Highly crystalline, small sized, monodisperse alpha-NiS nanocrystal ink as an efficient counter electrode for dye-sensitized solar cells

We report the synthesis of highly crystalline, small sized, alpha-NiS nanocrystal inks for the fabrication of a counter electrode for dye-sensitized solar cells. Monodisperse alpha-NiS nanocrystals (about 7 nm) are obtained via a noninjection, solution-phase chemical synthesis method. During the growth process of alpha-NiS nanocrystals, the Ni-oleate complex, which is generated in situ from the reaction of nickel chloride and sodium oleate, is decomposed and acts effectively as a growth source in synthesizing monodisperse nanocrystals. By controlling the reaction temperature, the resultant nanocrystal sizes and crystallinity can be well tuned. Compared to conventionally obtained NiS bulk materials, monodisperse alpha-NiS nanocrystals possess abundant catalytic reaction sites for dye-sensitized solar cells due to their small particle size and high crystallinity. First-principles calculations have been employed for the first time to investigate the adsorption energy of I3 molecules on the (111) surface of alpha-NiS with equilibrium shape. DSSCs based on monodisperse alpha-NiS nanocrystal ink with higher crystallinity display a power conversion efficiency of 7.33%, which is comparable to that based on the Pt cathode (7.53%), but significantly higher than that based on the bulk NiS (4.64%) and relatively low-crystalline alpha-NiS nanocrystals (6.32%). It can be attributed to more reaction catalytic sites due to the surface effect of small alpha-NiS nanocrystals, and the highest work function level (5.5 eV) that matched the redox shuttle potential. We believe that our method paves a promising way to design and synthesize advanced counter electrode materials for energy harvesting.