-dependent energy barrier originating from the difference between the energy levels of the pentacene and Alq3 layers controls the charge transfer at the heterointerface layer. Our approach suggests a simple way to fabricate heterointerface layers for organic non-volatile memory applications with high air stability and reliability.

Effect of growth rate on quality of Alq3 films and Co diffusion

The quality of organic semiconductor tris-(8-hydroxyquinoline)aluminum (Alq3) thin films, deposited at the rate of 5.0 Å s−1, 2.5 Å s−1, and 1.0 Å s−1 respectively, have been investigated using x-ray reflectivity (XRR) and grazing incidence small angle x-ray scattering (GISAXS) studies. While XRR provides information about structural details, GISAXS provides information about the morphology of the films. The film quality is very good with a surface roughness of only 8 Å. The electron density of the film increases from 0.39 e Å−3 to 0.43 e Å−3 due to reduction of growth rate from 5 Å s−1 to 1 Å s−1. The higher electron density (0.43 e Å−3) of the film deposited at the lower deposition rate (1 Å s−1) indicates the presence of less defects and/or porosity in the film. Average separation between pores increases from 200 Å to 500 Å and simultaneously pore depth decreases from 300 Å to 120 Å due to reduction of growth rate from 5 Å s−1 to 1 Å s−1. The mixing between Alq3 and Co at the Alq3/Co interface and diffusion of Co into Alq3 layer through Co/Alq3 interface have been studied by combined XRR and grazing incidence x-ray standing wave (GIXSW) measurements for the three Alq3/Co/Alq3/W/Si multilayers in which Alq3 films were deposited at the above mentioned three different rates. It is found that there is about 30 Å thick (∼ two monolayer of Alq3) mixing region at the Alq3/Co interface in three multilayer samples, which suggests that mixing at this interface is independent of the growth rate of Alq3 films. The depth of diffusion of Co into Alq3 through Co/Alq3 interface reduces from 300 Å to 160 Å due to the reduction of deposition rate from 5 Å s−1 to 1 Å s−1. This study suggests that by controlling the growth rate of Alq3, porosity as well as the diffusion of ferromagnetic Co in an organic semiconductor Alq3 can be reduced, which plays an important role in the performance of organic spin valve devices.

Study the Effect of the Volumetric Ratio of Alq3 on the Optical and Structural Properties of the Alq3:NPD Blend for the UV Photodetector Applications

The effect of the volumetric ratio of the tris(8-hydroxyquinoline) aluminum (Alq3) on its blend with the N,N'-Di [(1-naphthyl)-N,N'-diphenyl]-(1,1'-biphenyl)-4,4'-diamine (NPD) (Alq3:NPD) is investigated and optimized for the UV photodetectors fabrication. The optical and structural properties of Alq3:NPD blend with different volumetric ratios 1:1, 2:1, and 3:1 is studied in the context of the absorbance, transmittance, optical energy gap and XRD patterns. Results show that the absorbance is increased by 11% at A = 260 nm with the increase in the volumetric ratio. In contrast, the optical energy bandgap that is extrapolated from the Tauc’s plot is decreased with the increase in the volumetric ratio, and the 2:1 ratio shows the lowest energy in the UV region. In terms of the XRD investigation, the 2:1 volumetric ratio shows the highest intensity for the crystallinity peak at 36.6°. The fabricated photodetector with a different volumetric ratio of the active layer Alq3:NPD blend shows the best performance with the ratio 2:1.

Hybrid white light emitting devices (HWLEDs) from organic polymer and PbS nanocrystals by multiple excitons

Abstract Hybrid white light-emitting devices (HWLEDs) were fabricated using FTO/PEDOT: PSS/PbS/Alq3/Ni system and synthesized by phase separation process. In the present study, the multiple excitons generation in lead sulfide (PbS) NCs, which is characteristic of PbS NCs, was used to induce an effective and regulated energy transfer to an HWLED. The HWLED consisted of three layers successively deposited on FTO glass substrate; the first layer consisted of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) blended with polymethyl methacrylate (PMMA) organic polymer in the 1:1 ratio, while the second layer consisted of PbS NCs. Finally, above the layer of the PbS NCs, Tris (8-hydroxyquinoline) aluminum (Alq3) layer was deposited. The white light was generated with quite a good efficiency due to the confinement effect that makes the energy gap greater. The characteristics of the current-voltage (I-V) indicate acceptable conditions for the generation of white light by multiple excitons. It was found that the emission levels able to produce white luminescence, classified based on the coordinate system of chromaticity (CIE 1931), are x = 0.31, y = 0.33 while the correlated color temperature (CCT) is about 6250 K. The HWLEDs made from PbS NCs with hole injection from the organic polymer (PEDOT: PSS with PMMA), and electron injection from organic molecules (Alq3) are capable of white light generation.

Antimony Selenide Thin Film Solar Cells with an Electron Transport Layer of Alq3*Supported by the High Level Talents Project of Hainan Basic and Applied Research Program (Natural Science) (Grant No. 2019RC118), and the Open Fund of the State Key Laboratory of Molecular Reaction Dynamics in DICP (Grant No. SKLMRDK202005).

