Alq3 Films Research Articles

Effect of thermal annealing on the structural and optical properties of tris‐(8‐hydroxyquinoline)aluminum(III) (Alq3) films

Tris-(8-hydroxyquionoline)aluminum (Alq3 ) was synthesized and coated on to a glass substrate using the dip coating method. The structural and optical properties of the Alq3 film after thermal annealing from 50°C to 300°C in 50° steps was studied. The films have been prepared with 2 to 16 layers (42-324 nm). The thickness and thermal annealing of Alq3 films were optimized for maximum luminescence yield. The Fourier transform infrared spectrum confirms the formation of quinoline with absorption in the region 700 - 500/cm. Partial sublimation and decomposition of quinoline ion was observed with the Alq3 films annealed at 300°C. The X-ray diffraction pattern of the Alq3 film annealed at 50°C to 150°C reveals the amorphous nature of the films. The Alq3 film annealed above 150°C were crystalline nature. Film annealed at 150°C exhibits a photoluminescence intensity maximum at 512 nm when excited at 390 nm. The Alq3 thin film deposited with 10 layers (220 nm) at 150°C exhibited maximum luminescence yield.

Dependence of the dynamics of exciton transport, energy relaxation, and localization on dopant concentration in disordered C545T-doped Alq_3 organic semiconductors

The dynamics of exciton transport, energy relaxation, and localization in disordered Tris(8-quinolinolato)-aluminum (Alq3) organic semiconductors with different 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-lH, 5H, 11H-benzo[l] pyrano[6, 7, 8-і ј] quinolizin-11-one (C545T) dopant concentrations were reported. The increasing trend of the Stokes Shift (737~764 meV) with increasing dopant concentrations is consistent with the degree of disorder and a more effective Forster energy transfer from Alq3 to C545T. In addition, a dynamic scenario representing possible paths of the exciton transport (hopping) among host molecules and the competition of the exciton transport from host molecules into the deep site traps (localized states) and aggregations was proposed to elucidate the recombination dynamics in disordered C545T-doped Alq3 organic semiconductors. The early-stage decay times, decreasing with increasing emission photon energy, show the characteristic of the exciton hopping and energy relaxation processes within the inhomogeneously broadened density-of-states in organic semiconductors. Because the current-voltage (J-V) characteristics of the C545T-doped organic light emitting diode (OLED) fitted well with the power law J~Vm (m>2), the carrier transport behaviours can be described by the trapped-control mode and the tail state distribution can be approximated by the exponential trap distribution. With the approximation of an exponential distribution for the tail states, the characteristic energy (Em), radiative recombination lifetime (τrad), and localization depth (E0) associated with the dynamics of exciton energy relaxation and localization can be quantitatively determined. The much larger E0 (40~120 meV), increasing with the dopant concentration, than other disordered semiconductors (2~34 meV) indicates a strong localization effect in such doped organic semiconductors. Also, the strong dependence of Em on the dopant concentration shows that a relatively small dopant concentration can enhance the degree of disorder and greatly affect the recombination dynamics. Furthermore, the observed optical properties and dynamic scenario of C545T-doped Alq3 films are found to be consistent with the carrier transport and recombination dynamics of C545T-doped Alq3 OLEDs.

In Situ Monitoring of Thermal Crystallization of Ultrathin Tris(8-Hydroxyquinoline) Aluminum Films Using Surface-Enhanced Raman Scattering

Thermal crystallization of 3, 10, and 60 nm-thick tris(8-hydroxyquinoline)aluminum (Alq3) films is studied using surface-enhanced Raman scattering with a constant heating rate. An abrupt higher frequency shift of the quinoline-stretching mode is found to be an indication of a phase transition of Alq3 molecules from amorphous to crystalline. While the 60 nm-thick film shows the same crystallization temperature as a bulk sample, the thinner films were found to have a lower crystallization temperature and slower rate of crystallization. Non-isothermal kinetics analysis is performed to quantify kinetic properties such as the Avrami exponent constants and crystallization rates of ultrathin Alq3 films.

