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 (Rct of approximately 2.5 ohm) and series resistance (Rs of approximately 14 ohm) that are even better than those for Pt/FTO CEs (Rct=6.36 ohm and Rs=16.4 ohm). Owing to these excellent Rct and Rs 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.