Carl P. Tripp

Humidity-induced crystallization of tris (8-hydroxyquinoline) aluminum layers in organic light-emitting devices

We report electroluminescence degradation studies of tris (8-hydroxyquinoline) aluminum (Alq3) organic light-emitting devices (OLEDs) under ambient conditions. Alq3 films and organic bilayer anode/naphthyl-substituted benzidine derivative/Alq3/cathode devices are studied via electroluminescence, photoluminescence, polarization microscopy and atomic force microscopy, and via microscopic infrared spectroscopy. Results reveal that humidity induces the formation of crystalline Alq3 structures in originally amorphous films. The same phenomenon is found to occur in OLEDs and causes cathode delamination at the Alq3/cathode interface that results in the formation of black (nonemissive) spots in the devices.

DEGRADATION PROCESSES AT THE CATHODE/ORGANIC INTERFACE IN ORGANIC LIGHT EMITTING DEVICES WITH MG:AG CATHODES

We report electroluminescence degradation studies on tris(8-hydroxyquinoline) aluminum (Alq3)-based organic light emitting devices (OLEDs) with Mg:Ag cathodes in ambient conditions. The nonemissive spots in the OLEDs are studied via optical and fluorescence microscopy and via microscopic infrared spectroscopy. Studies reveal that a majority of the nonemissive spots are caused by the growth of Mg(OH)2 sites at the Alq3/Mg:Ag interface, associated with local degradation of the Alq3 layer. In addition, the growth of elevated cathode bubbles, which also lead to nonemissive spots, is found to be caused by gas evolution from the galvanic corrosion of the Mg/Ag couple as well as from the electrolysis of absorbed moisture.

Synthesis of high surface area monoclinic WO3 particles using organic ligands and emulsion based methods

Several synthetic approaches have been used to obtain nano-sized monoclinic WO3 (m-WO3) powders. All of these methods begin with a standard preparative method where H2WO4 is first generated by passing a Na2WO4 solution through a cation-exchange resin. It is shown that high surface area particles are produced by dripping the H2WO4 exiting from the ion-exchange column into a solution containing oxalate and acetate exchange ligands or alternatively, into a water-in-oil (w/o) based emulsion. In comparison to commercial WO3 powders, the surface area of the m-WO3 powders were higher by factors of 10 and 20 times when prepared in the presence of acetate/oxalate chelating agents and w/o emulsions, respectively. The much higher surface areas enable infrared spectroscopic identification of surface sites along with detection and monitoring of gaseous reactions and adsorbed species on the surface of this metal oxide. This is demonstrated with the adsorption of a nerve agent simulant, dimethyl methyl phosphonate. In general, little is known about the reactions of gaseous molecules on m-WO3 surfaces and the fabrication of high surface area m-WO3 particles will aid in gaining an understanding of the chemical processes occurring in WO3 based sensors.

Interaction of organophosphorous compounds with TiO2 and WO3 surfaces probed by vibrational spectroscopy

The interaction of organophosphorous compounds with TiO2 and WO3 high surface area powders has been studied by thin film infrared spectroscopy. Room temperature adsorption of dimethyl methyl phosphonate (DMMP), dimethyl hydrogen phosphonate (DMHP), and trimethyl methyl phosphonate (TMP) is through hydrogen bonding of the PO functional group to the hydroxyl groups of the metal oxide surface. At higher reaction temperatures, these hydrogen bonded organophosphorous compounds dissociate and form covalently attached species. Above 200°C, the methoxy groups desorb from the surface while the methyl groups remain stable. Above 300°C, a stable phosphate surface complex is formed. The presence of the phosphate complex may be responsible for poisoning effects observed during DMMP gas exposure of chemiresistive sensors operating in this temperature range.

Alkylchlorosilane reactions at the silica surface

Abstract This paper reviews recent work on the reaction of alkyltrichlorosilanes with a pure silica surface. In the absence of moisture these reactions do not occur at temperatures below 300°C. Specifically, it is shown that octadecyltrichlorosilane (OTS) in carbon tetrachloride solution does not react with the surface at room temperature. Reaction does occur with molecular water which is either adsorbed on the surface or dissolved in the CCl 4 . The polymeric species which is formed can then physically adsorb onto the silica surface causing perturbation and H-bonding of the surface hydroxyls, but not reaction. A room temperature reaction can be induced by the use of a two-step mechanism. In the first step a strong lone pair base such as triethylamine is H-bonded to the surface hydroxyl groups. This induces a strongly nucleophilic oxygen at the hydroxyl site, which is then able to interact with the incoming chlorosilane to form a genuine surface compound. The base is released as the chloride salt.

