Photochemical doping

With photoexcitation, it is possible to introduce charge to the PA chain without using chemical dopants. Through optical spectroscopy, the new subgap transitions can be detected and are vir-tualty the same as those found in chemically doped samples [152]. The concentration of long-lived excited states is low, however, and this is not a practical method for maintaining a PA film with stable conductivity. 

While other conducting polymers such as polypyrrole and polyaniline have found applications in industry, PA has not been employed widely in commercial applications due to its environmental instability. The difficulty in processing and the ease of oxidation, together with the high water/air sensitivity of doped PA, have re- 

A schematic drawing of a prototype solar cell containing doped 

---

Photochemical doping. This is accomplished by treating the polymer with a dopant species that is initially inert towards the materials, followed by irradiation. For example, diphenyliodonium hexafluoride arsenate in methylene chloride or triarylsulfonium salts, followed by ultraviolet irradiation. 

Note 1 (ohm-cm) = 1 S cm Chemical doping. Electrochemical doping. Photochemical doping. 

Among the most common alternative doping techniques used have been Ion Implantation, Photochemical Doping, Heat Treatment, Solution Doping, and "Dry Doping". These are briefly discussed in turn. 

Photochemical doping involves treating a CP with a dopant which is initially inert but rendered an active dopant by irradiation. These dopants are usable with common CPs. Examples are diphenyliodonium hexafluoroarsenate (in CH2CI2) or triaryl-sulfonium salts (in aqueous medium), both of which are rendered active by UV radiation [125, 126]. 

Dry" doping is similar to the photochemical doping described above, with heat used to generate an active dopant from an inert species. Angelopoulos et al. [129] have described such dry-doping of P(ANi) (used in photoresists) with amine triflate salts, which are then thermally decomposed to yield the active dopant (triflate). This eliminates the need for solution, electrochemical, radiation, or other methods to effect the doping. 

An interesting series of experiments in which diaryliodonium and triarylsulfonium salts have been used to carry out the photochemical doping of polyacetylene has been reported by workers at IBM Polymers with controlled conductivities in patterned areas can be obtained by irradiating through a mask. 

---