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Abstract:
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Photoconductivity (PC) processes may
be the most suitable technique for obtaining information about the states in the
gap. It finds applications in photovoItaics, photo detection and radiation
measurements. The main task in the area of photovoltaics, is to increase the
efficiency of the device and also to develop new materials with good optoelectronic
properties useful for energy conversion, keeping the idea of cost
effectiveness. Photoconduction includes generation and recombination of carriers
and their transport to the electrodes. So thermal relaxation process, charge
carrier statistics, effects of electrodes and several mechanisms of recombination
are involved in photoconductivity.A major effect of trapping is to make the experimentally observed decay
time of photocurrent, longer than carrier lifetime. If no trapping centers are
present, then observed photocurrent will decay in the same way as the density
of free carriers and the observed decay time will be equal to carrier lifetime. If
the density of free carriers is much less than density of trapped carriers, the
entire decay of photocurrent is effectively dominated by the rate of trap emptying
rather than by the rate of recombination.In the present study, the decay time of carriers was measured using
photoconductive decay (PCD) technique. For the measurements, the film was
loaded in a liquid Helium cryostat and the temperature was controlled using
Lakshore Auto tuning temperature controller (Model 321). White light was
used to illuminate the required area of the sample. Heat radiation from the light
source was avoided by passing the light beam through a water filter. The decay current. after switching off the illumination. was measured using a Kiethely
2000 multi meter. Sets of PCD measurements were taken varying sample
temperature, sample preparation temperature, thickness of the film, partial
pressure of Oxygen and concentration of a particular element in a compound.
Decay times were calculated using the rate window technique, which is a decay
sampling technique particularly suited to computerized analysis. For PCD curves
with two well-defined regions, two windows were chosen, one at the fast decay
region and the other at the slow decay region. The curves in a particular window
were exponentially fitted using Microsoft Excel 2000 programme. These decay
times were plotted against sample temperature and sample preparation
temperature to study the effect of various defects in the film. These studies
were done in order to optimize conditions of preparation technique so as to get
good photosensitive samples. useful for photovoltaic applications.Materials selected for the study were CdS, In2Se3, CuIn2Se3 and CuInS2•
Photoconductivity studies done on these samples are organised in six chapters
including introduction and conclusion. |