Photoconduction charge-transfer exciton

The use of interpenetrating donor-acceptor heterojunctions, such as PPVs/C60 composites, polymer/CdS composites, and interpenetrating polymer networks, substantially improves photoconductivity, and thus the quantum efficiency, of polymer-based photo-voltaics. In these devices, an exciton is photogenerated in the active material, diffuses toward the donor-acceptor interface, and dissociates via charge transfer across the interface. The internal electric field set up by the difference between the electrode energy levels, along with the donor-acceptor morphology, controls the quantum efficiency of the PV cell (Fig. 51). 

All photoeffects involve the absorption of photons to produce an excited state in the absorber or liberate electrons directly. With the direct release of electrons, photoemission may occur from the surface of solids. While the excited state may revert to the ground state, it may proceed further to a photochemical reaction to provide an electron-hole pair (exciton) as the primary photoproduct. The exciton may dissociate into at least one free carrier, the other generally remaining localized. In an externally applied electric field, photoconduction occurs. Photomagnetic effects arise in a magnetic field. Absorption of photons yield photoelectric action spectra which resemble optical absorption spectra. Photoeffects are involved in many biological systems in which charge transfer takes place (e.g., as observed in the chlorophylls and carotenoids) [14]. 

Yasunaga et al. (1979) studied photoconductivity of single crystals of PbPc doped with O. The photocurrent increased sharply with increasing O. The results were explained by the ionization of a charge-transfer state created by the dissociation of a triplet exciton. The exciton dissociation was suggested to occur at crystal sites occupied by O. 

At first, diarsenic triselenide was used as the photoconducting material. Poly(A -vinyl carbazole) is now used. This polymer absorbs ultraviolet light, producing an exciton, which ionizes in an electric field. Poly(vinyl carbazole) behaves as an insulator in visible light, but can, however, be sensitized with certain electron donors to form a charge transfer complex. 

Apart from these conceptual problems one has to consider that in a photoconduction experiment there is always an extrinsic contribution to photogeneration such as sensitization due to inadvertent impurities and exciton dissociation at an electrode. The former usually dominates at low to moderate electric fields. It originates from charge transfer from bulk excitons towards mostly acceptor-type of impurities such as oxidation products and can be rationalized in terms of Arkhipov et al. s theory [58]. It is worth mentioning, though, that at high electric fields, photogeneration is, in fact, intrinsic as evidenced by the concomitant decrease of fluorescence [26]. 

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