Influence of Preparation Conditions on Electrical Properties of the Al/Alq3/Si Diode Structures

Hybrid organic-inorganic diode structures, Al/Alq3/n-Si and Al/Alq3/p-Si based on thin films of tris(8-hydroxyquinoline) aluminum (Alq3) have been investigated. The Alq3 films were evaporated in vacuum and spin coated onto patterned areas of crystalline n- and p-type Si substrates with chemically removed native SiO2 layer. Current-voltage characteristics of the diode structures demonstrated improved rectification property compared to similar Al/n-Si and Al/p-Si device structures. Increased barrier height values (0.90 eV ÷ 1.1 eV and 0.77 eV ÷ 0.91 eV for the Al/Alq3/n-Si and Al/Alq3/p-Si device structures, respectively) certified presence of an interface dipole induced by the organic interlayer. Non-ideal behavior of forward current-voltage characteristics has been explained assuming non-uniformity of barrier height, presence of interface states, and influence of the organic film on diode series resistance and space charge limited current. DOI: http://dx.doi.org/10.5755/j01.ms.19.4.2733

A tris(8-hydroxyquinoline) aluminum-based organic bistable device using ITO surfaces modified by Ag nanoparticles

A tris (8-hydroxyquinoline) aluminum (Alq3)-based organic bistable device (OBD) using Al electrode and ITO electrode modified by Ag nanoparticles (NPs) was reported. The OBD exhibits high ON/OFF switching ratios in the range of 102–103 and long retention time over 104 s. The influence of the Ag NPs densities, as well as the Alq3 film thickness on the switch performance current–voltage (I–V) of the OBDs was studied. Correlation between filament formation mechanism and charge storage mechanism was observed by analysing the I–V characteristics of OBDs with different Alq3 film thickness. As for the Alq3 film with thickness of 300 nm, the trapping effect of Ag NPs leads to both ON and OFF states for OBD; for 100 nm thick Alq3 film, the effect of filamentation dominates in the ON and OFF states of OBD. For the case of 200 nm thick Alq3 film, however, the ON state results from the filamentation effect, while trapping effect is responsible for the OFF state. In addition, the diffusion effect of Al atoms in Alq3 film in the devices was discussed and was expected to explain this thickness-dependence relationship.

Determination of electron mobility in tris(8-hydroxy-quinolinato) aluminum by admittance spectroscopy

The electron mobilities of tris(8-hydroxy-quinolinato) aluminum (Alq3) thin films at various thicknesses (70–280 nm) have been determined by using admittance spectroscopy measurements. Our results show that the electric field dependence of electron mobilities exhibits different behaviors at different thicknesses of Alq3 films. Clearly, when the thickness of Alq3 film is less than 150 nm, the electron mobilities slightly decrease with increasing the electric field. For the thickness of Alq3 film more than 150 nm, however, the mobilities increase as the electric field increases. The phenomena are well discussed by energetic disorder. Using temperature dependent I–V characteristics, we further calculated the density of traps.

Investigation of Al2O3 barrier film properties made by atomic layer deposition onto fluorescent tris-(8-hydroxyquinoline) aluminium molecular films

Al2O3 films have been deposited at 85°C by atomic layer deposition onto single 100nm thick tris-(8-hydroxyquinoline) aluminium (AlQ3) films made onto silicon wafers. It has been found that a thick ALD-deposited Al2O3 layer (>11nm) greatly prevents the photo-oxidation of AlQ3 films when exposed to continuous UV irradiation (350mW/cm2). Thin Al2O3 thicknesses (<11nm) on the contrary yield lower barrier performances. Defects in the Al2O3 layer have been easily observed as non-fluorescent AlQ3 singularities, or black spots, under UV light on the system Si/AlQ3/Al2O3 stored into laboratory conditions (22°C/50% Relative Humidity (RH)) for long time scale (~2000h). Accelerated aging conditions in a climatic chamber (85°C/85% RH) also allow faster visualization of the same defects (168h). The black spot density grows upon time and the black spot density occurrence rates have been calculated to be 0.024h−1·cm−2 and 0.243h−1·cm−2 respectively for the two testing conditions. A detailed investigation of these defects did show that they cannot be ascribed to the presence of a detectable particle. In that sense they are presumably the consequence of the existence of nanometre-scaled defects which cannot be detected onto fresh samples. Interestingly, an additional overcoating of ebeam-deposited SiO2 onto the Si/AlQ3/Al2O3 sample helps to decrease drastically the black spot density occurrence rates down to 0.004h−1·cm−2 and 0.04h−1·cm−2 respectively for 22°C/50% RH and 85°C/85% RH testing conditions. These observations highlight the moisture sensitivity of low temperature ALD-deposited Al2O3 films and confirm the general idea that a single Al2O3 ALD film performs as an ultra-high barrier but needs to be overprotected from water condensation by an additional moisture-stable layer.