We fabricated Sb2Se3 thin film solar cells using tris(8-hydroxy-quinolinato) aluminum (Alq3) as an electron transport layer by vacuum thermal evaporation. Another small organic molecule of N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine (NPB) was used as a hole transport layer. We took ITO/NPB/Sb2Se3/Alq3/Al as the device architecture. An open circuit voltage (Voc) of 0.37 V, a short circuit current density (Jsc) of 21.2 mA/cm2, and a power conversion efficiency (PCE) of 3.79% were obtained on an optimized device. A maximum external quantum efficiency of 73% was achieved at 600 nm. The Jsc, Voc, and PCE were dramatically enhanced after introducing an electron transport layer of Alq3. The results suggest that the interface state density at Sb2Se3/Al interface is decreased by inserting an Alq3 layer, and the charge recombination loss in the device is suppressed. This work provides a new electron transport material for Sb2Se3 thin film solar cells.

Opto‐electrical properties of HAT‐CN based organic light emitting diode

1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN)-based organic light emitting diode (OLED) have been simulated, fabricated, and characterised. HAT-CN has been inserted as an organic interlayer at the indium tin oxide/organic interface. It has been observed that there is a significant increase in the current density of the device. Also, the electroluminescence (EL) intensity of the OLED device is in contrast with J–V characteristics, i.e. EL intensity and current density is maximum at 22 nm Alq3 layer thickness. There is an approximate increase of 0.13 A/cm2 in current density with respect to the applied voltage when the Alq3 thickness is varied from 20 to 22 nm. The maximum current density obtained is 0.4 A/cm2. EL intensity is approximately increased by 30% at the same Alq3 thickness. A comparison of fabricated and simulated optoelectronic devices has also been reported in this work. They have found proximity between the fabricated and simulated devices.

Influence of nonradiative Auger process in the lanthanide complexes lifetime near interfaces in organic light-emitting diode structures

The low efficiency of organic light-emitting diodes based on lanthanide complexes is generally attributed to the triplet-triplet annihilation processes in the regime of high concentration of excited states caused by their long lifetimes and optical losses near the interfaces of multilayer device structures. Despite the enormous effort to synthesize short-lived complexes and minimize the optical losses in the interfaces, it remains insufficient in understanding the exciton recombination processes that reduce the lifetime of these complexes. Herein, we investigated the influence of the exciton recombination processes on a Tb complex (Tb-C) lifetime in the regime of a highly excited state concentration as a function of the distance between the carrier layer and the interface by using a typical organic light-emitting diode structure. Our results show that a 10 nm-thick Alq3 layer decreases the exciton lifetime of the Tb-C, increasing approximately by 16 times the spontaneous emission decay rate of triplet exciton. The effects of interference and optical losses at the metallic interface contribute actively to the modulation of the emission intensity and lifetime decay. However, these effects alone do not explain the significant increase in the emission decay rate. The nonradiative Auger process at the Alq3/Tb-C interface seems to be largely accountable for the Tb-C lifetime reduction as the energy released by the terbium ion occurs by the excitation of an adjacent electron at higher energy. Furthermore, we propose a simple theoretical model to explain the observed effects. These results can provide a new approach to reduce the lanthanide complexes’ lifetime through the Auger electron process near the interface and thus improve the performance of organic light-emitting diodes.

Vacuum-Level Shift at Al/LiF/Alq3 Interfaces: A First-Principles Study.

Work function changes, or vacuum-level shifts (ΔVLS), in Al(001) surfaces by the adsorption of thin layers composed of tris(8-hydroxyquinolinato)aluminum (Alq3) and/or LiF are theoretically investigated. First-principles calculations reasonably reproduce experimentally obtained ΔVLS values, enabling us to discuss the underlying mechanism. Dipole moment of Alq3 and interfacial charge rearrangement (Pauli push-back effect) are the main reasons for ΔVLS at Al(001)–Alq3 and Al(001)–LiF interfaces, respectively. For a stacked Al(001)–LiF–Alq3 layer configuration, theory suggests a more complicated picture, which takes charge rearrangement between LiF and Alq3 layers into account, than a simple sum rule of dipole contributions from the two layers.

Negative impedance of organic light emitting diodes in AC electrical response

In previous papers, we introduced a novel analysis technique of the impedance spectroscopy of organic light emitting diodes to elucidate their layer-to-layer carrier dynamics and proposed a novel carrier transport model. In this article, we will discuss the carrier dynamics in the voltage region where the layer-to-layer dynamics cannot be distinguished. There appear several specific characteristic features in frequency- and voltage-dependence of complex impedance, the negative impedance, and the diffusion impedance. In the negative bias voltage region, where neither carrier injection nor accumulation occurs, conductance shows ω1/2 frequency dependence. On the other hand, at the higher voltage region where all the semicircles in Cole-Cole plot coalesce, there appear two types of negative impedance, which have ω1/2 and a Debye-like frequency dependence. We consider that the ω1/2 dependences are attributed to the diffusion impedance (Warburg impedance) coupled with and without the interfacial charge transfer reaction. The Debye-like negative impedance is attributed to the carrier trap at the interface of the Alq3 layer.