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.

The W18O49 nanowire (NW)‐reduced graphite oxide (W18O49‐rGO) nanocomposite was prepared by a facile solvothermal reaction. The W18O49 NWs were stably dispersed on the surface of rGO, and the content of W18O49 NWs in the composite can be tailored by controlling the mass ratio of reactants (WCl6 and graphite oxide). Employing the W18O49‐rGO nanocomposite with the proper W18O49 NW content of 33% as a counter electrode of dye‐sensitized solar cells (DSSCs), it gets a photoelectrical conversion efficiency (η) of 7.23%, which is close to that of Pt‐based DSSC (7.39%). The high efficiency of DSSCs based on W18O49‐rGO counter electrode is contributed to the intrinsic electrocatalytic activity of W18O49 NWs and excellent electron‐transfer characteristics of rGO. The first‐principles calculations have been first employed to investigate the adsorption energy between I3− molecule and W18O49 NW to explain its good catalytic activity. The cyclic voltammetry (CV), electrochemical impedance spectra (EIS), and Tafel polarization tests can further identify that the W18O49‐rGO counter electrode has high electrocatalytic activity for the reduction of I3− and low interface charge transfer impedance. It is believed that the W18O49‐rGO nanocomposite can be used as a promising counter electrode material for DSSCs.