Dependence of Sheet Resistivity on Urbach Energy of Nano TiO2 - Graphene-based Electrode for DSSC Application

Abstract

The importance of renewable energy cannot be over-emphasized. Titanium IV oxide (TiO2 ) is the most suitable semiconductor for dye-sensitized solar cells (DSSC) due to its chemical stability, non-toxicity and excellent optoelectronic properties. In this research, TiO2 is coated on Graphene to enhance its charge transport, aiming to reduce recombination, a main setback in DSSCs. Understanding Graphene- TiO2 contact is, therefore, essential for DSSC application. Using doctor blading, TiO2 thin films were deposited on single-layer graphene (SLG) and fluorine tin oxide (FTO). The films were annealed at 2 °C /min and 1 °C/min up to a temperature of 450 °C, then sintering at this temperature for 30 minutes. Four four-point probes SRM -232 were used to measure the samples' sheet resistance. The film thickness was obtained from transmittance using pointwise unconstrained minimization approximation (PUMA). UV–VIS spectrophotometer was employed to measure transmittance. The resistivity of TiO2 on both FTO and Graphene was of order 10-4 Ω cm. However, TiO2 annealed on graphene matrix exhibited a slightly lower resistivity, 5.6 x10-4 Ω cm, compared to 6.0 x10-4 Ω cm on FTO. Optical transmittance on the visible region was lower for TiO2 on FTO than on SLG, 71.48% and 80.11%, respectively. The annealing rate decreased the weak absorption region's Urbach energy (Eu). Urbach energies for 1oC/min TiO2 on FTO and SLG were 361 meV and 261 meV, respectively. This accounted for the decrease in film disorders due to annealing. A striking relation between sheet resistivity and Urbach was reported, suggesting SLG as a suitable candidate for the photoanode of a DSSC.

Keywords

• graphene, urbach energy, resistivity, annealing, titanium iv oxide