Rectifying bilayer electrodes

A p-n-heterojunction type device can be fabricated on the basis of the junction formed between poly(pyrrole) and PT derivatives (e.g. PT or PMT) [784]. A PODT/Cfio junction device with a photo-induced charge transfer between PODT and Cgo is shown in Fig. 22 [449]. Rectifying bilayer electrodes with sequential bilayer structures are prepared from any pair of PBT/poly(pyrrole) and poly(3-bromothiophene)/poIy(pyrrole) by anodic electropolymerization on platinum electrodes [785]. 

Rectifying bilayer electrodes with sequential bilayer structures are prepared from any pair of 3-butylPT/polypyrrole and 3-bromoPT/polypyrrole by anodic electropolymerization on platinum electrodes [130]. 

In the net sense, the bilayers provide a means for directed or unidirectional electron transfer from an external film to the electrode and in that sense they impart a rectifying character to the electrode-film interface. 

Between the bilayer and solution experiments it is possible to begin to show how to transfer the rectifying properties observed on photolysis at a semiconductor-solution interface to any electrode material. In a more general context, it seems clear that the single layer and bilayer interfaces have intrinsic characteristics usually associated with solid state electronics devices. 

A bilayer-coated electrode has two electroactive films, each having different reduction potentials. The inner layer is in direct contact with the carrier electrode surface and acts as a mediator to the outer layer which is mainly in contact with the solution. Provided that the redox levels in the two layers are appropriate, the interface between the two polymer films acts analogously to a semiconductor junction as a charge rectifying junction. The method of preparing first the inner layer on the electrode and then, in a second step, the outer layer may in principle be deduced from the conventional methods already described. 

Polymer pendant MtPcs (M Fe, Co, Np, Cu +) were used as films to produce a photoresponse. MV present in the solution was not effective as an acceptor, but Fe EDTA worked as an acceptor to enhance the photocurrent about 10 times A polymer bilayer coated electrode consisting of an electron-mediator film in the inner layer 49) and a sensitizer film (50) in the outer layer produced a rectified photocurrent In the presence of a sacrificial reductant (triethanolamine) in the solution, the bilayer coated electrode gave exclusively anodic photocurrent independent of the applied potential between —0.2 and —0.3 V vs SCE. 

---