JOURNAL OF MATERIALS SCIENCE 27 (1992) 4184-4188 Anomalous Poole-Frenkel effect observed in some polyenes in sandwich cell configuration D. GHOSH, S. HAZRA, P. PAL, T. N. MISRA Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Calcutta- 700 032, India Both the dark- and photo-current-voltage characteristics of zeaxanthin and lutein is ohmic in the lower field regime followed by non-ohmic behaviour in the higher voltage regime, which has been satisfactorily explained by the anomalous Poole-Frenkel effect. A single dominant donor level is the major contributor to the dark-current while a single dominant trap level is the major contributor to the photo-current, both levels have been identified from Arrhenius type plots. Photo-action spectra suggest that the predominant mechanism of charge carrier generation is the same both in dark and illuminated conditions. 1. Introduction Highly conjugated n-electronic structure of polyene molecules results in their semi- and photo-conductive properties in the solid state. Photo-conduction in polyenes is thought to be involved in photo-biological processes like photosynthesis [1 4]. Polyenes like lutein and zeaxanthin were detected in the macular region of the retina. Macular degeneration is one of the leading causes of blindness. The possible role of a light induced sensitized reaction in the degeneration process [5] has been suggested. These polyenes may act as a scavenger of radicals or a quencher of singlet oxygen generated in the photo-sensitization process. The generation of photo-carriers and their transport in these materials, is, therefore of considerable interest. In this paper we report the results of our investigation on zeaxanthin and lutein. 2. Experimental procedure High quality polyenes used in this investigation were obtained as a gift from Hoffmann-La Roche Switzerland. The purity of the samples was checked by absorption spectra and they were used without further purification. The powdered samples were used in a sandwich cell with a stainless steel and tin oxide coated glass electrode. The cell was placed in a suit- ably designed conductivity chamber [6] with a quartz window through which light can pass and illuminate the sample. For current measurements Keithley's pro- grammable electrometer model 617 was utilized. The electrometer has a built-in isolated d.c. voltage source which can be adjusted between - 100 to + 100 V. Voltages up to + 100 V were applied across the elec- trodes from this source and for higher voltages, a high voltage regulated power supply (model 7333 APLAB, India) was used. Temperature measurements were made by a copper-constant thermocouple and a panel meter (HIL 2301, India). A 100 W Xenon lamp was used to illuminate the sample for the steady state 4184 photo-conductivity study. A Shimadzu spectropho- tometer (model 210A) was used to run the absorption spectra. For photo-conductivity action spectra meas- urements a monochromator (Jobin Yvon H20 UV 1673, France) was used. The sandwich cell thickness and area were maintained at 0.0025 cm and 0.25 cm 2, respectively. After each voltage application sufficient time was allowed to attain equilibrium current values. Several measurements were made in order to ensure reproducibility of results both in vacuum and in a dry nitrogen atmosphere. 3. Results and discussion The observed current-voltage characteristics of zea- xanthin and lutein are ohmic at the low voltage regime followed by non-ohmic behaviour at higher voltages both in the dark and in the illuminated condition. The non-ohmic region does not satisfy the space-charge limited conduction relation. The data yielded a straight line when plotted as log I versus V 1/2. Such plots for zeaxanthin and lutein are shown in Figs 1 and 2, respectively. In the low voltage regime, the experimental points fall below the straight line drawn with the points in the higher voltage regime. Such a linear plot is attributed either to the Schottky effect or to the Poole-Frenkel effect [7-10]. But the temper- ature dependence measurements of dark- and photo- currents at various voltages and excitation wavelength measurements suggest the Poole-Frenkel effect to be operative. Moreover, different parameters evaluated lie in an acceptable range" only when the Poole-Frenkel effect is assumed to be operative. A photo-conductivity study of Poole-Frenkel effect is rather rare. The Poole-Frenkel effect operating simul- taneously in dark- and photo-conduction was reported by Ieda et al. [11] and Jonscher and Ansari [12]. In the Poole-Frenkel effect, electrons are thermally emitted from the randomly distributed traps to the conduction band of the insulator by the lowering of 0022-2461 9 1992 Chapman & Hall