S. G. Prakash

Photoconductivity and dark conductivity studies of (MgO-ZnO) mixed system

The photoconductivity and dark conductivity of (20% MgO-80% ZnO) mixed system in polystyrene binder layer have been studied and compared with (50% MgO-50% ZnO) and ZnO samples. For (20% MgO-80% ZnO) and (50% MgO-50% ZnO) samples, the dark current is found to be space charge limited at higher voltages. The photocurrent shows non-Ohmic behaviour at lower voltages and tends to saturation at higher voltages. For 100% ZnO system at higher voltages the photocurrent does not show any saturation effect and the dark current becomes linear. The change in photocurrent versus intensity curve from superlinear to sublinear indicates the presence of sensitizing centres near to the valence band and the existence of an exponential trap distribution in (20% MgO-80% ZnO) and (50% MgO-50% ZnO) samples. For 100% ZnO system, the photocurrent versus intensity curve changes from sublinear to linear indicating the presence of exponential trap distribution. Photocurrent decays more rapidly with increasing percentage of MgO in the mixed system. The X-ray diffraction pattern of these samples do not show formation of solid solutions.

Photoconductivity studies of (PbCrO4-HgO-ZnO) composites

This paper deals with the photoconductivity studies of (PbCrO4-HgO-ZnO) composites comprising of base materials of three different energy gaps. The response of (10%PbCrO4-80%HgO-10%ZnO) and (20%PbCrO4-60%HgO-20%ZnO) composites is faster than that of other compositions and of base materials. The dark current for (20%PbCrO4-60%HgO-20%ZnO) is trap limited-space charge limited at higher voltages while it shows non-ohmic behaviour at lower voltages. The photocurrent for (20%PbCrO4-60%HgO-20%ZnO) composition shows non-ohmic behaviour at lower voltages and tends to saturation at higher voltages. The photocurrent for the above composition varies sublinearly with the intensity of illumination. The (10% PbCrO4-80%HgO-10%ZnO) composition is the best suitable material for optical switching. The existence of discrete trap depths at 16°C appears plausible for different compositions.

Photovoltaic Studies of PbCrO4-HgO-ZnO Thick Binder Layers

In this paper PbCrO4-HgO-ZnO photovoltaic detectors have been reported, which were fabricated by embedding the sensitive material in polystyrene and sandwiching between the conducting surfaces of two glass plates. Comparative study of photovoltaic effect in mixed lattices taking PbCrO4 as one of the components ie PbCrO4-HgO-ZnO, PbCrO4-ZnO and PbCrO4-HgO has been carried out. The variations of photocurrent and photovoltage with time and variations of photovoltage with intensity of illumination are investigated. An increase in open circuit output photovoltage and short circuit photocurrent was observed by connecting cells in series and parallel respectively. The existence of discrete trap depths appears plausible for different compositions.

Electroluminescence in (ZnS-ZnSe)Cu, H systems

A series of copper doped sulpho-selenide binary solid solutions, giving electroluminescent emission, right from green to red, has been prepared using ZnS and ZnSe in varying proportions. The behaviour of such polycrystalline samples was studied by usual methods in audio-frequency range. While the hydrogen ion acted as the main donor impurity in the entire series, its comparative study was made with chlorine coactivated samples. As regards the electroluminescent emission, while in the case of hydrogen coactivation, it is the transition from donor levels to the copper level that predominates, Cl-coactivation favoured direct transition from conduction band to same fundamental copper level. On frequency variation while the samples with ZnS-ZnSe composition showed marked peak shifts in their respective emission bands, no such effect could be observed in samples with single base.