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.

Analysis of metal-oxide-based charge generation layers used in stacked organic light-emitting diodes

We present device model calculations for the current–voltage (I–V) characteristics of organic diodes and compare them with measurements of structures fabricated using MEH-PPV. The measured I–V characteristics have a Fowler–Nordheim (FN) functional form, but are more than three orders of magnitude smaller than the calculated FN tunneling current. We find that the low mobility of the organic materials leads to a large backflow of injected carriers into the injecting contact. These results account for the experimental observations and also demonstrate how transport layers in multilayer organic light-emitting diodes can be used to improve carrier injection.

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. The role of a bilayer structure in this case is to prevent the holes from traversing the device without recombining. In both cases, single layer device limitations can be overcome by employing a two organic layer structure. The results are discussed using the calculated spatial variation of the carrier densities, electric field, and recombination rate density in the structures.

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.

We present a device model to describe polymer light-emitting diodes (PLEDs) under bias conditions for which strong electrical injection does not occur (i.e., reverse, zero, and weak forward bias). The model is useful to interpret: capacitance–voltage measurements, which probe the charged trap density in the PLEDs; electroabsorption measurements on PLEDs, which probe the built-in electric field in the device; and internal photoemission measurements, which probe the effective Schottky barriers at the contacts of the PLED. The device model is based on the low-density nondegenerate continuum model for the electronic structure of polymers. Polarons and bipolarons are the principal charged excitations in this model. Polarons are singly charged excitations which play the primary role in charge injection and in experiments such as internal photoemission which probe single particle interface properties. Bipolarons are doubly charged excitations which can play an important role in establishing Schottky barriers at metal/polymer interfaces. In the device model, the region of the polymer near each contact is assumed to be in quasiequilibrium with that contact. The charge density as a function of position is found from the electrostatic potential and equilibrium statistics. Poisson’s equation is integrated to determine the electrostatic potential. We find that a large charge density is transferred into the polymer if the chemical potential of a contact is higher than the negative bipolaron formation energy per particle or lower than the positive bipolaron formation energy per particle. The transferred charge pins the Fermi level and establishes the effective Schottky barrier. If the contact chemical potential is between the formation energy per particle of the two types of charged bipolarons, there is little charge transfer into the polymer and the Fermi level is not pinned. The electric field in the device is found for different contacts and bias conditions. Capacitance as a function of voltage is calculated for various trap binding energies and densities. The calculated results are used to interpret recent measurements on PLEDs.

We describe a model for metal-polymer interfaces based on the nondegenerate continuum model of Brazovskii and Kirova for the electronic properties of polymers. The correct analytic equations for a bipolaron lattice in this model are stated and the electronic properties of the bulk polymer, i.e., the energy-level structure, the energy density, and the chemical potential as a function of electron density are obtained numerically. We find that the bipolaron lattice is unstable at high densities when the intrinsic gap parameter exceeds a critical fraction of the total energy gap. The electronic properties of the bulk polymer are used for modeling the metal-polymer interface. The charge density near a metal-polymer interface is found from the electrostatic potential and an analytic expression for the bipolaron chemical potential assuming that the contact is in equilibrium with the polymer layer. Poisson's equation is integrated to determine the electrostatic potential. We find that a large charge density is transferred into the polymer layer if the Fermi level of the metal contact is higher than the negative bipolaron formation energy per particle or lower than the positive bipolaron formation energy per particle. The transferred charge lies very close to the metal-polymer interface as a bipolaron lattice with charge density progressively decreasing away from the interface. The transferred charge gives rise to a region of rapid ''band bending,'' pins the Fermi level, and establishes the effective Schottky energy barrier. Upon increasing the metal Fermi level above the bipolaron formation energy per particle, the effective Schottky barrier saturates at the energy difference between the polaron formation energy and the bipolaron formation energy per particle. The model results are useful in interpreting recent measurements of internal photoemission, device electroabsorption, and capacitance-voltage characteristics in polymer light-emitting diodes.

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 vinylene͒ 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.