Film thickness using polythiophene

Polymerization at constant current is most convenient for controlling the thickness of the deposited film. Charges of ca. 0.3, 0.2, and 0.08 C cm-2 are required to produce 1 fim of polypyrrole,59 poly(3-methylthio-phene)60 (no data are available for polythiophene), and polyaniline 43 respectively. Although these values can reasonably be used to estimate the thicknesses of most electrochemically formed conducting polymer films, it should be noted that they have considerable (ca. 30%) uncertainties. For each polymer, the relationship between charge and film thickness can... 

Reductive Electropolymerization. Besides the oxidative anodic electropolymerization of the monomer, which is the most convenient and the most widely used method, polythiophene can also be prepared by a cathodic route involving the electroreduction of the complex Ni(2-bromo-5-thienyl)(PPh3)4Br in acetonitrile. This method, initially proposed for the synthesis of poly(p-phenylene) [374-376], has been extended to polythiophene [520]. The major drawback is that the polymer is produced in its neutral insulating form, which leads rapidly to a passivation of the electrode and limits the attainable film thickness to approximately 100 nm. On the other hand, this technique presents the advantage of being applicable to electrode materials subject to anodic corrosion such as small-bandgap semiconductors [521]. 

The materials (metals and conjugated polymers) that are used in LED applications were introduced in the previous chapter. The polymer is a semiconductor with a band gap of 2-3 eV. The most commonly used polymers in LEDs today are derivatives of poly(p-phenylene-vinylene) (PPV), poly(p-phenylene) (PPP), and polythiophene (PT). These polymers are soluble and therefore relatively easy to process. The most common LED device layout is a three layer component consisting of a metallic contact, typically indium tin oxide (ITO), on a glass substrate, a polymer film r- 1000 A thick), and an evaporated metal contact4. Electric contact to an external voltage supply is made with the two metallic layers on either side of the polymer. 

As an alternative to electrochemical oxidation, electrochemical reduction has emerged as a route to the synthesis of conducting polymers such as polythiophene [251] and polyparaphenylene [310]. Reductions can be used on substrates where oxidation cannot occur. Unfortunately, the thickness of the films produced by reduction has been limited to about 0.1 /u,m. However, electrochemical reduction is the only known electrochemical route for the synthesis of polyacetylene [384-386]. 

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