Device model for single carrier organic diodes

Abstract

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.

Device physics of single layer organic light-emitting diodes

“…We describe the measured I -V characteristics using a device model, described in detail elsewhere, that considers charge injection from the metal into the organic by thermionic emission and a backflowing interface recombination which is the time reversed process of thermionic emission. 10 We find that for most cases of interest for organic LEDs these two currents are separately much larger than the net device current and that they establish quasithermal equilibrium at the injecting contacts. Charge transport is described by continuity equations, with electric field dependent carrier mobilities and a drift-diffusion form for the current coupled to Poisson's equation.…”

“…There has been much work done recently to understand the basic principles of organic LED operation. [4][5][6][7][8][9][10][11] Our approach to studying the device physics of organic LEDs is to begin with simple devices in which we can separate, to as large a degree as possible, the fundamental processes of charge injection, transport, and recombination. The understanding gained from the simple devices can then be applied to more complex structures.…”

Bis(arylene-ethynylene)-s-tetrazines: A Promising Family of n-Type Organic Semiconductors?

“…38 A good ohmic contact between semiconductor materials and electrodes is generally expected only for potential barriers lower than 0.2 − 0.3 eV, 75,76 while for larger barriers interfacial effects such as metal reactivity, polarization processes and inter-diffusion within the metal-organic interface and temperature cannot be neglected. 38,[77][78][79] For this reason, the sole analysis of Φ and E LUMO values cannot provide a quantitative evaluation of the injection barrier but it nevertheless serves as a guide to predict the alignment of levels at the interface and the electron injection barrier, as well as to interpret trends within a set of related compounds.…”

Space-charge limited surface currents between two semi-infinite planar electrodes embedded in a uniform dielectric medium

“…Space-charge limited currents have always played a pertinent role in electronic devices, starting with the vacuum tube [1,2], subsequently in solid-state electronic devices [3] and more recently in organic electronic devices [4,5]. In a vacuum tube the space-charge limited electron current is found based on energy conservation and Poisson's equation and leads to the Child-Langmuir equation…”