We report the synthesis of highly crystalline, small size, α-NiS nanocrystal inks for the fabrication of counter electrode of dye-sensitized solar cells. The monodisperse α-NiS nanocrystals (about 7 nm) are obtained via a noninjection, solutionphase chemical synthesis method. During the growth process of α-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 crytallinity can be well tuned. Compared to conventional obtained NiS bulks, the monodisperse α-NiS nanocrystals possess an abundance of reaction catalytic sites for dye-sensitized solar cells due to the small particle size and high crystallinity. The first-principles calculations have been first employed to investigate the adsorption energy of I3 -molecule on (111) surface of α-NiS with equilibrium shape. The DSSCs based on monodisperse α-NiS nanocrystal ink with higher crystallinity display the power conversion efficiency of 7.33 %, which is comparable to that based on Pt cathode (7.53 %), but significantly higher than that based on the bulk NiS (4.64 %) and lower crystallinity α-NiS nanocrystals (6.32 %). It can be attributed to more reaction catalytic sites due to the surface effect of small α-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.

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