P. S. Davids

Device model for single carrier organic diodes

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 i...

Device model investigation of bilayer organic light emitting diodes

Organic materials that have desirable luminescence properties, such as a favorable emission spectrum and high luminescence efficiency, are not necessarily suitable for single layer organic light-emitting diodes (LEDs) because the material may have unequal carrier mobilities or contact limited injection properties. As a result, single layer LEDs made from such organic materials are inefficient. In this article, we present device model calculations of single layer and bilayer organic LED characteristics that demonstrate the improvements in device performance that can occur in bilayer devices. We first consider an organic material where the mobilities of the electrons and holes are significantly different. The role of the bilayer structure in this case is to move the recombination away from the electrode that injects the low mobility carrier. We then consider an organic material with equal electron and hole mobilities but where it is not possible to make a good contact for one carrier type, say electrons. Th...

The Schottky energy barrier dependence of charge injection in organic light-emitting diodes

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.

Device model investigation of single layer organic light emitting diodes

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.

Charge injection and transport in single-layer organic light-emitting diodes

We present experimental and device model results for the current–voltage characteristics of a series of organic diodes. We consider three general types of structures: electron only, hole only, and bipolar devices. Electron and hole mobility parameters are extracted from the corresponding single carrier structures and then used to describe the bipolar devices. The device model successfully describes the experimental results for: electron only devices as thickness is varied, hole only devices as the contact metals are varied, and bipolar devices are both the thickness and the contact metals are varied.

Probing electronic state charging in organic electronic devices using electroabsorption spectroscopy

In metal/organic-film/metal device structures with different metal contacts there is a built-in electrostatic potential at equilibrium due to the asymmetric contacts. At thermal equilibrium the electrochemical potential is constant across the device structure. The electrochemical potential can be divided into the sum of two parts, the electrostatic potential and the chemical potential. By measuring the built-in electrostatic potential change across a structure at equilibrium, one can determine the change in chemical potential across the structure. Measuring this built-in electrostatic potential for devices with different contact metals provides a way of changing the chemical potential (μ) in the organic material and identifying the values of μ at which charged excitations are populated. Such measurements can be used to gain information on the energy spectrum of intrinsic charged excitations, charged trap states and charged interface states. We apply an electroabsorption technique to measure the built-in potentials of metal/organic-film/metal structures fabricated from poly[2-methoxy,5-(2′-ethyl-hexyloxy)-1,4 phenylene vinylene] (MEH-PPV), C60-doped MEH-PPV, and 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) and discuss what is learned about the charged excitations in these materials from the results.

Correlation effects and electronic properties of fullerenes and carbon nanotubes

Abstract We calculated analytically the quasi-particle energy spectra for various fullerenes and graphitic microtubules within a mean-field theory. Correlation effects due to the Coulomb interaction between electrons are investigated by using the self-consistent quasi-particle energy spectra for microtubules. We also found a striking paramagnetic to diamagnetic ordering transition of a two-dimensional electron gas on a microtubule in an external magnetic field.

Magnetic response of certain curved graphitic geometries

Abstract The quasi-particle energy spectra associated with some members of buckyfamily (curved graphitic geometries), in particular C 50 , C 60 , C 70 and related fullerenes as well as coaxial helical microtubules of graphite, are obtained analytically within the mean-field approximation. These energy spectra are then used to calculate various response functions. Specifically, we calculate the specific heat, magnetization and magnetic susceptibility in the presence of an external magnetic field at low temperatures. For a single microtubule an extra peak superimposed on the first de Haas van Alphen (dHvA) oscillation in magnetic susceptibility is found in the 50 – 170 Tesla range depending on the radius which is possibly accessible in special (explosive flux compression) experiments. Finally, we point to important potential applications of these novel mesoscopic structures in nanotechnology.