High dark conductivity

Many organic solids and polymers are perfect insulators and are used to a considerable extent technically as such. There is, however, also a large number of organic semiconductors. Furthermore, there are organic sohds with a high dark conductivity or with a quasi-metallic conductivity (Chap. 9), as well as organic superconductors (Chap. 10). 

Organic crystals with high dark conductivities have become an active and extensive area of research in recent decades. We refer the reader to the review articles [Ml, M4, M5] and [4]. 

These polymers are mainly known for their conductivity, particularly when doped with suitable electron donors or acceptors. Photoconductivity is marginal, and therefore these polymers are of limited interest for the study of this phenomena. Undoped and doped polyacetylene has been studied in some detail [53-57]. Undoped ds-poly-acetylene is photoconductive in UV light [54] doping with AsFs shifts the photoresponse to the visible range, but the polymer also becomes highly dark-conductive. Under certain conditions, the ratio of photo to dark... 

The electrical properties (dark conductivity and photoconductivity) are reported to first decrease and then increase upon increasing power [361]. The optical bandgap increases with increasing power, due to the increase of the hydrogen content [63, 82, 362, 363]. However, at very high power levels, microcrystalline silicon is formed [364], which causes the hydrogen content (and, consequently, the bandgap) to decrease. 

In the conductance image, a dark color represents high electronic conductance, whereas a white color corresponds to areas of low or zero electronic conductance. Taking into account that the tip is in physical contact with the oxide, the magnitude of the current is determined by the local electronic properties of the electrode and the tip, and the tip-sample voltage difference. The surface conductance image of the cathode from the fresh cell... 

ELECTRICAL PROPERTIES. Rcsistivity studies on CdSe are much less widespread than on CdS films. The dark conductivity of undoped films is high (10 O-cm is typical), and the photocurrent sensitivity is less than for CdS films (even under illumination, the films are normally very resistive). 

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. 

Figure 8 Video images of the appearance of high-ionic-conductance pathways during iontophoresis of Fe(CN) and Fe. Counterdirectional iontophoresis was conducted across HMS for up to 2 hours at a current density of 40 pA/cm. The dark spots are localized deposits of Prussian blue dye at pore openings. (Reproduced with permission from Ref. 65.)...

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