Two-dimensional transition metal dichalcogenide-based counter electrodes for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) are gaining considerable interest as alternatives to semiconductor-based thin film solar cells. The noble metal platinum (Pt) is conventionally used as a counter electrode (CE) material for fabricating DSSCs, since Pt is expensive and scarce, therefore, new materials have been explored to develop cost-effective Pt-free counter electrodes. Two-dimensional (2D) graphene-based counter electrodes have achieved the highest power conversion efficiency (PCE, η) of 13%, which has stimulated research activities in 2D layered transition metal dichalcogenides (TMDs) for developing Pt-free DSSCs. In this review, progress made on alternative counter electrodes for fabricating low-cost Pt-free DSSCs, based on earth-abundant 2D TMDs including MoS₂, WS₂, TiS₂, FeS₂, CoS₂, NiS₂, SnS₂, MoSe₂, NbSe₂, TaSe₂, NiSe₂, FeSe₂, CoSe₂, Bi₂Se₃ and their based composites, are discussed and summarized. Also, the considerable progress made on thin films of MoS₂ and MoS₂ based carbon, graphene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) composites as efficient counter electrodes (CEs) for DSSCs are discussed, in terms of their electrochemical and photovoltaic properties. At present, PCE values higher than that of standard Pt CE have been recorded for a number of TMD-based CEs, which include MoS₂ and MoSe₂/thin films deposited on Mo foil, MoS₂/CNTs, MoS₂/graphene, MoS₂/carbon, MoSe₂/PEDOT:PSS, NbSe₂, FeS₂, FeSe₂ nanosheets, TiS₂/graphene, and NiS₂/graphene hybrid systems in DSSCs, for the reduction of triiodide (I₃⁻) to iodide (I⁻). The highest PCE (η = 10.46%) versus Pt CE (η = 8.25%) at 1 Sun (100 mW cm⁻², AM 1.5G) was measured for DSSCs having a low cost and flexible CoSe₂/carbon-nanoclimbing wall counter electrode deposited on a nickel foam. Though TMD-based materials show great potential for solar cell devices, their long-term stability is equally important. The DSSCs with a TiS₂/graphene hybrid, and TiS₂/PEDOT:PSS composite CEs, showed stability up to 20 to 30 days, respectively, without any measurable degradation in the photovoltaic performance. The long-term stability of TMDs-based DSSCs under different environmental conditions is also described in view of their commercial applications.