Photophysical properties of Alq3 thin films

This work contains investigation results of the photophysical properties of aluminum (III) tris(8-hydroxyquinoline) thin films. The Alq3 thin films were successfully fabricated by Physical Vapor Deposition technique. The films were grown on transparent: (quartz and glass) and semiconductor (n-type silica) substrates kept at room temperature during the deposition process. Selected films were annealed after fabrication in ambient atmosphere for 12h at the temperature equal to 100°C and 150°C.Morphology of the films was investigated by AFM technique. Photophysical properties were characterized via photoluminescence, transmission, second and third harmonic generation measurements. The thin films exhibit high structural quality regardless of the annealing process, but the stability of the film can be improved by using an appropriate temperature during the annealing process. Photoluminescence of Alq3 films obtained in air were efficient and stable. The measurements of transmission, SHG and THG spectra allowed us to determine optical constant of the films. We find that the photophysical properties were strictly connected with the morphology and the annealing process significantly changes the structural properties of the films.

Synthesis and characterization of pure and Tb/Cu doped Alq3 nanostructures

Tris (8-hydroxyquinoline) aluminum (Alq3) is an organic semiconductor molecule, widely used in optoelectronic devices. In this work we report on fabricating different nanostructures of Alq3 and characterize them using different techniques. Nanostructured films of Alq3 were grown using the physical vapor condensation and thermal-vapor transport methods. The as synthesized films were characterized by X-ray diffraction, scanning electron microscope, energy dispersive spectroscopy and absorption spectra. Nanoparticles and nanorods/nanowires are observed in the synthesized films. Tb and Cu doped Alq3 films were also produced and studied for their photoluminescence (PL) properties. When the original powder sample of Alq3 was excited by 378nm, one broad PL emission band is observed at around 515nm. The pure nanoparticles film shows similar band with a drastic increase in the intensity by a factor of 2. This has been attributed to the large specific surface area, which might has increased the absorption and then the quantum yields. The Tb and Cu doped films show also similar band with a slight shift in the peak position to the blue region, but with further enhancement in the peak intensity, particularly that of Cu. The PL intensity of Cu doped sample is around 1.5 times stronger than that of the pure Alq3 nanoparticles. This remarkable result on obtaining highly luminescent nanomaterial based on Cu doped Alq3 nanoparticles film might be useful for future organic light emitting diode display devices.

Role of intrinsic molecular dipole in energy level alignment at organic interfaces

The energy level alignment in metal-organic and organic-organic junctions of the widely used materials tris-(8-hydroxyquinoline)aluminum (Alq3) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) is investigated. The measured alignment schemes for single and bilayer films of Alq3 and NTCDA are interpreted with the integer charge transfer (ICT) model. Single layer films of Alq3 feature a constant vacuum level shift of ∼0.2–0.4 eV in the absence of charge transfer across the interface. This finding is attributed to the intrinsic dipole of the Alq3 molecule and (partial) ordering of the molecules at the interfaces. The vacuum level shift changes the onset of Fermi level pinning, as it changes the energy needed for equilibrium charge transfer across the interface.

Electron injection in magnesium-doped organic light-emitting diodes

We investigated alkali metal doping mechanism by comparative analysis between an Mg-Alq3 co-deposition (Mg:Alq3) and an Mg deposition on Alq3 films (Mg/Alq3). The operating voltage decreased by 0.4 V and the luminance increased by 60 cd/m2 at 11 mA/cm2 for devices constructed from the Mg:Alq3. However, the device characteristics of Mg/Alq3 samples were degraded. Our experimental results using an in-situ photoemission study showed that alkali metal doping in Alq3 did not induce band bending, but reduce electron injection barrier by charge transfer from alkali metals to Alq3 molecules.

Phase transitions in thermally annealed films of Alq3

Organic light emitting devices have improved greatly in recent years, so they are now widely used in displays and lighting. Annealing processes in different atmospheres strongly affect their performance, as when the active material is the small molecule Alq3. In particular, a significant increase in the photoluminescence is observed in thin films of this molecule between 150 and 180 °C, before physical destruction of the films sets in at higher temperatures. This phenomenon is attributed to a phase transition to a novel morphological aggregation of the molecules in the film, which has yielded a much improved optical material for luminescence applications, and seems to apply to other molecules as well.

