Transparent anode

Fig. 5.58 Scheme of a photogalvanic cell. The homogeneous photoredox process takes place in the vicinity of the optically transparent anode (a) or cathode (b)... 

Y Cao, G Yu, C Zhang, R Menon, and AJ Heeger, Polymer light-emitting diodes with polyethylene dioxythiophene polystyrene sulfonate as the transparent anode, Synth. Met., 87 171-174, 1997. 

The anode material is, most typically, transparent ITO coated onto a glass or plastic substrate. Chapter 5 describes the details of such transparent anode materials. The general requirements for an anode material are as follows ... 

The following sections will first describe the major components of a typical bottom-emitting OLED the transparent anode, the organic layers, and the metal cathode. Alternative device architectures are also briefly described. 

Figure 3.26. Structure of an OLED. S = substrate (glass), ANO = anode (e.g., ITO — indium tin oxide), HIL = hole injection layer (e.g., Cu phthalocyanine), HTL = hole transport layer, EML = emission layer, ETL = electron transport layer, EIL = electron injection layer (e.g., LiF), KAT = cathode (e.g., Ag Mg, Al). The light that is generated by the recombination of holes and electrons is coupled out via the transparent anode.

The zone of recombination can be very small as was shown by Aminaka et al. [225] by doping only a thin layer (5 nm) in the device by a red emission material. By observing the ratio of host and dopant emission, the authors were able to show that the recombination zone of the device was as thin as 10 nm. The emitted light is usually coupled out at the substrate side through the transparent anode. As a rule, the electroluminescence spectrum does not differ much from the photoluminescence spectrum. 

V. P. Verma, S. Das, I. Lahiri, W. Choi, Large-area graphene on polymer film for flexible and transparent anode in field emission device, Appl. Phys. Lett., vol. 96, p. 203108, 2010. 

Graphene films are promising candidates as electrodes in OLED devices as transparent anodes and cathodes. However, optimization of graphene film characteristics (thickness and quality) is necessary in order to obtain device results comparable to ITO based devices [254, 260]. 

Figure 4.3 Schematic representation of a simple monolayer organic light-emitting diode (OLED) incorporating an electroluminescent material between a transparent anode and a cathode.

This polymer is also water soluble, and hence, similar to PANI, can be used as a transparent anode. 

As mentioned above, this material can be used as a transparent anode. However, it is now also commonly deposited on ITO as an HTL in PLEDs. Indeed, it has recently become the HTL of choice in most efforts to develop PLEDs for commercial products. 

In the basic operating mode of an OLED, holes are injected from the (transparent) anode and electrons from the metal cathode (see Figure 1.6). There is typically a roughly triangular barrier for both h+ penetration into the HTL from the anode... 

OLEDs are normally fabricated on a transparent substrate and therefore on top of a transparent anode. However, several potential applications, such as micro-displays integrated on a crystalline silicon chip or a totally transparent OLED array for a heads-up display, require a transparent top electrode. There has been some work published describing the development of transparent cathodes. The most obvious approach is to use a very thin metal layer, such as Mg Ag, overcoated with a transparent conductor, such a.s ITO [94]. This is not so trivial as it appears, since the cathode metal must survive the reactive sputtering process employed to deposit the ITO. Another approach uses no metal but rather a CuPc layer between the electron-transporting Alqs and the ITO [95]. It is suggested dial the oxidative environment during ITO deposition results in heavy n-type doping near the CuPc interface. 

Fig. 1 Basic set-up of a layered OLED structure. Electrons and holes are injected from the respective electrodes (metal cathode, semiconducting and transparent anode). The charge carriers move from different sides into the recombination/emitter layer, where electrons and holes recombine and excite the doped emitter molecules (asterisks, e.g., or-ganometallic triplet emitters). For more details see Fig. 2. For clarity, light emission is only shown for one direction although the photons are emitted in all directions...

The fabrication of OLEDs involves the deposition of one or more layers of organic semiconductors onto the transparent anode, indium tin oxide (ITO) in most cases, often coated with a hole-injection layer. Generally, two classes of devices are distinguished, according to the materials involved and - as a result of this - the fabrication process ... 

---