Synthesis And Characterization Of Niobium Pentoxide Thin Films Prepared By Spray Pyrolysis Technique For Dye Sensitized Solar Cell Application

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.