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Introducing a hole-injection-tunable-anode-buffer-layer (HITABL) at the indium tin oxide anode contact of an organic light-emitting device can finely tune hole injection to establish proper charge balance, thus remarkably improves its operational stability. The HITABL consists of two sublayers: (i) an ∼2.5 nm thick metal (e.g., Ca, Mg, or Ag) sublayer and (ii) an ∼10 nm thick tetrafluorotetracyanoquinodimethane (F4TCNQ) doped N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine sublayer. Hole injection can be tuned by changing (i) the metal in the first sublayer and/or (ii) the concentration of the F4TCNQ dopant in the second sublayer. The choice of the metal used in the first sublayer and/or the concentration of F4TCNQ in the second sublayer affect the hole-injection efficiency. Therefore, by using the HITABL, one can make the necessary diminutive adjustments to the hole injection of a device and achieve proper charge balance, resulting in a significant improvement in operational stability. [ABSTRACT FROM AUTHOR]

To resolve the issue of which of the “indium migration” model and the “unstable AlQ3 cationic” model plays a more important role in luminescence degradation of organic light-emitting devices, we investigated the effect of the device structures on device operational stability. The results show that alterations in device layer structures can significantly affect the device operational stability, although they do not appear to noticeably change the magnitude of indium migrations. This suggests that the indium migration model may not play a dominant role in device degradation. On the other hand, the change in device stability with the alteration in the device structures can be plausibly explained by the unstable AlQ3 cationic model. [ABSTRACT FROM AUTHOR]

Presents a study which examined the polyimide films containing water by dielectric relaxation and [sup]hydrogen nuclear magnetic resonance spectroscopies. Materials and methods used; Characteristics of the dielectric loss of the Kapton polyimide; Details on the nuclear magnetic resonance of deuteron.

We report the effect of finer structures, or more levels, in a particular type of planar metallic fractal pattern on the stop bands or band gaps in the transmission spectrum of electromagnetic wave. It is found that the band gaps shift to low frequencies with the increase of levels, showing tunable band gap properties. In addition, we find from both experiments and theoretical calculations that the bands appearing in long wavelength range are determined by individual patterns which compose a 6×6 fractal array, while those bands located in the range of short wavelength are strongly influenced by their neighboring residents. Our experimental observations are in good agreement with those of finite difference time domain simulations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Negative dielectric constant has long been pursued for a possible revolution in electronics and photonics. It is usually found in systems containing free electrons under high frequency oscillating field, but not involving static charges or insulating materials. Here, we report the observation of the phenomenon in an insulating polymer containing static electricity, which lasts for several weeks, where negative capacitance persists from <1 Hz up to MHz frequency, also presenting an unusual spiral curve in impedance spectrum, producing inductors without bulky magnetic coils. [ABSTRACT FROM AUTHOR]

Ultra-low frequency AC field was employed to align p/n polymers during organic photovoltaic device fabrication. The resulting devices show 15% increase in power conversion efficiency and four-fold increase in parallel resistance. Supported by the transmission electron microscopy and atomic force microscopy images, the performance enhancement is attributed to the optimized morphology and enlarged p/n interface by AC field, which is more effective than DC, possibly explained by the argument of better mixing via back-and-forth shaking than a single swing. [ABSTRACT FROM AUTHOR]

Charge injection between the active layers in organic semiconducting devices is a key determinant of device function. Accordingly, understanding the effect of intermixing between the layers at these interfaces is of fundamental importance. In this letter, via the use of the time-of-flight method, a comparison is made between the charge injection across discrete versus intermixed interfaces of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine and 1,1-bis((di-4-tolylamino)phenyl)-cyclohexane doped polycarbonate, semiconducting thin-film layers. No perturbation to the overall charge transport was observed with the discrete interface; however, in contrast the rate of charge transport was clearly reduced through the intermixed interface. [ABSTRACT FROM AUTHOR]

The authors measured electric-field-induced fluorescence quenching (EFIFQ) in both undoped and fluorescent dye-doped tris(8-hydroxyquinoline)aluminum (AlQ3) layers of organic light-emitting devices. Results show that doped AlQ3 layers demonstrate smaller EFIFQ than undoped ones. The phenomenon is attributed to the narrower energy band gap of the guest molecule relative to that of the host material, which makes it less prone to electric-field-induced dissociation of the excited state. Results also show that increasing the concentration of the guest material or decreasing its band gap leads to a decrease in EFIFQ. [ABSTRACT FROM AUTHOR]

