Metal/metal sulfide functionalized single-walled carbon nanotubes: FTO-free counter electrodes for dye sensitized solar cellsElectronic supplementary information (ESI) available: X-ray diffraction (XRD) spectra of metal sulfides on CNT/glass. See DOI: 10.1039/c2cp40900c

The use of single-walled carbon nanotubes (CNT) thin films to replace conventional fluorine-doped tin oxide (FTO) and both FTO and platinum (Pt) as the counter electrode in dye sensitized solar cells (DSSC) requires surface modification due to high sheet resistance and charge transfer resistance. In this paper, we report a simple, solution-based method of preparing FTO-free counter electrodes based on metal (Pt) or metal sulfide (Co 8.4 S 8 , Ni 3 S 2 ) nanoparticles/CNT composite films to improve device performance. Based on electrochemical studies, the relative catalytic activity of the composite films was Pt > Co 8.4 S 8 > Ni 3 S 2 . We achieved a maximum efficiency of 3.76% for the device with an FTO-free counter electrode (Pt/CNT). The device with an FTO- and Pt-free (CoS/CNT) counter electrode gives 3.13% efficiency. Improvement in the catalytic activity of CNT thin films was observed when Ni 3 S 2 , Co 8.4 S 8 , or Pt nanoparticles are deposited in close contact with CNT surfaces as counter electrodes in DSSC.