Sensitization of Photoconductivity

Various types of the photoconductive polymers are available now. The photoconductivity of such materials may be essentially increased by means of the chemical and spectral sensitization [12-14]. Spectral sensitization is connected with the appearance of the photosensitivity in the new spectral bands and the chemical sensitization with the increase of the proper sensitivity. As a rule both types of sensitisation may take place in the photoconductor at the same turn. The first data about chemical and spectral sensitization in organic photoconductors appeared in [19, 20]. The example of the chemical and spectral sensitization of the photoconductivity by dyes in polymeric copper-phenyl-acetylenide is presented in Fig. 2. Later on it was proposed that not only low molecular weight compounds but polyconjugated polymers could also be used as sensitizers [21] having broad absorption tends and high thermostability compared with dyes. Now it is clear that various types of molecules may be used as a photosensitizers. 

The effectiveness of the spectral sensitization depends on many factors. As a rule, a spectral sensitization process needs the thermal activation energy of the 

Two main models are usually discussed for the mechanism of the spectral sensitization. The excitation of the sensitizer by absorbed light and electron transfer from the excited sensitizer to the semiconductor is the first model. The alternative mechanism consists of the transfer of the excitation energy from the sensitizer to the semiconductor. This energy is used for photogeneration of the charge carriers in the sensitized photoconductor. In the first case the excited singlet level of the sensitizers has to be located above the conduction band of the semiconductor for realization of the electron transfer. For hole transfer the basic sensitizer level has to be located lower than the valence band of the sensitized photoconductor. The energy transfer mechanism does not need a special mutual location of the semiconductor and sensitizer levels. 

Most of the modern theories of the photoconductivity sensitization consider that local electron levels play the decisive role in filling up the energy deficit The photogeneration of the charge carriers from these local levels is an essential part of the energy transfer model. Regeneration of the ionized sensitizer molecule due to the use of the carriers on the local levels takes place in the electron transfer model. The existence of the local levels have now been proved for practically all sensitized photoconductors. The nature of these levels has to be established in any particular material. A photosensitivity of up to 1400 nm may be obtained for the known polymer semiconductors. There are a lot of sensitization models for different types of photoconductors and these will be examined in the corresponding sections. 

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Quenching of the Intersystem Crossing to the Triplet State 521 Photoinduced 1RAV Studies 522 Time-Resolved Photoinduced Studies 524 Sensitization of Photoconductivity 525 Magnetic Properties 526... 

The extension of sensitivity of photoconductive polymers from the UV to the visible range by doping with electron acceptors was reported by Hoegl81 13 Since allknown... 

Bauer, W. (1977). Experimental studies on the spectral sensitization of photoconductivity on zinc oxide crystals. J. Phys. Chem. Solids 38 463-467. 

Fig. 9. Spectral sensitivity of detectors where the detector temperatures in K are in parentheses, and the dashed line represents the theoretical limit at 300 K for a 180° field of view, (a) Detectors from near uv to short wavelength infrared (b) lead salt family of detectors and platinum siUcide (c) detectors used for detection in the mid- and long wavelength infrared. The Hg CdTe, InSb, and PbSnTe operate intrinsically, the doped siUcon is photoconductive, and the GaAs/AlGaAs is a stmctured supedattice and (d) extrinsic germanium detectors showing the six most popular dopants.

West, W. Correlations between photographic and photoconductance sensitivity of silver halides.. Mitchell, J. W. Fundamental mechanisms of photographic sensitivity, p. 99. London Butterworth s Sci. Publ. 1951. 

As is the case for the dark resistivity, the dependence of the sensitivity of the photoconductivity (defmed here as the ratio between light and dark conductivity) on the deposition parameters is far from clear-cut. Some observations can be made, however. The first (obvious) one is that for a high sensitivity, the dark resistivity must be high. Apart from this, there does seem to be a general trend (clear-cut in the triethanolamine and citrate baths and seen also by the lack of appreciable photoconductivity in the one low- (room-) temperature-deposited film reported [40]) of an increase in photosensitivity (due to decrease in light resistivity) with increasing deposition temperature. 

Besides polyimides, photoconductive polymers with conjugated bonds also can be successfully used in liquid crystal spatial light modulators [262-264], The high resolution, mechanical and electrical stability and possibility for sensitization of the photosensitivity make the use of organic polymer photoconductors in spatial light modulators very attractive. Devices such as phase... 

As already shown, the sensitized spectra follow the dye absorption. Most of the dyes have sufficiently narrow absorption bands. This does not permit us to obtain the panchromatic sensitivity in the sufficiently broad spectral range. It was proposed to use the polymers with conjugated bonds as sensitizers [21]. The broad diffuse absorption spectra are inherent to such compounds. One can expect higher thermal stability from such sensitizers. In addition the application of binder may be omitted from the preparation of the photosensitive layers, for example, in electrophotography. Polymers with triple bonds, polyphenylenes and polyoxiphenylenes were used as sensitizers [10, 14, 278-280]. The typical results are shown in Fig. 47. The main rules for photoconductivity sensitized by polymers were the same as for the dyes. Optimum sensitization was obtained at the concentration of the sensitizer of 10 1-10-2 g/cm3 relative to the polymeric photoconductor weight. 

Besides spectral sensitization the chemical sensitization of PAC was revealed [281-282]. The treatment of the polymer with nucleic acids lead to an increase in the photocurrent by 3 orders of magnitude without changing the photoconductivity spectrum. The same results were obtained with adenine. The data obtained were explained by the model with new recombination centers leading to an increase in the life time of the predominant charge carriers. This was confirmed by kinetic investigations. 

Kellogg s photoconductivity measurements (quoted in ref. 107) showed that the photoconductivity of 0.2 pm AgBr grains increased with increasing degree of hypersensitization, passed through a maximum, then decreased. The sensitivity of the... 

In our experiments with the low-temperature sensitivity of monodisperse silver bromide emulsions having 0.5 ym cubic grains, the sensitivity of the (S+Au)-sensitized emulsion differed from that of the S-sensitized emulsion both with respect to the dependence on the degree of sensitization and the correlation with microwave photoconductivity measurements (16). 

For low, medium, and high levels of sensitization, as determined by their effects on room-temperature sensitivity, the photoconductivity at 77K decreased with increasing sensitization for... 

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

Fig. 12. Action spectrum of photoconductivity of sensitized PVK. (1) PVK + methylene blue. (2) PVK + methylene blue + TCNE124)...

Silver halide microcrystals are wide band gap semiconductors which exhibit weak photoconductivity. Early experiments demonstrated that dyes that sensitized silver halide photographic action also sensitized silver halide photoconductivity [6c]. Since the observation of photoconductivity necessitates the movement of free charge within the crystals, dye sensitization processes must inject charge into the silver halide lattice in some way. Initial theories of sensitization were based on the semiconductor view of silver halides, especially as espoused by Gurney and Mott [10]. Current ideas are based on thorough studies of the absorption spectroscopy and luminescence of silver halide emulsions and of adsorbed, sensitizing dyes, and the oxidation-reduction properties of the dyes at silver/silver halide electrodes [11]. 

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