Alq3 Thin Films Research Articles

Effect of titanyl phthalocyanine doping on opto-electrical properties of Alq3 thin films

The titanyl phthalocyanine (TiOPc) was doped in tris (8-hydroxy quinolinato) aluminum (Alq3) with concentration of 1, 2 and 3 % by weight. The thin film of undoped and doped Alq3 was studied extensively for optical and electrical properties. The refractive index of the studied material was found in the range of 1.5–2.07 for 300–1000 nm wavelength. The Urbach tail energy decreases by increase of doping concentration. The PL quenching ratio increases with doping that attribute charge transfer from Alq3 to the TioPc. The electrical properties of the thin film were studied by impedance spectroscopy over a frequency range of 100 Hz–1 MHz. The undoped and doped Alq3 shows single relaxation process. The Cole–Cole plots of undoped and doped device can be represented by a single parallel resistance RP and capacitance CP network with a series resistance RS. The value of RP and CP at zero bias are ~146 kΩ, 87 kΩ, 814 Ω and 22 kΩ and 68, 9, 30 and 29 nF for undoped, 1, 2 and 3 % doping, respectively. The resistance RP decreases with applied bias whereas the capacitance CP remains almost constant. At high frequency, the AC conduction of the film follows the universal power law and the onset frequency increases with increasing bias voltages.

Following the Island Growth in Real Time: Ag Nanocluster Layer on Alq3 Thin Film

The progress of organic electronics demands an increased participation of nanotechnology, and it has already been shown that the presence of metallic nanoparticles and/or nanostructured thin films can enhance the device performance. Nevertheless, to gain control over the device final performance, it is crucial to achieve a profound understanding of the nanostructure development and assembly. We investigate the growth kinetics of silver (Ag) on a tris(8-hydroxyquinolinato)aluminum (Alq3) thin film via sputter deposition. The increase of the average electron density of the Ag nanostructured film is observed to follow a sigmoidal shape development as a function of the deposited Ag thickness, as a consequence of dominant island-mediated growth. The nanoclustered film is percolated at around a thickness of 5.0 ± 0.1 nm. At this film thickness the effective film density is about 50%. Moreover, our simulation results indicate that the shape of the nanoclusters changes from truncated spheres to cylinders upon sur...

Characterization of facial and meridional Alq3 thin films fabricated from physical vapor transport at high substrate temperatures

During a thermal annealing process, the isomeric transformation from meridional (mer) into facial (fac) tris(8-hydroxyquinoline)-aluminum (Alq3) based on the infrared absorption was observed to be nearly complete around 375 °C. Then, in a physical vapor transport process with post-annealed fac-Alq3 as source powder (at source temperature of 375 °C), the effects from various substrate temperatures (Tsub from 285 to 346 °C) on the transformation from fac-Alq3 into mer-Alq3 isomers re-condensed on the substrate were studied again, and an isomeric transformation occurs at Tsub = 316 ± 2 °C. For Tsub below 316 ± 2 °C, the re-condensed mer-Alq3 isomers tend to aggregate to form close-packed micron-rods, which are composed of α-phase crystallites with an average coherent length L ∼ 46 nm. The photoluminescence (PL) spectra for these as-fabricated mer-Alq3 films show typical green emission. However, for Tsub above 316 ± 2 °C, the adsorbed fac-Alq3 molecules on the substrate preserve their corresponding isomeric structure from the post-annealed fac-Alq3 source and self-organize into flexible submicron-fibers. These samples are dominated by δ-phase crystallites with L ∼ 57 nm. The PL spectra for these as-fabricated fac-Alq3 films do emit blue luminescence. Besides, Raman scattering for these mer-Alq3 and fac-Alq3 molecules are also compared.

Study on n-type Doped Electron-Transporting Layers in OLEDs by Electron Spin Resonance

We present an electron spin resonance (ESR) study of Cs2CO3 and LiF doped tris(8-hydroxyquinoline) aluminum (Alq3) thin films. The g values of the Cs2CO3 and LiF doped Alq3 thin films were obtained as 2.0040 and 2.0028, repectively. The g value of the LiF doped film was very close to a theoretically calculated one for an isolated Alq3 radical anion, while Cs2CO3 doped film showed a large g shift. This feature can be explained by large spin-orbit interaction caused by heavy Cs atoms. While doping of Cs2CO3 did not showed cathode-metal (Al and Au) dependence, LiF doping required Al as cathode.

