Photovoltaicity and photoconductivity

Law (1993) has reviewed the widespread polymorphism in phthalocyanines and its relationship to their photoconductivity and use in xerographic applications. These are best demonstrated by the prototypical copper phthalocyanine CuPc (6-XIX, M = Cu), whose structural chemistry is discussed in detail in Section 8.3.3.1. The polarized absorption spectra of five forms are given in Fig. 6.9. One outstanding feature of these spectra is the rather intense red shifted band at 770 nm in the 5 and modifications compared to the solution absorption at 678nm (Law 1993). This 

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Photovoltaic and photoconductive phenomena for various types of CT complexes between saturated polymers and dopant molecules, heterojunctions between polymers and organic and inorganic photoconductors were also investigated in the last few years [86-92]. The quantum efficiency of the energy conversion of 10-3% was obtained for such systems and output power density of 3 x 102 mV cm-2. The mobilities of the heterogeneous polymer systems with despersed inorganic photoconductors reach the value — 10 3-10-4 m2 V 1s 1. 

Grey selenium exhibits both photovoltaic and photoconductive properties, which make it useful in the production of photocells and solar cells. Moreover,... 

The type of detector used in an FT-IR spectrometer is highly dependent upon the bandwidth (i.e. the spectral frequencies), the modulation rate of the interferometer, and the intensity of the radiant flux. Several types of detectors are used in the infrared regions photoconductive, photovoltaic, bolometers, pyroelectric and Golay cells. A detailed discussion of detectors may be found elsewhere.12 In general, the photovoltaic and photoconductive detectors can be used in the near- and mid-infrared regions as rapid response, high sensitivity detectors. Usually the bandwidths are limited and will not cover the total ran passed by the beamsplitter. Examples of such detectors are given in Table I. As can be seen from the... 

Photovoltaic and photoconductive effects in solids are widely used for detecting infrared radiation. These detectors offer very high detectivities, although they must often be cooled to achieve such performance. Their performance is high and continues to improve because of the development of highly purified, singlecrystal semiconductors as their active materials. However, several different materials appear to be competing for dominance, and it is not clear whether photovoltaic or photoconductive effects should be emphasized. We will attempt to put the situation into better perspective. 

The basic theory of photovoltaic and photoconductive detectors shall be presented in Section 4.1 in a unified form convenient for intercomparison of the two effects and of the various detector materials. Then Sections 4.2,4.3, and 4.4 shall cover photovoltaic, intrinsic photoconductive, and extrinsic photoconductive detectors, respectively, each of these sections including first a subsection in which the general theory of Section 4.1 is specialized to that class of detector, and then a subsection in which specific materials suitable for that class of detector are evaluated in terms of the theory. Finally in Section 4.5 we will draw some conclusions about the status and prospects of photovoltaic and photoconductive infrared detectors. Symbols used in this chapter which are not defined in the text are defined in Table 4.1. 

Photovoltaic and photoconductive effects result from direct conversion of incident photons into conducting electrons within a material. The two effects differ in the method of sensing the photoexcited electrons electrically. Detectors based on these effects are called photon detectors, because they convert photons directly into conducting electrons no intermediate process is involved, such as the heating of the material by absorption of photons in a thermal detector which causes a change of a measurable electrical property. 

The expressions (4.10) and (4.12) for the noise currents of photovoltaic and photoconductive detectors are both of the form... 

Equation (4.15) is the general relationship between Df and detector parameters it applies to both photovoltaic and photoconductive detectors. 

We shall use the above four conditions as a basis for analysis and comparison of the various detector materials and of the two modes, photovoltaic and photoconductive, in the remainder of this chapter. Conditions 1-4 provide a convenient framework for discussing the detector materials, because the parameters on the left sides of the four relationships are directly related to measurable and controllable material properties. 

Thus we shall review recent progress in these detectors, assess their present status, and analyze prospects for their future improvement, emphasizing the relationships of detector performance parameters to semiconductor material parameters and to fundamental limits. There shall be little discussion of detector fabrication technology. Of particular interest shall be the potential performance of the various detector materials if their properties can be optimized, and the comparison of photovoltaic and photoconductive effects in these materials. We shall treat only infrared detectors detectors of optical radiation in the visible region of the electromagnetic spectrum were reviewed recently by Seib and Aukerman [4.1]. Useful recent general references for this chapter are the reviews of infrared detectors and their applications by Dimmock [4.2] and of narrow-gap semiconductors by Hannan and Melngailis [4.3]. 

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