Carrier generation and transport in crocetin crystals in a sandwich cell: steady state semi- and photoconductivity measurements

Abstract

Dark conduction and photoconduction in crocetin have been studied as a function of applied field and temperature. The results suggest that a single discrete trapping level is involved in charge carrier generation and that this level is different for dark conduction and photoconduction. Space-charge-limited current theory has been used to evaluate various transport parameters. In crocetin two opposite activation processes are operative in photoconduction in the two temperature regimes. The negative activation energy at high temperature may indicate activated exciton trapping in deep traps. The dependence of photocurrent on light intensity suggests that carrier generation is by a one-photon process and is trap limited. The photoconductivity spectrum suggests two distinct mechanisms of photocarrier generation, one through photoinjection from the illuminated electrode and the other through direct electron-hole pair production.

Spectroscopic, dark and photoconductive properties of some polyene-iodine charge-transfer complexes

“…In the charge-transfer complex, on the other hand, the photoconduction peak at 320-450 nm region is very intense and the peak at 600-700 nm region appears to be very weak. Similar observation was reported for another polyene, crocetin in pure state (Pal and Misra 1989). In pure polyenes, the high energy photoaction band has been attributed to the photoinjection of charge carriers from the electrode and the low energy band indicates direct electron-hole pair production.…”

“…In our earlier communications (Pal et al 1988;Pal and Misra 1989) we reported the photocarrier generation and their transport mechanism in a number of conjugated polyenes such as voltage, temperature and excitation light intensity dependence of the photocurrent and the photoconduction action spectra.…”

Studies of photoconductivity in β-Apo-8′ carotenal

“…After T,,,x, I~', decreases with increase of temperature. Similar observation of maximum photoconductivity at a specific temperature has been found in chalcogenide glasses (Anderson and Spear 1977;Tripathi and Kumar 1988), in amorphous Si (Spear et al 1979) and in a biomaterial crocetene (Pal and Misra 1989). In chalcogenide glasses this has been shown to be due to different recombination processes in the two temperature regimes; the bimolecular recombination at high temperature regime and monomolecular recombination at low temperature regime.…”

“…The feature of the long wavelength band is distinctly different from the absorption spectrum. Such long wavelength peak has been reported for many other polyene materials (Rosenberg 1966;Chapmann et a11967;Hoshino and Tateishi 1979;Pal and Misra t989). The long wavelength tail of this photoconduction band when extrapolated, intersect the abscissa at an energy which gives the photoconduction threshold energy i.e.…”

“…In photobiological processes like photosynthesis the photoconduction of polyenes is thought to be involved (Goodwin 1980). The photoconductivity of fi-carotene and its various isomers and of some other polyenes has been studied extensively (Rosenberg 1961(Rosenberg , 1966Hoshino and Tateishi 1979;Pal and Misra 1989). The study of photoconductivity in such materials is interesting since they have a large photocurrent to dark current ratio which is a necessary criteria for photovoltaic effect.…”