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.

Thermal conductivity of organic semi-conducting materials using 3omega and photothermal radiometry techniques

Organic semiconductors for opto-electronic devices show several defects which can be enhanced while increasing the operating temperature. Their thermal management and especially the reduction of their temperature are of great interest. For the heat transfer study, one has to measure the thermal conductivity of thin film organic materials. However the major difficulty for this measurement is the very low thickness of the films which needs the use of very specific techniques. In our work, the 3-omega and photothermal radiometric methods were used to measure the thermal conductivity of thin film organic semiconducting material (Alq3). The measurements were performed as function of the thin film thickness from 45 to 785 nm and also of its temperature from 80 to 350 K. With the 3 omega method, a thermal conductivity value of 0.066 W.m −1 K −1 was obtained for Alq3 thin film of 200 nm at room temperature, in close agreement with the photothermal value. Both techniques appear to be complementary: the 3 omega method is easier to implement for large temperature range and small thicknesses down to a few tens of nanometers whereas the photothermal method is more suitable for thicknesses over 200nm since it provides additional information such as the thin film volumetric heat capacity.

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.

Demonstration of determination of electron and hole drift-mobilities in organic thin films by means of impedance spectroscopy measurements

The electron and hole drift-mobilities of tris(8-hydroxy-quinolinato) aluminium (Alq3) thin films have been determined using impedance spectroscopy (IS) measurements. The theoretical basis of drift-mobility measurement with IS rests on a theory for single-injection space-charge limited current. The electron drift-mobilities in Alq3 which measured from the frequency dependence of both capacitance and conductance had the same electric-field dependence and were identical to those measured by a time-of-flight (TOF) transient photocurrent technique. To estimate the hole drift-mobility in Alq3, a 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl thin film was inserted as an injection layer to reduce the hole injection barrier between an injection electrode and the Alq3 thin film. The hole drift-mobilities of Alq3 thin films measured with IS were identical to those measured by the TOF method. These results are essential for clarifying the mechanism of the carrier transport of organic light emitting diodes (OLEDs) with the aim of improving OLED performance.

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.

Direct Observation of Charge Carriers in Highly Magnesium-Doped Tris(8-hydroxyquinoline) Aluminum Thin Film by Electron Spin Resonance

We report on an electron spin resonance (ESR) study of a tris(8-hydroxyquinoline) aluminum (Alq3) thin film doped highly with Mg. We observed a clear ESR signal of radical anions in the film prepared by coevaporating Alq3 and Mg (Alq3:Mg= 1:3). The g value and peak-to-peak ESR linewidth ΔHpp are obtained as 2.0030 and 2.24 mT, respectively, which coincide well with the reported values of an Alq3 thin film with a low Mg doping concentration. The spin susceptibility of the film was found to obey Curie's law, which indicates that no interaction exists between spins of Alq3 anions. The doped charges are found to be localized at deep trapping sites by lineshape analysis and from the temperature dependence of ΔHpp. The doping concentration was evaluated to be 4.3%. This low doping concentration can be explained by the formation of Mg clusters in the film, which was directly confirmed by transmission electron microscopy.

Electron-injecting properties of Rb2CO3-doped Alq3 thin films in organic light-emitting diodes

Rubidium carbonate (Rb2CO3)-doped tris(8-quinolinolato)aluminum (III) (Alq3) thin films have been investigated as electron-injecting materials for organic light-emitting diodes (OLEDs). Electron-only devices consisting of glass/tin-doped indium oxide (ITO)/Rb2CO3-doped Alq3 (10 nm)/aluminum (Al) showed an electron-ohmic contact property between the electrode and the organic layer at the doping concentration of 10% and higher. The electron-injecting ability of these contacts was largely enhanced by the n-doping effect of Rb2CO3 into the Alq3 layer. The ultraviolet photoemission spectra revealed that when the doping concentration was increased, the n-doping effect reduced the carrier-injecting barrier height by lowering the work function at the Rb2CO3-doped Alq3 interfaces. Also, the x-ray photoemission spectra showed that as the doping concentration was increased at the interfaces, Alq3 molecules decomposed in a chemical reaction with Rb2CO3. The OLED device, having the glass/ITO/molybdenum oxide (MoOx, 25%)-doped N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB, 5 nm)/NPB (63 nm)/Alq3 (32 nm)/Rb2CO3-doped Alq3 (10%, 10 nm)/Al (100 nm) structure, showed the best performance at the optimal doping concentration of Rb2CO3-doped Alq3, both the maximum luminance of 114 400 cd/m2 at the bias voltage of 9.8 V and the power efficiency of 2.7 lm/W at the luminance of 1000 cd/m2 were obtained.

