Multilayered cathodes

O. Renault, O.V. Salata, M. Etchells, P.J. Dobson, and V. Christou, A low reflectivity multilayer cathode for organic light-emitting diodes, Thin Solid Films, 379 195-198, 2000. 

EA as a Probe of Internal Fields in Diodes with PEDOTiPSS Anodes and Multilayer Cathodes... 

Here we will discuss the results relative to two different sets of cathodic systems (1) LEDs with LiF/Al cathodes for different LiF thicknesses (2) LEDs with multilayer cathodes (including CsF/Al, LiF/Al, CsF/Ca/Al and LiF/Ca/Al). Further details on these investigations can be found in [109,120]. 

In the Al cathode case Vbi is (1-2 + 0.1) V. Inserting LiF, Ca, or CsF interlayers increases the Vbi sizably. The largest value for Vbi which we were able to determine, however, belongs to the LEDs fabricated with LiF/Ca/Al cathodes [Vbi = (2.71 + 0.05) Vj. The EA response of the CsF/Ca/Al, which deviates from the linear dependence and crosses the zero axis at biases lower than the other multilayer cathode devices, is reported in [109]. 

FIGURE 19.17 Schematic diagram (e = 1) of the average relative energy positions of the Fermi levels at flat band for the series of diodes with different multilayer cathode materials, drawn according to the built-in potential results reported in [109] and Table 19.1. 

FIGURE 19.18 Vpi and the operating voltage (Vqp) at which the luminance is 100 cd/m for PLEDs fabricated with different multilayer cathodes. (From Brown, T.M., et al. /. Appl. Phys., 93,6159-6172,2003. With permission.)... 

To clarify the capability of a NOx adsorption layer on the multilayer cathode, KHatminoto et.al. (Hamamoto et al., 2008) carried out the NOx decomposition measurements of the YSZ based cells with and without a NOx adsorbent at a fixed operating voltage, U=2.5 V, on the cells under various 02 concentrations at 500 C by passing a mixed gas with 1000 ppm of NO in He through the cell at a gas flow rate of 200 ml/ min (Fig. 19). As a result, NOx adsorption layers have improved the NOX decomposition properties though the current values of each cell were almost the same. 

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. 

Song HS, Hyun SH, Moon J, and Song RH. Electrochemical and microstructural characterization of polymeric resin-derived multilayered composite cathode for SOFC. J. Power Sources 2005 145 272-277. 

An optical microcavity produced by the latter process has been applied to tune the emission from erbium-doped PS [Zh6], Erbium compounds like Er203 are known to exhibit a narrow emission band at 1.54 pm, which is useful for optical telecommunications. Several methods have been used to incorporate erbium in PS. A simple and economical way is cathodic electrochemical doping. External quantum efficiencies of up to 0.01% have been shown from erbium-doped PS films under electrical excitation [Lo2]. The emission band, however, is much broader than observed for Er203. This drawback can be circumvented by the use of an optical cavity formed by PS multilayers. In this case the band is narrowed and the intensity is increased because emission is only allowed into optical cavity modes [Lo3]. 

Wang et al. [140] presented another example of multilayered MPLs that consisted of a basic MPL (Vulcan XC-72 and PTFE) on top of a DL and a mixfure of carbon black (Black Pearl 2000) and Nafion sprayed onfo fhe peripheral region of the MPL (i.e., around the active area). Tliis was done on both the anode and cathode sides. The extra MPL, also called a water transfer region (WTR), was used in order for fhe MEA to be self-humidified. The WTR would receive the excess water at the cathode and then the water... 

Perhaps more important than cost is the solution to the crucial problem of interfacial contacts that always plagues homogeneous GPE films prepared from traditional approaches. Since both cathode and anode composite materials are coated on their substrates with the same PVdF—HEP copolymer as the binder, the in situ gellification following the electrolyte activation effectively fuses the three cell components into an integrated multilayer wafer without physical boundaries, so that the interfaces between anode and electrolyte or cathode and electrolyte are well extended into the porous structures of these electrodes, with close similarity to the interfaces that a liquid electrolyte would access. 

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