Nanoclimbing-wall-like CoSe₂/carbon composite film for the counter electrode of a highly efficient dye-sensitized solar cell: A study on the morphology control

Three types of composite films of CoSe₂and carbon (CoSe₂/C), which have different morphologies, were obtained on fluorine-doped tin oxide (FTO) substrates via a simple two-step synthesis, i.e., first a pulse-potential electro-deposition and then an annealing process. A two-stage growth mechanism is proposed: stage 1 involves the formation of three types of CoSe₂/C films consisting of nanowalls through an electro-deposition process, by using bathes with different pH values; stage 2 involves "morphological transformation" of the films in an annealing process. After the annealing, nanograin (NG)-like, nanorock (NR)-like, and nanoclimbing-wall (NCW)-like films of CoSe₂/C were obtained on the FTO substrates. The film of CoSe₂/C-NCW exhibits a great conductivity, a good intrinsic heterogeneous rate constant, a large effective surface area, and thereby an excellent electro-catalytic ability toward triiodide (I3-) reduction. All the annealed CoSe₂/C films were used as the counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). The DSSC with the CoSe₂/C-NCW exhibits the highest solar-to-electricity conversion efficiency (η) of 8.92±0.03%, which is found to be even higher than that of the cell with a platinum (Pt) CE (8.25±0.03%). The CoSe₂/C-NCW was further electro-deposited onto two low-cost, highly porous, and flexible substrates, i.e., nickel foam (NF) and carbon cloth (CC). Each tiny part of the NF or the CC is observed to be covered with the CoSe₂/C-NCW, as in a core shell structure, where NF or CC is the core and CoSe₂/C-NCW is the shell. The DSSC with the CoSe₂/C-NCW on NF exhibits a very high η of 10.46±0.23% at 1 sun and an η of 7.90±0.05% at 0.2 sun. The low-weight CC rendered for its DSSC a high η of 9.87±0.06% at 1 sun and an η of 7.83±0.03% at 0.2 sun. The low cost CoSe₂/C-NCW is an attractive replacement for the expensive Pt for DSSCs, and is also suitable for outdoor, indoor, or wearable DSSCs.