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Notes: We present device model calculations of the current–voltage (I–V) characteristics of organic diodes and compare them with measurements of structures fabricated using MEH-PPV. The structures are designed so that all of the current is injected from one contact. The I–V characteristics are considered as a function of the Schottky energy barrier to charge injection from the contact. Experimentally, the Schottky barrier is varied from essentially zero to more than 1 eV by using different metal contacts. A consistent description of the device I–V characteristics is obtained as the Schottky barrier is varied from small values, less than about 0.4 eV, where the current flow is space-charge limited to larger values where it is contact limited. © 1998 American Institute of Physics.

Notes: We present a unified device model for single layer organic light emitting diodes (LEDs) which includes charge injection, transport, and space charge effects in the organic material. The model can describe both injection limited and space charge limited current flow and the transition between them. We specifically considered cases in which the energy barrier to injection of electrons is much larger than that for holes so that holes dominate the current flow in the device. Charge injection into the organic material occurs by thermionic emission and by tunneling. For Schottky energy barriers less than about 0.3–0.4 eV, for typical organic LED device parameters, the current flow is space charge limited and the electric field in the structure is highly nonuniform. For larger energy barriers the current flow is injection limited. In the injection limited regime, the net injected charge is relatively small, the electric field is nearly uniform, and space charge effects are not important. At smaller bias in the injection limited regime, thermionic emission is the dominant injection mechanism. For this case the thermionic emission injection current and a backward flowing interface recombination current, which is the time reversed process of thermionic emission, combine to establish a quasi-equilibrium carrier density. The quasi-equilibrium density is bias dependent because of image force lowering of the injection barrier. The net device current is determined by the drift of these carriers in the nearly constant electric field. The net device current is much smaller than either the thermionic emission or interface recombination current which nearly cancel. At higher bias, injection is dominated by tunneling. The bias at which tunneling exceeds thermionic emission depends on the size of the Schottky energy barrier. When tunneling is the dominant injection mechanism, a combination of tunneling injection current and the backflowing interface recombination current combine to establish the carrier density. We compare the model results with experimental measurements on devices fabricated using the electroluminescent conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] which by changing the contacts can show either injection limited behavior or space charge limited behavior. © 1997 American Institute of Physics.

Notes: We present calculations of single layer organic light emitting diode (LED) characteristics using a device model which includes charge injection, transport, recombination, and space charge effects in the organic material. Contact limited and ohmic contacts, high and low carrier mobilities, and device thicknesses from 5 to 200 nm are considered. The scaling of device current with applied voltage bias and organic film thickness is described for contact limited and ohmic contacts. Calculated device current, light output, and quantum and power efficiency are presented for representative cases of material and device parameters. These results are interpreted using the calculated spatial variation of the electric field, charge density, and recombination rate density in the devices. We find that efficient single layer organic LEDs are possible for a wide range of organic material and contact parameters. © 1998 American Institute of Physics.

Notes: We present experimental and device model results for electron only, hole only, and bipolar organic light-emitting diodes fabricated using a soluble poly (p-phenylene vinylene) based polymer. Current–voltage (I–V) characteristics were measured for a series of electron only devices in which the polymer thickness was varied. The I–V curves were described using a device model from which the electron mobility parameters were extracted. Similarly, the hole mobility parameters were extracted using a device model description of I–V characteristics for a series of hole only devices where the barrier to hole injection was varied by appropriate choices of hole injecting electrode. The electron and hole mobilities extracted from the single carrier devices are then used, without additional adjustable parameters, to describe the measured current–voltage characteristics of a series of bipolar devices where both the device thickness and contacts were varied. The model successfully describes the I–V characteristics of single carrier and bipolar devices as a function of polymer thickness and for structures that are contact limited, space charge limited, and for cases in between. We find qualitative agreement between the device model and measured external luminance for a thickness series of devices. We investigate the sensitivity of the device model calculations to the magnitude of the bimolecular recombination rate prefactor. © 1999 American Institute of Physics.

