Platinum-free metal sulfide counter electrodes for DSSC applications: Structural, electrochemical and power conversion efficiency analyses

Micro/Nanoscale binary (CoS, NiS, and ZnS) and ternary (CoNi₂S₄ and Zn_0.76Co_0.24S) metal sulfides were directly grown on FTO substrates by a novel one step hydrothermal method for low-cost DSSC counter electrode applications. XRD study confirms phase pure formation of CoS, NiS, ZnS, CoNi₂S₄ and Zn_0.76Co_0.24S. HRSEM images of CoS, NiS and CoNi₂S₄ films exhibits hierarchical flower, hexagonal microcage and flake like particulates respectively. The ZnS and Zn_0.76Co_0.24S films show spherical micro-nanoparticles. The nanoflake morphological CoS and CoNi₂S₄ revealed high surface area measured by BET analysis. XPS result confirms the chemical states of CoNi₂S₄ and Zn_0.76Co_0.24S. A detailed electrochemical cyclic voltammetry, Tafel polarization and electrochemical impedance spectroscopy (EIS) analyses reveals the ternary metal sulfide (CoNi₂S₄) possesses an excellent electrocatalytic activity and electrical conductivity than other counter electrodes for the reduction of tri-iodide to iodide. Photovoltaic performance of the devices was studied using standard solar simulator at 1 Sun intensity (AM 1.5 G) and dark conditions. The reduced dark current obtained in CoNi₂S₄ and Zn_0.76Co_0.24S confirms the less recombination of electrons in ternary metal sulfides based DSSCs. The power conversion efficiency of ~4.03% achieved in Pt-free CoNi₂S₄ counter electrode is comparable to standard Pt based DSSC (~4.59%). The CoNi₂S₄ exhibiting good device performance and high electrocatalytic activity can serve as efficient low-cost counter electrode for Pt-free DSSCs.