Materials for Organic Light-Emitting Diodes

Background and Overview

Electroluminescence (EL) is the generation of light by electrical excitation. In the simplest single-layer assembly, an organic or polymer LED contains an electroluminescent material (emitting layer) sandwiched between two electrodes (see "a" in figure below). When a sufficient bias is applied to an LED device, electrons and holes are injected respectively from the positive and the negative electrodes into the electroluminescent material. Electrons and holes recombine within the electroluminescent material, forming a neutral excited species (termed an exciton). Excitons decay to the ground state liberating energy. A fraction of the liberated energy is in the form of light. The color of the light emitted depends on the difference in energy between the excited and the ground states. In a first approximation, optimal device efficiency is achieved if the two electrodes possess Fermi levels (or electronic work functions, phi) that closely match respectively the valence (HOMO) and the conduction (LUMO) energy levels of the emitting material. In other words, the Fermi energy of the anode should match the valence band (HOMO) of the emitting material and the Fermi energy of the cathode should match the conduction band (LUMO) of the emitting material.

Practical considerations control and limit the choice of useful anode and cathode materials. Both electrodes must be highly conducting to provide ample currents during device operation, and at least one electrode must be transparent enough to out-couple the emitted light. The transparent conductor of choice is indium-tin oxide (ITO). ITO has a work function of ca. 4.7 eV, a value that is reasonably close to the HOMO energy level of most EL materials. Thus, ITO must be used as the anode (refer to the energy levels in "a" above). Consequently, low work function materials, such as Mg, Al, Ca, or Li must be used for the cathode. However, single layer devices tend to produce unbalanced charge injection (more electrons than holes) and this causes low efficiency. To prevent this, charge transport layers for both electrons and holes (ETL and HTL respectively) can be used. These additional layers have several effects: 1) If a barrier exists between the electrode work function and the HOMO (or LUMO) of the emitting layer, the transport layers provide intermediate energy states that allow holes (or electrons) to cascade through smaller gaps (Figure above "b" and "c") reducing the voltage required to drive the device. 2) A proper choice of orbital offsets can block the transmission of the majority carrier (electrons) balancing the charge transport. 3) The separation of the emitting layer from the ITO anode reduces the number of non-radiative decay pathways of excitons that would otherwise occur at the ITO surface, increasing the device efficiency. 4) Electron and hole transport layers offer the additional advantage of protecting the emitting layer from contaminants migrating from the electrodes (especially oxygenated species from the ITO).

Materials in Development and Testing at TDA

TDA has many interests in the field of OLEDs for displays and general illumination. We are currently developing materials that can act as HTLs, EL materials, and ETLs. Because each type of material needs a distinct electronic structure, well matched to the other layers in the device, we design and synthesize materials with structural motifs that target specific electronic properties. In other words, electron rich or electron deficient structures are designed to result in p-type or n-type materials respectively. We test these materials in house by fabricating simple prototype devices, which allow us to establish proof-of-concept. The figure above right shows the basic structure of the layers in an OLED device built at TDA using our new light emitting materials. These devices are fabricated by TDA empolyees using a clean room facility of the University of Colorado at Boulder. We are also preparing to have our materials evaluated in state-of-the-art devices and displays by our partners who are better equiped to prepare such prototypes.

The figure above right shows the top and the bottom of OLED devices constructed with TDA's materials. We are just beginning to characterize our prototype devices as to their efficiency and current-voltage characteristics. As was stated above, our goal thus far has not been to produce the best devices, but to show proof that our materials can work in these devices. The light ouput of a new EL material designed and synthesized at TDA is shown in the figure at right. While this device is by no means operating under optimal conditions or setting any records for efficiency, it is considered a great success because our researchers were able to go from molecular design and synthesis to device fabrication in less than six months. Other n-type materials are being evaluated for use as n-doped conductors and ETL materials.

| About | ICPs | OLEDs | N-type materials | Composites | Contact |