Functionalized multi-wall carbon nanotubes/TiO2 composites as efficient photoanodes for dye sensitized solar cells

Abstract

We report on the effects of incorporation of different concentrations of carboxyl group (COOH)-functionalized multi-wall carbon nanotubes (F-MWCNTs) into TiO₂ active layers for dye-sensitized solar cells (DSSCs). Standard DSSCs with bare TiO₂ exhibit a photo-conversion efficiency (PCE) of 6.05% and a short circuit current density (J_sc) of 13.3 mA cm⁻². The presence of 2 wt% F-MWCNTs in the photoanodes increases the PCE up to 7.95% and J_sc up to 17.5 mA cm⁻². The photoanodes were characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. The electrochemical behaviour of the solar cells was investigated by electrochemical impedance spectroscopy (EIS). We attribute the improved performances to the combined effect of increased dye loading and reduced charge recombination (as clarified by dye loading and EIS measurements), due to the conformal coverage of F-MWCNTs, which allows fast and efficient charge collection in operating solar cells. These results can help in improving the PCE in DSSCs in an elegant and straightforward way, minimizing the need of additional steps (e.g. pre- and post-treatment with TiCl₄) for photoanode preparation.