Solution Processed NixSy Films: Composition, Morphology and Crystallinity Tuning via Ni/S-Ratio-Control and Application in Dye-Sensitized Solar Cells

The design and facile fabrication of highly efficient, cost-effective and earth abundant counter electrode material on an electrode surface is highly desirable for the application of dye-sensitized solar cells (DSSCs). Herein, using a N,N-dimethylformamide based solution process, transition metal chalcogenide Ni x S y films have been obtained by spin-coating a NiCl 2 -thiourea (Ni-Tu) solution combining mild thermal treatment. XRD, SEM and TEM characterizations reveal that the current procedure allows for phase composition (Ni 3 S 2 , NiS and NiS-NiS 2 ), crystallinity , morphology (film uniformity and compactness) control of the films through simple adjusting Ni/S ratio in the precursor solution. Electrochemical analysis indicates that the FTO supported Ni 3 S 2 and NiS films exhibit excellent electrocatalytic activity toward the reduction of triiodide, resulting in higher photo-electric conversion efficiencies of 6.86% and 6.95% when used as counter electrode in DSSCs, versus 6.66% for Pt. In particular, even without the support of conductive FTO layer, Hall effect measurements and electrocatalytic analysis reveal that pristine Ni x S y films exhibit good electrical conductivity and electrocatalytic activity, yielding a highest photo-electric conversion efficiency of 4.41% when used as counter electrode in DSSCs. Our study thus provides a facile procedure which allows for composition, morphology optimization and high performance for low-cost, large-scale DSSC application.