Keith A. Higginson

Chemical and morphological stability of aluminum tris(8-hydroxyquinoline) (Alq/sub 3/): effects in light-emitting devices

Aluminum tris(8-hydroxyquinoline) (Alq/sub 3/) is presently considered one of the most reliable electron transporting and emitting materials for molecular-based organic light-emitting diodes (OLEDs). This paper summarizes the investigations that our group, over the past three years, have conducted into the physical and chemical response of Alq/sub 3/ in the presence of small amounts of moisture. The characteristic evolution of 8-hydroxyquinoline (8-Hq), a volatile byproduct of the hydrolysis of Alq/sub 3/, was quantified using thermogravimetric and gas chromatography/mass spectroscopy (GC/MS) analysis as a function of sample morphology. Annealed (more crystalline) samples exhibited greater stability to hydrolysis than freshly sublimed films, at the expense of photoluminescence (PL) efficiency. These phenomena are discussed with respect to failure mechanisms observed in Alq/sub 3/-based OLEDs.

Negative refractive index metamaterials in the visible spectrum based on MgB2∕SiC composites

An isotropic three-dimensional negative refractive index metamaterial has been fabricated and characterized in the visible regime. The metamaterial is based on a structure consisting of polycrystalline magnesium diboride (MgB2) as the host, providing negative permittivity, and silicon carbide (SiC) nanoparticles embedded randomly within the host, providing negative permeability. The metamaterial was fabricated using hot isostatic pressing to produce a fully dense solid with well-dispersed SiC nanoparticles. The properties of the resulting bulk metamaterial were evaluated using surface plasmon excitation, which showed coupling of both magnetic and electric plasmons, signifying both negative permeability and permittivity at 632nm.

Adaptive geometric optics derived from nonlinear acoustic effects

Tunable gradient index (GRIN) lenses were formed in a liquid-filled cavity supporting an ultrasonic standing wave. The refractive index gradient is based on the steady-state pressure component of the finite-amplitude acoustic signal. An acoustic lens was constructed which (1) focused and modulated a collimated light source and (2) formed an image. The properties of the lens are a function of the amplitude and frequency of the applied ultrasound. At the center of symmetry, the device can be approximated as a diverging lens with a single focal point of about −10 cm. In general, however, a series of focal lines are formed, to create a device resembling an axicon or an abberrating lens.

Tunable optics derived from nonlinear acoustic effects

Gradient index lenses were formed in a liquid-filled cavity supporting an ultrasonic standing wave. The constructed devices acted as diverging lenses or axicon lenses, depending on whether the center or edge region is interrogated. The focal length of the diverging lens was controllable with the frequency and amplitude of applied ultrasound from −100 mm to negative infinity. Experiments and models suggest that the primary process contributing to lensing is the steady-state density component of the finite-amplitude standing wave; sound amplitudes up to 150 MPa were calculated in glycerin, corresponding to a maximum contrast in the refractive on the order of 0.1%. This amplitude was also sufficient to move high index nanometer-scale particles via an acoustic radiation force and thereby create larger refractive index gradients. The segregation of suspended nanoparticles was found to enhance the lensing effects that occurred in the pure fluids. Concepts are also explored to manipulate the particle distributio...

Correlating physical and chemical degradation in the performance of aluminum tris(8-hydroxyquinoline) (Alq3)-Based OLEDs

Abstract Aluminum tris(8‐hydroxyquinoline) (Alq3), which is utilized as an electron transport layer (ETL) in organic light emitting diodes (OLEDs) is known to form crystalline domains over time. This morphological instability plays an important role in OLED degradation. In this study, crystallization was suppressed via entropic stabilization by blending Alq3 with a close adduct of the same i.e., aluminum tris(4 methyl, 8‐hydroxyquinoline) (4m‐Alq3). X‐ray diffraction studies at elevated temperatures indicate that the blend remains amorphous even after 96 hr of annealing at 160°C. OLED devices were fabricated and investigated for Alq3, 4m‐Alq3, and a co‐evaporated 50/50% mixture (blend) of Alq3 and 4m‐Alq3. As expected, the blend device possessed both the highest resistance and quantum efficiency over the entire current density range. Devices made with 4m‐Alq3 showed the highest current density, yet their quantum efficiency and lifetime were significantly lower than that of Alq3. In spite of the morphological stability of the blend, the intrinsic instability of 4m‐Alq3 was found to play a more important role in device degradation. This study indicates that while entropic stabilization is an effective way of obtaining brighter and more efficient OLEDs, care must be taken to ensure electrochemical stability and interfacial energy‐level optimization for both components.