Life extension of organic LED's by doping of a hole transport layer

Abstract We investigated the influence of a series of aromatic hydrocarbon dopants in the hole transport layer on the life of organic light emitting devices (OLED) with the following configuration: ITO/NPB+dopant/Alq 3 +fluorescent dye/Mg:Ag. The addition of selected dopants to NPB led to an increase in device lifetime by more than an order of magnitude. Half-life, t 1/2 , of the devices was measured at the current density of J =25 mA/cm 2 . Device life, at initial luminance of 100 cd/m 2 , was estimated by using the scaling law established by A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160, ( L initial × t 1/2 =constant). Typically extrapolated device life exceeded 10 000 h, with the best devices exceeding 50 000 h. Life extension by use of dopants is consistent with the recently proposed mechanism that OLED degradation is primarily caused by holes injected into Alq 3 .

Formation of calcium carbonate particles by direct contact of Ca(OH)2 powders with supercritical CO2

A novel method to obtain a high conversion of Ca(OH)2 powder to CaCO3 is reported. The method uses supercritical CO2 containing water that is passed over the dry Ca(OH)2 powder. Conversions greater than 98% are obtained. The dominant crystal structure is calcite. The process has the advantages of the conventional solution based process in terms of high conversion and the advantages of a gas–solid based process in that dry, non-agglomerated particles are obtained by simple venting of the solvent.

Dual WO3 based sensors to selectively detect DMMP in the presence of alcohols

A size selective approach to improving selectivity in semiconducting metal oxides (SMO) sensors was obtained by tailoring the architecture of WO(3) powders. The key for achieving high selectivity is based on using a dual sensor configuration where the response on a porous WO(3) powder sensor was compared to the response on a nonporous WO(3) powder sensor. Detection selectivity between methanol and dimethyl methylphosphonate (DMMP) is obtained because the access of a gas molecule in the interior pore structure of WO(3) is size dependent leading to a size dependant magnitude change in the conductivity of SMO sensor.

Structure and infrared (IR) assignments for the OLED material: N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (NPB)

Organic light-emitting diodes (OLEDs) are currently under intense investigation for use in next-generation display technologies. Research into the fundamental properties of the materials used in OLEDs, such as structure and vibrational modes, will help provide experimental probes which are required to gain insight into the processes leading to device degradation and failure. Calculations using the hybrid B3LYP functional and the split-valence polarized 6-31G(d) basis set have been carried out to assign the IR bands of the OLED hole transport material N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (NPB). Excellent agreement was found between the computed and experimental wavenumbers allowing the reliable assignment of major IR bands. Comparison of the reflection absorption IR (RAIRS) spectra obtained from room temperature and thermally annealed NPB thin films indicates that, upon annealing, structural changes occur and the average orientation of the NPB naphthyl groups become predominately flat with respect to the surface.

Formation of a thin TiO2 layer on the surfaces of silica and kaolin pigments through atomic layer deposition

Abstract Atomic layer deposition (ALD) is used to deposit a titanium dioxide (TiO 2 ) layer onto the surfaces of both a silica and kaolin pigment. The reactive vapors of TiCl 4 and H 2 O are used in a cyclic reaction sequence to grow the titania layer. Fourier transform infrared spectroscopy is used to monitor the cycle-to-cycle changes, in situ, during the deposition. Raman spectroscopy is used to determine the morphology of the deposited TiO 2 layer. The formation of SiOTi surface bonds, through the reaction with the silica surface silanols, is detected and shows that three complete reaction cycles are necessary to fully cover the silica surface. Detection of a covalent surface bond to the kaolin is difficult due to the intensity and number of kaolin lattice vibrations in the low frequency region. Differences in the structure of adsorbed water were, therefore, used to monitor the extent of titania surface coverage on the kaolin. Again it is found that three complete reaction cycles are necessary to fully cover the kaolin surface. The behavior of the modified kaolin in water is determined through measurement of its electrophoretic mobility and its dispersion stability.