Photoconductive polymers absorb electromagnetic radiation and produce an increase of electrical conductivity. Photoconductive polymers have been used in a wide variety of technical applications such as Xerography (electrophotography) and laser printing. Electrical conductivity is usually very small in organic compounds. Conductive polymers usually have large electrical conductivity. Photoconductive polymer is a smart material based on conductive polymer, and the electrical conductivity can be controlled by the amount of radiation. The basic parameters of photoconductivity are the quantum efficiency of carrier generation( Υ {\displaystyle \Upsilon } ), the carrier mobility( μ {\displaystyle \mu } ), electric field(E), temperature(T), and concentration(C) of charge carriers. The intrinsic properties of photoconductive polymers are the quantum efficiency ( Υ {\displaystyle \Upsilon } ) and carrier mobility( μ {\displaystyle \mu } ), which will determine the photocurrent. Photocurrent will be affected by these four kinds of processes: charge-carrier generation, charge injection, charge trapping, charge carrier transport. Hundreds of photoconductive polymers have been disclosed in patents and literature. There are mainly two types of photoconductive polymer: negative photoconductive polymers and magnetic photoconductive polymers.
Definition Photoconductivity is an optical and electrical phenomenon, which material's electrical conductivity increase by absorption of electromagnetic radiation (e.g. visible light, ultraviolet light, infrared light). Photoconductive polymers can serve as good insulators when the electricity, free electrons and holes are absent. In general, the polymers usually satisfy these two features. 1. Photoconductive polymers can absorb light to excite electrons from ground state to excited state. The photoexcited electron will form a pair of charge carriers, it can be separated by electric field. 2. Photoconductive polymers must allow migration of either photoexcited electrons or holes, or both, through the polymer in the electric field towards the appropriate electrodes. Photoconductive polymers act merely as charge-transporting media, and it can be p-type or n-type, however most known photoconductive polymers are p-type (only transport holes). Photocurrents usually observed are very small in organic compounds. The mobilities μ are typically 10−12-10−18 m2V−1s−1. And photocurrents are usually effected by charge-carrier generation, injection and transport. Photoconductive polymers have been developed into different types, there are two mainly types, one is negative photoconductivity, another one is magnetic photoconductivity. The photoconductive polymers have been greatly enriched the photoconductive material, and there are many applications (e.g. xerography, laser printers)
Negative Some materials exhibit decrease in photoconductivity upon exposure to illumination. One prominent example is hydrogenated amorphous silicon in which a metastable reduction in photoconductivity is observable. Other materials that were reported to exhibit negative photoconductivity include molybdenum disulfide, graphene, and metal nanoparticles.
Factors influencing the photocurrent When light is absorbed by a material, the number of free electrons and electron holes increases and raises its electrical conductivity. To cause excitation, the light that strikes to the materials must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap. And this process will involve four kinds of processes: charge-carrier generation, charge injection, charge trapping, charge carrier transport.
Charge-carrier generation
The charge-carrier generation can be affected in different aspects: photons absorbed, polymer itself, photoexcitation of photosensitive material. The mechanism for intrinsic photogeneration is as illustrated. As Onsager originally developed this theory: The encounter complex will be formed by photoexcitation with migration of the exciton to an acceptor site. The photogeneration efficiency is determined by the competition between carrier separation and geminate recombination. The photogeneration efficiency was defined by using the dissociation of ion pairs in weak electrolytes, which can be expressed as a function of electric field, temperature and the separation distance of the bound hole-electron pair. The overall photogeneration efficiency ϕ ( E ) {\displaystyle \phi (E)} can be given by
ϕ ( E ) = ϕ 0 ∫ p ( r , Θ , E ) g ( r , Θ ) d 3 r {\displaystyle \phi (E)=\phi _{0}\int p(r,\Theta ,E)g(r,\Theta )d^{3}r}
d 3 r {\displaystyle d^{3}r} is a volume element, ϕ {\displaystyle \phi } is the primary quantum yield, p ( r , Θ , E ) {\displaystyle p(r,\Theta ,E)} is the probability that a hole-electron pair separated by a distance r {\displaystyle r} at an angle Θ {\displaystyle \Theta } to the direction of electric field E {\displaystyle E} , g ( r , Θ ) {\displaystyle g(r,\Theta )} is the spatial distribution function between ions. Efficient injection of charge into the layer plays an important role in the operation with a photogeneration layer. Under quasi-steady state conditions, it can be written by the flowing equation:
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