Stability of 8-hydroxyquinoline aluminum films encapsulated by a single Al2O3 barrier deposited by low temperature atomic layer deposition

100nm thick 8‐AlQ3 films deposited onto silicon wafers have been encapsulated by mean of low temperature atomic layer deposition of Al2O3 (20nm). Investigation of the film evolution under storage test as harsh as 65°C/85% RH has been investigated up to ~1000h and no severe degradation could be noticed. The results have been compared to raw AlQ3 films which deteriorate far faster in the same conditions. For that purpose, fluorescence measurements and atomic force microscopy have been used to monitor the film evolution while transmission electron microscopy has been used to image the interface between AlQ3 and Al2O3. This concept of bilayer AlQ3/Al2O3 barrier films has finally been tested as an encapsulation barrier onto an organic light-emitting diode.

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Displacement current measurement (DCM) is a simple but powerful tool for exploring charge carrier dynamics in organic semiconductor devices. In the first section, we review the basic concept of DCM and how it detects the charge injection, extraction, accumulation, and trapping behaviors in organic semiconductor devices within a quasistatic regime. Subsequently, we present applications of this technique to investigate the device properties of tris-(8-hydroxyquinolate) aluminum (Alq3)-based organic light-emitting diodes. We observed that light irradiation during device fabrication induces additional negative space charges and charge traps in the Alq3 layer. In addition, the device containing the illuminated Alq3 film exhibits a lower luminous efficiency and shorter lifetime compared to the device fabricated in dark conditions, possibly because of the additional hole accumulation in the illuminated Alq3 film. DCM detects the formation of charge traps in the aged devices, decay of the negative space charge, and increase in hole injection voltage with device aging. The origins of these behaviors can be attributed to orientation polarization and charge traps in Alq3 film.

Photoluminescence of Alq3‐ and Tb‐activated aluminium–tris(8‐hydroxyquinoline) complex for blue chip‐excited OLEDs

The tris(8-hydroxyquinoline)-aluminium complex is the most important and widely studied as electron transporting and green light emitting material. Alq(3) and Tb(x) Al((1-x)) q(3) have been synthesized (where x = 0.1, 0.3, 0.5, 0.7 and 0.9) and blended films of Alq(3) and Tb(x) Al((1-x)) q(3) with PMMA and PS at different percentage weight (wt%) concentrations (e.g., 0.1, 1, 5, 10, 25 and 50 wt%) have been prepared. The synthesized materials and their blended thin films have been characterized by a photoluminescence (PL) technique; the synthesis and PL characterization are reported in this paper. The synthesized metal complex shows bright emission of green light with blue light excitation (440 nm) and the prepared Tb(x) Al((1-x)) q(3) phosphor may be applicable in blue chip-excited OLEDs for the newly developed wallpaper lighting technology.

Formation of aluminum tris (8‐hydroxyquinoline) solution in methanol and fabrication of thin films by ultrasonic spray‐assisted vapor deposition

AbstractSmall molecular material, aluminum tris (8‐hydroxyquinoline) (AlQ3), was found to be soluble in methanol under application of high ultrasonic power, and this has opened safe and environmental‐friendly solution‐based deposition of thin films. The chemical state of AlQ3 was analyzed by nuclear magnetic resonance study and the results showed there was no severe damage on AlQ3 atoms applied by high ultrasonic power. The AlQ3 films were fabricated by ultrasonic‐assisted mist deposition method on indium‐tin‐oxide. The low leakage current suggested well‐defined AlQ3 thin films without significant electrical defects such as pinholes.