A probe for moisture permeation using metal/organic composite films

Composite films made by simultaneous sublimation of metal and organic materials were used as sensors for moisture in overlying polymeric films, and polymer transport properties were obtained from a spatially resolved signal. In the probe film, the molecularly dispersed metal quenched the fluorescence of the organic dye. Upon exposure to water vapor, the metal was oxidized and the fluorescence of the dye recovered. The intensity profile development over time was modeled, and a diffusion coefficient for a ultraviolet-curable adhesive was obtained which compared well to other resins of its type. This technique offers several advantages in characterizing transport properties in confined geometries, such as those used for the passivation of light-emitting diodes or other electronic devices.

Chemical Degradation and Physical Aging of Aluminum(III) 8-Hydroxyquinoline: Implications for Organic Light-Emitting Diodes and Materials Design

A traditional barrier to marketing organic light-emitting diode (OLED) technology has been achieving devices capable of sustaining brightness levels with minimum drift over long periods of time as well as maintaining longevity while operating at elevated temperatures (e.g., 60–80°C). In many cases, product developers have designed around these problems (e.g., by extensive passivation1,2 or with the driving technology),3 but understanding of the decay processes is still by many means insufficient, especially with respect to the intrinsic limitations imposed by the materials in question. This type of knowledge gives upper-bound design constraints and should give insight in materials or device designs with greater stability or longevity.

Improving the Crystallization Resistance of Aluminum(III) 8-Hydroxyquinoline-Based Emitting Materials by Entropic Stabilization

The morphological stability of evaporated films of aluminum(III) 8-hydroxyquinoline (Alq 3 ) was investigated. Films which were found to be non-crystalline by x-ray diffraction upon deposition, crystallized rapidly upon annealing, especially where defects were present. Blends of Alq 3 with aluminum(III) 5-methyl-8-hydroxyquinoline were proposed for thermally stable amorphous emitting layers in light-emitting diodes. Films coevaporated at a 1:1 ratio did not show evidence of crystallization or phase separation even after long annealing periods at temperatures as high as 160°C.

Synthesis and characterization of colloidal mercury chalcogenide quantum dots

We describe the synthesis of colloidal mercury chalcogenide quantum dots (QDs) using a combination of strong Hg(II) coordinating ligands and precursor phase separation. This synthetic strategy provides a means of controlling the growth kinetics of mercury based II-VI QDs and addresses some of the problems which have heretofore made the synthesis of such compounds difficult. In particular, the simultaneous use of mercury coordinating ligands and precursor phase separation overcomes both the rapid precipitation of bulk mercury chalcogenides that occurs when only weak ligands are used and the reduction of Hg(II) when a strong ligand/high temperature combination is pursued. In the case of both HgS and HgSe this scheme has yielded one of the first examples of mercury chalcogenide QDs to date. The linear absorption/emission of HgS is size-dependent and ranges from 500 nm to 800 nm with corresponding sizes between 1 to 5 nm in diameter. For HgSe the band edge absorption/emission are also size dependent, ranging from 600 to 900 nm. The zincblende phase of both HgS and HgSe QDs is determined from wide angle x-ray diffraction experiments and reveals potentially large (band edge) spectral tunabilities for either material given their zero or slightly negative (bulk) band gaps.

Influence of shallow traps on the failure mechanism of aluminum (III) 8-hydroxyquinoline-based OLEDs

Conduction in aluminum(III) 8-hydroxyquinoline (Alq3)- based organic light-emitting diodes (OLEDs) was modeled based on trapped charge-limited conduction of electrons in the Alq3 bulk. This model was chosen since it can easily incorporate an arbitrary trap distribution such as may arise during the materials degradation. The evolution of a narrow Gaussian distribution of localized trap states below the lowest unoccupied molecular orbital (LUMO) of Alq3, lying against a natural exponential background, was used to explain changes in the current-voltage characteristic and external quantum efficiency with time observed by many researchers for organic light-emitting diodes. Based on the change of the shape of the DC current density vs. voltage (J-V) curve, the depth of the electron trap states that were formed during aging was about 0.25 eV below the LUMO of Alq3. This value is consistent with electrochemical measurements of known chemical degradation products. The J-V characteristics show behavior which is suggestive of a trap- filled limit, and this is discussed along with the general appropriateness of the model used.