Optimizing platinum-free counter electrodes for dye-sensitized solar cells: Multiwalled carbon nanotube forests covered with ruthenium layers

Multiwalled carbon nanotube (MWCNT) forests have extremely large surface areas and high catalytic activity. To use them as alternative materials for Pt/fluorine-doped tin oxide (FTO) conventional counter electrodes (CEs) in dye-sensitized solar cells (DSSCs), a unique Ru metal layer, prepared with 600-cycle atomic layer deposition, was employed. The best Ru-coated MWCNT forest (Ru/MWCNT) CE performance was achieved by examining the experimental conditions for the MWCNT forests, including deposition processes, catalyst thicknesses and synthesis time. The results revealed that 20μm-thick MWCNT forests with numerous defects/disordered sites exhibit excellent CE performance. In practice, Ru/MWCNT CEs prepared under optimal synthesis conditions show low charge transfer resistance (R_ct of approximately 2.5 ohm) and series resistance (R_s of approximately 14 ohm) that are even better than those for Pt/FTO CEs (R_ct=6.36 ohm and R_s=16.4 ohm). Owing to these excellent R_ct and R_s values, dye-sensitized solar cells with optimized Ru/MWCNT CEs showed better performance than those containing Pt/FTO CEs. Finally, post-heat treatment of Ru/MWCNT CEs increased the cell efficiency of the DSSCs.