Highly transparent ultrathin metal sulfide films as efficient counter electrodes for bifacial dye-sensitized solar cells

Highly transparent metal sulfides (Ni 3 S 4 , MoS 2 ) are directly deposited on fluorine doped tin oxide (FTO) substrates as effective Pt-free counter electrodes (CEs) for novel bifacial dye-sensitized solar cells (DSSCs). The blocky-like Ni 3 S 4 and petal-shaped MoS 2 nanoparticles (NPs) possess large surface area, which provides amount of active site to contact with electrolyte. The films not only show high transmittance at visible wavelengths, but also exhibit prominent electrocataltytic activity and outstanding metallic conductivity in iodine/iodide based bifacial DSSCs. Under front-side irradiation (100   mW/cm 2 ), the bifacial DSSC based on the transparent Ni 3 S 4 or MoS 2 CE presents remarkable photovoltaic conversion efficiency (E ff   =   6.56%, 6.11%, respectively), closing to that of conventional Pt CE (E ff   =   6.83%). Moreover, by appending an ordinary mirror as a photo reflector to achieve both-side illumination, the bifacial DSSC using the transparent Ni 3 S 4 or MoS 2 sample as CE obtained higher short-circuit current density (J sc ) and more attractive power conversion efficiency (PCE) (E ff   =   7.06%, 6.65%, respectively). It indicates that the as-prepared transparent Ni 3 S 4 and MoS 2 CEs are promising potential candidates to explore low-cost and high-efficiency bifacial DSSCs.