Multi-walled carbon nanotube forests covered with atomic-layer-deposited ruthenium layers for high-performance counter electrodes of dye-sensitized solar cells

Abstract On account of the high surface area and good catalytic activity of multi-walled carbon nanotube (MWNT) forests, they have great potential for use as alternative materials to conventional Pt/fluorine-doped tin oxide (FTO) counter electrodes (CEs). Nevertheless, the inferior charge collection efficiency of MWNT forest CEs is a drawback that needs to be overcome. Here, we uniquely introduce a Ru layer on MWNT forests by a typical atomic layer deposition (ALD) process. Under an optimal ALD cycle number of 600, Ru layers directly connect vertically grown MWNTs by acting as their bridges while also retaining the microporous structure of the MWNTs. As a result, deposition of the Ru layer under the optimal conditions causes a decrease in the resistances of as-synthesized MWNT forests (charge transfer resistance R ct : from 155.3 Ω to 4.12 Ω; series resistance R s : from 205.1 Ω to 12 Ω); these decreased resistances are better than even those of Pt/FTO CEs ( R ct  = 14.2 Ω and R s  = 17.6 Ω). Dye-sensitized solar cells with the optimally deposited Ru/MWNT forest CEs show performances equivalent to those of devices with Pt/FTO CEs on account of the excellent R ct and R s values of the former CEs.