One-step in situ growing CoS_1.097 nanoplates on flexible graphite paper as efficient and stable FTO-free counter electrodes for dye-sensitized solar cells

Hexagonal single crystal CoS_1.097 nanoplates with the size of 200~500 nm have successfully in situ grown on flexible graphite paper (GP) and FTO substrates with a one-step hydrothermal method, which are used as counter electrodes (CEs) of dye-sensitized solar cells (DSSCs) and show different microstructures and different electrocatalytic activities for I₃⁻ reduction. The DSSC based on CoS_1.097/GP CE shows larger fill factor and higher short-circuit current density than the DSSCs with Pt/FTO CE and CoS_1.097/FTO CE, which attribute to the excellent electrical conductivity of GP and predominant electrocatalytic activity of hexagonal single crystal CoS_1.097 nanoplates with the addition of a small electrocatalytic contribution of GP. Therefore, the DSSC with the CoS_1.097/GP CE shows the highest photoelectric conversion efficiency (6.99%) among these DSSCs. Furthermore, the CoS_1.097/GP CE still shows excellent electrochemical and mechanical stability in the iodine-based electrolyte after enduring the S-type mechanical perturbation. This work indicates the flexible CoS_1.097/GP electrode is a promising candidate to replace Pt/FTO CE as a Pt-free, FTO-free, low cost, exceptionally stable, and high-efficient CE of DSSC.