Selective Synthesis of FeS and FeS<sub>2</sub> Nanosheet Films on Iron Substrates as Novel Photocathodes for Tandem Dye-Sensitized Solar Cells

FeS and FeS<sub>2</sub> nanosheet films were selectively synthesized on iron substrates through one-step hydrothermal treatment of iron foil and sulfur powder in the presence or absence of hydrazine. The resulting FeS<sub><i>x</i></sub> (<i>x</i> = 1, 2) nanosheet films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) and used as novel photocathodes in tandem solar cells with dye-sensitized TiO<sub>2</sub> nanorod films as the corresponding photoanode. The photovoltaic properties of the tandem dye-sensitized solar cells were carefully studied. We found that the performance of the FeS nanosheet film photocathode was better than that of the FeS<sub>2</sub> one in the tandem dye-sensitized solar cells. In the case of the FeS nanosheet film photocathode, a short circuit photocurrent (<i>I</i><sub>sc</sub>) of 2.53 mA/cm<sup>2</sup>, an open circuit photovoltage (<i>V</i><sub>oc</sub>) of 0.60 V, a fill factor (FF) of 0.31, and conversion efficiency (η) of 1.32% were obtained under an illumination of 100 mW/cm<sup>2</sup>. This study suggests that these iron sulfide nanosheet films are attractive photocathodes for tandem dye-sensitized solar cells.