Introducing a thin Mg layer at the hole injection contact of organic light-emitting devices remarkably improves their operational stability. Devices in which a ∼2.5 nm thick Mg layer is inserted between the indium tin oxide anode and a tetrafluoro-tetracyanoquinodimethane-doped hole transport material layer exhibit a significantly longer lifetime compared to similar devices without the Mg layer. After 600 h of operation at a current density of 62.5 mA/cm2 with a 50% duty cycle, the luminance of devices containing the Mg layer decreases by only ∼10% of the initial value. The stability enhancement resulting from using the Mg layer is attributed to improved balance in charge injection at the anode and cathode contacts. [ABSTRACT FROM AUTHOR]

With existing methods it is difficult to measure the mobility of semiconducting thin films that have submicron thickness and submicrosecond charge-carrier transit time. To simplify these measurements we demonstrate a technique that is a combination of the time-of-flight and transient electroluminescence methods. The technique is fundamentally optical in that it decouples the carrier transient signal from the device charging circuit and hence removes the RC time constant constraint that limits existing methods. The technique was applied to measure electron mobility in a tris(8-hydroxyquinoline) aluminum (AlQ3) thin film. Results agree well with mobility values obtained using other methods. [ABSTRACT FROM AUTHOR]

Discrete interfaces between successive layers in an organic semiconducting device simplify any examination of interface barriers for charge transport. To form discrete interfaces between organic layers we propose lamination as an alternate approach to physical vapor deposition. Transient photocurrent measurements as a function of pressure, thickness, and electric field were performed on cells of 1,1-bis[(di-4-tolylamino)phenyl]-cyclohexane (TAPC), N,N-bis(3,4-dimethylphenyl)-4-aminobiphenyl (DMPAB), and N,N’-diphenyl-N,N’-bis(3-methylphenyl)-[1,1’-biphenyl]-4,4’-diamine (TPD). It was found that, in the range 0.8–3.0 MPa, a pressure-laminated interface between two identical materials causes no measurable perturbation to charge transport. This justifies the use of pressure lamination to study interfaces between nonidentical layers. [ABSTRACT FROM AUTHOR]

Poor environmental stability has been a major concern for organic light-emitting devices. Exposure to ambient conditions leads to the formation of nonemissive areas (dark spots) that result in a decrease in device luminescence. Although a number of mechanisms for the formation of dark spots have been proposed, the causes underlying their initiation, and their nucleation sites are still far from being clear. In this study, optical microscopy is used to investigate the sites of dark spots of devices in which the original cathodes are peeled off and replaced by newer cathodes. Results confirm that the growth of dark spots occurs primarily due to cathode delamination. The growth of dark spots is also associated with changes in the organic layers, especially at the organic/cathode interface. Results also suggest that the nucleation of dark spots takes place at the organic/cathode interface and originates during the deposition of the cathode. On the other hand, both the anode and the hole transport layer do not appear to play a role in the formation of dark spots. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

For many years, people believed that in conventional x-ray crystallography one can only record the scattering intensities but not the phases. In order to study structures at the atomic scale, one has to use multibeam measurement or to rely on additional knowledge such as bonding length and to fit the intensity data by trial structure models. In this letter, however, we show that the phases are in fact hidden in the intensity data and can be recovered by studying the peak shoulders. To demonstrate, the x-ray diffraction data of aluminum powder were used to recover the phases and to reconstruct the electron density map. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report electroluminescence degradation studies on tris(8-hydroxyquinoline) aluminum (Alq[sub 3])-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)[sub 2] sites at the Alq[sub 3]/Mg:Ag interface, associated with local degradation of the Alq[sub 3] 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. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report electroluminescence degradation studies of tris (8-hydroxyquinoline) aluminum (Alq[sub 3]) organic light-emitting devices (OLEDs) under ambient conditions. Alq[sub 3] films and organic bilayer anode/naphthyl-substituted benzidine derivative/Alq[sub 3]/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 Alq[sub 3] structures in originally amorphous films. The same phenomenon is found to occur in OLEDs and causes cathode delamination at the Alq[sub 3]/cathode interface that results in the formation of black (nonemissive) spots in the devices. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]