Metal sulfide nanosheet–nitrogen-doped graphene hybrids as low-cost counter electrodes for dye-sensitized solar cells

Transparent thin two-dimensional nitrogen-doped graphene (NG) nanosheets coated with Ni- and Cu-doped MoS₂ nanosheets (denoted as Ni-Mo-S@NG and Cu-Mo-S@NG, respectively) were synthesized by a simple hydrothermal process. The synthesized samples were coated by a simple spray pyrolysis technique onto fluorine-doped tin oxide substrates. Scanning transmission electron microscopy analysis showed that square-like Ni-Mo-S and hexagonal-like Cu-Mo-S nanosheets were anchored on the surface of NG without aggregation in Ni-Mo-S@NG and Cu-Mo-S@NG, respectively. Raman analysis confirmed the presence of graphene in both samples because of the high G-band intensity. Cyclic voltammetry analysis indicated that Ni-Mo-S@NG possessed superior catalytic activity to Cu-Mo-S@NG as an electrode in dye-sensitized solar cells; the separation between its oxidation and reduction peak currents (Epp) of about 382 mV was smaller than that of a platinum electrode (450 mV). A solar cell with Ni-Mo-S@NG showed a higher open circuit voltage (0.72 ± 0.02 V) and fill factor (0.57 ± 0.04) than those of solar cells with Cu-Mo-S@NG and Pt counter electrodes. The solar cell with Cu-Mo-S@NG showed higher current density and lower Epp (421 mV) than those of an equivalent cell with Pt. The solar cells with Ni-Mo-S@NG and Cu-Mo-S@NG showed power conversion efficiencies of 2.85% and 2.62%, respectively, which were equivalent to that of a cell with a Pt counter electrode (2.41%).