Surface Roughness, Band gap, Crystallite Size and Surface Potential Tuning and their Effect on Photocatalytic Activity of Titanium Dioxide Thin Films

Abstract

With the ever increasing demand for green energy, researchers have turned their focus on photovoltaic (PV) technology. Currently, dye-sensitized solar cells (DSSCs) have been reported as versatile and relatively less costly compared to the dominant silicon-based solar cells. In this regard, many nanostructured and nanoporous materials are now being investigated for their application in PV. Nanoporous Titanium Dioxide (TiO2) thin films for application in DSSCs have been under intensive study for many years. The films serve as the dye-supporting electron transporting electrode in the promising DSSCs. Recent interest has been raised regarding the tuning of anatase TiO2 films’ properties by doping it with Plasmonic metals and nonmetals. This work reports on a novel method of optimizing sol-gel-derived TiO2 thin films by their surface roughness, structural, optical, surface potential and photocatalytic activity for enhanced performance DSSCs. It also reports on the interdependence of the material’s properties. The films were synthesize using Doctor-blade technique and subsequently subjected to thermal treatment. The as-deposited and annealed thin films (1-step annealing, 1 oC/Min and 2 oC/Min) were studied using various methods including Optical Microscopy, UV-Vis Spectrophotometry, Scanning Kelvin Probe Microscopy (SKPM) and X-ray diffractometry. The Optical Microscopy exhibited an increase in surface roughness with decrease in annealing rate. A remarkable correlation between annealing rate and surface roughness was established. Further analysis of the films revealed optical band gap of 88 .3 , 72 .3 , 33 .3 , and 13.3 eV for the as-deposited, 1-Step, 2 oC/Min and 1 oC/Min annealing, respectively. The X-ray diffractometry technique revealed spectra which indicated enhancement in the films’ crystallinity upon annealing of the films, with the lowest annealing rate 1 oC/Min having the largest crystallite size of nm9633 .24 . Variation in surface potential was observed and was attributed to Smoluchowski-like dipole and local dipole moment of surface atoms. Good interdependence of surface roughness, optical band gap, crystallite size, energy loss, and surface potential has been observed. The DSSCs and DSSMs fabricated from TiO2 annealed at annealing 1 oC/Min rate exhibit the highest PCE due to high internal surface area which favour dye adsorption, and hence enhanced electron injection from the excited dye into the TiO2. Crystallinity was also found to enhance charge transport whereas large active area of DSSCs and DSSMs enhances carrier recombination rate. This slows charge transport rate. The effect is reduced power conversion efficiency (PCE) and is believed to result from deeper density of states (DOS) in large working area films. The material’s properties are tunable via annealing route and the interdependence models established are useful for rapid characterization of the thin films.