Cathode metal/polymer interfaces

As will be shown later, during cathodic delamination of a polymer from a metal surface due to ingress of an electrolyte into the metal/polymer interface, an additional liquid phase could be formed between the substrate and the organic layer. In this case, the metal/electrolyte interface can be treated as a conventional electrochemical interface, but an additional Galvani potential difference ADonnan potential or membrane potential [24—26]) has to be taken into account at the electrolyte/polymer interface. The latter is directly correlated with the incorporation of ions into the polymer membrane according to Eq. (14). 

Cathodic Delamination on Polymer-coated Iron Certain areas at the metal-polymer interface may become sufficiently cathodic to promote a cathodic reaction underneath the coating. This cathodic polarization might be the result of a purposely induced polarization, for example, cathodic protection of pipelines... 

Fig. 24 Principal corrosion model explaining the formation of a galvanic element in case of cathodic delamination on polymer-coated iron, (a) Cross section through a metal-polymer interface with a defect in the polymer coating (b) overview of the polarization curves at the defect (i), the intact interface fii) and the situation after galvanic coupling of the parts (c).

Oxygen reduction takes place in the defect with a rate that is controlled by the transport of oxygen through the electrolyte layer (a, i). Thus, a galvanic current is established between the anodic site (zinc within the delaminated zone) and the defect (cathode). In the area between the two potential steps (b, ii), no equilibrium potential surface is observed but the potential rises continuously from the borderline of the local anode to the potential jump, which indicates the intact metal-polymer interface. It can be assumed that the closer the zinc to the cathodic delamination front the smaller is the local anodic current while the... 

Two principle mechanisms that are discussed as possible corrosion protection mechanisms on mild steel are discussed in short. ICPs may induce the formation of a passive oxide [206]. The ICP will be reduced as a consequence of passivation and will be reoxidized by oxygen reduction. Consequently, the ICP may promote the cathodic oxygen reduction on the polymer surface rather than at the metal-polymer interface. On the basis of the good corrosion results gained by the combination of a molecular adhesion promoter and the subsequent electrodeposition of the polymethylthiophene film Rammelt and coworkers [207] concluded that the essential aspect of the corrosion protection by ICPs could be the local separation of iron oxidation and oxygen reduction. This would eliminate the local pH increase at the metal surface and subsequent cathodic disbondment. 

A number of explanations have been put forward for delamination mechanism whereby the alkaline environment under the film affects the integrity of the metal-polymer interface, or perhaps more properly the interface between the oxide and the polymer. Koehler [94] showed that this form of failure only occurs when there are alkali metal cations available in the environment to act as counter ions to the cathodically generated OH ions. 

XPS was used for determination of metal-polymer interaction and chemical state of atoms on metal-polymer interface after diffusion of Ag and Cu atoms in polyethyleneterephtalate (PET) and polyamide (PI) by Mackova et al. (2005). XPS measurement gives an evidence of Ag clustering in Ag-PET samples prepared by cathode sputtering. In PI the Cu atoms exhibit higher diffusivity than Ag atoms due to their lower atomic radius. 

CV is usually one of the first methods to be applied to new polymer films. As in the usual solution-based CV, a triangular-shaped potential is applied to the cell, but in this case the working electrode is coated with the polymer to be studied. The cell is filled with electrolyte solution that does not contain any electroactive solute. When current is flowing there is electron transfer across the metal-polymer interface and simultaneously ion transfer across the polymer-solution interface. The only diffusion-controlled process occurs inside the polymer film, where ions have limited mobility. If the polymer film is very thin, the diffusion time of ions is very short and we expect that the reverse electron transfer occurs exactly at the same potential on the return sweep of CV i.e., we should have a voltammogram with symmetrical and mirror-image cathodic and anodic waves. The current in the reversible case is... 

Up to now, only the stability of model metal/polymer interfaces has been discussed by the scanning Kelvinprobe. However, additional investigations have also been performed with industrial samples on the basis of steel and galvanized steel. Then the metal is pre-treated by phosphates and chromates, and polymer-coated either by cathodically electro-deposited paints or by coil-coating lacquers. The defect is either prepared in a similar manner as before (defect size typically 1 cm ), or the coating is locally destroyed by scratching, as in conventional industrial tests. Some aspects of these investigations are summarized below. 

Figure 1-3. In Ihis improved bilaycr device structure lor a polymer LED an extra ECHB layer has been inserted between the PPV and the cathode metal. The EC11B material enhances the How of electrons but resists oxidation. Electrons and holes then accumulate near the PPV/EC1113 layer interface. Charge recombination and photon generation occurs in the PPV layer and away from the cathode.

The direct evidence for this morphological dependence of A

hole-only devices.25,37 For example, it is observed that a hole-only device consisting of Au(anode POM contact)/polymer/Au(cathode MOP contact) has different I-V curves under forward and reversed biases.37 This phenomenon is also observable with other high-workfunction metals such as Cu and Ag.25 In these devices, if the anode/polymer and the cathode/polymer interfaces have the same

significantly different since the two metals have different workfunctions (4.5 eV for Cu and 4.3 eV for Al). However, it was found by Roman et al. that the I-V curves were almost identical under forward and reverse biases.36... 

Obviously, a large potential difference between the active metal surface in the defect (ca. —O.SVshe) and the intact zinc-polymer interface is observed. Between these areas, a steep potential increase marks the location of the delamination front. The potential maps indicate that, as for polymer-coated iron, a cathodic reaction leads to the delamination of the coating. Fiirbeth and Stratmann proved the cathodic mechanism by small spot XPS analysis of the delaminated surface [83]. While no chloride was detected in the delaminated region, the amount of sodium decreased from a high value near the defect to a small value at the front of the delamination. The distribution of sodium ions was in total agreement with the potential maps. 

The two main mechanisms [38,179,190,196,198,203] expected to be behind the reduction of <]> (and b) in the presence of the fluoride interlayer are (a) interaction of the fluoride with the metal and organic layers and its dissociation. The liberated Li would not only dope the polymer but, importantly, either create a low work function contact (Li has a low work function [204-206]) or, in the form of Li ions, build a doped region of space charge at the cathode/polymer interface [40,41] (b) a dipole-induced work function change due to either the large dipole moment of the oriented fluoride molecules [183,207,208] or the interfacial transfer of charge from the adsorbed fluoride layer [50] (in particular fi-om the alkali metal atoms [71]) to the A1 cathode. 

Inverted Device Structures The conventional device structure for PSCs is indium tin oxide (ITO)/PEDOT PSS/polymer blend/Al, where a conductive high-work-function PEDOTPSS layer is used for anode contact, and a low-work-function metal as the cathode. Both the PEDOTPSS layer and the low-work-function metal cathode can cause the degradation of PSCs [110-112]. The acidic PEDOTPSS was reported to etch the ITO and cause interface instability through indium diffusion into the polymer active layer. Low-work-fiinction metals, such as calcium and aluminum, are easily oxidized when exposed to air, increasing the series resistance at the metal/BHJ interface and degrading device performance. 

In failure analysis the possibility to record all elements is advantageous, not least in combination with 3D imaging (i.e., a TOF-SIMS instrument with dual-beam capability is the instrument of choice). An example is the investigation of black spots in OLEDs where a fluorine-based polymer was sandwiched between a metallic cathode consisting of Ba and A1 and a poly(3,4-ethylenedioxythiophene)/ITO anode. From the recorded raw data, depth profiles can be reconstructed as well as two-dimensional (2D) images in any depth or a 3D representation of all interesting signals. It was found that aluminum was oxidized at the Al/polymer interface [220]. 

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