Notes: The purpose of this study was to compare the evoked potential (EP) to the “imperative stimulus” (S2) in the contingent negative variation (CNV) paradigm with the EP to a comparable stimulus nut associated with a CNV. Twelve subjects performed a task that required reporting an occasional change in frequency of a tone. In one condition each change was paired with a warning Hash, thus generating a CNV. In another condition tone changes were only occasionally preceded by a flash and no CNV was observed. Three differences were noted in the vertex EP associated with a CNV: 1) all components after approximately 60 msec were shifted in a more positive direction, 2) negative components Nl and N2 were shorter in latency, and 3) negative components N1, N2, and N3 were attenuated in amplitude. Possible interpretations of these results are discussed. Conclusions are drawn concerning the methodological and theoretical importance of this interaction between the CNV and EP.

Notes: We show that piezoelectric effects can give rise to internal electric fields that modify the current–voltage characteristics of double barrier resonant tunneling devices, if suitable stresses are applied. Electric polarization fields in the direction perpendicular to the interfaces arise in heterostructures on (001)-oriented substrates under uniaxial stress parallel to the (110) or (11¯0) orientations. We measured current–voltage characteristics of (001)-oriented AlAs/GaAs/AlAs double barrier structures as a function of uniaxial external stress applied parallel to the (110) and the (11¯0) orientations. The substrate contact was grounded in all measurements. Under (110) stress, the resonance peaks shift to more positive voltages for both positive and negative bias. For (11¯0) stress, the peaks shift toward more negative voltages. We also calculated the current–voltage characteristics of resonant tunneling structures under uniaxial stress, taking into account stress effects on the band alignment and piezoelectric effects. We obtain satisfactory agreement between our calculations which contain no adjusted parameters and the measured data. © 1995 American Institute of Physics.

Notes: We report polarized optical transmission and reflection measurements which are used to determine the orientation of polymer chains in spin-cast thin films of soluble, electroluminescent polymers. We find that the polymer chains lie primarily in the plane of the film. This result has three important implications for polymer light emitting diodes: the electroluminescence is preferentially emitted propagating perpendicular to the polymer film; the relevant dielectric and electrical transport properties are those perpendicular to the polymer chains; and large area, uniform devices can be produced by spin casting. © 1995 American Institute of Physics.

Notes: Measured current density was compared as a function of bias voltage for (111)B and (100) oriented resonant tunneling structures consisting of GaAlAs barriers and a GaInAs quantum well. The GaInAs quantum well is in biaxial compression. In the (111)B, but not the (100), oriented devices an electric field is generated inside the quantum well by the piezoelectric effect. It is observed that this strain generated electric field leads to a characteristic asymmetry in the current density of the (111)B oriented devices for positive and negative voltage bias. Calculations are presented that show how the piezoelectric generated electric field leads to the observed asymmetry in the current density of the (111)B oriented device and which are in qualitative agreement with the experimental observations. © 1995 American Institute of Physics.

Notes: We report electrical impedance measurements of polymer light-emitting diodes employing the soluble, conjugated polymer poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as the light-emitting layer. The diode structures were metal-polymer-metal structures utilizing thin gold as the transparent, positive contact, and calcium as the negative contact. The devices were fabricated using undoped, polymer active layers ∼40 nm thick. The polymer light-emitting diodes are accurately modeled as a resistor and capacitor in parallel for frequencies from 100 Hz to 1 MHz and for bias conditions from reverse bias to forward current densities of 0.1 A/cm2. The diode capacitance as a function of bias voltage is qualitatively different from conventional Schottky or p-n junction diodes; in reverse bias, the polymer layer is fully depleted and the capacitance is independent of bias; at small forward bias, traps are charged near the metallic contacts and the capacitance increases; under large forward bias, with significant electron and hole injection, the traps are neutralized and the capacitance decreases. From the magnitude of the initial increase in capacitance with forward bias the trap density is estimated to be only a few times 1016 cm−3. © 1995 American Institute of Physics.