Photophysics and Energy Transfer Studies of Alq<SUB>3</SUB> Confined in the Voids of Nanoporous Anodic Alumina

We report on a hierarchical nanoarchitecture wherein distinct chromophores are deterministically placed at two different types of sites in a nanoporous metal oxide framework. One chromophore, namely Tris(8-hydroxyquinoline)aluminium(III) (Alq3), is embedded in the 1-2 nm sized nanovoids of anodic aluminum oxide (AAO) and another chromophore (carboxyfluorescein or pyrenebutyric acid) is anchored in the form of a monolayer to the surface of the walls of the cylindrical nanopores (- 20 nm in diameter) of AAO. We found the luminescence maximum to occur at 492 nm, blueshifted by at least 18 nm from the value in solutions and thin films. The excited state decay of Alq3 molecules in nanovoids was found to be biexponential with a fast component of 338 ps and a slower component of 2.26 ns, different from Alq3 thin films and solutions. Using a combination of steady state and time-resolved luminescence studies, we found that efficient Forster-type resonance energy transfer (FRET) from Alq3 in the nanovoids to the carboxyfluorescein monolayer could be used to pump the emission of surface-bound chromophores. Conversely, the emission of nanovoid-confined Alq3 could be pumped by energy transfer from a pyrenebutyric acid monolayer. Such intra-nanoarchitecture interactions between chromophores deterministically placed in different spatial locations are important in applications such as organic light emitting diodes, chemical sensors, energy transfer fluorescent labels, light harvesting antennas and organic spintronics.

Fabrication and Characterization of Flexible Organic Light Emitting Diodes Based on Transparent Flexible Clay Substrates

In the present work, transparent flexible polymer-doped clay (P-clay) substrates were prepared for flexible organic light emitting diode (OLED) applications. Nanocrystalline indium tin oxide (ITO) thin films were prepared on P-clay substrates by ion-beam sputter deposition method. The structural, optical, and electrical properties of as-prepared ITO/P-clay showed that the as-prepared ITO thin film was amorphous, and the average optical transparency and sheet resistance were around 84% and 56 Ω/□, respectively. The as-prepared ITO/P-clay samples were annealed at 200 and 270 °C for 1 h to improve the optical transparency and electrical conductivity. The average optical transparency was found to be maximum at an annealing temperature of 200 °C. Finally, N,N-bis[(1-naphthyl)-N,N '-diphenyl]-1,1'-biphenyl)-4,4'-diamine (NPB), tris(8-hydroxyquinoline) aluminum (Alq3) thin films, and aluminum (Al) electrode were prepared on ITO/P-clay substrates by thermal evaporation method. The current density–voltage (J–V) characteristic of Al/NPB/ITO/P-clay showed linear Ohmic behaviour. In contrast, J–V characteristic of Al/Alq3/NPB/ITO/P-clay showed non-linear Schottky behaviour. Finally, a very flexible OLED was successfully fabricated on newly fabricated transparent flexible P-clay substrates. The electroluminescence study showed that the emission intensity of light from the flexible OLED device gradually increased with increasing applied voltage.

Ultraviolet photodegradation of tris(8-hydroxy-quinolinate) aluminum (Alq3) thin films studied by electron and laser stimulated desorption

Alq3 has been the reference material used widely in the fabrication and characterization of efficient OLEDs due to its good properties as electroluminescent and electron transporting layer. Although the inclusion of these devices in commercial displays and lighting devices represents many benefits, the knowledge about the progressive loss of performance and efficiency with time for such devices is still limited. Therefore, it is an incentive to understand the mechanisms of Alq3 degradation when it is subjected to the influence of various extrinsic factors such as UV radiation. In the present work the degradation processes of Alq3 thin films as a result of 254nm UV light irradiation are presented and discussed. The degradation products produced by the action of UV light were evaluated by time-of-flight mass spectrometry using electron stimulated ion desorption and laser desorption ionization techniques. Strong evidence for carboxylic acid formation after photodegradation was observed by the electron desorption technique.

