Anodes, OLEDS

The materials described in the preceding section may be combined in an OLED de vice in a variety of different geometries and compositions. The simplest of these is a single oiganic layer sandwiched between two electrodes. In contrast to the convention used in surface science, it is customary to list the layers in the order of deposition. Thus, anode/organic/cathodc (for example, ITO/PPV/A1) implies that the anode (ITO) is deposited first on the (presumably transparent) substrate. 

Studies of double carrier injection and transport in insulators and semiconductors (the so called bipolar current problem) date all the way back to the 1950s. A solution that relates to the operation of OLEDs was provided recently by Scott et al. [142], who extended the work of Parmenter and Ruppel [143] to include Lange-vin recombination. In order to obtain an analytic solution, diffusion was ignored and the electron and hole mobilities were taken to be electric field-independent. The current-voltage relation was derived and expressed in terms of two independent boundary conditions, the relative electron contributions to the current at the anode, jJfVj, and at the cathode, JKplJ. 

Another issue that can be clarified with the aid of numerical simulations is that of the recombination profile. Mailiaras and Scott [145] have found that recombination takes place closer to the contact that injects the less mobile carrier, regardless of the injection characteristics. In Figure 13-12, the calculated recombination profiles arc shown for an OLED with an ohmic anode and an injection-limited cathode. When the two carriers have equal mobilities, despite the fact that the hole density is substantially larger than the electron density, electrons make it all the way to the anode and the recombination profile is uniform throughout the sample. 

Figure 13-13. (a) Currem-vollage data from MEH-PPV-bascd OLEDs willi Au anodes and various cathodes plotted according to E4. (13.5) (b) the external quantum efficiency for the diodes with Al and Ca cathodes The solid line represents the maximum efficiency of 2%. Reproduced with permission from 11511. Copy light 1998 by the American Physical Society. 

The materials used as the electron and hole injecting electrodes play a crucial role in the overall performance of the device and therefore cannot be neglected even in a brief review of the materials used in OLEDs. The primary requirements for the function of the electrodes is that the work function of the cathode be sufficiently low and that of the anode sufficiently high, to enable good injection of electrons and holes, respectively. In addition, at least one electrode must be sufficiently transparent to permit the exit of light from the organic layer. 

The characteristics of ITO described in the previous paragraph make it a useful material for use as the anode in an OLED. At the same time, they arc the cause of many difficulties which have been observed in the reproducibility and stability of OLED devices. We shall return to this topic in more detail later, but suffice it... 

Figure 13-11. (a) A diagram showing ihc spatial distribution of lire relative hole and electron currents in an OLED. The recombination efficiency h is equal to the fraction of the electron (hole) current that docs not make it to the anode (cathode) (b) cll icicncy-currcni balance diagram for OLEDs. Sec text for details. 

A typical multilayer thin film OLED is made up of several active layers sandwiched between a cathode (often Mg/Ag) and an indium-doped tin oxide (ITO) glass anode. The cathode is covered by the electron transport layer which may be A1Q3. An emitting layer, doped with a fluorescent dye (which can be A1Q3 itself or some other coordination compound), is added, followed by the hole transport layer which is typically a-napthylphenylbiphenyl amine. An additional layer, copper phthalocyanine is often inserted between the hole transport layer and the ITO electrode to facilitate hole injection. 

The simplest manifestation of an OLED is a sandwich structure consisting of an emission layer (EML) between an anode and a cathode. More typical is an increased complexity OLED structure consisting of an anode, an anode buffer or hole injection layer (HIL), a hole transport layer (HTL), a light-emitting layer, an electron transport layer (ETL), a cathode... 

Other materials such as gold (< = 4.9 eV), aluminum (< = 4.2 eV), indium-doped zinc oxide, magnesium indium oxide, nickel tungsten oxide, or other transparent conductive oxide materials, have been studied as anodes in OLEDs. Furthermore, the WF of ITO can be varied by surface treatments such as application of a very thin layer of Au, Pt, Pd, or C, acid or base treatments, self-assembly of active surface molecules, or plasma treatment. 

Unlike the constraints on anode material, the constraints on cathode materials are usually lower because typically they do not need to constitute the transparent electrode material. In certain instances, where a completely transparent OLED is needed (windshield and heads-up displays), ITO may also be used as the cathode with suitable modification [12]. In general, cathode materials are pure metals or metal alloys. The requirements for cathode materials are as follows ... 

Closely related to the anode modifications described above, the use of a HIL material to improve charge injection into the OLED device has spawned a number of materials, which have been shown to provide benefits, particularly in terms of lower operating voltages and extended lifetimes of devices. 

Gradient Refractive Index Anode for High Contrast OLEDs.517... 

ANODE MODIFICATION FOR ENHANCING OLED PERFORMANCE 6.2.1 Indium Tin Oxide Surface Treatment and Modification... 

Operating Voltage and Corresponding Luminous Efficiency of Identical OLEDs Made on ITO Anode with Different Alq3 Modification Layer Thicknesses, Measured at a Current Density of 100 mA/cm2... 

The primary effect of the anode modification on the enhancement in luminous efficiency and the increased stability of OLEDs can be attributed to an improved hole-electron current balance. By choosing an interlayer with a suitable thickness of a few nanometers, anode modification enables engineering of the interface electronic properties. The above results indicate that conventional dual-layer OLEDs of ITO/NPB/Alq3/cathode have an inherent weakness of instability that can be improved by the insertion of an ultrathin interlayer between ITO and HTL. The improvements are attributed to an improved ITO-HTL interfacial quality and a more balanced hole electron current that enhances the OLED performance. 

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