Bernard Kramer

Photovoltaic effects observed with thin films of zinc sulphide

Abstract Photovoltages (up to several hundred volts) have been observed along the surface of thin ZnS films evaporated at an angle to the substrate. This photovoltage is proportional to the exciting intensity. Upon illumination the resistance decreases by a factor of 4 from its dark value, and then remains independent of the intensity over a wide range of excitation. When strongly absorbed u.v. light is added to the homogeneously absorbed tungsten illumination, the photovoltage and resistance are considerably decreased. These results are discussed in light of a proposed model consisting of ZnS monocrystals separated by a ZnS material of different electrical characteristics. The latter material is considered to be non-conducting and non-photovoltaic except at high intensities (strongly absorbed u.v.) where it becomes conductive and displays a photovoltage opposite to that of the crystal.

Calculation of the photoconductivity from A.C. impedance changes induced in ZnS and ZnCdS phosphors

Abstract To determine the true induced conductivity of ZnCdS-type phosphors, a.c. impedance measurements are required so that the barrier effects of the grains and electrodes can be eliminated. Five phosphors have been investigated and from the capacitance change and the dissipation factor determined at various frequencies and intensities the conductivity of the sample was obtained for homogeneous excitation. For non-homogeneous excitation the actual conductivity cannot be as easily determined, but the dependence of the conductivity on excitation can be determined. A comparison of theoretical and experimental results is given for the samples investigated.

Long Time Phosphorescence of Organic Crystals

Long time phosphorescence (of the order of minutes) of anthracene, stilbene, durene, and naphthalene crystals at room temperature is reported. The decays are approximately hyperbolic and the observed phosphorescence is shifted to longer wavelengths than the usual fluorescent emission. This phosphorescence is not induced by the light strongly absorbed in the substance but mainly by the weak absorption of light of wavelengths corresponding to their own fluorescent light. There are indications that this phosphorescence stems from an approximately uniform, continuous trap distribution and that such a mechanism permits a description of some of the observed behavior. Wavelength measurements seem to indicate that this phosphorescent light emission does not originate from triplet to ground transitions.

Comparison of photovoltaic and photoconductive effects in insulating CdS crystals

Photovoltage measurements with insulating CdS crystals capacitively coupled to the external circuit have been performed at room temperature in 1 atm oxygen and 10−8 Torr vacuum, and at 100 K at 10−8 Torr vacuum, and have been interpreted self consistently with photoconductivity measurements. It was found that the Dember voltage is about 0.06 V at room temperature and 0.02 V at 100 K, that the oxygen chemisorption‐induced surface barrier is at least 0.15 eV at room temperature, and that photoconductivity measurements are a valuable tool in the self‐consistent interpretation of photovoltage measurements on an insulating specimen.