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.

Precise pixel patterning of small molecule organic light-emitting devices by spin casting

We demonstrate the fabrication of OLEDs with precisely patterned pixels as small as 10 μm by the spin-casting of Alq3 thin films. The solution-based patterning technique produces pixels via the segregation of organic molecules into micro-fabricated channels or wells. Segregation is controlled by a combination of weak adsorbing characteristics of aliphatic terminated self-assembled monolayers and by centrifugal force, which directs the organic solution into the channel or well. This novel patterning technique may resolve the limitations of pixel resolution of the method of thermal evaporation using shadow masks, and is applicable to the fabrication of large area displays. Furthermore, the patterning technique has the potential to produce pixel sizes to the limitation of photolithography and micromachining, thereby enabling the fabrication of high-resolution micro-displays. We discuss the patterning method and device fabrication, and characterize the morphological, optical, and electrical properties of the organic pixels.

Bipolar carrier transport in tris(8-hydroxy-quinolinato) aluminum observed by impedance spectroscopy measurements

We studied bipolar carrier transport in tris(8-hydroxy-quinolinato) aluminum (Alq3) thin films using impedance spectroscopy (IS). Two transit times were observed in the impedance spectra of the Alq3 double-injection diodes. The mobilities determined from the transit times are in good agreement with the electron and the hole mobilities in Alq3 measured by IS using single injection diodes and by the time-of-flight transient photocurrent technique. The bipolar carrier transport observed in Alq3 shows that the carrier recombination of Alq3 is weak on the basis of the simulation [M. Schmeits, J. Appl. Phys. 101, 084508 (2007)]. Simultaneous measurements of electron and hole mobilities are useful in the study of charge-carrier transport in active layers in organic light-emitting diodes and organic solar cells.

Optical investigation of photo-bleaching effects in organic Alq_3 thin films

The photo-bleaching effects of selective ultraviolet and blue light illumination on the photoluminescence spectra of thermally evaporated tris(8-hydroxy-quinoline) aluminum (Alq3) thin films exposed in air were investigated by fluorescence spectroscopy and advanced optical imaging techniques. The photoluminescence intensity time behavior observed in the Alq3 films at different excitation powers allows to establish that Alq3 molecules in their exciting state are involved in these complex phenomena. The photo-bleaching processes were exploited for direct laser patterning of the investigated Alq3 thin films.

Maple prepared organic heterostructures for photovoltaic applications

In this study, we present the deposition of ZnPc, Alq3, and PTCDA thin films using Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique. We also report the realisation of multilayer structures, made by the successive application of MAPLE. The films have been characterized by spectroscopic (UV–VIS and Photoluminescence) and microscopic (SEM and AFM) methods, and the effect of different deposition conditions such as fluence, number of pulses, and target concentration on the properties has been analysed. This paper also presents some investigations on the electrical conduction in sandwich type structures ITO or Si/organic layer/Au or Cu and ITO/double organic layer/Cu, emphasising the dominant effect of the height of the energetic barriers at the inorganic/organic and organic/organic interfaces.

Ultrasonic Spray-Assisted Solution-Based Vapor-Deposition of Aluminum Tris(8-hydroxyquinoline) Thin Films

Aluminum tris(8-hydroxyquinoline) (Alq3) thin films were fabricated by a vapor-deposition technique from its methanol solution, that is, by the ultrasonic-assisted mist deposition technique. The application of high ultrasonic power to the Alq3–methanol mixture resulted in a stable and transparent solution. Mist particles formed by ultrasonic atomization of the solution were used as the source for vapor-deposition at the substrate temperature of 100–200 °C. Optical absorption and photoluminescence characteristics indicated the formation of Alq3 thin films. The results promise the formation of thin films of a variety of organic materials by the solution-based technique.