Photo-Conducting Polymers

Unless polymers contain long sequences of double bonds, they are fairly good insulators, particularly in the dark. Nevertheless, a number of common polymers show measurable increase in conductivity, when irradiated with light. When polymeric materials, like poly (vinyl fluoride), poly (vinyl acetate), poly (vinyl alcohol), or poly (N-vinyl carbazole), are exposed to light, they develop charged species. The species can migrate under an electric field and thus conduct electricity. When poly (N-vinyl carbazole) is doped with photosensitizers or compounds that form charge-transfer complexes, the photosensitivity can be increased and even extended into the visible region of the spectrum. Since discovery in 1957 that poly(N-vinyl carbazole) has photoconductive properties, there has been increasing interest in the synthesis and study of this and other polymeric materials with similar properties that allow various photonic applications. Related polymers are presently utilized in photocopiers, laser printers, and electro-photographic printing plates. Photoconductive polymers can be p-type (hole-transporting), n-type (electron-transporting), or bipolar (capable of transporting both holes and electrons). To date, most photoconductive charge transporting polymers used commercially are p-type. Poly(vinyl carbazole) and other vinyl derivatives of polynuclear aromatic polymers, such as poly (2-vinyl carbazole) or poly(vinyl pyrene), have high photoconductive efficiencies. These materials may take up a helical conformation with successive aromatic side chains arranged parallel to each other in a stack. In such an arrangement, the transfer of electrons is facilitated. Also, it is believed that the primary mechanism for poly(vinylcarbazole) charge carrier generation is due to excitation of the carbazole rings to the first excited singlet state. This polymer absorbs ultraviolet light in the 360-nm region and forms an exciton that ionizes in the electric field. The excited state by itself is not a conductive species. The addition of an equivalent amount of an electron acceptor, like 2,4,7 trinitrofluorenone, shifts the absorption of this polymer into the visible region by virtue of formation of charge transfer states. The material becomes conductive at 550 nm. This associated electron positive hole pair can migrate through the solid polymeric material. Upon dissociation of this pair into charged species, an electron and a positively charged hole, the electron becomes a conductive state. To achieve this, additional energy is required and can be a result of singlet–singlet interaction [239], singlet–triplet interaction [240], singlet–photon interaction [239], triplet–photon interaction [239], and two–photon interaction [240]. Kepler carried out fluorescence quenching studies and concluded that the migration of the exciton is the most probable energy transfer mechanism of poly(vinyl carbazole) [241]. He, furthermore, suggested that the exiton can visit 1,000 monomer units during its lifetime [241]. This is a distance of about 200 A ˚. Kang and coworkers [242] also explored steady state and pulsed photo-conductivities in 4–8 mm thick films of trans-polyphenylacetylene and also trans-polyphenylacetylene films doped with inorganic and organic electron acceptors, particularly iodine and 2,3-dichloro-5,6-dicyano-p modulated by shallow electron traps in the undoped polymer and by trapping the charge-transfer complex in the doped polymer [242]. Guellet [94] states that photo-conductivity s is equal to the current density J divided by the applied field strength e, where J is aperture/unit electrode area. This is related to the number of negative-charge carriers (usually electrons) per unit volume, and p is the number or positive charge carriers (or positive holes) per unit [94]

where e is the charge on the electron, and µ_n and µ_p are the mobilities of the negative and positive carriers, respectively. Photo-conductivity and mobility of the charge carrying species can be determined from a relationship [94]:

µ= d²/Vt

where d is the thickness of the film, V is the applied voltage, and t is the carrier drift time. The photo-effect is evaluated in terms of the effective gain, G. It represents the number of generated carriers reaching the external circuit per unit time, compared with the number of photons absorbed at the same time [94]:

where J_p is the photocurrent, e is the electric charge, I0 is the number of incident photons per cm²/s, T is the optical transmittance of the film, and A is the area of the sample that is being illuminated.

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مواضيع ذات صلة

Polymer-Based Solar Cells

Photo-Conducting Polymers That Are Not Based on Carbazole

Photoconductive Polymers Based on Carbazole

Liquid Crystalline Alignment

Application to Optical Data Storage

Changes in Viscosity and Solubility of Polymeric Solutions

Photo-Isomerization of the Azo Group

Photo-Isomerization of the Olefinic Group

Copolymers of Methacrylates with Cholesteric Monomers

Polymers with Norbornadiene Moieties

Photo-Responsive Polymers

Polymers Designed to Cross-link Upon Irradiation with Laser Beams