Influence of Graphene on Sheet Resistivity and Urbach Enery of Nano TiO2 for DSSC Electrode

Abstract

Importance of renewable energy cannot be over emphasized. Titanium IV oxide (TiO₂) is the most suitable semiconductor for dye sensitized solar cell (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 which is a main set back in DSSCs. undestanding graphene- TiO₂ contact is therefore essential for DSSC application. TiO₂ thin films were deposited on single layer graphene (SLG) as well as on flourine tin oxide (FTO) using doctor blading technique. The films were annealed at rates of 2°C /min and 1°C/min up to a temperature of 450°C followed by sintering at this temperature for 30 minutes. Four point probe SRM-232 was used to measure sheet resistance of the samples. The film thickness were obtained from transmittance using pointwise unconstrained minimization approximation (PUMA). UV –VIS spectrophotometer was employed to measure transmittance. Resistivity of TiO₂ on both FTO and Graphene were of order 10^-4 Ωcm. However, TiO₂ annealed on graphene matrix exhibited a slightly lower resistivity 5.6 x10^-4 Ωcm as compared to 6.0x10^-4 Ωcm on FTO. Optical transmittance on visible region was lower for TiO₂ on FTO than on SLG, 71.48% and 80.11% respectively. Urbach energy (Eu) for weak absorption region decreased with annealing rate. Urbach energies for 1°C/min TiO₂ on FTO and SLG were 361 meV and 261meV respectively. This was used to account for decrease of disoders of films due to annealing. A striking relation between sheet resistivity and urbach was reported suggesting SLG as a suitable candidate for photoanode of a DSSC.