Characterization of nano-crystalline structure of TiO₂ for Dye-sensitized Solar Cells by Raman spectroscopy and spectroscopic ellipsometry

Abstract

Dye-sensitized solar cells (DSSCs) are attractive alternatives to conventional solid-state photovoltaic devices because of performance, stability, environmental compatibility and cost. In contrast to the conventional systems where the semiconductor assumes both the task of light absorption and charge carrier transport, these two functions are separate in DSSC and, therefore, efficiency is very sensitive to the cell structure/composition. High-efficiency DSCs based on mesoporous nanocrystalline titanium dioxide (TiO₂) electrodes have received considerable research attention in the past decade. Grain size and thickness of the mesoporous TiO₂ film have shown a dominant effect on the efficiency of the photovoltaic devices.

We have investigated screen printed TiO₂ films deposited on a textured fluorinated SnO₂ (FTO) glass substrate, using Raman spectroscopy and spectroscopic ellipsometry. Materials were prepared by repeating a same screen-printing procedure once, twice and three times, using TiO₂ paste with 10nm, 20 nm and 200nm particles sequentially. Raman spectra of PV devices cells taken at different excitation (266 nm, 325 nm, 364 nm, 532 nm, 633 nm, and 1064 nm) as well as pure TiO₂ oxides are presented. Different excitation wavelengths allow to probe different depth of the sample. It was found that there is strong correlation of the position and the width of E_2g mode of anatase at 144 cm^-1 and size of TiO₂ particles. The samples show that this peak shifts to the high frequency region and becomes broader for small size particles. The position and broadening of the peak can be described by optical confinement model that depends on the size of nano-crystals. Results showed varying grain sizes as correlated with different TiO₂ paste applied. Thickness, optical constants and porosity of TiO₂ films were determined by spectroscopic ellipsometry. In this work, we have demonstrated the use of Raman Spectroscopy and Spectroscopic Ellipsometry for non-destructive characterization of nanocrystalline TiO₂ films for dye-sensitized solar cells.