Electrical and optical characterization of cdxzn1-xs and pbs thin films for photovoltaic applications
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Date
2013-07-22
Authors
Mosiori, Cliff Orori
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Abstract
In this research an n-type CdxZn1-xS and p-type PbS thin films were optimised for solar cell
applications employing chemical bath deposition technique. The thin films were prepared
using thiourea and nitrates of cadmium, zinc and lead. Deposition of optimised CdxZn1-xS
was done by CBD at 820 C and in alkaline conditions while that of PbS was done at room
temperature and both films at normal atmospheric pressure utilizing aqueous conditions. This
study concentrated on optimising optical and electrical characterization of the films. Optical
constant suitable for photovoltaic applications were sort for and for this purpose a UV VIS IR
spectrophotometer 3700 DUV was utilised while the electrical properties were investigated
using a four point probe connected to a Keithley 2400 source meter interfaced with computer.
The optical band gap of the as deposited CdxZn1-xS films varied from 2.47eV (x =0.6) to 2.72
eV (x =1.0), and transmittance above 79% in the VIS - NIR region for the concentration
range of x = 0.6 to 1.0, that is, the band gap increased with increasing Zn concentration of the
alloy and Cd06Zn0.4S sample showed the widest band gap. It was obtained that the presence of
zinc increased optical band gap. The average extinction coefficients for the as deposited
CdxZn1-xS samples were very low revealing that they absorb very little radiation hence a good
window layer material. As measured by the four point probe connected to a Keithley 2400
source meter, electrical resistivity increased with increase in Zn in the bath in CdxZn1-xS and a
resistivity range of 9.5×101 – 1.22× 102 Ω-cm was obtained. These properties are appropriate
for window layers used for photovoltaic cell applications. PbS thin films had a band gap of
0.89 eV and a transmittance of below 55% appropriate for absorber layers of photovoltaic
cells and a resistivity range of 6.78 × 103 to 1.26 × 104 Ω-cm. The fabricated photovoltaic cell
had a short circuit current, Isc = 0.031 A, open voltage, Voc = 0.37V, efficiency, η = 0.9% and
a fill factor, FF = 0.66 implying that the two materials are appropriate for photovoltaic
applications especially in the VIS and IR light spectrum.