SYNTHESIS AND CHARACTERIZATION OF NIOBIUM PENTOXIDE THIN FILMS PREPARED BY SPRAY PYROLYSIS TECHNIQUE FOR DYE SENSITIZED SOLAR CELL APPLICATION
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Date
2019-03
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Abstract
There is a rapid increase in demand for energy worldwide and one of the most
promising approaches to resolve this crisis is through use of photovoltaic
technologies. Dye sensitized solar cells (DSSC) are attractive candidates in this
regard. This is because the materials they use are of low cost and non-toxic.
Niobium pentoxide (Nb2O5) thin films have been stable as photo anode for DSSC.
Methods that are used to deposit niobium pentoxide thin films include hydrothermal
method, solgel method, anodization technique, electro-deposition, sputtering, and
spray pyrolysis. Among the above mentioned methods, spray pyrolysis is the
simplest. It is a low cost method especially with regard to equipment. The technique
requires chemicals that can undergo pyrolysis. In this study, Niobium pentoxide
(Nb2O5) thin films for DSSC application were deposited on ordinary glass substrates
using the spray pyrolysis technique. In the synthesis of Nb2O5, Niobium
pentachloride (NbCl5) was used a precursor solution. This was prepared by
dissolving Niobium pentachloride in distilled water to which hydrogen peroxide and
acetic acid were added. The deposition parameters of the niobium thin films such as
spray duration and substrate temperature were varied. The sprayed solution
underwent thermal decomposition resulting in formation of niobium pentoxide
(Nb2O5) thin films. The optical characteristics of the films were determined using
the Shimadzu model DUV 3700 spectrophotometer. From the transmittance spectra
obtained, it was clear that the films were 60% to 90% transparent in the visible
region and the absorption edge occurred at around 300nm. Sheet resistivity of the
films determined by use of the four point probe method was 10-4Ωcm. XRD studies
revealed that the films were tetragonal in nature with well defined reflections at
(211) and (220). The average crystallite size was estimated to be 21.25nm. The
thicknesses of the films were determined using the KLA Tencor Alpha–step IQ
surface profiler and was found to range between 162nm and 517nm. Band gap
energy and absorption coefficient were estimated using point-wise unconstrained
minimization approach (PUMA) software. Band gap energy values ranged between
3.92 eV to 4.12 eV. From the obtained results, the films considered best for DSSC
application were those deposited at substrate temperature of 470oC and at duration
of 2 minutes.