Enhancing in the performance of dye-sensitized solar cells by the incorporation of functionalized multi-walled carbon nanotubes into TiO2 films: The role of MWCNT addition

TiO2–MWCNT composite electrodes were prepared by a direct mixing method. The presence of acid-treated multi-wall carbon nanotubes (MWCNT–COOH) into the titanium dioxide (TiO2) photoanode was investigated by Raman spectroscopy and X-ray diffraction (XRD). The morphological properties of the composite photoanodes were analyzed by field emission scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM). The performance of quasi-solid state dye-sensitized solar cells (DSSC) using TiO2–MWCNT photoanodes was dependent on the MWCNT loading. Compared with a DSSC based on conventional TiO2 electrodes, the TiO2–MWCNT film containing 0.02 wt.% of carbon nanotubes provided an increase of ca. 30% in deviceʹs efficiency, which was attributed to an enhanced short-circuit current density (Jsc). The improvement on Jsc was correlated with an enhanced interconnectivity between MWCNT–COOH and TiO2 nanoparticles. The carbonaceous materials introduced an alternative electrical conduction pathway which facilitates rapid electron transport in the photoelectrode, as suggested from Kelvin probe force microscopy (KPFM) measurements. At high MWCNT loading, we observed that the energy conversion efficiency decreased due to energy losses from the optical absorption of carbonaceous materials, and also due to an increase in charge recombination.