Improved electron injection of organic light-emitting diodes by using an ultrathin lithium-hydroxide buffer layer

A cathode buffer layer of lithium hydroxide (LiOH) was used to improve the electro-optical properties of organic light-emitting diodes (OLEDs). LiOH layers with various thicknesses were prepared by thermally evaporating LiOH powders. When a 1-nm-thick LiOH layer was inserted between the aluminum (Al) cathodes and the tris(8-hydroxyquinolinato)aluminum (Alq3) electrontransporting layers, device properties such as the turn-on voltage, the maximum luminance, and the device efficiency were improved, becoming better than those of the device with a LiF buffer layer. The surface of the Alq3 film became smoother after the LiOH layer had been deposited. Ultraviolet photoelectron spectroscopy results show that the reaction between LiOH and Alq3 results in an n-type doping effect, which moves the Fermi level close to the lowest unoccupied molecular orbital (LUMO) of Alq3. Thus, the electron-injection efficiency was enhanced due to a lower electron injection barrier, which improved the charge carrier balance in the OLEDs and led to a better device efficiency.

Doping Mechanism and Electronic Structure of Alkali Metal Doped Tris(8-hydroxyquinoline) Aluminum

We investigated the electronic structures of alkali metal doped Alq3 molecules prepared by codeposition of Na and tris(8-hydroxyquinolinato)aluminum (Alq3) (Na:Alq3) using in situ synchrotron radiation photoelectron spectroscopy. The doping ratio of Na to Alq3 (RNa) was 0, 0.3, 0.6, 0.8, 2.7, 3.4, or 3.9. The work function, calculated from photoemission spectra, remained at 3.6 eV ± 0.03 eV for all samples, while the energy of the highest occupied molecular orbital increased from 2.2 to 2.85 eV as the doping ratio increased from RNa = 0 to RNa = 2.7. The work function and valence band spectra indicated that there is no band bending or surface work function change due to an alkali doping effect, in contrast to the findings of previous reports. The doped layer was composed of an n-type organometallic complex according to the analysis of O 1s and N 1s spectra. The N-type doping effects shown in the N 1s spectra of coevaporation samples were reflected in the schematic band diagram, so the energy difference between the Fermi level (EF) and the lowest unoccupied molecular orbital (LUMO) decreased by 0.64 eV. The schematic band diagram demonstrates that a monotonic shift of the LUMO toward EF was observed with increasing doping, which is in contrast to general n-type doping effects in inorganic semiconductors. Also, we experimentally observed increased electron transport characteristics of alkali metal doped Alq3. The operating voltage at 100 mA/cm2 decreased from 11.9 V (RNa = 0) to 9.6 V, and the luminance at 10.5 V increased from 3575 cd/m2 (RNa = 0) to 9675 cd/m2 when the Na:Alq3 film (RNa = 2.7) was inserted between Alq3 and LiF/Al.

Role of K2CO3 as an n-type dopant in enhancing the electron injection and transport of organic light-emitting devices

Potassium carbonate (K2CO3) has demonstrated to be an efficient n-type dopant in improving the performance of the Alq3-based organic light-emitting diodes (OLEDs). K2CO3-doped tris(8-hydroxyquinolinato)aluminum (Alq3) layers with various concentrations were prepared by the co-evaporating method and the role of K2CO3 was investigated. When 10% K2CO3 was doped into an Alq3 electron-transporting layer, device properties such as the turn-on voltage, the maximum luminance, and the device efficiency were improved. K2CO3-doped Alq3 layers possess an enhanced electron injection and transporting ability. The higher mobility of the K2CO3-doped samples has been derived using space–charge–limited current (SCLC) measurements. X-ray and ultraviolet photoelectron spectroscopy results show that some electrons transfer from K2CO3 into Alq3, which increases the electron concentration in Alq3 films and moves the Fermi level close to the lowest unoccupied molecular orbital (LUMO) of Alq3. Therefore, the electron injection and transport efficiency are both enhanced due to a lower electron injection barrier and a higher electron mobility, which improve the charge carrier balance in OLEDs and lead to better device efficiency.

Fabrication of Organic Small Molecular Thin Films based on Ultrasonic Spray-Assisted Vapor-Deposition Method

ABSTRACTSmall molecular thin films using solution-based method is a challengeable subject in organic optical and electronic devices. In our previous research, we successfully deposited aluminum tris(8-hydroxyquinoline) (Alq3) films on glass substrate. In this paper, aiming at future exploration of electroluminescent devices, we deposited N, N ’-Bis(3-methylphenyl)-N,N’- diphenylbenzidine) (TPD) films on indium-tin-oxide (ITO) substrates using the vapor-deposition method. Photoluminescence characteristics evidenced the actual formation of TPD thin films. Together with the good surface morphology and low leakage current of the films, the results are promising for actual device fabrication at low cost and low material loss.