Photoconductivity dependence

In 2-arylidene-1,3-indandiones 6, which can be considered as ethylene derivatives with two end-carbon atoms attached to 1,3-indandione and to electron-acceptor and electron-donor groups (R), respectively, photoconductivity depends on the substituents of the arylidene group 81>. 

Some polymeric materials become electrically conductive when illuminated with light. For instance, poly(TV-vinylcarbazole) is an insulator in the dark, but when exposed to UV radiation it becomes conductive. The addition of electron acceptors and sensitizing dyes allows the photoconductive response to be extended into the visible and NIR regions. In general, such photoconductivity depends on the materials ability to create free-charge carriers, electron holes, through absorption of light, and to move these carriers when a current is applied. 

In 1977 it was observed that extended illumination with visible light of a-Si H produced a decrease in photoconductivity and dark conductivity (the Staebler-Wronski effect), which is reversible upon annealing, as shown in Fig. 7 (Staebler and Wronski, 1977,1980). The effect can be quite dramatic, producing a decrease in dark conductivity of over four orders of magnitude, though the extent of the decrease depends on the initial defect density and doping level of the sample. The degraded conductivity state is... 

Figure 3.13 The spectral dependence of the specific detectivity for several photoconduction detectors. The values corresponding to a typical thermopile and to a typical piroelectric detector are also shown.

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. 

Fig. 4.1 Time dependence of photoconductivity of CdS films deposited from tri-ethanolamine/ammonia/thiourea bath at two different temperatures (26°C and 75°C). The two plots at each temperature differ by the ratio between the Cd and thiourea concentrations [Thiourea] [Cd] = 0.25 for the upper plots at each temperature and 0.5 for the lower plots. (Adapted from Ref. 34 with permission from Elsevier Science).

Films were deposited from solntions of lead and tin salts (the salts used were not specified) with ammoninm acetate, ethylenediamine, and selenonrea at a pH > 9 (probably at least 11) [34]. To obtain thicker films, deposition was repeated a nnmber of times and the films were annealed therefore it is not known if solid so-Intion formation occnrred in as-deposited films. In annealed films, Pbi -j Snj Se solid solntions with x np to 0.11 were verified by XRD. The spectral response of the photoconductivity of the (annealed—as-deposited films were not photosensitive) films shifted from a peak at ca. 4 p,m (pure PbSe) to ca. 7.5 p,m (11% Sn), supporting solid solution formation of the annealed films. The room-temperature, dark resistance of the (probably annealed, but not certain) films varied from 1 to 300 kO, depending on deposition conditions. 

Traps and recombination centers which depend on purity, crystal defects and preparation, can exert an influence, and electrode contacts, carrier injections, and other factors can interfere with measurements. Yet there is no doubt that the photoconductive gain (quantum yield) G can be reproduced by different methods. As in the case of dark conductivity, the photoconductivity properties are related to the electronic and structural behavior of pure and doped organic compounds, also those in the polycrystalline state. 

Fig. 7. Dependence of photoconductivity on structure for aceanthraquinoxaline 5 /phot = f (Voltage)...

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